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

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

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

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

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

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

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