Optical properties of α-ZnAl2S4:Cr single crystals

The optical properties of the chromium doped spinel type semiconductor alpha-ZnAl2S4 have been examined over the temperature range 2-540 K. The intrinsic photoluminescence (PL) showed an intense ultraviolet peak at 381 nm at 296 K. From extrinsic absorption and emission spectra the transitions between the ground state (4)A(2g) and the excited levels E-2(g), T-2(1g), T-2(2g), T-4(2g), and T-4(1g) of Cr3+ ions were observed. A fit of the measured temperature dependence of the PL decay, using a four level model, yielded the lifetimes of the E-2(g),T- 2(1g), and T-4(2g) levels and the energy gaps between the E-2(g)-T-2(1g) and E-2(g)-T-4(2g) states which were found to be very close to the values obtained from steady-state measurements. A configurational coordinate diagram for Cr3+ ions in a alpha-ZnAl2S4: Cr spinel host has been constructed. Optical gain measurements at 198, 298, and 380 K displayed two separate spectral regions associated to the Cr3+ ions and possibly Cr4+ impurity centers. Gain values of up to 25 cm(-1) at 198 K were obtained. (C) 1998 American Institute of Physics.