# Terahertz Photoconductive Antenna Based on a Topological Insulator Nanofilm
## 2 Proposed System Design  
A TI sample of Bi\({}_{1.9}\)Sb\({}_{0.1}\)Te\({}_{2}\)Se (BSTS) was prepared with a composition near the Ren's curve at the composition-structure diagram. It was grown by metalorganic vapor-phase epitaxy (MOVPE) on a (0001) sapphire substrate with a 10-nm zinc telluride (ZnTe) buffer layer of orientation (111) at the atmospheric pressure of hydrogen in a horizontal quartz reactor. The sources of organometallic compounds of bismuth, antimony, zinc, tellurium, selenium, trimethyl bismuth, trimethyl antimony, diethyl zinc, diethyl tellurium, and diethyl selenium were used. The ZnTe buffer layer was grown in one technological cycle with a BSTS film at a temperature of 463 \({}^{\circ}\)C. To determine the elemental composition of the films, we used X-MaxN energy dispersive X-ray spectrometer with an electron microscope. A thin film of BSTS, 40 nm thick and of necessary composition, was selected for THz generation (Figure 1a). A dipole antenna was fabricated by patterning Ti/Pd/Au metal electrodes (of corresponding thicknesses 50/120/200 nm) onto BSTS film. The gap between electrodes was 20 \(\upmu\)m. Templates were preliminarily applied using the photolithography method. The fabricated TI-antenna had an impedance of 490 \(\Omega\). A hemispherical high-resistance silicon lens of 1 cm diameter was mounted on the surface of the Al\({}_{2}\)O\({}_{3}\) substrate to collect the generated THz radiation (Figure 1c). To analyse the generation efficiency, we compared the generated THz pulses from two photoconductive antennas with the same dipole structure, one fabricated on the BSTS film (TI PCA) and the other on a semiconductor multilayer heterostructure (semiconductor PCA). Another dipole photoconductive antenna, fabricated on a low-temperature grown In\({}_{0.5}\)Ga\({}_{0.5}\)As/In\({}_{0.5}\)Al\({}_{0.5}\)As superlattice, was used as a second antenna for reference. The parameters of similar spiral antenna were previously studied and reported in [33]. The multilayer heterostructure InGaAs/InAlAs for semiconductor PCA was grown on an InP (111) substrate (Figure 1b). Such structures are usually prepared by molecular beam epitaxy and exhibit a high conversion efficiency of optical radiation into the broadband THz radiation. The overall thickness of the InGaAs/InAlAs heterostructure was 2000 nm, and the total thickness of all InGaAs layers was 1200 nm. An impedance semiconductor PCA was 12.8 k\(\Omega\). We measured the room temperature absorption coefficient at the pump wavelength. It was 4300 cm\({}^{-1}\) for our InGaAs layer lattice and 120,000 cm\({}^{-1}\) for the BSTS sample. In both cases, the thickness of the antenna material was less than the pump field penetration depth. Thus, the selected film thicknesses, 40 nm in the case of BSTS and 1200 nm of the total thickness of the InGaAs layers, provided almost uniform pumping illumination throughout the depth of the samples. Figure 2a shows a photograph of the board. The electrodes were deposited on the samples of photoconductive films with five dipole gaps (Figure 2b). One of the five inter-electrode gaps, located above the most homogeneous region in the case of the topological insulator nanofilm, was chosen. Then, by heating with hot air from a soldering station, the electrodes were soldered to the board. A photograph of electrodes under a microscope is shown in Figure 2c. A hemispherical high-resistivity silicon lens 1 cm in diameter was pressed onto the substrate from above. The measurements of THz generation characteristics were conducted on an experimental setup schematically presented in Figure 3. An Er\({}^{3+}\)fiber laser was used as a source for optical pulsed pumping at a wavelength of 1560 nm, pulse repetition rate of 70 MHz,  
Figure 1: The (**a**) atomic force microscopy image of BSTS sample surface; (**b**) sketch of the InGaAs/InAlAs heterostructure; and (**c**) sketch of the antenna emitter.