Optical studies of localized excitons in symmetric coupled quantum wells

We present evidence for coupling between spatially separated excitonic states in a GaAs symmetric coupled quantum well. A split excitonic peak is observed in the photoluminescence spectrum of our sample and is identified as due to islands of monolayer fluctuation in the well width. We use the technique of far-infrared modulated photoluminescence to show that a resonant coupling exists between the excitonic states when the incident far-infrared photon energy is approximately equal to the splitting as measured in the photoluminescence spectrum. The most likely mechanism for this coupling is the dipole–dipole interaction.     

Subhertz spectral fluctuations from high-density excitons in coupled quantum wells

In 1994 Butov et al. [Phys. Rev. Lett. 73, 304 (1994)] reported large, extremely low-frequency fluctuations in the luminescence intensity from indirect excitons in a coupled quantum well. We have reproduced these fluctuations in a similar structure. Our observations indicate, however, that the fluctuations are spectral fluctuations rather than intensity fluctuations. We have eliminated several possible causes for the fluctuations and propose a possible cause.     

Dimensional effects and magnetic field influence on excitons in coupled quantum dots and coupled quantum wells

Direct and indirect excitons in coupled quantum wells and in coupled quantum dots are studied. We consider excitons with two-dimensional, quasi-two-dimensional and three-dimensional carriers. Problems were investigated for a wide range of characteristic parameters—confining to potential steepness, distances between quantum wells or dots, effective width of wells and magnetic fields. The mutual influence of the controlling parameters of the problem on exciton properties is analyzed. Energy and wave function spectra were calculated and dispersion law and effective masses were obtained.     

Optically induced phase transition of excitons in coupled quantum dots

The weak classical light excitations in many semiconductor quantum dots have been chosen as important solid- state quantum systems for processing quantum information and implementing quantum computing. For strong classical light we predict theoretically a novel phase transition as a function of magnitude of this classical light from the deformed to the normal phases in resonance ease, and the essential features of criticality such as the scaling behaviour, critical exponent and universality are also present in this paper.     

Charged excitons in GaAs quantum wells

Il Nuovo Cimento D - We implement optical spectroscopy to study charged excitons (trions) in modulation-doped GaAs/AlGaAs quantum wells. We observe for the first time several new trions: the...     

Stimulated scattering of indirect excitons in coupled quantum wells: signature of a degenerate Bose-gas of excitons.

We observe and analyze strongly nonlinear photoluminescence kinetics of indirect excitons in GaAs/AlGaAs coupled quantum wells at low bath temperatures, > or = 50 mK. The long recombination lifetime of indirect excitons promotes accumulation of these Bose particles in the lowest energy states and allows the photoexcited excitons to cool down to temperatures where the dilute 2D gas of indirect excitons becomes statistically degenerate. Our main result--a strong enhancement of the exciton scattering rate to the low-energy states with increasing concentration of the indirect excitons--reveals bosonic stimulation of exciton scattering, which is a signature of a degenerate Bose-gas of excitons.     

Phase transitions of indirect excitons in coupled quantum wells: The role of disorder

The theory of what happens to a superfluid in a random field, known as the “dirty boson” problem, directly relates to a real experimental system presently under study by several groups, namely excitons in coupled semiconductor quantum wells. We consider the case of bosons in two dimensions in a random field, when the random field can be large compared to the repulsive exciton–exciton interaction energy, but is small compared to the exciton binding energy. The interaction between excitons is taken into account in the ladder approximation. The coherent potential approximation (CPA) allows us to derive the exciton Green's function for a wide range of the random field strength, and in the weak-scattering limit CPA results in the second-order Born approximation. For quasi-two-dimensional excitonic systems, the density of the superfluid component and the Kosterlitz–Thouless temperature of the superfluid phase transition are obtained, and are found to decrease as the random field increases.     

A diffusion anomaly in dipole-oriented two-dimensional excitons in GaAs/AlGaAs coupled quantum wells

Il Nuovo Cimento D - We have observed an anomaly in the low-temperature photoluminescence of dipole-oriented long-lived excitons in a coupled quantum well under an electrical bias. A discrepancy...     

Bound excitons in p-doped GaAs quantum wells

Photoluminescence attributed to excitons bound to neutral impurities has been observed from GaAs quantum wells in Al_xGa_1?xAs-GaAs heterostructures grown by molecular beam epitaxy. The quantum wells were either doped with [Be] ≈ 10¹⁷ cm^-3 or Zn-diffused. At low temperatures both single and multiple quantum wells exhibited this extrinsic luminescence which is ascribed to the radiative recombination of the n=1 ground state heavy hole exciton E_1h bound to a neutral acceptor A^o. The dissociation energy E_D of the A^o-E_1h complex is obtained directly from the measured separation of this extrinsic peak from the intrinsic E_1h free exciton peak. For 46Å wide GaAs wells, E_D=6.5meV and E_D decreases with increasing well width.     

Maxwell relaxation of excitons in double quantum wells

The spreading of dipolar excitons in double quantum wells is described by a nonlinear continuity equation. It has been shown that the law of corresponding states holds, which allows one to find the dependence of the characteristic process parameters on the amplitude and width of the initial distribution. In contrast to usual (linear) diffusion, the spreading occurs much faster in the one-dimensional case than in the two-dimensional case.     

Bose condensation of interwell excitons in double quantum wells

The luminescence of interwell excitons in double quantum wells GaAs/AlGaAs (n-i-n heterostructures) with large-scale fluctuations of random potential in the heteroboundary planes was studied. The properties of excitons whose photoexcited electron and hole are spatially separated in the neighboring quantum wells were studied as functions of density and temperature within the domains on the scale less than one micron. For this purpose, the surfaces of the samples were coated with a metallic mask containing specially prepared holes (windows) of a micron size an less for the photoexcitation and observation of luminescence. For weak pumping (less than 50 μW), the interwell excitons are strongly localized because of small-scale fluctuations of a random potential, and the corresponding photoluminescence line is inhomogeneously broadened (up to 2.5 meV). As the resonant excitation power increases, the line due to the delocalized excitons arises in a thresholdlike manner, after which its intensity linearly increases with increasing pump power, narrows (the smallest width is 350 μeV), and undergoes a shift (of about 0.5 μeV) to lower energies, in accordance with the filling of the lowest state in the domain. With a rise in temperature, this line disappears from the spectrum (T _c ≤ 3.4 K). The observed phenomenon is attributed to Bose-Einstein condensation in a quasi-two-dimensional system of interwell excitons. In the temperature range studied (1.5–3.4 K), the critical exciton density and temperature increase almost linearly with temperature.