Asymmetric Coupled Quantum Wells Structure Research Articles

OPTICAL PROPERTIES AND CARRIER DYNAMICS IN ASYMMETRIC COUPLED InGaN MULTIPLE QUANTUM WELLS

Optical properties and carrier dynamics of InGaN/GaN asymmetric coupled quantum wells (ACQWs) are studied by excitation-power-dependent photoluminescence (PL), photoreflectance (PR) and time-resolved PL (TRPL) experiments. Under weak excitations, only the emission from the widest well is observed due to the tunneling from narrower to wider wells. Under strong excitations, the carrier distribution becomes more uniform and an enhanced emission from the mid well (2.5 nm well) is observed. Dependence of the PL intensity on excitation power is well explained by a rate equation model. The energy levels in the ACQW structure are clearly revealed by PR measurements and are in good agreement with calculations. Our results indicate that the enhanced emission from the mid well is ascribed to "reverse tunneling" from 3.0 to 2.5 nm well, which is confirmed by TRPL experiments.

Behaviours of excitonic optical nonlinearity associated with a large detuning rate photonic echo in biased asymmetric coupled quantum well structure

The third-order optical nonlinear susceptibility χ(3) responsible for phase-conjugate beam in non-degenerate four-wave mixing for a biased asymmetric coupled quantum well (ACQW) structure are given by the doubled-side Feynman diagram technique. It is shown that the photonic echo (PE) in excess of one terahertz pump-probe detuning rate can be achieved under low temperature condition due to electronic wavepaket oscillation when two laser beams interacts with the delocalized excitons in the ACQW structure. Also, the resonant peak position in the frequency domain of the large detuning rate PE depends strongly on an external inverse electric field strength but its peak intensity is insensitive to this one, resulting in the frequency conversion peak maintains a constant in a small bias range.

Tuneable frequency up-conversion based on biased asymmetric coupled quantum well structure

The behaviours of the optical nonlinear susceptibility χ(3) responsible for the phase-conjugate beam of frequency conversion in non-degenerate four-wave mixing (NDFWM) are studied for a biased asymmetric coupled quantum well (ACQW) structure. It is shown that the frequency up-conversion peak position determined by χ(3) is very sensitive to the external inverse electric field strength applied among the grown direction of quantum well but its value is insensitive to that. In other words, the frequency up-conversion peak has a large shift but its value maintains a constant when the electric field strength increases in a small bias range. The characteristics of the tuneable-frequency and the power balance of the ACQW structure may provide the high-efficient wavelength conversion in the optical communication system.

Optical characterization of intersubband transitions in ZnxCd1‐xSe/Znx′Cdy′Mg1‐x′‐y′Se asymmetric coupled quantum well structures by contactless electroreflectance

AbstractContactless electroreflectance (CER) was used to characterize the possible optical transitions in a ZnxCd1‐xSe/Znx′Cdy′Mg1‐x′‐y′Se asymmetric coupled quantum well (ACQW) structure grown by molecular beam epitaxy for quantum cascade laser (QCL) applications. CER spectrum revealed a wide range of the possible optical transitions in ACQW structure. The ground state transition was assigned by comparing with the photoluminescence emission signal taken from the same structure. A comprehensive analysis of the CER spectrum led to the identification of various interband transitions. The intersubband transitions were estimated and confirmed by FTIR measurements. The results demonstrate the potential of using CER as a complimentary technique for the contactless and nondestructive characterization of the wide band gap II‐VI ACQW structures for QCL applications (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Effect of hydrostatic pressure on the donor impurity states in GaN/AlGaN asymmetric coupled quantum wells

Based on the effective-mass approximation, the ground-state donor binding energy in zinc-blende(ZB) GaN/AlGaN asymmetric coupled quantum wells(QWs) is investigated variationally, considering the hydrostatic pressure effect. Numerical results show that the donor binding energy is highly dependent on the impurity positions, the asymmetric coupled QWs structure parameters and the hydrostatic pressure. It is found that the donor binding energy increases with increase in the hydrostatic pressure for any impurity position. The hydrostatic pressure has an obvious influence on the donor binding energy of impurity localized inside the wide well of the asymmetric coupled QWs. For any hydrostatic pressure, our results show that the donor binding energy is distributed asymmetrically with respect to the center of the asymmetric coupled QWs. In particular, for the impurity located inside the wide well, the donor binding energy is insensitive to the middle barrier width in ZB GaN/AlGaN asymmetric coupled QWs if the middle barrier width is large.

Photoreflectance and Fourier transform infrared spectroscopy study of intersubband transitions of a ZnxCd1−xSe/Znx′Cdy′Mg1−x′−y′Se asymmetric coupled quantum well structure for quantum cascade laser application

Photoreflectance (PR) and Fourier transform infrared (FTIR) spectroscopy were used to study the intersubband (ISB) transitions of a ZnxCd1−xSe/Znx′Cdy′Mg1−x′−y′Se asymmetric coupled quantum well (ACQW) structure for quantum cascade laser (QCL) application. The PR spectrum revealed a wide range of possible optical transitions in the ACQW structure. A comprehensive analysis of the PR spectrum led to the identification of various interband transitions. The ISB transitions were estimated and confirmed by FTIR absorption measurements. The results demonstrated the use of PR as a complimentary technique to FTIR for the contactless and nondestructive characterization of ACQW structures for QCL application.

Enhanced quantum‐confined Stark effect in ZnSe/ZnMgSSe asymmetric coupled quantum wells for blue‐ultraviolet optical modulators

We report on the enhanced quantum-confined Stark effects (QCSE) in ZnSe/ZnMgSSe asymmetric coupled quantum wells (ACQWs), and its application for blue-ultraviolet optical modulator. A new electronresonant-type of ACQW structure: [ZnSe(6ML)/ZnMgSSe(6ML)/ZnSe(12ML)] is designed and fabricated by molecular beam epitaxy (MBE). This new structure device exhibits a strong coupling of the electron envelope wavefunctions and a large Stark shift of ΔE ∼ 28 meV at room temperature. This field enhanced Stark shift is found to be superior to the usual heavy-hole-resonant type ACQW (ΔE ∼ 21 meV), which is consistent with the theoretical simulation based on the effective mass approximation. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Study of luminescence due to delocalized exciton recombination in asymmetric-coupled quantum well structure based on Ξ-type configuration

The luminescence properties due to the delocalized exciton recombination in the asymmetric-coupled quantum well (ACQW) structure have been studied based on Ξ-type configuration.We show that the luminescence has mainly two emission peaks:one peak corresponds to the red shift relative to the central transition frequency and the other to the blue shift.Compared with the single quantum well,this quantum-confined Stark shift is very sensitive to electric field.This means that the coupling between two wells for an ACQW structure leads to the enhancement of the quantum-confined Stark effect.From this optical property of ACQW,novel ultrafast optical modulation devices and optical switch fabricated with ACQW structure can be expected in the future optical communication system.

Exciton Binding Energy as a Function of Barrier Width in Coupled Quantum Well Structures

Excitons in a coupled quantum well structure (CQWS), consisting of two wells separated by a thick barrier, and in the two boundaries, the potential being infinite, are studied. The binding energy is calculated as a function of the barrier width. It is shown that there is a maximum in the binding energy as a function of the barrier width for an asymmetric CQWS, whereas the binding energy is a monotonically decreasing function of the barrier width for a symmetric CQWS. Moreover, the binding energy in an asymmetric CQWS is larger than that in a symmetric CQWS.