Plasmon-Triggered Ultrafast Operation of Color Centers in Hexagonal Boron Nitride Layers.

High-quality emission centers in two-dimensional materials are promising components for future photonic and optoelectronic applications. Carbon-enriched hexagonal boron nitride (hBN:C) layers host atom-like color-center (CC) defects with strong and robust photoemission up to room temperature. Placing the hBN:C layers on top of Ag triangle nanoparticles (NPs) accelerates the decay of the CC defects down to 46 ps from their reference bulk value of 350 ps. The ultrafast decay is achieved due to the efficient excitation of the plasmon modes of the Ag NPs by the near field of the CCs. Simulations of the CC/Ag NP interaction show that higher Purcell values are expected, although the measured decay of the CCs is limited by the instrument response. The influence of the NP thickness on the Purcell factor of the CCs is analyzed. The ultrafast operation of the CCs in hBN:C layers paves the way for their use in demanding applications, such as single-photon emitters and quantum devices.

Surface Plasmon Tunability of Core-Shell Au@Mo6 Nanoparticles by Shell Thickness Modification.

Plasmon resonances of noble metal nanoparticles are used to enhance light-matter interactions in the nanoworld. The nanoparticles' optical response depends strongly on the dielectric permittivity of the surrounding medium. We show that the plasmon resonance energy of core-shell Au@Mo6 nanoparticles can be tuned from 2.4 to 1.6 eV by varying the thickness of their Mo6 cluster shells between zero and 70 nm, when the core diameter is fixed at 100 nm. We probe their plasmonic response by performing nanometer-resolution plasmon mapping on individual nanoparticles, using electron energy-loss spectroscopy inside a transmission electron microscope. Our experimental results are corroborated by numerical simulations performed using boundary element methods. The simulations predict a similar dependency for the extinction energy, showing that this effect could also be observed by light-optical experiments outside the electron microscope, although limited by the size distribution of the nanoparticles in solution and the substantial scattering effects.

Strong coupling regime and bound states in the continuum between a quantum emitter and phonon-polariton modes.

We investigate the population dynamics of a two-level quantum emitter (QE) placed near a hexagonal boron nitride (h-BN) layer. The h-BN layer supports two energy phonon-polariton bands. In the case that the transition energy of the QE is resonant to them, its relaxation rate is enhanced several orders of magnitude compared to its free-space value and the population of the QE excited state shows reversible dynamics. We further show that for specific parameters of the QE/h-BN layer system, the QE population can be trapped in the excited state, keeping a constant value over long periods of time, thus demonstrating that the h-BN layer is a platform that can provide the strong light-matter interaction conditions needed for the formation of bound states in the electromagnetic continuum of modes. Semi-analytical methods are employed for determining whether such a bound state can be formed for given coupling conditions, as well as for computing the amount of initial population trapped in it. The bound states in the continuum are important for designing practical future quantum applications.

Entanglement of quantum emitters interacting through an ultra-thin noble metal nanodisk.

Ultra-thin metallic nanodisks, supporting localized plasmon (LP) modes, are used as a platform to facilitate high entanglement between distant quantum emitters (QEs). High Purcell factors, with values above 103, are probed for a QE placed near to an ultra-thin metallic nanodisk, composed of the noble metals Au, Ag, Al, and Cu. The disk supports two sets of localized plasmon modes, which can be excited by QEs with different transition dipole moment orientations. The two QEs are placed on opposite sides of the nanodisk, and their concurrence is used as a measure of the entanglement. We observe that the pair of QEs remains entangled for a duration that surpasses the relaxation time of the individual QE interacting with the metallic disk. Simultaneously, the QEs reach the entangled steady state faster than in the case where the QEs are in free space. Our results reveal a high concurrence value for a QES separation distance of 60 nm, and a transition energy of 0.8 eV (λ = 1550 nm). The robustness exhibited by this system under study paves the way for future quantum applications.

Non-Markovian spontaneous emission interference near a MoS2 nanodisk.

We propose a nanophotonic structure that gives high-degree quantum interference (QI) in the spontaneous emission (SE) of a quantum emitter (QE) in conjunction with strong light-matter coupling and non-Markovian dynamics. Specifically, we study the SE dynamics of a three-level V-type QE close to a MoS2 nanodisk (ND). We combine quantum dynamics calculations with electromagnetic calculations and find reversible population dynamics in the QE, together with high-degree QI created by the ND. A rich population dynamics is obtained, depending on the energy of the QE with respect to the energies of the exciton-polariton resonances of the MoS2 ND, the distance of the QE from the ND, and the initial state of the QE. Our results have potential applications in emergent quantum technologies.

Strong interaction of quantum emitters with a WS2 layer enhanced by a gold substrate.

We investigate the spontaneous emission (SE) effects of a two-level quantum emitter (QE) near a WS2 layer. The QE is placed above the WS2 layer in a host medium with constant dielectric permittivity. The material below the WS2 layer is either the same as the host medium or Au. We find that the Purcell factor of the QE near the dielectric/WS2 takes values up to 104. When the value of the dielectric permittivity of the host medium is increased, the enhancement of the Purcell factor diminishes. For a dielectric/WS2/dielectric structure, we obtain a Rabi splitting in the SE spectrum of the QE up to 75 meV at room temperature. This splitting is more pronounced for a WS2 layer of improved material quality which can be achieved by improved fabrication methods or operating at lower temperatures. When the WS2 layer is placed on top of an Au substrate, the hybrid exciton-surface plasmon polariton modes strongly interact with the QE, inducing coherent energy exchange between the two, as manifested by a pronounced Rabi splitting in the emission spectrum which is increased to 100 meV.

Nanoplasmonic Sensing at the Carbon-Bio Interface: Study of Protein Adsorption at Graphitic and Hydrogenated Carbon Surfaces.

Various forms of carbon are known to perform well as biomaterials in a variety of applications and an improved understanding of their interactions with biomolecules, cells, and tissues is of interest for improving and tailoring their performance. Nanoplasmonic sensing (NPS) has emerged as a powerful technique for studying the thermodynamics and kinetics of interfacial reactions. In this work, the in situ adsorption of two proteins, bovine serum albumin and fibrinogen, were studied at carbon surfaces with differing chemical and optical properties using nanoplasmonic sensors. The carbon material was deposited as a thin film onto NPS surfaces consisting of 100 nm Au nanodisks with a localized plasmon absorption peak in the visible region. Carbon films were fully characterized by X-ray photoelectron spectroscopy, atomic force microscopy, and spectroscopic ellipsometry. Two types of material were investigated: amorphous carbon (a-C), with high graphitic content and high optical absorptivity, and hydrogenated amorphous carbon (a-C:H), with low graphitic content and high optical transparency. The optical response of the Au/carbon NPS elements was modeled using the finite difference time domain (FDTD) method, yielding simulated analytical sensitivities that compare well with those observed experimentally at the two carbon surfaces. Protein adsorption was investigated on a-C and a-C:H, and the protein layer thicknesses were obtained from FDTD simulations of the expected response, yielding values in the 1.8-3.3 nm range. A comparison of the results at a-C and a-C:H indicates that in both cases fibrinogen layers are thicker than those formed by albumin by up to 80%.

Ag colloids and arrays for plasmonic non-radiative energy transfer from quantum dots to a quantum well.

Non-radiative energy transfer (NRET) can be an efficient process of benefit to many applications including photovoltaics, sensors, light emitting diodes and photodetectors. Combining the remarkable optical properties of quantum dots (QDs) with the electrical properties of quantum wells (QWs) allows for the formation of hybrid devices which can utilize NRET as a means of transferring absorbed optical energy from the QDs to the QW. Here we report on plasmon-enhanced NRET from semiconductor nanocrystal QDs to a QW. Ag nanoparticles in the form of colloids and ordered arrays are used to demonstrate plasmon-mediated NRET from QDs to QWs with varying top barrier thicknesses. Plasmon-mediated energy transfer (ET) efficiencies of up to ∼25% are observed with the Ag colloids. The distance dependence of the plasmon-mediated ET is found to follow the same d -4 dependence as the direct QD to QW ET. There is also evidence for an increase in the characteristic distance of the interaction, thus indicating that it follows a Förster-like model with the Ag nanoparticle-QD acting as an enhanced donor dipole. Ordered Ag nanoparticle arrays display plasmon-mediated ET efficiencies up to ∼21%. To explore the tunability of the array system, two arrays with different geometries are presented. It is demonstrated that changing the geometry of the array allows a transition from overall quenching of the acceptor QW emission to enhancement, as well as control of the competition between the QD donor quenching and ET rates.

Influence of plasmonic array geometry on energy transfer from a quantum well to a quantum dot layer.

A range of seven different Ag plasmonic arrays formed using nanostructures of varying shape, size and gap were fabricated using helium-ion lithography (HIL) on an InGaN/GaN quantum well (QW) substrate. The influence of the array geometry on plasmon-enhanced Förster resonance energy transfer (FRET) from a single InGaN QW to a ∼80 nm layer of CdSe/ZnS quantum dots (QDs) embedded in a poly(methyl methacrylate) (PMMA) matrix is investigated. It is shown that the energy transfer efficiency is strongly dependent on the array properties and an efficiency of ∼51% is observed for a nanoring array. There were no signatures of FRET in the absence of the arrays. The QD acceptor layer emission is highly sensitive to the array geometry. A model was developed to confirm that the increase in the QD emission on the QW substrate compared with a GaN substrate can be attributed solely to plasmon-enhanced FRET. The individual contributions of direct enhancement of the QD layer emission by the array and the plasmon-enhanced FRET are separated out, with the QD emission described by the product of an array emission factor and an energy transfer factor. It is shown that while the nanoring geometry results in an energy transfer factor of ∼1.7 the competing quenching by the array, with an array emission factor of ∼0.7, results in only an overall gain of ∼14% in the QD emission. The QD emission was enhanced by ∼71% for a nanobox array, resulting from the combination of a more modest energy transfer factor of 1.2 coupled with an array emission factor of ∼1.4.