Resonant enhancement of direct two-photon absorption inTb3+:LiYF4

Abstract

Resonant enhancement by a factor of over 20 of direct two-photon absorption from the ground state ${}_{}{}^7{}_{}{}^{}\mathrm{F}_6^{}{}_{}{}^{}$ to the excited state ${}_{}{}^5{}_{}{}^{}\mathrm{G}_6^{}{}_{}{}^{}$ of the 4$f^8$ configuration of the ${\mathrm{Tb}}^{3+}$ ion at 40 200 ${\mathrm{cm}}^{\mathrm{-}1}$ has been observed in time-resolved experiments with two separate pulsed lasers. The results provide clear evidence for resonant enhancement of two-photon absorption in rare-earth compounds and imply a similar enhancement for Raman scattering and nonlinear-optical mixing. Two separate contributions to the direct transition moment were observed. When a single laser frequency was used, the intermediate states which made the largest contribution were from excited configurations of opposite parity; those intermediate states were far from resonance. Detailed two-frequency experiments showed, however, that near a single photon ${}_{}{}^7{}_{}{}^{}\mathrm{F}_6^{}{}_{}{}^{}$ ${\mathrm{-}}^5$ ${\mathrm{D}}_4$ resonance, a much stronger contribution arose from the 4$f^8$ configuration ${}_{}{}^5{}_{}{}^{}\mathrm{D}_4^{}{}_{}{}^{}$ intermediate state. Two-photon absorption was clearly isolated from two-step (incoherent) excitation by varying the timing of two separately triggered lasers.