Search

The fluorescence of CaF 2 single-crystals containing Eu 2+ ions was studied under the excitation in the long-wavelength tail of the absorption spectrum which is due to electric dipole allowed transitions (“laser cooling regime”). Though no actual cooling was detected, spectra with a dominant anti-Stokes component were observed which demonstrate the possibility to employ electron-phonon bands of Eu 2+ f-d transitions for optical refrigeration. The absence of observable optical refrigeration is ascribed to Eu 2+ two-step photoionization processes that are responsible for the excited state absorption. The role of two-step excitation of impurity centers (excited state absorption) in laser cooling experiments is discussed.

Various insulating materials containing local centers with electric dipole allowed transitions were studied under excitation in the long-wavelength tail of the absorption spectrum (“laser cooling regime”). 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. The mechanisms responsible for the absence of observable optical refrigeration are discussed.