Sensitisation of the Tm3+-ion luminescence by Cr4+ ions in continuous 2-μm lasing in a YAG : Tm3+:Cr4+:Ca2+ crystal

Continuous lasing in the 2 μm wavelength range was achieved in a YAG : Tm³⁺:Cr⁴⁺:Ca²⁺ crystal as a result of a transition in the Tm³⁺ ion. The crystal was pumped longitudinally with radiation from a cw Nd laser at the wavelength of 1.06 μm, which was within the absorption band of the tetrahedrally coordinated Cr⁴⁺ ion. Quantum efficiency of the transfer process was estimated on the assumption that the excitation energy was transferred nonradiatively from the Cr⁴⁺ to the Tm³⁺ ions. The Cr⁴⁺ ions were used for the first time as the sensitisers of the luminescence of the active centres in a solid-state laser.