Photon antibunching in a cluster of giant CdSe/CdS nanocrystals

Plasmon Mediated Single Photon Emission from a Nanocrystal Ensemble

Quantum photonic devices require robust sources of single photons to perform basic computational and communication protocols. Thus, developing scalable, integrable, and efficient quantum light sources has become crucial for the realization of quantum photonic devices. Single quantum dots are promising sources of quantum light due to their tunable emission wavelength. Here, we show the emergence of quantum-emitter-like antibunched emission behavior when multiple quantum dots are located in the vicinity of plasmonic particles. To evaluate the robustness of this phenomenon, we consider both monometallic and bimetallic particles. We find that the photoluminescence intensity of the plasmon coupled quantum dots fits well to a single sublinear power law exponent that is distinct from the behavior of CQD aggregates. Significantly, we find that plasmon coupling results in reduced flickering, thus enabling the realization of a more stable and reliable single photon source. Possible roles of emergent excitonic interactions in the coupled system are discussed.

Engineering quantum dots for improved single photon emission statistics

High fidelity single photon sources are required for the implementation of quantum information processing and communications protocols. Although colloidal quantum dots (CQDs) are single photon sources, their efficacy is limited by their tendency to show finite multiphoton emission at higher excitation powers. Here, we show that wave function engineering of CQDs enables the realization of emitters with significantly improved single photon emission performance. We study the ZnS/CdSe/CdS system. It is shown that this system offers significantly improved probabilities of single photon emission. While conventional CQDs such as CdSe/CdS exhibit a g2(0) > 0.5 ± 0.02 at ⟨N⟩ = 2.17, ZnS/CdSe/CdS show a greatly improved g2(0) ≈ 0.04 ± 0.01. Improved single photon emission performance encourages the use of colloidal materials as quantum light sources in emerging quantum devices.

Light squeezing enhancement by coupling nonlinear optical cavities

In this paper, we explore the squeezing effect generated by two coupled optical cavities. Each cavity contains a second-order nonlinear material and coherently pumped by a laser. Our results show that light intensity is strongly improved due to the presence of the nonlinearities and mainly depends on the detunings between external laser frequencies and cavity modes. More interestingly, the proposed scheme could enhance light squeezing for moderate coupling between cavities : the squeezing generated by one cavity is enhanced by the other one. For resonant interaction, highest squeezing effect is obtained near resonance. When fields are non resonant, squeezing increases near resonance of the considered cavity, but decreases for large detunings relative to the second cavity. Further, when the dissipation rate of the second cavity is smaller than the first, the squeezing could be improved, attaining nearly the perfect squeezing. While the temperature elevation has a negative impact overall on the nonclassical light, squeezing shows an appreciable resistance against thermal baths for appropriate parameter sets.

Electrochemical Control over the Optical Properties of II-VI Colloidal Nanoplatelets by Tailoring the Station of Extra Charge Carriers

An electrochemical (EC) method has been successfully applied to regulate the optical properties of nanocrystals, such as reducing their gain threshold by EC doping and enhancing their photoluminescence intensity by EC filling of trap states. However, the processes of EC doping and filling are rarely reported simultaneously in a single study, hindering the understanding of their underlying interactions. Here, we report the spectroelectrochemical (SEC) studies of quasi-two-dimensional nanoplatelets (NPLs), intending to clarify the above issues. EC doping is successfully achieved in CdSe/CdZnS core/shell NPLs, with red-shifted photoluminescence and a reversal of the emission intensity trend. The injection of extra electrons (holes) into the conduction (valence) band edges needs high bias voltages, while the passivation/activation process of trap states with the shift of Fermi level starts at lower EC potentials. Then, we explore the role of excitation light conditions in these processes, different from existing SEC research studies. Interestingly, increasing the laser power density can hinder EC electron injection, whereas decreasing the excitation energy evades the passivation process of trap states. Moreover, we demonstrate that EC control strategies can be used to realize color display and anti-counterfeiting applications via simultaneously tailoring the photoluminescence intensity of red- and green-emitting NPLs.

Nanoscale imaging of quantum dot dimers using time-resolved super-resolution microscopy combined with scanning electron microscopy

Time-resolved super-resolution microscopy was used in conjunction with scanning electron microscopy to image individual colloidal CdSe/CdS semiconductor quantum dots (QD) and QD dimers. The photoluminescence (PL) lifetimes, intensities, and structural parameters were acquired with nanometer scale spatial resolution and sub-nanosecond time resolution. The combination of these two techniques was more powerful than either alone, enabling us to resolve the PL properties of individual QDs within QD dimers as they blinked on and off, measure interparticle distances, and identify QDs that may be participating in energy transfer. The localization precision of our optical imaging technique was ∼3 nm, low enough that the emission from individual QDs within the dimers could be spatially resolved. While the majority of QDs within dimers acted as independent emitters, at least one pair of QDs in our study exhibited lifetime and intensity behaviors consistent with resonance energy transfer from a shorter lifetime and lower intensity donor QD to a longer lifetime and higher intensity acceptor QD. For this case, we demonstrate how the combined super-resolution optical imaging and scanning electron microscopy data can be used to characterize the energy transfer rate.

Direct Detection of Long-Range Interdomain Auger Recombination in Dumbbell-Shaped Quasi-Type-II Nanoparticle

Multicarrier dynamics in heterostructured ZnS-AgInS₂ (ZAIS) dumbbell-like nanoparticle (nanodumbell), which consists of two visible-light absorptive domains (ellipsoidal tip domains) directly linked to each end of a 22 nm length rod domain of the ZAIS nanodumbell with a quasi-type-II heterostructure, was investigated by femtosecond transient absorption spectroscopy under variable excitation intensities. Quantitative analysis together with the numerical simulations for the excitation intensity dependence of the dynamics revealed that only one electron-hole pair survived in the overall dumbbell as a consequence of Auger recombination, even though multiple carriers were formed on both terminal tip domains. This result strongly suggested carrier-carrier interaction between the tip domains, leading to the long-range Auger recombination via tunneling across a rod potential barrier.

Nonblinking Colloidal Quantum Dots via Efficient Multiexciton Emission

Nonblinking colloidal quantum dots (QDs) are significant to their applications as single-photon sources or light-emitting materials. Herein, a simple heat-up method was developed to synthesize high-qualityWZ-CdSe/CdS core-shell colloidal QDs, which achieved a near-unity photoluminescence quantum yield (PLQY). It was found that the blinking behavior of such QDs was completely suppressed at high excitation intensities, and ultra-stable PL emission was observed. For this reason, a systematic investigation was conducted, revealing that the complete blinking suppression was attributed mainly to the efficient multiexciton emission at high excitation intensities. Such high-quality QDs with nonblinking behaviors and nearly ideal PL properties at high excitation intensities have massive potential applications in various robust conditions, including QD display screens, single-particle tracks, and single-photon sources.

Universal Existence of Localized Single-Photon Emitters in the Perovskite Film of All-Inorganic CsPbBr3 Microcrystals

All-inorganic halide perovskites have drawn a lot of research attention very recently owing to their potential solution to the instability issue currently faced by the organic-inorganic counterparts. Meanwhile, the halide perovskites in a solid film are manifested as microscale morphologies whose functionalities are unavoidably affected by the interior or exterior presence of various nanoscale entities. Here all-inorganic solid films are fabricated with varying densities of single CsPbBr₃ microcrystals, showing that very sharp photoluminescence peaks can be universally observed at 4 K with the linewidths being as narrow as hundreds of μeV. The single-photon emission nature is confirmed for such a photoluminescence peak, whose intensity is completely quenched above ≈30 K to suggest its possible origin from a low potential-energy region of the single microcrystal. The discovery of such a novel emitting species in halide perovskites, with the enriched structure-property relationship, will surely impart significant influences on the advancement of relevant optoelectronic devices and quantum-light sources.

How Blinking Affects Photon Correlations in Multichromophoric Nanoparticles

A single chromophore can only emit a maximum of one single photon per excitation cycle. This limitation results in a phenomenon commonly referred to as photon antibunching (pAB). When multiple chromophores contribute to the fluorescence measured, the degree of pAB has been used as a metric to "count" the number of chromophores. But the fact that chromophores can switch randomly between bright and dark states also impacts pAB and can lead to incorrect chromophore numbers being determined from pAB measurements. By both simulations and experiment, we demonstrate how pAB is affected by independent and collective chromophore blinking, enabling us to formulate universal guidelines for correct interpretation of pAB measurements. We use DNA-origami nanostructures to design multichromophoric model systems that exhibit either independent or collective chromophore blinking. Two approaches are presented that can distinguish experimentally between these two blinking mechanisms. The first one utilizes the different excitation intensity dependence on the blinking mechanisms. The second approach exploits the fact that collective blinking implies energy transfer to a quenching moiety, which is a time-dependent process. In pulsed-excitation experiments, the degree of collective blinking can therefore be altered by time gating the fluorescence photon stream, enabling us to extract the energy-transfer rate to a quencher. The ability to distinguish between different blinking mechanisms is valuable in materials science, such as for multichromophoric nanoparticles like conjugated-polymer chains as well as in biophysics, for example, for quantitative analysis of protein assemblies by counting chromophores.

Rational synthesis of novel "giant" CuInTeSe/CdS core/shell quantum dots for optoelectronics

"Giant" core/shell quantum dots (g-QDs) are promising candidates for emerging optoelectronic technologies thanks to their facile structure/composition-tunable optoelectronic properties and outstanding photo-physical/chemical stability. Here, we synthesized a new type of CuInTeSe (CITS)/CdS g-QDs and regulated their optoelectronic properties by controlling the shell thickness. Through increasing the shell thickness, as-prepared g-QDs exhibited tunable red-shifted emission (from 900 to 1200 nm) and prolonged photoluminescence (PL) lifetimes (up to ∼14.0 μs), indicating a formed band structure showing efficient charge separation and transfer, which is further testified by theoretical calculations and ultrafast time-resolved transient absorption (TA) spectroscopy. These CITS/CdS g-QDs with various shell thicknesses can be employed to fabricate photoelectrochemical (PEC) cells, exhibiting improved photoresponse and stability as compared to the bare CITS QD-based devices. The results indicate that the rational design and engineering of g-QDs is very promising for future QD-based optoelectronic technologies.

PbS/CdS Quantum Dot Room-Temperature Single-Emitter Spectroscopy Reaches the Telecom O and S Bands via an Engineered Stability

We synthesized PbS/CdS core/shell quantum dots (QDs) to have functional single-emitter properties for room-temperature, solid-state operation in the telecom O and S bands. Two shell-growth methods-cation exchange and successive ionic layer adsorption and reaction (SILAR)-were employed to prepare QD heterostructures with shells of 2-16 monolayers. PbS/CdS QDs were sufficiently bright and stable to resolve photoluminescence (PL) spectra representing both bands from single nanocrystals using standard detection methods, and for a QD emitting in the O-band a second-order correlation function showed strong photon antibunching, important steps toward demonstrating the utility of lead chalcogenide QDs as single-photon emitters (SPEs). Irrespective of type, few telecom-SPEs exist that are capable of such room-temperature operation. Access to single-QD spectra enabled a direct assessment of spectral line width, which was ∼70-90 meV compared to much broader ensemble spectra (∼300 meV). We show inhomogeneous broadening results from dispersity in PbS core sizes that increases dramatically with extended cation exchange. Quantum yields (QYs) are negatively impacted at thick shells (>6 monolayers) and, especially, by SILAR-growth conditions. Time-resolved PL measurements revealed that, with SILAR, initially single-exponential PL-decays transition to biexponential, with opening of nonradiative carrier-recombination channels. Radiative decay times are, overall, longer for core/shell QDs compared to PbS cores, which we demonstrate can be partially attributed to some core/shell sizes occupying a quasi-type II electron-hole localization regime. Finally, we demonstrate that shell engineering and the use of lower laser-excitation powers can afford significantly suppressed blinking and photobleaching. However, dependence on shell thickness comes at a cost of less-than-optimal brightness, with implications for both materials and experimental design.

Copper Doping in II-VI Semiconductor Nanocrystals: Single-Particle Fluorescence Study

Copper doping in II-VI semiconductor nanocrystals (NCs) has sparked enormous debate regarding the oxidation state of Cu ions and their hugely differing consequences in optoelectronic applications. The identity of a magnetically active Cu²⁺ ion or a magnetically inactive d¹⁰ Cu⁺ ion has generally been probed using optical techniques, and confusion arises from the spatial clutter that is part of the technique. One major probe that could declutter the data obtained from ensemble emission is single-particle fluorescence spectroscopy. In this work, using this very technique along with X-ray absorption spectroscopy probing the local environment of dopant ions, we study Cu-doped II-VI semiconductor NCs to find conclusive evidence on the oxidation state of Cu dopants and hence the mechanism of their emission. Detailed analysis of blinking properties has been used to study the single-particle nature of the NCs.

Long Range Energy Transfer in Self-Assembled Stacks of Semiconducting Nanoplatelets

Fluorescent emitters like ions, dye molecules, or semiconductor nanoparticles are widely used in optoelectronic devices, usually within densely packed layers. Their luminescence properties can then be very different from when they are isolated, because of short-range interparticle interactions such as Förster resonant energy transfer (FRET). Understanding these interactions is crucial to mitigate FRET-related losses and could also lead to new energy transfer strategies. Exciton migration by FRET hopping between consecutive neighbor fluorophores has been evidenced in various systems but was generally limited to distances of tens of nanometers and involved only a few emitters. Here, we image self-assembled linear chains of CdSe nanoplatelets (colloidal quantum wells) and demonstrate exciton migration over 500 nm distances, corresponding to FRET hopping over 90 platelets. By comparing a diffusion-equation model to our experimental data, we measure a (1.5 ps)^-1 FRET rate, much faster than all decay mechanisms, so that strong FRET-mediated collective photophysical effects can be expected.

Single Photons Emitted by Nanocrystals Optically Trapped in a Deep Parabolic Mirror

We investigate the emission of single photons from CdSe/CdS dots-in-rod which are optically trapped in the focus of a deep parabolic mirror. Thanks to this mirror, we are able to image almost the full 4π emission pattern of nanometer-sized elementary dipoles and verify the alignment of the rods within the optical trap. From the motional dynamics of the emitters in the trap, we infer that the single-photon emission occurs from clusters comprising several emitters. We demonstrate the optical trapping of rod-shaped quantum emitters in a configuration suitable for efficiently coupling an ensemble of linear dipoles with the electromagnetic field in free space.

Colloidal quantum dot molecules manifesting quantum coupling at room temperature

Coupling of atoms is the basis of chemistry, yielding the beauty and richness of molecules. We utilize semiconductor nanocrystals as artificial atoms to form nanocrystal molecules that are structurally and electronically coupled. CdSe/CdS core/shell nanocrystals are linked to form dimers which are then fused via constrained oriented attachment. The possible nanocrystal facets in which such fusion takes place are analyzed with atomic resolution revealing the distribution of possible crystal fusion scenarios. Coherent coupling and wave-function hybridization are manifested by a redshift of the band gap, in agreement with quantum mechanical simulations. Single nanoparticle spectroscopy unravels the attributes of coupled nanocrystal dimers related to the unique combination of quantum mechanical tunneling and energy transfer mechanisms. This sets the stage for nanocrystal chemistry to yield a diverse selection of coupled nanocrystal molecules constructed from controlled core/shell nanocrystal building blocks. These are of direct relevance for numerous applications in displays, sensing, biological tagging and emerging quantum technologies.

In analogy to the coupling of atoms into molecules, the authors fuse colloidal semiconductor nanocrystals into quantum dot dimers. These nanocrystal ‘molecules’ exhibit significant quantum coupling effects, making them promising for applications in devices and potential quantum technologies.

Supersymmetric approach to coherent states for nonlinear oscillator with spatially dependent effective mass

A nonlinear oscillator with variable mass is studied in the approach of Supersymmetric quantum mechanics. Ladder operators in association with the shape invariance condition allowed us to find the coherent states of the system in the sense of Barut and Girardello. The statistical properties of these particular states are studied using the probability of distribution. In addition, the vibrational partition function is calculated. We see that the thermodynamic functions of the system such as mean energy and entropy depend on the nonlinearity parameter, except for the specific heat.

Emergence of multiple fluorophores in individual cesium lead bromide nanocrystals

Cesium-based perovskite nanocrystals (PNCs) possess alluring optical and electronic properties via compositional and structural versatility, tunable bandgap, high photoluminescence quantum yield and facile chemical synthesis. Despite the recent progress, origins of the photoluminescence emission in various types of PNCs remains unclear. Here, we study the photon emission from individual three-dimensional and zero-dimensional cesium lead bromide PNCs. Using photon antibunching and lifetime measurements, we demonstrate that emission statistics of both type of PNCs are akin to individual molecular fluorophores, rather than traditional semiconductor quantum dots. Aided by density functional modelling, we provide compelling evidence that green emission in zero-dimensional PNCs stems from exciton recombination at bromide vacancy centres within lead-halide octahedra, unrelated to external confinement. These findings provide key information about the nature of defect formation and the origin of emission in cesium lead halide perovskite materials, which foster their utilization in the emerging optoelectronic applications.

Inorganic perovskite nanocrystals attract lots of research attention but the origin of their photoluminescence remains debatable. Here Zhang et al. show that behavior of both CsPbBr₃ and Cs₄PbBr₆ nanocrystals is like individual molecular fluorophores and independent of the structural dimensionalities.

A Multichannel Time-Tagged Time-Resolved (TTTR) Model for Quantification of Oligomer Concentrations Based on Antibunching Effect

Molecule/protein aggregation causes many devastating and incurable diseases in human bodies. For example, studies have revealed that protein oligomers formed at the early stage are toxic and may be mostly responsible for some diseases. In the fundamental research, differentiation of different protein oligomers and quantification of the concentrations are important and challenging. Here, we have developed a multichannel time-tagged time-resolved (TTTR) confocal fluorescence model based on antibunching effect to solve the problem. The key point of the model is that n-oligomers labeled with n-dyes cannot emit more than n photons at one time. By assuming that all labeling dyes behave perfectly as noninteractive individual dyes, the analytic relationship between photon-emission probability and oligomer concentrations has been derived. Simulations have been carried out to verify the model, in which differentiation and concentration quantification of up to tetraoligomers can be realized with a relative error <10% in an eight-channel TTTR confocal setup with eight single-photon detectors.