Free-electron lasers provide powerful scientific research tools for humans to understand and explore the world. Compared with traditional lasers, free-electron lasers have characteristics such as continuously adjustable wavelength over a significant range, high peak power, narrow bandwidth, and full coherence. They have also been widely used in frontier scientific research fields, such as advanced materials, condensed matter physics, atomic and molecular physics, chemistry, biology, and medicine.

In recent years, free-electron laser light sources have rapidly developed. CTFEL is the only high-power terahertz free-electron laser facility driven by a superconducting accelerator in China. However, its capabilities are limited. Currently, the Chinese Academy of Engineering Physics is developing an infrared terahertz free-electron laser facility based on CTFEL. The upgraded facility is a multifunctional user facility for frontier research in the fields of material science, spectroscopy, biology, and medicine.

The upgraded facility adopts resonant optical cavity free-electron laser technology. A straight section of the CTFEL with a 320 kV HV-DC photocathode electron gun and a 2×4-cell superconducting accelerator are used as the injector system. The driving laser and photocathode systems are upgraded for compatibility with both GaAs and Cs₂Te. The main accelerator system consists of two 2×9-cell superconducting accelerator modules that can increase the electron beam energy to a maximum of 50 MeV. In addition, two new undulators (U48 and U35) are added to expand the free-electron laser frequency coverage from 0.1‒125 THz, and improve the maximum marco-pulse power to over 100 W.

The upgraded facility has three different working modes FEL, FLASH, and ERL (Fig.5). In the FEL mode, four electron beam transmission paths pass through four undulators to generate FEL radiation in different frequency bands. Notably, the electron beams can choose only one path for transmission. In the FLASH mode, after passing through the main accelerator and the first 180° deflection section, the electron beams directly enter the R101 biomedical experimental station for the X-ray FLASH irradiation experiments. In ERL mode, the facility operates in a quasi-continuous wave mode and can be used as a small energy recovery linear accelerator experimental platform for key physical and technical research on ERL. The average beam current is 1‒3 mA. The injector energy is 6‒8 MeV, and the recirculation energy, approximately 20 MeV.

Two experimental user stations, a biomedical experimental station and a material spectrum experimental station, were developed. The biomedical experimental station was located on the first floor of the laboratory. According to its function, it is divided into two line stations, R101 and R102, which predominantly include the cell biology experiment, neurobiology experiment, and X-ray FLASH radiotherapy experiment platforms. The material spectrum experiment station is located on the second floor of the laboratory, and it is divided into three line stations R201, R203, and R204, including the transient excitation loading-ultrafast spectral detection system, multi-physical field time-resolved pump detection system, off-site lithium battery-infrared THz free-electron laser experimental platform, and terahertz parameter measurement calibration experimental platform. In addition, R202 is a FEL beam quality diagnosis platform for the online real-time diagnosis of FEL parameters.

This study primarily introduces the general design of the infrared terahertz free-electron laser facility of the Chinese Academy of Engineering Physics. The upgraded facility adds two 2×9-cell superconducting accelerator modules and two undulators based on CTFEL, to increase the maximum electron energy to 50 MeV, expand the spectrum coverage to 0.1 THz‒125 THz, and realize the maximum macro-pulse average power to greater than 100 W. Through a track-shaped beamline design, an experimental energy-recovery linear accelerator research platform will be built. In addition, two user experimental stations for material spectroscopy and biomedicine are under construction.

Terahertz (THz) radiation has emerged as a crucial tool in various research domains, including matter manipulation, electron acceleration, and biological physics. The distinct properties of THz radiation are attributed to its unique frequency range between the far infrared and microwave regions, which covers the vibrational and rotational frequency of numerous materials, whereby THz radiation acts as an effective tool for resonant manipulation of such materials. Furthermore, the smaller wavelength of THz radiation than microwaves and the short duration of pulsed terahertz sources result in high peak field strength, presenting an enormous potential for nonlinear matter control and electron acceleration.

Terahertz spectroscopy serves as a fundamental component of terahertz application research. However, its unique frequency range presents challenges in detecting terahertz spectrum, as traditional electronic and optical methods are not readily applicable. Intuitively, terahertz energy can be measured with energy meters such as bolometers or pyroelectric detectors. Through the scanning by the terahertz bandpass filter, the terahertz spectrum can be measured. The most prevalent technique for terahertz spectrum detection is the scanning free-space electro-optical sampling method, which necessitates multiple shots to measure the terahertz time-domain waveform. The frequency spectrum can be obtained with a Fourier transformation of the time-domain waveform. However, for low repetition rate terahertz sources and irreversible processes, the scanning method is challenging to utilize, necessitating the development of single-shot terahertz detection techniques.

This study presents various single-shot terahertz time-domain waveform and frequency spectrum measurement techniques. The first section discusses single-shot time-domain waveform measurement methods, where the fundamental concept is to encode terahertz time-domain information into the properties of the probe laser, including spectral-encoding and spatial-encoding methods. The chirped ultrafast laser pulse is utilized to map the time information of terahertz radiation onto the spectrum of the probe laser, which is relatively easy to implement with minor modifications of standard scanning terahertz time-domain waveform measurement techniques. However, the time resolution of this technique is limited by the chirped probe pulse duration due to the uncertainty principle. To maintain time resolution at the transform limit of the probe laser, spectral interferometry techniques have been introduced, but the setup, alignment, and data processing required for this approach are more complex than those for standard spectral-encoding methods. Spatial-encoding methods represent another type of encoding, where terahertz time-domain information is encoded into the spatial distribution of the probe laser beam. The first type of spatial-encoding method is non-collinear spatial encoding, which employs an oblique incident probe, and different parts of the beam arrive at the electro-optical crystal at different moments. The setup of this method is relatively simple, and no complex alignment is required in the experiment. However, the time resolution and time window of this approach are contingent on the incident angle and conflict with each other, necessitating careful consideration of the incident angle. The terahertz focal spot may also impact the time window and introduce distortion in the time-domain waveform. The second type of spatial-encoding method utilizes echelons, which have a stair-like shape, to introduce a time interval between different parts of the probe beam. Reflective echelons are more appropriate for measuring terahertz pulses with short pulse duration and may achieve higher time resolution than transmissive echelons. We conducted four types of single-shot terahertz time-domain waveform measurements in our experiment, and their results are presented. However, the spectrum bandwidth using the methods mentioned above may still be limited by the response of the electro-optical crystal and the pulse duration of the probe laser, even if the time resolution is sufficiently high through careful design of the terahertz time-domain waveform measurement system.

The second section discusses single-shot terahertz frequency spectrum measurement methods. For terahertz radiation, such as terahertz sources based on laser-solid density plasma interactions, whose frequencies can be up to 30 THz, our research team developed two types of ultrawide bandwidth single-shot spectrometers. The first spectrometer employs multiple terahertz energy detectors with varying terahertz bandpass filters. The terahertz beam is split using silicon wafers, enabling the measurement of the terahertz spectrum in a single shot. This approach has a relatively simple optical design, but the spectrum bandwidth and resolution are restricted by the bandpass filter. To achieve high spectrum resolution with wide spectrum bandwidth, we designed and experimentally demonstrated a novel terahertz autocorrelator, which has ultrawide bandwidth with high frequency resolution.

This study provides an overview of several single-shot terahertz time-domain waveform and frequency spectrum measurement methods developed at the Key Laboratory of Optical Physics in the Institute of Physics, Chinese Academy of Sciences. The emphasis is placed on the design principles and characteristics of each method. These techniques are particularly applicable to strong terahertz sources with low repetition rates and are expected to significantly contribute to the characterization of terahertz radiation, an enhanced understanding of terahertz generation mechanisms, and the advancement of terahertz application research.

The application of modern nanotechnology has been greatly facilitated by the studies of ultrafast dynamics at the nanoscale. Terahertz radiation occupies a unique position in the electromagnetic spectrum, making it a popular choice for the exploration of various physical properties. Through advancement in femtosecond laser technology and terahertz radiation source, the intensity of terahertz radiation has seen a tremendous increase, broadening its range of applications. In recent years, terahertz time-domain spectroscopy (THz-TDS) has achieved remarkable commercial success. Consequently, terahertz radiation has become a key diagnostic tool in the development of new technologies. However, the diffraction limit of hundred microns hinders the use of terahertz radiation at the nano- and sub-nanoscale, thus emphasizing the imperative for the development of terahertz microscopy to further promote terahertz technologies.

In 1981, the first scanning tunneling microscope (STM) was conceived in Zurich, enabling the real-time observation of individual atomic arrangements on matter surfaces and elucidating the physical and chemical properties related to the electrons’ behavior of the surface. This breakthrough was recognized by the global scientific community as one of the world’s top ten scientific and technological achievements of the 1980s. However, the time-resolution of STM is limited to milliseconds, making it impossible to track ultrafast dynamics in materials. To address this challenge, terahertz scanning tunneling microscopy (THz-STM) was developed. By using an atomic-scale tip to enhance and confine terahertz radiation, the spatial resolution of terahertz imaging can be improved by up to six orders of magnitude. This review outlines the historical progression, fundamental principles, system configuration, and potential applications of THz-STM, including ultrafast dynamics studies in semiconductor surfaces and single molecules. Finally, prospects for future advancements in this field are discussed.

THz-STM utilizes quantum tunneling effect akin to the conventional STM, but with the difference that it couples terahertz pulses to the STM tip, by using the time-varying terahertz electric field instead of the direct current (DC) bias voltage. Consequently, a comparable spatial resolution to that of STM is attained, along with sub-picosecond temporal resolution by adjusting the relative delay between the pumping and probing pulses. A THz-STM system comprises two primary parts: a strong-field terahertz system and a traditional STM system. Currently, large-area photoconductive antennas (Fig.2) and optical rectification in lithium niobate crystal (Fig.3) are utilized to generate strong terahertz field. To implement the terahertz pump-probe technique, the optimal optics guides terahertz pulses to a Michelson interferometer, which splits a terahertz pulse into the pumping and probing pulses. Varied delay is achieved by adjusting the length of one of the arms of the interferometer.

THz-STM finds primary applications in semiconductor and molecular research. Ten years ago, when the first THz-STM system was demonstrated in University of Alberta, it was explored to conduct an optical pump-terahertz probe investigation on InAs nanodots on GaAs substrate, successfully observing the ultrafast dynamics of carriers captured by an InAs nanodot. Subsequently, Jelic et al. improved the system by operating the THz-STM system in an ultra-high vacuum chamber, which increased the spatial resolution to the angstrom level along with a temporal resolution on the order of hundreds femtoseconds, allowing for single unit cell reconstruction on the surface of Si(111)-(7×7) (Fig.4). In 2021, the THz-STS was presented for graphene nanoribbons with atomic precision as well (Fig.5). In particular, THz-STM offers captivating possibilities for single-molecule investigations. In 2016, Cocker et al. achieved single-electron tunneling on a pentacene molecule with a THz-STM system, which triggers molecular resonance (Fig.6). Four years later, Peller et al. used THz-STM to achieve ultrafast control of single-molecule switching motion, proposing a more convenient observation method (Fig.7).

This article reviews the development of THz-STM. The ultrahigh spatiotemporal resolution of THz-STM makes it possible for researchers to observe ultrafast dynamics at the atomic scale, thus promoting the fusion of terahertz technology with other research fields. It is believed that direct observation of the microscopic world and further understanding of some basic physical processes can be achieved through the use of THz-STM in the future. Furthermore, the incorporation of new technologies holds the potential to improve the performance of THz-STM. Promising avenues for further exploration include time-resolved terahertz pulse induced electroluminescence, terahertz tunnel current induced single photon emission, ultrafast atomic force microscopy and multi-probe THz-STM system. In particular, a static STM has been shown to be compatible with the magnetic field. Research can also be conducted to study the nature of metasurfaces and provide solutions to the practical challenges associated with them.

In conclusion, the THz-STM reviewed in this paper has immense potential for application and will be a significant presence in microphysical research, thus requiring more researchers to explore the field.

Terahertz (THz) radiation, with the frequency lying between microwave and infrared, has rich information content, high temporal-spatial coherence, low photon energy, strong penetration, and high bandwidth. Therefore, the THz radiation holds immense application value in fields such as national security, satellite communications, non-destructive material testing, and medical imaging. However, the current THz sources with the small energy cannot meet the demand of scientific and application research. The strong-field THz source, with peak electric fields larger than 1 MV/cm, low photon energy, and high temporal resolution, has become the research target of many scientists.

Up to now, the THz radiation output through photonics has attracted enough interests due to its ultra-fast resolution and ultra-wide spectrum. The lithium niobate (LiNbO₃, LN) crystal, exhibiting weak THz absorption, a large second-order nonlinear coefficient, and stable physical and chemical properties, is one of the first materials to achieve THz pulse output. To be noted, since Hebling and co-workers proposed the tilted-pulse-front technique, the LN THz source has been greatly improved in terms of output power and conversion efficiency. Recently, Wu and co-workers achieved an ultra-strong LN-THz radiation with a single pulse energy of 13.9 mJ, a conversion efficiency of 1.2%, and a peak field strength of 7.5 MV/cm. Apart from the laser technique, the property of LN crystal is the main factor for the THz source. Moreover, the traditional LN crystal is a classical non-stochiometric crystal with high-concentration intrinsic defects, which greatly affects the crystal property. Correspondingly, some improvement methods have been exploited including magnesium-doped and near-stochiometric LN (SLN) crystals. Therefore, in order to promote the development of LN THz source, we analyze the performance requirements and development direction of LN crystals for strong-field THz sources.In 2008,Stepanov et al. used the large size MgO∶SLN crystal pumped by the pulsed laser with 6-mm-diameter spot, and got the single-period THz pulse with 30 μJ energy. Since then, the pump light with large-size spot with the small energy flow density became an effective method for strong-field THz radiation. Herein, the large-size and high-quality LN crystal is the key matrix material. In 2012, Fül?p and co-workers achieved the 0.4 mJ THz pulse from the MgO∶SLN crystal pumped by 186 mJ with the spot size of 8.1 mm×20 mm. The MgO∶SLN crystal was grown from Li-rich melt by the Czochralski method, which increased the growth difficulty of large-size crystal. Therefore, scientists started to use the Mg-doped congruent LN crystal (MgO∶CLN), and continued to explore the strong LN THz radiation. In 2021, Zhang et al. used a spliced MgO∶CLN crystal with the size of 64 mm×40 mm, and achieved a 1.4 mJ THz radiation with an energy conversion efficiency of 0.7% and a peak electromagnetic field of 6.3 MV/cm. Moreover, Wu and co-workers improved the technique and achieved an ultra-strong terahertz radiation with a single pulse energy of 13.9 mJ, a conversion efficiency of 1.2%, and a peak field strength of 7.5 MV/cm. This represents the highest values reported internationally using this method.

Though the LN crystal is one of the first materials to achieve THz pulse output, the LN THz radiation has not been improved due to the refractive index difference between the pump laser and the THz radiation in this crystal until the tilted-pulse-front technique. In 2002, Hebling and co-workers proposed the tilted-pulse-front technique for LN, allowing the propagation direction of the pump laser energy inside the crystal to phase-match with the group velocity of the THz waves, greatly improving the energy conversion efficiency from pulsed lasers to THz pulses. In 2003, Stepanov et al. used 2% Mg (molar fraction) doped MgO∶SLN crystal to obtain a THz pulse of 98 pJ under a 2.3 μJ, 200 kHz near infrared laser pump, and the energy conversion efficiency was only 0.0043%. The experimental results showed that the subpicosecond free space terahertz radiation can be generated by pumping LiNbO₃ crystals using femtosecond laser pulses based on tilted wavefront technology.

Based on the tilted-pulse-front technique, the LN crystal pumped by femtosecond laser is a promising material for strong-field THz sources. Uniformly magnesium-doped LN, with a large laser damage threshold, can meet the demand of high-power pumping laser. The low-concentration magnesium doped near-stoichiometric LN crystal, with a larger nonlinear coefficient and lower THz wave absorption coefficient, is a promising material for high-beam-quality, high-efficiency, and high-stability strong-field THz sources. To lower the energy density of the pump laser and reduce damage to the crystal, a large-aperture crystal is necessary to obtain high-energy strong-field THz sources. In the future, the x-axis LN crystal with ultra-large diameter (300 mm) may support the ultra-large aperture sample exceeding 200 mm, which holds promise for achieving extremely high-energy strong-field THz sources.

Terahertz (THz) waves refer to electromagnetic waves within the frequency range of 0.1‒10 THz, corresponding to wavelengths from 3 mm to 0.03 mm. THz techniques have found wide applications in fields such as materials science, non-destructive testing, biomedicine, security imaging, and next-generation communications, thus propelling the rapid development of THz photonics. However, whether in fundamental research or practical applications, effective manipulation of THz waves is essential to achieve functionalities such as frequency conversion, directional transmission, mode conversion, and phase control. Moreover, THz nonlinear effects are usually constrained by the electric field intensity and interaction distance.

For ionic crystals, incident THz waves couple with optical phonons in the material to form stimulated phonon polaritons, which introduces a novel mechanism for the interaction between THz waves and crystal materials. Stimulated phonon polaritons hold the potential to effectively control and dominate the interaction between THz waves and crystals, and offer a theoretical basis and technical means for studying the nonlinear effects of THz waves in crystals, thereby opening up new possibilities for the development of strong-field THz science and technology. Therefore, it is crucial and necessary to summarize research advances of stimulated phonon polaritons.

This paper reviews the research progress of THz waves transmission modulation and nonlinear effects based on stimulated phonon polaritons. Firstly, after a brief introduction, the basic physical concepts and optical properties of phonon polaritons and stimulated phonon polaritons are introduced. With the influences of external input THz waves, stimulated phonon polaritons are excited, which are described by nonlinear Huang equations. In comparison with the spontaneous phonon polaritons described by classic Huang equations, the stimulated ones show mainly three differences: stronger intensity, more coherence, and delocalization. This is mathematically described by nonlinear Huang equations.

Secondly, the three excitation methods of stimulated phonon polaritons and corresponding diagrams of experimental setups (Fig. 2) are introduced. Femtosecond laser pulse pumping ferroelectric crystals such as lithium niobate (LN) is one of the most popular methods of the excitation of stimulated phonon polaritons. Tilted pulse fronts and lateral excitation can meet the velocity matching conditions, improve the excitation efficiency of the excited stimulated phonon polaritons, and modify their center frequency and pulse width. Then, the detection of stimulated phonon polaritons is described. Spatiotemporal super resolution quantitative imaging system (Fig.3) can obtain the complete spatiotemporal evolution process of stimulated phonon polaritons by using pump-probe and phase contrast technique.

Thirdly, THz waves transmission modulation based on stimulated phonon polaritons is mainly achieved by three methods: topological valley transport (Fig.4), asymmetric transmission (Fig.6), and “frozen-phase” propagation (Fig.8). In the topologically protected state, THz waves exhibit valley Hall effect and make smooth detours when encountered with wide angle (120°) bends, while the trivial ones are majorly scattered. In the subwavelength waveguide with phase gradient metasurfaces, THz waves are capable of asymmetric propagation with bandwidth up to 100 GHz by mode conversion. Under lateral excitation, the first order dispersion of THz waves is eliminated, resulting in a phase-invariant propagation. These results lay the foundation for on-chip directional transmission, mode conversion and phase control of THz waves on chip, promoting the practical development of THz integrated devices.

Fourthly, in the ionic crystal, the delocalized stimulated phonon polaritons would lead to a giant enhancement of the optical nonlinearity at THz frequency by increasing the ionic polarization. Different from the heat-excited spontaneous phonon polaritons, the specialty of the stimulated phonon polaritons lies in the attendance of external coherent THz driving and the strong delocalization, which breaks the traditional light-matter interaction mechanism. Once THz waves are employed in the polar material, stimulated phonon polaritons are generated (Fig.10). They transport the ionic states by electromagnetic fields in the whole material, indicating a strong delocalization of the stimulated phonon polaritons and ionic states. Furthermore, the external driving field makes the noncoherent ionic oscillations in spontaneous phonon polaritons become coherent, which is guaranteed to confirm the external driving THz field and behaves in a regular temporal phase evolution. Moreover, the spatial coherence of the stimulated phonon polaritons is protected by the temporal coherence and strong delocalization. Therefore, THz waves can directly excite the ionic polarization via stimulated phonon polaritons-mediated light-matter interaction. This results in a significant nonlinear light-matter interaction and induces a series of phenomena at the THz frequencies. Such high nonlinearities may prove valuable in practical applications such as on-chip integration of THz waves.

Stimulated phonon polaritons in ionic crystals represent not only a continuation of elementary excitations in condensed matter physics, but also a crucial branch in the future development of THz nonlinear physics. Stimulated phonon polaritons exceed Born-Oppenheimer approximation in physics, enabling a novel mechanism of interaction between light and matter. This mechanism turns phonon polaritons, originally affecting material spectra and heat capacity, into active participants in the interaction between light and matter. Through synergizing with strong-field THz radiation, stimulated phonon polaritons can modify the light-matter interaction process, enhancing nonlinear susceptibility and potentially further boosting THz nonlinear effects. Moreover, nonlinear Huang equations suggest that stimulated phonon polaritons can achieve comprehensive control over materials, extending beyond the modulation of optical properties in the high-frequency range (visible and near-infrared) of crystalline materials. This control encompasses properties such as optical-thermal, opto-mechanical, energy levels, and polarization, thus promising significant advancements and breakthroughs in the development of strong-field THz science and technology in the future.

Terahertz (THz) radiation located between the optical and microwave frequency region is known as the “THz gap” (0.1‒10 THz). THz radiation has many unique characteristics, such as low photon energy, transmission of organic materials, and high spectral resolution. These unique properties confirm that THz radiation has significant application value in multiple fields such as information communication, biomedicine nondestructive testing, and scientific research. Traditional THz application is primarily confined to the weak field passive detection linear region, while the transient strong field THz can be used to actively regulate the state of matter. The high-field THz radiation source has strong application demand in nonlinear optics, quantum and condensed matter physics, and many other fields. On one hand, as a unique means of manipulation, strong field THz waves can be used for the coherent regulation of materials, such as that of the electrical and phonon states and phase transition induction. Meanwhile, it can also be utilized as a special diagnostic means for transient spectroscopy diagnosis and single imaging. The pump-probe technique corresponding to the strong field THz wave can be used to characterize ultrafast dynamic processes of energetic materials or plasmas under extreme conditions.

An ultrafast laser provides a stable reliable excitation source for THz generation and detection. THz waves can be generated using ultrafast laser pumping and various excitation media, such as photoconductive antennas, nonlinear crystals, metallic copper foil, air, and liquid water. With continuously increasing THz field strength, matter manipulation using high field THz and nonlinear spectroscopy in the THz region has been recently promoted. Furthermore, it has been demonstrated that THz absorption spectroscopy could assist in revealing excited state dynamics, and that THz emission spectroscopy could also be used to distinguish the strong coupling of carriers, excitons, phonons, and other elementary excitations.

In this study, THz sources based on an ultrafast laser are reviewed. Ultrafast laser-driven photoconductive antennas are a traditional THz source widely used for THz time-domain spectroscopy (TDS) (Fig. 1). Recently, Darrow et al. from Columbia University, demonstrated that large aperture antenna (LAPCA) produces a much higher THz field than a traditional photoconductive antenna. You et al. from Columbia University and Ropagnol et al. from the University of Quebec used different semiconductor materials as the LAPCA substrate, both of which produce high-filed THz radiation. Ultrafast laser-driven optical rectification (OR) (Fig.2) and difference frequency generation (DFG) in nonlinear crystals are important methods for high-field THz generation. Hebling et al. from the University of Pe˙cs, proposed the tilted pulse front technique (TPFP) to fulfill the phase mismatch condition between the infrared pump pulse and the generated THz pulse in lithium niobate (LN) crystal (Fig.3). The energy conversion efficiency from the pump to THz is further improved using cryogenic technology. Besides LN, organic crystals such as DAST, DSTMS, and OH1, have also been used for THz generation via OR owing to their high nonlinear coefficients and collinear phase-matching. Using DSTMS, Vicario et al. from Paul Scherer Institute, achieved an ultra-high THz energy of 0.9 mJ at a pumping wavelength of 1250 nm (Fig.4). Liu et al. from the Max Planck Institute for the Structure and Dynamics of Matter generated high power and wideband tunable THz waves using DFG in DSTMS crystals (Fig.5). Tunable THz is also generated from other crystals such as GaSe et al. based on DFG technology. THz radiation is emitted from the intense laser-driven plasma. Cook et al. from the University of Pennsylvania, first proposed that air plasma produced by a two-color laser field could generate THz radiation. Koulouklidis et al. from the Institute of Electronic Structure and Laser of Greece, obtained a record value of 0.185 mJ THz pulse energy, which corresponds to an electric field strength of 100 MV/cm based on a two-color field scheme (Fig.6). For liquid plasma, Jin et al. from University of Rochester generated THz radiation from liquid water film driven by an ultrashort laser pulse (Fig.7). The development of high-field THz generation based on ultrahigh laser pumped various liquid media has been initiated. For solid plasma, the working principle of high-filed THz radiation generated by solid target intense laser pumping is described (Fig.8). Liao et al. from Shanghai Jiaotong University, acquired extreme THz radiation from metal copper foil targets pumped by ultra-short and ultra-intense lasers. Liao et al. identified that intense laser pumping of metal targets into various materials can produce strong THz radiation.

The THz wave is a novel and powerful tool for investigating the fundamental physics process of vibration rotation, spin precession, and electron acceleration (Fig.9). The application of strong-field THz waves in matter manipulation is concluded. Furthermore, the application of time-resolved THz spectroscopy is introduced (Fig.10). Topological insulators and semiconductors pumped by the ultrafast laser may emit THz radiation which indicates the electron dynamics in materials. Broadband THz waves of different polarization directions radiate from a topological insulator surface induced by femtosecond laser pulses (Fig.11). In WSe₂/Si heterojunction, THz radiation is enhanced by drift current amplification (Fig.12). A THz wave is generated by the laser-induced polaron in FAPbI₃ (Fig.13). Therefore, THz radiation can be used for both manipulation and detection of material dynamics.

Recent process regarding THz generation based on an ultrafast laser is reviewed, including the working principle and existing problems. THz radiation applications in physical state regulation are summarized. THz radiation has broad application prospects in the characterization and control of matter properties.

Terahertz (THz) waves are electromagnetic waves with frequencies ranging from 0.1 to 10 THz, between microwave and infrared. With the development of femtosecond lasers, terahertz waves are gradually being widely used in imaging technology, communication technology, medical and health, biochemical technology, nondestructive testing, security inspection technology, and other fields. Currently, the energy required to generate terahertz waves is relatively low, and detection technologies with high sensitivity and bandwidth are urgently required. The most widely used techniques in the field of terahertz wave coherent detection are solid dielectric-based photoconductive sampling and electro-optic sampling. However, owing to the factors such as non-instantaneous response of dielectric carriers, phonon absorption, and the Reststrahlen frequency band, it is difficult for the detection bandwidth to cover the entire terahertz band. Gas media are not affected by these factors, and the coherent detection of sufficiently wideband terahertz waves can be achieved through air-biased and light field-biased coherent detection. However, to achieve high detection sensitivity, the femtosecond laser beam to be detected ionizes air into plasma. Owing to the high excitation threshold of plasma in air, the energy of the laser beam is usually several hundred microjoules (µJ). Solids and gases have been proven to be suitable media for detecting terahertz waves, and the potential use of liquids for the coherent detection of terahertz waves has been an important issue of interest for researchers in the terahertz field. Related research has confirmed that liquids can be used for the generation of terahertz waves, and the intensity of such generated terahertz waves is 1.8 times greater than that of air. Compared to gases, liquids have higher molecular densities and nonlinear coefficients, which results in higher free electron concentrations and lower ionization thresholds in liquid plasmas. Compared with solids, the fluidity of liquids increases their damage threshold and allows them to self-repair.

This article reviews the coherent detection of broadband terahertz pulses in pure water, salt solution, and ethanol. Water requires lower detection energy than air to generate the same level of terahertz induced second harmonic (TISH). The measurement results for a water film under 5 µJ probe light excitation for a terahertz wave with a frequency band of 18 THz and a field intensity of 1 MV/cm are shown in Fig. 8. When detecting in air, the energy of the detected light must be increased to 75 µJ to obtain a signal with the same signal-to-noise ratio level. These results indicate that it is necessary to increase the detection light energy by 1‒2 orders of magnitude to achieve the same TISH energy in air. Therefore, under the same experimental conditions, the sensitivity of liquid water detection is 1‒2 orders of magnitude greater than that of air (Figs. 8 and 9). Because of the high third-order nonlinear coefficient of salt solutions, the signal intensity of coherent detection increases with increasing solution concentration, and the slope of the signal intensity also changes accordingly. Salts with a higher refractive index have a higher signal amplitude; therefore, the improvement in detection sensitivity is attributed to the increase in the refractive index of high-concentration solutions (Figs. 10 and 11). Ethanol has a third-order nonlinear coefficient greater than that of water, making it easier to ionize. A lower detection energy is required to form liquid plasma, and ethanol has a higher molecular response than pure water in the terahertz band. When the detection light energy is fixed at 15 µJ, the sensitivity of ethanol in the terahertz band is higher than that of water. We compared the coherent detection signals of ethanol and pure water under different detection lenergies of 5‒30 µJ, and found that ethanol has a higher response than pure water under any detection light. Even if the detection energy is as low as 5 µJ, the time-domain waveform of ethanol still has a good signal-to-noise ratio, providing new research prospects for low-laser-energy terahertz coherent detection (Fig. 15). The liquid-based terahertz wave coherent detection scheme expands the variety of terahertz wave detectors, providing the possibility of revealing molecular interaction mechanisms in biological liquid environments.

Liquid detection of terahertz waves has unique advantages over gases and solids, providing a new perspective for the coherent detection of broadband terahertz pulses, which has great potential in terahertz time-domain spectral applications and remote sensing.

The terahertz (THz) radiation spectrum, lying between the infrared and microwave regimes, encompasses a wide range of energy levels of lattice vibrations and molecular rotations in matter. Hence, THz radiation has potential applications in fields including matter manipulation, nondestructive testing, and biomedical imaging. However, a key challenge hindering the application of THz radiation is the need to further improve its energy and intensity. Therefore, exploring techniques for producing high-energy THz radiation remains a major focus and hot topic.

One method for generating intense THz radiation involves the nonlinear interaction between high-energy, ultrafast laser pulses and gases, resulting in a remarkably wide bandwidth of approximately 200 THz. This exceeds other commonly used THz radiation sources, such as optical-conductive antennas or optical rectification crystals, which are usually limited to a bandwidth below 5 THz. Furthermore, air-based THz sources offer advantages over solid-state sources, including immunity to laser damage and renewable properties, making them a pivotal for generating powerful and wide-ranging THz radiation.

An early study involving air plasma-based THz sources employed a one-color femtosecond laser pulse (usually with a wavelength near 800 nm from a Ti∶Sapphire laser) for THz generation. A two-color scheme, combining the fundamental laser field and its second harmonic, has also been intensively studied over the past 20 years. For the two-color scheme, it was revealed that the asymmetry of the two-color optical field plays a crucial role in generating high-intensity THz radiation. Nevertheless, the asymmetry of the optical field provided by the two-color field remains limited. Hence, there is a strong incentive to utilize multi-color laser fields to optimize the conversion efficiency of THz radiation, while also enabling manipulation of its properties.

This paper provides a comprehensive review of THz generation techniques based on air plasma, with an emphasis on the evolution from one-color field excitation to an advanced excitation scheme with a multi-color optical field. The development and progression of these methodologies are discussed in detail. In 1993, Hamster et al. focused a one-color femtosecond laser onto a gas target and observed THz radiation emitted from the gaseous plasma. However, the energy conversion efficiency of THz radiation produced by one-color laser fields was low, of the order of 10^-7‒10^-5, limiting the further applications of this THz source.

In 2000, Cook et al. first employed a two-color laser field to produce THz waves from air plasma. Their findings demonstrated that the amplitude of the resulting THz waves was much stronger than that generated using a one-color field. In the two-color field experiments, a BBO crystal used to generate a second harmonic pulse was installed before the laser focus in the beam path. By adjusting the distance between the BBO crystal and the focal point, the relative phase of the two-color fields could be finely controlled. Later, K. Y. Kim et al. proposed a local current model to explain the underlying physical mechanism. With the two-color field, the breaking of the symmetry of the electron motion effectively produces a net transverse electron current in the plasma, resulting in THz electromagnetic radiation (Fig. 4).

Cleric et al. explored the impact of the pump-laser wavelength on THz generation in the near-IR to far-IR regime. Their research showed that the yield of THz radiation increases with the wavelength of the pump laser field (Fig.6). In the latest reported two-color field scheme with a pump laser in the mid-infrared regime, the strength of the THz radiation field reaches up to 100 MV/cm, with a corresponding energy conversion efficiency of 2.36%. Moreover, Vvedenskii’s group discovered that non-harmonic two-color lasers can also be used to generate THz radiation (Fig.8).

To further improve the conversion efficiency, the exploration of multi-color fields was proposed in a theoretical perspective. L. Bergé et al. proposed a multi-color field scheme with a sawtooth wave shape to generate THz radiation. The multi-color sawtooth field maximizes the electron drift velocity at the ionization instants, which increases the THz efficiency by up to 1 order of magnitude compared to a standard two-color field (Fig.9). However, the construction of the sawtooth wave requires many harmonic laser pulses, and its synthesis is currently challenging.

Experiments frequently exploit a three-color field to mimic a sawtooth-wave excitation. Jeremy A. Johnson’s group utilized a three-color field from an optical parametric amplifier (OPA), composed of a fundamental frequency, the variable IR signal, and idler outputs from the OPA. They achieved higher THz radiation intensities compared to a two-color laser field (Fig.12). With the three-color pulse from the OPA, the relative phases of the optical fields were not controlled, and the temporal waveform of the multi-color optical field was therefore random.

Recently, an inline setup for three-color field synthesis for THz excitation was demonstrated. This setup allows the relative phases between the three optical fields to be controlled independently with attosecond precision. It was found that both photoionization and THz emission can be controlled coherently. THz enhancement, relative to the widely used two-color scheme, was also confirmed.

Employing a multi-color laser field to excite air plasma for THz generation can improve conversion efficiency significantly. Furthermore, the properties of THz emission, such as its polarization and polarity, can be controlled precisely by manipulating the optical phase. Due to its high intensity and wide bandwidth, the air-based THz source holds potential for applications in the manipulation of matter, electron acceleration, and biomedicine. Moreover, utilizing phase-controlled multi-color optical fields enables coherent control of the nonlinear interaction between powerful ultrafast lasers and gases, offering potential benefits for various high-field physics studies, such as high-order harmonic generation, attosecond pulse generation, and laser micro- and nano-machining. This article reviews the progress in research on THz generation with multi-color laser fields, with emphasis on the evolution of the experimental methods from one-color excitation to two- and three-color schemes, the relevant theoretical models, as well as the effects of the laser parameters on THz energy enhancement.

Particle accelerators are scientific facilities that utilize electromagnetic waves to accelerate charged particles to speeds close to that of light. Over the last century,accelerator development has contributed significantly to the advancement of science,particularly for the investigation of microscopic constituents that comprise macroscopic matter,and broader application communities such as the construction of advanced light sources,material science,and medical therapy. Conventionally,radio frequency (RF) is employed to power mature accelerators. However,this acceleration scheme is constrained by an acceleration gradient of 100 MV/m and encounters challenges such as high price,significant footprint,and a lengthy construction time,all of which impede its promotion for wider scientific and technological applications. As a result,the search for new electron acceleration technologies has intensified in the pursuit of advanced accelerators.

To reduce accelerator size,increasing operation frequency provides an effective path for enhancing accelerator performance and availability. In such cases,energy transfer from electromagnetic waves to electrons can occur over shorter distances,resulting in larger accelerating gradients and a smaller device footprint. In particular,contrary to the long wavelength associated with RF,electron acceleration driven by terahertz (THz) waves and lasers on a dielectric grating can generate acceleration gradients up to GV/m in these higher frequency bands. Recent progress utilizing terahertz and optical waves has already demonstrated non-relativistic and relativistic electron acceleration and phase space manipulation. Moreover,the combination of a light-wave-driven electron source with a novel accelerator enables realization of an all-optical electron source,which unlocks new directions for small-scale and even integrated accelerator development.

Research on optical-field-driven electron acceleration originates from the laser invention in the last century. However,owing to electron beam distortion by scattering in the air,dielectric laser acceleration takes advantage of phase-matching between the electron and laser on a grating surface,which has only emerged over the past two decades,and was experimentally demonstrated by Byer et al. and Hommelhoff et al. in 2013. Subsequently,advancement toward the concept of an integrated accelerator has been proposed and extensively studied worldwide. Currently,two primary directions have emerged that utilize the distinct electromagnetic spectral bands of THz and laser waves. In dielectric laser acceleration,the average acceleration gradient witnessed rapid growth from 25 MV/cm (for sub-relativistic electrons) and 300 MV/m (for relativistic electrons) to near GV/m by careful design of the accelerator materials and structures (Figs.3 and 5). In general,the established designs rely on single- and double-sided grating,with the leading arrangement exploring flexible dual pillar rows and inverse-design structures. This could facilitate access to the required integrated electron accelerator.

Although increasing the operation frequency can significantly reduce accelerator size,the laser’s short wavelength also poses significant challenges for synchronization,stability,and acceleration of a substantial amount of charge. Therefore,THz radiation emerges as a suitable operating band for compact accelerators that alleviate these stringent requirements and the accelerator structure processing precision. Nanni et al. reported the first experimental demonstration of THz-driven acceleration in 2015. Meanwhile,developments in electron energy gain (Fig.1) and phase space manipulations (Fig.5) have emerged by increasing THz wave energy and optimizing accelerator structures,which also increases interaction distance. To further improve THz-driven electron acceleration,however,additional energy must be made available,necessitating the development of new THz wave generation and transmission techniques. The recently unraveled amplification of THz surface plasmons via free electron pumping (Fig.2) provides a novel method for accelerating electrons,enabling energy gain in the order of MeV.

This review examines research progress on electron acceleration driven by THz to optical bands in the electromagnetic spectrum,including related advances of coherent electron sources and beam control,and investigates the emerging novel concept of integrated electron accelerators. Despite significant progress in both the acceleration/manipulation of non-relativistic and relativistic electrons,current advances remain insufficient to enable the development of a compact accelerator for mature applications. It is anticipated that higher accelerator pump power availability will lead to breakthroughs in terms of energy gain reaching MeV or even GeV scales. Recent progress in the generation of millijoule or even subjoule scale THz radiation has already illuminated such prospects in THz-driven acceleration research. The intense surface wave,in particular,allows access to an integrated electron source device,which eliminates the necessity for bulky optics for free-space THz wave generation,transport,and mode conversion. Future processing precision refinement,on the other hand,would enable more precise control of tip-generated electrons and their phase-matching with optical waves. This is particularly critical for dielectric laser acceleration,which is dependent on electron generation and steering. In the future,integrated accelerators are anticipated as a viable alternative to large-scale RF particle accelerators in university laboratories. This will potentially trigger significant research development in physics,chemistry,medicine,and other disciplines.

Terahertz waves have wide applications in fields such as communication, astronomy, material detection, physical control, and charged particle acceleration and diagnostics. In recent years, the development of terahertz sources has become an important frontier topic in terahertz science. With the continuous development of 5G communication, the carrying capacity of the microwave band is insufficient to meet future communication demands, making terahertz waves with shorter wavelengths the focus of future communication solutions. Compared with microwaves, the terahertz band has a stronger information carrying capacity and is recognized as a band for the next-generation 6G communication that is yet to be further developed. Terahertz waves exhibit high absorption and non-biological damage effects in water molecules, making them suitable for non-destructive imaging and research analysis of biomolecules. Terahertz waves can resonantly couple with the partial motion states of ions, electrons, and spins in matter, making them useful for material excitation and spectroscopic analysis. In solids, the energy of terahertz photons is consistent with the energy required for lattice vibrations (phonons), collisions of free electrons, binding energy of electron-hole pairs (excitons), and decay of spin waves (magnons). Therefore, terahertz waves can probe the linear response of materials without causing changes in their properties, making them widely applicable in the study of fundamental processes in complex materials. In recent years, terahertz waves, as pump pulses, have been expanded to applications in biophotonics, material resonances, magnetization control, electron beam measurements, charged particle acceleration, and other areas of research. These applications have placed higher requirements on the intensity and quality of terahertz pulses.

As the application areas of terahertz continue to expand, there are increasing demands for the quality of terahertz waves. The quest for high-quality terahertz sources has always been a key scientific problem in the field of terahertz science. The generation of terahertz sources can be roughly divided into three categories: solid-state electronics, quantum cascade lasers, and secondary radiation sources based on intense lasers. Solid-state electronics employ electronic devices such as Schottky diodes, semiconductor terahertz sources, and coherent transition radiation devices to generate terahertz sources. These sources typically emit weak, narrowband (<3 THz) radiation with low field strength and low energy. Quantum cascade lasers are unipolar devices, and their radiation originates from intraband transitions in a stack of semiconductor quantum well heterostructures. The average power is in the milliwatt range, and although the frequency of terahertz sources generated by this method can be tuned, the tuning range is limited to a narrow band in the range of a few terahertz. Chirped pulse amplification (CPA) technology can significantly enhance laser power density, allowing femtosecond laser intensity to reach relativistic levels. Intense laser-plasma interactions can generate broad-spectrum, ultra-strong terahertz waves, which are of greater significance for terahertz spectroscopic research. This article provides an overview of several mechanisms for terahertz generation based on intense lasers, particularly the development status of plasma-based terahertz sources, and summarizes and predicts the development trends of terahertz sources.

Secondary terahertz sources based on intense lasers can be divided into solid-state and plasma terahertz sources based on the interacting medium. In the case of laser pulse interaction with solid-state materials, terahertz sources are generated through processes such as optical rectification, microscopic photoconductivity, and photoconductive antennas. Wynne et al. proposed a one-dimensional model for the laser-induced terahertz oscillation polarization of the medium. If the interaction between the laser and the medium satisfies conditions such as instantaneous response, no dispersion within the terahertz range, and perfect phase matching, the electric field of the terahertz pulse can be obtained from the time derivative of the intensity envelope of the optical pulse. The generated terahertz pulse exhibits a fixed carrier envelope phase, and for Gaussian envelope pulses, it produces a single-cycle waveform, which is highly valuable for many applications.

Regarding plasma terahertz sources, they can be classified into gas and dense plasma terahertz sources based on the density of the plasma. Gas target terahertz sources mainly rely on plasma waves generated by mass motion to produce terahertz radiation. Dense plasma terahertz sources, on the other hand, generate ultra-strong terahertz waves due to the dynamic behavior of relativistic electrons accelerated by the laser at the plasma-vacuum interface.

Solid materials such as semiconductors and organic crystals have energy damage thresholds. High-intensity pump lasers can cause thermal damage, vaporization, and even ionization. For photoconductive antennas, the terahertz field strength generally does not exceed MV/m level, and the terahertz energy saturates with increasing laser pulse intensity. Using LiNbO₃ tilted-wavefront optical rectification, terahertz waves with peak intensities of around 400 MV/m can be obtained, and using the new generation of organic crystals can further increase it to GV/m level. Additionally, overcoming the absorption of terahertz radiation by nonlinear crystals is also a consideration. Most semiconductors used to generate terahertz radiation are polar (such as ZnTe, GaP, GaSe, GaAs, or organic materials), resulting in a resonance effect between optical phonons and surrounding terahertz radiation, leading to strong attenuation of radiation in the Reststrahlen band between 1-15 THz. Terahertz pulse spectra generated by tilted-wavefront LiNbO₃ crystals are limited to <3 THz due to material absorption.

The interaction of laser with plasma overcomes the energy threshold limitations of crystals but still faces challenges. The photoconductive current mechanism reaches saturation for terahertz generation at laser intensities of 10¹⁵ W/cm², and at higher intensities, terahertz waves are mainly generated by plasma waves. A plasma wave density of 10¹⁶ cm^-3 corresponds to a terahertz frequency of about 10 THz, and low electron densities limit the energy and field strength of terahertz radiation generated through plasma waves. Therefore, it is difficult to significantly increase the intensity and pulse energy of gas plasma terahertz sources based on the current mechanisms.

Solid target and structured target terahertz sources are generated by the dynamic behavior of laser-accelerated electrons, and the quality of terahertz radiation is directly determined by the accelerated electron beam. Strong terahertz waves with energies greater than 50 mJ can be obtained using the planar solid target transition radiation scheme. In solid target schemes, the energy conversion efficiency from laser to ultrahot electrons is relatively low, resulting in low overall terahertz radiation efficiency.

Femtosecond laser filamentation in air refers to the technical approach of using the femtosecond laser to ionize air near its focal point, forming a plasma channel (also known as the optical filament) that emits terahertz (THz) waves. Due to the remote generation of plasma filaments and the broadband and high intensity characteristics of the emitted THz waves, femtosecond laser filamentation overcomes water vapor absorption losses in free-space THz transmission; thus, it is advantageous for applications such as remote sensing and communication in THz band. Therefore, the study of physical mechanisms of THz wave radiation by femtosecond laser filaments has become an important branch of THz science.

However, there currently remains controversy surrounding the mainstream models for THz wave generation, and important experimental observations such as THz polarization and angular dispersion distribution remain in disagreement. For example, the two mainstream models, i.e., four-wave mixing (4WM) and photocurrent (PC), have fundamental differences: when the THz wave yield is significant, the former assumes a relative phase difference of 0 between the fundamental and second harmonic laser fields, while the latter assumes a π/2 phase difference. Furthermore, regarding the dependence of the far-field divergence angle of THz waves on frequency, the mainstream viewpoint represented by the off-axis phase matching model suggests that only dual-color filaments longer than the dephasing length can radiate THz waves with high-frequency components distributed inside and low-frequency components distributed outside. However, this explanation fails to account for the similar radiation behavior observed in short filaments, and contradicts experimental findings in some literature, which report high-frequency outside, low-frequency inside, or frequency-independent angular distribution.

These unresolved contradictions pose significant challenges in the study of THz wave radiation from dual-color filaments. The reason for these contradictions could be that femtosecond laser filamentation is a complex optical phenomenon involving multiple nonlinear processes such as optical Kerr self-focusing and plasma defocusing. Therefore, a single physical model is likely insufficient to encompass the entire dynamic mechanism of filamentation-induced THz wave radiation. Accordingly, we propose a three-process theory that incorporates mainstream models and the recent experimental observation of spatial confinement of THz waves inside the laser filament.

We first divided the filamentation process into Kerr self-focusing and plasma defocusing before and after the laser intensity breaking the ionization threshold. Then, in the first process, when neutral gas molecules are not yet ionized, we primarily consider the 4WM effect of the pump laser and its harmonic in air, which generates THz waves. The second process occurs when the laser intensity exceeds the ionization threshold, resulting in the ionization of air and the formation of a plasma (free electrons). Under the drive of the time-asymmetric electric field of the dual-color laser pulse, the plasma oscillates and gives rise to a nonzero trailing current (also known as drift current or residual current) and the emission of THz waves. Finally, considering the time scale of THz waves (ps), which is much larger than the establishment time of the plasma filament (tens or hundreds of fs) and much smaller than the plasma lifetime (ns), the filament can be treated as a quasistatic waveguide for THz waves. During the transmission of THz wave, its interaction with the plasma free electrons leads to the spatial confinement of THz waves at the radial edge of the filament. This constitutes the third process.

Based on the analysis above, we explain the THz radiation regarding the three processes using the mainstream models of 4WM and PC, as well as the proposed one-dimensional negative dielectric waveguide model (1DND). As shown in Fig. 1, if we consider only the first or second process, the predictions of the THz orthogonal polarization components by the two mainstream models alone are unsatisfactory. If we incorporate the THz spatial confinement effect as the crucial third process, it is necessary to consider the spatial mode distribution, energy loss, and spectral changes of THz waves after interaction with the plasma. This consideration leads to the best fit to experimental results. The THz spatial confinement effect has also facilitated several novel applications and addressed key challenges in the field (Fig. 4), including super-resolution THz imaging, high conversion efficiency of THz strong electric fields, and flexible manipulation of broadband THz polarization states.

In conclusion, the proposed 4WM+PC+1DND model presents a new mechanism that overcomes the limitations of a single physical model, bridges the connections between mainstream models, and provides a unified framework for fundamental theories that were previously incompatible. This model could comprehensively and reasonably explain the unresolved evolution of THz polarization states with the rotation of the frequency-doubling crystal in dual-color field radiation, as well as experimental results such as near-field THz modes and far-field THz spatial chirping. It provides a fresh opportunity to reevaluate past experimental findings and resolve related contradictions in the field.

Future prospects of the THz spatial confinement effect can be focused on the following aspects. 1) Investigation of the physical mechanisms behind single-color and dual-color filamentation for THz wave generation: due to the universality of the THz spatial confinement effect, it can be incorporated into both single-color and dual-color THz generation processes. This might establish a foundation for a unified understanding of the common mechanisms in these two important research directions. 2) Exploration of new technologies: compressing THz waves into subwavelength spatial scales is expected to lead to technological innovations. For example, broadband THz all-optical computation techniques relying on spatial compression in subwavelength narrow channels can be developed utilizing THz wave spatial confinement for guidance and transmission, which has the potential for breakthroughs in this field. 3) Interdisciplinary exploration: the integration of the 1DND model with principles from micro and nanooptics can be explored. Concepts such as surface waves and epsilon near zero (ENZ) can be utilized to investigate THz spatial confinement across scales ranging from submillimeter to nanometer. This approach can provide new insights into laser-induced plasma filaments using the concept of surface plasmon polariton waveguides. 4) Laser filaments as self-balanced physical systems: based on the THz spatial confinement effect, laser filaments in the THz regime can become a new platform for self-balanced physical systems. Similar to the action of silicon-based fiber waveguides on light, this filament platform can greatly facilitate the study of new laser ionization mechanisms, THz wave transmission principles, and modulation methods from a new perspective.

As an important research method, spectroscopy exhibits versatile and unique advantages, such as contactless measurement, high sensitivity, and convenience, and thus is widely used in material science and engineering. With increasing progress of science and technology, the branches of spectroscopy have gradually broadened, and a variety of complex functional spectral analysis technologies have emerged. Among those, ultrafast spectroscopy is an important subject that has developed rapidly in recent years. It introduces the time degree of freedom on the basis of traditional steady-state spectroscopy, subdivides the interaction between light and matter at the picosecond time scale, and studies the time-resolved dynamics of such quasi-particles as hot carriers, phonons, polarons, and excitons in matter. At present, the wavelengths of light used in ultrafast spectroscopy have covered most of the bands in the electromagnetic spectrum, and its scope of application has also extended deeply into condensed matter physics, material science, biomedicine, military, and national defense. Terahertz spectroscopy is an important branch of ultrafast spectroscopy used in technology that developed in the 1980s that has important scientific research and application prospects. Generally, as commonly defined, terahertz (THz) waves refer to electromagnetic waves with frequencies in the range 0.1‒10 THz (wavelength 3 mm‒30 μm), also known as submillimeter waves or terahertz radiation. Electromagnetic waves in this band exhibit many useful properties, such as low photon energy, high penetration, a close match with molecular vibration and rotational energy levels, and no harmful radiation. Thus, THz radiation has found a wide range of use nowadays. Terahertz technology therefore has become a major emerging field of science and technology in the 21st century. It is rated as one of the top 10 technologies that will change the world of the future by the United States, and has been highly evaluated by governments around the world.

With the continued progress of terahertz technology, the terahertz emission efficiency of materials has continually increased, and the corresponding electromagnetic field intensity has also gradually increased. The electric field component of intense terahertz waves can easily reach MV/cm magnitudes, and its corresponding magnetic field component can reach Tesla magnitudes. When a sample is irradiated with such intense terahertz waves, the strong electromagnetic field can apparently regulate the internal physical properties of matter, such as its spin/electron/lattice structure, dielectric property, and susceptibility, resulting in a series of nonlinear responses, such as collision ionization, valley scattering, and the terahertz Kerr effect. Research on these nonlinear effects can clarify these phenomena, promoting the development of ultrafast optoelectronic devices.

Intense terahertz waves exhibit high peak power and correspondingly large amplitudes of the electric and magnetic field components, whereby they can induce numerous novel anomalous phenomena. In this paper, we first introduce some frequency used intense terahertz emission sources, including photoconductive antennas, optical rectification crystals, solid and gas plasma emissions, metamaterials, and tip enhancement. Some typical applications of intense terahertz technology in material science are also introduced, including collision ionization, intervalley scattering, coherent modulation, spin regulation, terahertz fluorescence, terahertz high order harmonic generation, terahertz Kerr effect, and biomedical uses.

With advances in technology, intense terahertz sources have become increasingly readily available, and terahertz detection techniques are also continuously growing. Currently, a wide range of experimental methods are being applied to detect a variety of nonlinear terahertz phenomena. In addition, the range of application of intense terahertz spectroscopy is also expanding, whereby measurements of physical properties of substances under extreme environmental conditions such as extremely low temperatures, strong electric and magnetic fields, and high pressure can now be performed. However, a series of problems also exists, such as the need for stable and efficient intense terahertz sources and a deep understanding of the physical mechanism underlying the interaction between intense terahertz waves and matter. In addition, a new terahertz theory to account for major breakthroughs in basic physics has not yet appeared, and there remains a so-called “terahertz gap” in theory. A combination of electromagnetic theory and quantum theory to solve new terahertz physics problems is needed. The answers to these questions are important not only for the progress of basic science, but also for the development of applied technology. We believe that in the near future, terahertz technology is bound to see major breakthroughs, whereupon terahertz devices will be as widely used in every corner of life as visible and near-infrared devices.

The strong-field terahertz (THz) time-domain spectroscopy is fundamental in strong-field THz science, technology, and applications. Strong-field THz time-domain spectroscopy is also indispensable in many fields including materials, physics, chemistry, and biology, which involve strong nonlinear interactions between strong-field THz and matter. However, the unavailability of a high-field free-space THz source with high repetition rate, excellent beam quality, and high stability hinders its development. In this study, we designed and independently developed a highly integrated strong-field THz nonlinear time-domain spectroscopy system based on a lithium niobate (LN) strong-field THz source. The proposed system is driven by a kilohertz femtosecond laser amplifier and exhibits the functions of a strong-field THz nonlinear spectrum, THz-pump THz probe (TPTP), strong-field THz-pump optical probe, optical-pump THz probe (OPTP), and THz emission spectrum. This highly integrated strong-field THz nonlinear time-domain spectroscopy system is a powerful tool for analyzing the nonlinear effects of strong-field THz waves.

We developed a strong-field THz nonlinear time-domain spectroscopy system. We employed a Ti∶sapphire femtosecond laser amplifier that provided laser pulses with a center frequency of 800 nm, pulse duration of 35 fs, repetition rate of 1 kHz, and maximum pump power of 5 mJ. The laser input to the system was split using an 80∶20 beam splitter. The transmitted beam (80%) was employed as a pump beam to generate strong-field THz waves from the LN crystal through optical rectification based on the tilted pulse front technique. The strong-field THz waves generated by the LN crystal was used to induce and probe the nonlinear effects. The residual 20% femtosecond laser energy was divided into three beams: the first for the optical pump, the second for generating a weak-field THz probing beam in a ZnTe emission crystal (ZnTe 1), and the third for electro-optic sampling of both the pump and probing THz temporal waveforms. Three delay lines were employed to synchronize the strong-field THz, optical pump, and electro-optic sampling. THz temporal waveforms were detected by an electro-optic sampling system consisting of a ZnTe crystal, quarter-wave plate, Wollaston prism, and two photodiodes for the coherent detection of THz pulses based on the principle of the electro-optic effect.

In this work, the study on LN strong-field THz nonlinear spectroscopy is summarized as follows. First, for the strong-field THz generation and detection system (Fig. 2), the energy conversion efficiency of near-infrared light to THz waves was approximately 0.22%. At the focus of off-axis parabolic mirror 2 (OAP2), the calculated peak electric field can reach 350 kV/cm. The focused THz beam profile can be detected using a THz camera and temperature-sensitive paper. Based on this strong-field THz static nonlinear spectroscopy system, we observed the nonlinear absorption caused by the intervalley scattering of doped silicon induced by strong-field THz using the Z-scan technique (Fig. 3). In addition, the strong-field THz-induced nonlinear transmission self-frequency modulation of the THz nonlinear metasurface further demonstrates the excellent ability of strong-field THz nonlinear spectroscopy in the frequency domain (Fig. 4).

Second, the pump-probe technique is an essential research method for performing strong-field THz nonlinear spectroscopy. The TPTP technique can be achieved by introducing a THz probing beam generated by a ZnTe emission crystal, a coaxially aligned THz pump (generated by LN), and a THz probe, followed by focusing on the sample with an off-axis parabolic mirror (Fig. 5). The THz probing beam was modulated by a chopper with a 500 Hz rotor frequency to obtain a pure probing signal. A THz polarizer was positioned behind the sample with its polarizing orientation perpendicular to the THz pump to further restrict the transmitted THz pump. Using the TPTP technology with a spectral resolution, we observed dynamic changes in the resonant frequency of the THz probe transmission spectra induced by a strong THz field on nonlinear THz metasurface samples. This phenomenon demonstrates that carrier production in this SRRs sample is caused by the impact ionization of high-resistance silicon.

Third, we realized the OPTP technique by introducing an 800 nm pumping beam. The strong-field THz waves generated by the LN crystal can be used as the probing beam. By adjusting the incident THz field strength, the transmission signal self-modulation induced by the strong-field THz under an 800 nm pump is measured (Fig. 6). For more common OPTP applications, the weak-field THz generated by the ZnTe emission crystal is used as the probing beam, which is primarily used to investigate the ultrafast dynamics of carriers in semiconductors (Fig. 7). By adding a spectral resolution, the photoexcitation dynamics with different frequencies can be analyzed more comprehensively.

Finally, the proposed strong-field THz nonlinear time-domain spectroscopy system exhibits THz emission spectral capability. When stimulated by a femtosecond laser, the induced THz pulse carries a significant amount of physical information in its waveform. Taking the spin THz emission as an example, we demonstrate the flexibility of the system in examining the emission characteristics of the W/CoFeB/Pt thin-film structure (Fig. 8).

Strong-field THz nonlinear spectroscopy has become a critical method for studying the nonequilibrium behaviors resulting from strong THz-matter interactions. Based on the self-developed LN strong-field THz nonlinear-time-domain spectroscopy system, various experimental methods of strong-field THz nonlinear spectroscopy were studied and explained, demonstrating the unique ability and essential role of strong-field THz nonlinear spectroscopy in basic research. In addition, with a slight adjustment, the system can also be used for two-dimensional THz spectroscopy, THz electron acceleration, and the THz Kerr effect. This highly integrated and miniaturized THz time-domain spectrometer provides comprehensive research capabilities and potential for nonlinear THz spectroscopy in physics, materials, biology, and engineering applications.

Terahertz (THz) waves offer a distinctive diagnostic method for detecting high energy density matter. However, realizing the THz time-domain spectral (THz-TDS) diagnosis of matter states under extreme conditions in large high-energy density devices remains a significant obstacle. To address this requirement, we designed and implemented an optical pump-THz single-shot detection system driven by a strong femtosecond laser. The system possesses the capability of THz single-shot detection under extreme conditions and diagnosis of irreversible processes with extreme transience using THz-TDS diagnosis under intense laser pumping.

We developed an integrated optically pumped terahertz (THz) single-shot detection system that utilizes a 45 TW Ti∶sapphire femtosecond laser with a pulse width of 30 fs, central wavelength of 800 nm, and spot diameter of approximately 38 mm. The laser pulses were initially directed to realize second harmonic generation (SHG) via KDP crystals and then separated into fundamental and SHG using a dichroic mirror (DM). The SHG was reflected into the pump time-delay line (TD2) and focused by a lens to ensure complete pumping of the target object with a focus size of approximately 2 mm in diameter consistent with the THz focus size. Meanwhile, the fundamental frequency laser transmitted by the DM was divided by the beam splitting mirror (BS) with 90% of the energy used as the driving laser of the lithium niobate wafer. An intense THz pulse was generated by collinear optical rectification effect, and an off-axis parabolic mirror (OAP) was utilized to focus it onto the target object. The THz pulses transmitted through the target object were focused by the OAP and reflected by indium tin oxide (ITO) to reach the surface of the ZnTe crystal. Moreover, 10% of the transmitted energy of the THz probe laser was directed into the time-delay line (TD1) incident with the surface normal of the reflective echelon at 14° and encoded time information into a one-dimensional space. The outgoing laser was spatiotemporal coincident with the THz on the surface of the ZnTe crystal. Finally, the orthogonal detection scheme was utilized to probe the THz waveform.

We present the design and implementation of an intense-field optical pump-THz time-domain spectroscopy single-shot detection system for measuring the irreversible non-equilibrium transient processes in high-energy and low-repetition-rate pumps of large laser devices. The system employs a reflective echelon and orthogonal detection scheme to detect pulses generated through the collinear optical rectification of a lithium niobate wafer with a diameter of 3 inch. The system consists of a THz generation-intense laser pumping module and a THz time-domain spectral single-shot detection module integrated into separate optical breadboards. The former can be placed in a vacuum chamber, and the latter in an atmospheric environment, making it easy to move and install and suitable for different laser-device application scenarios. The THz pulses have an energy of 7 µJ at 800 nm 1 J laser energy, can be easily adjusted, and have a detection capability of a 30 nm free-standing gold foils transmission spectrum at room temperature. We verify that the waveform obtained by the single-shot detection is the same as that obtained by traditional scanning. Based on this device, the variation of conductivity in the THz band of 30 nm free-standing gold foils with pumping delay measured under the 0.8 MJ/kg laser energy density of a 400 nm pump contributes to the further understanding of the generation and evolution of the warm dense state of gold.

With the advent of intense femtosecond lasers, it has become possible to investigate the state of matter in extreme conditions. The maturation of THz time-domain spectroscopy technology also provides a new tool for diagnosing extreme non-equilibrium states. To meet the demands of THz emission and state diagnosis under such extreme conditions, an intense-field optical pump-THz time-domain spectroscopy single-shot detection system with a simple THz path was designed and fabricated. The system was employed to measure the transient THz conductivity of 30 nm thick free-standing gold foils pumped by a 400 nm laser pulse. The obtained results serve as a potent platform for further exploration of irreversible processes including extreme condition THz emission-detection and the diagnosis of non-equilibrium states of matter under extreme conditions.

As a newly developed ultrafast optical spectroscopy, terahertz two-dimensional coherent spectroscopy (2DCS) has become a promising method to characterize the physical properties of optical excitations in various materials. It utilizes two or more THz pulses to detect nonlinear responses of materials, thereby incorporating multiple frequency variables. Experimentally, it has uncovered a host of interesting phenomena in quantum wells, electronic glasses, and superconductors. In this work, we theoretically investigate the 2DCS of the Josephson plasmon mode in layered high-temperature superconductors.

Layered high-temperature superconductors consist of alternating superconducting and insulating layers. The adjacent superconducting-insulating-superconducting layers form a Josephson junction. Consequently, the layered high-temperature superconductors possess a nonlinear mode, known as Josephson plasmon mode. The Josephson plasmon mode arises from Josephson tunneling of Cooper pairs across the neighboring superconducting layers separated by insulating block layers.

In this work, we compute the 2DCS of the Josephson plasmon mode and analyze the features therein. We expect that our findings will provide the theoretical basis for the future 2DCS experiments on layered high-temperature superconductors.

We begin with a semiclassical effective model of layered high-temperature superconductors. We derive and solve numerically the equation of motions of the Josephson mode coupled to the electrical field. Fourier transforming the time-domain data yields 2DCS. Meanwhile, assuming that the electrical field is weak, we are able to solve the equation of motion analytically by using the perturbation theory. Finally, we associate the peaks in the two-dimensional spectrum with the Liouville paths and clarify the response process in detail.

Figure 3 shows the 2DCS of the Josephson plasmon mode. The numerical results [Figs. 3(a) and 3(b)] are found to be consistent with the analytical results [Figs. 3(c) and 3(d)]. In the time domain [Figs. 3(a) and 3(c)], the signals decay exponentially with increasing t1 or t2. The decay rate is proportional to the resistance of the Josephson junctions in layered high-temperature superconductors, γ. Meanwhile, the signal oscillates with the frequency of the Josephson plasmon mode,ω. In the frequency domain [Figs. 3(b) and 3(d)], the 2DCS exhibits eight peaks. In Fig. 3(b), the blue circles label the pump-probe peaks. The red circles label the photon echo peaks, which can help to separate the homogeneous broadening and inhomogeneous broadening. The green circles label the dephasing peaks. The orange circles label the two-quantum (2Q) peaks, which correspond to the simultaneous excitation of two plasmon mode quanta.

We have investigated the 2DCS of Josephson plasmon mode in layered high-temperature superconductors. Our findings reveal that the 2DCS contains various peaks, including pump-probe peaks, photon echo peaks, dephasing peaks, and 2Q peaks. We have also clarified their origins by associating these peaks to the various optical transition processes.

We envision that our model may be extended and improved in various aspects. Firstly, by adding the spatial degrees of freedom, the corresponding equation of motions would become sine-Gordan equation. Sine-Gordan equation hosts a specific excitation: soliton. Soliton has been detected in experiments but its 2DCS signature is still unclear and requires further investigation. Secondly, in deriving the equation of motion, the resistance is introduced phenomenologically. It would be interesting to derive the dissipative term from the first principles.

Terahertz (THz) is a electromagnetic wave with the frequency range of 0.1‒10 THz, and is gradually playing an important role in many fields. However, because traditional electronic and optical design methods consider the adjacent microwave and infrared optical bands, the application of the THz band is not easy to directly expand, which will undoubtedly greatly hinder further development of THz technology. Thus, there is an urgent need for new THz device design methods to solve this difficulty. Metamaterials are composed of a series of micro- and nanostructures with artificially designed periodic arrangements, whose size, shape, and distribution can produce optical responses that natural materials do not exhibit after careful design. Two-dimensional metamaterials, i.e., metasurfaces, with a simple process flow and low processing cost, have gradually replaced metamaterials in recent years and have become a popular research topic. The application of metasurfaces to THz technology overcomes the limitations of traditional materials, contributing to their development. With the increasing demand for corresponding applications, researchers have shifted their attention from single passive hypersurfaces to tunable active metasurfaces. These tunable metasurfaces are often dependent on several tunable materials. In particular, in the THz band, vanadium dioxide (VO₂) is an excellent tunable material that is being actively investigated by researchers due to its abrupt change in conductivity of four to five orders of magnitude before and after the phase transition temperature, which allows it to complete the insulating to the metallic phase transition.

A metasurface comprising a periodic array of double-gap split-ring resonators, with VO₂ structures embedded in the gaps, was considered in this study. The spectral responses to different VO₂ conductivities and electric field distribution images of the corresponding modes were first simulated using the commercial simulation software, CST. Next, samples were obtained via conventional lithography and other micro- and nano-processing techniques, which were then characterized experimentally using THz time-domain spectroscopy (TDS). First, the samples were heated directly using a hot stage, followed by laser pumping, strong THz pumping, and THz detection for mode characterization.

The simulation results clearly show that with increasing VO₂ conductivity from 10 to 2×10⁵ S/m, the resonant frequency red-shifts from 0.75 to approximately 0.5 THz, the gap of the metal arm is approximately filled, and the whole structure completes the transition from mode 1 to mode 2. Field monitoring shows that before the phase transition, mode 1 is a magnetic dipole resonance with an enhanced electric field at the opening; whereas, after the phase transition, mode 2 is an electric dipole resonance after the conduction of the metal arm, and its electric field is mainly distributed in the upper and lower metal arm regions (Fig. 1). Direct characterization of the sample heating confirmed the simulation results. At temperatures lower than 57 ℃, the resonant frequency of the structure remained at approximately 0.7 THz, indicating that the temperature change at this time could not substantially affect the conductivity of VO₂, and the sample was in the mode 1 state. As the temperature increases further, the resonant frequency gradually red-shifts to 0.45 THz, accompanied by a gradual decrease in the amplitude of the resonant peak, reaching a minimum of approximately 0.4 at ~64 ℃. With continuing increase in temperature, the resonant frequency continues to red-shift, and the amplitude becomes larger, indicating that VO₂ is transitioning between the insulating phase and the metallic phase. For temperatures higher than 73 ℃, the resonance mode does not change significantly in both amplitude and resonance frequency, and tends to stabilize, at which time the VO₂ conductivity tends to saturate and completes the filling of the metal arm gap, indicating that the metasurface is in the state of mode 2. Therefore, by directly heating the sample, a conductive channel at the gap is successfully constructed, the transition from resonant mode 1 to mode 2 of the metasurface is completed, and the variation in the resonant frequency with temperature provides a more direct reflection of the mode switching (Fig.2). Laser pumping requires heating the respective sample to near the phase transition temperature; similarly, different laser powers can induce a VO₂ phase transition (Fig.3). Finally, strong THz pumping of samples with different intensities can also produce the VO₂ phase change. It is worth noting that although VO₂ is in the vicinity of the phase transition temperature, broadband modulation, such as temperature and laser pumping excitation, cannot be fully achieved under strong THz field excitation (Figs.5 and 6).

In this paper, a tunable, embedded VO₂ hybrid metasurface is proposed to realize the dynamic switching of resonant modes from a high frequency of around 0.7 THz to a low frequency of around 0.43 THz in the THz band. The VO₂ at the opening of the resonant ring undergoes a sudden change in conductivity by more than four orders of magnitude before and after the phase transition, constructing a conductive channel in the metal arm of the resonant ring and thus completing mode switching. The feasibility of this mode-switching was verified experimentally through various applications of thermal, laser, and strong-field THz excitations. Although the physical mechanisms of the former and latter two differ, the multimode dynamic excitation manipulation of THz waves presents a feasible idea for practical applications.

In recent years, significant attention has been paid to the nonlinear response of Weyl semimetals, in which either the inversion or time-reversal symmetry is broken. For example, it was confirmed that the second-order optical responses in the type-I Weyl semimetal TaAs, including the shift and injection current and second-harmonic generation (SHG), relate to the topological effects of Weyl semimetals. Remarkably, Dirac or Weyl semimetals have been proposed to support divergently large current-induced SHG when the Fermi level is located near the Dirac/Weyl points. Such current-induced SHG components have been demonstrated in TaAs using an optically pumped shift current. Compared to the optical pump, the terahertz (THz) pump is advantageous because extremely strong THz fields such as 1‒80 MV/cm can be applied without a significant heating effect. In this study, we used the intense THz generated by the tilted wavefront method as the pump beam and the SHG probe to explore the TaAs third-order nonlinear response.

A Ti sapphire femtosecond laser amplifier generates 35 fs pulses with a central wavelength of 800 nm, repetition rate of 1 kHz, and single-pulse energy of 7 mJ. 95% laser energy was employed to generate THz radiation from a MgO-doped LiNbO₃ crystal (doping concentration of MgO is 5%) using tilted wavefront technique. The THz radiation generated with 0.8 THz center frequency (pump beam) was focused on the sample surface using three off-axis parabolic mirrors. A pair of wire-grid polarizers was used to attenuate the THz field and change polarization. The remaining weak laser pulse (probe beam) was used to generate SHG from the sample. The probe pulses were focused on the sample surface through a hole in the last off-axis parabolic mirror at near-normal incidence. Polarization of the probe pulses was rotated using a half-wave plate. The SHG signal generated at approximately 400 nm was delivered through several bandpass filters and detected using a photomultiplier tube. The SHG polarization was filtered using a wideband wire-grid polarizer mounted on a motorized stage. High-quality single crystals TaAs were grown via a chemical vapor transport method using iodine as the agent.

The maximum pulse energy of THz radiation at the sample position was 6.4 μJ, measured by a commercial thermopile detector (Ophir, 3A-P-THz). The size of the focused THz spot was measured using an uncooled microbolometer THz camera (Swiss Terahertz, S2x). Assuming a Gaussian beam profile, a focus diameter of 530 and 600 μm at the sample position was obtained (Fig. 3). The THz electric field intensity was estimated to be 970 kV/cm at the peak from electro-optic sampling measurements. First, we determined the orientation of the high-symmetry axes on the (112) surface of the TaAs crystal using the static SHG pattern (Fig. 4). With the arrival of the pump pulse, the time-resolved THz field-induced SHG (TFISHG) signal followed the temporal profile of the pump THz pulse. The peak value of the TFISHG signal increases linearly with an increase in the THz electric field intensity (Fig. 5), which is probably owing to the existence of a large TaAs second-order susceptibility tensor. The TFISHG pattern was collected when the strong-field THz radiation was selectively pumped along the two in-plane [1,-1,0] and [1,1,-1] axes of the (112) face. The TFISHG signal was two orders of magnitude smaller than the static SHG signal, albeit exhibited a clear dependence on the THz electric field direction (Fig. 6). These results can be explained quantitatively by third-order nonlinear polarization introduced by the pump THz electric field. We analyzed the mathematical forms of the third-order susceptibility tensor and discussed the possibility of an exotic topological origin for certain components of this tensor. Finally, we briefly commented on the difference in the transient SHG signals induced by femtosecond laser and strong-electric-field THz pulses.

In summary, we report the measurements of the third-order nonlinear optical response in Weyl semimetal TaAs crystals driven by an intensity THz beam with a peak electric field intensity of 970 kV/cm. The TFISHG signal exhibited different polarization dependencies when the pumps THz electric field was along the two special axes of the TaAs (112) surface. The TFISHG signal can be quantitatively explained by third-order nonlinear polarization introduced by the THz electric field, where the topological nontrivial zzzz component may play a crucial role. Our results suggest that strong electric field terahertz can be applied to alter the symmetry of topological material on ultrafast time scale, thus providing the possibility to further control the topological properties that are associated with symmetry.

Terahertz waves are electromagnetic waves with frequencies of 0.1-10.0 THz. They have the characteristics of wide bandwidth,strong penetration,and low photon energy. Notably,the energy levels of the terahertz spectrum correspond to the rotational and vibrational energy levels of several biological macromolecules. Therefore,the terahertz spectroscopy technology can be used to study the properties of biomolecules,such as their molecular structures and their molecular interactions with the surrounding environment. Because the wavelength of a terahertz wave (0.03-3.00 mm) is not in the same order of magnitude as the characteristic size of common biological macromolecules,it is difficult to produce sufficient interaction between the trace levels of biological macromolecules contained in the sample and the terahertz wave,and the weak change in the terahertz spectral line is difficult to capture. Metamaterial is an artificial material whose electromagnetic properties can be manipulated artificially. By combining terahertz spectroscopy technology with metamaterials,the small disturbance to the metamaterial caused by trace-level objects can result in significant changes in the spectra of metamaterials,and thus make high-sensitivity detection of biomacromolecules possible. In this study,a resonant terahertz metamaterial is used to enhance the interaction between terahertz wave and the determinand,and a bovine serum albumin (BSA) solution is selected as the analyte. An efficient BSA sensor is constructed using terahertz spectroscopy technology. The relationship between the concentration of the BSA solution and the resonance frequency offset of the sensor is analyzed,and the limit of detection of the sensor is examined.

To achieve rapid and highly sensitive detection of BSA solution,the unit structure designed by Sengupta et al. is adopted. Because of the C4 symmetry of the structure,the metamaterial-based sensor is insensitive to polarization. The electromagnetic properties of the metamaterial are simulated by full-wave numerical simulation. According to the simulation results,the above structure has resonance absorption near 0.8 THz. When the refractive index of the material at the surface of the metamaterial changes,the resonance frequency of the metamaterial shifts. By combining the terahertz spectrum technology,the relationship between the resonance frequency shift of the metamaterial and the concentration of determinand is established. The schematic of the experimental setup is shown in Fig. 1. First,the transmission spectrum of the metamaterial without a determinand is measured with a terahertz spectrometer and used as the background. Second,quantitative BSA is dissolved in distilled water to prepare a certain concentration of BSA solution,and the 20 μL solution is collected through a microsampler and dropped onto the metamaterial. The metamaterial is heated at 70 ℃ for 10 min. The transmission spectra of the metamaterials are measured using a terahertz spectrometer to provide a sample group. During the experiment,to reduce error,the linear weighted average processing is carried out on the data of the adjacent points in a single scanning. The final data for each sample is the average of the three scanning results. A second-order Gaussian fitting method is used to fit the transmission spectral data of the background and sample groups nonlinearly,and the resonance frequency shift of the metamaterial is obtained by comparing the fitted curves.

A resonance absorption peak of approximately 0.8 THz for the metamaterial without a determinand is found (Fig.2). According to the fitting curves,a BSA solution with a volume mass of 2 mg/mL can induce a 10.43-GHz redshift in the resonant frequency of the metamaterial. Using the control variable method,several groups of solutions with different volume mass values (2,3,4,and 8 mg/mL) are configured,and their terahertz transmission spectra are measured and compared with the transmission spectrum of the bare metamaterial (Fig.3). With a gradual increase in the concentration of the determinand,the resonance frequency of the metamaterial gradually moves in the lower frequency direction (Table 1). When the solution volume mass is in the range of 2-8 mg/mL,a linear relationship exists between the resonance frequency shift of the metamaterial and the volume mass of the determinand (Fig.4). The concentration of the BSA solution is determined according to a linear relationship. When the concentration of the BSA solution is reduced continuously and when the volume mass is decreased to 0.3 mg/mL,the frequency offset of the resonance chip is 0.87 GHz. When the volume mass is further decreased to 0.2 mg/mL,the resonance shift is less than 0.04 GHz,which is significantly lower than the sampling step size of the spectrometer (Fig.5 and Table 2). Furthermore,0.3 mg/mL exhibites the lowest detection limit. By optimizing the experimental environment,considering the change in transmittance and resonance frequency,and shortening the frequency step size,the detection sensitivity can be further improved,the limit of detection can be reduced,and the sensor specificity can be enhanced.

Based on metamaterials and terahertz spectroscopy, an efficient sensor for biological macromolecules is constructed. The experimental results show that the addition of determinand can cause a significant shift in the terahertz transmission spectrum and the variation in the resonant frequency of the sensor increases with an increase in the concentration of the determinand. When the volume mass of the BSA solution is in the range of 2-8 mg/mL, the offset of the resonant frequency is linearly correlated with the volume mass, demonstrating the potential of the proposed sensor for detecting the substance concentration. In addition, the limit of detection is 0.3 mg/mL. In conclusion, the proposed BSA sensor has high sensitivity, simple operation, and high efficiency. Subsequently, by further optimizing the materials and parameters of metamaterials, comprehensively utilizing the position and amplitude information of the resonance peak, and improving the test environment, it should be possible to obtain biological macromolecule sensors with higher specificity and sensitivity.

The study of high-order nonlinear behavior,prompted by single-cycle or subcycle strong terahertz pulses,has been scarce,primarily due to the limitations of highly stable and high-intensity terahertz radiation sources. The strong terahertz field–matter interaction leads to new avenues for exploring and understanding various novel nonlinear phenomena. The employment of a femtosecond laser to pump a lithium niobate (LiNbO₃) crystal has been considered as a major strategy for generating strong terahertz radiation. The strong terahertz pulse acquired by this technique can coherently detect and manipulate the optical properties of materials on an ultrafast timescale and can play a crucial role in discovering new phenomena and revealing new mechanisms. In this study,to explore the nonlinear effects of materials under strong terahertz pulse irradiation,a strong terahertz pump-optical probe spectroscopy system is developed by utilizing the tilted wavefront technique based on a lithium niobate crystal. Specifically,the highest terahertz output energy of 2.6 μJ and a focused terahertz peak electric field strength of 632 kV/cm are realized. Meanwhile,the nonlinear crystal gallium selenide (GaSe) is examined with this system,and we demonstrate that the strong change in the terahertz pulse-induced refractive index of GaSe is due to Pockels and Kerr effects.

A strong terahertz pump-optical probe spectroscopy system is established (Fig.1). This system operates on a Ti∶sapphire laser amplifier that can deliver laser pulses with a single-pulse energy of 2 mJ,a pulse duration of 120 fs,and a repetition rate of 1 kHz. A small portion of the femtosecond laser is divided by a beam splitter to sample the birefringence signal of the material induced by the terahertz pulse,whereas the residual 90% of the laser energy is used to pump the lithium niobate crystal to generate a strong terahertz pulse. A diffraction grating of 1800 line/mm is used to tilt the wavefront of the pump laser. A half-wave plate behind the grating changes the polarization of the pump laser from horizontal to vertical,making it parallel to the optical axis of the lithium niobate crystal. A 4f-lens geometry is used to image the pump spot with a tilted pulse front onto lithium niobate. The polarization distribution of terahertz pulses delivered from the lithium niobate is vertical. Subsequently,strong terahertz pulses are focused on the surface of the sample using a series of off-axis parabolic mirrors. A weak 780-nm probe beam overlaps with the terahertz pumping beam on the sample,and it is transmitted through the sample. An assembly of a quarter-wave plate,a Wollaston prism,and a balanced detector is used to sample changes in the refractive index of the material. A pair of wire-grid polarizers is used to tune the electric field intensity of terahertz pulses.

The transient optical responses of the GaSe crystal are measured under different terahertz peak electric fields (Fig.3). The transient signal intensity increases as the terahertz peak electric field strength increases. In essence,when strong terahertz pulses are incident on the GaSe crystals,the refractive index is modulated. Subsequently,the delayed probe light experiences birefringence induced by intense terahertz pulses as it passes through the GaSe crystal,and the two polarization components parallel and perpendicular to the terahertz polarization direction produce a phase retardation in the propagation direction. The maximum transient signals under different terahertz electric fields are extracted to analyze the potential nonlinear behavior of the experimental phenomena. A function that considers the coexistence of χ⁽²⁾ and χ⁽³⁾ nonlinear processes is used to fit the extracted maximum. The fitted curve can reproduce the experimental data in a better manner,indicating that the strong terahertz pulses simultaneously cause Pockels and Kerr effects in the GaSe crystal. Finally,we present the relationship between the change in the refractive index,induced by strong terahertz pulses,and electro-optic coefficient,along with the nonlinear refractive index.

A strong terahertz pulse with a single pulse energy of 2.6 μJ and peak electric field strength of 632 kV/cm is generated by employing the tilted wavefront technique based on lithium niobate crystals. The nonlinear optical response of a gallium selenide crystal is examined using a terahertz pump-optical probe system. The optical birefringence induced by sub-cycle terahertz pulses is observed,and the analysis suggests that the changes in polarization are related to Pockels and Kerr effects. This study provides a pathway for the analysis of the nonlinear effect in a medium under strong terahertz pumping and offers a reference for the determination of the electro-optic coefficient and nonlinear refractive index of materials.