Effects of spin--orbit interaction on optical properties of narrow-band semiconductor quantum wells

We study effects of strong spin--orbit interaction on optical properties of narrow--gap semiconductor quantum films. Electron states in such materials ($e.g. ${\$}Pb{\_}xSn{\_}{\{}1-x{\}}(S,Se,Te){\$}) are well described by the two--band Kane model with the Dirac--type effective Hamiltonian [1]. It may be shown that electron dispersion in a film with identical boundaries still keeps spin degeneracy. In the present work we consider two types of (asymmetric) films with broken mirror symmetry: $i)$ with nonequivalent boundaries, and \textit{ii}) with a linear spatial variation of the forbidden band in the direction of epitaxial growth. It was shown that in both cases there is a noticeable spin--splitting of size--quantized subbands, strongly dependent on parameters in boundary conditions on film's surfaces. Under external illumination this results in an asymmetric distribution of photoexcited carriers in \textbf{k}--space, and therefore, in spin--polarized photocurrents. We also show that measurement of optical absorption coefficient may give direct information about the film surface structure. [1] J. O. Dimmock, G. B. Wright, Phys. Rev. \textbf{135}, A821 (1964). This work is supported by the Indiana 21$^{st}$ Century Research and Technology Fund.