Asymmetric Coupled Quantum Wells Research Articles

Characterization of recombination processes in multiple narrow asymmetric coupled quantum wells based on the dependence of photoluminescence on laser intensity

Continuous-wave intensity-dependent photoluminescence spectra of multiple narrow asymmetric coupled quantum wells at room temperature have been measured. At low laser intensity, the total photoluminescence intensity is primarily proportional to the square of the laser intensity due to dominant nonradiative recombination of free carriers at nearly saturated interface traps. At high laser intensity, however, the total photoluminescence intensity approaches a level proportional to the laser intensity due to radiative recombination of free carriers. Based on this transition behavior, which has been observed for the first time to the best of our knowledge, and our simple theory, the transition intensity, the nonradiative decay time of the carriers, and the intensity-dependent carrier density and photoluminescence quantum efficiency have been determined.

Intersubband relaxation of photoexcited carriers in asymmetric coupled quantum wells

Intersubband relaxation of hot carriers in III-V compound quantum well systems depends explicitly on the form of the polar optical coupling. In the GaAs/AlGaAs system, it is well known that the electrical and mechanical boundary conditions imposed on the atomic displacement at the interface lead to the formation of GaAs-like and AlAs-like slab and surface phonon modes. Thus, a variety of possible intersubband relaxation channels exist rather than a single phonon mechanism, as is the case with bulk-like phonons. Here the authors study the effect of such modes on the intersubband scattering rate in asymmetric coupled quantum wells in which an ensemble Monte Carlo simulation of electrons and holes is used to compare with recent time-resolved photoexcitation experiments. Their results show that, when energetically permitted, the dominant contribution to the transfer time between wells is due to AlAs modes originating in the barrier rather than GaAs-like modes in the well.

Dynamics of coherent electron tunnelling in asymmetric coupled quantum wells under longitudinal magnetic fields

The authors have studied the dynamics of electrons in a Ga1-xAlxAs-GaAs asymmetric double quantum well heterostructure under an external applied electric field. In this coupled quantum well system, they show that direct observation is possible of an oscillating luminescence signal with a quantum tunnelling period modulated by means of an applied longitudinal magnetic field.

Density dependence of nonresonant tunneling in asymmetric coupled quantum wells.

We have measured tunneling rates in a series of asymmetric coupled quantum wells as a function of the barrier thickness and carrier density between confined levels in which optical-phonon-assisted tunneling is energetically forbidden. Our measurements have demonstrated a strong density dependence of the tunneling rates. The measured dependence is qualitatively consistent with a model of excitons diffusing to a limited number of extrinsic tunneling sites.

Switching between two mid-infrared intersubband absorption energies in double quantum wells

We present a new efficient modulators structure in which electron tunneling between asymmetric coupled quantum wells is used to populate or deplete the ground level for the infrared absorption, leading to externally controlled infrared absorption at two different wavelengths (here 10.2 μm and 11.4 μm). The temperature dependence of the modulation depth is measured. The experimental results are corroborated by calculations based on a self-consistent model. Monochromatic electron transfer infrared modulators are also presented.

Resonance effects in the carrier-tunneling dynamics in asymmetric coupled quantum wells.

We present a detailed experimental study of the tunneling times, after picosecond excitation, of electrons and holes in a large set of unbiased GaAs/${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As asymmetric double-quantum-well structures, pointing out the role played by resonances in the tunneling process. In particular, fast hole tunneling is found when the quantum wells show band alignment both at ${\mathit{k}}_{\mathrm{?}}$=0 and at ${\mathit{k}}_{\mathrm{?}}$\ensuremath{\ne}0. Referring to electrons, a resonance is found in the tunneling time, which corresponds to phonon-assisted processes. Experimental data at high excitation intensity are also reported, suggesting band-filling effects in the wider well.

Observation of anomalously large blue shift of the heavy-hole photocurrent peak and optical bistability in narrow asymmetric coupled quantum wells

Blue shift of the heavy-hole peak of the photocurrent spectra has been observed, for the first time, in narrow GaAs/Al0.4Ga0.6As asymmetric coupled quantum wells near the heavy-hole resonance. With an external reverse bias of only −2.35 V, a maximum upward shift of the apparent peak position of ∼6.1 meV has been measured at 78 K. Sharp change of the inhomogeneous linewidth of the heavy-hole peak has also been observed. cw optical bistability has been observed with the external feedback.

Tunneling assisted modulation of the intersubband absorption in double quantum wells

We present a new efficient modulator structure in which electron tunneling between asymmetric coupled quantum wells is used to populate or deplete the ground level for the infrared (IR) absorption, leading to externally controlled IR absorption at two different wavelengths (here 10.2 and 11.4 μm). The temperature dependence of the modulation depth is measured. We have used photoluminescence and photoluminescence excitation spectroscopies to analyze the space charge effects in the double quantum wells, and to estimate the carrier concentration in each well as a function of bias. The behavior of the device has been simulated using a self-consistent model.

Strong excitonic nonlinearity in a P-I-N photodiode incorporating narrow asymmetric coupled quantum wells

Strong excitonic nonlinearity in the photoconductive response of a P-I-N photodiode incorporating narrow asymmetric coupled quantum wells has been observed at 78 K. When the P-I-N photodiode is overbiased, the heavyhole energy levels in the two coupled quantum wells are moved toward the resonance by increasing the laser intensity. Also, both the light-hole and the heavy-hole excitonic transitions undergo intensity-dependent shifts. Both these effects indicate intrinsic change of bias due to redistribution of photogenerated carriers and, therefore, the existence of an intrinsic feedback mechanism. The magnitude of the blue shift of the heavy-hole excitonic transitions significantly increases when the laser intensity is changed from 9.2 to ~270 mW/cm(2).

Coherent tunneling of mixed state hole wave packets in coupled quantum well structures

The time-dependent Schrödinger equation is solved numerically to study the coherent tunneling of hole wave packets in asymmetric coupled quantum wells. The importance of selection rules and band mixing is evident in the extremely low rates of the wave-packet leakage from heavy-hole state to a resonant light-hole state at zero in-plane wave vector (k∥). But these rates increase dramatically away from k∥=0, when the hole states acquire mixed character, and rapidly become comparable to the heavy-hole to heavy-hole resonant tunneling rates. The effect of inhomogeneous level broadening arising from well size fluctuations in multicoupled quantum well systems is shown to greatly reduce the effective tunneling rates near resonance.

Independent and ambipolar tunneling in asymmetric-coupled quantum well structures

We report femtosecond pump-probe transmission spectroscopy experiments on GaAs/AlGaAs asymmetric-coupled quantum wells, using a spectrally narrow optical excitation at 740 nm. The time-resolved evolution of the bleaching demonstrates the tunneling of both carriers through the barrier. At high densities a single tunneling time is observed in opposition to the low density regime, where the electron and hole contributions are well separated.

An enhanced and linear Stark shift in n-i-n AlGaAs/GaAs symmetric and asymmetric coupled quantum wells

Stark shift in AlGaAs/GaAs symmetric and asymmetric coupled quantum wells is compared experimentally and theoretically. Symmetric coupled quantum wells possess a linear and enhanced Stark shift which is relatively insensitive to small perturbations in quantum well width and mismatch ratio. Asymmetric coupled quantum wells also possess a linear and enhanced Stark shift. However, in contrast to symmetric wells, the Stark shift is quite sensitive to well width and mismatch ratio. The linear and enhanced Stark shift in AlGaAs/GaAs asymmetric and symmetric coupled quantum wells is experimentally confirmed using photoluminescence on an n-i-n device structure.

Electric field effects in AlGaAs-GaAs symmetric and asymmetric coupled quantum wells

A theoretical study of energy level shift and field-induced tunneling in symmetric and asymmetric coupled quantum wells is presented. Energy level shift is calculated from the time-dependent Schrodinger equation using the inverse power method. The time evolution of an electron wavepackage is shown by the application of the time-development operator of the time-independent Schrodinger equation. Energy level shift in coupled quantum wells is found to be enhanced in comparison to single quantum wells. The energy level shift in coupled quantum wells is found to be nearly linear with the applied field. Oscillation frequencies for electrons in symmetric and asymmetric coupled quantum wells are evaluated versus the applied field and compared to semiclassical prediction. Tunneling lifetimes in symmetric and asymmetric coupled quantum wells are evaluated versus the applied field. >

Electroreflectance studies of asymmetrically coupled quantum wells

Electroreflectance (ER) spectroscopy has been used to study asymmetric coupled quantum wells in the GaAs AlGaAs and InGaAs GaAs systems, having 5 nm and 10 nm wells separated by either 1.5 nm or 2.5 nm barriers. We find that the line shapes and relative intensities of the n = 1 allowed interband transitions are very sensitive to the coupling between the wells. This leads to changes in the ER line shape, which can be used to characterize the resonance of the coupling in terms of a modulated dielectric function. The modulation observed is due to both a gap modulation and an intensity modulation caused by the electric field. This allows for a probe of the behavior of the coupled system in terms of wave function transfer between adjoining wells.

Giant nonlinear optical rectification at 8–12 μm in asymmetric coupled quantum wells

We report giant, nonlinear optical rectification in asymmetric quantum wells weakly coupled by an intermediate potential barrier. This phenomenon originates from (i) macroscopic displacements (30 nm) of carriers during optical transitions and (ii) large storage times of excited electrons because of a slow transfer mechanism between the wells (≊6 ps at 77 K). The resulting rectification coefficient is 1.62×10−3 m/V per well, more than six orders of magnitude higher than in bulk GaAs. These structures really behave as giant ‘‘quasimolecules’’ optimized for infrared optical nonlinearities and their use may be envisioned for a new class of infrared detectors.

Electroabsorption studies on GaAs asymmetric coupled quantum wells

We report the results of electroabsorption measurement on GaAs asymmetric coupled quantum wells done at low temperature (4 K) and at room temperature (300 K). Rapid quenching and enhancement of excitonic absorption peaks is achieved at relatively low bias electric fields of the order 10 kV/cm. This is explained in terms of level anti-crossing and the subsequent switching of intrawell (spatially direct) and interwell transitions (spatially indirect) as the lowest lying conduction energy levels of the two wells are brought to resonance.

Electroabsorption in In0.53Ga0.47As/In0.52Al0.48As asymmetric coupled quantum wells grown on InP substrates.

We report the results of photocurrent measurements on ${\mathrm{In}}_{0.53}$${\mathrm{Ga}}_{0.47}$As/${\mathrm{In}}_{0.52}$${\mathrm{Al}}_{0.48}$As asymmetric coupled quantum wells lattice matched to InP substrates. We show that large shifts in excitonic absorptions with applied bias result from the resonant coupling of electron subbands. Results of our measurements agree well with calculations performed with use of a single-band envelope-function approximation that incorporates a scattering--phase-shift method for determining the quasibound electron- and hole-subband states of the coupled-well system in an electric field.

Direct measurement of resonant and nonresonant tunneling times in asymmetric coupled quantum wells.

We have directly measured the tunneling times in an asymmetric coupled double-quantum-well structure using subpicosecond luminescence spectroscopy. We observe a strong dependence of the nonresonant tunneling times on the barrier thickness. The most striking feature of the data is a drastic reduction of the tunneling time and then a further increase as an applied electric field sweeps electron levels in the two wells through a resonance. The measured resonant tunneling time is considerably longer than the estimated coherent tunneling time.

Electroabsorption and electrorefraction in GaAs/AlGaAs waveguides containing asymmetric coupled quantum wells

Changes in the optical phase and transmission with applied electric fields were measured in GaAs/AlGaAs waveguides containing asymmetric coupled quantum wells. At sufficiently high applied bias, the lowest energy optical transition supported by this system is that between the lowest energy hole subband of the wide well and the lowest energy electron subband of the narrow well. We show that this interwell transition produces significant contributions to both the electroabsorption and the electrorefraction in the coupled quantum well system.

Charge-transfer-state photoluminescence in asymmetric coupled quantum wells.

We have performed continuous and time-resolved photoluminescence experiments on novel double-quantum-well structures in Schottky diodes. We have directly observed the buildup of a charge-transfer (CT) state in which the electrons and holes are in separate wells because of the fact that they tunnel in opposite directions. We have studied the effect of an electric field on the CT state formation, and have observed a strong, linear Stark shift of the CT luminescence.