# Visible Measurement of Terahertz Power Based on Capsulized Cholesteric Liquid Crystal Film
## 3 Setup and Results  
Since we aim to propose a new method to detect THz power using our CLC device, the characterization of its performance in a large power range is more expected. Therefore, the THz pulse generation setup using the titled-pump-pulse-front method [15; 16], which is believed to be one of the most efficient THz sources [9; 13; 16], is adopted in our experiment. We generate THz pulses with pulse energy of 2.6 \(\upmu\)J by using pump beam with pulse energy of 4 mJ. The energy conversion efficiency is thus \(\sim\)0.7\(\%\). The advantage of our system is that the LiNbO\({}_{3}\) has a much higher pump-power tolerance and can generate high intensity THz waves. The much larger pump power compensates for the lack of the energy conversion efficiency. Our mW class THz source (pulse peak amplitude \(\sim\)500 kV/cm) can be  
Figure 2: Effect of temperature on THz absorption spectra of CCLCF. The inset shows the photos of color change from green (23.8 \({}^{\circ}\)C) to blue (25.5 \({}^{\circ}\)C). satisfactory for many researches. The setup and the THz power measurement with the CCLCF is shown in Figure 3a. The LiNbO\({}_{3}\) crystal is pumped by a regeneratively amplified Ti: sapphire laser (Legend Elite: 800-nm central wavelength, 4-W optical power, 100-fs pulse duration, 1-kHz repetition rate, Coherent) to generate the broadband THz pulse. A black polypropylene filter is used to block the remaining pumped laser and the radiation produced in the crystal along with the terahertz generation. The THz source intensity is tunable by rotating the THz polarizer. The CCLCF (1.5 cm \(\times\) 2.5 cm) is placed at the focal point. THz radiation is normally incident on the sample. Images and videos are taken by a smartphone camera with wireless Bluetooth technology to make it not only convenient but also to insure the detection position is fixed. The initial temperature of the CCLCF is 23.8 \({}^{\circ}\)C. Figure 3b shows the THz temporal profile. Single-cycle THz pulses with amplitudes of about 500 kV/cm are generated by optical rectification from LiNbO\({}_{3}\)[29]. Its corresponding THz spectral range is mainly 0-1 THz, which is shown in Figure 3c. The inset indicates that the THz beam spot size obtained with a THz camera (IRV-T0831, NEC, Tokyo, Japan) has a spatial profile of only about 300 \(\upmu\)m after focusing by the parabolic mirrors. A commercial pyroelectric detector (THz-5B-MT, Gentec-EO, Quebec City, Canada) was used to calibrate and measure the THz radiation power. The maximum average THz power (\(P_{\text{max}}\)) was 2.6 mW in our system. Thus, the maximum THz power density is as high as 4.0 \(\times\) 10\({}^{3}\) mW/cm\({}^{2}\). As the focused THz beam size is very small with a high power density, the color change increases in size to a large area in the CCLCF, which can be seen clearly. A direct visualization of different THz powers is demonstrated in Figure 4a. The images show an increase in alteration at the same area, suggesting the apparent temperature growth and thermal diffusion due to the increase in THz intensity. Without THz radiation, the green background color is a reference. Just two colors are involved, which is beneficial for our image analysis. For simple use, here we choose the diameter of the color change area as an evaluating parameter. Through the processing of our CCLCF images and videos based on hue, since hue increases monotonically with an increase in temperature [28], related parameters and data can be obtained by using software ImageJ [30]. The digital value of hue is 131. Figure 4b presents the relationship between the diameter and THz power in the steady state. The detectable THz field threshold value is about 0.07 mW, which can produce color changes visible to the naked eye. It has an approximately linear relationship above the threshold. The film still works well even when the THz intensity exceeds 4.0 \(\times\) 10\({}^{3}\) mW/cm\({}^{2}\). Therefore, we quantify the THz wave intensity by the diameter of the color change of the CCLCF.