Improvement in intersubband optical absorption and the effects of device parameter variations in quantum wells with an applied electric field

The intersubband optical absorption in symmetric and asymmetric, single and coupled, double GaAs/ Ga_{1 − x}Al_xAs quantum wells is calculated. The results have been obtained in the presence of a uniform electric field as a function of the potential symmetry, size of the quantum well, and coupling parameter of the wells. In coupled double quantum wells we obtain a large Stark effect that can be used to fabricate tuneable photodetectors. We show that the effect of an applied electric field on the intersubband optical absorption is similar to changes in the dimensions of the structure. This behaviour in the intersubband optical absorption for different wells and barrier geometries can be used to study these systems in regions of interest, without the need for the growth of many different samples.     

Intersubband relaxation of hot carriers in quantum well systems

We use an ensemble Monte Carlo simulation of coupled electrons, holes and nonequilibrium polar optical phonons in multiple quantum well systems to model the intersubband relaxation of hot carriers measured in ultra-fast optical experiments. We have investigated the effect of various models of confined photon modes on the energy relaxation and intersubband transition rate in single quantum well and coupled well systems. In particular, the symmetry of the atomic displacement with respect to the quantum well has a marked effect on the relative intersubband versus intrasubband scattering rates, depending on whether one considers electrostatic boundary conditions(slab modes) or mechanical boundary conditions(guided modes). In single quantum wells systems, the overall intersubband relaxation time is not found to be strongly dependent on the confined mode model used due to competing effects of hot phonons and the relative intrasubband scattering rates. For coupled well systems, the relaxation rate is much more dependent on the exact nature of the phonon amplitude. Large effects are found associated with localized AlAs interface modes which dominate the intersubband relaxation time.     

Intersubband transitions from the second subband in a coupled quantum wells structure

Asymmetrical coupled quantum wells structures with energy separation between the first two subbands of the order of 10–50 meV are key structures in the design of optically pumped intersubband lasers. In these structures the population of the second subband is not negligible and intersubband transitions from the second to higher excited subbands can be observed. In this work we investigate the temperature dependency of the intersubband transitions from the second subband in an asymmetrical coupled quantum wells structure. We show that this approach provides a direct way to measure the energy separation between the second subband and the Fermi energy which is a crucial parameter in the design of optically pumped intersubband lasers.     

Effects of intersubband interaction on multisubband electron transport in single and double quantum wells

The multisubband electron transport properties are studied for doped single quantum well and gated double asymmetric quantum well structures. The effects due to intersubband interaction and screening of the ionized impurity scattering are also investigated. We show that intersubband coupling plays an essential role in describing the screening properties as well as the effect of ionized impurity scattering on the mobility in a doped single quantum well. For coupled double quantum well structures, negative transconductance is found theoretically which is due to resonant tunneling between the two quantum wells.     

Electrically injected cavity polaritons

We have realized an electroluminescent device operating in the light-matter strong-coupling regime based on a GaAs/AlGaAs quantum cascade structure embedded in a planar microcavity. At zero bias, reflectivity measurements show a polariton anticrossing between the intersubband transition and the cavity mode. Under electrical injection the spectral features of the emitted light change drastically, as electrons are resonantly injected in a reduced part of the polariton branches. Our experiments demonstrate that electrons can be selectively injected into polariton states up to room temperature.     

Intersubband excitations in single-and double-layer electron systems in a parallel magnetic field

The spectrum of neutral intersubband excitations in single and double quantum wells has been studied by the inelastic light scattering method. It is shown that excitation energies in an external magnetic field have an anisotropic component proportional to the dipole moment of excitations along the growth axis of the quantum wells. Consequently, the measurement of excitation energy in a magnetic field makes it possible to experimentally estimate the quantitative measure of asymmetry of the quantum wells (dipole moment of the intersubband transition). In addition, a parallel magnetic field makes it possible to considerably extend the range of momenta studied since it shifts the dispersion curves in the momentum space by the value of the anisotropic component. A new method is proposed for determining the symmetry of double quantum wells. In asymmetric wells, intersubband excitations appear between the layers and have a large dipole moment along the growth axis. In symmetric wells, the magnetic field itself induces the dipole moment of intersubband excitations so that the excitation spectrum does not change upon magnetic field inversion. Analysis of energy anisotropy in intersubband excitations in double quantum wells makes it possible to determine the symmetry of double wells to a high degree of accuracy.     

Ultrafast all optical modulation based on intersubband transition in semiconductor quantum wells

Ultrafast modulation of interband-resonant light by intersubband-resonant light in n-doped GaAs/AlGaAs and GaN/AlGaN quantum wells was investigated by femtosecond pump-probe technique. A planar-type AlGaAs/GaAs modulation device shows a modulation speed of ～1 ps at room temperature. The observed modulation efficiency indicates that 99% modulation can be achieved with a control pulse energy of ～1 pJ when a waveguide-type device structure is utilized. The feasibility of the all-optical modulation in GaN/AlGaN quantum wells is also investigated. The intersubband carrier relaxation time, which mainly determines the modulation speed, is measured and is found to be extremely fast (130–170 fs). The results indicate that the optical modulation at a bit rate of over 1 Tb/s will be possible by utilizing the intersubband transition in GaN/AlGaN quantum wells. The modulation efficiency in GaN/AlGaN quantum wells is also discussed in comparison with that in GaAs/AlGaAs quantum wells.     

The dependence of the intersubband transitions in square and graded QWs on intense laser fields

The intersubband absorption in square and graded quantum wells under a laser field is calculated within the framework of the effective mass approximation. We conclude that, for quantum wells with different shapes, the laser field amplitude induces an important effect on the electronic and optical properties of the semiconductor structure. This gives a new degree of freedom in various device applications based on the intersubband transition of electrons.     

Simultaneous effects of laser field and hydrostatic pressure on the intersubband transitions in square and parabolic quantum wells

The intersubband transitions in square and parabolic quantum wells under simultaneous action of the hydrostatic pressure and high-frequency laser field have been investigated. We found that the laser-induced blueshift effect on the subband energy levels may be tuned by the pressure action. Our calculations revealed that the oscillator strength of the transition between the ground and the first excited levels depends on the quantum well width and shape, laser field intensity and hydrostatic pressure. This combined effect of pressure and laser field offers a new degree of freedom in the optoelectronic devices applications.     

Intersubband transitions in band gap engineered parabolic potential wells

Intersubband transitions in spike-inserted wide parabolic quantum wells are investigated. A thin potential barrier within the pure parabola devides the electron system in two well separated but strongly coupled layers, which in turn drastically changes the collective excitations scheme. In contrast to a pure parabolic quantum well where according to the generalised Kohn's Theorem only one fixed resonance is observed, the collective intersubband transitions recover the complex coupling and splitting scheme of the single particle states of a strongly coupled system. We interpret our experimental findings in terms of resonant tunnel processes and discuss them using simple model calculations.     

Femtosecond pump and probe measurement of all-optical modulation based on intersubband transition in n-doped quantum wells

An all-optical modulation of interband-resonant light (near-infrared signal light: 800 nm) by intersubband-resonant light (mid-infrared control light: 4–7 μm) in n-doped AlGaAs/GaAs multiple quantum wells is investigated by two-color femtosecond pump–probe experiments at room temperature. The modulation of the near-infrared signal light with an ultrafast recovery as short as 1 ps is successfully observed when the quantum wells are pumped by the mid-infrared control light pulse (4 fJ/μm²). The dependence of the modulation depth on the wavelength of the control light is also measured, which is shown to be consistent with the intersubband absorption spectrum of the quantum wells. The results indicate that the utilization of the intersubband transition is promising for the ultrafast all-optical modulation and switching.     

Optical second-harmonic generation in coupled double two-level quantum wells

On the baisis of a microscopic local-field theory, a non-local analysis of the second harmonic generation arising from intersubband transitions in coupled double quantum wells is presented. Taking as a start point an integral equation for the local field, the local fields at both the fundamental and second harmonic frequencies for a p-polarized incident field are determined. In a reflection geometry, the conversion efficiency of the second harmonic generation from the coupled double quantum wells is derived. Numerical calculations for different parameters show that the influence of the electronic non-local effects on the second harmonic generation can be significant.     

Photoinduced infrared spectroscopy of bound-to-bound and bound-to-continuum transitions in SiGe/Si quantum wells

Infrared spectroscopy of intersubband transitions in the valence band of undoped SiGe/Si quantum wells is presented. Optical pumping of interband transitions is used to generate carriers in the wells. The spectral features of bound-to-bound and bound-to-continuum transitions are analyzed and compared to those of GaAs quantum wells. In samples with only one heavy hole bound level, a ratio of 20:1 is observed between intersubband and free carrier absorption. Room temperature photo-induced absorption is only observed in samples with high germanium content (≈50%). The feasibility of normal-incidence infrared modulators based on s-polarized intersubband absorption is also demonstrated. Resonant dispersion associated with intersubband transitions is evidenced.     

Bragg polaritons: strong coupling and amplification in an unfolded microcavity

Periodic incorporation of quantum wells inside a one-dimensional Bragg structure is shown to enhance coherent coupling of excitons to the electromagnetic Bloch waves. We demonstrate strong coupling of quantum well excitons to photonic crystal Bragg modes at the edge of the photonic band gap, which gives rise to mixed Bragg polariton eigenstates. The resulting Bragg polariton branches are in good agreement with the theory and allow demonstration of Bragg polariton parametric amplification.     

Optimization of a solar-blind and middle infrared two-colour photodetector using GaN-based bulk material and quantum wells

This paper calculates the wavelengths of the interband transitions as a function of the Al mole fraction of Al_xGa_1-xN bulk material. It is finds that when the Al mole fraction is between 0.456 and 0.639, the wavelengths correspond to the solar-blind (250~nm to 280~nm). The influence of the structure parameters of Al_yGa_1-yN/GaN quantum wells on the wavelength and absorption coefficient of intersubband transitions has been investigated by solving the Schr?dinger and Poisson equations self-consistently. The Al mole fraction of the Al_yGa_1-yN barrier changes from 0.30 to 0.46, meanwhile the width of the well changes from 2.9~nm to 2.2~nm, for maximal intersubband absorption in the window of the air (3~μm <λ <5~μm). The absorption coefficient of the intersubband transition between the ground state and the first excited state decreases with the increase of the wavelength. The results are finally used to discuss the prospects of GaN-based bulk material and quantum wells for a solar-blind and middle infrared two-colour photodetector.     

Infrared laser based on intersubband transitions in quantum wells

In this paper, the current status of intersubband lasing in quantum wells is briefly reviewed, and the physical features related to intersubband infrared lasers are discussed. New device structures based on leaky quantum wells for the realization of intersubband lasing are proposed. It is shown that these newly proposed leaky quantum well device structures are promising for infrared lasers, not only in the creation of population inversion, but also in the suppression of nonradiative loss. Also, some future perspectives are discussed.     

A concept for a tunable antenna-coupled intersubband terahertz (TACIT) detector

Detectors based on intersubband transitions in quantum wells have great potential for use between one and several terahertz. We propose a tunable, antenna-coupled, intersubband terahertz (TACIT) detector that is both sensitive and fast, with a speed limited only by the intersubband relaxation rate (1 ns at at ). The detector is sensitive over a narrow range of frequencies, and the frequency of peak absorption can be tuned by applying a bias voltage to the device.     

Cross-phase-modulation-based wavelength conversion using intersubband transition in InGaAs/AlAs/AlAsSb coupled quantum wells

We report an ultrafast cross phase modulation (XPM) effect in intersubband transition (ISBT) of InGaAs/AlAs/AlAsSb coupled quantum wells, where the ISBT absorption of a transverse-magnetic mode pump signal induces phase modulation of a transverse-electric mode probe signal. Using waveguide-type ISBT devices, we have achieved XPM-based 10 Gbit/s wavelength conversion with a power penalty of 2.53 dB. Also, we propose XPM-based signal processing circuits for gate switching and modulation format conversion.     

加偏置电场的双曲线量子阱中线性与三阶非线性光学吸收系数的计算

利用紧致密度矩阵近似方法,研究了加偏置电场双曲线量子阱中的线性与三阶非线性光学吸收系数. 得到了该系统中的线性与三阶非线性光学吸收系数的解析表达式.分析了势阱的形状、外加电场的大小以及入射光场的强度对吸收系数的影响规律. 文章以典型的AlxGa1－xAs/ GaAs双曲线量子阱为例作了数值计算.结果表明：随着势阱宽度的增加,系统的吸收系数将减小;随着外加电场的增加,系统的非对称性增加,系统的吸收系数将增加;随着外加光场强度的增加,系统的吸收系数将减小,并且当光强增加到一定值时会出现明显的饱和吸收现象,这一结论为进一步的实验研究提供了相应的理论依据.