Mid-IR transitions of trivalent neodymium in low phonon laser crystals

Mid-IR Nd3+ transitions perspective for laser oscillation were analyzed in the CaGa2S4, PbGa2S4, and PbCl2 crystals and compared with low phonon fluoride crystals. Fluorescence kinetics decay of the high-lying 4G7/2 and 4G5/2; 2G7/2 and low-lying 4IJ levels and its temperature dependence in the range of 13–295 K were measured. For 5 μm mid-IR transitions of Nd3+ the radiative relaxation rates are found to be several times higher and multiphonon relaxation (MR) rates are several times lower in lead and calcium thiogallate crystals compared to low phonon fluoride crystals. MR rate dependence on the Nd3+ to the nearest ligand distance R0 is established experimentally and analyzed in the frame of point-charge model of nonlinear theory of multiphonon relaxation. Mid-IR Nd3+ 4–5.5 μm fluorescence spectra of 4Ii–4Ij transitions perspective for laser oscillation were measured for the first time in the studied crystals.