# Accurate Characterization of the Properties of the Rare-Earth-Doped Crystal for Laser Cooling
## 3 Experimental Set-Up  
A schematic diagram of the experimental set-up for LITMoS testing is shown in Figure 1. A tunable fiber laser with a wavelength range of 1010-1080 nm is used to pump the 5% Yb\({}^{3+}\)-doped LuLiF\({}_{4}\) crystal with size of 2 \(\times\) 2 \(\times\) 5 mm\({}^{3}\). The sample is placed in the vacuum chamber of 10\({}^{-5}\) Pa. The pump laser beam is collected by beam dump after passing through the sample. A calibrated thermal camera (FLIR A300) equipped with 100 \(\upmu\)m macro lens is used to measure the temperature change \(\Delta T\) of the sample in real time. A spectrometer (Ocean Optics Maya 2000 Pro-NIR, Dunedin, FL, USA) is used to measure the fluorescence spectra and the absorption power \(P_{abs}(\lambda)\) of the sample. The cooling efficiency \(\eta_{c}(\lambda)^{exp}\) of the sample at the different pump wavelength \(\lambda\) can be acquired by precisely measuring \(\Delta T\) and \(P_{abs}(\lambda)\). Figure 2 shows the picture of devices for the LITMoS test and the thermal camera calibration. The picture of the thermal camera and the cooling grade testing chamber (CGTC) is shown in Figure 2a. The infrared camera is used for non-contact temperature measurement in the LITMoS test. The camera's detector contains a focal plane array of uncooled micro-radiometric thermometers (320 \(\times\) 240 pixels) and a spectral response range of 7.5-13 \(\upmu\)m. The BK7 glass windows (25 mm diameter) for pump laser are AR coated at a wavelength of 1 \(\upmu\)m. The other pair of BaF\({}_{2}\) windows (25 mm diameter) is used for the sample temperature measurement with the thermal camera. The temperature of the CGTC can be adjusted in the range of 276-308 K with an accuracy of \(\pm\)0.05 K by the chiller. The interior of the CGTC is shown in Figure 2b. There are two copper blocks with a slit in the middle for accommodating a reference sample identical in size to the cooling one. A TEC is contact-fixed to the copper clamp for varying the sample temperature. A K-type thermocouple with very thin strings is connected to the copper clamp for temperature measurement. A low-evaporation silicone grease is used on all contact surfaces for good thermal conductivity. These electronic devices are wired to the outside of the CGTC through connectors on the sealed cap. The temperature of the reference sample, which is embedded  
in the sample holder, can be controlled by adjusting the current of the TEC and measured by the thermal camera at the same time. Figure 2c shows the side view of the CGTC. Figure 1: The schematic of the LITMoS test set-up. Figure 2: The photograph of the LITMoS test set-up and the thermal camera calibration. (**a**) The cooling grade testing chamber with the water chiller and the thermal camera (FIRE A300). (**b**) The interior of the cooling grade testing chamber (top view). The TEC and copper clam, including the reference sample (inside the area of the red dash box). (**c**) The interior of the cooling grade testing chamber (side view). It is difficult to directly measure the temperature change of the sample in the CGTC using thermal camera imaging because the sample has low thermal emissivity and is transparent at thermal wavelengths. In order to identify the proportional relationship between pixel brightness and temperature change of the sample, it is necessary to vary the temperature of the sample inside the CGTC and acquire its thermal images accordingly. The micro-radiometer technology is based on the principle that the change in pixel resistivity is proportional to the energy of the infrared radiation absorbed by the detector. After calibration, one can deduce the temperature of the emitting source from the pixel intensity variation of the output images. The thermal camera outputs an 8-bit depth grayscale image with intensity values between 0 (black) and 255 (white) for each pixel in the photo.