Cyclotron resonance in asymmetric double quantum wells

Far-infrared magnetotransmission measurements in magnetic fields are carried out on asymmetric coupled double wells. We observe a splitting in the cyclotron resonance (CR) line for a wide range of intermediate magnetic fields and only one line at high magnetic fields. Two peaks observed in the CR spectra correspond to transitions between Landau levels in individual wells. We propose that phase transition between weak and strong coupling regimes may be responsible for the features. The characteristics of the transition are studied via an analysis of CR masses, CR splitting and line widths as a function of the magnetic field.     

Electromagnetically induced transparency in asymmetric double quantum wells

We show how to compute nonlinear optical absorption spectra of an Asymmetric Double Quantum Well (ADQW) in the region of intersubband electronic transitions. The method uses the microscopic calculation of the dephasing due to electron-electron and electron-phonon scattering rates and the macroscopic real density matrix approach to compute the electromagnetic fields and susceptibilities. The polarization dephasing and the corrections to the Rabi frequencies due to the electron-electron interaction are also taken into account. For a proper choice of the QW widths and of the driving fields we obtain electromagnetically induced transparency. This transparency has a very narrow linewidth when a single driving field is applied resonant to the transition between the second and the third subband. In the case of two resonant driving fields or of a driving field resonant between the first and third subband we obtain a large transparency enhancement over the entire absorption spectrum. Results are given for GaAs/GaAlAs QWs and experiments are proposed. Received 21 June 2001 and Received in final form 21 January 2002     

Charge and energy transfer in double asymmetric quantum wells with quantum dots

The luminescence and luminescence excitation spectra of CdSe/ZnSe quantum dots are studied in a set of double quantum wells with the ZnSe barrier of width 14 nm, the same amount of a deposited CdSe layer forming a deep well and shallow wells with different depths. It is found that for a certain relation between the depths of shallow and deep wells in this set, conditions are realized under which the exciton channel in the luminescence excitation spectrum of a shallow well dominates in the region of kinetic exciton energies exceeding 10 longitudinal optical phonons above the bottom of the exciton band of the ZnSe barrier. A model is developed for the transfer of electrons, holes, and excitons between the electronic states of shallow and deep quantum wells separated by wide enough barriers. It is shown that the most probable process of electronic energy transfer between the states of shallow and deep quantum wells is indirect tunneling with the simultaneous excitation of a longitudinal optical phonon in the lattice. Because the probability of this process for single charge carriers considerably exceeds the exciton tunneling probability, a system of double quantum wells can be prepared in which, in the case of weak enough excitation, the states of quantum dots in shallow quantum wells will be mainly populated by excitons, which explains experimental results obtained.     

Nonlinear optical rectification in asymmetric double triangular quantum wells

The nonlinear optical rectification (OR) in the asymmetric double triangular quantum wells (DTQWs) is investigated theoretically. The dependence of OR on the right-well width of the DTQWs is studied, and the influence of the applied electric field on OR is also taken into account. The analytical expression of the OR susceptibility is analyzed by using the compact density-matrix approach and the iterative method and the numerical calculations are presented for the typical GaAs/Al_xGa_1-xAs asymmetric DTQWs. The results show that the OR susceptibility obtained in this coupled system can reach the magnitude of 10^-3 m/V, 2-3 orders of magnitude higher than that in single quantum systems. Moreover, the OR susceptibility is not a monotonic function of the width of the right well, but has complex relationship with it. The calculated results also reveal that an applied electric field has a great influence on the OR susceptibility. Applying an appropriate electric field to a quasi-symmetric or symmetric DTQW can result in a larger OR susceptibility as compared with that obtained in an optimal asymmetric DTQW without electric field.     

Interwell exciton relaxation in semimagnetic asymmetric double quantum wells

The possibility of magnetic field control of the spectral and polarization characteristics of exciton recombination is examined in Cd(Mg, Mn) Te-based asymmetric double quantum wells. At low fields, the exciton transition in a semimagnetic well is higher in energy than that in a nonmagnetic well and the interwell exciton relaxation is fast. In contrast, when the energy order of the exciton transitions reverses at high fields, unexpectedly slow relaxation of σ⁻ polarized excitons from the nonmagnetic well to the σ⁺-polarized ground state in the semimagnetic well is observed. Strong dependence of the total circular polarization degree on the heavy-light hole splitting Δ _hh-lh in the nonmagnetic well is found and attributed to the spin dependent interwell tunneling controlled by exciton spin relaxation. Such a slowing down of the relaxation allows separation of oppositely spin-polarized excitons in adjacent wells. The text was submitted by the authors in English.     

Mechanism of THz emission from asymmetric double quantum wells

Impulsive interband optical excitation of the lowest two conduction subbands of a suitably engineered GaAs/AlGaAs double quantum well can lead to coherent THz emission. We demonstrate that, contrary to previous expectations, the dominant emission mechanism can involve the beating of either continuum or exciton states, depending on excitation conditions. The coherence of the continuum beats persists for several picoseconds even for excitation an optical phonon energy above the band edge. We attribute this to the small energy difference between the component eigenstates, which substantially reduces the number of relevant scattering events.     

Relaxation of polarized excitons in asymmetric double quantum wells

Il Nuovo Cimento D - We consider the intrawell and interwell relaxation of circularly polarized excitons in an asymmetric double-quantum-well structure. For the intrawell case (photocreated light...     

Luminescence of excitons in slightly asymmetric double quantum wells

We report the first studies of exciton luminescence spectra from asymmetric double quantum wells (DQWs) of very similar width. The DQWs were of GaAs/AlGaAs and the differences in widths of the coupled wells were one or two monolayers. The coupled direct and indirect exciton states anticross with a resonance splitting of 1.33 meV. An additional luminescence line appearing at low temperatures is identified as a localized indirect exciton. Fiz. Tverd. Tela (St. Petersburg) 39, 735–739 (April 1997)     

Entanglement via tunable Fano-type interference in asymmetric semiconductor quantum wells

Entanglement is realized in asymmetric coupled double quantum wells (DQWs) trapped in a doubly resonant cavity by means of Fano-type interference through a tunneling barrier, which is different from the previous studies on entanglement induced by strong external driven fields in atomic media. We investigate the generation and evolution of entanglement and show that the strength of Fano interference can influence effectively the degree of the entanglement between two cavity modes and the enhanced entanglement can be generated in this DQW system. The present investigation may provide research opportunities in quantum entangled experiments in the DQW solid-state nanostructures and may result in a substantial impact on the technology for entanglement engineering in quantum information processing.     

Generation of infrared entangled light in asymmetric semiconductor quantum wells

We proposed a scheme to achieve two-mode CV entanglement with the frequencies of entangled modes in the infrared range in an asymmetric semiconductor double-quantum-wells (DQW), where the required quantum coherence is obtained by inducing the corresponding intersubband transitions (ISBTs) with a classical field. By numerically simulating the dynamics of system, we show that the entanglement period can be prolonged via enhancing the intensity of classical field, and the generation of entanglement doesn't depend intensively on the initial condition of system in our scheme. Moreover, we also show that a bipartite entanglement amplifier can be realized in our scheme. The present research provides an efficient approach to achieve infrared entangled light in the semiconductor nanostructure, which may have significant impact on the progress of solid-state quantum information theory.     

Optical second harmonic generation in asymmetric double triangular quantum wells

The second harmonic generation (SHG) in the asymmetric double triangular quantum wells (DTQWs) is investigated theoretically. The dependence of the SHG coefficient on the right-well width of the DTQWs is studied, and the influence of the applied electric field on SHG coefficient is also taken into account. The analytical expression of the SHG coefficient is analyzed by using the compact density-matrix approach and the iterative method. Finally, the numerical calculations are presented for the typical GaAs/Al_xGa_1−xAs asymmetric DTQWs. The results show that the calculated SHG coefficient in this coupled system can reach the magnitude of 10⁻⁵ m/V, 1–2 orders of magnitude higher than that in step quantum well, and that in double square quantum wells. Moreover, the SHG coefficient is not a monotonic function of the right-well width, but has complex relationship with it. The calculated results also reveal that an applied electric field has a great influence on the SHG coefficient. Applying an appropriate electric field to a DTQW with a wider right well can induce a sharper peak of the SHG coefficient due to the double-resonant enhancement.     

Modeling of electron tunneling times in asymmetric double quantum wells

A scattering process modeled by an imaginary potential V _I in the wide well of an asymmetric double quantum well structure (DQWS) is used to model the electron tunneling from the narrow well. Taking V _I –5 meV, the ground resonant level lifetimes of the narrow well in the DQWS are in quantitative agreement with the experimental resonance and non-resonance tunneling times. The corresponding scattering time 66 fs is much faster than the intersubband scattering time of LO-photon emission.     

Transient gain-absorption of the probe field in asymmetric semiconductor quantum wells

Under the weak-probe approximation, we theoretically investigate the transient gain-absorption property of the probe field in a four-level asymmetric semiconductor quantum well system. We find that the strength of Fano interference and the energy splitting affect the transient gain-absorption property of the weak continuous-wave (CW) probe field or Gaussian-pulse probe field dramatically. The dependence of transient gain-absorption property of the probe field on the intensity and the frequency detuning of the strong coupling field is also given. Our study is much more practical than its atomic counterpart due to its flexible design and the controllable interference strength. Thus, it may provide some new possibilities for technological applications.     

Optical bistability via tunable Fano-type interference in asymmetric semiconductor quantum wells

We analyze hybrid absorptive-dispersive optical bistability (OB) behavior via tunable Fano-type interference based on intersubband transitions in asymmetric double quantum wells (QWs) driven coherently by a probe laser field by means of a unidirectional ring cavity. We show that OB can be controlled efficiently by tuning the energy splitting of the two excited states (the coupling strength of the tunnelling), the Fano-type interference, and the frequency detuning. The influence of the electronic cooperation parameter on the OB behavior is also discussed. This investigation may be used for optimizing and controlling the optical switching process in the QW solid-state system, which is much more practical than that in atomic system because of its flexible design and the controllable interference strength.     

Self-consistent calculations of coherent tunneling oscillations in asymmetric double quantum wells

Coherent carrier tunneling in asymmetric double quantum well heterostructures is analytically investigated through a self-consistent method of calculation. Longitudinal electric fields are applied to reach the interwell resonance conditions. Exact wave functions in asymmetric double quantum well systems (modified Airy functions) are used to include the electron-hole interaction in the coherent tunneling process. Results are in good agreement with the available previous works.     

Transient and steady-state properties of asymmetric semiconductor quantum wells at telecom wavelength bands

The transient and steady-state dispersion and absorption properties of a three-subband asymmetric semiconductor quantum well system are investigated. In the steady-state regime, it is shown that by increasing the strength of Fano-interference as well as enhancement of energy splitting of two excited states the slope of dispersion changes from negative to positive which is corresponding to a switch between superluminal to subluminal light propagation. At the same time, the probe absorption reduces at telecommunication wavelength λ = 1550 nm. The influence of incoherent pumping fields on time-dependent susceptibility is then discussed. It is found that due to more transfer of population to the upper levels, increasing the rate of incoherent pump field leads to the reduction of probe absorption. In addition, it is realized that incoherent pumping has a major role in converting fast to slow propagation of light at long wavelength. We also introduce an extra controllability for the light pulse to be slow downed at Telecom wavelength just through the quantum interference arising from incoherent pumping fields. The obtained results may be practical in telecommunication applications.