# Terahertz Emission Enhanced by a Laser Irradiating on a T-Type Target
## 5 Robustness of the Scheme  
Taking into account the effects of the laser and target parameters in the realistic experiments, we performed a series of PIC simulations to investigate the robustness of our scheme. We scanned the effect of THz emission intensity by varying parameters such as the length and thickness of Target I in our simulations. It should be noted that in order to be able to compare effectively, the position of the detection point was unchanged relative to the position of Target II. The results are plotted in Figure 7. We found that although the parameters could slightly affect the maximum intensity of the radiated electromagnetic field, without exception, they were greatly improved compared to the case of SLT. We also plotted the relationship between the power of the THz emission and the length and thickness of Target-I, as shown in Figure 7b,d. We found that the THz emission was affected when changing the parameters, either the length or the thickness of Target-I. It can be concluded: (i) As the length of Target-I increased, the power of the THz emission increased  
Figure 6: The energy gain from the longitudinal (large \(W_{x}\)) and transverse (large \(W_{y}\)) acceleration. The red dashed line represents the \(W_{x}=W_{y}\), and the black dashed line represents the \(W_{x}+W_{y}=0\).  
gradually and tended to saturate slowly. This is because as the length increased, more electrons were produced. (ii) As the thickness of Target-I decreased, the power of THz emission also increased gradually. This is mainly because as the thickness decreased, it was closer to the center of the laser focal spot, when the laser interacted with the target surface. However, the thickness should not be too small, otherwise, the laser will ablate and destroy the Target-I. All of the above results demonstrate that this scheme is very robust.  
In addition, we performed numerical simulations on the effect of the laser duration on THz emission when keeping the laser input energy constant, as shown in Figure 8. As the laser duration increased, the intensity of the radiated electromagnetic field decreased, and the power of the corresponding THz wave also decreased. The main reason for the above results is that the intensity of the laser decreases with the increase of the laser duration under the condition that the laser input energy remains unchanged. The electron energy is proportional to the intensity of the laser, so this leads to a decrease in the THz wave's intensity when the electron energy decreases. Our results were consistent with the previously reported results, which investigated the effect of laser duration on THz emission [61]. Furthermore, we investigated the effects of SLT and TTT on THz radiation using the controlled variable method under the same conditions for all laser input parameters; so, the effect of changing laser duration on both cases was similar.  
Moreover, the laser's pointing stability is a very important parameter, since it may cause the misalignment between the laser focal spot center and the target center. The laser's pointing deviation of less than \(0.8r_{0}\) is currently achievable [44; 62; 63]. Therefore, we simulated the process of generating THz emission when the laser was \(\pm 5\lambda_{0}\) of the \(x\)-axis, as shown in Figure 9. This result indicated that in the case of laser pointing deviation, the radiated electromagnetic field and the corresponding THz emission passing through the detection point would not weaken but increase, and they were much larger than that of the SLT.  
Figure 7: Influence of the target parameters such as length and thickness on THz emission. The dependence of terahertz radiation on different lengths (**a**,**b**) and thicknesses (**c**,**d**) of Target I in the case of TTT. (**a**,**c**) are the time-domain waveforms of the radiated electromagnetic field, and (**b**,**d**) are the corresponding frequency-domain spectrum.