Two-photon transition fromH43toS01ofPr3+in LaF3

A direct two-photon absorption between crystal-field components of the ${}_{}{}^3{}_{}{}^{}H_4^{}{}_{}{}^{}$ ground-state configuration to the highest excited state ${}_{}{}^1{}_{}{}^{}S_0^{}{}_{}{}^{}$ in the $4f^2$ manifold of ${\mathrm{Pr}}^{3+}$ in La${\mathrm{F}}_3$ has been observed. The magnitude of the intensities and the polarization dependence require a third- and fourth-order theoretical analysis, similar to that developed for two-photon absorption of ${\mathrm{Gd}}^{3+}$ in La${\mathrm{F}}_3$.