Harmonic Generations and Sum-Frequency Spectroscopy

The centrosymmetry may, however, be broken by the application of an applied dc electric field such symmetry can also be broken on a surface either as a freely suspended film or by a surface aligrunent modification technique that induces the flexoelectric effect. Sukhov and Timashev have shown that the centrosymmetry can also be broken optically. As explained in Chapter 11, the main obstacle in getting efficient harmonic generation is the phase matching of the fundamental and second-harmonic wave vectors. 

Another possibiUty to break the centtosymmetry of hqirid crystal is to make use of the fiexoelecttic effect. There have been research earlier on second-harmonic generation in 

NONLINEAR OPTICAL PHENOMENA OBSERVED IN LIQUID CRYSTALS 

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