Quantum Dot Emission Research Articles

Electrical control of Förster resonant energy transfer across single-layer graphene

Abstract In artificial structures of molecular or quantum dot emitters in contact with single-layer graphene (SLG) Förster-type resonant energy transfer (FRET) can occur unconditionally due to the gapless band structure of SLG. A significant breakthrough for applications, however, would be the electrical modulation of FRET between arbitrary FRET pairs, using the SLG to control this process and taking advantage of the particular band structure and the monatomic thickness of SLG, far below the typical Förster radius of a few nanometers. For a proof of concept, we have therefore designed a Sandwich device where the SLG was transferred onto holey Si3N4 membranes and organic molecules were deposited on either side of the SLG. The relative photoluminescence (PL) intensities of donor and acceptor molecules changed continuously and reversibly with the external bias voltage, and a variation of about 6% of FRET efficiency has been achieved. We ascribe the origin of the electrical modulation of FRET to important doping-dependent nonlocal optical effects in the near field of SLG in the visible range.

In situ-Synthesized cadmium sulfide quantum dots in pore-forming protein and polysaccharide matrices for optical biosensing applications

The main limitation for practical implementation of quantum dots-based sensors and biosensors is the possible contamination of sensing media with quantum dots (QDs) moved out from the sensor structure, being critical for living systems measurements. Numerous efforts have addressed the challenge of pre-synthesized QDs incorporation into porous matrix provide, on the one hand, proper fixation of quantum dots in its volume and preserving a free analyte transfer from the sensing media to them - on the other hand. Here, we propose an alternative insight into this problem. Instead of using preliminary synthesized particles for doping a matrix, we have in situ synthesized cadmium sulfide QDs in porous biopolymeric matrices, both in an aqueous solution and on a mica substrate. The proposed technique allows obtaining QDs in a matrix acting simultaneously as a ligand passivating surface defects and preventing QDs aggregation. The conjugates were used as a photoluminescence sensor for the metal ions and glutathione detection in an aqueous media. Different kinds of sensor responses have been found depending on the analyte nature. Zinc ions' presence initiates the intraband QDs emission increases due to the reduction of non-radiative processes. The presence of copper ions, in contrast, leads to a gradual photoluminescence decrease due to the formation of the non-luminescent copper-based alloy in the QDs structure. Finally, the presence of glutathione initiates a ligand exchange process followed by some QDs surface treatment enhancing defect-related photoluminescence. As a result, three different kinds of sensor responses for three analytes allow claiming development of a new selective QD-based sensor suitable for biomedical applications.

Luminescent properties of spatially ordered Ge/Si quantum dots epitaxially grown on a pit-patterned “silicon-on-insulator” substrate

The results of studying the luminescent properties of epitaxially grown structures with spatially ordered Ge/Si nanoislands (quantum dots), in which the pit-patterned “silicon-on-insulator” substrates serve both for the spatial ordering of quantum dots and for the two-dimensional photonic crystal formation, are presented. It is found that with a certain choice of pit-patterned substrate parameters (diameter of pits and their spatial period), in the photoluminescence spectra of such structures, the multiple narrow peaks are observed. In addition, a significant increase in the quantum dot luminescence signal intensity occurs in the near-infrared range. The effects are related to resonance interaction of quantum dot emitters with photonic crystal modes. The luminescence enhancement effect persists up to room temperatures.

Self-assembled InAs/GaAs single quantum dots with suppressed InGaAs wetting layer states and low excitonic fine structure splitting for quantum memory

Abstract Epitaxial semiconductor quantum dots (QDs) have been demonstrated as on-demand entangled photon sources through biexciton–exciton (XX-X) cascaded radiative processes. However, perfect entangled photon emitters at the specific wavelengths of 880 nm or 980 nm, that are important for heralded entanglement distribution by absorptive quantum memories, remain a significant challenge. We successfully extend the QD emission wavelength to 880 nm via capping Stranski–Krastanow grown In(Ga)As/GaAs QDs with an ultra-thin Al x Ga1−x As layer. After carefully investigating the mechanisms governing the vanishing of wetting-layer (WL) states and the anisotropy of QDs, we optimize the growth conditions and achieve a strong suppression of the WL emission as well as a measured minor fine structure splitting of only ∼(3.2 ± 0.25) μeV for the exciton line. We further extend this method to fabricate In(Ga)As QDs emitted at 980 nm via introducing InGaAs capping layer, and demonstrate a two-photon resonant excitation of the biexciton without any additional optical or electrical stabilized source. These QDs with high symmetry and stability represent a highly promising platform for the generation of polarization entanglement and experiments on the interaction of photons from dissimilar sources, such as rare-earth-ion-doped crystals for solid quantum memory.

Mode Interference Effect in Optical Emission of Quantum Dots in Photonic Crystal Cavities

Radiation properties of a pointlike source of light, such as a molecule or a semiconductor quantum dot, can be tailored by modifying its photonic environment. This phenomenon lies at the core of cavity quantum electrodynamics (CQED). Quantum dots in photonic crystal microcavities have served as a model system for exploring the CQED effects and for the realization of efficient single-photon quantum emitters. Recently, it has been suggested that quantum interference of the exciton recombination paths through the cavity and free-space modes can significantly modify the radiation. In this work, we report an unambiguous experimental observation of this fundamental effect in the emission spectra of site-controlled quantum dots positioned at prescribed locations within a photonic crystal cavity. The observed asymmetry in the polarization-resolved emission spectra strongly depends on the quantum dot position, which is confirmed by both analytical and numerical calculations. We perform quantum interferometry in the near-field zone of the radiation, retrieving the overlap and the position-dependent relative phase between the interfering free-space and cavity-mode-mediated radiative decays. The observed phenomenon is of importance for realization of photonic-crystal light emitters with near unity quantum efficiency. Our results suggest that the full description of light-matter interaction in the framework of CQED requires a modification of the conventional quantum master equation by also considering the radiation mode interference.12 MoreReceived 20 October 2021Revised 12 February 2022Accepted 13 April 2022Corrected 22 June 2022DOI:https://doi.org/10.1103/PhysRevX.12.021042Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.Published by the American Physical SocietyPhysics Subject Headings (PhySH)Research AreasCavity quantum electrodynamicsLight-matter interactionNanophotonicsOpen quantum systemsPhotonic crystalsPhotonicsQuantum description of light-matter interactionQuantum interference effectsSingle photon sourcesPhysical SystemsQuantum cavitiesQuantum dotsTechniquesFinite-difference time-domain methodFinite-element methodInterferometryOptical spectroscopyPhotoluminescenceQuantum master equationTime-resolved photoluminescenceCondensed Matter, Materials & Applied PhysicsAtomic, Molecular & OpticalGeneral Physics

A facile reprecipitation method of Quantum-confined CsPbBr3 perovskite quantum dots for narrow-band deep-blue emission

Quantum-confined cesium lead tribromide perovskite quantum dots (PeQDs) are attractive photonic sources for versatile optoelectronic devices and thus the synthetic routes have been explored via various techniques. However, the lack of a general method to fabricate the quantum-confined PeQD with remarkable stability at colloidal state has been one of the major obstacles to device applications. Herein, we propose a simple and robust ligand-assisted reprecipitation method associated with didodecyl dimethyl ammonium bromide for quantum-confined PeQDs with remarkable stability at colloidal state. This method will shed a light on the field of colloidal PeQDs synthesis for the next generation of photonic sources.

Super-resolution Imaging of Plasmonic Near-Fields: Overcoming Emitter Mislocalizations.

Plasmonic nano-objects have shown great potential in enhancing applications like biological/chemical sensing, light harvesting and energy transfer, and optical/quantum computing. Therefore, an extensive effort has been vested in optimizing plasmonic systems and exploiting their field enhancement properties. Super-resolution imaging with quantum dots (QDs) is a promising method to probe plasmonic near-fields but is hindered by the distortion of the QD radiation pattern. Here, we investigate the interaction between QDs and “L-shaped” gold nanoantennas and demonstrate both theoretically and experimentally that this strong interaction can induce polarization-dependent modifications to the apparent QD emission intensity, polarization, and localization. Based on FDTD simulations and polarization-modulated single-molecule microscopy, we show that the displacement of the emitter’s localization is due to the position-dependent interference between the emitter and the induced dipole, and can be up to 100 nm. Our results help pave a pathway for higher precision plasmonic near-field mapping and its underlying applications.

Size Engineering of Trap Effects in Oxidized and Hydroxylated ZnSe Quantum Dots.

Environmentally friendly blue-emitting ZnSe quantum dots (QDs) are in high demand for next-generation light-emitting devices. Yet, they suffer longstanding optical instability issues under aerobic conditions. Herein, we have demonstrated the existence of oxidization or hydroxylation on the QD surface when QDs are subjected to oxygen exposure, which potentially introduces highly localized in-gap states. Those states result in a dense number of surface-related, weak-intensity "dark" exciton states at the emission edge. Remarkably, there exists a critical diameter (Dc ≈ 8.5 nm) at which the deepest trap level reaches resonance with the highest occupied molecular orbital state. Beyond this critical diameter, the effects of those trap states are minimized, and the emission edge is dominated by high-intensity, bulk-to-bulk-like "bright" exciton states. The present work provides a novel strategy for designing highly stable QD emitters via size engineering, which are broadly applicable to other closely related QD systems.

High‐Performance Blue Quantum‐Dot Light‐Emitting Diodes by Alleviating Electron Trapping

AbstractCurrently, blue quantum‐dot light‐emitting diodes (QLEDs) remain the bottleneck limiting the development of QLED‐based applications. To achieve high‐performance blue QLEDs, poly‐N‐vinylcarbazole (PVK) is usually employed as the hole‐transport layer (HTL) to reduce the hole injection barrier. However, fabrication of efficient blue QLEDs with PVK HTL remains challenging and empirical/accidental. Here, it is demonstrated that PVK layer can trap electrons and hence resulting in low device efficiency. This is why the performance of blue QLEDs is highly dependent on the PVK batch received from the manufacturers. As an interlayer, ZnSe/ZnS quantum dots (QDs) are inserted between PVK and blue ZnCdSe/ZnS QD emitters. The resulted device presents an external quantum efficiency of 20.6% under 2920 cd m−2, enhanced by ≈35% compared with that of PVK‐alone device. The remarkable enhancement is owing to the QD interlayer that not only suppresses the electron trapping in PVK, increasing the probability of exciton recombination within the emissive layer, but also facilitates the hole injection, improving the charge balance in blue devices.

Microcavity coupled quantum dot emission with detuning control.

Solution processed colloidal semiconductor quantum dots (QDs) have size-tunable optical transitions and high quantum efficiencies, enabling various applications in opto-electronic devices. To enrich the functionality of QD-based opto-electronic devices, colloidal semiconductor QDs have been frequently coupled with optical cavities to enable emission modulation. However, it remains a challenge to fully understand the interaction between the optical cavity resonance and the QD emission, especially for the planar optical microcavities. Here, we have investigated the light emission of colloidal semiconductor QDs in the planar Fabry-Perot microcavity consisted of two Ag mirrors. With the matched QD and cavity resonance, the microcavity coupled QD samples show a prominently narrower emission linewidth and emission angle range because of the efficient QD-cavity coupling, while with a slightly positive or negative energy detuning, the linewidth and angular distribution of the microcavity coupled QD emission both become broadened. Furthermore, with the standard lithography technique, the microcavity coupled QD sample can be patterned into arbitrary geometries, showing extra features of in-plane mode confinement. Our work highlights the important role of detuning in determining the coupling between colloidal semiconductor QDs and microcavities and paves the way for the future design of microcavity coupled QD devices.

Electron Delocalization in CsPbI3 Quantum Dots Enables Efficient Light‐Emitting Diodes with Improved Efficiency Roll‐Off

AbstractPerovskite quantum dot (QD) charging is prone to occur during light‐emitting diode (LED) operation due to the electron accumulation inside the QD emitters, which is caused by the unbalanced charge injection and carrier transportation. Here in order to address this issue, an electron attractor of phenethylammonium (PEA+) is introduced to the surface of perovskite QDs. The PEA+ conjugated ligands can reversibly store and release electrons driven by the electron local density of states redistribution, which enhances the delocalization of electrons in QDs, and thus suppresses QD charging at high carrier concentrations, as evidenced by the excitation energy dependent photoluminescence. Besides, PEA+ down‐shifts the Fermi level of perovskite QDs, consequently increases the hole concentration, leading to greatly improved hole conduction. Halogen‐rich synthesis environment also reduces the number of halogen vacancies, consequently narrows the distribution of electronic states at the band edges, leading to slightly improved electron conduction. Taking advantage of the enhanced electron delocalization and improved charge transportation, the CsPbI3 QDs LEDs show the maximum external quantum efficiency (EQE) of 15.6% with a low efficiency roll‐off, and the maximum luminance of 1634 cd m−2.

Converting single photons from an InAs/GaAs quantum dot into the ultraviolet: preservation of second-order correlations

Wavelength conversion at the single-photon level is required to forge a quantum network from distinct quantum devices. Such devices include solid-state emitters of single or entangled photons, as well as network nodes based on atoms or ions. Here we demonstrate the conversion of single photons emitted from a III-V semiconductor quantum dot at 853 nm via sum frequency conversion to the wavelength of the strong transition of Yb+ ions at 370 nm. We measure the second-order correlation function of both the unconverted and the converted photon and show that the single-photon character of the quantum dot emission is preserved during the conversion process.

In-plane resonant excitation of quantum dots in a dual-mode photonic-crystal waveguide with high β-factor

A high-quality quantum dot (QD) single-photon source is a key resource for quantum information processing. Exciting a QD emitter resonantly can greatly suppress decoherence processes and lead to highly indistinguishable single-photon generation. It has, however, remained a challenge to implement strict resonant excitation in a stable and scalable way, without compromising any of the key specs of the source (efficiency, purity, and indistinguishability). In this work, we propose a novel dual-mode photonic-crystal waveguide that realizes direct in-plane resonant excitation of the embedded QDs. The device relies on a two-mode waveguide design, which allows exploiting one mode for excitation of the QD and the other mode for collecting the emitted single photons with high efficiency. By proper engineering of the photonic bandstructure, we propose a design with single-photon collection efficiency of β > 0.95 together with a single-photon impurity of ϵ < 5 × 10−3 over a broad spectral and spatial range. The device has a compact footprint of and would enable stable and scalable excitation of multiple emitters for multi-photon quantum applications.

Mechanisms behind multicolor tunable Near-Infrared triple emission in graphene quantum dots and ratio fluorescent probe for water detection

Developing environmental-friendly and low-cost strategies of synthesizing red-emission graphene quantum dots (R-GQDs) is a considerable challenge. Herein, we present the green and facile preparation of R-GQDs by using o-phenylenediamine (o-PD) and catechol as precursors via oxidation/polymerization and Schiff base reaction at low temperature (80 °C) for 3 hrs. Results show that the prepared R-GQDs exhibit triple fluorescence emissions, which is enabled by the introduce of different nitrogen (pyrrolic N, pyridinic N, and amino N) species on the surface of R-GQDs. Moreover, the R-GQDs are implemented to detect the moisture in different organic solvent. A highly sensitive ratiometric sensing of water in organic solvents is achieved, and the relationship between the change of fluorescence signal caused by moisture and the corresponding internal emission site is also determined. In the end, the multicolor light emissions of R-GQDs are realized by simply adjusting the polarity of surrounding solvents. And based on the solvatochromism of R-GQDs, the multicolor solid fluorescent powder and ink are prepared for illumination application. All in all, the above research work presents a novel R-GQDs for wide application in detecting and illumination.

Photoneutralization of charges in GaAs quantum dot based entangled photon emitters

Semiconductor-based emitters of pairwise photonic entanglement are a promising constituent of photonic quantum technologies. They are known for the ability to generate discrete photonic states on-demand with low multiphoton emission, near-unity entanglement fidelity, and high single photon indistinguishability. However, quantum dots typically suffer from luminescence blinking, lowering the efficiency of the source and hampering their scalable application in quantum networks. In this paper, we investigate and adjust the intermittence of the neutral exciton emission in a GaAs/AlGaAs quantum dot under two-photon resonant excitation of the neutral biexciton. We investigate the spectral and quantum optical response of the quantum dot emission to an additional wavelength tunable gate laser, revealing blinking caused by the intrinsic Coulomb blockade due to charge capture processes. Our finding demonstrates that the emission quenching can be actively suppressed by controlling the balance of free electrons and holes in the vicinity of the quantum dot and thereby significantly increasing the quantum efficiency by $30%$.

Photoluminescence and Electron Paramagnetic Resonance Spectroscopy for Revealing Visible Emission of ZnO Quantum Dots

AbstractBoth PL and EPR are simultaneously adopted to systematically elucidate the defect centers of green luminescence (GL) as well as the EPR peak g = 1.96 of ZnO and the relationship between them. The PL of ZnO QDs reveals that GL of 2.21–2.31 eV disappears at excitation wavelengths &gt; 400 nm. This is related to the electronic transition from conduction band (CB), or shallow donor defect centers, to deep defects of VZn‐H complexes at ≈0.9 eV above the valence band (VB). The EPR peak g = 1.96 emerged only when irradiated with light shorter than 400 nm, which is explicitly correlated with the electrons trapped in the CB or the shallow donors participating in the GL. A spin‐spin relaxation time (T2) estimated from the peak‐to‐peak line width ΔHpp in the EPR signal is ≈17.5–52.5 ns, which is two orders of magnitude longer than known values for bulk or thin‐film ZnO.

High-fidelity multiphoton-entangled cluster state with solid-state quantum emitters in photonic nanostructures

We propose a complete architecture for deterministic generation of entangled multiphoton states. Our approach utilizes periodic driving of a quantum-dot emitter and an efficient light-matter interface enabled by a photonic crystal waveguide. We assess the quality of the photonic states produced from a real system by including all intrinsic experimental imperfections. Importantly, the protocol is robust against the nuclear spin bath dynamics due to a naturally built-in refocusing method reminiscent to spin echo. We demonstrate the feasibility of producing Greenberger-Horne-Zeilinger and one-dimensional cluster states with fidelities and generation rates exceeding those achieved with conventional ``fusion'' methods in current state-of-the-art experiments. The proposed hardware constitutes a scalable and resource-efficient approach towards implementation of measurement-based quantum computing and quantum communication.

Broadband, efficient extraction of quantum light by a photonic device comprised of a metallic nano-ring and a gold back reflector

To implement quantum light sources based on quantum emitters in applications, it is desirable to improve the extraction efficiency of single photons. In particular, controlling the directionality and solid angle of the emission are key parameters, for instance, to couple single photons into optical fibers and send the information encoded in quantum light over long distances, for quantum communication applications. In addition, fundamental studies of the radiative behavior of quantum emitters, including studies of coherence and blinking, benefit from such improved photon collection. Quantum dots grown via Stranski–Krastanov technique have shown to be good candidates for bright, coherent, indistinguishable quantum light emission. However, one of the challenges associated with these quantum light sources arises from the fact that the emission wavelengths can vary from one emitter to the other. To this end, broadband light extractors that do not rely on high-quality factor optical cavities would be desirable, so that no tuning between the quantum dot emission wavelength and the resonator used to increase the light extraction is needed. Here, we show that metallic nano-rings combined with gold back reflectors increase the collection efficiency of single photons, and we study the statistics of this effect when quantum dots are spatially randomly distributed within the nano-rings. We show an average increase in the brightness of about a factor 7.5, when comparing emitters within and outside the nano-rings, in devices with a gold back reflector, and we measure count rates exceeding 7 × 106 photons per second and single photon purities as high as 85% ± 1%. These results are important steps toward the realization of scalable, broadband, easy to fabricate sources of quantum light for quantum communication applications.

InAsP Quantum Dot-Embedded InP Nanowires toward Silicon Photonic Applications

Quantum dot (QD) emitters on silicon platforms have been considered as a fascinating approach to building next-generation quantum light sources toward unbreakable secure communications. However, it has been challenging to integrate position-controlled QDs operating at the telecom band, which is a crucial requirement for practical applications. Here, we report monolithically integrated InAsP QDs embedded in InP nanowires on silicon. The positions of QD nanowires are predetermined by the lithography of gold catalysts, and the 3D geometry of nanowire heterostructures is precisely controlled. The InAsP QD forms atomically sharp interfaces with surrounding InP nanowires, which is in situ passivated by InP shells. The linewidths of the excitonic (X) and biexcitonic (XX) emissions from the QD and their power-dependent peak intensities reveal that the proposed QD-in-nanowire structure could be utilized as a non-classical light source that operates at silicon-transparent wavelengths, showing a great potential for diverse quantum optical and silicon photonic applications.

Determination of spermine and spermidine in meat with a ratiometric fluorescence nanoprobe and a combinational logic gate

A ratiometric fluorescent nanoprobe is developed with a wide color variation for visual determination of spermine (SP) and spermidine (SD) in meat samples. The green emission provided from the combination of yellow emissive quantum dots and blue emissive carbon dots turns into pink when SP or SD are present. The results show that the developed sensor has good linearity in the range of 0.5–10 and 0.5–80 µM for SP and SD and suitable detection limits were achieved including 0.2 and 2.1 µM for SP and SD. The probe was highly selective in the presence of amino acids and other biogenic amines. RGB indices were extracted to build a combinational logic gate for visual and simultaneous detection of SP and SD. The dual functional logic gate was easy to design and convenient to operate. Finally, a portable sensor was fabricated for visual, rapid and on-site assessment of meat freshness.