ENHANCEMENT OF NONLINEAR-OPTICAL SUSCEPTIBILITY IN A SUPERLATTICE WITH RANDOM CONSTITUENT ASYMMETRIC MULTIQUANTUM WELLS

The optical third-order susceptibility of a superlattice consisting of randomly distributed asymmetric multi-quantum well units is theoretic ally studied. We carefully choose the randomness of thicknesses of the asymmetric multi-quantum well units to make sure that the spectrum ha s nearly equally separated energy minibands which give rise to the res onant enhancement of the third-order susceptibility owing to the multi -photon processes. Since the Bloch symmetry in the growth direction is partially violated due to the randomness, the inhibition of the trans itions between states with different Bloch vectors is removed. Thus, t he multi-photon processes are strengthened and the third-order suscept ibility is greatly enhanced. The absorption coefficient of the random superlattice is also enhanced, but the photoabsorption is mainly a one -order process, its increase is far less than the third-order suscepti bilities. Our calculation shows that, for the 150 layered superlattice s, the ratio of third-order susceptibility to the absorption coefficie nt of a random superlattice can be about twenty times larger than that of the periodic superlattice.