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We propose the dynamical Casimir effect in a time-modulated near-field system. The system consists of two bodies made of polaritonic materials, that are brought in close proximity to each other, and the modulation frequency is approximately twice the relevant resonance frequencies of the system. We develop a rigorous fluctuational electrodynamics formalism to explore the produced Casimir flux, associated with the degenerate as well as non-degenerate two-polariton emission processes. We have identified flux contributions from both quantum and thermal fluctuations at finite temperatures, with a dominant quantum contribution even at room temperature under the presence of a strong near-field effect. We have conducted a nonclassicality test for the total radiative flux at finite temperatures, and shown that nonclasscical states of emitted photons can be obtained for a high temperature up to $\sim 250\,$K. Our findings open an avenue for the exploration of dynamical Casimir effect beyond cryogenic temperatures, and may be useful for creating tunable nanoscale nonclassical light sources.

We explore near-field radiative heat transfer between two bodies under time modulation by developing a rigorous fluctuational electrodynamics formalism. We demonstrate that time modulation can results in the enhancement, suppression, elimination, or reversal of radiative heat flow between the two bodies, and can be used to create a radiative thermal diode with infinite contrast ratio, as well as a near-field radiative heat engine that pumps heat from the cold to the hot bodies. The formalism reveals a fundamental symmetry relation in the radiative heat transfer coefficients that underlies these effects. Our results indicate the significant capabilities of time modulation for managing nanoscale heat flow.

Cavity quantum electrodynamics (QED), wherein a quantum emitter is coupled to electromagnetic cavity modes, is a powerful platform for implementing quantum sensors, memories, and networks. However, due to the fundamental tradeoff between gate fidelity and execution time, as well as limited scalability, the use of cavity-QED for quantum computation was overtaken by other architectures. Here, we introduce a new element into cavity-QED - a free charged particle, acting as a flying qubit. Using free electrons as a specific example, we demonstrate that our approach enables ultrafast, deterministic and universal discrete-variable quantum computation in a cavity-QED-based architecture, with potentially improved scalability. Our proposal hinges on a novel excitation blockade mechanism in a resonant interaction between a free-electron and a cavity polariton. This nonlinear interaction is faster by several orders of magnitude with respect to current photon-based cavity-QED gates, enjoys wide tunability and can demonstrate fidelities close to unity. Furthermore, our scheme is ubiquitous to any cavity nonlinearity, either due to light-matter coupling as in the Jaynes-Cummings model or due to photon-photon interactions as in a Kerr-type many-body system. In addition to promising advancements in cavity-QED quantum computation, our approach paves the way towards ultrafast and deterministic generation of highly-entangled photonic graph states and is applicable to other quantum technologies involving cavity-QED.

We show that the spatial coherence of thermal radiation can be manipulated in time-modulated photonic systems supporting surface polaritons. We develop a fluctuational electrodynamics formalism for such systems to calculate the cross-spectral density tensor of the emitted thermal electromagnetic fields in the near-field regime. Our calculations indicate that, due to time-modulation, spatial coherence can be transferred between different frequencies, and correlations between different frequency components become possible. All these effects are unique to time-modulated systems. We also show that the decay rate of optical emitters can be controlled in the proximity of such time-modulated structure. Our findings open a promising avenue toward coherence control in thermal radiation, dynamical thermal imaging, manipulating energy transfer among thermal or optical emitters, efficient near-field radiative cooling, and engineering spontaneous emission rates of molecules.

A system consisting of two slabs with different temperatures can exhibit a non-equilibrium lateral Casimir force on either one of the slabs when Lorentz reciprocity is broken in at least one of the slabs. This system constitutes a photonic heat engine that converts radiative heat into work done by the non-equilibrium lateral Casimir force. Reversely, by sliding two slabs at a sufficiently high relative velocity, heat is pumped from the slab at a lower temperature to the other one at a higher temperature. Hence the system operates as a photonic heat pump. In this work, we study the thermodynamic performance of such a photonic heat engine and pump via the fluctuational electrodynamics formalism. The propulsion force due to the non-reciprocity and the drag force due to the Doppler effect was revealed as the physical mechanism behind the heat engine. We also show that in the case of the heat pump, the use of nonreciprocal materials can help reduce the required velocity. We present an ideal material dispersion to reach the Carnot efficiency limit. Furthermore, we derive a relativistic version of the thermodynamic efficiency for our heat engine and prove that it is bounded by the Carnot efficiency that is independent of the frame of reference. Our work serves as a conceptual guide for the realization of photonic heat engines based on fluctuating electromagnetic fields and relativistic thermodynamics and shows the important role of electromagnetic non-reciprocity in operating them., Comment: 26 pages, 7 figures, and supplementary materials

Negative refraction provides an attractive platform to manipulate mid-infrared and terahertz radiation for molecular sensing and thermal radiation applications. However, its implementation based on available metamaterials and plasmonic media presents challenges associated with optical losses, limited spatial confinement, and lack of active tunability in this spectral range. Here, we demonstrate gate-tunable negative refraction at mid-infrared frequencies using hybrid topological polaritons in van der Waals heterostructures with high spatial confinement. We experimentally visualize wide-angle negatively-refracted surface polaritons on {\alpha}-MoO3 films partially decorated with graphene, undergoing planar nanoscale focusing down to 1.6% of the free-space wavelength. Our atomically thick heterostructures outperform conventional bulk materials by avoiding scattering losses at the refracting interface while enabling active tunability through electrical gating. We propose polaritonic negative refraction as a promising platform for infrared applications such as electrically tunable super-resolution imaging, nanoscale thermal manipulation, and molecular sensing.

Controlling the charge carrier density provides an efficient way to trigger phase transitions and modulate the optoelectronic properties in natural materials. This approach could be used to induce topological transitions in the optical response of photonic systems. Here, we predict a topological transition in the isofrequency dispersion contours of hybrid polaritons supported by a two-dimensional heterostructure consisting of graphene and $\alpha$-phase molybdenum trioxide ($\alpha$-MoO3). By chemically changing the doping level of graphene, we experimentally demonstrate that the contour topology of polariton isofrequency surfaces transforms from open to closed shapes as a result of doping-dependent polariton hybridization. Moreover, by changing the substrate medium for the heterostructure, the dispersion contour can be further engineered into a rather flattened shape at the topological transition, thus supporting tunable polariton canalization and providing the means to locally control the topology. We demonstrate this idea to achieve extremely subwavelength focusing by using a 1.2-$\mu$m-wide silica substrate as a negative refraction lens. Our findings open a disruptive approach toward promising on-chip applications in nanoimaging, optical sensing, and manipulation of nanoscale energy transfer.

Probing optical excitations with high resolution is important for understanding their dynamics and controlling their interaction with other photonic elements. This can be done using state-of-the-art electron microscopes, which provide the means to sample optical excitations with combined meV--sub-nm energy--space resolution. For reciprocal photonic systems, electrons traveling in opposite directions produce identical signals, while this symmetry is broken in nonreciprocal structures. Here, we theoretically investigate this phenomenon by analyzing electron energy-loss spectroscopy (EELS) and cathodoluminescence (CL) in structures consisting of magnetically biased InAs as an instance of gyrotropic nonreciprocal material. We find that the spectral features associated with excitations of InAs films depend on the electron propagation direction in both EELS and CL, and can be tuned by varying the applied magnetic field within a relatively modest sub-tesla regime. The magnetic field modifies the optical field distribution of the sampled resonances, and this in turn produces a direction-dependent coupling to the electron. The present results pave the way to the use of electron microscope spectroscopies to explore the near-field characteristics of nonreciprocal systems with high spatial resolution., Comment: 7 pages, 6 figures, 52 references

Transient optical heating provides an efficient way to trigger phase transitions in naturally occurring media through ultrashort laser pulse irradiation. A similar approach could be used to induce topological phase transitions in the photonic response of suitably engineered artificial structures known as metamaterials. Here, we predict a topological transition in the isofrequency dispersion contours of a layered graphene metamaterial under optical pumping. We show that the contour topology transforms from elliptic to hyperbolic within a subpicosecond timescale by exploiting the extraordinary photothermal properties of graphene. This new phenomenon allows us to theoretically demonstrate applications in engineering the decay rate of proximal optical emitters, ultrafast beam steering, and dynamical far-field subwavelength imaging. Our study opens a disruptive approach toward ultrafast control of light emission, beam steering, and optical image processing.

The ultrafast response of metals to light is governed by intriguing non-equilibrium dynamics involving the interplay of excited electrons and phonons. The coupling between them gives rise to nonlinear diffusion behavior on ultrashort timescales. Here, we use scanning ultrafast thermo-modulation microscopy to image the spatio-temporal hot-electron diffusion in a thin gold film. By tracking local transient reflectivity with 20 nm and 0.25 ps resolution, we reveal two distinct diffusion regimes, consisting of an initial rapid diffusion during the first few picoseconds after optical excitation, followed by about 100-fold slower diffusion at longer times. We simulate the thermo-optical response of the gold film with a comprehensive three-dimensional model, and identify the two regimes as hot-electron and phonon-limited thermal diffusion, respectively., Comment: 20 pages, 8 figures

The physics of electrons, photons, and their plasmonic interactions changes greatly when one or more dimensions are reduced down to the nanometer scale. For example, graphene shows unique electrical, optical, and plasmonic properties, which are tunable through gating or chemical doping. Similarly, ultrathin metal films (UTMFs) down to atomic thickness can possess new quantum optical effects, peculiar dielectric properties, and predicted strong plasmons. However, truly two-dimensional plasmonics in metals has so far 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-nanometer gold UTMFs, with clear evidence of new dispersion regimes and large electrical tunability. Resonance peaks at 1.5-5 micrometer wavelengths are shifted by hundreds of nanometers 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, bio-sensing, and smart windows., Comment: 11 pages, 3 figures

Nanoscale photothermal sources find important applications in theranostics, imaging, and catalysis. In this context, graphene offers a unique suite of optical, electrical, and thermal properties, which we exploit to show self-consistent active photothermal modulation of its nanoscale response. In particular, we predict the existence of plasmons confined to the optical landscape tailored by continuous-wave external-light pumping of homogeneous graphene. This result relies on the high electron temperatures achievable in optically pumped clean graphene while its lattice remains near ambient temperature. Our study opens a new avenue toward the active optical control of the nanophotonic response in graphene with potential application in photothermal devices., Comment: 11 pages, 12 figures

Optical excitation and subsequent decay of graphene plasmons can produce a significant increase in charge-carrier temperature. An efficient method to convert this temperature elevation into a measurable electrical signal at room temperature can enable important mid-infrared applications such as thermal sensing and imaging in ubiquitous mobile devices. However, as appealing as this goal might be, it is still unrealized due to the modest thermoelectric coefficient and weak temperature-dependence of carrier transport in graphene. Here, we demonstrate mid-infrared graphene detectors consisting of arrays of plasmonic resonators interconnected by quasi one-dimensional nanoribbons. Localized barriers associated with disorder in the nanoribbons produce a dramatic temperature dependence of carrier transport, thus enabling the electrical detection of plasmon decay in the nearby graphene resonators. We further realize a device with a subwavelength footprint of 5*5 um2 operating at 12.2 um, an external responsivity of 16 mA/W, a low noise-equivalent power of 1.3 nW/Hz1/2 at room temperature, and an operational frequency potentially beyond gigahertz. Importantly, our device is fabricated using large-scale graphene and possesses a simple two-terminal geometry, representing an essential step toward the realization of on-chip graphene mid-infrared detector arrays.

Plasmons --the collective oscillations of electrons in conducting materials-- play a pivotal role in nanophotonics because of their ability to couple electronic and photonic degrees of freedom. In particular, plasmons in graphene --the atomically thin carbon material-- offer strong spatial confinement and long lifetimes, accompanied by extraordinary optoelectronic properties derived from its peculiar electronic band structure. Understandably, this material has generated great expectations for its application to enhanced integrated devices. However, an efficient scheme for detecting graphene plasmons remains a challenge. Here we show that extremely compact graphene nanostructures are capable of realizing on-chip electrical detection of single plasmons. Specifically, we predict a twofold increase in the electrical current across a graphene nanostructure junction caused by the excitation of a single plasmon. This effect, which is due to the increase in electron temperature following plasmon decay, should persist during a picosecond time interval characteristic of electron-gas relaxation. We further show that a broad spectral detection range is accessible either by electrically doping the junction or by varying the size of the nanostructure. The proposed graphene plasmometer could find application as a basic component of future optics-free integrated nanoplasmonic devices.

The two-dimensionality of graphene and other layered materials can be exploited to simplify the theoretical description of their plasmonic and polaritonic modes. We present an analytical theory that allows us to simulate these excitations in terms of plasmon wave functions (PWFs). Closed-form expressions are offered for their associated extinction spectra, involving only two real parameters for each plasmon mode and graphene morphology, which we calculate and tabulate once and for all. Classical and quantum-mechanical formulations of this PWF formalism are introduced, in excellent mutual agreement for armchaired islands with $>10\,$nm characteristic size. Examples of application are presented to predict both plasmon-induced transparency in interacting nanoribbons and excellent sensing capabilities through the response to the dielectric environment. We argue that the PWF formalism has general applicability and allows us to analytically describe a wide range of 2D polaritonic behavior, thus facilitating their use for the design of actual devices.

Highly nonlinear optical processes, such as multiphoton photoemission, require high intensities, typically achieved with ultrashort laser pulses and, hence, were first observed with the advent of picosecond laser technology. An alternative approach for reaching the required field intensities is offered by localized optical resonances such as plasmons. Here, we demonstrate localized multiphoton photoemission from plasmonic nanostructures under continuous-wave illumination. We use synthesized plasmonic gold nanostars, which exhibit sharp tips with structural features smaller than 5 nm, leading to near-field-intensity enhancements exceeding 1000. This large enhancement facilitates 3-photon photoemission driven by a simple continuous-wave laser diode. We characterize the intensity and polarization dependencies of the photoemission yield from both individual nanostars and ensembles. Numerical simulations of the plasmonic enhancement, the near-field distributions, and the photoemission intensities are in good agreement with experiment. Our results open a new avenue for the design of nanoscale electron sources.

Fast modulation and switching of light at visible and near-infrared (vis-NIR) frequencies is of utmost importance for optical signal processing and sensing technologies. No fundamental limit appears to prevent us from designing wavelength-sized devices capable of controlling the light phase and intensity at gigaherts (and even terahertz) speeds in those spectral ranges. However, this problem remains largely unsolved, despite recent advances in the use of quantum wells and phase-change materials for that purpose. Here, we explore an alternative solution based upon the remarkable electro-optical properties of graphene. In particular, we predict unity-order changes in the transmission and absorption of vis-NIR light produced upon electrical doping of graphene sheets coupled to realistically engineered optical cavities. The light intensity is enhanced at the graphene plane, and so is its absorption, which can be switched and modulated via Pauli blocking through varying the level of doping. Specifically, we explore dielectric planar cavities operating under either tunneling or Fabry-Perot resonant transmission conditions, as well as Mie modes in silicon nanospheres and lattice resonances in metal particle arrays. Our simulations reveal absolute variations in transmission exceeding 90% as well as an extinction ratio >15 dB with small insertion losses using feasible material parameters, thus supporting the application of graphene in fast electro-optics at vis-NIR frequencies., Comment: 17 pages, 13 figures, 54 references

The excitation of localized or delocalized surface plasmon polaritons in nanostructured or extended graphene has attracted a steadily increasing attention due to their promising applications in sensors, switches, and filters. These single resonances may couple and intriguing spectral signatures can be achieved by exploiting the entailing hybridization. Whereas thus far only the coupling between localized or delocalized surface plasmon polaritons has been studied in graphene nanostructures, we consider here the interaction between a localized and a delocalized surface plasmon polariton. This interaction can be achieved by two different schemes that reside on either evanescent near- field coupling or far-field interference. All observable phenomena are corroborated by analytical considerations, providing insight into the physics and paving the way for compact and tunable optical components at infrared and terahertz frequencies., Comment: 6 pages, 4 figures