## 3 Numerical Results and Discussion
### External Temperature  
First of all, it is worth mentioning that the tunability in such a PC medium is considered a highly practical advantage. The more tunable this device, the better it can optimally be utilized in the applications of interest under the desired conditions. Secondly, as this device could also be exploited in thermos optical usages like thermal energy harvesting and management, thermal switches, transistors, etc., the thermal tunability of the proposed structure could pave the way for an efficient performance of that optical device. In the far-infrared (FIR) range of the THz spectrum, the temperature-dependent permittivity, \(\varepsilon_{C}(\omega)\), of InSb exhibits remarkable sensitivity to variations in temperature. This sensitivity gives rise to intriguing and anomalous optical features in 1 DPCs composed of InSb and a dielectric material. The temperature dependence is shown in Figure 2 for both real and imaginary parts. In this figure, the temperature varies between 150 K and 250 K. At low temperatures, roughly ranging from 150 K to 200 K, the permittivity dispersion is relatively weak, resulting in both the real and imaginary parts of \(\varepsilon_{C}(\omega)\) being positive. However, as temperatures rise, the real part of \(\varepsilon_{C}(\omega)\) can exhibit both positive and negative values, while the imaginary part remains consistently positive. Consequently, manipulating the tunability of \(\varepsilon_{C}(\omega)\) could serve to achieve a temperature-dependent composite. The numerical data clearly depict the proposed structure's temperature dependency as follows. The transmittance spectrum of the structure at three different temperatures under the usual incoming beam is shown in Figure 2a-c. The lattice constants are \(d_{A}=28\text{ }\upmu\text{m}\), \(d_{B}=12\text{ }\upmu\text{m}\) (\(\varepsilon_{B}=2.1\text{ }\mu_{B}=1\)), \(d_{C}=3\text{ }\upmu\text{m}\), and \(d_{D}=12\text{ }\upmu\text{m}\) (\(\varepsilon_{D}=4.41\text{ }\mu_{D}=1\)). Additionally, the damping coefficients are considered to be \(\gamma=0\) and \(\Gamma=0.034\text{ THz}\). Also, the number of periods is \(M=4\). As shown in Figure 2, increasing the applied external temperature causes the PBGs to shift towards higher frequencies. This event demonstrates that the transmittance spectrum is temperature adjustable. For example, the mean frequency point of band-gap I is 0.10 THz with a width of 0.21 THz at T = 150 K, while the mean frequency point and the width of gap III are 0.09 THz and 0.19 THz at T = 250 K, respectively. However, the change is not significantly noticeable, and all of the lower and upper frequency limits of the band-gaps are shifted towards the higher frequency by increasing the temperature. Complete gaps, which totally stop the propagation of the incident beam through the structure, can be shown in Figure 2. At lower frequencies, the band-gaps are practically as narrow as they are at higher frequencies. As a result, the width can be adjusted according to the frequency.  
The above analysis indicates that a THz mirror can be achieved within multilayer structures comprising the semiconductor material InSb. The upper and lower band-edge frequencies, as well as the bandwidth of the PBG, can be adjusted by modifying the external temperature. THz mirrors play a significant role in future THz communication systems.