Coupled GaAs Quantum Wells Research Articles

The Role of Spin-Flip Collisions in a Dark-Exciton Condensate

We show that a Bose-Einstein condensate consisting of dark excitons forms in GaAs coupled quantum wells at low temperatures. We find that the condensate extends over hundreds of micrometers, well beyond the optical excitation region, and is limited only by the boundaries of the mesa. We show that the condensate density is determined by spin-flipping collisions among the excitons, which convert dark excitons into bright ones. The suppression of this process at low temperature yields a density buildup, manifested as a temperature-dependent blueshift of the exciton emission line. Measurements under an in-plane magnetic field allow us to preferentially modify the bright exciton density and determine their role in the system dynamics. We find that their interaction with the condensate leads to its depletion. We present a simple rate-equations model, which well reproduces the observed temperature, power, and magnetic-field dependence of the exciton density.

Defect Proliferation at the Quasicondensate Crossover of Two-Dimensional Dipolar Excitons Trapped in Coupled GaAs Quantum Wells.

We study ultracold dipolar excitons confined in a 10 μm trap of a double GaAs quantum well. Based on the local density approximation, we unveil for the first time the equation of state of excitons. Specifically, in this regime and below a critical temperature of about 1K, we show that for a local density n∼(2-3)×10^{10} cm^{-2} a coherent quasicondensate phase forms in the inner region of the trap, encircled by a more dilute and normal component in the outer rim. Remarkably, this spatial arrangement correlates directly with the concentration of defects in the exciton density, which is strongly decreased in the quasicondensed region, consistent with a superfluid phase. Thus, our observations point towards a Berezinskii-Kosterlitz-Thouless crossover for two-dimensional excitons.

Dynamical Control over the Confinement of Spatially Indirect Excitons in Electrostatic Traps Composed of Coupled GaAs Quantum Wells

Exploring collective quantum phenomena is aided by model systems of cold gases of (quasi)particles, such as spatially indirect excitons in a semiconductor---but a proper control system is needed. This study provides a technique to manipulate the confinement energy of indirect excitons, relying only on dipolar coupling to the evanescent electric field inside a field-effect device, with no need for impedance matching. A microscopic trap offers control with nanosecond precision at subkelvin temperatures, without incidental heating, for direct application in studying excitonic Bose-Einstein condensates, and in tuning excitonic transition energies in semiconductor quantum optics.

Collective electronic excitation in a trapped ensemble of photogenerated dipolar excitons and free holes revealed by inelastic light scattering

Photogenerated excitonic ensembles confined in coupled GaAs quantum wells are probed by a complementary approach of emission spectroscopy and resonant inelastic light scattering. Lateral electrostatic trap geometries are used to create dense systems of spatially indirect excitons and excess holes with similar densities in the order of 10$^{11}$ cm$^{-2}$. Inelastic light scattering spectra reveal a very sharp low-lying collective mode that is identified at an energy of 0.44 meV and a FWHM of only ~50 $\mu$eV. This mode is interpreted as a plasmon excitation of the excess hole system coupled to the photogenerated indirect excitons. The emission energy of the indirect excitons shifts under the application of a perpendicular applied electric field with the quantum-confined Stark effect unperturbed from the presence of free charge carriers. Our results illustrate the potential of studying low-lying collective excitations in photogenerated exciton systems to explore the many-body phase diagram, related phase transitions, and interaction physics.

Overflow of a dipolar exciton trap at high magnetic fields

We study laterally trapped dipolar exciton ensembles in coupled GaAs quantum wells at high magnetic fields in the Faraday configuration. In photoluminescence experiments, we identify three magnetic field regimes. At low fields, the exciton density is increased by a reduced charge carrier escape from the trap, and additionally, the excitons' emission energy is corrected by a positive diamagnetic shift. At intermediate fields, magnetic field dependent correction terms apply which follow the characteristics of a neutral magnetoexciton. Due to a combined effect of an increasing binding energy and lifetime, the exciton density is roughly doubled from zero to about 7 T. At the latter high field value, the charge carriers occupy only the lowest Landau level. In this situation, the exciton trap can overflow independently from the electrostatic depth of the trapping potential, and the energy shift of the excitons caused by the so-called quantum confined Stark effect is effectively compensated. Instead, the exciton energetics seem to be driven by the magnetic field dependent renormalization of the many-body interaction terms. In this regime, the impact of parasitic in-plane fields at the edge of trapping potential is eliminated.

Bias voltage dependence of two-step photocurrent in GaAs/AlGaAs quantum well solar cells

We investigated photoresponses of AlGaAs solar cells in which coupled GaAs quantum wells were embedded in the i-region of p-i-n diodes; we studied how the bias voltage Vb affects the normal photocurrent I generated by the visible light and a “two-step” photocurrent ΔI generated by the absorption of visible and infrared photons. We found that as Vb exceeds −0.2 V, ΔI rises and peaks at 0.6 V, while the normal photocurrent I falls to about half of its saturated level. These findings are discussed in terms of a rate equation model to show that ΔI is mainly determined by the balance of escape and recombination of photogenerated carriers.

Long-lived spin coherence of indirect excitons in GaAs coupled quantum wells

We study spatially indirect excitons confined in a symmetric GaAs double quantum well. We show that the spin polarisation of very dilute gases can be optically imprinted, in both pure and superposition states. In the regime where indirect excitons can be localized, we observe at 350 mK that the excitons spin degree of freedom is frozen with a relaxation time comparable to the coherence time and to the radiative lifetime .

Control of intersubband transitions in multiple quantum well systems

The dynamics of intersubband transitions in a multiple quantum well system consisting of GaAs coupled quantum wells separated by an AlxGa1−xAs barrier is studied in the presence of a combined static and a dynamic laser field. The eigenenergies and eigenfunctions of the system under the effect of static field are solved by a time independent Schrödinger equation using a Finite Difference method. Calculations included the effect of the intensity of static field, effect of the number of wells on various intersubband transitions, the energies and wavefunctions of the levels involved. We initiated the study with the periodic structure consisting of a few quantum wells (N=2) and studied the bound–bound and bound–continuum transitions in the mini bands formed in the wells. We have analyzed how the features characteristic of long periodic systems are formed as the number of quantum well increases. These are then used to formulate the laser–multiple quantum well interaction using a time dependent Schrödinger equation which is solved with the help of an efficient fourth order Runge Kutta method.

Light-emitting properties of a strain-tuned microtube containing coupled quantum wells

Pre-stressed multi-layer nanomembranes are rolled-up into a microtube in order to tune the strain applied to the contained coupled GaAs quantum wells. Additional GaAs/AlAs adjusting layers were deposited on the top of the nanomembrane to alter the thickness/stiffness of the to-be-rolled nanomembrane. In this way, microtubes with an adjustable diameter and strain are possible from a single initial grown sample. The internal strain state in the microtube affects the energy levels of the quantum wells and their coupling, which can be probed sensitively by photoluminescence. We measure different strain relaxation in rolled-up nanomembranes which we explain using a gradual change of the longitudinal relaxation as the distance of the nanomembrane from the etching front varies.

Spin injection in a heterovalent structure with coupled quantum wells GaAs/(Al,Ga)As/(Zn,Mn)Se/ZnSe

AbstractIn this report we present the study, by means of low‐temperature magneto‐optical measurements, of the spin transport dynamics in a heterovalent MBE‐grown III‐V/II‐VI structure under optical pumping. The structure consists of two coupled GaAs quantum wells (QWs) of 100 Å and 50 Å surrounded by (Al,Ga)As barriers, a heterovalent (Al,Ga)As/ZnSe interface, a 10 nm (Zn,Mn)Se spin aligner, and a 70 nm thick ZnSe cap layer. We obtain values of about 30% at 4.5 T for the degree of circular polarization. These results give some hints on the design improvements needed for higher spin‐filtering efficiency. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Kinetics of the inner ring in the exciton emission pattern in coupled GaAs quantum wells

We report on the kinetics of the inner ring in the exciton emission pattern. The formation time of the inner ring following the onset of the laser excitation is found to be about 30 ns. The inner ring was also found to disappear within 4 ns after the laser termination. The latter process is accompanied by a jump in the photoluminescence (PL) intensity. The spatial dependence of the PL-jump indicates that the excitons outside of the region of laser excitation, including the inner ring region, are efficiently cooled to the lattice temperature even during the laser excitation. The ring formation and disappearance are explained in terms of exciton transport and cooling.

Loading indirect excitons into an electrostatic trap formed in coupled GaAs quantum wells

We demonstrate how to load indirect excitons into an electrostatic trap, which is formed in a field-effect structure based on coupled GaAs quantum wells. Within the plane of a double quantum well, indirect excitons are trapped at the perimeter of a SiO 2 area sandwiched between the surface of the GaAs heterostructure and a semi-transparent metallic top gate. The trapping mechanism is well explained by a combination of the quantum confined Stark effect and local field enhancement. We find that the trap can be filled with indirect excitons, as soon as the distance between the laser excitation and the trap is shorter than the effective diffusion length of the indirect excitons.

Micropatterned electrostatic traps for indirect excitons in coupled GaAs quantum wells

We demonstrate an electrostatic trap for indirect excitons in a field-effect structure based on coupled GaAs quantum wells. Within the plane of a double quantum well indirect excitons are trapped at the perimeter of a SiO2 area sandwiched between the surface of the GaAs heterostructure and a semitransparent metallic top gate. The trapping mechanism is well explained by a combination of the quantum confined Stark effect and local field enhancement. We find the one-dimensional trapping potentials in the quantum well plane to be nearly harmonic with high spring constants exceeding 10 keV/cm^2.

Drift mobility of long-living excitons in coupled GaAs quantum wells

The authors report on high-mobility transport of indirect excitons in coupled GaAs quantum wells. A voltage-tunable in-plane potential gradient is defined for excitons by exploiting the quantum confined Stark effect in combination with a lithographically designed resistive top gate. Excitonic photoluminescence resolved in space, energy, and time provides insight into the in-plane drift dynamics. Across several hundreds of microns an excitonic mobility of 105cm2∕eVs is observed for temperatures below 10K. With increasing temperature the excitonic mobility decreases due to exciton-phonon scattering.

Leakage current induced anomalies in the photoluminescence of GaAs coupled quantum wells

We have performed photoluminescence experiments on GaAs/AlGaAs coupled quantum wells as a function of bias voltage and photoexcitation intensity. Spatially indirect recombination appears at higher electric fields for weak photoexcitation instead of the spatially direct one which is dominant at zero bias voltage. However, for strong photoexcitation with photon energy larger than the band gap of the barrier layers, the spatially direct recombination remains dominant over a wide range of bias voltage. Under this condition, the exciton ring emission is observed in the PL image at positive biases for which a large leakage current flows through the device. These results suggest that the 2DEGs induced by the large leakage current is important for interpreting the behavior of the ring emission. (C) 2004 WILEY-VCH Verlag GmbH & Co. KGaA. Weinheim.

Excitonic binding in coupled quantum wells

We study excitonic states in the presence of applied electric field in 8-nm GaAs coupled quantum wells (QW's) separated by a 4-nm ${\mathrm{Al}}_{0.33}{\mathrm{Ga}}_{0.67}\mathrm{As}$ barrier and in 6-nm ${\mathrm{In}}_{0.1}{\mathrm{Ga}}_{0.9}\mathrm{As}$ coupled QW's separated by a 4-nm GaAs barrier in which effects attributed to macroscopically ordered excitonic states have been recently reported. We discuss the differences in the nature of the states and in the origin of confinement which determines the change of excitonic properties with increase in the applied electric field in both structures. We have found that the indirect exciton binding energy for the field amplitude used in the experiment with InGaAs QW's is around 3.5 meV, much less than the previously reported 10 meV value. This suggests that the optically induced ring structure, reported to persist to near 100 K, might not be caused by collective excitonic transport.

Surface-acoustic-wave-induced carrier transport in quantum wires

The ambipolar transport of photogenerated electron-hole pairs by surface acoustic waves (SAW's) in coupled GaAs quantum wells (QW's) and quantum wires (QWR's) is investigated by spatially and time-resolved photoluminescence. Experimental configurations for SAW propagation direction parallel or perpendicular to the QWR's have been studied. In the first configuration, the QWR confinement potential inhibits lateral carrier diffusion. The carriers are then efficiently transported along the wire as well-defined charge packages with a repetition rate corresponding to the SAW frequency. In the second configuration, we demonstrate that the SAW can be used to transfer electron-hole pairs generated in the QW into the QWR. We also provide clear evidence for the extraction of carriers from the QWR into the QW, when the SAW piezoelectric field is sufficiently strong to overcome the QWR confinement potential.

Huge density-dependent blueshift of indirect excitons in biased coupled quantum wells

We measure a dynamical blueshift of the indirect exciton in GaAs-coupled quantum wells of over 20 meV for carrier densities in the range ${10}^{9}--{10}^{12}{\mathrm{cm}}^{\ensuremath{-}2}.$ We show that this shift is a many-body effect, which is proportional to the indirect exciton density over a broad range of densities, and it is accompanied by homogeneous broadening, which is proportional to the indirect exciton density.

Photon-assisted tunneling in coupled double quantum wells

Using different techniques to individually contact two closely spaced electron gases, we study the tunneling characteristics between weakly coupled GaAs quantum wells, with and without resonant far-infrared excitation. We find that for barriers as thick as 300 Å, the alignment between the subbands in the wells can be observed as an increased tunneling conductivity. To study photon-assisted tunneling in our samples, we use the cyclotron resonance as a strong, tunable electronic excitation in the far-infrared. When the Landau-level spacing [ hstrok] ω c corresponds to the laser energy [ hstrok] ω L, the carriers are effectively pumped to higher Landau levels, which leads to a reduced resistance across the tunnel barrier. This photo-conductive signal is “doubly resonant” in that it is at maximum when ω c coincides with ω L, and at the same time the subbands of the two wells are aligned.

Quasi-phase-matched second-harmonic generation from asymmetric coupled quantum wells

Optical second-harmonic generation of visible light in a superlattice of asymmetric coupled GaAs quantum wells has been measured for the first time to our knowledge. The orientation of the asymmetric quantum wells was periodically reversed such that the quantum-well contribution to the radiated second-harmonic signal was quasi-phase matched when it was excited by a lambda = 1.319 microm fundamental in the reflection geometry. The second-harmonic intensity generated in this superlattice was approximately 3.6 times that generated in a homogeneous GaAs reference. The measured superlattice second-harmonic susceptibilities are[Equation] and[Equetion].