Photoluminescence investigations of Tm~(3+) doped SrF_2 single crystals for visible and infrared laser applications

摘要

This paper reports the optical properties of Tm3+ doped SrF2 single crystals. These crystals are obtained with a good optical quality by use of the standard Bridgman technique. Absorption, photoluminescence excitation and Stokes emission spectra were recorded at room temperature. Since we have good quality absorption spectra, we have used Judd-Ofelt (JO) analysis to determine the spectroscopic properties of these crystals. By applying a least-square fitting between the measured and calculated transition strengths, we determine the three JO parameters for 4f-4f transitions of Tm3+ ions. The best adjustment gives the following values: Omega(2) = 0.225, Omega(4) = 1.667, Omega(6) = 0.593 in 10(-20) cm(2) units. As Omega(2) parameter is sensitive to the local environment, the lower value of Omega(2) indicates a more ionic bond due to the charge compensation with interstitial Fanions in fluorite structure when doped with rare earth. The three obtained values of JO parameters are in a good agreement with those obtained for other fluoride hosts. They have been used to determine the transition probabilities which are in turn used to calculate the radiative lifetimes of the main emitting levels and the branching ratios of the corresponding transitions. The F-3(4)- H-3(6) infrared transition has a maximum branching ratio and a metastable radioactive lifetime greater than 10 ms as predicted in literature and has a bigger value of optical gain. A particular attention is also paid to transitions emitting in blue when we use infrared laser diode as excitation source. Such transitions present relatively efficient laser parameters.