Effects of Current Flow on the Optical Absorption of a Semiconductor

The effect of electric-field-induced current flow on the direct optical absorption in a semiconductor is calculated. The calculations are performed specifically for InSb, under the assumption that the only effect of the electric field is to shift the distribution of carriers in k space; however, the method may be applied to any semiconductor for which the electron wave functions have a sufficiently simple analytic form, and the substitution of other angularly dependent distribution functions would not alter the interesting characteristics of the results. A change in the absorption is found which is dependent on the photon energy and varies approximately as the square of the electric field. It is also found that the absorption depends on the relative orientation of the dc electric field and the polarization vector of the light. This polarization effect is, in turn, found to vary as the square of the carrier drift velocity, and its magnitude and sign are found to depend on the relative amounts of $X$ and $Z$ components in the atomic $p$ orbitals in the carrier wave functions. Curves showing the results of calculations of the absorption change and the polarization effect for both fundamental and inter-valence-band absorption are presented. The possibility of measuring current-flow effects and the extraction from such measurements of information concerning the band structure and distribution functions of semiconductors are discussed. A comparison is made between the effect considered here and a similar effect studied experimentally by Bray and Pinson in $p$-type germanium.