Laser cooling of Er3+-doped solids

We consider theoretically a mechanism for laser cooling in rare-earth-doped low-phonon materials based simultaneously on two cooling cycles: a traditional cooling cycle with an anti-Stokes fluorescence transition as well as an infrared-to-visible upconversion cycle, to overcome the self-termination effects in either anti-Stokes or upconversion cooling on its own. Our simulations, performed for erbium-doped potassium-lead chloride crystal (Er3+:KPl2Cl5) known to be an extremely low phonon energy host, uses two pump wavelengths corresponding to the long wavelength tails of the absorption spectra of the 4I15/2 → 4I13/2 and 4I15/2 → 4I9/2 transitions. The contribution of each pump source to the cooling process is comprehensively investigated. We show that, although the energy gap between 4I15/2 and 4I9/2 levels exceeds the energy gap between 4I15/2 and 4I13/2 levels and cooling process is more efficient with the cycle based on the 4I15/2 → 4I13/2 transition, the second cooling cycle based on the 4I15/2 → 4I9/2 transition can be used as a supplementary one.