# Terahertz Photoconductive Antenna Based on a Topological Insulator Nanofilm
## 3 Results  
A waveform of THz radiation, recorded from TI PCA at a bias voltage of 20 V, and its spectrum, obtained using fast Fourier transform, are shown by red curves in Figure 4a,b, respectively. The spectra in Figure 4b are \"bumpy\" due to re-reflection in the substrates and absorption by the water vapor. They extend up to 2 THz, which corresponds to the spectral range of the antenna-detector. The dynamic range of the obtained TI PCA radiation was approximately 60 dB.  
Figure 4: The (**a**) time-domain waveforms of dipole antennas based on BSTS (red) and InGaAs/InAlAs (blue); (**b**) fast Fourier transform spectra of BSTS (red) and InGaAs/InAlAs (blue) antennas; and (**c**) THz electric field amplitude versus bias voltage for BSTS antenna.  
Figure 3: Schematics of the terahertz time-domain spectroscopy setup.  
The blue curve in Figure 4a shows a waveform of THz radiation from a similar dipole semiconductor PCA biased at the same voltage U = 20 V (the curves were vertically shifted artificially). Though the thicknesses of active media differed by almost an order of magnitude (40 nm and 2000 nm), the amplitudes of the generated THz fields were comparable to each other. Even the peak-to-peak amplitude was 1.5 times more in case of thin TI PCA. Hence, we can conclude that, with the unit thickness of the medium, generation using BSTS is much more efficient than that using InGaAs/InAlAs. Figure 4c shows the dependence of the electric field amplitude on the bias voltage for TI PCA. The linear character of this dependence is clearly seen in the wide voltage range. At higher voltages, the THz signal was not recorded to avoid possible damage to the sample due to electrical breakdown.  
As the generated photocurrents are responsible for the generation of THz radiation, it was interesting to measure them in the range of acting bias voltages of 0-20 V. First, the steady current-voltage characteristic was recorded, i.e., the dependence of the dark current on the voltage in the absence of incident optical pumping (Figure 5a). This characteristic was well approximated up to 13 V by a linear curve. However, at higher voltages some deviation from the linear fit was observed, which can be explained by a decrease in the resistance of the sample due to heating. We recorded the heating of the BSTS from room temperature to 50 \({}^{\circ}\)C by a thermocouple at a maximal bias voltage, U\({}_{\text{b}}\) = 20 V. Figure 5b demonstrates the dependence of the average photocurrent on the bias voltage. It was also well described by a linear law over the entire range of U\({}_{\text{b}}\). For comparison, a similar dependence was shown for the semiconductor antenna, which lies well below the curve for the TI PCA. Figure 5c shows the obtained dependence of the TI PCA photocurrent on the pump power. It is linear; this is a typical situation for photoconductive antennas operating far from breakdown voltages and carrier screening effects.  
Figure 6a demonstrates the phase change of the generated THz radiation by 180\({}^{\circ}\) when the polarity of the bias voltage U\({}_{\text{b}}\) =15 V was reversed. If we summarize the two signals obtained at inverse voltages, we realise practically the same waveform as in the absence of an external voltage (Figure 6b). Similar results were observed at a higher voltage of 20 V.  
Figure 5: The **(a)** I-V characteristic of TI PCA measured without pump radiation; **(b)** filled circles: voltage dependence of the time-averaged photo-excited current of TI PCA; open circles: the same dependence for InGaAs/InAlAs PCA; and **(c)** dependence of the time-averaged photo-excited current on the pump power for TI PCA.