The neurovascular unit (NVU), a critical component of the brain, regulates almost all physiological process. The precision of the morphology and function presentation regarding the NVU provides hope for advancing research on basic neuroscience, as well as diagnosing brain diseases, which are common desires of the “Brain Project” worldwide. Accordingly, high temporal and spatial resolution visualization techniques are required. Fluorescence microscopic imaging technology has significant advantages in terms of specificity, diversity, image contrast, and spatio-temporal resolution; however, due to the limited penetration depth of light in tissue, use of noninvasive fluorescence imaging to obtain high-resolution structural and functional information of NVU is difficult in deep brain regions in vivo. As a result, fluorescence endoscopic microscopy imaging technologies based on micro probes are becoming more popular among brain science researchers.

Over the last two decades, a series of neurobehavioral studies in vivo have been conducted using fluorescence endoscopic microscopy. With endoscopic probes implanted into the brain, the NVU in most deep regions can be observed clearly in living mice, including the hippocampus, dorsal striatum, amygdaloid nucleus, and epithalamus. Incorporating an upright microscope or a head-mounted mini microscope, gradient refractive index (GRIN) lenses have been widely employed as an implantable probe, with the advantage of excellent stability, high resolution, and low cost. In addition, a potential strategy for implantable imaging of the brain in vivo involves using a single multimode fiber, based on modulation of the light field, to focus and scan spot at the end of multimode fiber. This reduces tissue damage, with resolution at the cellular level. Herein, the recent progression of implantable fluorescence endoscopic microscopy is reviewed based on both GRIN lens and a single multimode fiber, besides application research in vivo including blood velocity, neurons growth, calcium ion conduction, and so on. Finally, fluorescence endoscopic microscopy imaging technologies for clinical diagnosis of brain tumors are also introduced, demonstrating that these advanced optical imaging methods expand the toolbox for brain science research and disease diagnosis.

Endoscopic probes have been miniaturized, providing greater flexibility while maintaining high performance; thus, probes can be implanted at different depths in the living brain to carry out functional modulation studies in specific deep brain regions. With micromachining or adaptive optics technologies, GRIN lens provides an effective method to obtain high resolution images. Although the nonmechanical scan imaging through a single multimode fiber is a relatively new exploration for brain research in vivo, it has already exhibited the unique advantages of minimally invasive and flexibility. In future, the following considerations are worth exploring: (1) development of a high-performance multimode fiber with enhanced anti-interference ability to external disturbances; (2) processing of a microlens on the face of multimode fiber with precise 3D printing technology, to optimize imaging resolution, depth of field, and field of view; (3) introduction of fluorescence polarization and fluorescence lifetime imaging modes to analyze neuronal physiological information, such as protein dipoles and cellular microenvironment.

Owing to its non-invasiveness and high specificity, fluorescence microscopy is widely utilized in biomedical research to investigate the structures and functions of biological systems. Limited by the diffraction of light, the resolution of conventional fluorescence microscopy is ~250 nanometer (nm) and ~800 nm on the lateral and axial axes, respectively, and it cannot resolve nanostructures beyond this limit. To overcome the resolution limit, many super-resolution fluorescence microscopy techniques have been developed, enabling biologists to record the dynamics of the fine structures of organisms and cells in their active states. This offers the potential to elucidate the crucial details of biological phenomena.

Nevertheless, in super-resolution fluorescence microscopy, trade-offs exist between resolution, speed, and imaging depth. Although these trade-offs can be moderated by optimizing the microscopy hardware, certain strict physical limitations cannot be easily overcome. Therefore, enhancing microscopy performance via computational imaging methods is particularly important. For instance, the application of deconvolution algorithms can transcend physical limits without changing the optical hardware, thereby improving the dissection of biological information.

This review introduces the technical principles of various deconvolution methods. Deconvolution techniques are applied to four modes of super-resolution fluorescence microscopy: structured illumination microscopy (SIM), image scanning microscopy (ISM), stimulated emission depletion (STED) microscopy, and super-resolution optical fluctuation imaging (SOFI). Various modalities have been used for live cell imaging applications. For example, researchers have designed deconvolution algorithms to eliminate the reconstruction artifacts produced during the reconstruction of SIM and to improve its resolution. Additionally, for SOFI, deconvolution techniques can be applied as pre- or post-processing steps to further enhance the efficiency of utilizing statistical information and to improve resolution. The recently developed advanced deconvolution algorithm, sparse deconvolution, is stable and robust to various noise conditions and can effectively improve the three-dimensional resolution two-fold. Furthermore, it can be combined with different variants of fluorescence microscopy to enhance their contrast and resolution in situ without any changes. Owing to significant advances in the corresponding super-resolution reconstruction techniques, live-cell super-resolution microscopy has been effectively enhanced.

In the outlook section, considering the unrolling algorithm as an example, this review discusses the prospects of deconvolution methods based on deep learning. The combination of deep learning algorithms and microscopy imaging techniques may become a future development trend in the field of live-cell super-resolution microscopy. This review briefly describes the Fourier ring correlation (FRC) image resolution measurement method and its application in image reconstruction. Finally, a rolling FRC (rFRC) method is introduced to quantitatively detect the reconstruction uncertainties of super-resolution techniques at the corresponding super-resolution scale.

Owing to hardware limitations, extensive super-resolution microscopy methods have introduced computational steps to achieve the optimal quality of super-resolution imaging. This review can serve as a bridge between the super-resolution microscopy and computation communities to facilitate the application of novel computational techniques toward improved resolution, accuracy, and image processing.

Optics, which is a significant sub-discipline of physics, focuses on the study of the phenomena, properties, and applications of light. Optics has evolved into an independent discipline over time. Optical imaging plays a crucial role in optical research by utilizing the phenomena and properties of light to record images of objects. Optical imaging has extensive applications in diverse fields, including astronomy, medicine, communication, and photography. For example, with the ongoing advancements in biomedical research, optical imaging has progressively showcased its distinctive advantages. First, optical imaging offers high resolution that is free from ionizing radiation, making it safer than X-rays or gamma rays that pose the potential risk of cancer. In addition, optical imaging can be flexibly configured to provide rich biomedical information based on the amplitude, phase, wavelength, polarization, and other characteristics of light. Another advantage of optics is their exceptional sensitivity, which enables the precise and sensitive detection of interactions between light and tissue components or molecules. Finally, the application of contrast agents further enhances the imaging specificity and contrast, thereby improving the visualization of desired targets and opening new avenues for disease diagnosis and treatment.

These have spurred the development of a vast range of high-resolution optical imaging technologies, such as confocal microscope, multiphoton microscope, and super-resolution imaging, which have been achieved by exciting fluorescence signals and/or utilizing gating or nonlinear optical effects in tissue samples. However, these implementations without exception have encountered fundamental challenges in thick biological tissues. This limitation stems from the strong scattering of light in tissue due to the inherent inhomogeneous spatial distribution of the refractive index of the medium encompassing diverse tissue constituents and functions. As a result, when light propagates within biological tissues, the light beam spreads quickly and is accompanied by the accumulated scattering of light (approximately one scattering event per 0.1-mm optical path length at visible wavelengths), which also rapidly weakens the intensity of non-scattered light in situ. In combination, these result in an intrinsic trade-off between spatial resolution and penetration depth for optics in biological tissues. This is also why optical techniques that utilize ballistic or quasi-ballistic photons typically have an effective penetration depth of less than or approximately 1 mm beneath the skin, which corresponds to 10 times the transport mean free path in the visible and near-infrared regimes. Excessive laser power may further enhance tissue penetration depths, but it also poses a risk of damaging biological tissues, particularly the skin and subsurface.

In the past two decades, numerous studies have been conducted to address these challenges, including switching to longer wavelengths to obtain lower tissue scattering coefficients, converting diffused light into non-scattered ultrasound at the signal detection side, and creating a minimally invasive optical path via ultrathin fibers to deep tissue regions. We believe that summarizing these advancements is not only worthwhile, but also critical for inspiring further research aimed at greater penetration depths and faster speeds toward wider applications.

In this review, we summarize the recent efforts in deep-tissue optics from various perspectives based on the mechanism of operation, including physical, computational, learning, and fiber optics. Note that this is not a complete list but only an empirical one.

Regarding physical-optics-based efforts, relevant research has primarily focused on the three aspects of wavelength engineering, energy conversion, and phase compensation. Wavelength engineering, such as multiphoton imaging and up-conversion imaging, involves the transformation of the input light wavelength into a different output wavelength to enhance the penetration depth. In multiphoton fluorescence imaging, two or more photons with longer wavelengths but lower energies are absorbed almost simultaneously before exciting the target fluorescent molecules at depth, generating one photon with shorter wavelength but higher energy. The longer wavelength in excitation and elevated photon energy in emission both contribute positively to the increased penetration depth for imaging. Up-conversion imaging entails the sequential absorption of multiple low-energy photons and their conversion into a single high-energy photon, thereby increasing the penetration depth.

Among approaches based on energy conversion, the photoacoustic (PA) effect, which converts input pulsed light into ultrasonic waves, has been extensively studied. When a biological tissue absorbs light energy, it undergoes thermal transformation, leading to localized expansion in the region of interest. Conversely, when the optical illumination is switched off, the local temperature decreases, causing the tissue region to contract. When the activation and deactivation of optical illumination (such as pulsed light) are manipulated, the expansion and contraction of tissues can be controlled, generating periodic mechanical waves in the ultrasonic frequency (MHz) range. These are usually referred to as photoacoustic or optoacoustic signals and are detected by one or an array of ultrasound transducers positioned outside the tissue sample. Because the generation of PA signals relies on the optical absorption of light, optical absorption contrast is obtained in PA imaging. However, the generation of signals does not distinguish between ballistic or diffused photons, and the detection of signals is based on ultrasound, which scatters much less (~1/1000) than light in the tissue. In combination, these features lead to a considerably boosted balance between imaging resolution and penetration depth and enable many exciting applications that are not possible with pure optical technologies.

In phase compensation, optical devices are utilized to measure and compensate for the optical phase distortion induced by light scattering. One representative example of phase compensation is optical phase conjugation, which captures the phase distortion of the wavefront emitted by a guide star within the scattering medium and compensates for it by conjugately adjusting the incident wavefronts and then refocusing light onto the position of the guide star. The phase-conjugation mirror, which is typically a photorefractive material, is responsible for recording the incident wavefront pattern and generating conjugated light that propagates along the optical path opposite the original transmission path.

Computational optics is an interdisciplinary field that merges optics and computers to leverage physics and algorithms, thereby enabling applications beyond those that can be achieved using traditional optical systems. The primary computational optics-based efforts in deep-tissue optics include digital optical phase conjugation (DOPC), iterative wavefront shaping, and transmission and reflection matrices. In DOPC, the phase-conjugation mirror previously discussed is replaced by the integration of a digital camera, computer, spatial light modulator, and algorithms for determining and generating the phase-conjugated wavefront. In iterative wavefront shaping, the phase of the incident light wavefront is adjusted based on feedback signals and the focusing performance is iteratively optimized. Feedback signals can take various forms, such as focal intensity, peak-to-background ratio (PBR) in the captured pattern, and photoacoustic signal strength. In the transmission matrix, a linear mathematical model is used to describe the relationship between the incident and scattered output wavefronts to characterize the scattering medium. If we denote the input wavefront as ein and the output wavefront as eout, the transmission matrix (MTM) can be characterized as eout=MTM⋅ein. By measuring the transmission matrix, we can focus the diffused light, project specific patterns through a scattering medium, or retrieve images from speckles. The reflection matrix establishes the relationship between the incident and reflected wavefronts from a scattering medium. In deep tissues, it is typically impractical to define or position guidestars or obtain guidestar signals within or on the opposite side of a tissue sample. Thus, applications of transmission matrices are limited. The introduction of a reflection matrix addresses this challenge by utilizing a reflected wavefront instead of a transmitted wavefront. In this scenario, both the incident and reflected light detectors are present on the same side of the scattering medium, thereby circumventing the need for guidestars to be placed on the opposite side of the scattering medium.

These computational optics-based efforts typically rely on intricate physical models to achieve the focusing or imaging of simple targets, such as letters, numbers, and other basic patterns, through scattering media. With recent advances in artificial intelligence, complicated problems involving speckles can now be addressed using deep learning. For example, deep-learning-based speckle imaging has powerful learning capabilities and data-driven characteristics. Deep neural networks can be trained using known data pairs, including ground-truth images and corresponding speckles, to extract various dimensions of information features. This can enable the high-fidelity reconstruction of target images, such as human face images. In addition, by training the speckle patterns obtained under different states of perturbed scattering media, the generalization capabilities of deep neural networks can be further improved, and the robustness of handling perturbed scattering media exceeds that of transmission-matrix-based methods.

In addition to these endeavors, which are all aimed at noninvasive deep-tissue optics, minimally invasive solutions that employ ultrathin optical multimode fibers as light guides into the tissue are also attractive and have seen promising advancements in recent years. Multimode fiber-based imaging is advantageous due to its minimally invasive nature, flexibility, and affordability. However, because of mode dispersion and coupling within multimode fibers, the optical field output from the fiber appears to be similar to a speckle pattern from tissue-like scattering media, making it infeasible to directly interpret the transmitted spatial information. Nevertheless, if multimode fibers are treated as scattering media, the aforementioned wavefront shaping approaches can be applied to multimode fibers. Thus, with the integration of wavefront shaping, the speckled output from a lensless multimode fiber can be focused onto a single optical mode, and then the raster can scan at a high speed within the field of view of the fiber. The excited or responding signals can also be detected and relayed using the same fiber for further use. This creates a scenario very similar to laser confocal microscope, except that the probe is inserted deep into the tissue. As a result, spatially and/or temporally resolving optical signals from deep tissues can be excited and detected with high resolution, which opens avenues for exciting new optical practices that require high resolution at depths in tissue. This capability can also be extended beyond imaging, such as for optogenetics, where wavefront shaping-empowered multimode fibers can deliver light precisely to targeted neurons within deep tissues and pick up fluorescence signals reflecting neuronal activities, enabling precise activation or inhibition of neurons to study brain functions.

Optics have gained significant attention in the study of deep biological tissues due to their non-ionizing radiation, exceptional contrast, exquisite specificity, and heightened sensitivity. In addition, the integration of computational optics and deep learning with conventional optics has substantially enhanced penetration depths while preserving moderate resolution in deep biological tissues. Despite these remarkable advancements, the practical implementation of deep-tissue optics still encounters critical challenges that must be addressed before moving forward.

The first is the penetration depth. With photoacoustic efforts and wavefront shaping techniques, which are sometimes further aided by computational optics and deep learning, current practices have achieved high-resolution optical focusing and/or imaging far beyond the optical diffraction limit. While most experimental research efforts to date still concentrate on small animal models such as mice, future studies are anticipated to improve the depth capability and extend to large animal models such as rabbits and monkeys. This transition is necessary for assessing the practicality, safety, and reliability of clinical diagnostics and therapeutic applications before working with human patients.

Speed is another crucial factor in the operation of deep-tissue optics. To reverse or compensate for the scattering-induced wavefront distortion, the scattering medium or multimode fiber should theoretically remain stationary to maintain the medium status, equivalent to the transmission matrix, during the wavefront optimization process. However, in practical applications, this requirement is hardly met, particularly for living biological tissues, whose optical field decorrelates rapidly on the order of milliseconds or even faster due to factors such as blood flow and respiration. Although some operations based on physical optics, such as optical phase conjugation, can reach this time scale, the majority of wavefront shaping implementations to date, consume seconds or hundreds of milliseconds, which is mainly limited by the response rate of the hardware such as spatial light modulators.

Over the past few years, deep learning has significantly affected deep-tissue optics. By leveraging the power of deep neural network models, it excels in extracting features and establishing nonlinear relationships between the target information (the ground truth) and the corresponding speckles, enabling high-fidelity retrieval of the original information from speckles. In addition, the use of deep learning has expanded the scope of speckle imaging, enabling breakthroughs in scattering, virtual staining, optical encryption, optogenetic networks, etc. The integration of deep learning with deep-tissue optics is expected to improve the speed, penetration depth, and immunity to system and medium disturbances. In addition, the combination of deep learning with physics-based scattering models holds great potential for accurately understanding and modeling multiple scattering processes, which is essential for designing efficient computation algorithms.

Finally, noninvasive deep-tissue optics in vivo still remains limited in some respects and may require a few more years to achieve technical maturity. Accordingly, a temporary yet effective alternative is to integrate wavefront shaping with ultrathin multimode fibers. Because the diameter of the multimode fiber can be 100‒200 μm, close to the typical hair diameter of adults, this integration can create a minimally invasive optical path into deep biological tissue, enabling high-resolution and fast-scanned optical focusing, imaging, stimulation, and manipulation at depths in tissue. Although it is not a perfect solution, it is practically useful in many studies, particularly for those at the early and preclinical stages, or when the insertion of a fiber-based probe is accompanied by invasive surgery, and the insertion of the probe does not considerably increase the degree of invasion or discomfort to the patient.

The developments to date in this field have demonstrated the feasibility and potential of deep-tissue optics. With continuing efforts and progress in related areas, technical barriers, such as the speed bottleneck associated with the response rate of spatial light modulators and the insufficient generalization capability of neural networks, can be overcome. It is strongly envisioned that in the near future, deep-tissue optics will reach practical maturity and be usable in vivo, which can extend many exciting optical applications to tissue regions that are currently optically inaccessible. This could reshape the landscape of light use in biomedicine and many other areas.

Ytterbium-doped large-mode-area photonic crystal fibers (LMA PCFs) have attracted extensive attention owing to their important applications in high-peak-power ultrafast laser amplifiers. Fiber lasers are widely used in advanced manufacturing, medicine, national defense, and scientific research owing to their compact structure, high conversion efficiency, high reliability, and low cost. However, with the development of fiber lasers, particularly the development and application of ultrafast lasers in the field of fine processing in recent years, higher requirements have been placed on the output power and beam quality of fiber lasers. Currently, the output power of internationally commercialized fiber lasers has reached 100 kW. IPG Photonics Corporation uses a double-clad fiber with a core diameter of 30 μm to achieve a 10 kW single-mode single-fiber laser output. However, owing to the limitations of the physical mechanisms, such as nonlinear effects, optical damage, and thermal damage, it is very difficult to further increase the output power of a single laser module. The nonlinear effect of the optical fiber is related to the mode field area of ​​the optical fiber. The larger the mode field area, the weaker the nonlinear effect of the optical fiber, and the higher the threshold of the nonlinear effect. Therefore, large-mode field fibers are one of the most direct and effective ways of overcoming nonlinear effects and fiber laser damage to further increase laser power. However, an increase in the core diameter of large-mode field fibers inevitably causes competition among multiple transverse modes, degrading the beam quality of the laser.

Consequently, various fiber structure designs have been proposed to maintain a satisfactory beam quality with large core diameters, such as rod-type photonic crystal fibers, photonic bandgap fibers, leakage channel fibers, large-pitch fibers, chirally coupled-core fibers, and other microstructure fibers. Among them, the Yb-doped PCF has the most classic architecture, with an ordered array of microscopic air holes. These microscopic air holes favor convenient regulation of the effective refractive index of the cladding. Nevertheless, it is difficult to manipulate the refractive index so that it is close to that of pure silica glass cladding and maintain good uniformity in Yb-doped silica core glass. The commercial method of modified chemical vapor deposition (MCVD) combined with solution doping has some limitations in terms of the core size, refractive index uniformity in the radial and axial directions, and ultralow numerical aperture. Other non-MCVD fabrication technologies have also been developed and reported, including direct nanoparticle deposition (DND), reactive powder sintering of silica (REPUSIL), and sol-gel methods. Heraeus Quarzglas made great progress in the preparation of Yb³⁺/Al³⁺/F^--co-doped silica bulk glasses with the F^--doping-induced refractive index (RI) reduction being evident.

The sol-gel technique is a well-known method for producing centimeter-sized long glassy silica rods. Our group has committed to the preparation of large Yb³⁺-doped silica glass rods with a low refractive index and high optical homogeneity using a modified sol-gel method combined with high-temperature sintering. The sol-gel process ensures dopant mixing in the solution and consequently high doping uniformity, and high-temperature powder sintering allows the preparation of large-sized bulk glass. Fluorine incorporation during the sol-gel process is used to compensate for the increased refractive index caused by ytterbium and aluminum co-doping, and phosphorus is used to suppress the formation of Yb²⁺ and photodarkening. The sol-gel method combined with high-temperature sintering provides a cost-effective method for fabricating the core glass of a Yb-doped LMA PCF.

In this paper, we briefly introduce the progress of research on ytterbium-doped LMA PCF at home and abroad, as well as the design of ytterbium-doped LMA PCF. The effects of thermal history on the refractive index of the Yb/Al/P/F co-doped silica glass and the beam quality of the PCF are demonstrated. For comparison, the design and preparation methods of the polarization-maintaining ytterbium-doped PCF are presented. This paper focuses on the progress of research on Yb-doped LMA PCF in the past ten years at the Shanghai Institute of Optics and Fine Mechanics (SIOM) (Table 1 and Fig. 20). This includes accurate control of the refractive index value, homogeneity of the fiber core glass, and structure of the PCF. The output laser beam quality is significantly improved owing to the optimization of the Yb-doped core-glass rod.

With the rapid development of the domestic ultrafast laser processing industry, the demand for domestically produced Yb-doped LMA PCF by domestic ultrafast laser companies has increased. This paper summarizes the progress of ytterbium-doped large-mode-field photonic crystal fibers in SIOM over the past decade. Ytterbium-doped large-mode field photonic crystal fibers with core diameters of 40 μm, 50 μm, 75 μm, and 100 μm were prepared. Using a 40 μm /200 μm polarization-maintaining ytterbium-doped photonic crystal fiber, we independently designed and prepared an all-fiber amplification module, and achieved picosecond pulse amplification with an average power exceeding one hundred watts and high beam quality. The beam quality factor M² was less than 1.5, the polarization degree was greater than 12 dB, and the power fluctuation was less than 1.3% in 2 h under a 100 W amplification power operation. Using ytterbium-doped LMA PCF with a core diameter of 100 μm as the gain fiber, picosecond pulse amplification with a beam quality factor M²<1.3 and polarization degree greater than 95% was achieved. In the future, the performance of LMA PCFs should be further optimized to meet the requirements for high-average-power ultrafast fiber lasers.

Visible lasers, with wavelengths ranging from 380 nm to 780 nm, have important applications in the fields of display, biomedicine, precision processing, precision spectroscopy, optical communication, and military defense. Among all the different visible lasers currently available, the rare-earth-doped fiber ones attract considerable attention due to their advantages of high efficiency, excellent performance, compact structure, and maintenance-free nature. In this study, different types of lasers, including visible continuous-wave (CW) fiber lasers, visible Q-switched fiber lasers, and visible mode-locked fiber lasers, are discussed comprehensively, along with their output characteristics. The latest research progress indicates that these lasers can cover the entire visible wavelength range and present different colors, such as blue (~480 nm), cyan (~491 nm), green (~520 nm), yellow (~573 nm), orange (~605 nm), red (~635 nm), and deep-red (~717 nm). The output power approaches 10 W for the all-fiber visible lasers, and the pulse duration of the mode-locked pulse is less than 200 fs. Thus, the all-fiber visible lasers play an increasingly important role in underwater optical communication, material processing, laser welding, and spatiotemporal super-resolution imaging. This study summarizes the progress in the research on visible fiber lasers, which provides a strong basis for any future research and application on visible fiber lasers.

With continuous research on fluoride fibers doped with rare-earth metal ions like Pr³⁺, Ho³⁺, Er³⁺, Dy³⁺, Tm³⁺, and Nd³⁺, visible CW fiber lasers, visible Q-switched fiber lasers, and visible mode-locked fiber lasers have been actively developed. After nearly 30 years of development, the outputs of blue, green, yellow, red, and deep-red fiber lasers have been scaled up to Watt-level. Notably, the maximum output powers of red (~635 nm) and green (~521 nm) fiber lasers reach ~5 W and ~3.6 W, respectively, as shown in Fig.8 and Fig.11.

Visible mode-locked fiber lasers have the advantages of higher peak power and shorter response time than visible Q-switched fiber lasers. The development of visible mode-locked fiber lasers has been accelerated by the development of high-performance rare-earth-doped fluoride fibers. In 2020, Zou et al. reported the first all-fiber visible-wavelength (635 nm) passively mode-locked picosecond laser with a pulse duration as short as ~96 ps. In the following two years, red-light mode-locked fiber lasers were further developed. As shown in Fig.15, a 635-nm spatiotemporal mode-locking (STML) picosecond fiber laser with the implementation of a Pr³⁺/Yb³⁺ co-doped few-mode fiber and nonlinear polarization rotation (NPR) technology was reported by Ruan et al. in 2022. By further incorporating a visible ultrafast fiber amplifier, the average power at 635 nm was boosted up to 440 mW, corresponding to a maximum pulse energy and a peak power of 4 nJ and 280 W, respectively, while the pulse duration was shortened to 9 ps. This fills the knowledge gap of STML in the visible fiber lasers. By integrating the NPR scheme into Dy∶ZBLAN and Ho∶ZBLAN fiber lasers, Luo et al. obtained dissipative soliton resonance pulses at ~575 nm and ~545 nm, respectively. The average output power at 575 nm reached a maximum of ~240 mW, which represents an improvement of almost two orders of magnitude compared to those reported for the latest mode-locked visible fiber lasers. The minimal pulse duration at 575 nm is 83 ps as shown in Fig.16. Furthermore, by using a shorter gain fiber (Ho∶ZBLAN), the smallest pulse duration of 19.7 ps is achieved for the ultrafast true-green passively mode-locked fiber laser. The average output power at 545 nm reaches a maximum of ~288 mW, thus filling the “green gap” of semiconductor materials. To obtain mode-locked femtosecond pulses in the visible spectrum, a team from the Laval University reported a mode-locked fiber laser with a compressed external cavity that produced ultrafast pulses at 635 nm. The passively mode-locked ring cavity is based on nonlinear polarization evolution in a single-mode Pr³⁺-doped fluoride fiber and runs in an all-normal dispersion regime. The compressed pulses at 635 nm have a duration of 168 fs, a peak power of 0.73 kW, and a repetition rate of 137 MHz (Fig.17). Furthermore, the pulses directly emitted in a visible fiber oscillator by a phase-biased nonlinear amplifying loop mirror have durations less than 200 fs.

In this study, we review the current progress in research on directly emitting visible fiber lasers prepared from rare-earth-doped fluoride fibers. In summary, among the rare-earth-doped fluoride fiber lasers, the Pr³⁺-doped one is particularly useful for fabricating visible lasers because it can efficiently produce blue, green, orange, red, and deep-red spectra, pumped by GaN semiconductor laser. With fluoride fibers doped with rare-earth metal ions like Ho³⁺, Dy³⁺, Tb³⁺, Tm³⁺ and Pr³⁺/Yb³⁺, the wavelength can cover the entire visible spectrum. Significant progress has been made in the development of CW, Q-switched, and mode-locked fiber lasers. However, there remain some unsolved problems associated with visible fiber lasers, such as high power, large pulse energy, and femtosecond pulse generation. For visible CW fiber lasers, the highest possible output power is ~5 W at 635 nm. Further improvement of the output power, beam quality, slope efficiency, and ability to cover more visible wavelengths is the key to promoting the development and application of visible CW fiber lasers. Therefore, the research and numerical simulations of new visible rare-earth fibers with high damage thresholds, high-performance visible fiber devices, visible beam combiners, etc. will be of great significance. For visible pulsed fiber lasers, the highest pulse energy that can be obtained is ~3.17 mJ at 543 nm, and the shortest pulse duration is 168 fs at 635 nm. The research on STML, femtosecond pulse generation, all-fiber configuration operating in more visible wavelengths needs to be performed. Improving the pulse energy, average power and stability, and realizing the visible femtosecond all-fiber lasers are key to promoting the development and application of visible pulsed fiber lasers. Therefore, the new visible rare-earth-doped fibers, saturable absorber materials, and mode-locking technologies need to be explored. Through the innovation of breakthrough technologies, we believe that the visible CW/ultrafast fiber lasers will find widespread applications in the fields of biomedicine, optical communication, material processing, optical microscopy, and scientific research in the future owing to their advantages of miniaturization, high performance, maintenance-free nature, and low cost.

The rapid development of ultra-intense and ultra-short lasers has provided unprecedented new experimental methods and extreme physical conditions, made it possible to reach new frontiers of ultra-fast and intense interactions between lasers and matter, and given birth to a large number of new principles, new phenomena, and revolutionary techniques. Plasma-based acceleration driven by an ultra-intense and ultra-short laser may contribute to the emergence of new particle-acceleration technologies and generation of novel ultra-fast radiation sources. These novel particle and radiation sources can provide new means and opportunities for frontier interdisciplinary studies in areas such as high-energy particle physics, nuclear photonics, materials science, and biomedicine, making it a hot spot and emerging field on the world scientific and technological frontiers.

The accelerating electric field of a laser-driven plasma wakefiled can reach 100 GV/m, which is more than three orders of magnitude higher than that of a traditional electron accelerator. A high-energy GeV electron beam can thus be produced over a centimeter-scale acceleration length, thus greatly reducing the scale and cost of the accelerator. The electron beams produced via laser wakefield acceleration also have the advantages of an ultrashort pulse duration and inherent high-precision synchronization with the driving laser. In addition, by designing an appropriate and effective scheme, the electron-injection and acceleration processes can be optimized to produce high-quality and high-energy ultrafast electron sources with ultrahigh brightness comparable to that from a traditional accelerator.

Laser-wakefield-driven electron beams can be used as low-cost and desktop femtosecond radiation sources such as for betatron X-ray radiation, inverse Compton scattering, bremsstrahlung radiation, and undulator radiation. These novel radiation sources usually have high brightness, good collimation, a femtosecond pulse duration, and energy tunability, covering a wide spectral range from extreme ultraviolet to gamma rays. Therefore, research in this area is occurring around the world, and this is an important research topic for high-field laser physics and new accelerators. Such laser wakefield acceleration and novel radiation sources are thus of great scientific significance for the development and application of synchrotron radiation, free electron lasers, and high-energy particle physics.

After nearly 20 years of development, great progress has been made in both experimental and theoretical studies on laser-driven plasma acceleration. It is now transitioning from laser acceleration to laser accelerators. On one hand, the energy gain of laser wakefield electron acceleration has been significantly extended to 7.8 GeV. On the other hand, the specific qualities of the accelerated electron beams produced via laser wakefield acceleration, such as the energy spread, divergence, emittance, and stability of the electron beam, are also being optimized to a great extent. However, the comprehensive performance has to meet higher requirements for practical application, and there are still many key scientific issues and technical difficulties that need to be further explored and solved in the future. In particular, the energy spread of the electron beam is usually on the order of several percent, and such a large energy spread has greatly hindered its practical application. In order to obtain more stable and brighter high-energy electron beams, the electron injection and acceleration in the plasma wakefield should be accurately controlled and optimized to minimize the energy spread and divergence, which can also improve the application performance of novel radiation sources. Therefore, the basic principles and parameter characteristics of a plasma wakefield driven by a femtosecond intense laser are first briefly introduced. The mechanisms and characteristics of different electron injection methods are then analyzed and compared (Table 1). Second, based on the research results and progress made by our group in recent years, the schemes and technologies for exploring energy chirp control in a plasma wakefield with a structured plasma profile are summarized and analyzed in relation to the generation of ultrahigh-brightness electron beams with an ultralow energy spread at a per-mille level (Fig.5). Third, we discuss how these high-quality electron beams are used to produce novel radiation sources and greatly improve their application performance, including enhanced betatron X-ray radiation (Fig.9), quasi-monoenergetic all-optical self-synchronized Compton scattering γ‑rays (Fig.18), and free-electron lasing in an undulator (Fig.22). Some of the progress in other related frontier research fields is also discussed in relation to laser wakefield electron acceleration and novel radiation source generation. Finally, the prospects for a laser wakefield electron accelerator and its further practical applications are outlined.

A high-quality electron beam source and novel radiation source based on laser wakefield acceleration have the advantages of a compact size, easy tuning, small source size, femtosecond pulse duration, high brightness, good collimation, and high-precision synchronization control, which can provide new methods and tools for frontier interdisciplinary research such as high-energy particle physics, nuclear photonics, materials science, and biomedicine. Although significant progress has been made in the past decade in improving the quality of an electron beam such as its energy spread and six-dimensional brightness, the wakefield accelerator is still in a very early phase in view of the energy spread and stability of the electron beam, especially for electron beams with energy levels below 100 MeV or above 1 GeV, when compared with traditional accelerators. This dilemma is mainly limited by the scalability and stability of the existing schemes. The key issue or challenge facing the wakefield acceleration community is to devise more effective schemes to generate electron beams with an ultralow energy spread (0.1%‒0.01%), ultralow emittance (~1 μm·mrad), high repetition rate, and stability. Benefiting from the rapid and continuous development of ultrashort pulse laser technology in terms of the repetition rate, waveform control, and stability of the high-power laser, it is believed that the qualities and brightness of these high-energy ultrafast electron beams will be further improved by advancing the existing schemes, which will further facilitate the development of novel radiation sources. All these advances will greatly promote the continuous development of high-quality laser wakefield electron accelerators and their practical applications in the years to come.

Fiber-mode-locked lasers play an important role in generating ultrashort pulses of picosecond or even femtosecond durations, featuring high peak power and broad spectral characteristics. These pulses have important applications in precision machining, spectroscopic measurements, high-capacity optical communications, terahertz technology, and nonlinear optical imaging. Ultrashort pulses generated by fiber-mode-locked lasers result from a double balance between dispersion and nonlinearity, as well as between gain and loss. Grelu et al. extended the concept of dissipative solitons to include ultrashort pulses generated in nonconservative systems such as fiber-mode-locked lasers. These stable dissipative solitons exist in a continuous exchange of energy with the environment and a dynamic energy redistribution between the components of the soliton. In a mode-locked fiber laser, the process of mode phase-locking to produce periodic short pulses is known as mode locking, and the resulting pulses are generally termed optical dissipation solitons. A comprehensive understanding of the mechanisms underlying dissipative soliton generation holds great promise for advancing mode-locked fiber lasers in both scientific and practical applications, offering greater innovation and possibilities across a wider range of fields.

The peak pulse power generated by single-mode fiber mode-locked lasers approaches its limitations in the megawatt (MW) order, which are limited by the number of modes and the mode-field area of the single-mode fibers. To further enhance the performance of mode-locked fiber lasers, it is essential to consider higher dimensions (i.e., introducing spatial dimensions) and explore the impact of increasing the spatial modes (i.e., transverse modes) on soliton mode-locking in multimode fiber lasers. Consequently, mode-locked fiber lasers have evolved from traditional single-mode to multi-mode configurations, and mode-locking mechanisms have transitioned from one-dimensional (1D) temporal dissipative soliton mode-locking to (3+1)D spatiotemporal dissipative soliton mode-locking. The expansion of spatial dimensions results in complex nonlinear spatiotemporal interactions and rich physical spatiotemporal phenomena. Spatiotemporal dissipative solitons not only exhibit periodic pulse output in the time domain but also show the distribution characteristics of multiple transverse modes in the space domain. Spatiotemporal dissipative solitons achieved using multimode fiber lasers have potential applications in precision ranging, laser processing, nonlinear spectroscopy, optical tweezers, and scattering medium imaging, offering new possibilities in information transmission and imaging.

In this review, we focus on the study of dissipative soliton generation mechanisms in fiber mode-locked lasers, trace the development of fiber mode-locked lasers, and review the principles of generating one-dimensional temporal dissipative solitons in single-mode fiber lasers to generate three-dimensional spatiotemporal dissipative solitons in multimode fiber lasers. First, we explore the generation mechanisms of temporally dissipative solitons in single-mode fiber lasers with different chromatic dispersions. Temporal dissipative solitons can form when the positive chirp (owing to self-phase modulation) balances the negative chirp (owing to an anomalous even-order dispersion). Early studies generally considered the case in which self-phase modulation balances second-order dispersion, resulting in the formation of a second-order dispersion soliton (Fig.2). Higher-order even-order dispersive solitons can also be levelled with self-phase modulation and form the corresponding solitons, which are referred to as higher-even-order dispersive solitons (Fig.3). Notably, stable temporally dissipative solitons can be generated even in the absence of dispersion, and this type of soliton is referred to as a dispersionless soliton (Fig.4). The different types of solitons have different properties, and their formation involves various physical processes.

Subsequently, we delved into the latest achievements in spatiotemporal dissipative soliton mode-locking in multimode fiber lasers. In contrast to temporal dissipative solitons in single-mode fiber lasers, spatiotemporal dissipative solitons in multimode fiber lasers add spatial dimensions by incorporating multiple transverse modes. In this case, the dispersion consists of both chromatic (intramode) and intermode (modal) dispersions. Therefore, balancing the modal dispersion is important to generate spatiotemporal dissipative solitons in a multimode fiber laser. We discuss compensation methods for modal dispersion (Fig.5) and reveal their rich spatiotemporal mode-locking mechanisms and potential application scenarios. Finally, we provide an outlook on the research prospects for mode-locked fiber lasers.

In this paper, we present the research history from the traditional single-mode temporal dissipative soliton to a more complex multimode spatiotemporal dissipative soliton and summarize the generation mechanisms of various dissipative solitons in fiber lasers. Through this development, we aim to summarize the differences between temporal and spatiotemporal dissipative solitons, emphasize the significance of understanding the mode-locking mechanism for conducting research and the application of fiber-mode-locked lasers, and show the potential application scenarios of fiber-mode-locked lasers in the future. In summary, focusing on temporal/spatiotemporal dissipative solitons in fiber lasers not only enhances our understanding of the principle of fiber mode-locking lasers but also opens up more opportunities for fiber laser applications. With the continuous progress of technology and theoretical improvements, we believe that fiber-mode-locked lasers will continue to play an important role in the future and provide more possibilities for applications in optical frequency combs, material processing, medical diagnostics, and other fields.

With the rapid advancement of laser technology, the laser intensity reaches approximately 10²² W/cm², and charged particles can be accelerated to hundreds of MeV or even several GeV. These energetic particles can trigger nuclear reactions and generate neutrons. Compared with traditional neutron sources, such as reactors, spallation neutrons, and radioactive neutron sources, laser-driven neutron sources (LDNS) have interesting features, such as short duration, which is approximately tens or hundreds of ps, and ultrahigh flux, which is 10¹⁸‒10²¹ /(cm²·s). Moreover, the neutron energy is easy to adjust by manipulating the laser accelerating process. Therefore, studies on LDNS have attracted considerable interest and have shown unique potential for innovative investigations and applications in the past two decades, particularly after the significant progress achieved by Roth et al. in 2013. LDNS is expected to be a powerful alternative to traditional neutron sources and may play an essential role in specific applications, such as the fast neutron resonance radiography and rapid neutron capture. This study briefly reviews the historical development and status of laser-driven neutron sources. Significant attention is given to the recent progress in beam-target neutron sources.

First, this study reviews the technical approaches to increase the yield of laser-driven neutron sources, which mainly include nuclear reaction channels and ion acceleration efficiency. Compared with deuterium-deuterium and proton-lithium reactions, deuterium-lithium nuclear reactions result in larger nuclear reaction cross-sections and, thus, have received special attention in this field. After determining the nuclear reaction channel, the improvement of the neutron yield mainly depends on the optimization of the deuterium acceleration efficiency. Various new schemes for eliminating the contamination layer within the target normal sheath acceleration (TNSA) acceleration process, such as target heating, laser cleaning, and heavy water spraying, have been established. The use of advanced acceleration mechanisms, such as break-out afterburner and collisionless shock acceleration, has also been proposed to increase the cut-off energy and charge of deuterium ions, and the neutron yield eventually reaches as high as 10¹⁰ /sr (Fig.2). In addition to yield, neutron directionality is also a critical parameter that influences neutron application. New schemes such as the stripping of D-Li reaction and reverse kinematic effects of heavy ions have also been proposed to generate directional neutron sources. By applying the inverse kinematic effect, the proof-of-principle experiments conducted thus far have achieved a neutron angular distribution with a significant forward impulse and full width at half maximum (FWHM) of 40° (Fig.6), which is nearly half lower than those of the D-D and D-Li reactions. In addition to optimizing the quality of the laser neutron source, the accurate characterization of laser neutron source parameters is also an integral process of the neutron application. This study introduces the experimental diagnostic methods of laser neutron source yield, angular distribution, energy spectrum, and source size. The analysis method of the pulse width is also explained. The wide range of energy spectrum and ultrashort pulse-width characteristics are suitable for fast-neutron resonance analysis applications based on the time-of-flight method. Finally, this study reviews the application status of laser neutron sources. Current applications mainly focus on traditional application scenarios, such as fast neutron photography, fast neutron moderation, and thermal neutron resonance absorption. However, the high flux and short pulse of laser-driven neutron sources also make them valuable in fast-neutron resonance imaging and rapid neutron capture.

Research on laser neutron sources has aroused significant interest and demonstrates unique potential in terms of innovative research and application prospects. However, because of the limited yield, most of the current application experiments mainly focus on the application scenarios of the traditional neutron source, in which the LDNS does not have unique advantages in terms of neutron fluence. However, with the development of high repetition rate and high average-power laser technology, miniaturized laser neutron sources can gain advantages in terms of economy and flexibility to cope with more complex applications. In addition, because of the nonsubstitutable unique advantages of the short pulse width and high flux rate of LDNS, it also has potentials for new applications, such as fast neutron capture, diagnosis of the state of warm dense matter, and fusion material research. Finally, lasers have advantages in generating various particle sources, which can flexibly satisfy the needs of multiple application scenarios. For example, lasers can simultaneously generate multiple radiation sources, such as electrons, ions, γ-rays, and neutrons. The unique effects of combining radiation fields can lead to new applications, such as radiography implemented with thermal neutrons and X-rays. Overall, laser-driven neutron sources are expected to be widely used in scientific and industrial fields and can expand more distinctive application scenarios by adopting more stable and efficient neutron generation methods and more accurate neutron-source parameter characterization techniques.

The 2-5 μm mid-infrared wavelength range is a crucial region in an electromagnetic spectrum. Lasers that operate within this range play a critical role in various fields, such as defense, medical, environmental monitoring, and materials science. The generation of 2-5 μm lasers mainly includes solid-state lasers, quantum cascade lasers (QCL), inter-band cascade lasers, optical parametric oscillators (OPO), and fiber lasers. Fiber lasers have unique advantages, such as good beam quality, excellent thermal management capabilities, and robustness, which make them irreplaceable for various mid-infrared laser applications. Three methods are mainly used to generate 2-5 μm fiber lasers: 1) rare-earth-doped fiber lasers, which are the simplest and fundamental; 2) nonlinear fiber lasers based on nonlinear effects, which are effective for extending the laser wavelength, filling the spectral gaps not covered by rare-earth-doped fiber lasers owing to transition-level limitations; 3) gas-filled fiber lasers, which utilize energy-level transitions in gas molecules (N₂O, HBr, and CO₂) to achieve mid-infrared laser outputs.

This study comprehensively reviews the research and power-scaling progress in mid-infrared fiber lasers based on all-solid-state fibers. It covers three main types of mid-infrared fiber lasers: rare-earth-doped, Raman, and mid-infrared super-continuum fiber lasers. Table 1 in the main text presents representative achievements of rare-earth-doped fiber lasers in the 2-5 μm wavelength range. The continuous-wave laser output power within this range has been significantly improved, from milliwatt to watt/kilowatt levels. The highest output power values obtained using fiber lasers doped with Tm³⁺, Er³⁺, Ho³⁺, and Dy³⁺ ions are 1100, 41.6, 7.2, and 10.1 W, respectively. In particular, the longest wavelength tunability of the rare-earth-doped fiber lasers is 700 nm. Tables 2 and 3 present representative results for mid-infrared Raman fiber lasers and tunable mid-infrared Raman soliton lasers, respectively. Currently, by using tellurite, fluoride, or chalcogenide glass fibers as the Raman gain media, a second-order-cascaded Raman fiber laser operating at 3.77 μm and a tunable Raman soliton fiber laser covering the range of 2.8-4.8 μm, with an average watt-level power output in the 3-3.8 μm region, have been developed. Tables 4 and 5 list the representative research progress on germania fiber- and soft glass fiber-based supercontinuum lasers, respectively. The output power of the supercontinuum laser using germania fiber as a nonlinear medium exceeds 41.9 W, and the spectral width is 1.9-3.5 μm. The maximum output power values of the fluorotellurite fiber- and fluoride fiber-based supercontinuum laser are 50.2 W and 11.8 W, respectively, and the spectral widths are 1.22-3.74 μm and 1.9-4.9 μm, respectively.

Since the beginning of 21 century, continuous improvements in semiconductor laser technology, mid-infrared glass-fiber drawing techniques, and pumping schemes have propelled the rapid development of mid-infrared fiber laser sources. In the field of high-power mid-infrared fiber lasers operating within the range of 2.5-5.0 μm, research groups worldwide have achieved significant milestones in the past decade. Nevertheless, compared with the advanced near-infrared waveband, a significant gap still exists in the output power of mid-infrared fiber lasers. The primary challenge lies in the development of mid-infrared fibers with high damage thresholds, broad transmission windows, and advanced ion-doping capabilities. The lack of high-quality mid-infrared fiber functional devices also hinders an effective increase in the output power of mid-infrared fiber lasers. The solution lies in the development of mid-infrared fiber functional devices with high damage thresholds and broad operating bandwidths. The heat load is another critical factor limiting the enhancement of laser power, and damage to laser systems is mostly related to excessive heat loads. Therefore, new methods for suppressing heat generation and regulating heat loads are required to achieve high-power mid-infrared fiber lasers.

Colloidal quantum dots exhibit a unique electronic structure and solution-processing characteristics, making them suitable for the development of low-cost and high-performance lasers. However, colloidal quantum dot lasers have not yet been commercialized, indicating a lack of understanding of the fundamental physics and a failure to master the key fabrication technologies. In recent years, significant progress has been made in CdSe-based colloidal quantum dot lasers, including the achievement of continuous wave-pumped lasers and electrical injection light amplification. These recently achieved milestones suggest that colloidal quantum dot lasers may become practical instruments in our daily lives in the near future. On this basis, this article reviews the various types of colloidal quantum dot lasers developed in recent years and discusses the inherent challenges. Then, we propose feasible solutions for the issues and summarize the technological advancements in this field. We highlight the essential need to investigate the fundamental physics and key technologies to develop electrically pumped quantum dot lasers. Moreover, this review provides insights for future research directions, such as developing new theories, disruptive technologies, novel gain mechanisms, and unprecedented cavity structures. The article suggests the necessity of utilizing state-of-the-art simulation techniques to predict the physical properties and performance of quantum dot lasers. Finally, we emphasize the need to investigate potential applications of colloidal quantum dot lasers, such as integrating them with flexible substrates to create bendable and foldable laser devices and exploring new application areas in displays and sensors.

This article provides a summary of the recent progress in research on semiconductor lasers based on colloidal quantum dots, along with their prospects (Fig.1). First, the advantages of colloidal quantum dots as gain materials for lasers are introduced. Then, we focus on the study of electrically pumped colloidal quantum dot lasers. The discussion begins with research on continuous-wave pumped lasers (Figs.2‒4) and is then extended to optically pumped liquid lasers (Figs.5 and 6) with the potential for commercial applications. Next, we discuss the recent progress on environmentally friendly colloidal quantum dot lasers (Figs.7 and 8). Finally, the latest advancements in electrically pumped quantum dot lasers are discussed in detail (Figs.9‒12). The article points out that the current challenge lies in obtaining quantum dot gain media with low thresholds, high gain coefficients, long gain lifetimes, and high stability. Representative works in the field of Cd-based colloidal quantum dots (Table 1) and emerging colloidal quantum dot laser research (Table 2) in recent years are presented. Because of the lack of standardized synthesis and characterization methods for colloidal quantum dot lasers, there are substantial variations in the reported gains and laser performances across different countries and laboratories. This lack of consistency impairs reproducibility and consequently hinders the development of high-gain quantum dot media. Currently, electrically pumped quantum dot lasers have not been realized, indicating that challenges still exist in understanding the fundamental physics and mastering the key technologies needed for quantum dot laser devices. Considering the advantages of the distinct electronic structure and cost-effective processing methods of colloidal quantum dots compared to those of organic materials and epitaxial semiconductors, it is imperative to develop novel quantum dots, high-quality resonators, and new lasing mechanisms to push colloidal quantum dot lasers into commercial products.

Colloidal quantum dot lasers processed using wet solutions will potentially play important roles in various fields. This paper focuses on the significance of colloidal quantum dots as gain media and highlights the recent advances in the field of quantum dot lasers, focusing on four main types: continuous-wave pumped colloidal quantum dot lasers, optically pumped colloidal quantum dot lasers in a liquid phase, environmentally friendly colloidal quantum dot lasers, and electrically pumped colloidal quantum dot lasers. The article discusses the universal approaches to suppressing nonradiative Auger recombination, methods for managing Joule heating, the modulation of optical gain mechanisms, and the selection of different types of cavities. In future work, optically pumped and electrically pumped quantum dot lasers should be investigated simultaneously, and both may play important roles in fundamental research and practical applications. Several issues need to be addressed in the process of commercializing colloidal quantum dot lasers. Further, it is still necessary to explore how to fully exploit the unique properties and functions of colloidal quantum dot lasers. There is no doubt that new theories and technological instruments such as AI are required to accelerate the advance of colloidal quantum dot lasers. Finally, greater effort should be devoted to the exploration of the potential applications of colloidal quantum dot lasers.

Refractory high entropy alloy (RHEA) has superior properties such as high strength, high hardness, high temperature resistance and high corrosion resistance, which is expected to become a new material of high-temperature structure. RHEA has huge application prospects in aerospace, nuclear engineering, weapons and other fields. At present, the RHEA prepared by vacuum arc melting technology has some problems, such as large size limitation, difficult formation of complex structure, serious component segregation and long development cycle. RHEA formed by laser additive manufacturing has obvious advantages such as uniform composition, excellent microstructure and properties, integral forming of complex shape, etc. It has application potential in raw material development and high-performance parts preparation, so it has received hot attention in the research field.

The primary task of RHEA formed by laser additive manufacturing is defect control. The microstructure defects of RHEA produced by laser additive manufacturing can be divided into cracks and pores. The crack defects can be divided into hot crack and cold crack. The addition of a small number of nanoparticles, process optimization and post-treatment can eliminate the crack defects. Pore defects can be divided into four types: unfused pore, metallurgical pore, micro pore and shrinkage pore. The main control method of pore defects is process optimization. By controlling the defects, the forming of RHEA and its properties are improved greatly. Additive manufacturing of RHEAs is an effective method to develop new alloys quickly. The process parameters of laser additive technology are numerous, strongly coupled and nonlinear. Single-factor experiment, numerical overlap optimization of molten pool and numerical simulation are the rapid optimization strategies for laser additive manufacturing of RHEA. Laser additive manufacturing enables rapid development iterations of RHEA. By mechanical mixing powder or preforming alloy powder, laser additive manufacturing technology can realize the formation of various metal powders by varying component ratios, and realize the rapid screening and performance evaluation of alloy composition. NbMoTaW alloys have very high strength but poor ductility, which can be strengthened and toughened by appropriately adding low melting point ductility elements and reducing brittle elements in the matrix. HfNbTaZr alloy has good plasticity, but the yield strength is generally low, which can be strengthened by adding strengthening elements. Due to the difficulty of controlling the forming crack defects, there are few studies on the tensile properties of RHEA formed by laser additive manufacturing. And more research work is needed, especially major breakthroughs in material mechanism and forming process. Finally, the complex structural parts formed by laser additive manufacturing for RHEA is still in the initial attempt stage.

In this paper, the research on RHEAs formed by laser additive manufacturing is reviewed, and the development approaches, forming process and defect control, and the main research and challenges on mechanical properties of refractory high entropy alloys at multiple temperature stages are summarized. The research progress of complex parts of refractory high entropy alloy is introduced. Finally, the future application and development trend of RHEA formed by additive manufacturing are discussed. The integrated manufacturing of materials and shape is essential for RHEA, and laser additive manufacturing is the development direction of component manufacturing. The progress of materials and processes is the prerequisite for the integral forming of complex components in RHEA formed by laser additive manufacturing. At present, it has the ability to form simple samples, so more in-depth research is needed to accelerate the industrial application process of integrated manufacturing of shape and properties for RHEA in laser additive manufacturing.

Wettability is as a crucial physical and chemical property of solid surfaces. Surfaces with unique wettability, especially, attract considerable attention. Their significant impact spans various domains, including energy use, environmental protection, chemical engineering, healthcare, sustainable development, military defense, manufacturing, and agricultural breeding. Consequently, special wettability, particularly extreme wettability (i.e., superwettability), is emerging as a hot research topic in the field of micro- and nano-manufacturing. The study of superwettability originates from observing nature’s unique wetting phenomena and deeply investigating their formation mechanisms. Numerous plants and animals have evolved surfaces with special wetting properties to adapt to their environments. Inspired by natural superwettability, a range of micro/nano-manufacturing technologies have been employed to create various superwetting materials. These technologies include machining, photolithography, chemical etching, template replication, plasma etching, vapor deposition, electrochemical methods, the sol-gel process, electrospinning, electrochemical deposition, self-assembly, and spray/dip coating. Although existing microfabrication methods can produce superwetting structures with outstanding properties, traditional approaches face several technical challenges in achieving superwettability. These include complex preparation steps, constraints to specific substrate materials, and a lack of flexibility. Notably, most micromachining methods are limited to processing certain materials (for example, lithography is restricted to photosensitive polymers) or struggle with the precise design of micro/nanostructures (such as chemical etching, which can rapidly create large areas of uniform microstructures but faces difficulties in patterning these structures). These limitations significantly hinder the practical application of surfaces with engineered superwettability. Developing a versatile microfabrication technology capable of preparing various superwetting surfaces remains a significant challenge.

The characteristics of ultrashort pulse width and ultrahigh peak power establish femtosecond lasers as pivotal tools in modern extreme and ultra-precision manufacturing. Given that surface microstructure significantly influences the wettability of solid materials, femtosecond laser processing can create a variety of superwettability by constructing specialized microscale and nanoscale structures on material surfaces. Superhydrophilicity can be realized by forming sufficiently rough microstructures on inherently hydrophilic materials. In the case of superhydrophobicity, materials are generally categorized into two types. For intrinsically hydrophobic materials, superhydrophobicity can be directly achieved by preparing hierarchical micro/nanostructures on the substrate surfaces. For inherently hydrophilic materials, after forming surface microstructures with a femtosecond laser, it is often necessary to further reduce the surface energy via chemical modification. On a superhydrophilic surface, water droplets spread rapidly, while a superhydrophobic surface functions to repel water, offering waterproofing. Superoleophobic surfaces are categorized into two types, effective in air and underwater, respectively. To create superoleophobic surfaces in air, re-entrant bending microstructures are introduced, combined with stringent low-surface-energy chemical modifications. These microstructures are directly crafted onto the surface of hydrophilic substrates to realize underwater superoleophobicity. Superoleophobic surfaces repel oily liquids and some organic liquids with low surface energy. Generally, superhydrophilic surfaces exhibit superaerophobicity underwater, and superhydrophobic surfaces demonstrate superaerophilicity underwater. The superaerophobic surface effectively repels bubble adhesion, while the superaerophilic surface can adsorb tiny bubbles in water. Slippery surfaces created using femtosecond laser-induced porous network microstructures enable droplet contact with the material surface in a liquid/liquid mode, repelling various liquids. Underwater superpolymphobicity is achieved by constructing micro/nanostructures on the surface of hydrophilic materials. This property is useful for preventing the adhesion of liquid polymers to solid materials and assisting in the design of polymer shapes. Irrespective of superhydrophobicity or superhydrophilicity, femtosecond laser-induced microstructures exhibit supermetalphobicity. By designing patterned microstructures on the surface of flexible materials using a femtosecond laser, liquid metals can be transformed into circuits, enabling the creation of flexible electronic devices. Superwetting surfaces with controllable adhesion are achievable through the femtosecond laser-based design of surface micro/nanostructures. The adhesion level of these prepared surfaces to droplets can range from very low to very high. Anisotropic wettability is attainable on the anisotropically structured surfaces crafted by the femtosecond laser. Reversibly switchable wettability on these laser-structured surfaces can be achieved through three approaches: adjusting surface chemistry, modifying surface microtopography, and altering the ambient environment. The special wettability endows femtosecond laser-treated materials with a range of practical applications, such as waterproofing, self-cleaning, droplet manipulation, liquid patterning, buoyancy enhancement, tiny drop and bubble release, oil-water separation, water/gas separation, anti-icing, anti-corrosion, underwater drag reduction, water/fog collection, microfluidics, flexible circuits/electronics, cell engineering, biomedical engineering, seawater desalination, surface-enhanced Raman scattering, and more.

This review comprehensively outlines the advancements in femtosecond laser processing for manipulating the surface wettability of materials. By employing femtosecond lasers to design micro/nanostructures on various material surfaces, a range of unique wettabilities has been achieved. These include superhydrophilicity, superhydrophobicity, superoleophobicity, underwater superaerophobicity and superaerophilicity, slippery liquid-infused porous surfaces, underwater superpolymphobicity, supermetalphobicity, controllable adhesion, anisotropic wettability, and smart switchable wettability. The practical applications of these femtosecond laser-structured superwetting materials have been diverse and significant.

Currently, the technology of femtosecond laser-controlled surface wettability faces several challenges. A major bottleneck is processing efficiency, which still restricts the broader application of femtosecond laser micromachining technology. Despite new strategies such as laser parallel processing and light-field regulation, efficiency falls short of industrial application requirements. Additionally, if the laser focus deviates significantly from the material surface, then the desired microstructures cannot be effectively prepared. This defocusing issue also makes it difficult to create uniform superwetting micro/nanostructures on complex curved surfaces. Moreover, similar to surfaces prepared by other methods, femtosecond laser-induced superwettability surfaces encounter stability issues in practical applications. These surfaces often lose their initial extreme wettability when exposed to friction or specific operating environments. Thus, future research in this field should address these bottlenecks, enhancing the practicality and scalability of superwetting materials prepared by femtosecond lasers for real-world applications.

With the increasing demand for structural functionality and lightweight in aerospace and marine engineering fields, a growing requirement has emerged for diversity in material and structural properties. For some aerospace components, different structural parts are required to operate in different environments, and traditional homogeneous materials are inadequate. Especially in extreme service environments, materials and structures must integrate multiple properties to address specific engineering or scientific requirements. For example, the same part may exhibit significantly different thermal, mechanical, acoustic, and electrical properties in different locations. Such material combinations with significant physical differences, including metal-metal, metal-polymer, metal-ceramic, and polymer-ceramic, are referred to as materials with significant differences in their physical properties. Development of such materials is critical for weight reduction, and product performance and reliability improvement. With multi-material laser additive manufacturing science and technology development, it is possible to integrate the preparation of materials and components with significant differences in physical properties.

However, the interface problem that occurs in multi-material laser additive manufactured materials in this specific context is particularly important. The interfacial bonding quality of materials with significant differences in physical properties remains a significant problem. Interface defects, excessive residual stress, and cracking severely limit the multi-material laser additive manufacturing of these types of materials. Therefore, this study reviews research advances in laser additive manufactured multi-materials with significant differences in physical properties, focusing on interface problems, optimization methods, modeling and simulation.

Currently, multi-material laser additive manufacturing technology primarily includes: laser powder bed fusion, laser-directed energy deposition, laser-induced forward transfer, multiphoton fabrication, and hybrid multi-material laser additive manufacturing. In the multi-material laser additive manufacturing process, interface problems arise from laser absorption rate differences, thermophysical properties and brittle phase formation at the interface between materials with significant physical property differences. These issues lead to the formation of defects, cracks and residual stress at the interface during fabrication and may even result in interface material delamination and debonding. Therefore, this study investigates interface problems based on the three aforementioned aspects.

A literature analysis is conducted on interface optimization methods for laser additive manufacturing in this context in terms of process optimization, functional gradient design, and integrated manufacturing systems (Fig.10). These provide methods for achieving high-quality formation of materials with significant differences in physical properties. Process optimization primarily includes: parameter optimization, heat treatment, and laser re-melting. Functional gradient design primarily includes: transition and gradient bonding, and interface structure design. Integrated manufacturing systems primarily includes: laser wavelength selection and multi-energy field hybrids. This study provides a detailed explanation for process optimization and functional gradient design, which are widely used optimization methods. Research progress regarding modeling and simulation of laser additive manufacturing of multi-materials with significant differences in physical properties is expounded. Modeling and simulation are important methods for investigating the influence of material property differences on heterogeneous interface formation. By simulating the effects of laser powder bed additive manufacturing parameters on the thermal behavior of heterogeneous interfaces at both macroscopic and mesoscopic scales, optimization of formation parameters can be achieved.

This study reviews research advances on interfaces in the laser additive manufacturing of multi-materials with significant differences in physical properties. This includes multi-material laser additive manufacturing technologies, interface problems and optimization methods, and modeling and simulation in this specific context. Interface optimization methods are also summarized to identify high-quality heterogeneous material formation and to promote the research and application of multi-material laser additive manufacturing.

Consisting of sub-wavelength scatterers or holes arranged on a plane, the metasurface, as a two-dimensional form of the metamaterial, permits flexible and efficient modulation of the amplitude, phase, polarization and other characteristics of the light with an unprecedented degree of freedom, which is expected to break through the limitations of traditional optics and realize ultra-light, ultra-thin, high-performance, and novel-functional optical devices. In recent years, metasurfaces have attracted increasing research interest in both academia and industry, and a wide range of applications have been achieved in the field of imaging, holography, quantum optics, displacement metrology, virtual reality, optical encryption, and ultrafast optics. Based on the elucidation of the basic design principles of metasurfaces, this review covers the main development directions, research progress, and challenges of current metasurface applications.

The design principles of metasurfaces are now well established and can be understood in three dimensions the most basic meta-atom and its scattering effect, metasurface as an array of meta-atoms, and the topmost design methods. For the first dimension, the physical image of the electromagnetic modulation of the meta-atom is explained, and the three main types of phase modulation mechanisms are introduced, that are, the resonant phase, the propagation phase, and the geometric phase. The selection of appropriate phase modulation mechanisms is significant for realizing the design of metasurfaces with different functions. For the second one, generalized refraction and reflection laws are introduced. For the last one, the forward design method and its theoretical basis are presented.

How these mechanisms are utilized to realize a variety of applications is described in detail, including polarization multiplexing, wavelength multiplexing, wide bandwidth, large field-of-view, multilayer cascades, and nonlocal metasurfaces, covering the most important and recent developments. 1) Polarization multiplexed devices (Fig.3). When operating at a single wavelength, it is theoretically elucidated that a hybrid phase modulation mechanism can achieve arbitrary polarization and phase modulation under the ideal situation of sufficient design freedom of meta-atoms. Based on this theory, the latest research progresses are presented, such as the multiplexing for arbitrary orthogonal states of polarization, multichannel polarization multiplexing, etc.2) Wavelength multiplexed devices (Fig.4). To achieve independent phase modulation of the incident light at different wavelengths, intelligent strategies of space division multiplexing, decoupling with other multiplexing channels, and other methods have been proposed, leading to applications such as full-color holographic displays. 3) Broadband devices (Fig.5). The problem of achromaticity in metalenses has been a difficult problem in this field for several years. Through dispersion engineering, theories and methods for designing achromatic metalenses have been developed, and both polarization-sensitive and polarization-insensitive achromatic lenses have been realized. Recently, the novel idea of quasi-achromatic metalenses has also been proposed to relax the bandwidth limitation of achromaticity. 4) Incident angle multiplexed and wide field-of-view devices (Fig.6). Incident angle multiplexed metasurfaces can be designed by both forward methods and inverse methods such as topology optimization. By selecting the suitable target phase distribution for wide field-of-view imaging and introducing the concept of effective aperture or virtual aperture, metalenses with wide field-of-view have been designed and realized. 5) Multilayer cascaded metasurfaces (Fig.7). The distance between layers in cascaded metasurfaces determines the relationship between adjacent layers, according to different theoretical models and design methods. By cascading multiple layers of metasurfaces, design tasks that are difficult to achieve with a single metasurface can be achieved, such as hysteresis and chromatic aberration correction of metasurface lenses, and novel functions such as dual-wavelength and dual-focus metasurface lenses can be obtained. 6) Nonlocal metasurfaces (Fig.8). By exploiting the nonlocal effect of metasurfaces, the transverse-momentum-dependent electromagnetic response can be modulated to realize novel functions that are difficult to achieve for local metasurfaces, such as image differentiation and free-space compression, which is a latest trend in metasurface design.

Metasurfaces still face many challenges from science and engineering. In terms of the metasurface design, it is still a common problem of the field to realize devices with higher performance and larger size. On the one hand, it is important to clarify the theoretical performance limits of metasurfaces and the constraints of design methods to guide the future development of the metasurface design. On the other hand, it is also essential to make breakthroughs in design methods, such as the further development and promotion of inverse design. In terms of fabrication and manufacturing, there is still a long way to go for the industrialization and commercialization of metasurfaces due to the limitations of the fabrication accuracy, process compatibility, large-scale manufacturing cost, etc. We believe that metasurfaces will play a transformative role in the near future of optics contributing to their ultra-light and ultra-thin planar architectures, powerful electromagnetic modulation properties to support flexible device designs, ease for integration and miniaturization of optical systems, and the promise of low-cost, high-volume manufacturing.

This article provides an overview and analysis of the funding trends in the disciplines of optics and optoelectronics within the Information Science Department of the National Natural Science Foundation from the beginning of the 13th Five-Year Plan (2016‒2020) to the initiation of the 14th Five-Year Plan in 2021. It summarizes the proposed and approved projects of exploratory, guiding, and talent types, considering aspects such as the number of projects, funding amounts, supporting institutions, and disciplinary domains. It also analyzes the overall characteristics, structural changes, and developmental trends in this field. Based on the exemplary research outcomes during this period, the article analyzes the funding effectiveness. Additionally, it provides a prospective outlook on the prioritized developmental areas and funding management for the disciplines of optics and optoelectronics in line with the goals outlined in the 14th Five-Year Plan.

Coherent beam combining (CBC) is an effective method to improve the output power of fiber lasers while maintaining good beam quality. As interdisciplinary research continues to deepen, the CBC technology of fiber lasers is constantly revitalized, and its application scenarios are becoming increasingly diverse. Reviewing the evolution of fiber laser CBC development and outlining prospective directions for their future development are crucial. This article discusses the development trajectory of the active phase control CBC of fiber lasers, and systematically outlines its development stages from the perspective of literature metrics. The characteristics of each development stage are summarized, and the outlook for future development trends is provided.

Based on an analysis of the literature indexed in the Web of Science Core Collection, the development process for the active phase control CBC of fiber lasers can be divided into four stages: the early stage of academic development, the period of rapid academic development, the stage of stable academic development, and the key stage of technological development.

In the early stage of academic development before 2005, research focused on passive CBC and played an important role in promoting the research on the CBC of fiber lasers. During the period of rapid academic development from 2006 to 2011, researchers proposed various phase-control methods, which solved the basic prerequisite of achieving active phase control. Subsequently, with an increase in the output power, researchers pursued greater combining efficiency and proposed various structures to achieve this goal. At the same time, researchers also explored expanded applications of CBC technology of fiber lasers. During the stage of stable academic development from 2012 to 2016, significant progress was made in the CBC of pulsed fiber lasers. A co-aperture CBC system with higher theoretical efficiency received widespread attention. Researchers proposed many methods to further increase the numbers of combined channels. To improve the combining efficiency, researchers conducted studies on controlling multiple parameters that affect the combining efficiency. In addition, based on the research on the CBC of fiber lasers, researchers gained greater confidence in using CBC technology to obtain ultra-high energy and power, and have proposed various concepts for large scientific installations. In the key stage of technological development after 2017, with the development of CBC technology of fiber lasers, practical breakthroughs have been achieved in various areas such as the numbers of combined channels and combined power. CBC structures with tiled apertures have been widely used in optical-field manipulation. The evolution of nascent disciplines such as artificial intelligence intersects with the development of CBC. The structures and methods continue to improve, and various related products are gradually emerging.

With the development of CBC technology of fiber lasers, the overall trend is characterized by an increasing array scale, improved combined power, improved control parameters, the deepening of interdisciplinary research, and modularization. In terms of the array scale, CBC with thousands or even tens of thousands of beams is a further development trend, driven by various large scientific installations. Regarding combined power, using tiled aperture structures to achieve output power values ranging from hundreds of kilowatts to megawatts has become a practical reality. In terms of controllable parameters, automatic alignment devices with the polarization state adjustment capability and two- or three-dimensional fiber end position adjustment capability are important devices for achieving multidimensional control of fiber laser arrays. In the field of interdisciplinary research, CBC technology has been enriched and developed along with advancements in related technologies. CBC has been shown to be an effective approach to improve laser brightness, which can be utilized in various scenarios. Regarding modularization, modular development can meet different application scenarios and requirements, enabling the rapid construction of fiber laser CBC systems and promoting their application development.

The development process for the active phase control CBC of fiber lasers exhibits significant stage characteristics, with many excellent research achievements emerging at the interval of approximately 5 years. Domestic research institutions have played important roles in promoting its development, such as achieving kilowatt output power and arrays of thousands of beams for the first time. In terms of CBC modules such as single-frequency lasers and narrow linewidth lasers, the highest publicly reported technical indicators have also been achieved. With the continuous maturation of academic development and gradual improvement of technology in the field of the CBC of fiber lasers, unique application value has been demonstrated in areas such as basic scientific research and industrial processing, gradually integrating this technology into social productivity and daily life. Currently, in other countries, scientific devices and products related to the commercial use of CBC are being developed as light sources for basic scientific research and industrial processing. These developments are of great significance in promoting basic scientific research and supporting laser intelligent manufacturing. In the stage where the academic and technological development of the CBC of fiber lasers tends to mature, challenges and opportunities coexist. It is foreseeable that the “CBC+” model, driven by applications and uses in interdisciplinary research, will promote the development of relevant disciplines in the future. This will also lead to a new scientific understanding and technological progress in fiber laser CBC itself.

Quantum metrology, as one of the primary applications of contemporary quantum mechanics, has emerged as a crucial area of research in quantum technology in recent years. The fundamental objective of quantum metrology is to utilize quantum resources to enhance the precision of measuring unknown parameters in physical systems. Compared with other physical systems, photon-based systems possess distinct advantages such as long coherence time and low interaction with the environment, making them an ideal platform for processing quantum information. Improving sensing precision through photon-based sensors stands as a pivotal task within optical quantum metrology.

The general process of quantum metrology can be abstracted into four steps 1) preparation of a probe state; 2) interaction of the probe with the system to be measured; 3) measurement; 4) classical estimation, as shown in the main text (Fig.1). In terms of unitary evolution, Lloyd et al. compared the precision bound provided by four strategies, i.e., classical-classical (CC) strategy, classical-quantum (CQ) strategy, quantum-classical (QC) strategy and quantum-quantum (QQ) strategy (Fig.2). The QC and QQ strategies provide the ultimate precision of parameter estimation beyond the CQ and CC strategies. By analyzing the different strategies, one can easily find the relationships among the Fisher information, the QFI, the SQL and the HL (Table 1).

The fundamental theory of quantum metrology is based on the principles of parameter estimation theory. The quantum Cramér-Rao bound (QCRB) serves as a widely utilized mathematical tool in quantum metrology for evaluating the ultimate limit of precision. In the context of single parameter estimation, it is inversely proportional to the quantum Fisher information (QFI). By scrutinizing the QFI associated with different quantum states encoding parameters, one can determine which type of quantum states would be the most optimal for a given sensing task involving an unknown parameter.

The optical interferometer is a crucial apparatus in optics and plays an indispensable role in quantum metrology. Its applications range widely, from spectroscopic interferometric techniques to remarkable examples involving stellar interferometry and gravitational wave detection. Classical theory does not provide precise analysis of phase shift estimation in the interferometer, while the semi-classical theory, considering the quantized detection process, establishes the shot noise limit or standard quantum limit (SQL) of precision with N detected photons. However, it should be noted that SQL should not be considered as the fundamental bound when non-classical states of light, such as squeezed states, are injected into the interferometer. The maximally entangled photon number state known as NOON state can achieve a precision of 1/N referred to as the Heisenberg limit (HL). In comparison with SQL, HL exhibits scaling improvement and represents the fundamental bound for parameter estimation. Quantum resources like squeezing and entanglement can genuinely enhance phase estimation precision, while employing a sequential strategy where probe states undergo a sequential process can also yield maximal precision.

In recent years, quantum metrology has experienced rapid development, witnessing the generation of numerous non-classical quantum states with inherent metrological advantages and the invention of various interferometer structures aimed at enhancing phase estimation precision. Consequently, it is imperative to comprehensively and meticulously summarize existing research in order to provide guidance for future advancements in this field.

The fundamental principles of quantum metrology are elucidated, albeit in an abstract manner. To provide a more concrete illustration, we will consider the phase estimation problem as a prime example, which represents the most captivating application within quantum metrology. The Mach-Zehnder interferometer (MZI) serves as the prevailing structure (Fig.4). Numerous studies propose that by introducing distinct non-classical quantum states such as squeezed states and NOON states into the MZI, it is possible to enhance the estimation precision of phase shift in its two arms. Other types of interferometers like Michelson interferometer and Sagnac interferometer (Fig.6) also play significant roles in gravitational wave detection and quantum gyroscope applications. In practical scenarios, noise inevitably exists within interferometers. For instance, losses are almost unavoidable and can compromise precision levels. This review introduces several approaches aimed at mitigating the impact of loss.

In most realistic sensing scenarios, the system to be measured typically encompasses multiple unknown parameters. Multiparameter quantum metrology is also a crucial research aspect within the field of quantum information science, encompassing tasks such as estimating multiple phases, distributed quantum sensing, phase and phase diffusion, and so on. Besides addressing the phase estimation problem, optical quantum metrology finds numerous other applications including imaging and magnetometer. Finally, we provide a brief overview of some relevant works.

Photon, as an excellent information carrier, is suitable to be applied in quantum metrology, thereby establishing optical quantum metrology as a pivotal and burgeoning field of research. Further comprehensive and meticulous investigations are imperative to advance the theoretical and experimental development of optical quantum sensing. This review gives an in-depth and detailed introduction of recent progress in optical quantum process and we hope it will inspire some interest of readers.

Generally, the terahertz (THz) radiation spectrum is defined as the electromagnetic spectrum between 100 GHz to 30 THz (wavelength of 3 mm‒10 μm). THz waves have a wide bandwidth and low photon energy, their energy spectra cover the vibrational-rotational characteristics of numerous molecules, and THz waves can penetrate many non-polar media. These properties make them attractive for astronomical observations, public safety, biomedicine, and wireless communication. According to the different generation and detection methods of THz waves, THz technology can be divided into two categories: one is based on electronic technology-mainly related to microwaves in the low-frequency THz band, and the other is based on optical and photonics technology-mainly related to infrared light at higher frequencies. Further technical developments will bridge the gap between electronics and optics, enabling new THz spectroscopy and imaging methods for scientific exploration in physics, chemistry, biology, materials, devices, engineering, and other interdisciplinary fields.

Terahertz quantum cascade lasers (THz QCLs) and terahertz quantum-well photodetectors (THz QWPs) are semiconductor devices based on electron transitions within subbands. These devices are advantageous due to their small size, adjustable frequency, and fast response time. Over the past few years, their performance has improved and many techniques related to high-resolution spectroscopy, terahertz imaging, and wireless broadband communication have received considerable attention. Here, we review recent advances and discuss future research directions.

THz QCLs are the only practical and compact laser systems at this frequency and have significant impacts on the THz field. The first THz QCL was developed in 2002. After more than 20 years of development, device performance has made significant progress, including the emission frequency range from 1.2 THz to 5.6 THz, the maximum output power of 2.4 W, a single mode continuously tuning range of 650 GHz, a broad bandwidth of 2.6 THz, the maximum operating temperature of 261 K, and a great improvement in far-field beam quality (Figs. 3 and 4). The development of phase-locking technology has significantly improved the frequency stability of lasers, with the linewidth reaching a quantum limit of 100 Hz. This technology has been employed in high-resolution spectroscopy (Figs. 8 and 9). THz QCLs exhibit strong optical nonlinearity and short inter-subband transition lifetime, resulting in various applications. Room-temperature THz laser radiation is demonstrated through intra-cavity difference frequency generation in mid infrared lasers. Moreover, scientists have achieved active mode-locking, optical frequency combs, high-order wave mixing, inter-subband plasmons, and fast modulation and detection. These technologies have also been successfully used in metrology and THz imaging (Figs. 10 and 11).

THz QWPs are an extension of quantum-well infrared detectors to the THz band, with advantages such as fast response speed and small size. In 2004, the first THz QWP was successfully developed with a central detection frequency of 7.1 THz. Subsequently, by adjusting the energy of inter-subband transitions, the spectral response of THz QWP gradually covered the range of 1.5‒7.5 THz and 8.8‒15.0 THz. However, it is well documented that THz QWPs require low-temperature conditions, usually below 10 K, because the thermally activated current can rapidly exceed the photocurrent at high temperatures. Recently, the combination of device structure and metamaterials has effectively increased the operating temperature of the device to 60 K (Fig.7). In terms of applications, high-speed imaging and information transmission are demonstrated in the THz QCL and THz QWP systems (Fig.12), and THz QWPs are also used in broadband spectral measurements and nonlinear two-photon detection.

THz QCLs and THz QWPs have a significant impact on the THz field; however, their large-scale applications have not been realized. Currently, improving the high-temperature performance of devices is one of the main challenges because thermally excited carriers can obtain sufficient kinetic energy for transport between subbands at high temperatures. THz QCLs demolish the population inversion, whereas THz QWPs lead to an increase in the dark current. Additionally, a further increase in the frequency range poses challenges. Expanding to the low-frequency side becomes increasingly difficult owing to further reduction in photon energy, whereas expanding to the high-frequency side is limited by the reststrahlen band. The investigation of new active region designs, low-loss waveguide materials, metasurface structures, and photonic crystals is expected to improve the device performance. Moreover, wide-bandgap materials such as GeSi and quantum dots can be chosen for high-temperature devices. Additionally, high-power and low-noise optical frequency combs and ultrashort THz pulse technology have garnered considerable attention. Notably, understanding the phase relationship between the comb teeth and the mechanisms of pulsed operation can be helpful. Saturation absorbers based on inter-subband polaritons and harmonic combs have also been presented. THz QCLs with topological structures have opened up new directions. Topological structures result in unidirectional propagation of light, immunity to defects, and high-order light field regulation. Topological protection can facilitate the development of robust laser arrays, and topological chirality can modify far-field beam patterns and polarization for novel photonic devices.

The application and commercialization of THz QCLs and THz QWPs will become a driving force for the development of new methods for high-resolution spectroscopy, hyperspectral imaging, and terahertz communication. THz QCLs and THz QWPs combined with near-field microscopy can be employed for nanometer scale detection and used in fields such as material science and biomedicine.