High-pressure laser spectroscopy of ${\mathrm{Cr}}^{3+}$:${\mathrm{Gd}}_3$${\mathrm{Sc}}_2$${\mathrm{Ga}}_3$${\mathrm{O}}_{12}$ and ${\mathrm{Cr}}^{3+}$:${\mathrm{Gd}}_3$${\mathrm{Ga}}_5$${\mathrm{O}}_{12}$

We report on the effect of high pressure on the room-temperature emission spectra and lifetimes of ${\mathrm{Cr}}^{3+}$:GSGG (${\mathrm{Gd}}_3$${\mathrm{Sc}}_2$${\mathrm{Ga}}_3$${\mathrm{O}}_{12}$) and ${\mathrm{Cr}}^{3+}$:GGG (${\mathrm{Gd}}_3$${\mathrm{Ga}}_5$${\mathrm{O}}_{12}$). In both systems we observed a dramatic change of the overall emission band shape upon increasing pressure, from a nearly structureless broadband ${(}^4$${\mathit{T}}_2$${\to }^4$${\mathit{A}}_2$) to a highly structured narrow band ${(}^2$E${\to }^4$${\mathit{A}}_2$). From the peak energy of the broadband emission, we estimated the pressure-induced blueshift of the ${}_{}{}^4{}_{}{}^{}$${\mathit{T}}_2$${\to }^4$${\mathit{A}}_2$ transition to be 10 (±2) ${\mathrm{cm}}^{\mathrm{-}1}$/kbar. High-resolution measurements in the R-line region (∼700 nm) revealed that the ${}_{}{}^2{}_{}{}^{}$E${\to }^4$${\mathit{A}}_2$ transition hardly shifts at low pressures (<40 kbar), whereas at higher pressures (>60 kbar) a nearly linear redshift of 0.65 (±0.05) ${\mathrm{cm}}^{\mathrm{-}1}$/kbar is observed. Besides pressure-induced spectral changes, an enormous increase in the emission lifetime with increasing pressure was found for both systems. In the case of ${\mathrm{Cr}}^{3+}$:GSGG, the lifetime changed from 110 μs at ambient pressure to 4.4 ms at 125 kbar. For ${\mathrm{Cr}}^{3+}$:GGG, the lifetime increased from 168 μs to 7.3 ms for the same pressure range. The pressure-induced spectral and lifetime changes are described by a single configurational coordinate model that considers the effect of pressure on the thermal and spin-orbit coupling of the ${}_{}{}^2{}_{}{}^{}$E and ${}_{}{}^4{}_{}{}^{}$${\mathit{T}}_2$ states. A previously reported pressure-induced R-line-shift reversal in ${\mathrm{Cr}}^{3+}$:GSGG and the effect of high pressure on the lifetime in ${\mathrm{Cr}}^{3+}$:YAG are also discussed within the same framework.