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The ability of surface polaritons (SPs) to enhance and manipulate light fields down to deep-subwavelength length scales enables applications in optical sensing and nonlinear optics at the nanoscale. However, the wavelength mismatch between light and SPs prevents direct optical excitation of surface-bound modes, thereby limiting the widespread development of SP-based photonics. Free electrons are a natural choice to directly excite strongly confined SPs because they can supply field components of high momentum at designated positions with subnanometer precision. Here, we theoretically explore free-electron–SP coupling mediated by small scatterers and show that low-energy electrons can efficiently excite surface modes with a maximum probability reached at an optimum surface–scatterer distance. By aligning the electron beam with a periodic array of scatterers placed near a polariton-supporting interface, in-plane Smith–Purcell emission results in the excitation of surface modes along well-defined directions. Our results support using scattering elements to excite SPs with low-energy electrons. [ABSTRACT FROM AUTHOR]

Light carries momentum and, upon interaction with material structures, can exert forces on them. Here, we show that a planar structure with asymmetric optical response is spontaneously accelerated when placed in an environment at a different temperature. This phenomenon originates from the imbalance in the exchange rates of photons between both sides of the structure and the environment. Using a simple theoretical model, we calculate the force acting on the planar structure and its terminal velocity in vacuum, and analyze their dependence on the initial temperature and the geometrical properties of the system for different realistic materials. Our results unravel an alternative approach to manipulating objects in the nano and microscale that does not require an external source of radiation. [ABSTRACT FROM AUTHOR]

Spontaneous light emission is central to a vast range of physical systems and is a founding pillar for the theory of light–matter interactions. In the presence of complex photonic media, the description of spontaneous light emission usually requires advanced theoretical quantum optics tools such as macroscopic quantum electrodynamics, involving quantized electromagnetic fields. Although rigorous and comprehensive, the complexity of such models can obscure the intuitive understanding of many quantum-optical phenomena. Here, we review a method for calculating spontaneous emission and other quantum-optical processes without making explicit use of quantized electromagnetic fields. Instead, we introduce the concept of transition currents, comprising charges in matter that undergo transitions between initial and final quantum states. We show how predictions that usually demand advanced methods in quantum electrodynamics or quantum optics can be reproduced by feeding these transition currents as sources to the classical Maxwell equations. One then obtains the relevant quantum observables from the resulting classical field amplitudes, without washing out quantum optical effects. We show that this procedure allows for a straightforward description of quantum phenomena, even when going beyond the dipole approximation and single emitters. As illustrative examples, we calculate emission patterns and Purcell-enhanced emission rates in both bound-electron and free-electron systems. For the latter, we derive cathodoluminescence emission and energy-loss probabilities of free electrons interacting with nanostructured samples. In addition, we calculate quantum-beat phenomena in bound-electron systems and wave function-dependent optical coherence in free-electron systems. Remarkably, the transition-current formalism captures more complex phenomena, such as many-body interference effects and super-radiance of both bound- and free-electron systems, second-order processes such as two-photon emission, and quantum recoil corrections to free-electron radiation. We review a variety of light–matter interactions in fields ranging from electron microscopy to nanophotonics and quantum optics, for which the transition-current theoretical formalism facilitates practical simulations and a deeper understanding of novel applications. [ABSTRACT FROM AUTHOR]

Two-dimensional materials give access to the ultimate physical limits of photonics with appealing properties for ultracompact optical components such as waveguides and modulators. Specifically, in monolayer semiconductors, a strong excitonic resonance leads to a sharp oscillation in permittivity from positive to even negative values. This extreme optical response enables surface exciton–polaritons to guide visible light bound to an atomically thin layer. However, such ultrathin waveguides support a transverse electric (TE) mode with low confinement and a transverse magnetic (TM) mode with short propagation. Here, we propose that realistic semiconductor–insulator–semiconductor superlattices comprising monolayer WS2 and hexagonal boron nitride (hBN) can improve the properties of both TE and TM modes. Compared to a single monolayer, a heterostructure with a 1-nm hBN spacer separating two monolayers enhances the confinement of the TE mode from 1.2 to around 0.5 μm, while the out-of-plane extension of the TM mode increases from 25 to 50 nm. We propose two simple additivity rules for mode confinement valid in the ultrathin film approximation for heterostructures with increasing spacer thickness. Stacking additional WS2 monolayers into superlattices further enhances the waveguiding properties. Our results underscore the potential of monolayer-based superlattices as a platform for visible-range nanophotonics with promising optical, electrical, and magnetic tunability. [ABSTRACT FROM AUTHOR]

Polaritons are well-established carriers of light, electrical signals, and even heat at the nanoscale in the setting of on-chip devices. However, the goal of achieving practical polaritonic manipulation over small distances deeply below the light diffraction limit remains elusive. Here, we implement nanoscale polaritonic in-plane steering and cloaking in a low-loss atomically layered van der Waals (vdW) insulator, α-MoO3, comprising building blocks of customizable stacked and assembled structures. Each block contributes specific characteristics that allow us to steer polaritons along the desired trajectories. Our results introduce a natural materials-based approach for the comprehensive manipulation of nanoscale optical fields, advancing research in the vdW polaritonics domain and on-chip nanophotonic circuits.Polaritons – hybrid light-matter excitations – in van der Waals materials hold promise for photonics applications below the diffraction limit. Here, the authors demonstrate in-plane steering and cloaking of phonon polaritons in assembled micro-structures based on α-MoO3 films with misaligned crystallographic orientations. [ABSTRACT FROM AUTHOR]

Time-resolved electron microscopy aims to track nanoscale excitations and dynamic states of matter at a temporal resolution ultimately reaching the attosecond regime. Periodically time-varying fields in an illuminated specimen cause free-electron inelastic scattering, which enables the spectroscopic imaging of near-field intensities. However, access to the evolution of nanoscale fields and structures within the cycle of light requires sensitivity to the optical phase. Here we introduce free-electron homodyne detection as a universally applicable approach to electron microscopy of phase-resolved optical responses at high spatiotemporal resolution. In this scheme, a phase-controlled reference interaction serves as the local oscillator to extract arbitrary sample-induced modulations of a free-electron wavefunction. We demonstrate this principle through the phase-resolved imaging of plasmonic fields with few-nanometre spatial and sub-cycle temporal resolutions. Due to its sensitivity to both phase- and amplitude-modulated electron beams, free-electron homodyne detection measurements will be able to detect and amplify weak signals stemming from a wide variety of microscopic origins, including linear and nonlinear optical polarizations, atomic and molecular resonances, and attosecond-modulated structure factors. Free-electron homodyne detection allows measuring phase-resolved optical responses in electron microscopy, demonstrated in the imaging of plasmonic fields with few-nanometre spatial and sub-cycle temporal resolutions. [ABSTRACT FROM AUTHOR]

The transformation of electromagnetic energy into matter represents a fascinating prediction of relativistic quantum electrodynamics that is paradigmatically exemplified by the creation of electron-positron pairs out of light. However, this phenomenon has a very low probability, so positron sources rely instead on beta decay, which demands elaborate monochromatization and trapping schemes to achieve high-quality beams. Here, we propose to use intense, strongly confined optical near fields supported by a nanostructured material in combination with high-energy photons to create electron-positron pairs. Specifically, we show that the interaction between near-threshold γ-rays and polaritons yields higher pair-production cross sections, largely exceeding those associated with free-space photons. Our work opens an unexplored avenue toward generating tunable pulsed positrons from nanoscale regions at the intersection between particle physics and nanophotonics. The authors propose electron-positron creation by scattering of gamma-rays and polaritons, enabling the synthesis of ultrafast, localized positron sources and introducing the possibility to exploit nanophotonics for particle physics. [ABSTRACT FROM AUTHOR]

Single-photon emitters are a pivotal element in quantum technologies, but the generation of single photons along well-defined directions generally involves sophisticated configurations. Here, we propose a photon source capable of generating single photons with high efficiency along guided modes. Specifically, we consider a quantum emitter placed in a periodically patterned linear waveguide. The latter is designed to host a single guided mode over the spectral range of interest and display a divergence in the photonic density of states at an emission wavelength close to the period. Photons are preferentially emitted along the waveguide near that spectral region. We predict that nearly all of the emission can be made to occur along the waveguide with a reduction in the temporal uncertainty by two orders of magnitude. Our study opens a conceptually new direction in the production of single photons with a high degree of directionality and reduced temporal uncertainty. [ABSTRACT FROM AUTHOR]

The generation of nonclassical light states bears a paramount importance in quantum optics and is largely relying on the interaction between intense laser pulses and nonlinear media. Recently, electron beams, such as those used in ultrafast electron microscopy to retrieve information from a specimen, have been proposed as a tool to manipulate both bright and dark confined optical excitations, inducing semiclassical states of light that range from coherent to thermal mixtures. Here, we show that the ponderomotive contribution to the electron–cavity interaction, which we argue to be significant for low-energy electrons subject to strongly confined near-fields, can actually create a more general set of optical states, including coherent and squeezed states. The postinteraction electron spectrum further reveals signatures of the nontrivial role played by A2 terms in the light–matter coupling Hamiltonian, particularly when the cavity is previously excited by either chaotic or coherent illumination. Our work introduces a disruptive approach to the creation of nontrivial quantum cavity states for quantum information and optics applications, while it suggests unexplored possibilities for electron beam shaping. [ABSTRACT FROM AUTHOR]

High‐harmonic generation (HHG) in graphene heterostructures, consisting of metallic nanoribbons separated from a graphene sheet by either a few‐nanometer layer of aluminum oxide or an atomic monolayer of hexagonal boron nitride, is investigated. The nanoribbons amplify the near‐field at the graphene layer relative to the externally applied pumping, thus allowing the observation of third‐ and fifth‐harmonic generation in the carbon monolayer at modest pump powers in the mid‐infrared. The dependence of the nonlinear signals on the ribbon width and spacer thickness, as well as pump power and polarization are studied, and enhancement factors (EF) relative to bare graphene reaching 1600 and 4100 for third‐ and fifth‐harmonic generation, respectively, are demonstrated. The work supports the use of graphene heterostructures to selectively enhance specific nonlinear processes of interest, an essential capability for the design of nanoscale nonlinear devices. [ABSTRACT FROM AUTHOR]

Tuning electrical, optical, and thermal material properties is central for engineering and understanding solid-state systems. In this scenario, atomically thin materials are appealing because of their sensitivity to electric and magnetic gating, as well as to interlayer hybridization. Here, we introduce a radically different approach to material engineering relying on the image interaction experienced by electrons in a two-dimensional material when placed in proximity of an electrically neutral structure. We theoretically show that electrons in a semiconductor atomic layer acquire a quantum phase resulting from the image potential induced by the presence of a neighboring periodic array of conducting ribbons, which in turn modifies the optical, electrical, and thermal properties of the monolayer, giving rise to additional interband optical absorption, plasmon hybridization, and metal-insulator transitions. Beyond its fundamental interest, material engineering based on the image interaction represents a disruptive approach to tailor the properties of atomic layers for application in nanodevices. Existing approaches to modulating the properties of 2D materials typically involve heterostructuring or exposure to external fields. Here, the authors propose a gate-free non-contact approach to tuning the properties of a 2D semiconductor via the image interaction due to proximity to a neutral patterned structure. [ABSTRACT FROM AUTHOR]

Strain relaxation mechanisms during epitaxial growth of core-shell nanostructures play a key role in determining their morphologies, crystal structure and properties. To unveil those mechanisms, we perform atomic-scale aberration-corrected scanning transmission electron microscopy studies on planar core-shell ZnSe@ZnTe nanowires on α-Al2O3 substrates. The core morphology affects the shell structure involving plane bending and the formation of low-angle polar boundaries. The origin of this phenomenon and its consequences on the electronic band structure are discussed. We further use monochromated valence electron energy-loss spectroscopy to obtain spatially resolved band-gap maps of the heterostructure with sub-nanometer spatial resolution. A decrease in band-gap energy at highly strained core-shell interfacial regions is found, along with a switch from direct to indirect band-gap. These findings represent an advance in the sub-nanometer-scale understanding of the interplay between structure and electronic properties associated with highly mismatched semiconductor heterostructures, especially with those related to the planar growth of heterostructured nanowire networks. Planar growth of nanowire arrays involves interactions between materials that affect the electronic behavior of the effective heterojunction. Here, authors show how core curvature and cross-section morphology affect shell growth, demonstrating how strain at the core-shell interface induces electronic band modulations in ZnSe@ZnTe nanowires. [ABSTRACT FROM AUTHOR]

Given the symmetry of the waveguide, the radial and azimuthal components of the transverse field associated with each mode only depend on radial distance I R i , apart from an overall phase factor HT ht . 4.2 Field produced by an inner line dipole in the region outside the waveguide We consider a line dipole placed at a transverse position B R b SB 0 sb = ( I x i SB 0 sb , I y i SB 0 sb ) within the waveguide and represent it by a dipole density HT ht (dipole per unit length) extending along the line defined by varying I z i SB 0 sb in ( B R b SB 0 sb , I z i SB 0 sb ). In particular, a simple discrete-transform analysis leads to the conclusion that we can select a specific I m i = I m i SB 0 sb component while canceling out those of all other I m i 's in the | I m i | <= I N i range by combining 2 I N i + 1 incident waves along azimuthal directions I i SB I j i sb = 2 I j i /(2 I N i + 1) with amplitudes HT ht for 0 <= I j i <= 2 I N i . [Extracted from the article]

Nanoscale nonlinear optics is limited by the inherently weak nonlinear response of conventional materials and the small light–matter interaction volumes available in nanostructures. Plasmonic excitations can alleviate these limitations through subwavelength light focusing, boosting optical near fields that drive the nonlinear response, but also suffering from large inelastic losses that are further aggravated by fabrication imperfections. Here, we theoretically explore the enhanced nonlinear response arising from extremely confined plasmon polaritons in few-atom-thick crystalline noble metal films. Our results are based on quantum-mechanical simulations of the nonlinear optical response in atomically thin metal films that incorporate crucial electronic band structure features associated with vertical quantum confinement, electron spill-out, and surface states. We predict an overall enhancement in plasmon-mediated nonlinear optical phenomena with decreasing film thickness, underscoring the importance of surface and electronic structure in the response of ultrathin metal films. [ABSTRACT FROM AUTHOR]

Monolayer hexagonal boron nitride (hBN) is attracting considerable attention because of its potential applications in areas such as nano‐ and opto‐electronics, quantum optics and nanomagnetism. However, the implementation of such functional hBN demands precise lateral nanostructuration and integration with other two‐dimensional materials, and hence, novel routes of synthesis beyond exfoliation. Here, a disruptive approach is demonstrated, namely, imprinting the lateral pattern of an atomically stepped one‐dimensional template into a hBN monolayer. Specifically, hBN is epitaxially grown on vicinal Rhodium (Rh) surfaces using a Rh curved crystal for a systematic exploration, which produces a periodically textured, nanostriped hBN carpet that coats Rh(111)‐oriented terraces and lattice‐matched Rh(337) facets with tunable width. The electronic structure reveals a nanoscale periodic modulation of the hBN atomic potential that leads to an effective lateral semiconductor multi‐stripe. The potential of such atomically thin hBN heterostructure for future applications is discussed. [ABSTRACT FROM AUTHOR]

Entanglement engineering plays a central role in quantum-enhanced technologies, with potential physical platforms that outperform their classical counterparts. However, free electrons remain largely unexplored despite their great capacity to encode and manipulate quantum information, due in part to the lack of a suitable theoretical framework. Here we link theoretical concepts from quantum information to available free-electron sources. Specifically, we consider the interactions among electrons propagating near the surface of a polariton-supporting medium and study the entanglement induced by pair-wise coupling. These correlations depend on the controlled interaction interval and the initial electron bandwidth. We show that long interaction times of broadband electrons extend their temporal coherence. This in turn is revealed through a widened Hong–Ou–Mandel peak and is associated with an increased entanglement entropy. We then introduce a discrete basis of electronic temporal modes and discriminate between them via coincidence detection with a shaped probe. This paves the way for ultrafast quantum information transfer by means of free electrons, rendering the large alphabet that they span in the time domain accessible. [ABSTRACT FROM AUTHOR]

Tunable sources of X-ray radiation are widely used for imaging and spectroscopy in fundamental science, medicine and industry. The growing demand for highly tunable, high-brightness laboratory-scale X-ray sources motivates research into new fundamental mechanisms of X-ray generation. Here, we demonstrate the ability of van der Waals materials to serve as a platform for tunable X-ray generation when irradiated by moderately relativistic electrons available, for example, from a transmission electron microscope. The radiation spectrum can be precisely controlled by tuning the acceleration voltage of the incident electrons, as well as by our proposed approach: adjusting the lattice structure of the van der Waals material. We present experimental results for both methods, observing the energy tunability of X-ray radiation from the van der Waals materials WSe2, CrPS4, MnPS3, FePS3, CoPS3 and NiPS3. Our findings demonstrate the concept of material design at the atomic level, using van der Waals heterostructures and other atomic superlattices, for exploring novel phenomena of X-ray physics. Tunable X-ray generation, from ultrathin van der Waals materials impacted by relativistic electrons, is demonstrated. [ABSTRACT FROM AUTHOR]

The physics of electrons, photons and their plasmonic interactions changes greatly when one or more dimensions are reduced down to the nanometre scale1. For example, graphene shows unique electrical, optical and plasmonic properties, which are tunable through gating or chemical doping2–5. Similarly, ultrathin metal films (UTMFs) down to atomic thickness can possess new quantum optical effects6,7, peculiar dielectric properties8 and predicted strong plasmons9,10. However, truly two-dimensional plasmonics in metals has so far been elusive because of the difficulty in producing large areas of sufficiently thin continuous films. Thanks to a deposition technique that allows percolation even at 1 nm thickness, we demonstrate plasmons in few-nanometre-thick gold UTMFs, with clear evidence of new dispersion regimes and large electrical tunability. Resonance peaks at wavelengths of 1.5–5 μm are shifted by hundreds of nanometres and amplitude-modulated by tens of per cent through gating using relatively low voltages. The results suggest ways to use metals in plasmonic applications, such as electro-optic modulation, biosensing and smart windows. Nanometre thick metal films enable electrical tuning of plasmons. [ABSTRACT FROM AUTHOR]

Light--electron interaction is the seminal ingredient in free-electron lasers and dynamical investigation of matter. Pushing the coherent control of electrons by light to the attosecond timescale and below would enable unprecedented applications in quantum circuits and exploration of electronic motions and nuclear phenomena. Here we demonstrate attosecond coherent manipulation of a free-electron wave function, and show that it can be pushed down to the zeptosecond regime. We make a relativistic single-electron wavepacket interact in free-space with a semi-infinite light field generated by two light pulses reflected from a mirror and delayed by fractions of the optical cycle. The amplitude and phase of the resulting electron--state coherent oscillations are mapped in energy-momentum space via momentumresolved ultrafast electron spectroscopy. The experimental results are in full agreement with our analytical theory, which predicts access to the zeptosecond timescale by adopting semiinfinite X-ray pulses. [ABSTRACT FROM AUTHOR]

The speed of solid-state electronic devices, determined by the temporal dynamics of charge carriers, could potentially reach unprecedented petahertz frequencies through direct manipulation by optical fields, consisting in a million-fold increase from state-of-the-art technology. In graphene, charge carrier manipulation is facilitated by exceptionally strong coupling to optical fields, from which stems an important back-action of photoexcited carriers. Here we investigate the instantaneous response of graphene to ultrafast optical fields, elucidating the role of hot carriers on sub-100 fs timescales. The measured nonlinear response and its dependence on interaction time and field polarization reveal the back-action of hot carriers over timescales commensurate with the optical field. An intuitive picture is given for the carrier trajectories in response to the optical-field polarization state. We note that the peculiar interplay between optical fields and charge carriers in graphene may also apply to surface states in topological insulators with similar Dirac cone dispersion relations. [ABSTRACT FROM AUTHOR]

Control over the optical response of metal nanoparticles and their associated plasmons is currently enabling many promising applications in areas as diverse as biosensing and photocatalysis. In this context, experiments based upon colloid synthesis and nanofabricated structures are assisted by numerical electromagnetic modeling, which supplies predictive simulations, but not the kind of physical intuition needed for exploration of new ideas, such as one finds when simple mathematical expressions can describe a problem. This tutorial review presents and extends a simple analytical simulation method that allows us to accurately describe the optical response of metal nanoparticles, including retardation effects, without the requirement of large computational resources. More precisely, plasmonic extinction spectra and near-field enhancement are described through a small set of real numbers for each nanoparticle shape, which we tabulate for a wide selection of common morphologies. Remarkably, these numbers are independent of size, composition and environment. We further present a compilation of nanoplasmonic experimental data that are excellently described by the simple mathematical expressions here introduced. [ABSTRACT FROM AUTHOR]

Topological optical states exhibit unique immunity to defects, rendering them ideal for photonic applications. A powerful class of such states is based on time-reversal symmetry breaking of the optical response. However, existing proposals either involve sophisticated and bulky structural designs or can only operate in the microwave regime. Here we show a theoretical demonstration for highly confined topologically protected optical states to be realized at infrared frequencies in a simple two-dimensional (2D) material structure—a periodically patterned graphene monolayer—subject to a magnetic field of only 2 tesla. In our graphene honeycomb superlattice structures, plasmons exhibit substantial nonreciprocal behavior at the superlattice junctions under moderate static magnetic fields, leading to the emergence of topologically protected edge states and localized bulk modes. This approach is simple and robust for realizing topologically nontrivial optical states in 2D atomic layers, and could pave the way for building fast, nanoscale, defect-immune photonic devices. [ABSTRACT FROM AUTHOR]

Spatial nonlocality in the photonic response of metallic nanoparticles is actually known to produce near-field quenching and significant plasmon frequency shifts relative to local descriptions. As the control over size and morphology of fabricated nanostructures is truly reaching the nanometer scale, understanding and accounting for nonlocal phenomena is becoming increasingly important. Recent advances clearly point out the need to go beyond the local theory. We here present a general formalism for incorporating spatial dispersion effects through the hydrodynamic model and generalizations for arbitrary surface morphologies. Our method relies on the boundary element method, which we supplement with a nonlocal interaction potential. We provide numerical examples in excellent agreement with the literature for individual and paired gold nanospheres, and critically examine the accuracy of our approach. The present method involves marginal extra computational cost relative to local descriptions and facilitates the simulation of spatial dispersion effects in the photonic response of complex nanoplasmonic structures. [ABSTRACT FROM AUTHOR]

The article presents views of various professionals from the field of chemistry on topics related to quantum plasmonics, expressed during the Faraday Discussions meeting of the Royal Society of Chemistry in 2015. Thomas Ebbesen talks about the effect of position of the detection wavelength on plasmon in the excittaion spectra. It also noted views of Ortwin Hess on oscillatory dynamics in the ultra-strong coupled state, and Pavel Ginzburg on expression for vacuum Rabi splitting.

The article presents views of various professionals from the field of chemistry on topics related to surface plasmon enhanced spectroscopies, expressed during the Faraday Discussions meeting of the Royal Society of Chemistry in 2015. Jeremy Baumberg talks about effect of plasmon frequencies and spatial distributions in sub-nanometer gaps on plasmons. Javier Aizpurua describes the role of ligands in plasmonic tunnelling or quenching the hot spots.

The article presents views of various professionals from the field of chemistry on topics related to plasmonics, expressed during the Faraday Discussions meeting of the Royal Society of Chemistry in 2015. Mark Brongersma shares his views on presence of quantum mechanical effects at the final stages of the metal nanowire connection in memristor devices. Stefan Maier talks about application of plasmonics in photovoltaics while Evgenii Narimanov describes the Dirac diffraction in hypercrystals.

The observation and electrical manipulation of infrared surface plasmons in graphene have triggered a search for similar photonic capabilities in other atomically thin materials that enable electrical modulation of light at visible and near-infrared frequencies, as well as strong interaction with optical quantum emitters. Here, we present a simple analytical description of the optical response of such kinds of structures, which we exploit to investigate their application to light modulation and quantum optics. Specifically, we show that plasmons in one-atom-thick noble-metal layers can be used both to produce complete tunable optical absorption and to reach the strong-coupling regime in the interaction with neighboring quantum emitters. Our methods are applicable to any plasmon-supporting thin materials, and in particular, we provide parameters that allow us to readily calculate the response of silver, gold, and graphene islands. Besides their interest for nanoscale electro-optics, the present study emphasizes the great potential of these structures for the design of quantum nanophotonics devices. [ABSTRACT FROM AUTHOR]

The much sought after optical transistor-the photonic counterpart of the electronic transistor-is poised to become a central ingredient in the development of optical signal processing. The motivation for using photons rather than electrons comes not only from their faster dynamics, but also from their lower crosstalk and robustness against environmental decoherence, which enable a high degree of integration and the realization of quantum operations. A single-molecule transistor has recently been demonstrated at cryogenic temperatures. Here, we demonstrate that a single nitrogen-vacancy centre at room temperature can operate as an optical switch under non-resonant continuous-wave illumination. We show an optical modulation of more than 80% and a time response faster than 100 ns in the green-laser-driven fluorescence signal, which we control through an independent near-infrared gating laser. Our study indicates that the near-infrared laser triggers a fast-decay channel of the nitrogen-vacancy mediated by promotion of the excited state to a dark band. [ABSTRACT FROM AUTHOR]