Three-photon absorption in Nd-doped yttrium aluminum garnet

Resonantly enhanced, three-photon absorption has been observed in Nd-doped yttrium aluminum garnet (Nd: YAG). This process probes the energy-level structure of the Nd $4f^{2}5d^1$ electron configuration. The measured maximum value of the three-photon absorption cross section is ${\sigma }^{\mathrm{}\left(3\right)\mathrm{}}=1\mathrm{}\times {10}^{-81\mathrm{}}$ ${\mathrm{cm}}^6$ ${\mathrm{s}}^2$/${\mathrm{photon}}^2$, which is in good agreement with the expected size of the effect.