J. M. Hvam

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).

Phase diagram of the Bose condensation of interwell excitons in GaAs/AlGaAs double quantum wells

The luminescence of interwell excitons in GaAs/AlGaAs double quantum wells (n-i-n heterostructures) with large-scale fluctuations of random potential in the heteroboundary planes was studied at low temperatures down to 0.5 K. The properties of excitons whose photoexcited electron and hole are spatially separated in the neighboring quantum wells by a tunneling barrier were studied as functions of density and temperature. The studies were performed within domains about one micron in size, which played the role of macroscopic traps for interwell excitons. For this purpose, the sample surface was coated with a metal mask containing special openings (windows) of a micron size or smaller. Both photoexcitation and observation of luminescence were performed through these windows by the fiber optic technique. At low pumping powers, the interwell excitons were strongly localized because of the residual charged impurities, and the corresponding photoluminescence line was nonuniformly broadened. As the laser excitation power increased, a narrow line due to delocalized excitons arose in a threshold-like manner, after which its intensity rapidly increased with growing pumping and the line itself narrowed (to a linewidth less than 1 meV) and shifted toward lower energies (by about 0.5 meV) in accordance with the filling of the lowest exciton state in the domain. An increase in temperature was accompanied by the disappearance of the line from the spectrum in a nonactivation manner. The phenomenon observed in the experiment was attributed to Bose-Einstein condensation in a quasi-two-dimensional system of interwell excitons. In the temperature interval studied (0.5–3.6) K, the critical exciton density and temperature were determined and a phase diagram outlining the exciton condensate region was constructed.

Collective state of interwell excitons in GaAs/AlGaAs double quantum wells under pulse resonance excitation

The time evolution and kinetics of photoluminescence (PL) spectra of interwell excitons in double GaAs/AlGaAs quantum wells (n-i-n structures) have been investigated under the pulse resonance excitation of intrawell 1sHH excitons using a pulsed tunable laser. It is found that the collective exciton phase arises with a time delay relative to the exciting pulse (several nanoseconds), which is due to density and temperature relaxation to the equilibrium values. The origination of the collective phase of interwell excitons is accompanied by a strong narrowing of the corresponding photoluminescence line (the line width is about 1.1 meV), a superlinear rise in its intensity, a long time in the change of the degree of circular polarization, a displacement of the PL spectrum toward lower energies (about 1.5 meV) in accordance with the filling of the lowest state with the exciton Bose condensate, and a significant increase in the radiative decay rate of the condensed phase. The collective exciton phase arises at temperatures T<6 K and interwell exciton densities n=3×1010 cm−2. Coherent properties of the collective phase of interwell excitons and experimental manifestations of this coherence are discussed.

Exciton-exciton collisions and conversion of interwell excitons in GaAs/AlGaAs superlattices

The ratio of the densities of intra-and interwell excitons in a symmetric system of coupled quantum wells — a superlattice based on a GaAs/AlGaAs heterostructure — is investigated over a wide range of optical excitation power densities. Conversion of interwell excitons into intrawell excitons as a result of exciton-exciton collisions is observed at high exciton densities. Direct evidence for such a conversion mechanism is the square-root dependence of the interwell exciton density on the optical excitation level. The decrease in the lifetime of interwell excitons with increasing excitation density, as measured directly by time-resolved spectroscopy methods, confirms the explanation proposed for the effect.

Dynamics of excitonic states in GaAs/AlGaAs quantum wells

The influence of photoexcited carriers on the dynamics of the absorption spectra of GaAs/AlxGa1−2x As multilayer quantum wells is investigated experimentally. It is found that at quasiparticle densities all the way up to 1011 cm−2 the saturation of the excitonic absorption is due to both a decrease of oscillator strength and broadening of the excitonic lines. It is shown that in the case of femtosecond resonance laser exci-tation the decrease of oscillator strength is due to free electron-hole pairs, while the broadening and energy shift of the excitonic lines are due to the exciton-exciton interaction. The lifetimes of free electron-hole pairs and excitons (≈65 ps and ≈410 ps, respectively) are determined from the exponential decrease of the change in the oscillator strength and in the width and energy position of the excitonic lines.

Long-time luminescence kinetics of localized excitons and conduction band edge smearing in ZnSe(1−c)Tec solid solutions

It is shown that the integrated luminescence intensity of localized excitons in solid solutions ZnSe(1−c)Tec has a component slowly decaying with time. After the excitation above the mobility threshold, the long-time intensity decreases exponentially, with a fractional exponent changing from a value corresponding to the critical index of anomalous diffusion to the index of normal diffusion as the temperature increases from 5 to 80 K. This change allows estimation of the energy scale for the fluctuation tail of the conduction band.

Room-temperature dephasing in InAs-GaAs quantum dots

Summary form only given. Semiconductor quantum dots (QDs) are receiving increasing attention for fundamental studies on zero-dimensional confinement and for device applications. Quantum-dot lasers are expected to show superior performances, like high material gain, low and temperature-independent threshold current and chirp-free operation, due to the delta-like density of states (DOS). We have measured the dephasing time at room temperature of InAs QDs embedded in a waveguide to estimate the lower limit for the energy-broadening of the DOS given by the homogeneous linewidth. The sample consists of 3 stacked layers of InAs-InGaAs-GaAs quantum dots.

Biexciton dephasing in a semiconductor microcavity

Summary form only given. The experimental observation of biexcitons in microcavities has been addressed recently. A well-resolved polariton-biexciton transition was observed in a high-quality GaAs single quantum well (QW) /spl lambda/-microcavity of 25 nm well width using a pump-probe experiment (Borri et al, Phys. Rev. B vol. 62, p. R7765, 2000). In this microcavity, the heavy-hole (HH) vacuum Rabi splitting is 3.6 meV, more than three times larger than the biexciton binding energy in the bare QW (1.1 meV). Due to the narrow linewidth of the polariton resonances, a well-resolved pump-induced optical absorption associated with biexcitons was observed. In this work, we investigate the coherent properties of the biexciton in the same structure using a two-beam four-wave mixing (FWM) experiment at 10 K.

Coherence of excitonic secondary emission quantified by time resolved speckle analysis

Summary form only given. Near band-edge optical properties in undoped semiconductor nanostructures at low temperatures are often dominated by excitonic processes. Present interest is focusing on the coherence properties of light emission after short-pulse excitation. Emission in a non-specular direction involves scattering processes, and the temporal emission dynamics allows to look into the corresponding microscopic mechanisms such as interface roughness, phonon emission, or exciton-exciton interaction. If static disorder dominates, the scattering is elastic (Rayleigh scattering) and preserves the coherence. The other extreme is incoherent luminescence which has lost all phase memory. A clear distinction between the two cases is not possible in experiments where the emission is collected within a wide detection angle. The key point for the novel method presented here is that the existence of speckles in a one-pulse experiment can be taken as an indicator for coherence in the emitted radiation.

Effect of the coherence of free electron-hole pairs on excitonic absorption in GaAs/AlGaAs superlattices

The effect of photoexcited free carriers on the absorption spectra dynamics of GaAs/AlxGa1−xAs superlattices is investigated experimentally by the pump-probe method. A sharp change in the shift of the excitonic resonance energy from the low-to the high-energy direction is found to occur at the moment that the electromagnetic radiation of the pump and probe beams overlap in the case of band-band excitation. This phenomenon is explained in a model of scattering of high-energy electron-hole pairs. The dephasing time of free high-energy particles is experimentally estimated to be several tens of femtoseconds.