Abstract

Direct-gap semiconductors show very large increases in the two-photon absorption (2PA) coefficient, α₂, and in the nonlinear refractive index, n₂, when the photon energies become greatly different. For example, the 2PA coefficient of ZnSe increases 270× over that for the degenerate α₂ for photon energies having a ratio of ~10. This is the case where the sum of the photon energies in the two cases are equal, i.e., average photon energies equal. These nonlinear coefficients are measured using standard pump-probe techniques. Additionally, the n₂ for ZnSe increases by ~10× for a similar photon energy ratio. We measure this enhanced nonlinear refraction using a new beam-deflection method which will be discussed. We will show results for several semiconductors along with their modeling including both frequency degenerate and nondegenerate nonlinearities. These enhancements are in good agreement with our theoretical predictions and allow for IR detection using wide-gap semiconductors, 2-photon 3D LIDAR imaging, possible enhancement of all-optical switching, and the possibility of a 2-photon laser. This latter potential comes from the fact that 2-photon stimulated emission is the inverse process of 2PA.

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