Monte Carlo fluorescence ray tracing simulation for laser cooling of solids

We propose an approach to evaluate solid-state media for laser cooling by anti-Stokes fluorescence employing a Monte Carlo-based simulation of fluorescence ray tracing. This approach prompted a revisit of the experimental method, laser-induced thermal modulation spectroscopy (LITMoS), showing that the external quantum efficiency and the background absorption coefficient can be retrieved solely from the two wavelengths where neither cooling nor heating is observed. Our simulation can accurately compute two experimentally inaccessible quantities essential to evaluate laser-cooling media: the mean fluorescence wavelength and the fluorescence escape efficiency. These computed quantities in combination with LITMoS results allow us to retrieve the internal quantum efficiency which is a performance indicator independent of various factors such as the sample size and doping level. Using the proposed approach, we thoroughly investigate the impact of doping level, sample geometry, and refractive index on the fluorescence escape efficiency and reveal its temperature dependency for the example of Yb:YLF. Through comprehensive numerical analysis, we demonstrate that the reduction of sample symmetry is crucial in achieving lower cooling temperatures.