Multiphoton Transitions in Ionic Crystals

Photoconductivity measurements in uncolored single crystals of NaCl, KCl, and KI are reported. The exciting source is the beam of a $Q$-switched ruby laser ($\ensuremath{\hbar}\ensuremath{\omega}=1.78$ eV) and its second harmonic ($\ensuremath{\hbar}\ensuremath{\omega}=3.56$ eV). The experimental results show clear evidence of multiphoton absorption processes. The nonlinear cross sections ${\ensuremath{\gamma}}_{n}$ of two-, three-, four-, and five-photon transitions have been measured and compared with the cross sections predicted by the existing theories.

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