Resonant enhancement effect on two-photon absorption due to excitons in alkaline-earth fluorides excited with synchrotron radiation and laser light

Exciton states in wide-gap materials ${\mathrm{CaF}}_2$ and ${\mathrm{BaF}}_2$ have been studied with the technique of two-photon spectroscopy using synchrotron radiation and laser. The two-photon excitation spectrum of ${\mathrm{CaF}}_2,$ and that of ${\mathrm{BaF}}_2$ as well, exhibited a clear peak on the high-energy side of the $1S$ exciton energy. The peaks are attributed to the $2P$ states of the valence excitons. According to this assignment, the binding energies of the excitons have been determined to be 0.8 and 0.9 eV for ${\mathrm{CaF}}_2$ and ${\mathrm{BaF}}_2,$ respectively. The signal intensities were larger than that in the case of one-beam excitation by a factor of about 500, which is attributed to a resonant effect on two-photon absorption.