Nonequilibrium absorption in semiconductors and the dynamical Franz-Keldysh effect

Abstract

We theoretically study free electron light absorption for a sample which is placed in a strong, time-dependent uniform electric field. In the case of static fields one observes the Franz-Keldysh effect: finite absorption for photon energies below the band gap. We refer to this phenomenon as the Franz-Keldysh tail. Modulation of the spectra above the band gap is also found. These static effects are observed in both 3D, 2D and 1D systems. Our analysis, based on nonequilibrium Green function techniques, shows that an analogous effect takes place in time-dependent fields: the dynamical Franz-Keldysh effect. Specifically, we relate the nonequilibrium absorption coefficient to the generalized density of states and analyze how it is affected by an external harmonically oscillating electric field. The dynamical Franz-Keldysh tail should be experimentally observable in the THz regime in 1D, 2D and 3D systems. Modulation above the band gap is weak in the 3D case but considerable fine structure is predicted for 1D and 2D systems. A characteristic feature of the dynamical Franz-Keldysh effect is that the absorption edge will be shifted up by an amount corresponding to the average kinetic energy of an free electron placed in the oscillating external electric field.