Optical properties of undoped and Ho3+, Er3+, and Tm3+-doped BaAl2S4 and BaAl2Se4 single crystals

The optical energy band gaps of BaAl2S4 and BaAl2Se4 single crystals at 300 K were found to be 3.98 and 3.35 eV, respectively, and the optical energy band gaps of Ho3+, Er3+, and Tm3+-doped BaAl2S4 and BaAl2Se4 single crystals were smaller than those of the undoped single crystals. Photoluminescence spectra peaked at 459 and 601 nm in the BaAl2S4 and at 486 and 652 nm in the BaAl2Se4. The photoluminescence emission peaks were attributed to donor–acceptor pair recombinations. Photoluminescence spectra of the Ho3+, Er3+, and Tm3+-doped BaAl2S4 and BaAl2Se4 at 5 K were measured in the wavelength range of 400–900 nm. Sharp emission peaks due to Ho3+, Er3+, and Tm3+ ions were observed and their transition mechanisms were proposed.