# Terahertz Emission Enhanced by a Laser Irradiating on a T-Type Target
## 4 Discussion  
Section 3 demonstrated that the TTT can enhance THz emission generation. Since the principle of generating THz emission was mainly dominated by the CTR in this work, the parameters, such as the number density, distribution, and energy of fast electrons behind the target, were the origin of the generation of THz emission. Figure 4 shows the two-dimensional spatial number density distribution of fast electrons for both the SLT and TTT at different times, \(t=90T_{0}\) and \(t=120T_{0}\), respectively. To clarify the physical mechanism by which the TTT enhanced THz emission, we used some settings in the EPOCH codes to distinguish the fast electrons from Target I and Target II. Among them, Figure 4c,d shows the number density distribution of fast electrons in Target II at different time, while Figure 4e,f shows those in Target I. First, we can observe that almost no electrons moved behind the target II, either earlier or later. This means that the electrons in Target II did not gain much energy from the laser, and the corresponding contribution to the THz emission generation was almost non-existent. In addition, we can also observe that the electrons in Target II moved along the negative direction of the \(x\)-axis in Target I, which was due to the forward movement of electrons in Target I leading to the electrons' return current in Target II. On the contrary, a large number of electrons were pulled out from Target I and moved to the backside of the target, which means that the fast electrons that enhanced the THz emission originated from Target I. At the same time, it is also noticed that the fast electrons appeared in two regions behind the target at an earlier moment from Figure 4e: (i) the fast electrons with lower energy were located around the laser irradiation spot at the rear side; (ii) the electrons with higher energy were distributed symmetrically in the two wing regions. The above fast electrons' density distribution led to the appearance of two sets of THz emission, as shown in Figure 2c. Over time, most high-energy electrons appeared to be distributed at both wings of the target backside, causing the intensities of THz emission to be higher on the wings than that of the central region, which is, respectively, shown in Figures 4f and 2d. Yet, the fast electrons from both forward and backward of the SLT had a transverse Gaussian profile in the central region similar to the shape of the laser, as shown in Figure 4a,b, which explains why the generated THz emission radiates to the surroundings with this point as the source center. In addition to the modulation of the spatial profile of the THz emission by the fast electrons' number density distribution, the energy of the fast electrons can have a significant effect on the intensity of the THz emission. Figure 5 shows the energy spectrum distribution of the fast electrons for both the SLT and TTT at different times, \(t=90T_{0}\) and \(t=120T_{0}\). Compared with the case of SLT, the fast electron energy generated in the case of TTT was significantly higher: the maximum cutoff energy of the fast electrons generated in TTT was about 140 MeV, while that in SLT was only about 20 MeV. The fast electrons of such high energy in TTT naturally radiate higher THz wave intensity through the CTR when they pass through the vacuum interface behind the target. Meanwhile, we can also observe  
Figure 3: The radiated electromagnetic field passing through the space point (\(146.67\lambda_{0},-20.0\lambda_{0}\)) at the \((x,y)\) plane for the SLT and TTT. (**a**) The time-domain waveforms of the radiated electromagnetic field; (**b**) The frequency-domain spectra in logarithmic scale. from Figure 2a,c, that a part of the laser pulse was obviously not absorbed but reflected at the range 60 \(\upmu\)m \(<x<\) 70 \(\upmu\)m in the case of SLT, while it did not appear in the case of TTT. From this point of view, it also shows that the TTT can significantly enhance the energy coupling between the laser and plasma.