Electronic structure of interdiffused GaInAs(P)/GaInAsP quantum wells

Band structure calculations are performed by k·p theory on lattice-matched (LM) and strain-compensated (SC) interdiffused GaInAs(P)/GaInAsP quantum wells (QWs) designed for 1.55 μm wavelength response. The evolution of the in-plane band structures as a function of the diffusion length (up to 5 nm) is presented and discussed. The subbands are tightened with consecutive changes in their curvatures and interactions. In the case of LM structures, a densely packed valence subband structure is found with spiky singular behaviors in the curves of density of states. These arise from electron-like subbands and are strongly modified after interdiffusion. The in-plane effective masses of carriers involved in the fundamental excitonic transitions increase by 15% (electrons) and 25% (holes). The subband tightening trend is strongly marked in the case of SC structures with uniform cationic composition, for which narrow QWs are required. On the other hand, this trend is weak in the case of SC structures with uniform anionic composition, in which QWs are rather wide. In this latter case, a good stability in the optical properties of the structures after thermal processing is expected. However, due to the tensile strain in the wells, only heavy holes (HHs) can be confined. Since in-plane motion of confined holes involves HH and light hole (LH) mixing, this restricts the dynamic characteristics expected for these structures.     

Interdiffusion at the BaCuSeF/ZnTe interface

BaCuSeF/ZnTe is a model system to investigate physical and chemical properties of the interfaces of non-oxide wide-bandgap p-type semiconductors with materials used in chalcogenide solar cells. The BaCuSeF/ZnTe interface was studied using electron microscopy and photoelectron spectroscopy. Both techniques indicate that Se and Cu from BaCuSeF diffuse into ZnTe creating an interdiffused layer between these two materials. The interdiffusion may be attributed to the differences in materials formation enthalpies and to Fermi level pinning in BaCuSeF.     

Magnetic Fields From Nuclear Polarization in Parabolic Quantum Wells

We present analytical results for the position and time dependence of dynamic nuclear polarization in parabolic quantum wells. As a result of the hyperfine interaction, induced nuclear fields in the sample can be as high as a few gauss, leading to a nuclear magnetic resonance frequency shift of the order of kilohertz. We find that the particular geometry of the parabolic quantum well permits efficient electric field control of the position dependence of the nonequilibrium magnetic field due to the nuclei.     

Magnetic Fields From Nuclear Polarization in Parabolic Quantum Wells

We present analytical results for the position and time dependence of dynamic nuclear polarization in parabolic quantum wells. As a result of the hyperfine interaction, induced nuclear fields in the sample can be as high as a few gauss, leading to a nuclear magnetic resonance frequency shift of the order of kilohertz. We find that the particular geometry of the parabolic quantum well permits efficient electric field control of the position dependence of the nonequilibrium magnetic field due to the nuclei.     

Interdiffusion effect on quantum-well structures grown on GaSb substrate

We have modeled the effect of compositional interdiffusion on the optical properties of GaSb/AlGaSb and InGaAsSb/AlGaAsSb quantum-well structures grown on GaSb substrate. Blue shifts of emission wavelength as large as 270 nm and 700 nm are predicted from a 6 nm wide interdiffused GaSb/AlGaSb quantum-well for a diffusion length of 3 nm, and from a 10 nm wide interdiffused InGaAsSb/AlGaAsSb quantum-well for a diffusion length of 5 nm, respectively. The effects of the as-grown quantum-well width and applied electric field on the emission wavelength and their relationship to the interdiffusion are also investigated.     

GaxIn1-xAs/GaInAsP应变量子阱结构能带的计算

对含有Luttinger-Kohn哈密顿量的有效质量方程，利用S.L.Chuang提出的传递矩阵法，计算了量子阱中不同Ga组分的GaxIn1-xAs/GaInAsP应变量子阱结构的能带，该结构可被选作980nm光通信泵浦激光器的有源层，研究还得到了GaxIn1-xAs/GaInAsP双应变量子阱结构中电子和空穴的能级以及能级的色散关系。     

Mid/far-infrared few-cycle-pulse emission via resonant mixing in semiconductor heterostructures

Abstract

Mid/far-infrared emission from a semiconductor multiple quantum well structure under femtosecond optical pulse excitation is studied. It is shown that resonant nonlinear-wave mixing in the quantum wells can be used for the generation of ultra-short mid/far-infrared pulses with a duration of a few cycles or even a single cycle. Explicit analytical formulas for the mid/far-infrared radiation field and polarization in a simple three-level model of a quantum well are presented and compared with numerical simulations. The power of the mid/far-infrared emission and the down conversion efficiency of the resonant nonlinear-wave mixing are discussed.     

Mixing of states in quantum wells for terahertz polariton emitters

Two possible InGaAs/GaAs quantum-well structures ensuring the presence of radiative transitions between the polariton states in a microresonator with a quantum well, which are accompanied by generation of terahertz photons, are discussed in this work. For the first structure, symmetry breakdown that is required for the emission of a terahertz photon is conducted in a quantum well with refractive index gradient profile, which results in mixing of the states of a polariton and a dark exciton. Parameters of the quantum well, in which the energy of the second exciton level corresponds to the upper polariton energy, are determined. A double quantum well with exciton states split due to quantum-mechanical tunneling through a barrier is used in the second structure. Symmetry breakdown, which allows one to mix an exciton with a “dark” exciton, is ensured by adjusting the energy of electron levels in a double quantum well by applying an electric field to the structure. A hole remains localized in one of the wells.     

Injection laser with a functionally integrated frequency modulator based on spatially shifted quantum wells

A method for designing injection lasers with functionally integrated optical-radiation frequency modulators based on spatially shifted quantum wells in conduction and valence bands is proposed. The structure and variants of band diagrams of functionally integrated laser modulators are considered. It is shown that, in the proposed nanoheterostructures, maximum modulation frequencies are determined by the time of controlled relocation of charge-carrier density maxima in quantum domains and correspond to the terahertz range.     

Quantitative analysis of molecular interaction in a microfluidic channel: the T-sensor

The T-sensor is a recently developed microfluidic chemical measurement device that exploits the low Reynolds number flow conditions in microfabricated channels. The interdiffusion and resulting chemical interaction of components from two or more input fluid streams can be monitored optically, allowing measurement of analyte concentrations on a continuous basis. In a simple form of T-sensor, the concentration of a target analyte is determined by measuring fluorescence intensity in a region where the analyte and a fluorescent indicator have interdiffused. An analytical model has been developed that predicts device behavior from the diffusion coefficients of the analyte, indicator, and analyte--indicator complex and from the kinetics of the complex formation. Diffusion coefficients depend on the local viscosity which, in turn, depends on local concentrations of all analytes. These relationships, as well as reaction equilibria, are often unknown. A rapid method for determining these unknown parameters by interpreting T-sensor experiments through the model is presented.     

光孤子对量子态的高速调制

从高速调制的角度,研究了量子光通信系统。提出用孤子调制承载量子态的方案。方案着重从调制量子态的通信信道出发,分别论证了孤子在NLSE光纤和SIT媒介中的传输演变;得出量子压缩态和纠缠态可以在二级传输线(由传统光纤和支持自激励透明孤子的二级媒介组成)中,由孤子的传输演变产生;从而实现孤子对量子态的调制。这种调制方案,利用了光的波粒二重性,融合了量子和孤子的特性,是未来量子光通信的一种发展趋势。     

Spin Polarization by Tilted Magnetic Field in Wide Ga1?xAlxAs Parabolic Quantum Wells

Wide parabolic wells can be created by properly controlling the Al content during the growth of successive Ga_1–xAl_xAs thin layers. Under a tilted magnetic field these systems present interesting transport properties, which are associated to their composition dependent g-factor. We present an exact solution for the eigenstates of an electron gas inside such a quantum well. We calculate the renormalized cyclotron frequencies as functions of the angle of tilt, as well as the density of states, and the Fermi level. We discuss the conditions for the existence of spin-polarized charge.     

Spin Polarization by Tilted Magnetic Field in Wide Ga<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Al<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>As Parabolic Quantum Wells

Wide parabolic wells can be created by properly controlling the Al content during the growth of successive Ga_1–xAl_xAs thin layers. Under a tilted magnetic field these systems present interesting transport properties, which are associated to their composition dependent g-factor. We present an exact solution for the eigenstates of an electron gas inside such a quantum well. We calculate the renormalized cyclotron frequencies as functions of the angle of tilt, as well as the density of states, and the Fermi level. We discuss the conditions for the existence of spin-polarized charge.     

Conduction intersubband transitions at normal incidence in Si(1-x)Ge(x) quantum well devices

We show theoretically that it is possible to design SiGe-based quantum well structures in which conduction intersubband transitions are induced by normal incidence infrared radiation. A sp(3)d(5)s(*) tight binding model has been adopted to evaluate the electronic states and optical transitions between lowest conduction confined states of a superlattice composed of one pure Ge quantum well separated by SiGe alloys, grown along the [001] direction. We find that significant optical coupling between confined states in the Ge wells is achieved at normal incidence radiation by the off-diagonal elements of the mass tensor. The minimum energy Ge conduction valleys are, in fact, tilted with respect to the [001] growth axis. For comparison we show that no such coupling can be realized for the conduction states confined in a similar structure composed by Si quantum wells because the ellipsoids of the lowest conduction valleys are oriented along the growth direction.     

Broadband and polarization-insensitive metamaterial absorber based on hybrid structures in the infrared region

In this work, a new type of metamaterial absorber is proposed in the infrared region. This structure consists of metal-dielectric-metal. But the difference is that a square groove in the dielectric has been dug, and replaced by a metal. The simulated results show that this structure can achieve a broadband absorption. And the absorption bandwidth can be realized from 60.5 to 115.5 THz when the absorption efficiency is larger than 90%. And this structure is polarization-insensitive for incident electromagnetic waves. In addition, the structure can also achieve better absorption effect for a large incident angle, especially for TM polarized wave. What is more, a remarkably enhanced bandwidth can be realized by using a metal to fill the square groove which is dug in the dielectric. To further explain the mechanism of high absorption, the distribution of the electromagnetic field and power loss density at the resonance frequencies are analysed. And these novel properties make the absorbers have many applications including sensor, cloaking, etc.     

The impact of electron beam damage on the detection of indium-rich localisation centres in InGaN quantum wells using transmission electron microscopy

High-resolution transmission electron microscope (HRTEM) lattice fringe images and lattice parameter maps are used to reveal the rapid modification of InGaN quantum wells by the electron beam in a TEM. Images acquired within seconds of first irradiating a region of quantum well do not exhibit the strong nanometre-scale strain contrast which has been reported to signify the presence of very indium-rich regions in InGaN quantum wells. However, after a very short period of irradiation by a relatively low electron beam current density, images of the specimen could be interpreted as indicating the presence of these indium “clusters”. The beam damage is shown to occur for the InGaN quantum wells grown in our lab as well as those in a commercial blue light emitting diode (LED) and in TEM specimens prepared only using mechanical polishing. Possible mechanisms for the beam damage are discussed and we make suggestions as to what may cause exciton localisation in quantum wells that do not contain gross composition fluctuations.     

Interdiffusion studies in bulk Au–Ti system

The performance of the contacts, where Au/Ti layers are used in the metallization scheme, largely depends on the product phases grown by interdiffusion at the interface. It is found that four intermetallic compounds grow with narrow homogeneity range and wavy interfaces in the interdiffusion zone. The presence of wavy interfaces is the indication of high anisotropy in diffusion of the product phases. This also reflects in the deviation of parabolic growth from the average. Further, we have determined the relevant diffusion parameters, such as interdiffusion coefficient in the penetrated region of the end members and integrated diffusion coefficients of the intermetallic compounds.     

Quantum encoder and decoder for practical quantum key distribution using a planar lightwave circuit

To launch quantum key distribution (QKD) into the commercial market, it is important to develop a system that is simpler and more reliable using current technology. This report proposes quantum encoders and decoders using a passive planar lightwave circuit (PLC) that is useful for implementing optical-fiber-based QKD systems. Our encoders and decoders are based on an asymmetric Mach–Zehnder interferometer and allow us to prepare and analyze various photonic time-bin qubits reliably. The system can be stable and polarization-insensitive merely by stabilizing and controlling the device temperature. Our PLC-based devices enables us to simplify the QKD system and increase its reliability.     

Analysis of optical gain and threshold current density of 980 nm InGaAs/GaAs compressively strained quantum well lasers

The valence hole subbands, optical gain spectra and threshold current density of InGaAs/GaAs compressive-strained quantum wells (QWs) were studied using a numerical approach. We found that a higher In composition in the quantum well and a thicker well give longer emitting wavelength; a narrower well and higher In composition lead to higher TE mode peak gain. The result also shows a suitable combination of In composition, QW thickness and number of QWs should be selected to achieve low threshold current density.     

Selective modification of the band gaps of GaInNas/GaAs structures by quantum well intermixing techniques

We report the unambiguous demonstration of controlled quantum well intermixing (QWI) in the technologically important GaInNAs/GaAs 1.3 μm material system. QWI is a key technique to selectively modify the band gap of quantum wells, which has found broad application in semiconductor lasers and photonic integrated circuits (PICs). Extending such technology to GaInNAs/GaAs structures is highly desirable due to the technologically advantageous properties of this material system. Here, we investigate well-characterized GaInNAs quantum well material which has been annealed “to saturation” before QWI processing to allow unambiguous interpretation of results. After RTA at 700 °C for ∼180 s, controlled shifts in band-gap at room temperature of up to 200 nm have been observed in sputtered SiO₂-capped samples, whilst uncapped and PECVD SiO₂-capped samples demonstrated negligible shift. This selective modification of the band gap has been confirmed by detailed photoluminescence (PL) and photoluminescence excitation (PLE) spectroscopy. Analysis of composition profile by SIMS revealed that the QWI is due to the interdiffusion of In–Ga between the quantum wells and the barriers enhanced by the point defects generated during the sputtering process. Investigation of a series of samples of differing N concentrations will be presented, which provides extra information about the intrinsic properties of GaInNAs.