# Terahertz Emission Enhanced by a Laser Irradiating on a T-Type Target
## 1 Introduction  
The terahertz (THz) wave is in a special frequency band in the transition from electronics to photonics and has the characteristics of low radiation dose, good transmittance, etc. Therefore, THz radiation sources are widely used in frontier basic science fields, such as nonlinear THz optics [1], condensed matter physics, material science [2, 3], and life science research [4, 5, 6], as well as their important applications in non-destructive testing [7], THz wave radar detection, [8] and other technical fields. Among the THz wave generation methods, there are THz emission sources driven based on the electrons accelerated by conventional accelerators [9] and THz wave radiation sources driven by optical rectification [10, 11, 12] and photoconductive antennas [13]. In addition, with the rapid development of laser-plasma interaction physics, the generation of THz emission based on laser-plasma has been extensively studied, because the plasma itself does not have a breakdown threshold. For example, [PERSON] et al. proposed a scheme of using a two-color laser with 800 nm fundamental frequency and 400 nm doubled frequency light to ionize air into filaments to enhance the generation of THz emission [14]. The energy conversion efficiency in the experiments was three orders of magnitude higher than that of 800 nm fundamental frequency light. Subsequently, many researchers have proposed a series of schemes to enhance or modulate THz emission, such as changing the frequency ratio of the two-color laser [15], filling different types of gases in the vacuum tube [16], using a three-color laser field [17], applying an external axial magnetic field [18] and optimization parameters, and so on. However, the laser defocusing caused by plasma and the absorption by the air make the generated THz emission appear saturated, resulting in the intensity and energy of the THz emission no longer increasing with the increase in the laser intensity. Recent studies have shown that based on the ultra-intense laser irradiating on solid targets, THz emission with higher intensities and more energies can be obtained [19; 20; 21]. For instance, [PERSON] et al. [22] simulated that the half-cycle THz wave, based on the coherent transition radiation (CTR) mechanism [23; 24], could be radiated by the forward (or backward) fast electrons passing through the front (or rear) target-vacuum interfaces when a laser interacted with a solid foil. Subsequently, a large number of experiments have been carried out to study the generation of THz emission by the femtosecond (fs) or picosecond (ps) laser irradiating a solid target [25; 26; 27], and the obtained energy has reached \(\sim\)50 mJ [26]. Meanwhile, [PERSON] et al. [28] have also experimentally and theoretically investigated how to manipulate the THz spectrum by varying the laser pulse duration or target size when the ultra-intense laser pulse interacts with a foil target. In addition, in order to regulate or improve the efficiency of THz emission, researchers have carried out a large number of theoretical or experimental studies based on the interaction of lasers and structural targets to generate THz emission. For example, high energy and well-collimated electron bunches, produced and accelerated by the microplasma waveguide (MPW), can be converted to strong THz emission via coherent diffraction radiation (CDR) [29] when exiting the MPW, corresponding to efficiency exceeding 1% [30]. Apart from the MPW, the scheme of laser-irradiated wires, microtapes, or a surface of a solid target, has been widely applied in the theoretical, simulation, and experimental studies of laser plasma physics, such as the electron acceleration by the longitudinal field of a surface plasma wave (SPW) [31; 32; 33; 34], dense electron generation and acceleration by a wire [35; 36; 37; 38; 39; 40], protons acceleration [41; 42; 43; 44], \(\gamma\)-ray and electron-positron pair generation [45; 46], and so on.