# Terahertz Photoconductive Antenna Based on a Topological Insulator Nanofilm
## 1 Introduction  
Currently, topological insulators (TIs) are considered to be promising new 2D materials for terahertz (THz) generators and detectors [1, 2, 3]. TIs are characterized by the presence of stable (topologically protected against backscattering), conducting surface states of an electronic gas. TIs can be used in the development of various interdisciplinary connections in many modern areas, including spintronics [4], laser physics [5], and quantum information [6]. Angle-resolved photoemission spectroscopy (ARPES) detects the presence of Dirac electrons with nondegenerate spins in many solids [7]. In addition to ARPES, there are other methods that are sensitive to edge electronic states. In [8], a new method was proposed for detecting the surface states of electrons based on the photo-electromagnetic effect (PEM), which is not sensitive to bulk conductivity. In [9], ultrafast manipulation of topologically enhanced surface transport was demonstrated using THz and mid-IR pulses in bismuth selenide. Correspondingly, the generation of THz radiation can also be sensitive to surface electron transfer. [PERSON] et al. demonstrated a circular anisotropy of the photon drag effect (PDE) in Bi\({}_{2}\)Se\({}_{3}\), which proves optical coupling with topological surface states [10]. In a number of previous studies, the generation of THz radiation under the action of pulsed optical pumping was investigated in bismuth chalcogenides of double [11, 12] and quaternary [13] chemical compositions. A comprehensive review of the photovoltaic effects in Bi\({}_{2}\)Te\({}_{3}\) and Sb\({}_{2}\)Te\({}_{3}\) was given in [14]. A number of conclusions were drawn regarding the prevailing mechanisms of the generation of THz radiation, due to the excitation of photocurrents in the epitaxial films of topological insulators in the absence of any external electric bias field. In [15], it was shown that the contribution to the photocurrent from the PDE is an order of magnitude greater than the contribution from the diffusion current, and two times higher than the contribution from the drift current. Topological insulators of quaternary compositions Bi\({}_{2-x}\)Sb\({}_{x}\)Te\({}_{3-y}\)Se\({}_{y}\) are actively studied in connection with the possibility of smooth changes in their properties over a wide range. In [16], it was shown that there is a certain optimal curve (the \"Ren's curve\") in the composition-structure diagram y(x), where the properties of electronic surface states aremost pronounced. This is associated with a significant suppression of the bulk contribution to the conductivity, as the actions of acceptors and donors cancel each other, which leads to the predominance of surface electron transport. Bi\({}_{2-x}\)Sb\({}_{x}\)Te\({}_{3-y}\)Se\({}_{y}\) films with compositions located under the Ren's curve, as a rule, predominantly have p-type conductivity; while compositions located above this curve have conductivity of the n-type. At present, research is continuing on the most efficient methods for generating and detecting terahertz radiation. Photoconductive semiconductor antennas are effective devices in the THz range for applications in spectroscopy, and various imaging and data transmission systems [17; 18]. Due to the availability of components in the telecommunications industry, modern systems often use optical excitation at a wavelength of 1550 nm with femtosecond pulses of an erbium fiber laser [19; 20]. Semiconductor structures, based on indium-gallium arsenide, were well studied as photoconductive materials for THz due to their suitable optical absorption wavelength of 1.5 \(\upmu\)m. Recently, THz detectors fabricated from rhodium (Rh)-doped In\({}_{0.53}\)Ga\({}_{0.47}\)As showed a record peak dynamic range of 105 dB and a bandwidth up to 6.5 THz [21].