Single Quantum Dot Research Articles

Enhancing Carrier Diffusion Length and Quantum Efficiency through Photoinduced Charge Transfer in Layered Graphene-Semiconducting Quantum Dot Devices.

Hybrid devices consisting of graphene or transition metal dichalcogenides (TMDs) and semiconductor quantum dots (QDs) were widely studied for potential photodetector and photovoltaic applications, while for photodetector applications, high internal quantum efficiency (IQE) is required for photovoltaic applications and enhanced carrier diffusion length is also desirable. Here, we reported the electrical measurements on hybrid field-effect optoelectronic devices consisting of compact QD monolayer at controlled separations from single-layer graphene, and the structure is characterized by high IQE and large enhancement of minority carrier diffusion length. While the IQE ranges from 10.2% to 18.2% depending on QD-graphene separation, ds, the carrier diffusion length, LD, estimated from scanning photocurrent microscopy (SPCM) measurements, could be enhanced by a factor of 5-8 as compared to that of pristine graphene. IQE and LD could be tuned by varying back gate voltage and controlling the extent of charge separation from the proximal QD layer due to photoexcitation. The obtained IQE values were remarkably high, considering that only a single QD layer was used, and the parameters could be further enhanced in such devices significantly by stacking multiple layers of QDs. Our results could have significant implications for utilizing these hybrid devices as photodetectors and active photovoltaic materials with high efficiency.

Tip-Induced Strain Engineering of a Single Metal Halide Perovskite Quantum Dot.

Strain engineering of perovskite quantum dots (pQDs) enables widely tunable photonic device applications. However, manipulation at the single-emitter level has never been attempted. Here, we present a tip-induced control approach combined with tip-enhanced photoluminescence (TEPL) spectroscopy to engineer strain, bandgap, and the emission quantum yield of a single pQD. Single CsPbBrxI3-x pQDs are clearly resolved through hyperspectral TEPL imaging with ∼10 nm spatial resolution. The plasmonic tip then directly applies pressure to a single pQD to facilitate a bandgap shift up to ∼62 meV with Purcell-enhanced PL increase as high as ∼105 for the strain-induced pQD. Furthermore, by systematically modulating the tip-induced compressive strain of a single pQD, we achieve dynamical bandgap engineering in a reversible manner. In addition, we facilitate the quantum dot coupling for a pQD ensemble with ∼0.8 GPa tip pressure at the nanoscale estimated theoretically. Our approach presents a strategy to tune the nano-opto-electro-mechanical properties of pQDs at the single-crystal level.

A double quantum dot defined by top gates in a single crystalline InSb nanosheet* *Project supported by the National Key Research and Development Program of China (Grant Nos. 2017YFA0303304, 2016YFA0300601, 2017YFA0204901,and 2016YFA0300802), the National Natural Science Foundation of China (Grant Nos. 91221202, 91421303, 11874071, 11974030, and 61974138), the Beijing Academy of Quantum Information Sciences (Grant No. Y18G22), the Key-Area Research

We report on the transport study of a double quantum dot (DQD) device made from a freestanding, single crystalline InSb nanosheet. The freestanding nanosheet is grown by molecular beam epitaxy and the DQD is defined by the top gate technique. Through the transport measurements, we demonstrate how a single quantum dot (QD) and a DQD can be defined in an InSb nanosheet by tuning voltages applied to the top gates. We also measure the charge stability diagrams of the DQD and show that the charge states and the inter-dot coupling between the two individual QDs in the DQD can be efficiently regulated by the top gates. Numerical simulations for the potential profile and charge density distribution in the DQD have been performed and the results support the experimental findings and provide a better understanding of fabrication and transport characteristics of the DQD in the InSb nanosheet. The achieved DQD in the two-dimensional InSb nanosheet possesses pronounced benefits in lateral scaling and can thus serve as a new building block for the developments of quantum computation and quantum simulation technologies.

Numerical investigation on threading dislocation bending with InAs/GaAs quantum dots* *Project supported by the National Natural Science Foundation of China (Grant Nos. 61874148, 61974141, and 61674020), the Beijing Natural Science Foundation, China (Grant No. 4192043), the National Key Research and Development Program of China (Grant No. 2018YFB2200104), the Fund from the Beijing Municipal Science & Technology Commission, China (Grant No.

The threading dislocations (TDs) in GaAs/Si epitaxial layers due to the lattice mismatch seriously degrade the performance of the lasers grown on silicon. The insertion of InAs quantum dots (QDs) acting as dislocation filters is a pretty good alternative to solving this problem. In this paper, a finite element method (FEM) is proposed to calculate the critical condition for InAs/GaAs QDs bending TDs into interfacial misfit dislocations (MDs). Making a comparison of elastic strain energy between the two isolated systems, a reasonable result is obtained. The effect of the cap layer thickness and the base width of QDs on TD bending are studied, and the results show that the bending area ratio of single QD (the bending area divided by the area of the QD base) is evidently affected by the two factors. Moreover, we present a method to evaluate the bending capability of single-layer QDs and multi-layer QDs. For the QD with 24-nm base width and 5-nm cap layer thickness, taking the QD density of 1011 cm−2 into account, the bending area ratio of single-layer QDs (the area of bending TD divided by the area of QD layer) is about 38.71%. With inserting five-layer InAs QDs, the TD density decreases by 91.35%. The results offer the guidelines for designing the QD dislocation filters and provide an important step towards realizing the photonic integration circuits on silicon.

Applied electric and magnetic fields effects on the nonlinear optical rectification and the carrier's transition lifetime in InAs/GaAs core/shell quantum dot

In this paper, we have focused our research on the design of a resonant lens-shaped InAs/GaAs core/shell quantum dot structure with intra-band transitions in the conduction band. We have studied the combined effects of the structural dimensions, the hydrostatic pressure (P), the temperature (T), the applied electric (F) and magnetic (B) fields on the nonlinear optical rectification (NOR) and the carrier's transition lifetime (τ). This study is done under the compact density matrix formalism and the effective mass approximation by using the Finite Difference Method (FDM). Obtained Results show that the NOR coefficient is improved by increasing the core radius, as the NOR magnitude increases and a red shift is achieved. Moreover, the application of an electric and a magnetic field enhances the NOR coefficient where its resonant energy exhibits a red shift (blue shift) with increasing the applied magnetic field (applied electric field). Besides, a comparative study between the core/shell structure and a single quantum dot, for B = 15 T, shows that the cap layer thickness can be controlled and reduced to obtain the optimized NOR coefficient. Consequently, our theoretical results will be useful for designing new optoelectronic core/shell devices with an improved performance by the adjustment of the applied external proofs.

Wet-Etched Microlens Array for 200 nm Spatial Isolation of Epitaxial Single QDs and 80 nm Broadband Enhancement of Their Quantum Light Extraction.

Uniform arrays of three shapes (, , and ) of GaAs microlenses (MLs) by wet-etching are demonstrated, ∼200 nm spatial isolation of epitaxial single QDs embedded (: 890–990 nm) and broadband ( nm) enhancement of their quantum light extraction are obtained, which is also suitable for telecom-band epitaxial QDs. Combined with the bottom distributed Bragg reflector, the -shaped ML forms a cavity and achieves the best enhancement: extraction efficiency of 26%, Purcell factor of 2 and single-photon count rate of counts per second at the first lens; while the -shaped ML shows a broader band (e.g., longer ) enhancement. In the MLs, single QDs with featured exciton emissions are observed, whose time correlations prove single-photon emission with multi-photon probability ; some QDs show both biexciton and exciton X emissions and exhibit a perfect cascade feature. This work could pave a step towards a scalable array of QD single-photon sources and the application of QD photon-pair emission for entanglement experiments.

Enhanced emission from a single quantum dot in a microdisk at a deterministic diabolical point.

We report on controllable cavity modes by controlling the backscattering by two identical scatterers. Periodic changes of the backscattering coupling between two degenerate cavity modes are observed with the changing angle between two scatterers and elucidated by a theoretical model using two-mode approximation and numerical simulations. The periodically appearing single-peak cavity modes indicate mode degeneracy at diabolical points. Interactions between single quantum dots and cavity modes are then investigated. Enhanced emission of a quantum dot with a six-fold intensity increase is obtained in a microdisk at a diabolical point. This method to control cavity modes allows large-scale integration, high reproducibility and flexible design of the size, the location, the quantity and the shape for scatterers, which can be applied for integrated photonic structures with scatterer-modified light-matter interaction.

A broad-band planar-microcavity quantum-dot single-photon source with a solid immersion lens

The integration of single quantum dots (QDs) into a planar Fabry–Pérot 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.

Systematic study of the emission spectra of nanowire quantum dots

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.

Wafer-Scale Epitaxial Low Density InAs/GaAs Quantum Dot for Single Photon Emitter in Three-Inch Substrate.

In this work, we successfully achieved wafer-scale low density InAs/GaAs quantum dots (QDs) for single photon emitter on three-inch wafer by precisely controlling the growth parameters. The highly uniform InAs/GaAs QDs show low density of within the radius of 2 cm. When embedding into a circular Bragg grating cavity on highly efficient broadband reflector (CBR-HBR), the single QDs show excellent optoelectronic properties with the linewidth of , the second-order correlation factor , and an exciton life time of 323 ps under two-photon resonant excitation.

Effect of Coulomb interaction on the transient optical response of electrons in field-coupled quantum dots

In this work, we develop a physically transparent Coulomb-interaction model from simplification of a general many-body theory and apply this model to study interacting electron dynamics for transient occupation and quantum coherence in both single and double quantum dots under laser irradiation. Our theory considers self-consistently the Coulomb-renormalized Rabi coupling to light by using an induced optical-depolarization field, corresponding to dynamical exchange interaction between two electrons within the same quantum dot. Meanwhile, we employ evanescent-field coupling for dynamical Coulomb interaction between two electrons in different quantum dots based on a surface-plasmon model. In particular, we explore the quantum interference between a pair of laser-induced quantum coherence in three-level quantum-dot systems, which gives rise to indirect transition of electrons for sum- and difference-frequency transient optical responses. By varying laser frequency detuning, the control of laser-dressed electronic states becomes possible and can be utilized for switching among off-state, partial, and complete on-states. This study will be useful for controlling the phase entanglement of two laser-dressed states of quantum dots, as well as for enhancing the electro-optical performance through sum- and difference-frequency transitions in many optoelectronic devices.

Self-Induced Dark States in Two-Dimensional Excitons

We have obtained an ultralong lifetime exciton emission in InAs/GaAs single quantum dots (QDs) when the QD films are transferred onto the Si substrate covered by Ag nanoparticles. It is found that when the separation distance from the QD layer (also the wetting layer) to the Ag nanoparticles is around 19 nm, the QD emission lifetime changes from approximately 1 to 2000 ns. A classical dipole oscillator model is used to quantitatively calculate the spontaneous radiation decay rate of the excitons in the wetting layer (WL), and the simulated calculation result is in good agreement with the experimental one, revealing that the long lifetime exciton emission is due to the existence of the dark state in the WL. The self-induced dark state stems from the destructive interference between the exciton emission field and the induced dipole field of the Ag nanoparticles.

Second-order hyperpolarizability and all-optical-switching of intensity-modulated spatial self-phase modulation in CsPbBr1.5I1.5 perovskite quantum dot

The averaged second-order hyperpolarizability of perovskite (CsPbBr1.5I1.5) quantum dots (QDs) was characterized using the intensity-dependent spatial self-phase modulation (SSPM). The radial intensity variation of the Gaussian beam displayed the diffraction profile of concentric rings at the far-field due to the coherent superposition of transverse wave vectors with characteristic spatial nonlinear phases. The number of concentric rings as a function of input-intensity revealed the nonlinear refraction coefficients of the QDs. The nonlinear refraction coefficient or the real part of third-order nonlinear susceptibility as a function of concentration of QDs characterized the averaged second-order hyperpolarizability which is the nonlinear refraction coefficient of a single perovskite QD. The vertically asymmetric diffraction ring of SSPM indicates the phase distortion of the optical field due to the heat convection. All-optical switching characteristics of perovskite QDs is attributable to the intensity-modulated SSPM. The input intensity of optical switching by the SSPM was estimated to be ~1.05 MW/m2.

Photoluminescence Enhancement, Blinking Suppression, and Improved Biexciton Quantum Yield of Single Quantum Dots in Zero Mode Waveguides.

The capability of quantum dots to generate both single and multiexcitons can be harnessed for a wide variety of applications, including those that require high optical gain. Here, we use time-correlated photoluminescence (PL) spectroscopy to demonstrate that the isolation of single CdSeTe/ZnS core-shell, nanocrystal quantum dots (QDs) in Zero Mode Waveguides (ZMWs) leads to a significant modification in PL intensity, blinking dynamics, and biexciton behavior. QDs in aluminum ZMWs (AlZMWs) exhibited a 15-fold increase in biexciton emission, indicating a preferential enhancement of the biexciton radiative decay rate as compared to the single exciton rate. The increase in biexciton behavior was accompanied by a decrease in blinking events due to a shortening in the dark state residence time. These results indicate that plasmon mediated enhanced decay rates of QDs in AlZMWs lead to substantial changes in the photophysical properties of single quantum dots, including an increase in biexciton behavior.

RF reflectometry for readout of charge transition in a physically defined p-channel MOS silicon quantum dot

We have embedded a physically defined p-channel MOS silicon quantum dot (QD) device into an impedance transformer RC circuit. To decrease the parasitic capacitance of the device which emerges in MOS devices that have a top gate, we fabricate a new device to reduce the device’s top gate area from 400 to 0.09 μm2. Having a smaller top gate eliminates parasitic capacitance problem preventing the RF signal from reaching QD. We show that we have fabricated a single QD properly, which is essential for RF single-electron transistor technique. We also analyze and improve the impedance matching condition and show that it is possible to perform readout of charge transition at 4.2 K by RF reflectometry. This will enable fast readout of charge and spin states.

The single and coupled BHZ quantum dots with spin–orbit interaction

We investigate theoretically electronic structures of the Bernevig–Hughes–Zhang (BHZ) quantum dots (QDs) with both Rashba spin–orbit interaction (RSOI) and Dresselhaus spin–orbit interaction (DSOI) in the topological insulator regime. For a single QD, the ground state’s electron distribution shows a crescent shape along with specific crystallographic directions ±π∕4 due to the competition between RSOI and DSOI. The single BHZ QD can be tuned to coupled QDs with increasing the gate voltage, and the competition between SOIs and double parabolic confinement potential could rotate the electron distribution of the ground state. Furthermore, we find that the BHZ QDs with SOIs produce similar electronic density distribution to the non-topological quantum ring with SOIs. The BHZ QDs with SOIs could be utilized to explore the evolutionary properties of the BHZ model and may implement some topological electronics applications.

Real-time molecular-level visualization of mass flow during patterned photopolymerization of liquid-crystalline monomers

Single-particle fluorescence imaging is used to monitor dynamic processes that occur during patterned photopolymerization of liquid-crystalline monomers. A spatial gradient of chemical potential can be created at the border of bright and dark regions by structured illumination in the photopolymerization process, leading to mutual diffusion of polymers and monomers. Analysis of the fluorescence from single quantum dots doped into the monomers at minute concentrations enables visualization of highly directional flow from the illuminated region where the photopolymerization proceeds toward a masked unpolymerized region. This directional mass flow causes flow-induced orientation of the polymers that is subsequently fixed by completion of the polymerization reaction, resulting in a mesoscopic aligned area of the polymer film.

Photoinduced Kerr rotation spectroscopy for microscopic spin systems using heterodyne detection.

We develop a transient photoinduced Kerr rotation spectroscopy technique using a heterodyne detection scheme to study spin dynamics of microscopic quantum states in solids, such as single quantum dots and spin helixes. The use of the heterodyne beat note signal generated by the interference of the frequency-shifted probe and reference pulses realizes the Kerr rotation measurements in combination with micro-spectroscopy, even when the probe pulse propagates collinearly with the strong pump pulse, which resonantly excites the probing state. In addition, the interference gives an optical amplification of the Kerr signal, which provides a clear observation of the photoinduced spin dynamics by the weak probe intensity. Here, we present results of Kerr rotation measurements for a single quantum dot exciton, which shows a maximum rotation angle of few µrad.

Multi-Quantum Dots-Embedded Silica-Encapsulated Nanoparticle-Based Lateral Flow Assay for Highly Sensitive Exosome Detection.

Exosomes are attracting attention as new biomarkers for monitoring the diagnosis and prognosis of certain diseases. Colorimetric-based lateral-flow assays have been previously used to detect exosomes, but these have the disadvantage of a high limit of detection. Here, we introduce a new technique to improve exosome detection. In our approach, highly bright multi-quantum dots embedded in silica-encapsulated nanoparticles (M–QD–SNs), which have uniform size and are brighter than single quantum dots, were applied to the lateral flow immunoassay method to sensitively detect exosomes. Anti-CD63 antibodies were introduced on the surface of the M–QD–SNs, and a lateral flow immunoassay with the M–QD–SNs was conducted to detect human foreskin fibroblast (HFF) exosomes. Exosome samples included a wide range of concentrations from 100 to 1000 exosomes/µL, and the detection limit of our newly designed system was 117.94 exosome/μL, which was 11 times lower than the previously reported limits. Additionally, exosomes were selectively detected relative to the negative controls, liposomes, and newborn calf serum, confirming that this method prevented non-specific binding. Thus, our study demonstrates that highly sensitive and quantitative exosome detection can be conducted quickly and accurately by using lateral immunochromatographic analysis with M–QD–SNs.

Near-field modulation of single photon emitter with a plasmonic probe

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.