Normal-incidence linear and nonlinear infrared optical response ofL-valley quantum-well structures

We present full-scale microscopic band structure calculations of the linear and nonlinear optical response of L-valley ${\mathrm{Ga}}_{0.7}$ ${\mathrm{Al}}_{0.3}$Sb-AlSb and ${\mathrm{Ga}}_{0.7}$ ${\mathrm{Al}}_{0.3}$Sb-${\mathrm{Ga}}_{0.4}$ ${\mathrm{Al}}_{0.6}$Sb superlattices. We report the magnitude and spectral form of the first- and second-order optical susceptibilities. A maximum normal-incidence linear absorption coefficient of 2180 ${\mathrm{cm}}^{\mathrm{-}1}$, with a linewidth of 15 meV, is obtained at 10 μm. In addition, we predict that a large second-harmonic response can be induced by normally incident radiation. An account is given of the dependence of the response on the carrier concentration, and on microscopic features in the superlattice band structure. The significant enhancement of the normal-incidence response compared to other miniband absorption spectra reported in the literature is due to a combined effect of the large density of states, dispersion, and the degree of confinement.