Enhanced laser cooling of Tm-doped solids by upconversion pumping

We present a physical scheme of upconversion-assisted anti-Stokes fluorescence cooling in Tm doped low phonon materials. The upconversion pumping from the metastable level to the upconversion excited level is introduced to produce extra anti-Stokes cooling cycles which can improve the laser cooling performance. Both the limitation of the cooling cryogenic temperature and the range of the cooling wavelength can be extended notably in contrast to the traditional anti-Stokes cooling.