**3. Terahertz generation at room temperature**

Because of the expansion of the communication systems, information processing, and transmission fields are the most susceptible [19–22]. In terms of the frequency range, the lower Terahertz range may benefit air transmission, while the higher frequency range enables faster signal transfer and can be used to create ultra-high bandwidth data links. A fundamental component of numerous fields, including information processing and transmission, security screening, and biomedical applications, are emerging from Terahertz research [19, 21, 23, 24]. The future applications of Terahertz radiations have prompted an influx of scientific and technical research into creating Terahertz pulses for bio-material imaging, ultrafast dynamics, and nonlinear Terahertz optics [21, 25], among other things. This frequency may be generated in various ways, one of which is by using an ultrafast and ultrashort laser pulse. The interaction of short laser pulses with various objects makes it more challenging to produce adjustable, compact, coherent, and high-magnitude Terahertz radiation sources. The two-color filamentation in gases is a simpler way for a generation since it can scale the magnitude to incredible levels of interest and complexity.

However, the scaling was limited by the laser pulse energy constraints, and as a result, the search for alternate target materials was essential for these applications to succeed. It is necessary to search in a varied and uncluttered study region to locate the target, which exhibits Terahertz generations of high energies. The different crystals are the subject of this study due to their crystalline nature, as they can be used for large-scale broadband and high-frequency terahertz generation. Accordingly, phonon dynamics of topological insulators single crystals provide an attractive benefit in terahertz generator performance.

The terahertz frequency range of 0.03 to 5.2 THz is created by a single crystal of Bi2Se3 [14, 26–38]. In summary, phonons, which are the COP in bismuth selenide, are responsible for generating the terahertz frequency, as mentioned in the TRUS section above. The terahertz spectroscopy directly validates the 2 THz frequency generation, while the TRUS was the initial method by which these oscillations were originally seen [14, 18, 26, 30, 32, 34, 36, 37, 39–53] and was used to confirm the frequency of the 2 THz frequency. In addition, Prince Sharma et al. in 2020 contrasted assessing the frequency determined through TRUS, revealing the COP oscillations [34, 48] to evaluate the frequency from the kinetic spectrum. **Figure 1** depicts oscillations obtained from the kinetics of a flake cleaved out of the large single crystal of Bi2Se3 that was recorded at the CSIR-NPL. The FFD (filtering the high-frequency component and fitting the data) analysis investigates the generated frequency. These oscillations are removed from the kinetic data profile of charge carriers by applying a high pass filter in the 1 to 10 ps period. The data that has been filtered out is fitted with the sinusoidal damped function to determine the frequency associated with these oscillations. The frequency associated with them is determined to be ~2.42 terahertz (THz) from FFD analysis.

#### **Figure 1.**

*(a) Illustration of optical phonon oscillations by filtering with a high-pass Fourier filter with a cutoff frequency of 2.32 THz. (b) The optical phonons vibrations are fitted with a damped sinusoidal function. (c) The amount of experimental fitting data has skyrocketed. Reprinted from ref. [48], with permission of springer nature.*

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

A significant expansion on the assessment of terahertz production is also carried out, where the terahertz frequency may be changed just by altering the interaction of electromagnetic radiation in a single crystal. First and foremost, the probe energy is monitored while the excitation energy is maintained consistently. We detected strong oscillations at 1100 nm with a frequency of 2.42 THz in this system. A continuous probe at 1100 nm is then monitored by changing the excitation wavelength, which is the second step. In **Figure 2**, the tunable nature of the single-crystalline flake of bismuth selenide [34] is shown in detail. It is discovered that the frequency of phonon vibrations may be controlled by tuning the carriers in the crystal by appropriate doping, as well [45, 54]. Consequently, the adjustable nature of terahertz in single crystals might be beneficial in the growth of optoelectronics and Terahertz applications.

Although the primary emphasis is on the COP dynamics, Yuri D. Glinka et al. return to the distinct relaxations, namely, CAP [55]. The paper noted an increase in CAP frequency from 35 to 70 GHz. When the thickness of TI was reduced, the interaction of two processes caused this frequency difference. As the film thickness goes below 15 nm, lamb wave excitations (elastic waves whose propagation is plane to plane) become visible. Above this critical thickness, the system operates in the bulk acoustic resonator mode (indirect inter-surface coupling). Apart from performing TRUS measurements at ambient temperature, Yi-Ping Lai et al. conducted a temperature-dependent examination between 11–294 K [44]. To begin, they summarized the physical processes occurring at various time scales predicted by pump-probe experiments as fast oscillations (1012 Hz– COP), slow oscillations (1010 Hz–CAP), non-oscillatory signal (1011 Hz – electrons and incoherent phonons), and constant residual (1011 Hz – slow electron dynamics).

The temperature-dependent evolution of the COP revealed that when the sample approaches room temperature, the optical phonon frequency reduces from 2.25 to 2.17 THz [44]. This investigation also determines the electron–phonon coupling constant, demonstrating that the observed signal dominates the bulk. Additionally, Sung Kim et al. [37] studied the compatibility of the thin films with various polycrystalline and crystalline substrates and the resonance impact of the same at various thicknesses ranging from 3 to 30 nm. At 2.1 and 5.2 THz, the intensity of the differential reflectance (DR) and two distinct phonon modes exhibited some resonant behavior within a specific range of a crucial number of quintuple layers (QL). This resonant effect is substrate-independent, and the crucial QL value is between 6 and 9 QLs. Apart

#### **Figure 2.**

*The terahertz frequency is dependent on the probe wavelength and pumps excitation energy. Reprinted from ref. [34], with permission of springer nature.*

from this resonant effect, Jianbo Hu et al. [49] regulated the strong Raman mode of bismuth selenide using a two-pump laser in TRUS studies. They demonstrated phonon chirping due to the two-pump arrangement, which confirms the earlier thesis of carrier-lattice coupling. These two pumps effectively tune the amplitude, but the frequency stays constant.

Pump-probe spectroscopy is used to investigate single excitations with a specific energy. However, Giriraj Jnawali et al. employed the mid-infrared femtosecond TRUS on a nanoflake of TI for the first time [50]. The energy range is 0.3–1.2 eV. This range includes both the original BCB and the extended BCB. As a result, the paper modeled the ultrafast photoexcited carriers and holes across the BCB. Theoretical modeling of 10 K DR indicates a strong probability of the Fermi level being present much above the lowest conduction band. These experiments established a firm knowledge of the carriers inside TI's BCB, establishing a direct connection between carriers and Dirac SS [50].

Nonetheless, the TRUS provides no evidence for spin-polarized charges in SS through these broad energy probes. M.C. Wang investigated spin-dependent transitions that occurred solely in topologically protected SS using time-resolved Kerr rotation measurements [41]. Transitions between topologically protected SS may induce net spin polarization due to the presence of spin-protected carriers inside these SS. However, the study demonstrates that a circularly polarized pump may create net polarized spin, but the transitions responsible for this polarized spin are not just between the two SS but also between the first SS and the second BCB located near the second SS [41]. While the TRUS predicts just the energy levels of the different bulk conduction bands at ambient temperature. The surface state-related transitions are readily seen when investigating the temperature-dependent dynamics.
