Carrier excitations and nonlinear optical effects in semiconductor quantum wells.

摘要

Density matrix formulation is used to investigate the nonlinear optical properties of semiconductor quantum-well devices. The generation of terahertz electromagnetic pulses from ultrafast optical pulse excitations of semiconductor heterostructures is studied in the time domain with a three-level model. Coupled optical Bloch equations are derived with a rotating wave approximation and solved by the fifth-order Runge-Kutta method.;Two types of quantum-well structures, the asymmetrically coupled quantum wells and a single quantum well, are studied. An instantaneous radiation transient of the generated terahertz pulse follows the time evolution of the exciting laser pulse and is due to the optical rectification effect. Ringing oscillations at later times observed for both structures are attributed to the quantum beats of the electron oscillations between the coupled quantum wells or of the light-hole and heavy-hole intersubband transitions. Our theoretical results agree very well with the experimental data published by researchers at AT&T Bell Laboratories. Both the laser bandwidth and the exciton dephasing time are crucial in determining the waveform of the terahertz signals. This problem is also studied in the frequency domain with an exciton Green's function approach, which takes into account the Coulomb interactions between electrons and holes. The second-order nonlinear susceptibility is enhanced when the laser photon energy is resonant with a specific exciton level.;Selective control of the charge oscillations by coherent multipulse sequences is modeled by a second-order perturbation method. An analytical solution for the two-pulse excitation is derived with the delta-function assumption, and the modulation of the charge oscillations is dependent on the detuning energy, the pulse delay time, and the relative phase between the two pulses. Numerical results, taking into account the finite pulse widths and relaxation times, are also calculated from the optical Bloch equations and agree with those for the analytical solution.;The influence of many-body effects on intersubband excitations is studied with a new formulation. It was found that the depolarization and excitonic shifts are of comparable magnitude and must be taken into consideration to account for the experimental spin-density and charge-density excitation spectra.