Electroreflectance study of linear electro-optic effect in the organic solid 2-methyl-4-nitroaniline

Electroreflectance spectra have been measured on organic nonlinear optical crystal MNA (2-methyl-4-nitroaniline) over the exciton absorption region. From the observed electric-field-modulated reflectivities, the figure of merit (change of refractive index) and its dispersion were numerically evaluated up to the excitonic absorption bands. The large electro-optic effect in the transparent region is shown to be due to the field-induced energy shift of intramolecular charge-transfer excitations at {~}2.7 eV.