# Visible Measurement of Terahertz Power Based on Capsulized Cholesteric Liquid Crystal Film  
[PERSON]  
1 College of Electronic and Optical Engineering & College of Microelectronics, Nanjing University of Posts and Telecommunications, Nanjing 210023, China; [EMAIL_ADDRESS] (P.L.); [EMAIL_ADDRESS] (K.Y.); [EMAIL_ADDRESS] (W.W.) 12 Institute of Laser Engineering, Osaka University, Suita, Osaka 565-0871, Japan;[EMAIL_ADDRESS] (H.Q.); [EMAIL_ADDRESS] (T.N.K.P.); [EMAIL_ADDRESS] (K.K.); [PERSON]AIL_ADDRESS] (K.T.) 13 Center for Advanced Meta-Materials, Daejeon 305-343, Korea; [EMAIL_ADDRESS] 3 National Laboratory of Solid State Microstructures, Collaborative Innovation Center of Advanced Microstructures and College of Engineering and Applied Sciences, Nanjing University, Nanjing 210093, China; [EMAIL_ADDRESS] 3 Department of Electronic Engineering, Xiamen University, Xiamen 361005, China; [EMAIL_ADDRESS] 3 Research Institute of Superconductor Electronics (RISE), School of Electronic Science and Engineering, Nanjing University, Nanjing 210093, China; [EMAIL_ADDRESS] 3 Department of Electronic Engineering, Xiamen University, Xiamen 361005, China; lujianchen@xmu.  
radiation through the optical method, but it is fragile and sensitive to mechanical vibration, has a slow response, and has too low a THz power (maximum \(\sim\)10 \(\upmu\)W) to be detected [23]. THz detectors still have outstanding challenges to address before becoming cost-effective, user-friendly, and widely used.  
Recently, alternative thermal detection techniques using cholesteric liquid crystals (CLCs) have emerged. CLCs have the intrinsic self-organizing ability of a helical structure, the pitch of which depends on temperature, and the wavelength of the selective reflection depends on the pitch [24; 25]. THz radiation increases the temperature of CLCs, which changes the pitch. Therefore, THz power is detected through measurement of the reflective wavelength. THz waves induce color change of CLCs, which can be observed directly by the naked eye, thus makes electronic devices, power supplies, or connecting cables unnecessary. This simplicity enables us to make cheap and portable THz imagers. The first CLC imager for 337-\(\upmu\)m radiation was demonstrated by [PERSON] and [PERSON] in 1971 [26]. In 2013, [PERSON] et al. reported a CLC imager for THz quantum cascade lasers [27]. Both are for single-frequency THz radiation. With the rapid development of various broadband and high-power THz sources pumped by table-top lasers, we have used a temperature-sensitive CLC to measure the THz beam profile. We have measured the THz beam profiles at low power density under which condition the influence of thermodiffusion-effect CLC film can be neglected [28]. Under a high THz power density, the thermochromatic and thermodiffusion effect always exists simultaneously in the CLC. Especially, while the color does not change at the center of the CLC film at the saturation, the color change can still diffuse to a larger area in the CLC, which is stable, easy to investigate, and can be used for designing new THz devices.  
Here, we present new methods for detecting THz waves in high THz intensity using a temperature-supersensitive capsulized CLC film (CCLCF). We utilize not only the thermochromic effect but also the thermal diffusion effect of the CLCs to quantify the THz absorption-induced increase in the size of the color change area. By focusing the THz beam, the power density reaches as high as 4.0 \(\times\) 10\({}^{3}\) mW/cm\({}^{2}\). In the equilibrium state, the diameter of the color change area is linearly related to the THz power, whereas in the transient state, for 1 sec of THz radiation, the THz power can also be clearly detected.