The role of two-step excitation processes in laser cooling experiments: CaF 2 :Eu 2+

Abstract 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.