Abstract

We report what we believe to be the first experimental determination of nonlinear optical coefficients due to carrier-generation processes in a narrow-gap IV–VI alloy, Pb_0.8Sn_0.2Se grown by hot-wall epitaxy, Third-order nonlinear susceptibilities χ⁽³⁾ were determined from nondegenerate four-wave mixing using a pair of grating-tuned synchronously Q-switched CO₂ lasers which generate ≈150-ns pulses. Growth of the films on BaF₂ substrates leads to significant Fabry-Perot enhancement of the nonlinear response. Since Pb_1−xSn_xSe has a large index of refraction (≈5.2)¹ while that of the BaF₂ is low, both the front surface and interface with the substrate have high reflectivities. For the 1-μm thick sample discussed below, the round-trip optical path length of 10.4 μm is close to the laser wavelength of 10.6 μm. Constructive interference of the internally reflected light then leads to a strong amplification of the electromagnetic fields in the effective cavity formed by the air and substrate interfaces. Calculation of the nonlinear response in an etalon was performed, showing that in the present example the observed four-wave signal is enhanced by over an order of magnitude by the presence of the etalon.

© 1990 Optical Society of America