Nonlinear Optical Properties of Narrow-Gap Type-II Superlattices

We report here the first experimental and theoretical investigation of free carrier nonlinear optical processes in Type-II superlattices with narrow energy gaps. A general analysis of the effects of semiconductor band structure on nonlinear response at long wavelengths shows that the most favorable nonlinearities may be achievable in heterostructures with an indirect band alignment, where the indirectness may be in either real or momentum space.1 One of the most relevant figures of merit for devices is An/a, where An is the nonlinear modulation of the index of refraction and a is the absorption coefficient. However, An is limited in narrow-gap materials such as Hgo.78Cdo.22Te because the large third-order nonlinear susceptibilities (y(3)) observed at CO2 wavelengths tend to severely saturate at high intensities due to the dynamic Burstein shift.2