**5. Temperature-dependent terahertz frequency generation**

The kinetic profile is used in order to investigate the temperature dependence of the oscillation frequencies generated in the terahertz regime. In addition to being in the terahertz frequency range, other frequency modes are also observed in the TI. These modes lie in the gigahertz frequency range due to CAP. While the terahertz frequency is generated because of the vibration of COP. These modes are analyzed using filtering the high-frequency component and fitting the data (FFD) [48]. This detailed analysis helps exclude the charge carrier's relaxation from phonon-associated vibrations. The COP-associated frequency is found to have lied in the terahertz regime. Moreover, these frequency modes are dependent on excitation and probe energy, as mentioned above. In order to visualize the changes of these modes with temperature, the kinetic profile at 100 nm is considered. The micro flakes of different TI are excited

#### **Figure 5.**

*Frequency dependence of three TIs on temperature. The COP modes in TIs are discovered to be temperature sensitive, implying that their vibration frequency is as well [53, 62].*

with pump energy of 3.02 eV at 1.0 mW average power in order to observe the kinetic profile at this particular wavelength. The temperature-dependent investigation is carried out at these pumps and probe energy at different temperatures. With the use of this FFD analysis, it is possible to identify the terahertz frequency at which the phonons oscillate. Each of the TIs, namely Bi2Te3, Sb2Te3, and Bi2Se3, exhibits oscillations of the order of terahertz frequency in the range of 1–3 THz, as seen in **Figure 5**.
