Picosecond study of charge-carrier-induced nonlinear refraction in intermediate band-gap semiconductors

Abstract

Free carriers generated by absorption of light in semiconductors change the complex dielectric constant and give rise to irradiance-dependent refraction and absorption. In addition to the refractive-index change arising from the low frequency dipole moment induced by the charge carriers (the Drude contribution), there can be contributions to the refractive-index change arising from the band-gap resonant saturation of the conduction band states. The amount of this additional contribution has been theoretically predicted.1 Verification of this prediction for two-photon absorption-generated charge carriers needs accurate determination of the two-photon absorption coefficient (β) of the semiconductors as well as careful measurement of the induced irradiance-dependent refractive index. We present a study of this nonlinear refraction; the method of measurement is external self-action. From the changes in the near-field shapes of spatially and temporally Gaussian picosecond laser beams incident on the semiconductor samples, measured as functions of the incident irradiance and pulse width, the total induced nonlinearity is estimated. Both 1.06- and 0.53-μm radiation are used to study two-photon absorption in a variety of semiconductors. This knowledge of the relative roles of different mechanisms contributing to the nonlinearity is not only of theoretical interest but also is important for practical applications in optical devices.