Two-Photon Absorption as a New Test of the Judd-Ofelt Theory

Observation by fluorescence detection of direct two-photon absorption within the $4f$ configuration of the ${\mathrm{Gd}}^{3+}$ ion following excitation with a continuous-wave laser is reported. Quantitative intensity measurements test assumptions behind the Judd-Ofelt theory of lanthanide transition intensities in a new way, and significant discrepancies are found. The discrepancies indicate that the radial integrals $〈4f\left|\mathcal{r}\right|5d〉$ vary markedly for different $J$ multiplets, and that ligands significantly influence two-photon intensities.