**2. Transient reflectance ultrafast spectroscopy (TRUS)**

J.Qi. et al. performed the first TRUS measurements on Bi2Se3 crystals [14], where they demonstrated the time-resolved behavior of the crystal at room temperature. Three distinct relaxations of carriers induced by photons were identified in this experiment. The first two are phonon interactions between excited charge carriers of coherent optical and acoustic phonon interactions. In contrast, the third is a negative amplitude process generated by ultrafast carrier trapping of selenium. They also observed the frequency of 2.13 THz from their optical phonon oscillations. They also concluded that the atmosphere impacts these charge carriers and phonons since air promotes band bending, which causes an elevation in the Fermi level [14]. Nardeep Kumar et al. [15] utilized the ultrafast pulse to analyze the carrier's behavior by pump-probe spectroscopy. The Bi2Se3 TI exhibits two distinct oscillations, with the rapid oscillations occurring at 2.167 THz and the slow oscillations occurring at 0.033 THz. Coherent optical and acoustic phonons are responsible for this frequency [15]. In these systems, the terahertz frequency is not dependent on the size of the laser spot, and the ambipolar carrier diffusion coefficient is also determined to be 500 cm2 /s [15]. Nardeep et al. have shown that the COP-induced terahertz frequency production has an uneven dependency.

### *Temperature-Dependent Evaluation of Charge Carriers and Terahertz Generation in Bismuth… DOI: http://dx.doi.org/10.5772/intechopen.102887*

The ultrafast carrier dynamics were further investigated in the limited thickness of the TI, using pump-probe spectroscopy in thin films by Yuri D. Glinka et al. [16]. The thicker films have similar relaxation lifetimes to bulk crystals, while the thinner films exhibit fastened relaxation of excited charge carriers. The longevity of SS and bulk states might be related to their contribution. The shorter carrier lifetimes in thin films are associated with reducing bulk contributions of carriers in dominant surface states. A resonance-like property is also seen in 10 nm films, but the study does not provide conclusive evidence for how the property is generated. Yuri D. Glinka et al. also investigated the thin films of Bi2Se3 (6–40 nm), in which they confirmed the presence of radiative and non-radiative processes and described resonance phenomena at 10 nm films in terms of these processes [17]. This article establishes unequivocally that bismuth selenide contains a second SS. After the detection of the second SS, the primary trend observed in TRUS is the presence of three distinct processes: (a) electron–electron and electron longitudinal optical phonons in the 1–8 ps range, (b) a metastable bulk conduction band that continuously feeds charge carriers to the second SS for approximately 10 ps, and (c) a quasi-equilibrium carrier population. The thin layer is activated by 1.51 eV photons that excite carriers from the bulk conduction band (BCB) to the second surface states (SS). From the second surface to the first SS, carriers undergo intra-band and inter-band relaxation. The relaxations occur between the second SS and BCB and first SS, indicating that the ultimate recombination happened in these states. The relaxation of the excitation charge carriers results in forming a new Fermi level that is displaced away from the original Fermi energy. This relocation is a result of carrier localization. Again, the resonance phenomenon in a 10 nm film is found in this pump-probe spectroscopy, which is explained by the depletion of electrons caused by the connection between two film surfaces. The confinement of these 3D electrons is due to the existence of surface defects caused by selenium vacancies; however, they grew the 10 nm film many times and obtained this resonance in each of these films. This repeatability lends credence to the selenium vacancy theory. This resonance effect in 10 nm TI is unknown at the moment. However, recent work by Glinka et al.; explains the discrepancy by suggesting several surface state amendments [18].
