Spectral and luminescent properties of forsterite single crystals heavily doped with chromium: II. Luminescence

The polarised luminescence spectra of forsterite single crystals grown from a melt containing from 0.015 to 0.97 wt% of chromium are studied. Most of the crystals were grown by the Czochralski method under standard oxidation conditions (1.4–2.4 vol% of oxygen). Some crystals were grown in the atmosphere with a higher content of oxygen (~12 vol%) or in the neutral atmosphere (100% of Ar). The luminescence of Cr⁴⁺ ions excited to the ³T₂ level at 980 nm and to the ³T₁ level at 632.8 nm exhibited no quenching over the entire range of crystal doping. In crystals absorbing above 70% of laser energy, the self-absorption of luminescence of Cr⁴⁺ ions was observed. The reabsorption of luminescence of Cr³⁺(2) ions located in sites of the forsterite crystal with the mirror symmetry occurs stronger because of a greater overlap of the luminescence band of Cr³⁺(2) ions with the absorption bands of Cr⁴⁺ ions. The luminescence of Cr⁴⁺ ions is found to be polarised predominantly along the b axis for any polarisation of exciting radiation. The broadband luminescence of Cr³⁺(2) ions is polarised predominantly along the c axis for all the crystals studied, irrespective of the polarisation of exciting radiation and the oxygen concentration in the crystal-growth atmosphere. The luminescence of Cr³⁺(2) ions polarised along the c, b, and a axes has maxima at ~900, 860, and 850 nm, respectively.