Spontaneous temperature and fluorescence oscillations in CaS:Eu2+ crystals excited in the long-wavelength tail of the absorption spectrum

The electric dipole allowed 4f65d1-f7 fluorescence of CaS:Eu2+ crystals was studied under excitation in the long-wavelength tail of the absorption spectrum (the regime typical for laser cooling experiments). Though no actual cooling was detected, spectra with a strong anti-Stokes component were observed which demonstrate the possibility to employ electron-phonon bands of electric dipole allowed transitions for optical refrigeration. Under the strong laser excitation, the heating of thermally-isolated crystals was observed accompanied by optical bistability and spontaneous fluorescence oscillations. These unusual effects in fluorescence are ascribed to strong nonlinearities existing in the system.