Deterministic GHz-rate single photon sources at room-temperature would be essential components for various quantum applications. However, both the slow intrinsic decay rate and the omnidirectional emission of typical quantum emitters are two obstacles towards achieving such a goal which are hard to overcome simultaneously. We solve this challenge by a hybrid approach, using a complex monolithic photonic resonator constructed of a gold nanocone responsible for the rate enhancement, enclosed by a circular Bragg antenna for emission directionality [1,2]. A repeatable process accurately binds colloidal quantum dots to the tip of the antenna-embedded nanocone. As a result we achieve simultaneous 20-fold emission rate enhancement and record-high directionality leading to an increase in the observed brightness by a factor as large as 450 (80) into an NA = 0.22 (0.5). We project that these miniaturised on-chip devices can reach photon rates approaching 1.4´10⁸ single photons/second thus enabling ultrafast light-matter interfaces for quantum technologies at ambient conditions.

We also demostrate a significant progress towards a practical plug-and-play single photon source using giant colloidal quantum dots that show blinkingless emission which is stable over hours, on an antenna device which allows back optical pumping of the quantum dots and a front directional single photon emission with high single photon and an almost unity colllection efficiency [3,4].

Keywords: Single photon sources, quantum dots, quantum optics, quantum information, plasmonics, colloidal nanocrystals

References:

[1] Hamza Abudayyeh and Ronen Rapaport, Quantum Sci. Technol. 2 034004 (2017)

[2] Abudayyeh et al., ACS Nano 15, 11, 17384 (2021)

[3] Abudayyeh, et al.,  APL Photonics 6, 036109 (2021)

[4] Abudayyeh, et al., ACS Photonics 6, 446 (2019)

Lead-halide perovskite APbX₃ (A=Cs or organic cation; X=Cl, Br, I) quantum dots (QDs) are subject of intense research due to their exceptional properties as both classical¹ and quantum light sources.^2-4 Here, we report a comprehensive investigation of the room temperature single QD optical properties. The results reveal the origin of the QD homogeneous PL linewidths, and the peculiar size-dependent exciton photoluminescence line broadening and the exciton and multi-excitons recombination dynamics. Experimental results are corroborated by ab-initio molecular dynamics. 

Such findings guide the further design of robust single photon sources operating at room temperature. 

References

[1] Akkerman et al., Genesis, challenges and opportunities for colloidal lead halide perovskite nanocrystals. Nat. Mater. 17, 394–405 (2018).

[2] Becker et al., Bright triplet excitons in caesium lead halide perovskites. Nature 553, 189–193 (2018).

[3] Rainò et al., Superfluorescence from lead halide perovskite quantum dot superlattices. Nature 563, 671–675 (2018).

[4] Utzat et al., Coherent single-photon emission from colloidal lead halide perovskite quantum dots. Science 363, 1068–1072 (2019).

Multiply-excited states in semiconductor nanocrystals feature intriguing physics and play a crucial role in nanocrystal-based technologies. While photo-luminescence provides a natural probe to investigate these states, room temperature single-particle spectroscopy of their emission has so far proved elusive due to the temporal and spectral overlap with emission from the singly excited and charged states. In this work, we perform heralded spectroscopy of single quantum dots by incorporating the rapidly developing technology of single-photon avalanche diode arrays in a spectrometer setup. This allows us to directly observe the biexciton-exciton emission cascade and to measure the biexciton binding energy of single nanocrystals at room temperature, even though it is well below the scale of thermal broadening of the transitions due to finite temperature and that of spectral diffusion, the shift of the transition energy due to fluctuating electric fields. Single-particle heralded spectroscopy enables us to identify correlations of the biexciton binding energy with both charge-carrier confinement and fluctuations of the local electrostatic potential, which are masked in ensemble measurements, and to overcome artifacts due to inhomogeneous broadening [1]. Time-resolved spectrometry, as demonstrated here, has the potential of greatly extending our understanding of charge carrier dynamics in multielectron systems and of parallelization of quantum optics protocols.

Periodic arrays of nanoholes are patterned by electron beam lithography (EBL) upon a 170 µm thick glass coverslip, after the deposition of a sputter coated Indium Tin Oxide (ITO) and a spin coated polymethyl methacrylate (PMMA) electronic resist thin films. Colloidal semiconductor giant core/shell CdSe/CdS nanocrystals, shielded by an additional silica layer, are then trapped with remarkable efficiency into the PMMA holes by a capillary assembly method based on a finely tuned drop casting process. The resulting planar solid state arrays of semiconductor quantum dots demonstrate single photon emission properties, studied in detail by photoluminesce and second order photon correlation measurements.

Multiply-excited states in semiconductor nanocrystals (NCs) exhibit intriguing physics and are key in nanocrystal-based quantum technologies. While simultaneous multiple photon emission is typically quenched in quantum dots, in nanoplatelets (NPLs) its probability can be tuned according to size and shape. In particular, multiple excitons strongly bound in just one dimension are free to re-arrange in the lateral plane, reducing the probability for multi-body collisions. In this work, we analyze multi-exciton dynamics in two-dimensional CdSe/CdS core/shell NPLs of various sizes through the measurement of second-, third-, and fourth-order photon correlations.¹ Thus, for the first time, we could directly probe the dynamics of the two, three, and four exciton states at the single NC level. Remarkably, although higher orders of correlation vary substantially among a NPLs sample, they strongly correlate with the value of second-order antibunching. The scaling of the higher-order moments with the degree of antibunching presents a small yet clear deviation from the accepted model of Auger recombination through binary collisions. This deviation suggests that many-body contributions are present already at the level of tri-excitons.

In addition, multi-exciton interactions can be visually expressed in the emission patterns of different excitonic states. Type-II ZnSe/CdS dot-in-rod NCs, presenting a dipole radiation pattern due to anisotropy, can be used to demonstrate this phenomenon. A defocused imaging setup coupled to a novel single-photon avalanche detector (SPAD) array containing 23 pixels makes it possible to directly observe and analyze the out-of-focus dipole emission patterns of single excitons and bi-excitons in a single NC, allowing a deeper understanding of the underlying multi-exciton dynamics in semiconductor NCs.

Colloidal Quantum Dot (CQD) Molecules represents a new class of artificial molecules where the electron wavefunction in two neighboring CQD artificial atoms hybridizes within the connected nanocrystals. Like naturally occurring molecules, the electronic coupling strength is expected to manifest widely varied properties in CQD molecules, altering not only single-particle characteristics but also complex many-body interactions in CQDs. Herein, we discuss in detail the signatures of single CQD molecules at different coupling strengths, as reflected on their emitted photon statistics at room temperature. The neck diameter in CdSe/CdS core/shell CQD homodimer molecules allowed to tune the hybridization of the electron wavefunction at a fixed center-to-center distance. At small neck diameter, the electronic between two CQDs are weak, which increases progressively with neck filling.  We show that the photophysical properties of CQD molecules at weak coupling regime are governed by strong localization of electron wavefunction at individual CQD atoms, whereas in the strong coupling regime, due to facile delocalization the CQD molecules resemble the properties of single artificial atoms. A radiative multi-exciton configuration is preferred in the photo-excited weakly coupled molecules mostly governed by dipole-interactions, leading to strong photon bunching. In the strong coupling regime, electron tunneling activates a unique Auger process leading to single-photon emission via dissociation of biexciton, from a system consisting of two CQDs. This study validates the analogy of CQD molecules with the bonding nature of naturally occurring molecules that exhibit distinct ionic and covalent types of bonding in different coupling regimes, manifesting different properties. A detailed spectroscopic signatures following the time-tagged-time-resolved data will be discussed. The demonstration of multitude opto-electronic configuration tuning the extent of hybridization brands the CQD molecules as novel building blocks for many applications such as in low-threshold lasing and single-photon emitter in quantum applications.

The spectrum of multiexciton emission from colloidal quantum dots at room temperature is important for their use in high-power applications, but an in-depth characterization has not been possible until now. We present and apply a novel spectroscopic method to quantify the biexciton linewidth and biexciton binding energy of single CdSe/CdS/ZnS colloidal QDs. In our method, we select photons emitted by a biexciton emission cascade and reconstruct their spectrum. We find a biexciton linewidth of 86 meV on the single-QD scale, similar to that of the exciton. Variations in the biexciton energy are correlated with but are more narrowly distributed than variations in the exciton energy. Using a simple quantum-mechanical model, we conclude that inhomogeneous broadening in our sample is primarily due to variations in the CdS shell thickness.

Recent and nearly simultaneous developments in the fields of nanocrystal semiconductors, nanoplasmonics and nanophotonics may conspire to address deficiencies of available single-photon sources (SPSs). Namely, SPSs may now be accessible that operate in the telecommunications bands at room temperature, that are bright, and that can be easily integrated into quantum-advantaged communications, networking and sensing technologies. I will discuss our efforts to understand, design and synthesize heterostructured colloidal semiconductor quantum dots (QDs) that can meet the rigorous requirements for single-photons-on-demand in the infrared (IR) and at room-temperature.^1-4 In general, QD emitters, especially those that emit in the infrared, are slow to cycle photons. In order to dramatically enhance the rate of spontaneous emission, such emitters can be combined with nanoscale plasmonic materials or antennas. Traditionally, nanoscale noble metals such as gold and silver have been used to achieve the targeted enhancements in light-matter interactions that result from the presence of localized surface plasmons (LSPs). However, interest has recently shifted to intrinsically doped semiconductor nanocrystals (NCs) for their ability to display LSP resonances (LSPRs) over a much broader spectral range, including the IR. Among semiconducting plasmonic NCs, spinel metal oxides (sp-MOs) are an emerging material with distinct advantages in accessing the telecommunications bands in the IR and affording useful environmental stability. Here, I will discuss the plasmonic properties of several sp-MO NCs, known previously only for their magnetic functionality, and demonstrate their ability to modify the light-emission properties of telecom-emitting QDs, achieving Purcell enhancement factors up to ~50-fold for telecom-emitting QDs in simple plasmonic-spacer-emitter sandwich structures or significantly higher radiative rate enhancement aided by more sophisticated plasmonic nanoantenna architectures. Finally, in collaborative work,⁵ we have demonstrated prospects for on-chip integration and addressing losses in brightness associated with the omnidirectionality of free-standing emitters. In particular, using a scanning-probe method we directly place QDs into hybrid metal-dielectric antenna that cause their photons to emit in a highly focused, directional stream, dramatically enhancing collection efficiency and utility as a SPS. Taken together, these advances in nanomaterial chemistry, nanointegration and hybrid-material design are showing the way to practical utilization of nano-enabled SPSs for quantum applications.

Luminescent colloidal CdSe quantum dots (QDs) provide an opportunity to cover a wide range of emission wavelengths across the visible spectrum. Their high-temperature synthesis procedures offer crystalline materials with precise dimensions and well-defined optoelectronic properties. However, the core-only CdSe QDs show low photoluminescence (PL) quantum efficiency (QE) due to inefficient surface passivation by organic ligands, leading to charge trapping. Core/shell CdSe/CdS nanostructures with wurzite CdSe cores are known to improve the PL QE and long-term stability. A small lattice mismatch of 4% between the crystal structures of CdSe and CdS offers to grow a thick or giant CdS shell. Due to the electron delocalization in CdSe/CdS giant shell nanocrystals (NCs), the PL lifetime can be tuned from a few nanoseconds to microseconds. In this project, we studied a straight forward approach to synthesize high quality CdSe/CdS NCs with near-unity PL QE. Design of experiments (DOE), a statistics supported approach, is used to define the influence of reaction parameters in order to achieve a better synthesis procedure. Thirteen experiments were performed according to the DOE and monitored at 13 respective points for each experiment. PL QE and CdS shell thickness were analyzed with respect to all the variables used in DOE. PL QE up to 97 % has been achieved. Positive and negative effects were analyzed and optimal condition for the factors has been discussed.

Keywords  Colloidal quantum dots, core/shell nanostructures, near unity PL QY, Design of experiments

The low-temperature colloidal production of II-VI nanoplatelet heterostructures has stimulated the interest of researchers due to the possible uses of these materials in various optoelectronic devices. Here, we report a  room-temperature coating by CdS or ZnS dots of pre-prepared CdSe nanoplatelets. The dot coating process made use of a synthesis developed for the formation of free-standing CdS and ZnS species, involving injection of a metal precursor into reactive sulfur-amine solutions at room temperature. CdSe structured nanoplatelets with 1.75 nm thickness were used as the core constituent. The structural properties were investigated using Fourier transform infrared spectroscopy and advanced electron microscopy, while the elemental mapping was verified using high-angle annular dark field transmission electron microscopy. The results showed a dots-on-plate structure, resembling an intermediate configuration between core/crown or core/shell heterostructures. A thorough study of optical properties demonstrated a dramatic spectral shift of the absorption edge to lower energies, regardless of the uniformity of the coating layer, maintaining spectral stability over two months while stored at ambient conditions. Other optical measurements included examination of temperature-dependent photoluminescence and transient photoluminescence decay, from room temperature down to ~ 4 K. These measurements revealed excitons, trions, and trapped carrier recombination emission with variable relative intensities following the temperature change. The dots-on-plate structures studied displayed a short photoluminescence decay (< nanosecond), compatible with that of core/shell (crown) structures prepared at high temperatures. As a result, the presented techniques are alternate pathways that eliminate the requirement for excessive temperatures and/or multi-step processes, instead providing rapid, inexpensive, and scalable procedures with practical advantages.

This talk will address the influence of doping on the optical and magneto-optical properties of lead halide perovskites, revealing possibilities of their implementation in various spin-based applications. The Ni²⁺ ions were embedded into cesium lead halide-perovskite via a combined cation/anion exchange process. This study uses a thorough investigation of Ni²⁺ doping effects on the magneto-optical properties of CsPb(Br_xCl_1-x)₃ nanocubes, monitored by extensive spectroscopic methods such as steady-state, transient and magneto photoluminescence (PL) spectroscopy which recorded at various temperatures. The PL decay curve showed evidence for charge carrier trapping. The magneto-PL measurements revealed the occurrence of multiple recombination events with different degrees of circular polarization (DCP), with an apparent contrasting behavior in the undoped and doped samples. These observations unfold energy splitting of the bright triplet states (m_s = ±1 components) and the existence of emission processes related to delayed fluorescence by carrier localization at trapping sites (also confirmed by ODMR). The study reveals the role of the dopant in stiffening the entire crystal to the degree that enabled resolving the identification of carriers' localization sites.

Layer-by-layer (LBL) assembly is an appealing approach for the processing of nanoparticles and nanocolloids into thin films suitable for various applications¹. One of the major advantages of this method is its simplicity: the process requires neither sophisticated hardware nor high-purity components. This simplicity is combined with the high quality of the resulting coatings, whose thicknesses can be controlled at the nanometer level. In the field of light-emitting diodes (LEDs), thickness of the emissive material film is very important to assure the copious recombination of the opposite charges injected with resulting high photoemission². In addition, for Quantum Dot-based LEDs it’s crucial to have a good conductivity within the thin layer of emissive nanocrystals to facilitate the carrier mobility². Therefore, to merge these two important requirements, we developed a ligand exchange process on CsPbX₃ perovskite nanocrystal films which enables Layer-by-layer deposition.

We replaced, in solid state, oleate and oleylamine ligands (long insulant chains generally used for the synthesis of CsPbX₃ nanocrystals), with Didodecyldimethylammonium Bromide, Ammonium Thiocyanate and other ligands that enhance the photoluminescence quantum yield and improve the performances of LEDs. The exchange has been carried out through a spin-coating technique, using solvents with strategic polarity to avoid NCs dissolution or damaging. Maintaining the quick and simple method of spin-coating, we deposited an increasing number of layers and, through AFM measurements, we noticed considerable thickening of material.

The easy handling of this twofold process makes it very attractive for a laboratory-to-industry scale-up transition and the potential applicability to many nanoscale materials opens the door to a large variety of layer permutations.

Transition metal dichalcogenide (TMD) nanosheets have become an intensively investigated topic in the field of 2D nanomaterials, due to their semiconductor nature, the direct band gap transition and the broken inversion symmetry going from bulk to monolayer. These properties makes TMDs suitable for different technological applications such as photovoltaics, valleytronics, or hydrogen evolution reactions (HER), or transistors. Among them, MoX₂ (X = S, Se) are only direct-gap semiconductors when their thickness is reduced to a monolayer, hence an important effort is devoted to obtain single layer TMDs. Colloidal synthesis of TMDs has been developed in recent years as it provides a cost-efficient and scalable way to produce few-layer TMDs having homogenous size and thickness, yet obtaining a monolayer has proved challenging. Here we present a general method for the colloidal synthesis of mono- and few-layer MoX₂ (X = S, Se) using elemental chalcogenide and metal chloride as precursors. Using a synthesis with slow injection of the MoCl₅ precursor under nitrogen atmosphere, and optimizing the synthesis parameters with a Design Of Experiments (DOE) approach, we obtained a monolayer MoX₂ sample with the required semiconducting (2H) phase, a band gap of 1.96 eV for 2H-MoS₂ and 1.67 eV for 2H-MoSe₂, respectively, both displaying fluorescence at cryogenic and elevated temperatures. A correlation between the blue shifted absorption spectrum and the spectral difference between the Raman modes was established and confirmed that a single-layer thickness was obtained.

The recent advances in the colloidal synthesis of strongly emitting lead halide perovskite nanocrystals (NCs) with precise size and composition control open new possibilities for the fabrication of optoelectronic devices. Yet, the physics of the band-edge exciton fine structure of perovskites is still a subject of discussion [1], despite its central impact on the optical properties of these materials and therefore on their potential use in various applications, especially as quantum light sources. At cryogenic temperatures, we show that single cesium lead halide (CsPbX3) perovskite NCs display bright photoluminescence even though their band-edge exciton fine structure presents a dark ground singlet state. We introduce resonant PL excitation of the excitonic sublevels to investigate the indistinguishability character of the photons emitted by these NCs. Importantly, we demonstrate that the presence of a long-lived ground exciton state favors the formation of biexcitons and thus the emission of pairs of correlated photons. We show that this is a general behavior that can be observed for perovskite NCs as well as for conventional CdSe quantum dots. This property makes single CsPbI3 NCs versatile bright, quantum light sources with photon statistics that can be tuned from bunching to antibunching, using magnetic coupling or thermal mixing between dark and bright exciton states [2].

Finally, I’ll describe a recent study [3] of the photophysical properties of copper indium sulfide quantum dots which are emerging as promising alternatives to cadmium- and lead-based chalcogenides systems in various applications and for which the nature of the emission pathways remains a subject of debate.

Nanometric semi-conductor colloidal nanocrystals, like CdSe/CdS ones, are excellent single photon sources. Their emission is stable and bright, with a spectral bandwidth of the order of Dl=20-30nm. In specific conditions of high excitation or high confinement, their emission is dramatically changed. They lose their single photon source quality, the dynamic of the emission is accelerated.  the spectrum becomes very large and can reach a few 100nm. Such condition can be achieved either by exciting at high power the emitters or by coupling them to antennas [1], increasing optical confinement.

By in situ optical lithography using  high order Laguerre Gaussian mode, we deterministically positioned a single colloidal CdSe/CdS emitter inside a plasmonic patch antenna. The single nanocrystal embedded in a thin dielectric medium, is sandwiched between a thick gold layer and thin gold patch. We achieve inside plasmonic antennas a high interaction between the emitters and the confined field excited inside the antenna. We observe large modification of emission for quantum inside antennas illustrated by directive emission, high brightness and increased efficiency. We evidence a dramatic increase of absorption cross-section of emitters inside these antennas. At higher excitation, multi excitation recombination becomes radiative and the emission intensity becomes non linear.

[1] Amit Raj Dhawan, Cherif Belacel, Juan U. Esparza-Villa, Michel Nasilowski, Zhiming Wang, Catherine Schwob, Jean-Paul Hugonin, Laurent Coolen, Benoît Dubertret, Pascale Senellart, Agnès Maître, Extreme multiexciton emission from deterministically assembled single emitter subwavelength plasmonic patch antennas, Light: science and application, 9, 33 (2020)

[2] A. R. Dhawan, M. Nasilowski, Z. Wang, B. Dubertret, A. Maitre , sunder review, ^,https://hal.archives-ouvertes.fr/hal-03376851, Efficient single-emitter plasmonic patch antenna fabrication by deterministic in situ optical lithography using spatially modulated light (under review)