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We characterize InSb quantum dots induced by bottom finger gates within a nanowire that is grown via the vapor-liquid-solid process. The gates are separated from the nanowire by an exfoliated 35\,nm thin hexagonal BN flake. We probe the Coulomb diamonds of the gate induced quantum dot exhibiting charging energies of $\sim 2.5\,\mathrm{meV}$ and orbital excitation energies up to $0.3\,\mathrm{meV}$. The gate hysteresis for sweeps covering 5 Coulomb diamonds reveals an energy hysteresis of only $60\mathrm{\mu eV}$ between upwards and downwards sweeps. Charge noise is studied via long-term measurements at the slope of a Coulomb peak revealing potential fluctuations of $\sim 1\,\mu \mathrm{eV}/\mathrm{\sqrt{Hz}}$ at 1\,Hz. This makes h-BN the dielectric with the currently lowest gate hysteresis and lowest low-frequency potential fluctuations reported for low-gap III-V nanowires. The extracted values are similar to state-of-the art quantum dots within Si/SiGe and Si/SiO${_2}$ systems.

Solid-state qubits integrated on semiconductor substrates currently require at least one wire from every qubit to the control electronics, leading to a so-called wiring bottleneck for scaling. Demultiplexing via on-chip circuitry offers an effective strategy to overcome this bottleneck. In the case of gate-defined quantum dot arrays, specific static voltages need to be applied to many gates simultaneously to realize electron confinement. When a charge-locking structure is placed between the quantum device and the demultiplexer, the voltage can be maintained locally. In this study, we implement a switched-capacitor circuit for charge-locking and use it to float the plunger gate of a single quantum dot. Parallel plate capacitors, transistors, and quantum dot devices are monolithically fabricated on a Si/SiGe-based substrate to avoid complex off-chip routing. We experimentally study the effects of the capacitor and transistor size on the voltage accuracy of the floating node. Furthermore, we demonstrate that the electrochemical potential of the quantum dot can follow a 100 Hz pulse signal while the dot is partially floating, which is essential for applying this strategy in qubit experiments.

The unbalanced carrier injection is a key factor that deteriorates the performance of blue InP quantum dot light-emitting diodes (QLEDs). Therefore, to achieve efficient blue InP QLEDs, an effective strategy that balances carrier injection through enhancing the hole injection and transport is in demand. In this study, we introduced an ultrathin MoO3 electric dipole layer between the hole injection layer and the hole transport layer (HTL) to form a pair of dipole-induced built-in electric fields with forward resultant direction to enhance hole injection and facilitate the balance of carrier injection. Meanwhile, the p-doping effect by MoO3 leads to increased carrier concentration and decreased trap density of interfacial HTL, therefore improved its effective hole mobility. Consequently, the maximal external quantum efficiency of blue InP QLEDs was enhanced from 1.0% to 2.1%. This work provides an effective method to balance carrier injection by modulating hole injection and transport, indicating the feasibility to realize high-efficiency QLEDs.

The well-established silicon-on-insulator platform is very promising for large-scale integrated photonic and quantum photonic technologies due to the mature manufacturing technology and integration density. Here, we present an efficient and stable fiber-to-chip coupling, which enables the injection of single photons from telecom quantum dots into a silicon-on-insulator photonic chip. Two additional fibers further couple the chip to single-photon detectors. The approach chosen to achieve steady fiber-chip coupling is based on the use of grating couplers steadily packaged with angled single-mode fibers. Using this technique, coupling efficiencies between the fiber and the SOI chip as high as 69.1% per grating coupler (including the taper losses) are reached. The effective interface between the quantum light generated by quantum dots and the silicon components is verified via the measurement of the second-order correlation function using a Hanbury–Brown and Twiss setup. With g(2)(0)=0.051±0.001, it clearly proves the single-photon nature of the injected QD photons. This demonstrates the reliability of the interfacing method and opens the route to employ telecom quantum dots as non-classical light sources with high complexity silicon photonic functionalities.

We hetero-paired a ∼200 nm thin film consisting of colloidal 2.5 nm PbS quantum dots deposited on semi-insulating GaAs. By exciting the thin film with laser pulses (26 ps, 10 Hz) at 1064 nm, we observed the two-photon stimulated emission of the PbS quantum dots and the GaAs host. At a certain intensity of the optical stimulus, the absorption capability of the quantum dots collectively saturates, and more photons of the laser beam reach the GaAs host, causing a bistable-like up-switch in the GaAs photoluminescence intensity. The work further addresses the determination of the two-photon absorption coefficient, which was found to be 8.6 × 10−6 m/W.

Mechanical strain plays a key role in the physics and operation of nanoscale semiconductor systems, including quantum dots and single-dopant devices. Here we describe the design of a nanoelectronic device where a single nuclear spin is coherently controlled via nuclear acoustic resonance (NAR) through the local application of dynamical strain. The strain drives spin transitions by modulating the nuclear quadrupole interaction. We adopt an AlN piezoelectric actuator compatible with standard silicon metal-oxide-semiconductor processing, and optimize the device layout to maximize the NAR drive. We predict NAR Rabi frequencies of order 200 Hz for a single $^{123}$Sb nucleus in a wide region of the device. Spin transitions driven directly by electric fields are suppressed in the center of the device, allowing the observation of pure NAR. Using electric field gradient-elastic tensors calculated by density-functional theory, we extend our predictions to other high-spin group-V donors in silicon, and to the isoelectronic $^{73}$Ge atom., 6 pages, 5 figures

Since the absorption zone of ultraviolet (UV) photons with high energy is limited to a few tens of nm on the surface, the high defect density caused by the processes, such as ion implantation, leads to a weak response of the silicon avalanche photodiode (APD) in the ultraviolet band. In this work, the integration of the inorganic perovskite quantum dots (QDs) film by drop-casting as the down-shifting layer is reported for enhancing the UV response of Si APD. The light generated current increases 100% under the 365 nm light emitting diode. The response of the Si APD is improved in the entire ultraviolet band. In particular, the responsivity of APD is increased by 78% at 340 nm with an exceedingly EQE of 92%. In summary, the perovskite QDs film as a down-shifting layer provides an effective and low-cost method to improve the UV response of Si APD.

Feedback-controlled electric breakdown of graphene in air or vacuum is a well-established way of fabricating tunnel junctions, nanogaps, and quantum dots. We show that the method is equally applicable to encapsulated graphene constrictions fabricated using hydrogen silsesquioxane. The silica-like layer left by hydrogen silsesquioxane resist after electron-beam exposure remains intact after electric breakdown of the graphene. We explore the conductance switching behavior that is common in graphene nanostructures fabricated via feedback-controlled breakdown and show that it can be attributed to atomic-scale fluctuations of graphene below the encapsulating layer. Our findings open up routes for fabricating encapsulated room-temperature single-electron nanodevices and shed light on the underlying physical mechanism of conductance switching in these graphene nanodevices.

An optical refractive index (RI) nanosensor with a high sensitivity and figure of merit (FOM), good stability, and biocompatibility is of great significance for biological detection and sensing in narrow spaces. However, the current optical RI nanosensors are mainly fabricated using metals, semiconductors, and quartz, which are not biocompatible and are even biotoxic, and often face a trade-off between a high sensitivity and a high FOM. Moreover, the sensors are mainly based on surface plasmon resonance, photonic crystals, fiber grating, etc., and, thus, most of them usually require a laser source with a specific optical wavelength or harsh excitation conditions, which are likely to cause photodamage and are unfavorable for biological applications. Hence, polylactic acid (PLA), a flexible dielectric material with good biocompatibility, is functioned by doping high refractive index quantum dots (QDs) and fabricated as a nanowire RI sensor. Doping the QDs into a PLA nanowire can improve the light confinement ability and then enhance Mie resonant scattering of the PLA nanowire, which is very beneficial to obtain a higher quality factor and then a higher-performance nanowire sensor. Under irradiation of a white light source, a high sensitivity with 833.78 nm/RIU (per refractive index unit) and the highest FOM of 9.64 RIU−1 are obtained. The good reliability and reproducibility of the sensors are further demonstrated. By choosing a proper diameter, the scattering peak of the nanosensor can be tuned into a biofriendly spectral range (600–900 nm), which predicts that the PLA nanowire RI sensors have a great potential in biological microenvironment monitoring, biosensing, and biomedical treatment.

Photoconductive antennas deposited onto GaAs substrates that incorporate InAs quantum dots have been recently shown to efficiently generate both pulsed and CW terahertz radiation. In this Letter, we determine the operational limits of these antennas and demonstrate their extreme thermal breakdown tolerance. Implanted quantum dots serve as free carrier capture sites, thus acting as lifetime shorteners, similar to defects in low-temperature grown substrates. However, unlike the latter, defect-free quantum-dot structures possess perfect lattice quality, thus not compromising high carrier mobility and pump intensity stealth. Single gap design quantum dot based photoconductive antennas are shown to operate under up to 1 W of average pump power ( ∼1.6 mJ cm−2 energy density), which is more than 20 times higher than the pumping limit of low-temperature grown GaAs based substrates. Conversion efficiency of the quantum dot based photoconductive antennas does not saturate up to 0.75 W of pump power ( ∼1.1 mJ cm−2 energy density). Such a thermal tolerance suggests a glowy prospect for the proposed antennas as a perspective candidate for intracavity optical-to-terahertz converters.

Exploring the temperature-dependent electroluminescence (EL) properties of quantum dots (QDs) is not only critical for the fundamental understanding of charge recombination processes in quantum dot light-emitting diodes (QLEDs) but also important to expand their particular applications at cryogenic temperatures. Herein, the temperature-dependent EL properties of typical CdSe/ZnS core/shell QDs were systematically studied for both the standard and inverted QLEDs in the temperature range of 100–300 K. It was found that EL intensity and efficiency were gradually enhanced and exhibited a pronounced blue shifting in EL spectra with the decrease in temperature. We demonstrated that temperature-dependent EL behaviors mainly originated from the inherent properties of QDs, while the different device structures could induce some fine temperature-dependent behaviors on this basis. Moreover, the effective Joule heat dissipation at low temperatures protects the charge transport and QD layers from thermal decomposition and damage, which prolongs the operational lifetime of devices.

Detection of explosive traces in the vapor phase is of primary importance for safety and security in several environments. Different detection methods with high sensitivity are available in the market, but they are typically expensive and require specialized personnel to be operated. Here, we propose a compact, low-cost sensor for explosive detection based on the photoluminescence (PL) quenching of solid-state PbS quantum dot solids cast from the solution phase on a silicon substrate. We demonstrate the sensor capability to detect nitrobenzene vapor at a concentration as low as 445 ppb in air at room temperature, overcoming the performance of other state-of-the-art quantum dot-based PL sensors for nitroaromatic compounds. Moreover, the proposed system can be realized with off-the-shelf electronics and does not need any additional laboratory equipment to be operated, thus paving the way for its deployment in distributed sensor networks.

Electron spins in semiconductor quantum dots are a promising platform for quantum-information processing applications because their quantum phase coherence can persist for extremely long times. Nearest-neighbor electron spins naturally interact with each other via Heisenberg exchange coupling. Heisenberg exchange coupling results from the interplay of the electrostatic confinement potential together with the Pauli exclusion principle, which requires that no two electrons can have the same quantum number. Exchange coupling enables a host of useful capabilities, including the generation of different types of qubits, multi-qubit gates, ways to increase connectivity in systems of quantum-dot spin qubits, and routes to explore intriguing many-body phenomena.

The luminescent solar concentrator (LSC) has attracted worldwide attention as a sunlight manager for harvesting solar energy in building integrated photovoltaic structures. However, until now the relatively low optical efficiency (ηopt) still severely hinders the advancement of the LSCs. In this work, we report the perovskite quantum dots (QDs) integrated into the thiol-ene polymer for LSC applications. After encapsulated by the thiol-ene polymer, the CsPbBr3 QDs exhibit a slight blue-shift of the PL peak position and increasing FWHM because of the dielectric screen effect. With the different QDs' concentrations from 8.3 × 10−3 to 2.5 × 10−2 mg/ml, the LSC performance is evaluated by the direct optical characterization and the indirect photo-electrical measurement in detail. After coupling the PV cells, the as-fabricated LSC shows the optimal optical efficiency of 2.11% and the power conversion efficiency of 1.06%.

Optical localization in strongly disordered photonic media is an attractive topic for proposing novel cavity-like structures. Light interference can produce random modes confined within small volumes, whose spatial distribution in the near-field is predicted to show hot spots at the nanoscale. However, these near-field speckles have not yet been experimentally investigated due to the lack of a high spatial resolution imaging techniques. Here, we study a system where the disorder is induced by random drilling air holes in a GaAs suspended membrane with internal InAs quantum dots. We perform deep-subwavelength near-field experiments in the telecom window to directly image the spatial distribution of the electric field intensity of disordered-induced localized optical modes. We retrieve the near-field speckle patterns that extend over few micrometers and show several single speckles of the order of λ/10 size. The results are compared with the numerical calculations and with the recent findings in the literature of disordered media. Notably, the hot spots of random modes are found in proximity of the air holes of the disordered system., This work was supported by the FET project FP7 618025 CARTOON. This work is part of the research programme of the Foundation for Fundamental Research on Matter (FOM), which is financially supported by the Netherlands Organisation for Scientific Research (NWO), and it is also supported by the Dutch Technology Foundation STW, applied science division of NWO, the Technology Program of the Ministry of Economic Affairs under Project No. 10380. F.B. acknowledges funding from the Italian Ministry for Education, University and Research within the Futuro in Ricerca (FIRB) program (project DeLIGHTeD, Protocollo RBFR12RS1W).

Twistronic assembly of 2D materials employs the twist angle between adjacent layers as a tuning parameter for designing the electronic and optical properties of van der Waals heterostructures. Here, we study how interlayer hybridization, weak ferroelectric charge transfer between layers, and piezoelectric response to deformations set the valence and conduction band edges across the moir{\'e} supercell in twistronic homobilayers of MoS$_2$, MoSe$_2$, WS$_2$ and WSe$_2$. We show that, due to the lack of inversion symmetry in the monolayer crystals, bilayers with parallel (P) and anti-parallel (AP) unit cell orientations display contrasting behaviors. For P-bilayers at small twist angles we find band edges in the middle of triangular domains of preferential stacking. In AP-bilayers at marginal twist angles ($\theta_{AP} < 1^\circ$) the band edges are located in small regions around the intersections of domain walls, giving highly localized quantum dot states., Comment: 21 pages, 15 figures

Epitaxially grown semiconductor quantum dots are promising candidates for pure single photon and polarization-entangled photon pair emission. Excellent optical properties can typically be ensured only if these so-called “artificial atoms” are buried deep inside the semiconductor host material. Quantum dots grown close to the surface are prone to charge carrier fluctuations and trap states on the surface, degrading the brightness, coherence, and stability of the emission. We report on high-purity single photon emission [g(2)(0) = 0.016 ± 0.015] of GaAs/AlGaAs quantum dots that were grown only 20 nm below the surface. Chemical surface passivation with sulfur compounds such as octadecanethiol has been performed on quantum dots with 20, 40, and 98 nm from the surface. The reduction of the density and influence of surface states causes improvements in linewidth and photoluminescence intensity as well as a well-preserved single photon emission. Therefore, the realization of hybrid nanophotonic devices, comprising near-field coupling and high-quality optical properties, comes into reach.

We demonstrate a technique to quickly build and spatially map the frequency response of optoelectronic devices. The transfer function of a linear system is the Fourier transform of its impulse response. Such an impulse response is obtained from transient photocurrent measurements of devices such as photodetectors and solar cells. We introduce and apply Fourier transform impedance spectroscopy (FTIS) to a PbS colloidal quantum dot SiC heterojunction photodiode and validate the results using intensity-modulated photocurrent spectroscopy. Cutoff frequencies in the devices were as high as ∼10 kHz, showing their utility in advanced thin film and flexible electronics. The practical frequencies for FTIS lie in the mHz–kHz range, ideal for composite materials such as quantum dot films that are dominated by interfacial trap states. These can lead to characteristic lengths for charge collection ∼20–500 μm dominated by transmission line effects, rather than intrinsic diffusion and drift length scales, enabling extraction of interfacial capacitances and series/parallel resistances.

In this work we demonstrate a triggered single-photon source operating at the telecom C-band with photon extraction efficiency exceeding any reported values in this range. The non-classical light emission with low probability of the multiphoton events is realized with single InAs quantum dots (QDs) grown by molecular beam epitaxy and embedded directly in an InP matrix. Low QD spatial density on the order of 5x108 cm-2 to ~2x109 cm-2 and symmetric shape of these nanostructures together with spectral range of emission makes them relevant for quantum communication applications. The engineering of extraction efficiency is realized by combining a bottom distributed Bragg reflector consisting of 25 pairs of InP/In0.53Ga0.37Al0.1As layers and cylindrical photonic confinement structures. Realization of such technologically non-demanding approach even in a non-deterministic fashion results in photon extraction efficiency of (13.3+/-2)% into 0.4 numerical aperture detection optics at approx. 1560 nm emission wavelength, i.e., close to the center of the telecom C-band., Comment: 18 pages, 3 figures

In this study, we evaluated the degradation mechanism in quantum dot light-emitting diodes (QLEDs) to improve the device lifetime. We measured the hole mobility using the delay time of transient electroluminescence for three types of hole transport layer (HTL) materials. In addition, we estimated the degradation of luminance efficiency and hole mobility under constant current drive. As a result, the HTL material with a higher hole mobility yielded longer QLED device lifetimes. Through substitution of the HTL material from poly (9-vinylcarbazole) (PVK) to poly [(9,9-dioctylfluorenyl-2,7-diyl)-co-(4,4′-(N-(4-sec-butylphenyl)diphenylamine)] (TFB), the hole mobility and the 95% luminance lifetime from initial luminance improved from 0.5 × 10−5 cm2/V⋅s and 2.90 h at J = 10 mA/cm2 to 1.1 × 10−5 cm2/V⋅s and 179 h, respectively. Moreover, we clarified that the degradation of the luminescent efficiency is correlated with the hole mobility.

International audience; To deliver an optimal performance for photonic quantum technologies, semiconductor quantum dots should be integrated in a carefully designed photonic structure. Here, we introduce a nanowire optical nanocavity designed for free-space emission. Thanks to its ultrasmall mode volume, this simple structure offers a large acceleration of spontaneous emission (predicted Purcell factor of 6.3) that is maintained over a 30-nm bandwidth. In addition, a dielectric screening effect strongly suppresses the emission into the 3D continuum of radiation modes. The fraction of spontaneous emission funneled into the cavity mode reaches 0.98 at resonance and exceeds 0.95 over a 100-nm spectral range. Close-to-optimal collection efficiency is maintained over an equivalent bandwidth and reaches a predicted value of 0.54 at resonance for a first lens with a numerical aperture (NA) of 0.75. As a first experimental demonstration of this concept, we fabricate an Au–SiO2–GaAs device embedding isolated InAs quantum dots. We measure a maximal acceleration of spontaneous emission by a factor as large as 5.6 and a bright quantum dot emission (collection efficiency of 0.35 into NA¼0.75). This nanowire cavity constitutes a promising building block to realize advanced sources of quantum light for a broad range of material systems.

The integration of single quantum dots (QDs) into a planar Fabry–Perot microcavity has been established as a direct and viable approach to vertically steer photons emitted from the quantum emitters, resulting in a strong increase in the source brightness, which becomes particularly evident when a lens with a low numerical aperture is used. However, the spectral bandwidth of QD–microcavity structures is limited and determined by their intrinsic quality factor; these structures are, thus, not ideal for the extraction of entangled photon pairs or for studies of exciton dynamics. We have found that, when a deterministic low-index solid immersion lens is placed on top of the QD in a QD–microcavity structure, the structure exhibits an enhancement in the bandwidth to 27 nm and a source brightness of 23%. The solid immersion lens is deterministically fabricated via two-photon absorption and can be remade several times without perturbing the QD, ensuring that the QD's intrinsic properties are preserved and ensuring its long-term reliability.

In this study, quantum dot color conversion layers (QDCCLs) for full-color micro-LED display were successfully fabricated using microfluidics to conduct red and green perovskite quantum dots to the position of the pixel array. The QDCCL with full-color pixel size of 200 × 200 μm and sub-pixel size of 140 × 50 μm was achieved. Perovskite quantum dots with high quantum yield and narrow full width at half-maximum were used to achieve a wide color gamut, which was 131% of National Television Systems Committee standard. The proposed microfluidics-based QDCCL featured easy fabrication, low cost, high performance, and good integration prospects.

With the advent of the big-data era, conventional memory technologies and devices are facing enormous challenges. Resistive random access memory (RRAM) is an emerging memory technology that has aroused widespread interest for its immense potential. However, there remain some problems in resistive switching devices, such as high switching voltages, random voltages distribution, wide variation in resistance states, and poor endurance. In this work, molybdenum disulfide quantum dots are applied to resistive switching devices. The resulting devices exhibit improved performance. They have ultra-low and centralized switching voltages, uniformly distributed resistance states, good endurance, and extremely large on/off ratios. This performance optimization may derive from the convergence of electric field distribution around molybdenum disulfide quantum dots, which enhances the formation of localized conductive filaments. In this Letter, we propose an approach for improving resistive switching properties, significantly facilitating the development of data storage and related applications.

A systematic study of the emission spectra of single InAsP nanowire quantum dots in position-controlled InP photonic nanowire waveguides is presented. Using excitation power-dependent photoluminescence and correlation measurements, we distinguish between the different excitonic complexes responsible for s-shell emission. From measurements of over 40 nominally identical devices, we obtain a standard deviation of ∼ 5 meV in the emission energy of excitons, biexcitons, and charged exciton photons. The mean biexciton binding energy was 1.9 meV with a standard deviation of ∼ 0.8 meV. The experimental spectra are understood using atomistic multi-million atom theory of neutral and charged multi-exciton complexes implemented in the design tool QNANO.

Multiple exciton generation (MEG) increases the short circuit current of solar cells and is, therefore, often cited as a candidate scheme for surpassing the efficiency limit of single junction solar cells. Conventionally, limiting efficiencies for MEG solar cells have been calculated using quasi-equilibrium models that implicitly assume an effective separation of timescales between different processes. We show here that this separation of timescales is not possible for MEG solar cells, with Auger recombination, the inverse process to multi-exciton generation, needing to be considered explicitly in the efficiency limits of an MEG solar cell. We assess the impact of Auger recombination using a non-equilibrium model of a quantum dot solar cell that satisfies microscopic reversibility and can approximate experimental external quantum efficiency (EQE) curves of MEG solar cells. Recombination—both Auger and radiative—is treated in a quasi-equilibrium approach, which can be justified with a clear model for the separation of timescales. A key insight of this model is that the achievable voltage of the device, and hence the solar energy conversion efficiency, depends on the absolute values of the impact ionization rate and the rate at which electrons lose energy through phonon scattering. By contrast, the EQE profile at short circuit depends only on the ratio of these two rates. This shows that the potential of certain MEG solar cell approaches cannot be assessed from EQE improvements alone, which highlights the importance of considering non-equilibrium processes in models of solar energy conversion devices.

We report the fabrication of highly ordered arrays of GaAs/AlGaAs quantum dots (QDs) by droplet epitaxy using in situ direct laser interference patterning. Two-dimensional arrays of Ga droplets with a periodicity of ∼300 nm are initially formed on nanoisland structured AlGaAs surfaces due to the localized surface diffusion under the influence of a thermal gradient imposed by the light pulse. After crystallization under an arsenic flux, precisely ordered arrays of GaAs single dots are obtained. The size distribution and optical properties of the ordered GaAs QDs are shown to be optimized by the careful choice of parameters for nucleation and droplet formation.

Silicon-Germanium (SiGe) is a material that possesses a multitude of applications ranging from transistors to eletro-optical modulators and quantum dots. The diverse properties of SiGe also make it attractive to implementations involving superconducting quantum computing. Here we demonstrate the fabrication of transmon quantum bits on SiGe layers and investigate the microwave loss properties of SiGe at cryogenic temperatures and single photon microwave powers. We find relaxation times of up to 100 $\mu$s, corresponding to a quality factor Q above 4 M for large pad transmons. The high Q values obtained indicate that the SiGe/Si heterostructure is compatible with state of the art performance of superconducting quantum circuits., Comment: 5 pages, 3 figures

We used the measurements of the photo-luminescence spectra vs photon pumping and Coulomb blockade charge tuning together with Kelvin probe microscopy to study piezo-electric fields in self-organized InP/GaInP2 quantum dots (QDs) having a strong Wigner localization regime. These exciton/electron state-filling measurements together with the surface potential imaging and the band structure calculations demonstrate a piezo-electric doping and type-I optical transitions induced in these dots by an atomic ordering (AO) of GaInP2. Our results clarify a critical role of AO in the formation of natural Wigner and anyon molecules and represent an important step for realization of the topological quantum gates using these QDs.

Single solid-state quantum dots have significant potential as bright single-photon sources for scalable photonic quantum information technologies. Engineering their radiative relaxation properties is of significant importance for their practical applications. In this study, we demonstrate a cavity-free, broadband approach for modulating and collecting the fluorescence of a single-photon emitter using a fiber taper–silver nanowire plasmonic probe. When the plasmonic probe is located above a single colloidal quantum dot at approximately 20 nm, the photon-emitter interaction increased rapidly and a significant decrease, by an average factor of 3.38, in the lifetime of the quantum dot was observed. The fluorescence signal of the quantum dots was collected by the hybrid probe, with significantly higher efficiency than that of the traditional metal-coated near-field probe. The results of the numerical simulation were in good agreement with the experimental results. The proposed near-field modulation method can be applied to other single-photon sources and proved to be a flexible method for manipulating the luminescence of systems based on single-photon emitters.

Silicon quantum dot spin qubits have great potential for application in large-scale quantum circuits as they share many similarities with conventional transistors that represent the prototypical example for scalable electronic platforms. However, for quantum dot formation and control, additional gates are required, which add to device complexity and, thus, hinder upscaling. Here, we meet this challenge by demonstrating the scalable integration of a multilayer gate stack in silicon quantum dot devices using self-alignment, which allows for ultra-small gate lengths and intrinsically perfect layer-to-layer alignment. We explore the prospects of these devices as hosts for hole spin qubits that benefit from electrically driven spin control via spin–orbit interaction. Therefore, we study hole transport through a double quantum dot and observe current rectification due to the Pauli spin blockade. The application of a small magnetic field leads to lifting of the spin blockade and reveals the presence of spin–orbit interaction. From the magnitude of a singlet-triplet anticrossing at a high magnetic field, we estimate a spin–orbit energy of ∼ 37 μ eV, which corresponds to a spin–orbit length of ∼ 48 nm. This work paves the way for scalable spin-based quantum circuits with fast, all-electrical qubit control.

Triplet excitons in organic materials are nonradiative and tightly bound as self-trapped ones due to the strong electron-lattice coupling. They exhibit a finite radius and long lifetime and so can be regarded as the isolated quantum dots. In this work, we theoretically demonstrate that the spin polarization of self-trapped triplet excitons can be coherently controlled by the short circularly polarized laser pulses, acting as a helicity-dependent all-optical switching effect. Such a switching can be achieved within dozens of femtoseconds and conducted in a reproducible manner. The pulse parameter dependence on the switching ratio is further investigated. Our calculation provides a theoretical foundation for exploring the ultrafast all-optical recording and information processing technique based on the organic materials.

In this Letter, we demonstrate coupled double-quantum dot (DQD)-like transport in an ∼ 30 nm-wide controllably doped graphene nanoribbon (GNR). Controlled doping is introduced from hydrogen silsesquioxane by changing its electron exposure dose. The proximity effect, which brings in additional dose accumulation, is utilized to introduce two charge puddles with stronger p-doping at the two ends of the moderately p-doped GNR, which act as two quantum dots. By electrostatically isolating these two charge puddles with simplified overlapping dual gates, DQD-like transport features are measured in the doped GNR at a temperature of 5 K. Moreover, the transition from strongly to weakly coupled DQDs is observed due to electrically tunable inter-dot coupling.

A germanium (Ge) or germanium-on-silicon (Ge-on-Si) substrate is an attractive yet not well-studied platform for developing long-wave infrared photonics devices such as lasers and photodetectors. In this paper, we report a long-wave infrared quantum cascade photodetector grown on the Ge substrate with a submonolayer InAs/GaAs quantum dot as the infrared absorber. At 77 K under zero bias, the detector shows a differential-resistance area ( R 0 A) product of 298.7 Ω·cm2. The normal-incident peak responsivity is 0.56 mA/W observed at 8.3 μm, corresponding to a Johnson noise limited detectivity of 1.5 × 108 cm·Hz1/2/W. In addition, the effect of the periodic stage number of active regions on device's performance is discussed in detail. The device characteristics presented in this work demonstrate the potential for monolithic integration of this quantum cascade detector with the Ge or Ge-on-Si substrate for large-scale, cost-effective sensing and imaging applications.

When we applied colloidal quantum dots (QDs) for quantum dot light emitting diodes, it was well known that shell thickness played an important role in core protection, confinement of electrons and holes, and charge injection efficiency. However, although the shell thickness dependence of electroluminescence properties was reported, carrier injection efficiency has not been discussed in detail. In this paper, we investigated the effect of shell thickness on the carrier injection efficiency that was evaluated by photoelectrochemical measurements. By comparing the product of internal quantum yield of photoluminescence and the evaluated carrier injection efficiency with external quantum efficiency (EQE) for QDs with various shell thicknesses, we found that the optimal shell thickness for increasing EQE is determined by the balance between protection of QD's surface and carrier injection efficiency.

Development of alternating current (AC)-driven devices with versatile architecture is a potential approach to realize multifunctional light-emitting sources. Planar AC-powered electroluminescent (pAC-EL) devices using lateral placement of electrodes, instead of conventional stacking, are an emerging design that manifests promising applications beyond displays. While phosphors and organic light-emitting materials have been applied in pAC-EL devices, further enhancing the color purity and brightness remains a daunting challenge. In this Letter, we explore the utilization of quantum dots as the emitting layer for pAC-EL single-insulation devices without external injection. In such architecture, light is produced by the recombination of internally generated holes and field-induced electrons in the emissive layer of two in-plane light-emitting units alternately. The developed pAC-QLEDs exhibited a maximum brightness of 2023, 6327, and 613 cd/m2 for red, green, and blue (RGB) emissions at 150 kHz, respectively. Furthermore, a white-emitting pAC-QLED and a bi-color pAC-QLED were also constructed by stacking the R/G/B QD layers in serial and putting the R/G QD layers in parallel, respectively. In addition, compatibility of the proposed device configuration with flexible substrates is also manifested. The development of pAC-QLEDs provides an effective route to achieve high brightness without external injection, indicating diverse applications of these light sources for health monitoring sensors and phototherapy.

Imaging of optical phenomena at the sub-nanometer scale can offer fundamental insights into the electronic or vibrational states in atomic-scale defects, molecules, and nanoparticles, which are important in quantum information, heterogeneous catalysis, optoelectronics, and structural biology. Several techniques have surpassed the traditional Abbe diffraction limit and attained spatial resolutions down to a few nanometers, but sub-nanometer scale optics has remained elusive. Here, we propose an approach that combines spectrally specific photoabsorption with sub-nanometer scale resolution transmission electron microscopy (TEM) of photoexcited electrons. We first estimate the signal level and conditions required for imaging nanoscale optical phenomena in core-shell quantum dots (QDs) like CdS/CdTe. Furthermore, we show the possibility of imaging photoexcited states of atomic-scale defects in a monolayer hexagonal boron nitride (h-BN) using ab initio and high resolution (HR)TEM simulations. The ability to directly visualize photoexcited states at the sub-nanometer scale opens opportunities to study properties of individual quantum dots and atomic defects.

This Letter studies the effect of the nitrogen capture ability of quantum dots on resistive switching characteristics of AlN-based resistive random access memory. We prepared a single layer AlN device and four types of AlN/PbS quantum dot stacked structure devices with different concentrations. Compared with the single layer AlN device, the AlN/PbS quantum dot stacked structure devices exhibit excellent resistive switching characteristics, such as forming-free, low power consumption, and excellent stability. We propose that the resistive switching process is determined by the migration of nitrogen ions and the lead sulfide (PbS) quantum dot layer as a natural nitrogen ion reservoir, which can improve the resistive switching characteristics. Moreover, the size of the natural nitrogen ion reservoir can be modulated by changing the concentration of quantum dots.

The ability of two photons to interfere lies at the heart of many photonic quantum networking concepts and requires that the photons are indistinguishable with sufficient coherence times to resolve the interference signals. However, for solid-state quantum light sources, this can be challenging to achieve as they are in constant interaction with noise sources in their environment. Here, we investigate the noise sources that affect InAs/InP quantum dots emitting in the telecom C-band by comparing their behavior on a wetting layer for Stranski–Krastanov grown quantum dots with a nearly wetting layer-free environment achieved with the droplet epitaxy growth mode. We show that the droplet epitaxy growth mode is beneficial for a quiet environment, leading to 96% of exciton transitions having a coherence time longer than the typical detector resolution of 100 ps, even under non-resonant excitation. We also show that the decay profile indicates the presence of slow dephasing processes, which can be compensated for experimentally. We finally conduct Hong–Ou–Mandel interference measurements between subsequently emitted photons and find a corrected two-photon interference visibility of 98.6 ± 1.6% for droplet-epitaxy grown quantum dots. The understanding of the influence of their surroundings on the quantum optical properties of these emitters is important for their optimization and use in future quantum networking applications.

Efficiency and response speed are key figures of merit for high-performance photodetectors, with high efficiency often obtained at the expense of slow photoresponse. Here, we report on germanium quantum dot photodetectors (Ge QD PDs) with a 25-nm-thick active layer that possesses both high internal quantum efficiency (IQE) and fast photoresponse, yet is still based on simple design and fabrication. We characterize these devices with continuous wave (CW) and pulsed excitation at room temperature as a function of incident power and applied bias. Under the reverse bias of –4 V, the IQE approaches ∼2000% over a broad spectral range (λ = 500–800 nm). The transient photoresponse speed to a 4.5 ns laser pulse at λ = 640 nm is under 20 ns. Furthermore, we observe an interesting phenomenon: by superimposing a weak CW HeNe laser beam ( λ = 632.8 nm) on the laser pulse, we obtain an optically tunable photoresponse while retaining fast speed. This study elucidates the role of photocarrier generation, trapping, and hopping in the percolative Ge QD oxide matrix and helps explain the observed high gain and fast response speed. The demonstrated IQE and nanosecond response time render our devices suitable for low-light detection and imaging.

Most quantum communication schemes aim at the long-distance transmission of quantum information. In the quantum repeater concept, the transmission line is subdivided into shorter links interconnected by entanglement distribution via Bell-state measurements to overcome inherent channel losses. This concept requires on-demand single-photon sources with a high degree of multi-photon suppression and high indistinguishability within each repeater node. For a successful operation of the repeater, a spectral matching of remote quantum light sources is essential. We present a spectrally tunable single-photon source emitting in the telecom O-band with the potential to function as a building block of a quantum communication network based on optical fibers. A thin membrane of GaAs embedding InGaAs quantum dots (QDs) is attached onto a piezoelectric actuator via gold thermocompression bonding. Here the thin gold layer acts simultaneously as an electrical contact, strain transmission medium and broadband backside mirror for the QD-micromesa. The nanofabrication of the QD-micromesa is based on in-situ electron-beam lithography, which makes it possible to integrate pre-selected single QDs deterministically into the center of monolithic micromesa structures. The QD pre-selection is based on distinct single-QD properties, signal intensity and emission energy. In combination with strain-induced fine tuning this offers a robust method to achieve spectral resonance in the emission of remote QDs. We show that the spectral tuning has no detectable influence on the multi-photon suppression with $g^{(2)}(0)$ as low as 2-4% and that the emission can be stabilized to an accuracy of 4 $\mu$eV using a closed-loop optical feedback.

Localization of carriers in a self-organized quantum dot is a problem of quantum mechanics to be solved with the localizing potential for electrons and holes determined by the geometry, chemical composition, and built-in mechanical stress–strain field. We show that changes in the aspect ratio of a buried pyramidal quantum dot result in a substantial redistribution of the mechanical field components, which in turn provides certain non-trivial conditions for the strongest carrier localization at any given volume of the quantum dot.

Recently, a series of single-layer metal–organic frameworks (MOFs) was theoretically predicted to be two-dimensional organic topological materials. However, the experimental evidence of their nontrivial topological states has not been found. Here, combining the use of angle-resolved photoemission spectroscopy and scanning tunneling microscopy, we report the electronic structure studies on a single-layer Cu-coordinated 2,4,6-tri(4-pyridyl)-1,3,5-triazine (Cu-T4PT) MOF supported by a Cu(111) substrate and identify periodic surface states with the period of the Cu-T4PT reciprocal lattice. These periodic surface states, which have identical features to the Cu(111) Shockley surface states, can be attributed to the quantum confinement of the surface states of the underlying Cu(111) substrate by the network lattices of the Cu-T4PT MOF. Our work indicates that the surface states of the metal substrate can be tailored in a controlled manner by the network structures of MOFs with different periodic lattices. The lack of intrinsic bands and the possible topological properties of the single-layer Cu-T4PT MOF may be attributed to the strong electronic coupling between the Cu-T4PT MOF and the Cu(111) substrates. In order to exploit organic topological materials predicted in MOFs, it is necessary to grow them on weak van der Waals interaction substrates in the future.

Quantum dots (QDs) epitaxially grown on Si are promising for monolithic integration of light sources on a Si photonics platform. Unlike quantum well (QW) lasers on Si, 1.3 μm InAs QD lasers on Si show similar threshold current to those grown on GaAs owing to their better dislocation tolerance. To date, research on dislocation-tolerant QDs has exclusively focused on materials emitting at telecom wavelengths. In this work, we report visible InP QDs on Si with photoluminescence (PL) intensity similar to their counterparts grown on GaAs despite high threading dislocation density (TDD). In contrast, visible InGaP QWs grown on Si with the same TDD value show 9 × degradation in PL intensity compared to QWs grown on GaAs. The dislocation tolerance of InP QDs arises from their high density relative to TDD and the lateral carrier confinement that they provide. InP QDs on Si with bright PL are promising for low-cost light emitters and integrated photonics applications requiring monolithic red-light sources.

We have demonstrated an improved quantum dot (QD) and silica hybrid nanocomposite by interfacial optimization for simultaneous enhancement of fluorescence retention and stability. This nanocomposite was synthesized by using silica spheres as cores, adsorbing gradient alloy QDs (GA-QDs) as the first shell, and then coating a silica layer as the other outmost shell (termed SiO2-GA-QD-SiO2). The retaining ratio of pristine fluorescence intensity after silica coating was found to be significantly improved by the QDs' shell interfacial optimization due to the suppression of surface defects. The mechanism of the QDs' surface trap states capturing the excitons before and after silica coating was analyzed in detail. The results show that the optimized SiO2-GA-QD-SiO2 nanocomposite provides the highest resulting fluorescence intensity of 70%, which is 62% and 33% higher than those of the other two conventional structures. Photoluminescent liquid crystal display backlight samples were prepared with this hybrid nanocomposite to show the robustness against high temperature and humid environment. Even when immersed in water and heated to 80 °C, the backlight samples still retained 85% of the initial fluorescence, which was 40% higher than that with bare GA-QDs. High fluorescence and long-term stability highlight the potential of using this nanocomposite in displays or lighting applications.

In this work, we present a biexciton state population in (111)B oriented site-controlled InGaAs quantum dots (QDs) by resonant two photon excitation. We show that the excited state recombines emitting highly pure single photon pairs entangled in polarization. The discussed cases herein are compelling due to the specific energetic structure of Pyramidal InGaAs QDs - an antibinding biexciton - a state with a positive binding energy. We demonstrate that resonant two-photon excitation of QDs with antibinding biexcitons can lead to a complex excitation-recombination scenario. We systematically observed that the resonant biexciton state population is competing with an acoustic-phonon assisted population of an exciton state. These findings show that under typical two-photon resonant excitation conditions deterministic biexciton state initialization can be compromised. This complication should be taken into account by the community members aiming to utilise similar epitaxial QDs with an antibinding biexciton.

A hybrid device comprising a (Al)GaAs quantum dot heterostructure and a LiNbO$_3$ surface acoustic wave resonator is fabricated by heterointegration. High acoustic quality factors $Q>4000$ are demonstrated for an operation frequency $f\approx 300$ MHz. The measured large quality factor-frequency products $Q\times f>10^{12}$ ensures the suppression of decoherence due to thermal noise for temperatures exceeding $T>50\,\mathrm{K}$. Frequency and position dependent optomechanical coupling of single quantum dots and the resonator modes is observed., Accepted manuscript

Bright emission from non-classical light sources is a key requirement for their practical use in quantum optics. In this Letter, we report on an alternative approach to realize high-brightness nanowire emitters in the telecom band. We discuss the growth and optical properties of a single InA s 0.68 P 0.32 quantum dot in an InA s 0.50 P 0.50 quantum rod, all embedded in an InP nanowire waveguide. Modifying the bandgap energy of the matrix surrounding the quantum dot by inserting it into an InA s 0.50 P 0.50 quantum rod, instead of InP, reduces the barrier height for carriers in the dot. As a result, light emission at λ = 1310 nm is reached from an InA s 0.68 P 0.32 dot grown with the same deposition conditions as that used for λ = 950 nm emission in the conventional structure. We demonstrate that the dot-in-a-rod (DROD) configuration increases (up to fivefold) the emission rate of the emitters at 1310–1550 nm as compared to those grown with the higher dot aspect ratio required when not using the DROD structure. Carrier generation localized to the dot (quasi-resonant scheme) is achieved by optically pumping the rod below the InP bandgap.

Self-assembled Cd(Se,Te) quantum dots with various Se compositions embedded in the ZnTe matrix are grown by molecular beam epitaxy. A huge redshift of the near band edge emission, from 2.1 eV to 1.5 eV, with an increasing Se content in the dots is observed. It is accompanied by an increase in the excitonic lifetime by the factor of 10. We associate these effects with a gradual change from the direct type I confinement character in CdTe/ZnTe quantum dots to the staggered type II band alignment in the case of Cd(Se,Te)/ZnTe dots. This interpretation is consistent with the micro-photoluminescence study of several individual quantum dots, which reveals a gradual decrease in the biexciton–exciton energy difference with the increasing content of Se in the dots, which leads ultimately to the change from the binding to antibinding character of biexcitons. The latter effect originates, most likely, from the increasing Coulomb repulsion between excitons forming dipoles at the dot/barrier interface.

We have measured the electrochemical capacitance in a quantum coherent capacitor consisting of a quantum dot connected to an electron reservoir by a gate tunable quantum point contact and Coulomb coupled to a gate. The results show that for a weak transparent channel, the capacitance exhibits sharp Coulomb blockade oscillations with gate voltage related to the oscillations of the dot density of states, which is almost independent of the applied magnetic field. As the transmitting channel becomes more transparent, the capacitance oscillations decrease in magnitude reflecting the broadening of the resonances. Remarkably, we find that the capacitance oscillation persists even for a fully transparent quantum channel. From the magnetic field and temperature dependence, we conclude that such a capacitance oscillation is a mesoscopic coherent effect that arises due to the interference of electrons in a nearly open dot.