Multiphoton Absorption and OpticalHarmonic Generation in Highly Absorbing Molecular Crystals
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abstract
In crystals of the appropriate symmetry, it is to be expected that multiphoton absorption is accompanied by optical-harmonic generation. A simple relationship between the cross sections of the two processes has been derived for molecular crystals which are essentially transparent to the incident (laser) frequency but are highly absorbing at the multiphoton frequency. An additional process of interest, namely excitation of the crystal by direct absorption of the optical-harmonic photons is found to be of appreciable magnitude for crystals with the appropriate symmetry. The use of these relationships to provide information concerning the space-group symmetry of the crystal and the nature of the intermediate states is discussed. In the case of two-photon absorption and second-harmonic generation, an order-of-magnitude agreement between theory and experiment has been established.
