Large nonresonant light-induced refractive-index changes in thin films of amorphous arsenic sulfide

We have measured a light-intensity-dependent change in the refractive index of thin films of the amorphous semiconductor arsenic sulfide. In a novel type of nonlinearity, strongly absorbed light produces a relatively large change in the optical properties of the material at wavelengths where the absorption is weak. This represents a compromise between the large resonant nonlinearities found in crystalline semiconductors, which have the disadvantage of large signal attenuation, and the nonresonant nonlinearities, which occur at wavelengths with low absorption but are small.