Showing papers on "Quantum well published in 2020"
TL;DR: True-blue LEDs employing quasi-two-dimensional cesium lead bromide with a narrow size distribution of quantum wells are reported, achieved through the incorporation of a chelating additive.
Abstract: Metal halide perovskites have emerged as promising candidates for solution-processed blue light-emitting diodes (LEDs). However, halide phase segregation – and the resultant spectral shift – at LED operating voltages hinders their application. Here we report true-blue LEDs employing quasi-two-dimensional cesium lead bromide with a narrow size distribution of quantum wells, achieved through the incorporation of a chelating additive. Ultrafast transient absorption spectroscopy measurements reveal that the chelating agent helps to control the quantum well thickness distribution. Density functional theory calculations show that the chelating molecule destabilizes the lead species on the quantum well surface and that this in turn suppresses the growth of thicker quantum wells. Treatment with γ-aminobutyric acid passivates electronic traps and enables films to withstand 100 °C for 24 h without changes to their emission spectrum. LEDs incorporating γ-aminobutyric acid-treated perovskites exhibit blue emission with Commission Internationale de l'Eclairage coordinates of (0.12, 0.14) at an external quantum efficiency of 6.3%. Metal halide perovskites have been studied as promising materials for blue light-emitting diodes (LEDs) but the stability is still a bottleneck. Here Wang et al. develop a chelating additive strategy to increase efficiency, operational stability and color stability of blue perovskite LEDs.
126 citations
TL;DR: In this paper, the authors demonstrate a fast universal quantum gate set composed of single-qubit gates with a fidelity of 99.3 per cent and a gate time of 20 nanoseconds.
Abstract: Universal quantum information processing requires the execution of single-qubit and two-qubit logic. Across all qubit realizations1, spin qubits in quantum dots have great promise to become the central building block for quantum computation2. Excellent quantum dot control can be achieved in gallium arsenide3–5, and high-fidelity qubit rotations and two-qubit logic have been demonstrated in silicon6–9, but universal quantum logic implemented with local control has yet to be demonstrated. Here we make this step by combining all of these desirable aspects using hole quantum dots in germanium. Good control over tunnel coupling and detuning is obtained by exploiting quantum wells with very low disorder, enabling operation at the charge symmetry point for increased qubit performance. Spin–orbit coupling obviates the need for microscopic elements close to each qubit and enables rapid qubit control with driving frequencies exceeding 100 MHz. We demonstrate a fast universal quantum gate set composed of single-qubit gates with a fidelity of 99.3 per cent and a gate time of 20 nanoseconds, and two-qubit logic operations executed within 75 nanoseconds. Planar germanium has thus matured within a year from a material that can host quantum dots to a platform enabling two-qubit logic, positioning itself as an excellent material for use in quantum information applications. Spin qubits based on hole states in strained germanium could offer the most scalable platform for quantum computation.
115 citations
TL;DR: The results show that it is important to control the morphology of the quasi-2D films to achieve efficient energy transfer, which is a critical requirement for light-emitting devices.
Abstract: Quasi-2D Ruddlesden-Popper halide perovskites with a large exciton binding energy, self-assembled quantum wells, and high quantum yield draw attention for optoelectronic device applications. Thin films of these quasi-2D perovskites consist of a mixture of domains having different dimensionality, allowing energy funneling from lower-dimensional nanosheets (high-bandgap domains) to 3D nanocrystals (low-bandgap domains). High-quality quasi-2D perovskite (PEA)2 (FA)3 Pb4 Br13 films are fabricated by solution engineering. Grazing-incidence wide-angle X-ray scattering measurements are conducted to study the crystal orientation, and transient absorption spectroscopy measurements are conducted to study the charge-carrier dynamics. These data show that highly oriented 2D crystal films have a faster energy transfer from the high-bandgap domains to the low-bandgap domains (<0.5 ps) compared to the randomly oriented films. High-performance light-emitting diodes can be realized with these highly oriented 2D films. Finally, amplified spontaneous emission with a low threshold 4.16 µJ cm-2 is achieved and distributed feedback lasers are also demonstrated. These results show that it is important to control the morphology of the quasi-2D films to achieve efficient energy transfer, which is a critical requirement for light-emitting devices.
97 citations
TL;DR: An exciton-based ultrafast response of 2D perovskites opens up large avenues for a wide range of scalable dynamic photonic devices with potential applications in flexible photonics, ultrafast wavefront control, and short-range wireless terahertz communications.
Abstract: In recent years, two-dimensional (2D) Ruddlesden-Popper perovskites have emerged as promising candidates for environmentally stable solar cells, highly efficient light-emitting diodes, and resistive memory devices. The remarkable existence of self-assembled quantum well (QW) structures in solution-processed 2D perovskites offers a diverse range of optoelectronic properties, which remain largely unexplored. Here, we experimentally observe ultrafast relaxation of free carriers in 20 ps due to the quantum confinement of free carriers in a self-assembled QW structures that form excitons. Furthermore, hybridizing the 2D perovskites with metamaterials on a rigid and a flexible substrate enables modulation of terahertz fields at 50-GHz modulating speed, which is the fastest for a solution-processed semiconductor-based photonic device. Hence, an exciton-based ultrafast response of 2D perovskites opens up large avenues for a wide range of scalable dynamic photonic devices with potential applications in flexible photonics, ultrafast wavefront control, and short-range wireless terahertz communications.
84 citations
TL;DR: This work proposes and demonstrates experimentally active nanophotonic topological cavities incorporating III–V semiconductor quantum wells as a gain medium in the structure and shows room-temperature lasing with a narrow spectrum, high coherence, and threshold behaviour, a step towards topologically controlled ultrasmall light sources with nontrivial radiation characteristics.
Abstract: The study of topological phases of light underpins a promising paradigm for engineering disorder-immune compact photonic devices with unusual properties. Combined with an optical gain, topological photonic structures provide a novel platform for micro- and nanoscale lasers, which could benefit from nontrivial band topology and spatially localized gap states. Here, we propose and demonstrate experimentally active nanophotonic topological cavities incorporating III–V semiconductor quantum wells as a gain medium in the structure. We observe room-temperature lasing with a narrow spectrum, high coherence, and threshold behaviour. The emitted beam hosts a singularity encoded by a triade cavity mode that resides in the bandgap of two interfaced valley-Hall periodic photonic lattices with opposite parity breaking. Our findings make a step towards topologically controlled ultrasmall light sources with nontrivial radiation characteristics. Active topological cavities that can lase at room temperature could bring new opportunities for controlling light in integrated nanophotonic circuits. Smirnova et al. etched a special pattern of nanoscale holes into a 250 nm thick slab of the compound semiconductor InGaAsP to create a triangle-shaped cavity with optical behaviour governed by the band topology. The presence of quantum wells in the slab provides the cavity with optical gain allowing it to lase in the near-infrared when excited with nanosecond pump pulses at 980 nm. The laser emission is observed to be spectrally narrow with high coherence and hosts a donut-shaped singularity in Fourier space. The achievement opens the way to a new type of nanophotonic light source which exhibits unusual radiation characteristics and suits integration with metasurfaces.
78 citations
TL;DR: An observation of long-distance carrier transport over 2 to 5 micrometers in 2D perovskites with various well thicknesses is reported and highlights their potential application as an efficient energy/charge-delivery material.
Abstract: Layered two-dimensional (2D) hybrid perovskites are naturally formed multiple quantum well (QW) materials with promising applications in quantum and optoelectronic devices. In principle, the transport of excitons in 2D perovskites is limited by their short lifetime and small mobility to a distance within a few hundred nanometers. Herein, we report an observation of long-distance carrier transport over 2 to 5 μm in 2D perovskites with various well thicknesses. Such a long transport distance is enabled by trap-induced exciton dissociation into long-lived and nonluminescent electron-hole separated state, followed by a trap-mediated charge transport process. This unique property makes 2D perovskites comparable with 3D perovskites and other traditional semiconductor QWs in terms of a carrier transport property and highlights their potential application as an efficient energy/charge-delivery material.
48 citations
TL;DR: In this article, an inverted HgTe quantum well was used to guide edge channels from opposite sides of a device into a quasi-one-dimensional constriction, and it was shown that when edge states are brought close together, additional gaps appeared in the spectrum.
Abstract: Electrical currents in a quantum spin Hall insulator are confined to the boundary of the system. The charge carriers behave as massless relativistic particles whose spin and momentum are coupled to each other. Although the helical character of those states is already established by experiments, there is an open question regarding how those edge states interact with each other when they are brought into close spatial proximity. We employ an inverted HgTe quantum well to guide edge channels from opposite sides of a device into a quasi-one-dimensional constriction. Our transport measurements show that, apart from the expected quantization in integer steps of 2e2/h, we find an additional plateau at e2/h. We combine band structure calculations and repulsive electron–electron interaction effects captured within the Tomonaga–Luttinger liquid model and Rashba spin–orbit coupling to explain our observation in terms of the opening of a spin gap. These results may have direct implications for the study of one-dimensional helical electron quantum optics, and for understanding Majorana and para fermions. Little is known about how edge states in topological materials interact with each other. Here, a quantum spin Hall insulator is used to show that when edge states are brought close together, additional gaps appear in the spectrum.
46 citations
TL;DR: In this paper, the performance of a quantum well and a quantum-dot laser monolithically grown on on-axis Si (001) substrates, both experimentally and theoretically, under the impact of the same density of threading dislocations, is compared.
Abstract: High-performance III–V quantum-dot lasers monolithically grown on Si substrates have been demonstrated as a promising solution to realize Si-based laser sources with very low threshold current density, high output power, and long lifetime, even with relatively high density of defects due to the material dissimilarities between III–Vs and Si. On the other hand, although conventional III–V quantum-well lasers grown on Si have been demonstrated after great efforts worldwide for more than 40 years, their practicality is still a great challenge because of their very high threshold current density and very short lifetime. However, the physical mechanisms behind the superior performance of silicon-based III–V quantum-dot lasers remain unclear. In this paper, we directly compare the performance of a quantum-well and a quantum-dot laser monolithically grown on on-axis Si (001) substrates, both experimentally and theoretically, under the impact of the same density of threading dislocations. A quantum-dot laser grown on a Si substrate with a high operating temperature (105 °C) has been demonstrated with a low threshold current density of 173 A/cm2 and a high single facet output power >100 mW at room temperature, while there is no lasing operation for the quantum-well device at room temperature even at high injection levels. By using a rate equation travelling-wave model, the quantum-dot laser's superior performance compared with its quantum well-based counterpart on Si is theoretically explained in terms of the unique properties of quantum dots, i.e., efficient carrier capture and high thermal energy barriers preventing the carriers from migrating into defect states.
45 citations
TL;DR: In this article, the coupling between metallic terahertz metasurfaces and Landau-level transitions in high mobility 2D electron and hole gases was investigated using heavy-hole cyclotron resonances in InSb quantum wells, both within highly non-parabolic bands.
Abstract: We probe ultra-strong light matter coupling between metallic terahertz metasurfaces and Landau-level transitions in high mobility 2D electron and hole gases. We utilize heavy-hole cyclotron resonances in strained Ge and electron cyclotron resonances in InSb quantum wells, both within highly non-parabolic bands, and compare our results to well known parabolic AlGaAs/GaAs quantum well (QW) systems. Tuning the coupling strength of the system by two methods, lithographically and by optical pumping, we observe a novel behavior clearly deviating from the standard Hopfield model previously verified in cavity quantum electrodynamics: an opening of a lower polaritonic gap.
37 citations
TL;DR: A blue-emitting InGaN quantum well is incorporated between the quantum dot active region and the p-GaN, wherein electrons escaping from the device active region can recombine with holes and contribute to white-light emission.
Abstract: We investigated the effect of coupled quantum wells to reduce electron overflow in InGaN/GaN dot-in-a-wire phosphor-free white color light-emitting diodes (white LEDs) and to improve the device performance. The light output power and external quantum efficiency (EQE) of the white LEDs with coupled quantum wells were increased and indicated that the efficiency droop was reduced. The improved output power and EQE of LEDs with the coupled quantum wells were attributed to the significant reduction of electron overflow primarily responsible for efficiency degradation through the near-surface GaN region. Compared to the commonly used AlGaN electron blocking layer between the device active region and p-GaN, the incorporation of a suitable InGaN quantum well between the n-GaN and the active region does not adversely affect the hole injection process. Moreover, the electron transport to the device active region can be further controlled by optimizing the thickness and bandgap energy of this InGaN quantum well. In addition, a blue-emitting InGaN quantum well is incorporated between the quantum dot active region and the p-GaN, wherein electrons escaping from the device active region can recombine with holes and contribute to white-light emission. The resulting device exhibits high internal quantum efficiency of 58.5% with highly stable emission characteristics and virtually no efficiency droop.
36 citations
TL;DR: Optical patch antennas sandwiching dielectrics between metal layers can function to synchronize the optical phases for promoting coherent resonance, not only as electrical conductors, and optimal wire interconnects design is reported for controlling the optical properties.
Abstract: Optical patch antennas sandwiching dielectrics between metal layers have been used as deep subwavelength building blocks of metasurfaces for perfect absorbers and thermal emitters. However, for applications of these metasurfaces for optoelectronic devices, wiring to each electrically isolated antenna is indispensable for biasing and current flow. Here we show that geometrically engineered metallic wires interconnecting the antennas can function to synchronize the optical phases for promoting coherent resonance, not only as electrical conductors. Antennas connected with optimally folded wires are applied to intersubband infrared photodetectors with a single 4-nm-thick quantum well, and a polarization-independent external quantum efficiency as high as 61% (responsivity 3.3 A W−1, peak wavelength 6.7 μm) at 78 K, even extending to room temperature, is demonstrated. Applications of synchronously wired antennas are not limited to photodetectors, but are expected to serve as a fundamental architecture of arrayed subwavelength resonators for optoelectronic devices such as emitters and modulators. Applications of metasurfaces for optoelectronic devices require wiring to each isolated antenna for biasing and current flow. Here, the authors report optimal wire interconnects design for controlling the optical properties and present antenna-enhanced mid-infrared photodetection incorporating a single quantum well.
TL;DR: In this article, a single electron is carried in a potential minimum of a surface acoustic wave (SAW) and is transported to a region of holes to form an exciton.
Abstract: The long-distance quantum transfer between electron-spin qubits in semiconductors is important for realising large-scale quantum computing circuits. Electron-spin to photon-polarisation conversion is a promising technology for achieving free-space or fibre-coupled quantum transfer. In this work, using only regular lithography techniques on a conventional 15 nm GaAs quantum well, we demonstrate acoustically-driven generation of single photons from single electrons, without the need for a self-assembled quantum dot. In this device, a single electron is carried in a potential minimum of a surface acoustic wave (SAW) and is transported to a region of holes to form an exciton. The exciton then decays and creates a single optical photon within 100 ps. This SAW-driven electroluminescence, without optimisation, yields photon antibunching with g(2)(0) = 0.39 ± 0.05 in the single-electron limit (g(2)(0) = 0.63 ± 0.03 in the raw histogram). Our work marks the first step towards electron-to-photon (spin-to-polarisation) qubit conversion for scaleable quantum computing architectures. Electron-spin to photon-polarisation conversion is a promising technology for achieving free-space or fibre coupled quantum transfer. Here, the authors demonstrate acoustically-driven single photons from single electrons, without the need for self-assembled quantum dots, using a SAW-driven lateral n-i-p junction.
TL;DR: In this paper, the quantum well-dots (QWDs) are formed by metalorganic vapor phase epitaxial deposition of 4-16 monolayers of InxGa1−xAs of moderate indium composition.
Abstract: We review epitaxial formation, basic properties, and device applications of a novel type of nanostructures of mixed (0D/2D) dimensionality that we refer to as quantum well-dots (QWDs). QWDs are formed by metalorganic vapor phase epitaxial deposition of 4–16 monolayers of InxGa1−xAs of moderate indium composition (0.3 < x < 0.5) on GaAs substrates and represent dense arrays of carrier localizing indium-rich regions inside In-depleted residual quantum wells. QWDs are intermediate in properties between 2D quantum wells and 0D quantum dots and show some advantages of both of those. In particular, they offer high optical gain/absorption coefficients as well as reduced carrier diffusion in the plane of the active region. Edge-emitting QWD lasers demonstrate low internal loss of 0.7 cm−1 and high internal quantum efficiency of 87%. as well as a reasonably high level of continuous wave (CW) power at room temperature. Due to the high optical gain and suppressed non-radiative recombination at processed sidewalls, QWDs are especially advantageous for microlasers. Thirty-one μm in diameter microdisk lasers show a high record for this type of devices output power of 18 mW. The CW lasing is observed up to 110 °C. A maximum 3-dB modulation bandwidth of 6.7 GHz is measured in the 23 μm in diameter microdisks operating uncooled without a heatsink. The open eye diagram is observed up to 12.5 Gbit/s, and error-free 10 Gbit/s data transmission at 30 °C without using an external optical amplifier, and temperature stabilization is demonstrated.
TL;DR: Colloidal quantum wells, also called nanoplatelets, are nanoscopic materials displaying quantum confinement in two dimensions as discussed by the authors, which can translate many advantages of colloidal nanocrsytals or other solution-processable materials, such as scalable synthesis and substrate-agnostic deposition, without sacrificing material uniformity.
Abstract: Colloidal quantum wells, also called nanoplatelets, are nanoscopic materials displaying quantum confinement in two dimensions. Unlike colloidal quantum dots, colloidal quantum well ensembles have no inhomogeneous broadening due to an atomically-precise definition of the short axis, a fact which results in much narrower ensemble absorption and emission. Thus, colloidal quantum wells can translate many advantages of colloidal nanocrsytals or other solution-processable materials, such as scalable synthesis and substrate-agnostic deposition (particularly compared to epitaxial quantum wells), without sacrificing material uniformity. Due to very narrow photoluminescnece peaks, these materials have found a home in applications involving light emission, such as downconversion enhancement films, light-emitting diodes, and lasers, in which they represent some of the best performers among solution-cast materials. As argued in this review, the full spectrum of epitaxial quantum well devices offers a roadmap to other potential applications, such as detection, electronics, electro-optics, non-linear optics, or intersubband devices, in which only nascent efforts have been made.
TL;DR: The authors leverage the thickness of exfoliated 2D crystals to control the quantum well dimensions in few-layer semiconductor InSe and investigate the resonance features in the tunnelling current, photoabsorption and light emission spectra.
Abstract: Control over the quantization of electrons in quantum wells is at the heart of the functioning of modern advanced electronics; high electron mobility transistors, semiconductor and Capasso terahertz lasers, and many others. However, this avenue has not been explored in the case of 2D materials. Here we apply this concept to van der Waals heterostructures using the thickness of exfoliated crystals to control the quantum well dimensions in few-layer semiconductor InSe. This approach realizes precise control over the energy of the subbands and their uniformity guarantees extremely high quality electronic transport in these systems. Using tunnelling and light emitting devices, we reveal the full subband structure by studying resonance features in the tunnelling current, photoabsorption and light emission spectra. In the future, these systems could enable development of elementary blocks for atomically thin infrared and THz light sources based on intersubband optical transitions in few-layer van der Waals materials.
TL;DR: In this article, a laser-like phonon emission in a hybrid system that optomechanically couples polariton Bose-Einstein condensates (BECs) with phonons in a semiconductor microcavity was demonstrated.
Abstract: Efficient generation of phonons is an important ingredient for a prospective electrically-driven phonon laser. Hybrid quantum systems combining cavity quantum electrodynamics and optomechanics constitute a novel platform with potential for operation at the extremely high frequency range (30–300 GHz). We report on laser-like phonon emission in a hybrid system that optomechanically couples polariton Bose-Einstein condensates (BECs) with phonons in a semiconductor microcavity. The studied system comprises GaAs/AlAs quantum wells coupled to cavity-confined optical and vibrational modes. The non-resonant continuous wave laser excitation of a polariton BEC in an individual trap of a trap array, induces coherent mechanical self-oscillation, leading to the formation of spectral sidebands displaced by harmonics of the fundamental 20 GHz mode vibration frequency. This phonon “lasing” enhances the phonon occupation five orders of magnitude above the thermal value when tunable neighbor traps are red-shifted with respect to the pumped trap BEC emission at even harmonics of the vibration mode. These experiments, supported by a theoretical model, constitute the first demonstration of coherent cavity optomechanical phenomena with exciton polaritons, paving the way for new hybrid designs for quantum technologies, phonon lasers, and phonon-photon bidirectional translators. Efficient generation of phonons is an important ingredient for a prospective electrically-driven phonon laser for coherent control of quantum systems. Here, the authors report on laser-like phonon emission in a hybrid semiconductor microcavity that optomechanically couples BEC polaritons with phonons.
TL;DR: Decoupled high-order MQW superlattices are desired to design miniaturized photonic sources but they are yet to be realized in scalable ways using multiple-stacked colloidal lead halide perovskite quantum wells separated by atomically thin quantum barriers.
Abstract: Miniaturized photonic sources based on semiconducting two-dimensional (2D) materials offer new technological opportunities beyond the modern III-V platforms. For example, the quantum-confined 2D electronic structure aligns the exciton transition dipole moment parallel to the surface plane, thereby outcoupling more light to air which gives rise to high-efficiency quantum optics and electroluminescent devices. It requires scalable materials and processes to create the decoupled multi-quantum-well superlattices, in which individual 2D material layers are isolated by atomically thin quantum barriers. Here, we report decoupled multi-quantum-well superlattices comprised of the colloidal quantum wells of lead halide perovskites, with unprecedentedly ultrathin quantum barriers that screen interlayer interactions within the range of 6.5 A. Crystallographic and 2D k-space spectroscopic analysis reveals that the transition dipole moment orientation of bright excitons in the superlattices is predominantly in-plane and independent of stacking layer and quantum barrier thickness, confirming interlayer decoupling. Decoupled high-order MQW superlattices are desired to design miniaturized photonic sources but they are yet to be realized in scalable ways. Here Jagielski et al. achieve this goal using multiple-stacked colloidal lead halide perovskite quantum wells separated by atomically thin quantum barriers.
17 Dec 2020
TL;DR: In this paper, the authors introduce quantum well structures, their basic physics, their structure, fabrication technologies, and their elementary linear optical properties, which can be seen at room temperature and can be exploited in real devices.
Abstract: This chapter introduces quantum wells by discussing their basic physics, their structure, fabrication technologies, and their elementary linear optical properties. Many of the physical effects in quantum well structures can be seen at room temperature and can be exploited in real devices. All of the physics and devices that are based on properties of direct gap semiconductors near the center of the Brillouin zone. Quantum wells are one example of heterostructures–structures made by joining different materials, usually in layers, and with the materials joined directly at the atomic level. The chapter discusses only one class of effects, namely those related to optical absorption saturation near to the band-gap energy. The behaviour of the electro absorption for electric fields perpendicular to the quantum well layers is quite distinct from that in bulk semiconductors. The Self Electro-optic Effect Device principle is to combine a quantum well modulator with a photodetector to make an optically controlled device with an optical output.
TL;DR: It is found that the resonant tunneling can modulate the third-order and fifth-order of susceptibilities via detuning frequency of coupling light to useful for optical switching and optical sensors based on semiconductor nanostructures.
Abstract: We study the nonlinear optical properties in an asymmetric double AlGaAs/GaAs quantum well nanostructure by using an external control field and resonant tunneling effects. It is found that the resonant tunneling can modulate the third-order and fifth-order of susceptibilities via detuning frequency of coupling light. In presence of the resonant tunneling and when the coupling light is in resonance with the corresponding transition, the real parts of third-order and fifth-order susceptibilities are enhanced which are accompanied by nonlinear absorption. However, in off-resonance of coupling light, real parts of third-order and fifth-order susceptibilities enhance while the nonlinear absorption vanishes. We investigate also the two-dimensional electromagnetically induced grating (2D-EIG) of the weak probe light by modulating the third-order and fifth-order susceptibilities. In resonance of coupling light, both amplitude and phase grating are formed in the medium due to enhancement of third-order and fifth-order probe absorption and dispersion. When the coupling light is out of resonance, most of probe energy is transferred from zero-order to higher-order directions due to resonant tunneling effect. The efficiency of phase grating for third-order of susceptibility is higher than phase grating for fifth-order susceptibility. Our proposed model may be useful for optical switching and optical sensors based on semiconductor nanostructures.
TL;DR: A well-refilling mechanism providing a quasi-four-level system leading to multi-nanosecond lasing and record low room temperature lasing thresholds (~6 μJ cm−2 pulse−1) for III–V nanowire lasers is demonstrated.
Abstract: Continuous room temperature nanowire lasing from silicon-integrated optoelectronic elements requires careful optimisation of both the lasing cavity Q-factor and population inversion conditions. We apply time-gated optical interferometry to the lasing emission from high-quality GaAsP/GaAs quantum well nanowire laser structures, revealing high Q-factors of 1250 ± 90 corresponding to end-facet reflectivities of R = 0.73 ± 0.02. By using optimised direct-indirect band alignment in the active region, we demonstrate a well-refilling mechanism providing a quasi-four-level system leading to multi-nanosecond lasing and record low room temperature lasing thresholds (~6 μJ cm-2 pulse-1) for III-V nanowire lasers. Our findings demonstrate a highly promising new route towards continuously operating silicon-integrated nanolaser elements.
TL;DR: In this article, a detailed theoretical analysis of the electronic and optical properties of c-plane InGaN/GaN quantum well structures with In contents ranging from 5% to 25% is presented.
Abstract: We present a detailed theoretical analysis of the electronic and optical properties of c-plane InGaN/GaN quantum well structures with In contents ranging from 5% to 25%. Special attention is paid to the relevance of alloy induced carrier localization effects to the green gap problem. Studying the localization length and electron-hole overlaps at low and elevated temperatures, we find alloy-induced localization effects are crucial for the accurate description of InGaN quantum wells across the range of In content studied. However, our calculations show very little change in the localization effects when moving from the blue to the green spectral regime; i.e. when the internal quantum efficiency and wall plug efficiencies reduce sharply, for instance, the in-plane carrier separation due to alloy induced localization effects change weakly. We conclude that other effects, such as increased defect densities, are more likely to be the main reason for the green gap problem. This conclusion is further supported by our finding that the electron localization length is large, when compared to that of the holes, and changes little in the In composition range of interest for the green gap problem. Thus electrons may become increasingly susceptible to an increased (point) defect density in green emitters and as a consequence the nonradiative recombination rate may increase.
TL;DR: In this paper, the authors describe a spectroscopic method for probing excited states in isolated Si/SiGe double quantum dots using standard baseband pulsing techniques, easing the extraction of energy spectra in multiple-dot devices.
Abstract: Silicon quantum dot qubits must contend with low-lying valley excited states which are sensitive functions of the quantum well heterostructure and disorder; quantifying and maximizing the energies of these states are critical to improving device performance. We describe a spectroscopic method for probing excited states in isolated Si/SiGe double quantum dots using standard baseband pulsing techniques, easing the extraction of energy spectra in multiple-dot devices. We use this method to measure dozens of valley excited state energies spanning multiple wafers, quantum dots, and orbital states, crucial for evaluating the dependence of valley splitting on quantum well width and other epitaxial conditions. Our results suggest that narrower wells can be beneficial for improving valley splittings, but this effect can be confounded by variations in growth and fabrication conditions. These results underscore the importance of valley splitting measurements for guiding the development of Si qubits.
TL;DR: In this paper, the carrier confinement mechanism through nanostructures is studied in a copper-zinc-tin-sulfide/Cu2ZnSnSe4-type kesterite material, resulting in a remarkable performance enhancement of solar cells.
Abstract: In this work, the carrier confinement mechanism through nanostructures is studied in a copper-zinc-tin-sulfide/Cu2ZnSnSe4-type kesterite material, resulting in a remarkable performance enhancement of solar cells. The effect of the quantized energy band, recombination rate, and escape mechanism on the spectral response of solar cells is explored in detail. The mathematical model for carrier dynamics and performance measuring parameters are analyzed and optimized. Moreover, the number of quantum wells is incorporated gradually up to 100 and the corresponding performances are explored. It is observed that with the increase in the number of wells, photogenerated current density enhances significantly up to a saturation point and then deteriorates. A remarkable efficiency of 24.8% and more than 80% of quantum efficiency are achieved from 50 numbers of quantum wells with 79.8% of fill factor.
TL;DR: In this work, models of Al 0.75 Ga 0.25 N/AlN QWs constructed with variable lattice orientations were used to investigate the orbital intercoupling among atoms between the well and barrier regions and the barrier potential and transition rate at the band edge were enhanced through orbital engineering.
Abstract: AlGaN has attracted considerable interest for ultraviolet (UV) applications With the development of UV optoelectronic devices, abnormal carrier confinement behaviour has been observed for c-plane-oriented AlGaN quantum wells (QWs) with high Al content Because of the dispersive crystal field split-off hole band (CH band) composed of pz orbitals, the abnormal confinement becomes the limiting factor for efficient UV light emission This observation differs from the widely accepted concept that confinement of carriers at the lowest quantum level is more pronounced than that at higher quantum levels, which has been an established conclusion for conventional continuous potential wells In particular, orientational pz orbitals are sensitive to the confinement direction in line with the conducting direction, which affects the orbital intercoupling In this work, models of Al075Ga025N/AlN QWs constructed with variable lattice orientations were used to investigate the orbital intercoupling among atoms between the well and barrier regions Orbital engineering of QWs was implemented by changing the orbital state confinement, with the well plane inclined from 0° to 90° at a step of 30° (referred to the c plane) The barrier potential and transition rate at the band edge were enhanced through this orbital engineering The concept of orbital engineering was also demonstrated through the construction of inclined QW planes on semi- and nonpolar planes implemented in microrods with pyramid-shaped tops The higher emission intensity from the QWs on the nonpolar plane compared with those on the polar plane was confirmed via localized cathodoluminescence (CL) maps
TL;DR: In this paper, two different quantum Hall effects in HgTe quantum wells dilutely alloyed with Mn were reported, where a novel quantum Hall state emerged from the quantum spin Hall state at an exceptionally low magnetic field of approximately 50$ mT, where transport is dominated by van Hove singularity in the valence band.
Abstract: The realization of the quantum spin Hall effect in HgTe quantum wells has led to the development of topological materials which, in combination with magnetism and superconductivity, are predicted to host chiral Majorana fermions. However, the large magnetization ($\sim$ a few tesla) in conventional quantum anomalous Hall system, makes it challenging to induce superconductivity. Here, we report two different emergent quantum Hall effects in HgTe quantum wells dilutely alloyed with Mn. Firstly, a novel quantum Hall state emerges from the quantum spin Hall state at an exceptionally low magnetic field of $\sim 50$ mT. Secondly, tuning towards the bulk $p$-regime, we resolve multiple quantum Hall plateaus at fields as low as $20 - 30$ mT, where transport is dominated by a van Hove singularity in the valence band. These emergent quantum Hall phenomena rely critically on the topological band structure of HgTe and their occurrence at very low fields make them an ideal candidate for interfacing with superconductors to realize chiral Majorana fermions.
TL;DR: In this paper, a horizontal p−i−n ridge waveguide emitter on a silicon (100) substrate with a Ge0.91Sn0.09/Ge multi-quantum well (MQW) active layer was fabricated by molecular beam epitaxy.
Abstract: A horizontal p−i−n ridge waveguide emitter on a silicon (100) substrate with a Ge0.91Sn0.09/Ge multi-quantum-well (MQW) active layer was fabricated by molecular beam epitaxy. The device structure was designed to reduce light absorption of metal electrodes and improve injection efficiency. Electroluminescence (EL) at a wavelength of 2160 nm was observed at room temperature. Theoretical calculations indicate that the emission peak corresponds well to the direct bandgap transition (n1Γ−n1HH). The light output power was about 2.0 μW with an injection current density of 200 kA/cm2. These results show that the horizontal GeSn/Ge MQW ridge waveguide emitters have great prospects for group-IV light sources.
TL;DR: In this article, metal halide perovskite quantum wells (PQWs) are defined as quantum and dielectrically confined materials exhibiting strongly bound excitons.
Abstract: Metal halide perovskite quantum wells (PQWs) are quantum and dielectrically confined materials exhibiting strongly bound excitons. The exciton transition dipole moment dictates absorption strength ...
TL;DR: In this paper, a high-temperature performance quantum detector of terahertz (THz) radiation based on three-dimensional metamaterials is demonstrated, which consists of an inductor-capacitor (LC) resonator laterally coupled with antenna elements.
Abstract: We demonstrate a high-temperature performance quantum detector of Terahertz (THz) radiation based on three-dimensional metamaterial. The metamaterial unit cell consists of an inductor-capacitor (LC) resonator laterally coupled with antenna elements. The absorbing region, consisting of semiconductor quantum wells, is contained in the strongly ultra-subwavelength capacitors of the LC structure. The high radiation loss of the antenna allows strongly increased collection efficiency for the incident THz radiation, while the small effective volume of the LC resonator allows intense light-matter coupling with a reduced electrical area. As a result, our detectors operate at much higher temperatures than conventional quantum well detectors demonstrated so far.
TL;DR: This work demonstrates a photonic concept that fulfills the seemingly incompatible requirements for both strong electromagnetic confinement and impedance matching to free space and opens important perspectives for ultra-low dark current quantum detectors.
Abstract: Many photonic and plasmonic structures have been proposed to achieve ultrasubwavelength light confinement across the electromagnetic spectrum. Notwithstanding this effort, however, the efficient funneling of external radiation into nanoscale volumes remains problematic. Here, we demonstrate a photonic concept that fulfills the seemingly incompatible requirements for both strong electromagnetic confinement and impedance matching to free space. Our architecture consists of antenna-coupled meta-atom resonators that funnel up to 90% of the incident radiation into an ultrasubwavelength semiconductor quantum well absorber of volume V = λ310-6. A significant fraction of the coupled electromagnetic energy is used to excite the electronic transitions in the quantum well, with a photon absorption efficiency 550 times larger than the intrinsic value of the electronic dipole. This system opens important perspectives for ultralow dark current quantum detectors and for the study of light-matter interaction in the extreme regimes of electronic and photonic confinement.
TL;DR: In this paper, a detailed theoretical analysis of the electronic and optical properties of the $c$-plane quantum-well structures with alloy-induced carrier-localization effects was presented, with an emphasis on the green gap problem.
Abstract: We present a detailed theoretical analysis of the electronic and optical properties of $c$-plane $\mathrm{In}\mathrm{Ga}\mathrm{N}$/$\mathrm{Ga}\mathrm{N}$ quantum-well structures with $\mathrm{In}$ contents ranging from 5% to 25%. Special attention is paid to the relevance of alloy-induced carrier-localization effects to the ``green gap'' problem. Studying the localization length and electron-hole overlaps at low and elevated temperatures, we find alloy-induced localization effects are crucial for the accurate description of ($\mathrm{In}$,$\mathrm{Ga}$)$\mathrm{N}$ quantum wells across the range of $\mathrm{In}$ content studied. However, our calculations show very little change in the localization effects when moving from the blue to the green spectral regime; that is, when the internal quantum efficiency and wall-plug efficiencies reduce sharply, for instance, the in-plane carrier separation due to alloy-induced localization effects changes weakly. We conclude that other effects, such as increased defect densities, are more likely to be the main reason for the green-gap problem. This conclusion is further supported by our finding that the electron localization length is large, when compared with that of holes, and changes little in the $\mathrm{In}$ composition range of interest for the green-gap problem. Thus, electrons may become increasingly susceptible to an increased (point) defect density in green emitters and as a consequence the nonradiative-recombination rate may increase.