# Visible Measurement of Terahertz Power Based on Capsulized Cholesteric Liquid Crystal Film
## 2 Materials and Methods  
The configuration of the CCLCF is illustrated in Figure 1. It consists of three layers. The CLCs are encapsulated in droplets of \(\sim\)10 \(\upmu\)m in size dispersed in an organic solvent (RM2325, Capsular Products, Tokyo, Japan), shown in the inset of Figure 1, which is directly coated on the acrylic resin layer. The carbon black layer adheres to the CCLC layer from the back side, in order to clearly change the color of the CCLCF. This thin film is flexible and pressure-insensitive, which makes it easy to scale up for operation.  
Figure 1: Schematic and working principle of the capsulized cholesteric liquid crystal film (CCLCF). The inset shows a micrograph of the CCLCF, which is produced with a color 3D laser scanning microscope (VK-8710, KEYENCE, Osaka, Japan).  
We harness the heat produced by the localized absorption of the THz radiation to increase the temperature of the CCLCF. Thus, the helical pitch of the CCLC is shortened, simultaneously resulting in both a color change and increase in size. This is the process of producing an image of the color change that is dependent on the THz intensity. Especially when the THz radiation is focused, the high THz power density clearly reveals the diffusion of the color change. The THz radiation is measured by analyzing the image. The device is not restricted by the saturation of the color change and is stable and practical without any additional components required to measure the temperature.  
The narrow temperature range of the color change is better for the sensitivity of the THz power measurement. Our thermochromic LC is temperature-supersensitive, which is from about 22 \({}^{\circ}\)C to 25.5 \({}^{\circ}\)C, as shown in Figure 2, at a room temperature of 23.8 \({}^{\circ}\)C, and green light is reflected while the color changes to blue (corresponding to the saturation) when the temperature reaches 25.5 \({}^{\circ}\)C, increasing by just 1.7 \({}^{\circ}\)C. The inset pictures were taken by a common commercial digital camera and show that the film is very homogeneous. The curves represent the THz absorptance spectra at different temperatures. The absorption \(A(\omega)\) is calculated by \(1-T(\omega)-R(\omega)\), where \(T(\omega)\) and \(R(\omega)\) are the transmittance and reflectance of the CCLCF, respectively, measured by conventional THz time-domain spectroscopy. Compared to our earlier work [28], here we improved the sample by removing two layers Polyethylene terephthalate and double-sided tape to obtain a higher THz absorption. We can see that the THz absorption of the CCLCF is almost temperature-independent, where the absorption at 25.5 \({}^{\circ}\)C is a little larger than that at 23.8 \({}^{\circ}\)C. The THz polarization-independent characteristic is also measured and confirmed, which is not given here. The absorption increases over the entire THz region from 0 to 3 THz. The oscillation in the THz absorptance spectrum is an interference effect caused by multiple reflections of the CCLCF. The thermal conductivity of the sample we measured is about 0.5 J/msK. All the measurements indicate that the device is stable, which is suitable for our THz power detection.