Terahertz wave refers to the electromagnetic wave whose frequency and wavelength are in the ranges of 0.1-10.0 THz and 30-3000 μm, respectively. Throughout the entire electromagnetic spectrum, the terahertz band is located between electronics and optical bands, so it is also called the "terahertz gap". Terahertz wave has some special properties due to its special band, such as low energy, high penetration, and transient. With the development of research and the maturity of technology, terahertz wave has shown bright application prospects in material science, biomedicine, imaging, and communication in recent years. Broadband terahertz radiation source is an important basis for spectral technology in the terahertz band. Therefore, it is of great significance to develop broadband terahertz radiation sources. Since GaSe crystal has a high light transmission and a small absorption coefficient, it has a good application in the generation and detection of broadband terahertz radiation. In this study, GaSe crystal is used to generate tunable broadband terahertz pulses through the optical rectification effect, which provides new ideas for the generation of tunable broadband terahertz pulse radiation.

In light of nonlinear optics, this paper studies the tunable broadband terahertz radiation generated through the optical rectification effect of shaped ultrashort laser pulses interacting with GaSe crystal, and the process is simulated by numerical calculations. It is found that by adjusting the laser spectrum component with a 4F shaping system, some components whose wavelengths are greater than λ1 in the frequency domain are blocked and truncated. Different shaped ultrashort laser pulses are obtained by varying the value of λ1. The adjustable shaped pulses and the optical rectification effect of GaSe crystal can be utilized to produce tunable broadband terahertz radiation. This scheme makes the center frequency of the terahertz pulse modulated from the high frequency to the low frequency, and the bandwidth is also changed.

The terahertz radiation from GaSe crystal with different pulse durations is given by numerical methods (Fig. 2). Tunable terahertz pulses pumped by the shaped ultrashort laser pulses with a duration of 50 fs and a center wavelength of 800 nm are obtained, and their properties are given in detail. When the cutoff wavelength λ1 is changed, the center frequency and bandwidth (full width at half maximum) of the generated terahertz pulses are also changed accordingly (Fig. 3 and Fig. 4). In the shaping process, some energy of the laser pulse is lost. More energy of the pump pulse is lost at a smaller cutoff wavelength (Fig. 5). When half of the energy is blocked, the lost energy accounts for half of the total energy. Such energy loss also results in the energy loss of the resulting terahertz pulses (Fig. 6). The smaller the cutoff wavelength is, the smaller the terahertz pulse energy is. The proposed method is simple and feasible, and can change the envelope of the high-peak-power laser pulse without requiring any light modulator, thus avoiding the damage of the high-peak-power laser pulse to the modulator. Such pulse shaping methods provide a good theoretical reference for future experimental studies.

In summary, this paper presents a simple way to generate tunable terahertz pulse radiation by using the optical rectification effect of shaped ultrashort laser pulses on the GaSe crystal. The pump laser spectrum component in the 4F shaping system is shielded by an opaque baffle so that the high-peak-power laser pulse can be shaped, and the envelope of the laser pulse and the terahertz spectrum distribution generated by it can be changed. It is found that the center frequency and bandwidth of the generated terahertz pulse can be adjusted by changing the cutoff wavelength. This idea might offer new methods to generate tunable broadband terahertz pulses based on the optical rectification effect of GaSe crystal, which provides a good reference for related experiments.