Double Quantum-well System Research Articles

Tunable double-beam optical bistability in an asymmetric double quantum-well system

We have proposed a scheme for double-beam optical bistability in a tunnel-coupled asymmetric double quantum-well driven by two optical fields circulating inside two independently unidirectional ring cavities. In contrast to the single-cavity case where single-photon saturated absorption and self Kerr-nonlinearity are dominant, the two-photon absorption and cross phase modulation can be enhanced via tunneling induced interference and have an important influence to the formation of bistability. The bistable behavior can be controlled effectively via the system parameters such as the input and detuning of control field, the detuning of the probe field, Fano interference strength and cooperation parameter. Furthermore, the proposed scheme has the ability to manipulate the outputs of two optical cavities simultaneously. Due to the flexible design of semiconductor quantum well, our scheme is more practical than atomic system, therefore it can be utilized to achieve dual all-optical switching which has application in optical communication and computing.

Donor states in a semimagnetic [formula omitted] Double Quantum Well

The theoretical investigation has been carried out on the binding energy of donor associated with the electrons confined in a Cd1−xinMnxinTe/Cd1−xoutMnxoutTe Double Quantum Well (DQW) as a function of central barrier width for various well dimensions and impurity locations in the barrier and the well. The magnetic field can act as a tool to continuously change the interwell coupling inside this DQW systems and its effect on donor binding has also been studied. Moreover, the polaronic corrections, which is due to the strong exchange interaction between the magnetic moment of Mn2+ ion and the spin of the confined carrier, to the binding energy of the hydrogenic donor impurity has also been estimated with and without the application of magnetic field. The binding energy of the donor impurity is determined by solving the Schrodinger equation variationally in the effective mass approximation and the effect due to Bound Magnetic Polaron (BMP) is included using mean field theory with the modified Brillouin function. The results are reported and discussed.

Asymmetric effects on the optical properties of double-quantum well systems

Linear, nonlinear, and total absorption coefficient and refractive index changes of double-quantum well (DQW) systems are studied theoretically in the presence of external static electric field applied along the growth direction. The analytical expression for the linear and nonlinear optical properties is obtained using density matrix method. Emphasis is laid on the effect of asymmetry in the shapes of DQW system on optical properties. Some interesting results are obtained and explained.

Coherent control of position-dependent probe absorption spectrum in a solid

We investigate the two-dimensional (2D) probe absorption spectrum in a four-subband coupled double quantum-well system driven by two orthogonal standing-wave lasers. It is found that, due to the quantum interference effect of interacting dark resonances, the 2D spatial distribution of probe absorption spectrum can be easily controlled via adjusting the system parameters. Thus, our scheme shows the underlying probability for the formation of the 2D electron localization in a solid.

Crossover from negative to positive magnetoresistance in the double quantum well systemwith different starting disorder

Magnetotransport measurements were performed in two widely separated double quantumwell systems with different starting disorders. In the weak magnetic field regime, acrossover from negative to positive magnetoresistance in the longitudinal resistivity wasobserved in the system with weak disorder when the electron densities in the neighboringwells were significantly unbalanced. The crossover was found to be the resultof the exchange-energy-assisted interactions between the electrons occupyingthe lowest subbands in the neighboring wells. In the case of the system withstrong disorder short range scattering dominated the scattering process and nosuch transition in longitudinal resistivity in the low magnetic field regime wasobserved. However, at high magnetic fields, sharp peaks were observed in the Hallresistance due to the interaction between the edge states in the quantum Hall regime.

THE OPTICAL KERR EFFECT IN WURTZITEGaN-BASED DOUBLE QUANTUM WELLS: INFLUENCES OF PIEZOELECTRICITY AND SPONTANEOUS POLARIZATION

Taking the influences of piezoelectricity and spontaneous polarization into consideration, the nonlinear optical Kerr effect in a nitride semiconductor coupling double quantum well (DQW) has been theoretically investigated by using the compact density matrix approach and iterative treatment. The electronic eigenstates in a nitride DQW are exactly solved based on the built-in electric field model already constituted in recent reference. The band non-parabolicity effect of nitride heterostructures has been taken into account. A typical wurtzite GaN / AlGaN DQW is chosen to perform numerical calculations. The calculated results reveal that the optical Kerr coefficients sensitively depend on the structural parameters of the coupling DQW system. Moreover, a strong optical Kerr effect can be realized in the nitride DQW by choosing a group of optimum structural parameters and doped fraction.

Microscopic Theory of Coupled Quantum Well Structures in Photovoltaics

AbstractWe use our recently developed microscopic approach to a quantum theory of photovoltaic processes in nanostructures [1]to investigate geometry effects in quantum well photovoltaics, focussing on the role of asymmetry and inter-well coupling in double quantum well (DQW) systems. For that purpose, the IV and power characteristics for DQW systems with different asymmetry and degree of coupling are calculated numerically in the vicinity of the maximum power point. In order to assess the transport properties of a specific QW structure, we isolate these from the effects of absorption by normalizing to the absorptivity. The results obtained from this procedure reveal the escape regime dominating at room temperature and confirm previous experimental observations.

Experimental methods and analysis of cold and dense dipolar exciton fluids

We review various aspects of our recent work on dipolar excitons in double quantum wellsystems. We describe and analyse different possible avenues for obtaining high density andcold dipolar exciton fluids that may enable an observation of quantum phase transitions inexcitonic systems. These avenues include free dipolar exciton fluids, dipolar exciton fluidsin electrostatic traps and in excitonic rings. We present our experimental and modellingwork on the exciton dynamics in such systems, and discuss our current view of theadvances made and the challenges that remain in this fast evolving and promising field ofresearch.

Plasmons in asymmetric double quantum well structures

We investigate the effects of spatial asymmetry, tunneling coupling, and exchange-correlation correction on the plasmon modes in asymmetric double quantum well (DQW) structures in a time-dependent local-density approximation. Special attention is paid to the properties of the ω - mode which is always damped in symmetric DQW systems. In addition, the results on the spectral weight of the excitations are also presented. In general, all the modes carry finite spectral weights and should be observable in resonant inelastic light scattering experiments for the specified values of the parameters.

Effects of tunneling coupling on plasmon modes in asymmetric double-quantum-well structures

The collective charge density excitations in asymmetric double-quantum-well (DQW) structures with different tunneling strengths are systematically studied. In particular, the damping properties of the plasmon modes in various tunneling strengths are investigated in detail. It is shown that plasmon modes in asymmetric DQW structures are quite different from those in symmetric DQW systems. In weak tunneling regime, an intra-subband mode ω- with an acoustic-like dispersion relation which is damped in symmetric DQW structures arises and coexists with the optical-like mode ω+ while the inter-subband mode ω10 is highly damped. With the tunneling strength being increased, the ω10 branch gradually becomes undamped and emerges out of the (1-0) single-particle continuum, whereas the ω- branch gradually approaches the (0-0) single-particle continuum. In intermediate coupling regime, these three branches of modes coexist undamped. In strong tunneling regime, ω- enters the (0-0) single-particle continuum and becomes damped. Consequently, only the ω+ and ω10 modes exist in this regime.

Landau-level interplay in InGaAs double quantum wells

Shubnikov–de Haas (SdH) and Hall measurements have been used to investigate a pair of adjacent two-dimensional electron gases (2DEGs) which were formed in two n0.53Ga0.47As quantum-wells, separated by a thin In0.52Al0.48As barrier, grown lattice-matched on InP. This double quantum-well system consists of two asymmetric InGaAs quantum wells, 9 nm and 7 nm respectively, separated by a 4.5 nm InAlAs barrier. The existence of two occupied electronic subbands with differing electron densities can clearly be identified by beating effects in the SdH oscillations. By applying a substrate bias the electron densities can be tuned and the beating is shifted. In the simultaneously performed Hall measurements additional features can be observed: Hall measurements with different total electron densities reveal plateaus for integer filling factors ν (with ν=ν1+ν2, ν1and ν2both integers, corresponding to the two subbands). Some even filling factors become suppressed and recover with changing electron density. Also, for some densities an odd filling factor is observed. The systematic tuning of the electron densities via the application of a bias voltage to the front gate reveals two Landau fans, one for each electronic system, respectively, crossing each other. The electron densities for both electronic systems can be identified by analysing the SdH spectra. As a function of the front-gate voltage, these densities seem to show evidence for an anticrossing of the two electronic states and therefore for a strong coupling between the states.

A study of the relative strengths of spin-pseudospin phases in a strongly coupled double quantum well system

We perform an experimental and theoretical investigation of the rich phase diagram of quantum Hall paramagnetic and ferromagnetic ground states in a double quantum well (DQW) system, and investigate how the structure of this diagram results from the inter-play between spin and pseudospin exchange energies. We use a comparison with a DQW quantum Hall Hartree calculation to identify the nature of these different phases and assess the effect of exchange and correlation energies. We find that the strengths of different phases and the nature of crossings between them can be understood by the dependence of spin and pseudospin exchange energies on subband wave function delocalisation which we tune in the DQW system by a surface gate bias.

Plasmon-LO-phonon coupling effects on the drag rate in double quantum-well systems

We study the Coulomb drag rate for electrons in a double-quantum-well structure taking into account the electron-optical phonon interactions. The full wave vector and frequency dependent random-phase approximation (RPA) at finite temperature is employed to describe the effective interlayer Coulomb interaction. The electron-electron and electron-optical phonon couplings are treated on an equal footing. The electron-phonon mediated interaction contribution is investigated for different layer separations and layer densities. We find that the drag rate at high temperatures (i.e., T ≥ 0.3 E F ) is dominated by the coupled plasmon-phonon of the system. Including the local-field effects in an approximate way we estimate the importance of intralayer correlations to be significant.

Correlation effects in double quantum-well systems

An interacting double-layer electron system, with uniform positive backgrounds, is considered at finite temperatures and in the presence of a strong perpendicular magnetic field for integer filling factors. The low-energy physics of this system is described using an effective action formalism. The density-density correlation functions and the long-wavelength collective excitations of the electronic system (magnetoplasmons) are obtained. The results are valid beyond the random-phase approximation. Finally, the response of the system to electromagnetic interactions is considered and the current induced in one layer (transconductivity), when a current is driven only through the other layer, is evaluated. All results take into account explicitly interlayer and intralayer correlations.

Coulomb scattering lifetime of a two-dimensional electron gas.

Motivated by a recent tunneling experiment in a double quantum-well system, which reports an anomalously enhanced electronic scattering rate in a clean two-dimensional electron gas, we calculate the inelastic quasiparticle lifetime due to electron-electron interaction in a single loop dynamically screened Coulomb interaction within the random-phase-approximation. We obtain excellent quantitative agreement with the inelastic scattering rates in the tunneling experiment without any adjustable parameter, finding that the reported large ($\geq$ a factor of six) disagreement between theory and experiment arises from quantitative errors in the existing theoretical work and from the off-shell energy dependence of the electron self-energy.

Energy spectrum and charge-density-wave instability of a double quantum well in a magnetic field.

Treating the electrons in the Hartree-Fock approximation, we study the wave-vector dependence of the spin-density excitations and the charge-density excitations of a double-quantum-well (DQW) system in a strong magnetic field with total \ensuremath{\nu}=1. We conclude that as the distance between the wells increases, the DQW system undergoes a phase transition, probably to a charge-density-wave state. We discuss the possible effects of this phase transition on the quantum Hall effect.