V. G. Lyssenko

Binding of quasi two-dimensional biexcitons

Summary form only given. In this presentation we report on a determination of the biexciton binding energies in GaAs-AlGaAs quantum wells of different widths and the results of a theoretical calculation of the ratio of the biexciton binding energy to that of the exciton. We determine the binding energies using spectrally resolved transient four-wave mixing (TFWM) and photoluminescence (PL).

InGaAs/GaAs?quantum-dot?quantum-well heterostructure formed by submonolayer deposition

Discrete emission lines from self-assembled InGaAs quantum dots (QDs) grown in the submonolayer (SML) deposition mode have been observed in micro-photoluminescence (PL) spectra at 10 K. For the first time, the SML-grown InGaAs/GaAs QD heterostructure is verified to be a quantum-dot–quantum-well (QDQW) structure, by using high power PL and selective PL with excitation energies below the band gap of the GaAs barriers and temperature dependent PL. As the temperature is increased from 10 to 300 K, a narrowing of the full width at half-maximum at intermediate temperatures and a sigmoidal behaviour of the peak energy of PL band of the SML QD ensemble are observed and attributed to thermally activated carrier transfer between QDs via QW states.

Ultrafast nonlinear optics in GaAs/AlGaAs quantum wells

By degenerate four-wave mixing experiments in a two-beam geometry, we have investigated the initial coherence and dephasing of quasi two-dimensional excitions and biexcitons in GaAs multiple quantum wells. The dephasing has contributions from phonon scattering, interface-roughness scattering and exciton-exciton scattering. Inhomogeneous broadening and generation of coherent wavepackets play a significant role in the coherent exciton dynamics. The incoherent exciton dynamics, diffusion and recombination, is studied by three-beam transient grating experiments. A significant difference in the interface roughness scattering of coherent and incoherent (thermalized) excitons is observed.

Nonlinear Response of Localized Excitons: Effects of the Excitation‐Induced Dephasing

The polarization dependence of the ultrafast nonlinear response of localized excitons in GaAs/AlGaAs quantum wells has been investigated by spectrally resolved, transient four-wave mixing. The role of the excitation-induced dephasing in the delay dependence of the signal is discussed in detail. In particular, we show experimentally that the excitation-induced dephasing gives rise to a photon echo in inhomogeneous systems. We develop a simple analytical model for the delay dependence of the signal, which enables us to deduce the dephasing time from the four-wave mixing decay.

Interwell excitons in GaAs multiple quantum wells and superlattices

SummaryThe optical properties of multiple quantum wells in the transition from isolated wells to a superlattice are investigated theoretically and experimentally. For superlattices with a miniband width similar to the binding energy of the 2s exciton of the isolated quantum well we find an exciton state energetically between the 1s exciton state and the onset of the miniband absorption. We find this state to be an interwell exciton, with the electron confined to one well and the hole to the neighboring well. The interwell exciton resembles the first Wannier-Stark localized exciton of a biased superlattice. However, the localization in the present case is mediated by the Coulomb interaction of the electron and hole. The state has considerable oscillator strength and is observed experimentally in linear and nonlinear experiments.

Ultrafast exciton dynamics in semiconductors: Effects of disorder and confinement

The dynamics of excitonic transitions in semiconductors have been investigated by degenerate four-wave mixing experiments. We have studied the coherence, interference and dephasing of free, bound and localized excitons in bulk semiconductors and of quasi-two-dimensional excitons in quantum well structures. The influence of inhomogeneous broadening is investigated and compared with quantum interference in a continuum of states. The nature of four-wave mixing beats in a system of bound excitons and biexcitons is discussed.

Optical coherent control in semiconductors

Summary form only given. The developments with coherent control (CC) techniques in optical spectroscopy have recently demonstrated population control and coherence manipulations when the induced optical phase is explored with phase-locked laser pulses. These and other developments have been guiding the new research field of quantum control including the recent applications to semiconductors and nanostructures. We study the influence of inhomogeneous broadening in semiconductors on CC results. Photoluminescence (PL) and the coherent emission in four-wave mixing (FWM) is recorded after resonant excitation with phase-locked laser pulses. The sample is a narrow GaAs single quantum well.

Continuum Contribution to Excitonic Four-Wave Mixing

Summary form only given. We present an experimental and theoretical investigation of ultrafast transient four-wave mixing (TFWM) of GaAs-AlGaAs quantum wells for simultaneous excitation of exciton and continuum states. Recent TFWM experiments on semiconductors have shown unexpected results when both discrete exciton states and continuum states have been excited simultaneously by ultrafast laser pulses. The decay of the signal, as a function of delay, is significantly faster than the dephasing time of the excitons.Summary form only given. We present an experimental and theoretical investigation of ultrafast transient four-wave mixing (TFWM) of GaAs-AlGaAs quantum wells for simultaneous excitation of exciton and continuum states. Recent TFWM experiments on semiconductors have shown unexpected results when both discrete exciton states and continuum states have been excited simultaneously by ultrafast laser pulses. The decay of the signal, as a function of delay, is significantly faster than the dephasing time of the excitons.