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

Abstract

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.