Picosecond beats in coherent optical spectra of semiconductor heterostructures: photonic Bloch and exciton--polariton oscillations

摘要

Propagation of short pulses of light in multilayer semiconductor structures containing excitons is modelled by solving time-dependent Maxwell equations with the use of the scattering state technique. This approach allows this problem to be reduced to finding the eigenstates (scattering states) of the system, subject to the stationary Maxwell equations with appropriate boundary conditions. Then, the time-dependent electric field in the system can be found by Fourier integration of the scattering states. Application of this technique to the problem of light-propagation in laterally confined Bragg mirrors reveals the effect of photonic Bloch oscillations, i.e., oscillations of photons between two inclined mini--gaps of the optical superlattice, analogous to the well-known electronic Bloch oscillations. The scattering state technique is applied to the problem of propagation of exciton-polaritons in semiconductor films. The pulsed excitation induces a grating of dielectric polarization in the direction of propagation of light, which arises through interference between two exciton--polariton branches. The grating evolves backwards relative to the light propagation direction because of the multiple re-emission and re--absorption of photons by excitons. Inhomogeneous broadening of excitons exerts a dramatic influence on the time-resolved coherent optical spectra of semiconductor structures through the vertical motional narrowing effect in bulk crystals and multiple quantum wells (MQWs). The polariton interference governs the resonant Rayleigh scattering (RRS) spectra of the MQWs. In particular, a drastic difference between the RRS spectra of Bragg-arranged and anti-Bragg-arranged MQWs is predicted.