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The theories of three non-linear optical processes are given: (i) the creation of excitons by simultaneous absorption of two photons of different frequency; the particular case of Cu2O is treated in detail; (ii) the generation of second-harmonic radiation from a monochromatic primary beam in a crystal without a centre of inversion symmetry; the case of quartz is treated as an example; (3) the generation of third-harmonic radiation from a monochromatic primary beam in a crystal having a centre of inversion.
R. Loudon (Mon,) studied this question.
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