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

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

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

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

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