Two-photon excitation of the lowest 4${\mathit{f}}^2$→4f5d near-ultraviolet transitions in ${\mathrm{Pr}}^{3+}$:${\mathrm{Y}}_3$${\mathrm{Al}}_5$${\mathrm{O}}_{12}$

Two-photon transitions from the ${}_{}{}^3{}_{}{}^{}\mathrm{H}_4^{}{}_{}{}^{}$ ground-state manifold of the 4${\mathit{f}}^2$ configuration to the lowest state of the 4f5d configuration of ${\mathrm{Pr}}^{3+}$ in ${\mathrm{Y}}_3$ ${\mathrm{Al}}_5$ ${\mathrm{O}}_{12}$ have been studied with a tunable dye laser as a function of the excitation wavelength, intensity, temperature, and focusing geometry. The two-photon excitation spectrum is vibronic in nature and covers the 555–615-nm excitation wavelength range. Both the direct nonresonant and the resonantly enhanced transitions are observed. The measured two-photon-absorption cross section varies over 3 orders of magnitude with a maximum of 1×${10}^{\mathrm{-}51}$ ${\mathrm{cm}}^4$ s at the peak of the resonantly enhanced transition at 581 nm. The parity selection rule which forbids these two-photon transitions in a free ion is relaxed by odd-parity crystal-field mixing of the wave functions, making these transitions possible. Two-photon excitation measurements are complemented by one-photon-absorption and -emission measurements.