Growth mechanism of Cr:forsterite laser crystal with high Cr concentration

Abstract High concentration Cr:forsterite (Cr:Mg2SiO4) laser crystals were grown out successfully by a medium-frequency induction-heating Czochralski method in an atmosphere containing oxygen. In the crystal, most of Cr ions appear as trivalent Cr3+ and a small part of Cr ions appear as tetravalent Cr4+. The doping of trivalent Cr3+ is mainly limited by its segregation coefficient. The formation of tetravalent Cr4+ is related not only to the initial Cr content in the melt and the oxygen partial pressure in the growth chamber, but also most probably to the concentration of an oxygen vacancy existing in the crystallized Cr:forsterite. Under this circumstance, those holes released from the recombination of the oxygen vacancies determine the oxidation from trivalent Cr3+ to tetravalent Cr4+ in the crystal. It was proved that an as-grown Cr:forsterite crystal with such an oxygen vacancy could possess much higher Cr4+ absorption intensity in the near-infrared wavelength range after an oxidation annealing.