Single Quantum Dot Research Articles

Planarized spatially-regular arrays of spectrally uniform single quantum dots as on-chip single photon sources for quantum optical circuits

A long standing obstacle to realizing highly sought on-chip monolithic solid state quantum optical circuits has been the lack of a starting platform comprising scalable spatially ordered and spectrally uniform on-demand single photon sources (SPSs) buried under a planar surface. In this paper, we report on the first realization of planarized SPS arrays based on a unique class of shape-controlled single quantum dots (SQDs) synthesized on mesa top (dubbed MTSQDs) using substrate-encoded size-reducing epitaxy (SESRE) on spatially regular arrays of patterned nanomesas with edge orientation chosen to drive symmetric adatom migration from the nanomesa sidewalls to the top, thereby enabling spatially selective growth. Specifically, on GaAs(001) square nanomesas with edges along ⟨100⟩, we synthesized binary GaAs/InAs/GaAs MTSQDs emitting around 1120 nm with 1.8 nm standard deviation and single photon emission purity >99.5%. SESRE based MTSQDs are shown for the first time to lend themselves to planarization of the surface morphology when grown on pedestal shape mesas. We demonstrate that the planarizing overgrowth process over arrays of InGaAs SQDs largely maintains the SQDs’ high single photon emission purity (>98%) and spectral uniformity (∼5 nm). Such planarized SQD arrays offer the long-sought platform for on-chip integration with light manipulating structures to realize quantum optical circuits.

Cavity-assisted enhanced and dephasing-immune squeezing in the resonance fluorescence of a single quantum dot

We theoretically demonstrate the enhanced and dephasing immune squeezing in the resonance fluorescence of a single quantum dot (QD) confined to a pillar-microcavity and driven by a continuous wave laser. We employ a formalism based on Polaron master equation theory for incorporating the influence of exciton-phonon coupling quite accurately in the dot-cavity system. We show a significant enhancement of squeezing due to cavity coupling of the QD as compared to that of an ideal single two-level system in free space. Particularly, we show a four-fold enhancement in squeezing as compared to that of a single QD without cavity coupling. We further demonstrate the persistence of squeezing even when the pure dephasing becomes greater than the radiative decay rate. These novel features are attributed to the cavity-enhanced coherence causing partial reduction of the deteriorating effects of phonon-induced incoherent rates. We also show that the deteriorating effects of phonon-induced incoherent rates on squeezing can be partially circumvented by properly adjusting the detunings.

Hybrid DNA-Carbon Dot-Poly(vinylpyrrolidone) Hydrogel with Self-Healing and Shape Memory Properties for Simultaneous Trackable Drug Delivery and Visible-Light-Induced Antimicrobial Photodynamic Inactivation.

A two-step methodology for simultaneous conjugation of DNA and poly(vinylpyrrolidone) (PVP) polymer to a single carbon quantum dot (CD) is demonstrated for the first time to fabricate a pH-responsive DNA-CD-PVP hybrid hydrogel. Cross-linking in the hydrogel was achieved using CD as the common nucleus through the formation of DNA I-motif conformation at neutral to acidic pH and noncovalent interaction of PVP that infuse self-healing and shape memory properties in the hydrogel. The hydrogel is capable of loading and sustained delivery of drugs for more than 2 weeks as demonstrated using a model drug, Hemin. The quenching of fluorescence of CD by Hemin was trackable even through simple visual monitoring, which showed that Hemin can diffuse from the loaded part to the unloaded part of the hydrogel during the self-healing process. Most significantly, the chosen CD generates reactive oxygen species (ROS) upon visible light irradiation, armoring the hydrogel with worthy antimicrobial activity. Biocompatibility of the DNA-CD-PVP hydrogel was established on human fibroblast cells, indicating their potential use in biomedical area pertaining to wound healing.

Third-order photon cross-correlations in resonance fluorescence

We investigated third-order correlations between photons from a single quantum dot's resonance fluorescence in the strong excitation regime originating from opposite sidebands of the power spectrum. The three-time correlation measurements for photons filtered using three independent Fabry-Perot etalons resulted in correlation maps with ``antibunching'' features as a consequence of correlations originating from the same sideband, and ``bunching ridges'' due to opposite sideband correlated photons.

Bimodal size distribution immuno-quantum dots for fluorescent western blotting assay with high sensitivity and extended dynamic range.

A highly sensitive quantum dot (QD)-based western blot assay with extended dynamic range was developed. Bimodal size distribution QD (BQ) immunoprobes composed of small size single QD (7.3 nm) and big size QD nanobead (QB) (82.9nm) were employed for fluorescent western blot immunoassay on a membrane. Small size QD immunoprobes contributed to wider dynamic range of assay, while big size QB immunoprobes provided higher detection sensitivity. This BQ-based western blot assay can achieve a wide dynamic range (from 7.8 to 4000ng IgG) and is nearly as sensitive as commercial available ultrasensitive chemiluminescent methods, just using a simple gel imager with UV light (365nm) excitation and red light filter (610nm). The fluorescent signals of BQ western blot were stable for 10 min, while chemiluminescent signals faded after 1min. Moreover, this BQ immunoprobe was utilized for the detection of housekeeping protein and specific target proteins in complex cell lysate samples. The limit of detection of housekeeping protein is 0.25μg of cell lysate, and the signal intensities were proportional to loading protein amount in a wide range from 0.61 to 80μg. We believe that this new strategy of bimodal size distribution nanoparticles can also be expanded for other functional nanoparticle-based biological assays to improve the sensitivity and extend the dynamic range. Graphical abstract.

On-chip integration of Si/SiGe-based quantum dots and switched-capacitor circuits

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.

Color-adjustable CsPbBr3-xIx quantum dots glasses for wide color gamut display

CsPbBr3-xIx quantum dots (QDs) glasses have been fabricated by melt-quenching technique. Emission peaks are adjustable in 500–530 nm and 615–692 nm for CsPbBr3 and CsPbBr3-xIx respectively. FWHM varies from 20 to 50 nm. Emission peak and line-shape are independent of excitation wavelength. PL decay shows biexponential behavior with short and long lifetimes in the level of 10 ns and 100 ns respectively. Under excitation of 450 nm, PLQY is large for single anion CsPbBr3 QDs (46%) and CsPbI3 QDs (61%), but small (1.6%-25%) for double anion QDs. PL intensity decreases with temperature from ~25 to 125 °C and recovers after one cycle. Three primary colors for wide color gamut display have been researched using blue LED, green and red QDs glasses. Color purity is calculated as 98%, 91% and 100% (for both 630 nm and 670 nm red QDs) respectively. Color gamut areas 119% and 130% of NTSC have been realized for the case of 630 nm and 670 nm QDs.

Nonperturbative renormalization of the interaction of quantum dots with the phonon reservoir

The quantum dynamics of a single quantum dot with phonon reservoir is investigated by making use generalized quantum dynamics equation [Gainutdinov R.Kh. 1999 J. Phys. A 32 5675]. The self-energy function is calculated nonperturbatively taking into account the renormalization of the vertex function. In this paper, we show that the nonperturbative renormalization of the vertex function significantly affects the nature of the interaction of the quantum dot with the phonon reservoir.

Assembling Palladium and Cuprous Oxide Nanoclusters into Single Quantum Dots for the Electrocatalytic Oxidation of Formaldehyde, Ethanol, and Glucose

To effectively manipulate the electronic structure of the catalysts, we present here a simple bottom-up synthesis protocol for agglomerating palladium and cuprous oxide ultrasmall nanoclusters into...

Turning a hot spot into a cold spot: polarization-controlled Fano-shaped local-field responses probed by a quantum dot

Optical nanoantennas can convert propagating light to local fields. The local-field responses can be engineered to exhibit nontrivial features in spatial, spectral and temporal domains, where local-field interferences play a key role. Here, we design nearly fully controllable local-field interferences in the nanogap of a nanoantenna, and experimentally demonstrate that in the nanogap, the spectral dispersion of the local-field response can exhibit tuneable Fano lineshapes with nearly vanishing Fano dips. A single quantum dot is precisely positioned in the nanogap to probe the spectral dispersions of the local-field responses. By controlling the excitation polarization, the asymmetry parameter q of the probed Fano lineshapes can be tuned from negative to positive values, and correspondingly, the Fano dips can be tuned across a broad spectral range. Notably, at the Fano dips, the local-field intensity is strongly suppressed by up to ~50-fold, implying that the hot spot in the nanogap can be turned into a cold spot. The results may inspire diverse designs of local-field responses with novel spatial distributions, spectral dispersions and temporal dynamics, and expand the available toolbox for nanoscopy, spectroscopy, nano-optical quantum control and nanolithography.

Cavity quantum electro-dynamics with solid-state emitters in aperiodic nano-photonic spiral devices

Integrated quantum devices are at the basis of the realization of scalable, high-performance quantum technology, including quantum computers and quantum communication schemes, where single photons are emitted, guided, manipulated, and detected on a chip. Engineered nano-devices enable the efficient confinement of light and, ultimately, the control of the spontaneous emission dynamics of single emitters, which is crucial for cavity quantum electrodynamics experiments and for the development of classical and quantum light sources. Here, we report on the demonstration of enhanced light-matter interaction and Purcell effects on a chip, based on bio-inspired aperiodic devices fabricated in gallium arsenide. Indium arsenide single quantum dots are used as internal light sources to image, by means of micro-photoluminescence spectroscopy, the optical modes supported by photonic membranes with Vogel-spiral geometry. These emitters are also used to probe the density of optical states, modified by the aperiodic devices, by means of time-resolved spectroscopy. Our results show cavity quantum electrodynamics effects providing strong modifications of the spontaneous emission decay of single optical transitions. In particular, thanks to the significant modification of the density of optical states demonstrated in Vogel-spiral photonic structures, we show control of the decay lifetime of single emitters with a dynamic range reaching 20, thus opening the path to the implementation of aperiodic geometries in active classical and quantum devices.

A hybrid (Al)GaAs-LiNbO3 surface acoustic wave resonator for cavity quantum dot optomechanics

A hybrid device comprising a (Al)GaAs quantum dot heterostructure and a LiNbO3 surface acoustic wave resonator is fabricated by heterointegration. High acoustic quality factors Q>4000 are demonstrated for an operation frequency f≈300 MHz. The measured large quality factor-frequency products Q×f>1012 ensure the suppression of decoherence due to thermal noise for temperatures exceeding T>50 K. Frequency and position dependent optomechanical coupling of single quantum dots and resonator modes is observed.

Electrical control of single-photon emission in highly charged individual colloidal quantum dots.

Electron transfer to an individual quantum dot promotes the formation of charged excitons with enhanced recombination pathways and reduced lifetimes. Excitons with only one or two extra charges have been observed and exploited for very efficient lasing or single-quantum dot light-emitting diodes. Here, by room-temperature time-resolved experiments on individual giant-shell CdSe/CdS quantum dots, we show the electrochemical formation of highly charged excitons containing more than 12 electrons and 1 hole. We report the control over intensity blinking, along with a deterministic manipulation of quantum dot photodynamics, with an observed 210-fold increase in the decay rate, accompanied by 12-fold decrease in the emission intensity, while preserving single-photon emission characteristics. These results pave the way for deterministic control over the charge state, and room-temperature decay rate engineering for colloidal quantum dot-based classical and quantum communication technologies.

Toward all on chip optical detection in the few molecule regime

Integrated optics devices are one of the most promising technologies in many fields such as biosensing, optical monitoring, and portable devices. They provide several advantages such as unique sensitivity and the possibility of the well-established and developed silicon photonics technology. However some challenges still remain open, as the implementation of multiplex assay able to reach the single particle sensitivity. In this context, we propose a new design for a Si-based photonic structure that enables the realization of on chip sub-wavelength optical sources. The idea is based on the insertion of opportunely designed nanometric holes in the photonic circuit, which are available for analyte detection with high efficiency. We propose three different configurations in which both excitation and detection are obtained through the same waveguide thus simplifying the detection scheme and potentially enabling multiplexed detection. We proved the high confinement of the electromagnetic field in the holes both by theoretical modelling and spectroscopic measurements. We investigate the possibility of inserting an arbitrary number of optical sources by using a resonator and evaluate advantages and drawbacks of resonating and non-resonating solutions. Finally, we report the proof-of-concept experiment, where detection sensitivity down to single Quantum Dots is obtained by combining the novel design with fluorescence-based techniques. Importantly, the presented results are achieved by a simple modification of photonic sensing chips which are already on the market thus having an excellent translational perspective.

Radiometric characterization of a triggered narrow-bandwidth single-photon source and its use for the calibration of silicon single-photon avalanche detectors

The traceability of measurements of the parameters characterizing single-photon sources, such as photon flux and optical power, paves the way towards their reliable comparison and quantitative evaluation. In this paper, we present an absolute measurement of the optical power of a single-photon source based on an InGaAs quantum dot under pulsed excitation with a calibrated single-photon avalanche diode (SPAD) detector. For this purpose, a single excitonic line of the quantum dot emission with a bandwidth below 0.1 nm was spectrally filtered by using two tilted interference filters. Since high count rates are essential for many metrological applications, we optimized the setup efficiency by combining high overall transmission of the optical components with a geometrical enhancement of the extraction efficiency of a single quantum dot by a monolithic microlens to reach photon fluxes up to photons per second at the SPADs. A relative calibration of two SPAD detectors with a relative standard uncertainty of 0.7% was carried out and verified by the standard calibration method using an attenuated laser. Finally, an Allan deviation analysis was performed giving an optimal averaging time of 92 s for the photon flux.

Multiplexed Single Photons from Deterministically Positioned Nanowire Quantum Dots

Solid-state quantum emitters are excellent sources of on-demand indistinguishable or entangled photons and can host long-lived spin memories, crucial resources for photonic quantum information applications. However, their scalability remains an outstanding challenge. Here we present a scalable technique to multiplex streams of photons from multiple independent quantum dots, on-chip, into a fiber network for use off-chip. Multiplexing is achieved by incorporating a multi-core fiber into a confocal microscope and spatially matching the multiple foci, seven in this case, to quantum dots in an array of deterministically positioned nanowires. First, we report the coherent control of the emission of biexciton-exciton cascade from a single nanowire quantum dot under resonant two-photon excitation. Then, as a proof-of-principle demonstration, we perform parallel spectroscopy on the nanowire array to identify two nearly identical quantum dots at different positions which are subsequently tuned into resonance with an external magnetic field. Multiplexing of background-free single photons from these two quantum dots is then achieved. Our approach, applicable to all types of quantum emitters, can readily be scaled up to multiplex $>100$ quantum light sources, providing a breakthrough in hardware for photonic based quantum technologies. Immediate applications include quantum communication, quantum simulation, and quantum computation.

Observation of Cooperative Purcell Enhancements in Antenna-Cavity Hybrids.

Localizing light to nanoscale volumes through nanoscale resonators that are low loss and precisely tailored in spectrum to properties of matter is crucial for classical and quantum light sources, cavity QED, molecular spectroscopy, and many other applications. To date, two opposite strategies have been identified: to use either plasmonics with deep subwavelength confinement yet high loss and very poor spectral control or instead microcavities with exquisite quality factors yet poor confinement. In this work we realize hybrid plasmonic–photonic resonators that enhance the emission of single quantum dots, profiting from both plasmonic confinement and microcavity quality factors. Our experiments directly demonstrate how cavity and antenna jointly realize large cooperative Purcell enhancements through interferences. These can be controlled to engineer arbitrary Fano lineshapes in the local density of optical states.

Emission States Variation of Single Graphene Quantum Dots.

Graphene quantum dots (GQDs) are nanoparticles that consist of a nanometer-sized core of graphene with diverse chemical groups on its boundary. Due to their advantageous properties, they are considered to be a promising material for optoelectronics, bioimaging, or photovoltaics. Despite considerable efforts that have been focused on unraveling the mechanism of their photoluminescence, many fundamental details are still unclear. Here, we report on a single-particle multimodal study that provides new insight into the photoluminescence properties of emission centers of GQDs in various local chemical environments. In particular, we show that the properties that are associated with emission centers of GQDs are significantly more sensitive to the structure of the particle itself than to a nonuniform local chemical environment. A better understanding of the dependence of GQDs' emission states on the complex local chemical environment is an important step toward finding new ways of controlling the optical properties of GQDs and of optimizing their use in various applications.

Algorithm for automated tuning of a quantum dot into the single-electron regime

We report an algorithm designed to perform computer-automated tuning of a single quantum dot with a charge sensor. The algorithm performs an adaptive measurement sequence of sub-sized stability diagrams until the single-electron regime is identified and reached. For each measurement, the signal processing module removes the physical background of the charge sensor to generate a binary image of charge transitions. Then, the image analysis module identifies the position and number of lines using two line detection schemes that are robust to noise and missing data.

Surface passivation and oxide encapsulation to improve optical properties of a single GaAs quantum dot close to the surface

Epitaxial GaAs quantum dots grown by droplet etching have recently shown excellent properties as sources of single photons as well as entangled photon pairs. Integration in some nanophotonic structures requires surface-to-dot distances of less than 100 nm. This demands a surface passivation scheme, which could be useful to lower the density of surface states. To address this issue, sulphur passivation with dielectric overlayer as an encapsulation is used for surface to QD distances of ≲40 nm, which results in the partial recovery of emission linewidths to bulk values as well as in the increase of the photoluminescence intensity.