Two-photon excitation spectra of exciton luminescence in CaF2 obtained by using synchrotron radiation and laser

Charge centers inCaF2:Ab initiocalculation of elementary physical properties

“…Even in an ideal material, where no point defects are present, electromagnetic radiation can be absorbed with a small but finite probability via a two photon process 5,6 . Such an absorption process will therefore quadratically increase with increasing radiation intensity and becomes of strong importance with increasing radiation intensity.…”