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The first moments of a series of Kramers-Kronig-transformed experimental electro-reflectance spectra are calculated in order to estimate critical-point contributions to the second- and third-order nonlinear optical susceptibilities, χ⁽²⁾ and χ⁽³⁾. Analysis of data for Ge and GaAs provides the first direct experimental evidence that the electronic resonant processes responsible for electroreflectance contribute negligibly to χ⁽²⁾, but significantly affect the value of χ⁽³⁾.
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D. E. Aspnes (1971) studied this question.
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