New possibilities for Cr3+ ions as activators of the active media of solid-state lasers

An analysis is made of the relationship between the excited-state lifetime of chromium and the ratio of the luminescence intensities in the ⁴T₂ – ⁴A₂ and ²E – ⁴A₂ channels, on one hand, and the Cr³⁺ → acceptor energy transfer efficiency, on the other. A method is proposed for determination of the energy gap ΔE between the ²E and ⁴T₂ levels of Cr³⁺ from the temperature dependence of the Cr³⁺ → acceptor nonradiative transfer efficiency. Two independent methods were used to determine ΔE for gadolinium gallium garnet (GGG) crystals doped with chromium and with chromium and neodymium. The probabilities of Cr³⁺ intracenter relaxation and typical parameters of the Cr³⁺ – Nd³⁺ energy transfer process were also determined. It was found that the values ΔE in gadolinium scandium gallium (GdScGaG) and lanthanum lutetium gallium garnet crystals are comparable with the values of kT at liquid helium temperature. It is shown that the high efficiency of the Cr³⁺ → Nd³⁺ nonradiative transfer in GGG and GdScGaG crystals doped with chromium and neodymium at room temperature is due to the relatively low value of ΔE. It is noted that there are many crystals having the garnet structure and low values of ΔE that are potentially suitable for developing lasers utilizing the ⁴T₂ – ⁴A₂ electronic-vibrational transition in Cr³⁺ and having an emission frequency continuously tunable over a wide spectral range.