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. Spontaneous optical cooling 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. Our initial results demonstrate that the erbium dopants substantially reduce the quality factor of our resonators at the erbium absorption wavelengths, which is where we will then position the silica Stokes peak, thus reducing the ratio of Stokes to Anti-Stokes emission in our sample and leading towards possible net cooling effects.