Two-photon-absorption cross section ofNd3+in yttrium aluminum garnet and yttrium lithium fluoride near 1.06μm

We have measured the spectrally integrated two-photon-absorption (TPA) cross sections for the ${}_{}{}^4{}_{}{}^{}\mathrm{I}_{9/2}^{}{}_{}{}^{}$→${\mathrm{}}^4$ ${\mathrm{G}}_{7/2}$ transitions of ${\mathrm{Nd}}^{3+}$ and obtained values of 1.2×${10}^{\mathrm{-}40}$ and 0.15×${10}^{\mathrm{-}40}$ ${\mathrm{cm}}^4$ for ${\mathrm{Nd}}^{3+}$-doped yttrium aluminum garnet (YAG) and yttrium lithium fluoride (YLF), respectively. These results are in satisfactory agreement with theoretical calculations based on the properties of ${\mathrm{Nd}}^{3+}$ free-ion wave functions. The difference between YAG and YLF, however, is not accounted for by the free-ion theory and suggests that the intermediate-state energies and wave functions are considerably host dependent. In addition, we conclude, based on our measurements, that rare-earth TPA will not contribute significantly to either losses or the nonlinear refractive index in typical laser media employing rare-earth ions.