Towards Raman Cooling in Erbium Doped Microresonators

Optical excitation of matter commonly results in heating processes due to light absorption and inelastic phonon production processes such as Raman scattering. Optical cooling of solids has been achieved for certain materials through processes dependent on the sample’s electronic structure or through optical engineering. However, fluorescent cooling of any solid remains a challenge that has not been fully solved. We propose a new approach for achieving optical cooling in solids using whispering gallery modes. Optical absorption of rare-earth dopants in a micro-resonator is used to eliminate the Purcell enhancement of the heat-producing Stokes light scattering in a high-Q resonator with large Purcell enhancement of the phonon-absorbing Anti-Stokes scattering. We use several methods to fabricate erbium-doped Silica microsphere resonators with Q factors of 10^7, evanescently coupled to a biconical tapered fiber waveguide. We measure the effect of erbium absorption on the resonator Q for spontaneous Raman scattering. We will align the silica Stokes peak with the erbium absorption, thus reducing the ratio of Stokes to Anti-Stokes emission in our sample and leading towards possible net cooling effects. We will then measure the resultant effect on Stokes and Anti-Stokes emission due to our device’s Purcell enhancement and the coupled Rare-Earth atoms’ absorption.