Étalon enhancement of nonlinear optical response in Bi1−xSbx

Measurements of the nondegenerate four-wave mixing of CO2 laser beams in a Bi1−xSbx film have yielded the largest high-power third-order nonlinear susceptibilities ever reported at that wavelength (χ(3)≳6×10−4 esu at P0≳2×105 W/cm2). Furthermore, an étalon effect resulting from the high reflectivity of the Bi1−xSbx films at both the air and substrate interfaces leads to an additional enhancement of the four-wave signal by as much as a factor of 30. A theoretical model based on optical modulation of the free-carrier susceptibility gives results which are in excellent agreement with the data.