Anti-Stokes laser-induced cooling in rare-earth doped low phonon materials

In this work we discuss the anti-Stokes laser-induced cooling of two matrices doped with Yb3+ and Er3+: a low phonon KPb2Cl5 crystal and a fluorochloride glass. In order to assess the presence of internal cooling in these systems we used photothermal deflection and conventional excitation spectroscopic techniques, whereas the bulk cooling in the Er3+-doped materials was detected by means of a calibrated thermal sensitive camera. Furthermore, we also consider some of our findings on cooling processes occurring in Yb3+-doped low phonon materials from a theoretical perspective. The experimental results are in good agreement with the predictions of a model based on the presence of a second order process in the cooling mechanism. The fluorescence excess shown by the excitation spectra of Yb3+-doped sample obtained at high fluences, by pumping at wavelengths in the cooling region, has been explained in the framework of the configurational coordinate model by considering that the frequencies of the vibrational modes in the ground and excited states change at high pumping intensities (quadratic coupling mode). In the case of Er3+ ion, it is worthwhile to mention that the cooling was observed in the spectral region where some upconversion processes that initiate at the pumped 4I9/2 level occur. Together with the spectroscopic characterization, a short discussion on the experimental and theoretical background of the cooling process including the possible influence of upconversion processes is presented.

► Laser cooling of rare-earth doped low phonon materials is demonstrated.
► Anti-Stokes cooling in solids is strongly dependent on the phonon density of states.
► Efficient endothermic IR-to-VIS upconversion enhances the optical cooling efficiency.