Two-photon excitation of the lowest $4f\to 5d$ near-ultraviolet transition in ${\mathrm{Ce}}^{3+}$:Ca${\mathrm{F}}_2$

The two-photon transition from the $4f$ ground state to the lowest ${\mathrm{Ce}}^{3+}5d$ state in ${\mathrm{Ce}}^{3+}$:Ca${\mathrm{F}}_2$ has been studied with a tunable dye laser as a function of laser wavelength, intensity, polarization, and focusing geometry. At liquid-helium temperatures, a no-phonon resonance line is observed and is accompanied by its vibrational sideband. The peak cross section for the two-photon absorption is of order 5×${10}^{-54\mathrm{}}$ ${\mathrm{cm}}^4$ sec, indicating significant crystal-field parity mixing for the $5d$ levels. The nophonon line shows a polarization anisotropy consistent with a ${\mathrm{Ce}}^{3+}$ site of $C_{4v}$ symmetry. The phonon sideband has a frequency-dependent polarization anisotropy.