Interwell Excitons Research Articles

Interwell exciton dispersion engineering, coherent phonons generation and optical detectionof exciton condensate

AbstractThis issue's Editor's Choice [1] discusses interwell excitons in coupled quantum wells as a candidate for observation of different phases in an exciton system, including the very interesting phenomenon of Bose condensation.The cover picture shows schematically how the generation of coherent phonons and the angular distribution of the exciton photoluminescence (PL) from the quantum well system can be controlled by the external electric and magnetic fields.The first author, Yurii E. Lozovik, is head of the Laboratory of Spectroscopy of Nanostructures at the Institute of Spectroscopy and also Professor of Physics at the Moscow Physical and Technical Institute. His main interests are electron and electron–hole systems in nanostructures, cluster physics, quantum electrodynamics in a cavity, matter in strong magnetic fields, nanotechnology, ultrafast and near field optics, and computer simulations.

Interwell exciton dispersion engineering, coherent phonons generation and optical detection of exciton condensate

AbstractWe propose to use dispersion engineering of interwell excitons in coupled quantum wells with external electric and magnetic fields in order to generate coherent phonons and to detect exciton condensate. A parallel magnetic field moves the dispersion minimum of interwell excitons away from the radiative zone and thus reduces their recombination rate. Normal electric field moves an interwell excitons dispersion minimum on the energy scale. These two fields effect can be used to tune the resonance condition of the interwell excitons recombination process via an in‐well excitons level, which results in acoustic phonon emission. We show, that one can change recombination rate as well as intensity and angular distribution of the interwell excitons photoluminescence in the wide range by controlling the external fields. Based on this principle we propose and theoretically evaluate a procedure to detect the condensate of interwell excitons, as well as a scheme to obtain a coherent and monochromatic phonon beam (saser). The statistics of the phonon emission from the condensate of interwell excitons is studied. Numerical estimate for GaAs/AlGaAs coupled quantum wells is provided. (© 2003 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

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.

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.

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.

Condensation of interwell excitons in GaAs/AlGaAs double quantum wells

Experimental observations of the collective behavior of interwell excitons in the binary quantum wells with inclined bands under bias are discussed.

From spatially indirect excitons to momentum-space indirect excitons by an in-plane magnetic field

An in-plane magnetic field is found to change drastically the photoluminescence spectra and kinetics of interwell excitons in $\mathrm{GaAs}/{\mathrm{Al}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{As}$ coupled quantum wells. This effect is due to the in-plane magnetic-field-induced displacement of the interwell exciton dispersion in momentum space, which results in the transition from the momentum-space direct exciton ground state to the momentum-space indirect exciton ground state. An in-plane magnetic field is, therefore, an effective tool for exciton dispersion engineering.

Interwell excitons in GaAs/AlGaAs double quantum wells and their collective properties

Luminescence spectra of interwell excitons in GaAs/AlGaAs double quantum wells with electric-field-tilted bands (n-i-n) structures were studied. In these structures the electron and the hole in the interwell exciton are spatially separated between neighboring quantum wells by a narrow AlAs barrier. Under resonant excitation by circularly polarized light the luminescence line of the interwell excitons exhibited appreciable narrowing as their concentration increased and the degree of circular polarization of the photoluminescence increased substantially. Under resonant excitation by linearly polarized light the alignment of the interwell excitons increased as a threshold process with increasing optical pumping. By analyzing time-resolved spectra and the kinetics of the photoluminescence intensity under pulsed excitation it was established that under these conditions the rate of radiative recombination increases substantially. The observed effect occurs at below-critical temperatures and is interpreted in terms of the collective behavior of the interwell excitons. Studies of the luminescence spectra in a magnetic field showed that the collective exciton phase is dielectric and in this phase the interwell excitons retain their individual properties.

Collective behavior of interwell excitons in GaAs/AlGaAs double quantum wells

Photoluminescence spectra of interwell excitons in double GaAs/AlGaAs quantum wells (n-i-n structures) have been investigated (an interwell exciton in these systems is an electron-hole pair spatially separated by a narrow AlAs barrier). Under resonance excitation by circularly polarized light, the luminescence line of interwell excitons exhibits a significant narrowing and a drastic increase in the degree of circular polarization of photoluminescence with increasing exciton concentration. It is found that the radiative recombination rate significantly increases under these conditions. This phenomenon is observed at temperatures lower than the critical point and can be interpreted in terms of the collective behavior of interwell excitons.

Superradiant Coupling of Excitons in (In,Ga)As Multiple Quantum Wells

The third order nonlinear optical response at the heavy-hole (hh) exciton resonance in a wedge-shaped multiple quantum well structure close to the Bragg resonance is investigated using time-integrated and spectrally resolved degenerate four-wave mixing. Effective interwell exciton–exciton interaction due to photon exchange is demonstrated by studying the signals in backward direction for different widths of the barriers.

Interwell and Intrawell Magnetoexcitons in GaAs/AlGaAs Superlattices

The formation of spatially indirect (interwell) excitons in superlattices (Sls) with different barrier widths (different tunneling coupling) is experimentally investigated in a strong enough magnetic field with the use of photoluminescence (PL), photoluminescence excitation (PLE), reflectance spectra and time-resolved technique. The experimental conditions for the observation of interwell excitons in SLs without externally breaking inversion symmetry are identified and the distinct excitonic state is found to be located between 1s-exciton and the onset of the miniband absorption. Magnetic field, perpendicular to the layers, strongly modifies the interwell and intrawell excitonic states and transforms the system to the weak coupling regime. The effect of interwell exciton conversion into intrawell one due to exciton–exciton collisions has been observed at a high excitation regime and confirmed by time-resolved spectroscopy experiments under pulsed picosecond excitation.

Interwell excitons in GaAs superlattices

The formation of spatially indirect excitons in superlattices with narrow minibands is investigated experimentally. The interwell exciton is similar to the first Wannier–Stark localized exciton of an electrically biased superlattice. However, in the present case the localization is mediated by the Coulomb interaction of the electron and the hole without external fields.

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.

Coherent dynamics of interwell excitons in GaAs/AlxGa1−xAs superlattices

Coherent exciton dynamics in a GaAs/AlxGa1−xAs narrow-miniband superlattice is studied by spectrally resolved transient four-wave mixing. Coherent optical properties of the investigated structure are found to be strongly affected by the existence of two different heavy-hole excitonic states. One of them, the 1s heavy-hole exciton, is almost identical to the same state in noninteracting quantum wells, while the other, the heavy-hole interwell exciton, is composed of an electron and a heavy hole in adjacent wells. The interwell exciton leads to a resonant enhancement in the four-wave mixing spectra and exhibits quantum beats with the 1s heavy-hole exciton. The dephasing of the interwell exciton is one order of magnitude faster than that of the heavy-hole exciton and is mostly due to intensity-independent scattering mechanisms.

Interwell excitons in GaAs superlattices.

The formation of spatially indirect excitons in superlattices with narrow minibands is theoretically and experimentally investigated. We identify the experimental conditions for the observation of interwell excitons and find a distinct excitonic state energetically located between the 1s exciton and the onset of the miniband absorption. The interwell exciton is similar to the first Wannier-Stark localized exciton of an electrically biased superlattice. However, in the present case the localization is mediated by the Coulomb interaction of the electron and the hole without external fields. \textcopyright{} 1996 The American Physical Society.

Interwell excitons in GaAs multiple quantum wells and superlattices

The 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.

Resonant quenching of exciton photoluminescence in coupled GaAs/AlAs quantum wells: Effect of exciton binding energy

We have studied the electric field dependence of the photocurrent and of the excitonic photoluminescence (PL) associated with the first and second conduction subbands in weakly coupled GaAs/AlAs quantum wells. Resonant subband alignment between adjacent QWs results in a quenching of the PL intensity from the first subband, an enhancement of the PL from the second subband, and a maximum of the photocurrent. These resonant extrema are observed at slightly different electric fields. We show that this behavior arises from the different binding energies of intra- and inter-well excitons.

Exciton effects in sequential resonant tunneling of photocarriers in GaAs/AlAs multiple quantum wells.

We have studied the sequential resonant tunneling characteristics of coupled GaAs/AlAs quantum wells (QW's) by measuring the electric-field dependence of the photocurrent and of the excitonic photoluminescence (PL) associated with the first and second conduction subband. If the first subband of a QW is resonantly aligned with the second subband of the adjacent QW, a maximum of the photocurrent, a strong reduction of the PL intensity from the first subband, and a pronounced enhancement of the PL associated with the second subband are observed. Unexpectedly, these resonant extrema are observed at different electric fields. This phenomenon is attributed to the different binding energies of the intrawell and interwell excitons.

Optical Studies of free and Acceptor-Bound Excitons in GaAs/AlGaAs Symmetric Coupled Double Quantum Well Structures

Discrete peaks, due to the excited 2S state of the n=l symmetric heavy hole (hh) exciton, are observed in the photoluminescence excitation spectra of GaAs/AlGaAs coupled double quantum well (CDQW) samples. The observations of the 2S state allow a precise determination of 1S-2S splitting, and the experimental results show a good agreement with a simple calculation which is based on a model of fractional-dimension of the exciton state. The obtained values for the 1S-2S splitting are significantly reduced compared with the corresponding single (or multiple) quantum wells (QW). Central acceptors confined in CDQWs have also been studied. Due to built in electric fields, interwell and intrawell excitons bound to neutral acceptors are simultaneously observed in weakly coupled doped double QWs. The diamagnetic shift and binding energy of the bound excitons are similar to results of the single QW’s, while the diamagnetic shift of the free exciton differs from single QW’s.

Acceptor-bound excitons in GaAs/AlxGa1−xAs symmetric coupled-double-quantum-well structures

The effect of an electric-field perturbation on excitons bound to Be acceptors in symmetric coupled-double-quantum-well (CDWQ) structures has been studied with photoluminescence (PL) and photoluminescence excitation spectroscopy. The Be acceptors were positioned in the central region of each well. With an electric field perpendicular to the well layers, interwell and intrawell free excitons are observed in the PL spectra of CDQW's. In weakly coupled DQW's, interwell and intrawell excitons bound to neutral acceptors can be observed simultaneously. Only one (interwell) bound exciton is observed in a strongly coupled system. Interwell and intrawell bound excitons are found to have the same bound-exciton (BE) binding energies. This indicates that it is mainly the hole of the exciton which is attracted by the neutral acceptor impurity. No variations of BE binding energies with the strength of the electric field were observed for the weakly coupled DQW's, while the binding energy increased with the electric field in a strongly coupled DQW. It was found that the BE binding energies at zero electric field in CDQW's are equal to those obtained for center-doped single-quantum-well structures with the same well widths.