Abstract
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.
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