Search

A theoretical model of a semiconductor nanostructure consisting of a single-mode microresonator containing two quantum dots is considered. It is shown that the Coulomb interaction between electrons localized in the quantum dots modifies a spectral response of the system to an external laser field. The possibility of its use for detecting an elementary charge in the third (optically inactive) quantum dot is discussed. The influence of both diagonal (Stark effect) and non-diagonal (Foerster effect) Coulomb matrix elements of the Hamiltonian on the detection accuracy is studied. The dependences of a measuring contrast on the parameters of the resonator and the quantum dots are calculated. The existence of such structural configurations for which the contrast retains an optimal value even at large distances to the measured dot is established. [ABSTRACT FROM AUTHOR]

We introduce a convenient and nondestructive method for the quantitative study of symmetry-related factors in low-dimensional semiconductor structures. By experimentally monitoring the intrinsic valence-bands mixing effect (VBME) and its variation with external strain modulation, together with the application of effective-mass theory, intrinsic shear strain and asymmetric interface conditions can be simultaneously determined in InGaAs/InAlAs superlattices. The observed shear strain is of the order of 10 − 3 . Further analysis reveals that the VBME induced by asymmetric interface conditions is weaker than that induced by intrinsic shear strain. In addition, they exhibit a compensating relationship in the studied superlattices. The proposed method can be generalized for quantitative investigation of symmetry-related factors in many other semiconductor nanostructures in the framework of k ⋅ p perturbation theory. [ABSTRACT FROM AUTHOR]

Manipulation of the optical spin orientation in semiconductors is a key technology for realizing spin‐based photoelectric information processing. Application of magnetic field is a simple method to control the spin polarization degree through Zeeman splitting. However, this effect can only be achieved at cryogenic temperatures and in a strong magnetic field. Here, room‐temperature voltage control of optical polarization in the range of 3–15%, corresponding to a 12–60% change in relative spin polarization is demonstrated. For this, a III‐V semiconductor quantum dot tunnel coupled with a quantum well spin reservoir is used. The spin‐flip scattering rate within quantum dots is electric‐field‐controlled in the time domain of several tens of picoseconds. This is achieved by precise control of the tunnel injection efficiency of electrons and holes via coupled potential modification. The findings will pave the way for the generation of ultrafast spin‐modulated optical signals by the electric field effect. [ABSTRACT FROM AUTHOR]

Permanent dipole moments of electronic states in non-centrosymmetric materials play a pivotal role in many phenomena. Correctly evaluating them presents an arduous task and usually requires full knowledge of the band structure as well as understanding the intricate concepts of Berry curvature. Here, we show that in a few cases (e.g., zinc blende and wurtzite) a rather facile first-principle analytical derivation of the permanent dipole moments using L'Hôpital's rule can be performed, and the values and dispersion of these dipoles near high symmetry points can be found using just a couple of widely available material parameters. The results will hopefully contribute to better understanding of shift currents, optical rectification, and other electro-optical phenomena. [ABSTRACT FROM AUTHOR]

Single-charge pumps are the main candidates for quantum-based standards of the unit ampere because they can generate accurate and quantized electric currents. In order to approach the metrological requirements in terms of both accuracy and speed of operation, in the past decade there has been a focus on semiconductor-based devices. The use of a variety of semiconductor materials enables the universality of charge pump devices to be tested, a highly desirable demonstration for metrology, with GaAs and Si pumps at the forefront of these tests. Here, we show that pumping can be achieved in a yet unexplored semiconductor, i.e. germanium. We realise a single-hole pump with a tunable-barrier quantum dot electrostatically defined at a Ge/SiGe heterostructure interface. We observe quantized current plateaux by driving the system with a single sinusoidal drive up to a frequency of 100 MHz. The operation of the prototype was affected by accidental formation of multiple dots, probably due to disorder potential, and random charge fluctuations. We suggest straightforward refinements of the fabrication process to improve pump characteristics in future experiments. [ABSTRACT FROM AUTHOR]

We use optical orientation technique to monitor the degeneracy control of exciton states in a single InAs/GaAs quantum dot, achieved by applying an in-plane electric field. Under circularly polarized quasiresonant excitation, the exciton photoluminescence shows a pronounced maximum of circular polarization at electric field corresponding to zero fine structure splitting. By analyzing the width of this maximum we are able to determine the homogeneous linewidth of the excitonic transition. This experimental method is shown to be very efficient to test and possibly tune the photonic properties of an individual quantum dot for the emission of entangled photon pairs. [ABSTRACT FROM AUTHOR]

In the past decades, low-dimensional semiconductors received intensive research interest. By introducing intentionally size-confined nanostructures or crystal imperfections, low-dimensional semiconductors have been broadly exploited as zero-dimensional quantum dots (QDs) for high-performance quantum emitters. The QD-based nonclassical light sources allow not only the deterministic generation of single photons but also entangled-photon pairs. However, the randomness in strain, shape and composition in semiconductors results in unpredictable transition energies for different QDs. This complication impedes the generation of single and entangled photons with well-defined energies, which fundamentally limits the success probability of scalable quantum information technologies. Strain engineering, a unique and powerful method to reshape the electronic states of semiconductors, has advanced the development of all-solid-state low-dimensional semiconductor based single and entangled-photon sources. In this review, the recent progress of employing mechanical strain field to control the electronic states and optical properties of low-dimensional semiconductors is reviewed. A comprehensive summary of diverse strain engineered devices for engineering the exciton binding energy, the coherent coupling of electronic states, the optical properties of low-dimensional semiconductors including single and entangled photons are provided. In addition, prospects and challenges of deploying the strain-engineering technique for future scalable quantum networks and photonic quantum circuits are discussed. [ABSTRACT FROM AUTHOR]

The strong coupling of light with magnetoexcitons in a quantum well within a semiconductor microcavity under the action of an electric field parallel to the quantum well plane, is theoretically studied. Such a phenomenon is described within the Stahl–Balslev real-space density-matrix approach using a system of coupled equations for the coherent-wave amplitude and the electromagnetic fields. In the study, both s- and p-polarization geometries as well as the Coulomb interaction potential between electrons and holes are considered. It is shown that the optical reflectivity spectra for a heterostructure, having an InGaAs/GaAs quantum well inside the semiconductor microcavity with Bragg mirrors of alternating GaAs and InGaAs layers, exhibit well-discernible resonant dips. The strong magnetoexciton–photon coupling occurs when the magnetoexciton resonance frequency and that of the confined photon are close to each other. The application of a static electric field, parallel to the interfaces of the layers, allows for the optical excitation of magnetoexcitons, having nonzero angular momentum projection, and noticeably alters the resonance structure of both s- and p-polarization optical spectra. The strong magnetoexciton–photon coupling is observed until sufficiently large magnitudes of the applied parallel electric field are reached. [ABSTRACT FROM AUTHOR]

We study the interaction of a quantum dot in the Voigt configuration with a laser pulse and particularly analyze the potential for rapid spin initialization by putting the quantum dot near a molybdenum disulfide (MoS 2 ) monolayer. The MoS 2 monolayer influences the spontaneous decay rates of the quantum dot, leading to anisotropically enhanced decay rates, for the quantum dot's electric dipole moments parallel and perpendicular to the layer. By solving the relevant density matrix equations, we find that high spin initialization fidelity is obtained at short times. The fidelity is significantly higher than when the quantum dot is in free-space vacuum. We examine two different cases of the interaction of the quantum dot with the applied optical field. First, we use a continuous wave laser field and determine for various quantum dot—MoS 2 layer distances the field strength that leads to acceptable fidelity levels. The effect of the quality of the MoS 2 material on the fidelity of spin initialization is also examined. We also study the interaction of the quantum dot with a laser pulse and apply numerical optimal control to obtain the time-dependent field strength, which leads to maximum final fidelity for short time intervals. The latter approach gives beneficial results in comparison to the continuous wave field excitation. [ABSTRACT FROM AUTHOR]

The temperature-dependent electron spin relaxation of positively charged excitons in a single InAs quantum dot was measured by time-resolved photoluminescence spectroscopy at zero applied magnetic fields. The experimental results show that the electron-spin relaxation is clearly divided into two different temperature regimes: (i) at T < 50 K, spin relaxation depends on the dynamical nuclear spin polarization and is approximately temperature-independent, as predicted by Merkulov et al. [Phys. Rev. B 65, 205309 (2002)] (ii) T > about 50 K, spin relaxation speeds up with increasing temperature. A model of a two longitudinal optical phonon scattering process coupled with hyperfine interaction is proposed to account for the accelerated electron spin relaxation at higher temperatures. [ABSTRACT FROM AUTHOR]

We perform high-resolution photocurrent (PC) spectroscopy to investigate resonantly the neutral exciton ground-state (X0) in a single InAs/GaAs self-assembled quantum dot (QD) embedded in the intrinsic region of an n-i-Schottky photodiode based on a two-dimensional electron gas (2DEG), which was formed from a Si δ-doped GaAs layer. Using such a device, a single-QD PC spectrum of X0 is measured by sweeping the bias-dependent X0 transition energy through that of a fixed narrow-bandwidth laser via the quantum-confined Stark effect (QCSE). By repeating such a measurement for a series of laser energies, a precise relationship between the X0 transition energy and bias voltage is then obtained. Taking into account power broadening of the X0 absorption peak, this allows for high-resolution measurements of the X0 homogeneous linewidth and, hence, the electron tunneling rate. The electron tunneling rate is measured as a function of the vertical electric field and described accurately by a theoretical model, yielding information about the electron confinement energy and QD height. We demonstrate that our devices can operate as 2DEG-based QD photovoltaic cells and conclude by proposing two optical spintronic devices that are now feasible. [ABSTRACT FROM AUTHOR]

This paper develops an efficient bond orbital model with analytical expressions to investigate the optical properties of InAs/GaSb superlattices. In contrast to the conventional bond orbital model, the proposed method expresses the momentum matrix elements entirely in terms of Hamiltonian matrix elements without introducing any additional optical parameters. The existence of In–Sb and Ga–As heterobonds across the InAs/GaSb superlattice interfaces causes giant optical variations to be manifested in the momentum matrix element, absorption coefficient, and refractive index. [ABSTRACT FROM AUTHOR]

Optical orientation in the strained semiconductor superlattices (SL) is investigated theoretically. The dependence of the features in spin-polarization spectra on the structure parameters is clarified. The value of polarization in the first polarization maximum in the SL structures is shown to grow with the splitting between the hh and lh states of the valence band, the joint strain and confinement effects on the hh1-lh1 splitting being strongly influenced by the tunneling in the barriers. In strained structures with high barriers for the holes initial polarization can exceed 95%. Calculated polarization spectra are close to the experimental spectra of polarized electron emission. [ABSTRACT FROM AUTHOR]

Presents a study which investigated the vertical conductivity and localization effects in strongly coupled semiconductor periodical structures using Raman scattering measurement. Structures of periodical semiconductor; Experimental details and discussion.

By employing Tamm optical states—states localized between a thin metal microdisk and a semiconductor distributed Bragg reflector—we demonstrate near 1300 nm (O-band), narrow wavelength (20 nm FWHM), highly spatially localized photodetection. By varying the size of the microdisk, we show spectral tuning (7 nm) of the peak optical response. Furthermore, by reducing the symmetry of the microstructures, we lift the degeneracy of the polarization, producing a polarization sensitivity in our detector. These confined Tamm states are useful not only for sensing but also for photoconductive switch applications such as optical microwave switches and terahertz generation. [ABSTRACT FROM AUTHOR]

Focuses on a method of evaluating interface states for coupled superlattices. Overview of semiconductor superlattices coupled by a tunnel barrier; Localization problems involving superlattices; States of the surface state in the minigaps.

Presents a study which examined the Wannier-Stark effect in gallium arsenide/GaAlAs short-period superlattices under applied electric field perpendicular to the layers by room- and low-temperature photocurrent (PC) measurements. Method used in PC measurements; Structures in the PC spectra of a gallium arsenide/Ga[sub0.5]Al[sub0.5]As.

The tunneling resonance technique is used to study the electronic surface state (Tamm state) [Phys. Rev. Lett. 64, 2555 (1990)] under external electric field in semiconductor superlattices. The localization characteristics of the confined surface states depend on the direction and strength of the applied electric field. In the weak-field regime the surface states behave delocalized due to the distortion of the periodic medium by the applied field. The calculations presented can provide eigenenergy, wave function, and lifetime for the quasibound surface states, which are related directly to the optical properties of surface states under an electric field. [ABSTRACT FROM AUTHOR]

Effective utilization of excitation energy in nanoemitters requires control of exciton flow at the nanoscale. This can be readily achieved by exploiting near-field nonradiative energy transfer mechanisms such as dipole-dipole coupling (i.e., Förster resonance energy transfer) and simultaneous two-way electron transfer via exchange interaction (i.e., Dexter energy transfer). In this feature article, we review nonradiative energy transfer processes between emerging nanoemitters and exciton scavengers. To this end, we highlight the potential of colloidal semiconductor nanocrystals, organic semiconductors, and two-dimensional materials as efficient exciton scavengers for light harvesting and generation in optoelectronic applications. We present and discuss unprecedented exciton transfer in nanoemitter-nanostructured semiconductor composites enabled by strong light-matter interactions. We elucidate remarkably strong nonradiative energy transfer in self-assembling atomically flat colloidal nanoplatelets. In addition, we underscore the promise of organic semiconductor-nanocrystal hybrids for spin-triplet exciton harvesting via Dexter energy transfer. These efficient exciton transferring hybrids will empower desired optoelectronic properties such as long-range exciton diffusion, ultrafast multiexciton harvesting, and efficient photon upconversion, leading to the development of excitonic optoelectronic devices such as exciton-driven light-emitting diodes, lasers, and photodetectors. [ABSTRACT FROM AUTHOR]

Effects that are related to deviations from thermodynamic equilibrium have a special place in modern physics. Among these, nonequilibrium phenomena in quantum systems attract the highest interest. The experimental technique of spin-noise spectroscopy has became quite widespread, which makes it possible to observe spin fluctuations of charge carriers in semiconductors under both equilibrium and nonequilibrium conditions. This calls for the development of a theory of spin fluctuations of electrons and electron-hole complexes for nonequilibrium conditions. In this paper, we consider a range of physical situations where a deviation from equilibrium becomes pronounced in the spin noise. A general method for the calculation of electron and exciton spin fluctuations in a nonequilibrium state is proposed. A short review of the theoretical and experimental results in this area is given. [ABSTRACT FROM AUTHOR]

Despite the large number of theoretical III-V semiconductor studies reported every year, our atomistic understanding is still limited. The limitations of the theoretical approaches to yield accurate structural and electronic properties on an equal footing, is due to the unphysical self-interaction problem that mainly affects the band gap and spin-orbit splitting (SOC) in semiconductors and, in particular, III-V systems with similar magnitude of the band gap and SOC. In this work, we report a consistent study of the structural and electronic properties of the III-V semiconductors by using the screening hybrid-density functional theory framework, by fitting the α parameters for 12 different III-V compounds, namely, AlN, AlP, AlAs, AlSb, GaN, GaP, GaAs, GaSb, InN, InP, InAs, and InSb, to minimize the deviation between the theoretical and experimental values of the band gap and SOC. Structural relaxation effects were also included. Except for AlP, whose α = 0.127, we obtained α values that ranged from 0.209 to 0.343, which deviate by less than 0.1 from the universal value of 0.25. Our results for the lattice parameter and elastic constants indicate that the fitting of α does not affect those structural parameters when compared with the HSE06 functional, where α = 0.25. Our analysis of the band structure based on the k ⋅p method shows that the effective masses are in agreement with the experimental values, which can be attributed to the simultaneous fitting of the band gap and SOC. Also, we estimate the values of g-factors, extracted directly from the band structure, which are close to experimental results, which indicate that the obtained band structure produced a realistic set of k ⋅ p parameters. [ABSTRACT FROM AUTHOR]

We present an exact diagonalization approach for atomistic calculation of excitonic properties of semiconductor nanostructures under piezoelectric field. The method allows for efficient treatment of both single particle and many-body states at a small computational cost and results in a very good agreement with the full diagonalization treatment. We illustrate our approach by analyzing the effect of a piezoelectric field on a spectra of a self-assembled InAs/GaAs lens-shaped quantum dot. We study the influence of linear and quadratic piezoelectric terms on the quantum dot electronic structure and importantly we found that the non-linear, density functional based theory of piezoelectricity produces results very similar to those obtained by a well-established linear approach utilizing empirical parameters. [ABSTRACT FROM AUTHOR]

Strong correlations in the fluctuations of the intensity of emission from a semiconductor microcavity under resonant laser excitation are observed. The intensity correlation function exhibits an unusual oscillatory behavior with an unexpectedly long oscillation period and decay time. The visibility of the correlation function reaches 0.81. Long oscillation times are attributed to the Rabi frequency characterizing weak coupling between the electromagnetic field of the semiconductor microcavity mode and long-lived exciton states localized by the random potential of the quantum well. For a laser excitation power density of 400 W/cm, the power density of the radiation emitted by the microcavity is 12 W/cm, which corresponds to the total flux of nonclassical light of 1.5 × 10 photons/s from an excited spot 50 μm in diameter. Thus, a microcavity can serve as a bright emitter of nonclassical light. [ABSTRACT FROM AUTHOR]

Photonic trumpets are broadband dielectric antennas that efficiently funnel the emission of a point-like quantum emitter--such as a semiconductor quantum dot--into a Gaussian free-space beam. After describing guidelines for the taper design, we present a "giant" photonic trumpet. The device features a bottom diameter of 210nm and a 5µm wide top facet. Using Fourier microscopy, we show that 95% of the emitted beam is intercepted by a modest numerical aperture of 0.35. Furthermore, far-field measurements reveal a highly Gaussian angular profile, in agreement with the predicted overlap to a Gaussian beam Mg=0.98. Future application prospects include the direct coupling of these devices to a cleaved single-mode optical fiber. The calculated transmission from the taper base to the fiber already reaches 0.59, and we discuss strategies to further improve this figure of merit. [ABSTRACT FROM AUTHOR]

This article gives an overview on the advance of spin noise spectroscopy (SNS) in semiconductors in the past 8 years from the first measurements in bulk n-GaAs [Oestreich et al., Phys. Rev. Lett. 95, 216603 (2005)] up to the recent achievement of optical detection of the intrinsic spin fluctuations of a single hole confined in an individual self-assembled quantum dot [Dahbashi et al., arXiv:1306.3183 (2013)]. We discuss the general technical implementation of optical SNS and the invaluable profit of the introduction of real-time fast Fourier transform analysis into the data acquisition. By now, the full spin dynamic from the milli- to picosecond timescales can be addressed by SNS and the technique quickly strides ahead to enable real quantum non-demolition measurements in semiconductors. Spin noise spectra recorded in 2005 in bulk n-GaAs with approximately 109 electron spins (Oestreich et al.) and 2013 (Dahbashi et al.) for a single hole spin. The integration time for the latter is more than a factor of 40 shorter due to the significant advances in the measurement technique. [ABSTRACT FROM AUTHOR]

The ability of using onchip microcoils to control the electron-nuclear spin system in semiconductors is demonstrated. Electrically generated magnetic fields of several tens of mT can be obtained on a micrometer length scale, which are switchable on a sub-ns time scale due to the low complex coil impedance. This allows one to electrically (i) manipulate the nuclear spins by means of nuclear magnetic resonance in n-GaAs and (ii) control the hyperfine flip-flop rate in CdSe/ZnSe quantum dots. [ABSTRACT FROM AUTHOR]

The paper reviews the interplay of Rashba/Dresselhaus spin splittings in various two-dimensional systems made of zinc-blende III-V, wurtzite, and SiGe semiconductors. We discuss the symmetry aspects of the linear and cubic in electron wavevector spin splitting in heterostructures prepared on (001)-, (110)-, (111)-, (113)-, (112)-, and (013)- oriented substrates and address the requirements for suppression of spin relaxation and realization of the persistent spin helix state. In experimental part of the paper, we overview experimental results on the interplay of Rashba/Dresselhaus spin splittings probed by photogalvanic spectroscopy: The method based on the phenomenological equivalence of the linear-in-wavevector spin splitting and several photogalvanic phenomena. [ABSTRACT FROM AUTHOR]

The first functional light-emitting diodes (LEDs) based on quasi 2D colloidal core/shell CdSe/CdZnS nanoplatelets (NPLs). The solution-processed hybrid devices are optimized with respect to their electroluminescent characteristics, first, by improving charge injection through exchanging the as-synthesized NPL long-chain ligands to shorter ones such as 3-mercaptopropionic acid, and second, by comparing different hole-transporting layers. NPL-LEDs exhibit a maximum luminance of 4499 cd m-2 and external quantum efficiencies of 0.63%. In particular, over different applied voltages, systematically narrow electroluminescence of full width at half maximum (FWHM) in the range of 25-30 nm is observed to be independent from the choice of device configuration and NPL ligands. As spectrally narrow electroluminescence is highly attractive in terms of color purity in the context of LED applications, these results emphasize the unique potential of this new class of colloidal core/shell nanoplatelet in achieving bright and functional LEDs of superior color purity. [ABSTRACT FROM AUTHOR]

We have experimentally and theoretically investigated negative differential conductance regions in the current-voltage characteristic of undoped semiconductor superlattice surrounded by barriers and illuminated by laser light. The negative differential conductances are nonlinear and show bistable behavior as a function of the applied voltage. These phenomena are quantitatively described by a self-consistent analysis of the field-dependent Wannier-Stark absorption and accumulation of photocarriers in the presence of barriers at the superlattice borders. [ABSTRACT FROM AUTHOR]

III-nitride nanostructures have recently emerged as promising materials for new intersubband (ISB) devices in a wide variety of applications. These ISB technologies rely on infrared optical transitions between quantum-confined electronic states in the conduction band of GaN/Al(Ga)N nanostructures, namely quantum wells or quantum dots. The large conduction band offset (about 1.8 eV for GaN/AlN) and sub-picosecond ISB relaxation of III-nitrides render them appealing materials for ultrafast photonic devices in near-infrared telecommunication networks. Furthermore, the large energy of GaN longitudinal-optical phonons (92 meV) opens prospects for high-temperature THz quantum cascade lasers and ISB devices covering the 5-10 THz band, inaccessible to As-based technologies due to phonon absorption. In this paper, we describe the basic features of ISB transitions in III-nitride quantum wells and quantum dots, in terms of theoretical calculations, material growth, spectroscopy, resonant transport phenomena, and device implementation. The latest results in the fabrication of control-by-design devices such as all-optical switches, electro-optical modulators, photodetectors, and lasers are also presented. [ABSTRACT FROM AUTHOR]

Effects of quantum size and potential shape on the spectra of an electron and a hydrogenic-donor at the center of a permeable spherical cavity have been calculated, using linear variational method. B-splines have been used as basis functions. By extensive convergence tests and comparing with other results given in the literature, the validity and efficiency of the method were confirmed. [ABSTRACT FROM AUTHOR]

The ultrafast gain and refractive index dynamics of tunnel injected quantum dot based semiconductor optical amplifiers in the 1300 nm range are investigated using a heterodyne pump probe technique. In the gain regime, ground state wavelengths exhibit full gain recovery in less than 10 ps up to 3 times transparency, attributed to enhanced carrier refilling via the injector layer. The effect of the injector can also been seen in unusual phase dynamics at excited state wavelengths at this injection level. [ABSTRACT FROM AUTHOR]

The band structure of InP is excellently produced using sp3d5s* tight-binding model. The spin-orbit splitting in the whole Brillouin zone derived from the InP Γ-valley of the lowest electronic subband, heavy hole, light hole and split-off hole is calculated. Considering the hot electron effect, the cases of L and X-valleys for the lowest electronic subband are also discussed. We then further present the electron spin-orbit coupling coefficient around the corresponding valley bottom. Our results should provide a promising direction for future research on spintronics. [ABSTRACT FROM AUTHOR]

We present a selection of our studies on CdTe/ZnTe quantum dots considered as spin qubits. Discussed experiments are related to processes of spin reading, writing and evolution. We show that CdTe/ZnTe system is well suitable for studying effects important for optical quantum computing on single spins. [ABSTRACT FROM AUTHOR]

This paper summarizes recent progress achieved in the field of semiconductor cavity quantum electrodynamics with single quantum dots with the focus being on micropillar cavities. Light-matter interaction both in the strong and weak coupling regime is presented. Resonance tuning of the quantum dot by temperature, electric fields and magnetic fields is demonstrated while the strong coupling regime can be reached. Additionally, deterministic device integration of single positioned quantum dots is reported by a combination of site controlled quantum dot growth via directed nucleation and subsequent device alignment to overcome the degree of randomness of the quantum dot position in so far most common quantum dot-cavity systems. [ABSTRACT FROM AUTHOR]

Modern band structure engineering is based both on the important discoveries of the past century and modern epitaxial technology. The general goal is to control the behavior of charge carriers on an atomic scale, which affects how they interact with each other and their environment. Starting from the basic semiconductor heterostructure, band structure engineering has evolved into a powerful discipline, employing lower dimensionality to demonstrate new material properties. Several modern technologies under development are used as examples of how this discipline is enabling new types of devices and new functionality in areas with immediate application. [ABSTRACT FROM AUTHOR]

The electron spin decoherence by nuclear spins in semiconductor quantum dots is caused by quantum entanglement between the electron and the nuclei. The many-body dynamics problem of the interacting nuclear spins can be solved with the pair-correlation approximation which treats the nuclear spin flip-flops as mutually independent. The nuclear spin dynamics can be controlled by simply flipping the electron spin so that the electron is disentangled from the nuclei and hence its lost coherence is restored. [ABSTRACT FROM AUTHOR]

Quantum dots show excellent promise as triggered sources of both single and polarization entangled photons for quantum information applications. Our recent progress developing nonclassical light sources with single quantum dots is presented in this paper. Following radiative emission of an exciton confined in a quantum dot, there is a finite delay before reexcitation can occur; this results in an anti-bunching of the photons emitted providing a source of single photons. Excitation of a quantum dot with two electrons and two holes leads to the emission of a pair of photons; we show here that, provided the spin splitting of the intermediate exciton state in the decay is erased, the photon pair is emitted in an entangled polarization state. The fidelity of this entangled state is shown to exceed 70%. using quantum dots to generate quantum light allows contacts for electrical injection to be integrated into a compact and robust device. A cavity may also be integrated into the semiconductor structure to enhance the photon collection efficiency and control the recombination dynamics. We detail a process to form a submicrometer current aperture within an electrical device, allowing individual quantum dots to be addressed electrically in devices. [ABSTRACT FROM AUTHOR]

The radiative biexciton-exciton decay in a semiconductor quantum dot (QD) has the potential of being a source of triggered polarization-entangled photon pairs. However, in most cases the anisotropy-induced exciton fine structure splitting destroys this entanglement. Here, we present measurements on improved QD structures, providing both significantly reduced inhomogeneous emission linewidths and near-zero fine structure splittings. A high-resolution detection technique is introduced which allows us to accurately determine the fine structure in the photoluminescence emission and therefore select appropriate QDs for quantum state tomography. We were able to verify the conditions of entangled or classically correlated photon pairs in full consistence with observed fine structure properties. Furthermore, we demonstrate reliable polarization-entanglement for elevated temperatures up to 30 K. The fidelity of the maximally entangled state decreases only a little from 72% at 4K to 68% at 30 K. This is especially encouraging for future implementations in practical devices. [ABSTRACT FROM AUTHOR]

Exact analytical expressions for the eigenvalues and eigenvectors of the six-band k · p matrix Hamiltonian for narrow-gap III–V semiconductors are used to derive an analytical expression for the transmission coefficient T in type-II heterojunctions. The remote band coupling is included via the two Luttinger-type parameters, γ1 and $\tilde{\gamma}$. The expression for T contains two separate contributions: one from electron to light-hole tunneling and the other from electron to heavy-hole tunneling. Using the expression for T, the net recombination current density J is defined and evaluated for the specific case of an InAs/GaSb heterojunction. Ohmic behavior is observed in J for small applied voltages V approximately in the range -0.15 to 0.075 eV. Outside of this range, J changes nonlinearly with respect to changes in V. Comparison of the heavy-hole and light-hole contributions to J shows that the heavy-hole contribution increases J by approximately 10% at room temperature. The results for the InAs/GaSb heterojunction are in qualitative agreement with recent experimental observations. [ABSTRACT FROM AUTHOR]

Recent progress in the generation and spectroscopic application of femtosecond pulses in the wavelength range between 3 and 25 μm has led to new insight into low-energy excitations of solids, in particular semiconductors, semiconductor nanostructures, and high-TC superconductors. The ultrafast nonequilibrium dynamics of such excitations has been observed in real-time and the underlying microscopic interactions have been analyzed. This article gives a brief overview of this exciting field of ultrafast science with the main emphasis on pulse generation and applications in semiconductor research where femtosecond intersubband Rabi flopping in quantum wells and quantum coherent electron transport in quantum cascade devices have been observed. [ABSTRACT FROM AUTHOR]

We propose a scheme to carry out quantum phase gate in one step by bichromatic radiation method with semiconductor quantum dots (QDs) embedded in a single mode microcavity. The spin degrees of freedom of the only excess conduction band electron are employed as qubits and excitonic states are used as auxiliary states. The nearest-neighbor coupling is not required because the cavity mode plays the role of data bus. We show how to perform quantum computing with properly tailored laser pulses and Pauli-blocking effect, without exciting the cavity mode. [ABSTRACT FROM AUTHOR]

We reconstruct a ‘test’ wave function in a strongly coupled, graded well-width superlattice by resolving the spatial extension of the interband polarisation and deducing the wave function employing non-linear optical spectroscopy. The graded gap superlattice allows us to precisely control the distance between ‘test’ and ‘probe’ wave functions. By spatially tuning one wave function with respect to the other and recording the amplitude and the sign of the modulation of the spectrally resolved four-wave-mixing (FWM) signal with respect to delay, we are able to reconstruct the ‘test’ wave function. Our numerical simulation of the third-order response of an inhomogeneously broadened system reproduces the experimental data in great detail. The wave function used for the modelling is computed by a one-dimensional transfer matrix model including electron–hole Coulomb interaction. Our experimental scheme inherently allows us to quantitatively distinguish between non-linear mechanisms leading to the FWM signal, namely phase-space filling and excitation-induced dephasing. [ABSTRACT FROM AUTHOR]

The electronic conduction processes in the minibands and excited states of semiconductor superlattices are reviewed and discussed. The paper includes an introduction on miniband conduction and hopping conduction between localized states. It focusses subsequently on interminiband transport, both in the context of theoretical approaches and of experimental observations of these effects. Rabi oscillations between interacting Wannier-Stark levels are considered in detail. The role of scattering, and the manifestation of multipeaked velocity-field relationships on carrier and electric field profiles are evidenced. The final part concerns transport above the superlattice barriers, and quasi-ballistic transport in excited superlattice minibands. [ABSTRACT FROM AUTHOR]

Describes how artificial semiconductor superlattices have permitted the observation of the Wannier-Stark ladder. The Bloch oscillator and the motion of an electron in a periodic potential; Stark-splitting; Formation of electronic states in superlattices; Applications in optical devices; Implications of Stark ladder formation.