Energy spectrum, density of states and optical transitions in strongly biased narrow-gap quantum wells

We study theoretically the effect of an electric field on the electron states and far-infrared optical properties in narrow-gap lead salt quantum wells. The electron states are described by a two-band Hamiltonian. An application of a strong electric field across the well allows the control of the energy gap between the two-dimensional (2D) states in a wide range. A sufficiently strong electric field transforms the narrow-gap quantum well to a nearly gapless 2D system, whose electron energy spectrum is described by linear dispersion relations εσ(k) ∼ ±(k − kσ), where kσ are the field-dependent 2D momenta corresponding to the minimum energy gaps for the states with spin numbers σ. Due to the field-induced shift of the 2D subband extrema away from k = 0 the density of states has inverse-square-root divergencies at the edges. This property may result in a considerable increase of the magnitude of the optical absorption and in the efficiency of the electrooptical effect. PACS: 78.66.-w, 73.20.Dx, 78.20.Jq 1