Th. Östreich

THEORY OF EXCITON-EXCITON CORRELATION IN NONLINEAR OPTICAL RESPONSE

We present a systematic theory of Coulomb interaction effects in the nonlinear optical processes in semiconductors using a perturbation series in the exciting laser field. The third-order dynamical response consists of a phase-space filling correction, a mean-field exciton-exciton interaction, and two-exciton correlation effects expressed as a force-force correlation function. The theory provides a unified description of effects of bound and unbound biexcitons, including memory effects beyond the Markovian approximation. In the degenerate four-wave-mixing experiments, correlation effects are shown leading to polarization mixing, ringing, etc. The strong interaction, a nonperturbative theory of the correlation function, is numerically evaluated for a one-dimensional model. Approximations for the correlation function are presented.

Collective Oscillations Driven by Correlation in the Nonlinear Optical Regime

We present an analytical and numerical study of the coherent exciton polarization including exciton-exciton correlation. The time evolution after excitation with ultrashort optical pulses can be divided into a slowly varying polarization component and novel ultrafast collective modes. The frequency and damping of the collective modes are determined by the high-frequency properties of the retarded two-exciton correlation function, which includes Coulomb effects beyond the mean-field approximation. The overall time evolution depends on the low-frequency spectral behavior. The collective mode, well separated from the slower coherent density evolution, manifests itself in the coherent emission of a resonantly excited excitonic system, as demonstrated numerically. {copyright} {ital 1999} {ital The American Physical Society}

Exciton interaction and spin dynamics in semiconductor heterostructures

Abstract A consistent theory of exciton interaction which affects the coherent exciton spin dynamics and its dephasing is constructed. For the low-density exciton regime, the nonlinear equation for the exciton polarization adds to the Gross–Pitaevskii equation spin-dependent correlation terms, permitting an examination of the correlation effects on the optically excited spin dynamics. We apply the theory to investigate the collective spin oscillations under optical excitation.

Numerical study of the a.c.-Stark shift in quasi one-dimensional ring clusters

We investigate the excitonic a.c.-Stark shift in a chain of atoms on a ring with periodic boundary conditions and modified Coulomb type potential. The exact low excitation spectra can be calculated using the Lanczos algorithm. The results for non-resonant excitation are compared with perturbation theory and the existing mean-field approach. In the subspace of two electron-hole pairs with zero total momentum the model shows no formation of a bound excitonic molecule (biexciton) for arbitrary cluster size.

Spin coherence in semiconductor heterostructures

Abstract The addition of the spin degree of freedom to the nonlinear optical processes creates interesting possibilities for spin optoelectronics. In this theory paper, we attempt to explain in simple terms the concepts of optical and spin coherence, how the simultaneous occurrence of both types of coherence can be brought about by suitable arrangement of the magnetic field and polarization of exciting coherent light, and how the time development is governed by the nonlinear response of the system. We explain under what circumstances the single-particle effects dominate and under what circumstances the interaction effects between two excitons come to the fore. We relate to experiments by a theoretical formulation of how these effects are seen in a particular nonlinear optical experiment, namely the time-resolved Faraday rotation spectroscopy.

Non-stationary excitonic-insulator states in photoexcited semiconductors

Time dependent solutions of the semiconductor Bloch equations (SBE) are presented for zero external field which can be identified with a possible dynamics of the so called excitonic-insulator state (EIS) of a semiconductor after the laser pulse has switched off. The collective oscillation of the macroscopic polarization locks into a definite frequency depending on the energy absorbed during the interaction time with the ultra short laser pulse. This dynamical state is different from a stationary EIS proposed previously to be the final state for detunings above the exciton instability and will be demonstrated to appear already for off-resonant excitation. Numerical results are presented for a quasi 1d-model with long-range Coulomb interaction.