Tunable plasmons in atomically thin gold nanodisks

Chiral Light-Matter Interactions with Thermal Magnetoplasmons in Graphene Nanodisks

We investigate the emergence of self-hybridized thermal magnetoplasmons in doped graphene nanodisks at finite temperatures upon being subjected to an external magnetic field. Using a semianalytical approach, which fully describes the eigenmodes and polarizability of the graphene nanodisks, we show that the hybridization originates from the coupling of transitions between thermally populated Landau levels and localized magnetoplasmon resonances of the nanodisks. Owing to their origin, these modes combine the extraordinary magneto-optical response of graphene with the strong field enhancement of plasmons, making them an ideal tool for achieving strong chiral light-matter interactions, with the additional advantage of being tunable through carrier concentration, magnetic field, and temperature. As a demonstration of their capabilities, we show that the thermal magnetoplasmons supported by an array of graphene nanodisks enable chiral perfect absorption and chiral thermal emission.

Thickness-dependent optical properties of low-loss transdimensional plasmonic Sr0.82NbO3 thin films

To develop alternative plasmonic materials for nanophotonic applications, the thickness-dependent optical properties of ultrathin plasmonic Sr0.82NbO3 (SNO) films deposited on MgO are investigated. As the thickness decreases from 10 to 2 nm, the film exhibits less metallic, epsilon-near-zero (ENZ) wavelength redshift and higher optical loss due to increased scattering. Nevertheless, the thinnest film still has a high carrier concentration of 1022 cm-3, and the real part of the dielectric functions of all films is less than zero in the near-infrared (NIR) wavelength region, indicating that the samples possess relatively high metallicity and plasmonic characteristics in the NIR. It is found that the carrier concentration dominates the electron effective mass and Drude plasma frequency. Although Au is a commonly used plasmonic material, at a wavelength of 1550 nm, the loss of SNO is 85.8% lower than that of Au, and its plasmonic performance metrics is significantly higher than TiN, Al:ZnO and Sn:In2O3, demonstrating the great potential of SNO in NIR plasmonic device applications.

Voltage-controlled nonlinear optical properties in gold nanofilms via electrothermal effect

Dynamic control of the optical properties of gold nanostructures is crucial for advancing photonics technologies spanning optical signal processing, on-chip light sources and optical computing. Despite recent advances in tunable plasmons in gold nanostructures, most studies are limited to the linear or static regime, leaving the dynamic manipulation of nonlinear optical properties unexplored. This study demonstrates the voltage-controlled Kerr nonlinear optical response of gold nanofilms via the electrothermal effect. By applying relatively low voltages (~10 V), the nonlinear absorption coefficient and refractive index are reduced by 40.4% and 33.1%, respectively, due to the increased damping coefficient of gold nanofilm. Furthermore, a voltage-controlled all-fiber gold nanofilm saturable absorber is fabricated and used in mode-locked fiber lasers, enabling reversible wavelength-tuning and operation regimes switching (e.g., mode-locking-Q-switched mode-locking). These findings advance the understanding of electrically controlled nonlinear optical responses in gold nanofilms and offer a flexible approach for controlling fiber laser operations.

Large area single crystal gold of single nanometer thickness for nanophotonics

Two-dimensional single crystal metals, in which the behavior of highly confined optical modes is intertwined with quantum phenomena, are highly sought after for next-generation technologies. Here, we report large area (>104 μm2), single crystal two-dimensional gold flakes (2DGFs) with thicknesses down to a single nanometer level, employing an atomic-level precision chemical etching approach. The decrease of the thickness down to such scales leads to the quantization of the electronic states, endowing 2DGFs with quantum-confinement-augmented optical nonlinearity, particularly leading to more than two orders of magnitude enhancement in harmonic generation compared with their thick polycrystalline counterparts. The nanometer-scale thickness and single crystal quality makes 2DGFs a promising platform for realizing plasmonic nanostructures with nanoscale optical confinement. This is demonstrated by patterning 2DGFs into nanoribbon arrays, exhibiting strongly confined near infrared plasmonic resonances with high quality factors. The developed 2DGFs provide an emerging platform for nanophotonic research and open up opportunities for applications in ultrathin plasmonic, optoelectronic and quantum devices.

Ultraconfined Plasmons in Atomically Thin Crystalline Silver Nanostructures

The ability to confine light down to atomic scales is critical for the development of applications in optoelectronics and optical sensing as well as for the exploration of nanoscale quantum phenomena. Plasmons in metallic nanostructures with just a few atomic layers in thickness can achieve this type of confinement, although fabrication imperfections down to the subnanometer scale hinder actual developments. Here, narrow plasmons are demonstrated in atomically thin crystalline silver nanostructures fabricated by prepatterning silicon substrates and epitaxially depositing silver films of just a few atomic layers in thickness. Specifically, a silicon wafer is lithographically patterned to introduce on-demand lateral shapes, chemically process the sample to obtain an atomically flat silicon surface, and epitaxially deposit silver to obtain ultrathin crystalline metal films with the designated morphologies. Structures fabricated by following this procedure allow for an unprecedented control over optical field confinement in the near-infrared spectral region, which is here illustrated by the observation of fundamental and higher-order plasmons featuring extreme spatial confinement and high-quality factors that reflect the crystallinity of the metal. The present study constitutes a substantial improvement in the degree of spatial confinement and quality factor that should facilitate the design and exploitation of atomic-scale nanoplasmonic devices for optoelectronics, sensing, and quantum-physics applications.

Tuning of Ultra-Thin Gold Films by Photoreduction

Ultrathin metal films (UTMFs) are used in a wide range of applications, from transparent electrodes to infrared mirrors and metasurfaces. Due to their small thickness (<5 nm), the electrical and optical properties of UTMFs can be changed by external stimuli, for example, by applying an electric field through an ion gel. It is also known that oxidized thin films and nanostructures of Au can be reduced by irradiating with short-wavelength light. Here we show that the resistance, reflectance, and resonant optical response of Au UTMFs is changed significantly by ultraviolet light. More specifically, photoreduction and oxidation processes can be sequentially applied for continuous tuning, with observed modulation ranges for sheet resistance (Rs) and reflectance of more than 40% and 30%, respectively. The proposed method has the potential for achieving reconfigurable UTMF structures and trimming their response to specific working points, e.g., a predetermined resonance wavelength and amplitude. This is also important for large scale deployment of such surfaces as one can compensate material nonuniformity, morphological, and structural dimension errors occurring during fabrication.

Schottky barrier effect on plasmon-induced charge transfer

Plasmon-induced charge transfer causes electron-hole spatial separation at the metal-semiconductor interface, which plays a key role in photocatalytic and photovoltaic applications. The Schottky barrier formed at the metal-semiconductor interface can modify the hot carrier dynamics. Taking the Ag-TiO₂ system as an example, we have investigated plasmon-induced charge transfer at the Schottky junction using quantum mechanical simulations. We find that the Schottky barrier induced by n-type doping enhances the electron transfer and that induced by p-type doping enhances the hole transfer, which is attributed to the shift of the Fermi energy and the band bending of the Schottky junction at the interface. The Schottky barrier also modifies the layer distribution of hot carriers. In particular, for the system with a large band bending, there exists electron-hole spatial separation inside the TiO₂ substrate. Our results reveal the mechanism and dynamics of charge transfer at the Schottky junction, and pave the way for manipulating plasmon-assisted photocatalytic and photovoltaic applications.

Massive and massless plasmons in germanene nanosheets

Atomically thin crystals may exhibit peculiar dispersive electronic states equivalent to free charged particles of ultralight to ultraheavy masses. A rare coexistence of linear and parabolic dispersions yields correlated charge density modes exploitable for nanometric light confinement. Here, we use a time-dependent density-functional approach, under several levels of increasing accuracy, from the random-phase approximation to the Bethe-Salpeter equation formalism, to assess the role of different synthesized germanene samples as platforms for these plasmon excitations. In particular, we establish that both freestanding and some supported germenene monolayers can sustain infrared massless modes, resolved into an out-of-phase (optical) and an in-phase (acoustic) component. We further indicate precise experimental geometries that naturally host infrared massive modes, involving two different families of parabolic charge carriers. We thus show that the interplay of the massless and massive plasmons can be finetuned by applied extrinsic conditions or geometry deformations, which constitutes the core mechanism of germanene-based optoelectronic and plasmonic applications.

Active metasurface in the near-infrared region by gating ultrathin TiN films

Ultrathin titanium nitride (TiN) films have become a novel material flatform for constructing active metasurfaces in the near-infrared region. In this Letter, we numerically achieved the dual functions of switchable linear dichroism (LD) and tunable perfect absorption in a G-shape gold resonators/TiN film hybrid metasurface by gating ultrathin TiN films. As the carrier density of TiN decreases, the modulation depth for LD strength is about 70% at 1211 nm. Meanwhile, the response wavelength of perfect absorption (∼1) shifts to the blue by around 130 nm with a change of carrier density of 12%. Our proposed active metasurface with the capability of strength-switchable LD and wavelength-tunable perfect absorption has considerable potential in dynamic electro-optic modulation and flat photonic devices with reconfigurable functionalities.

Optimizing Plasmonic Gold Nanorod Deposition on Glass Surfaces for High-Sensitivity Refractometric Biosensing

Owing to high surface sensitivity, gold nanorods (AuNRs) are widely used to construct surface-based nanoplasmonic biosensing platforms for label-free molecular diagnostic applications. A key fabrication step involves controlling AuNR deposition onto the target surface, which requires maximizing surface density while minimizing inter-particle aggregation, and is often achieved by surface functionalization with a self-assembled monolayer (SAM) prior to AuNR deposition. To date, existing studies have typically used a fixed concentration of SAM-forming organic molecules (0.2-10% v/v) while understanding how SAM density affects AuNR deposition and resulting sensing performance would be advantageous. Herein, we systematically investigated how controlling the (3-aminopropyl)triethoxysilane (APTES) concentration (1-30% v/v) during SAM preparation affects the fabrication of AuNR-coated glass surfaces for nanoplasmonic biosensing applications. Using scanning electron microscopy (SEM) and UV-visible spectroscopy, we identified an intermediate APTES concentration range that yielded the highest density of individually deposited AuNRs with minimal aggregation and also the highest peak wavelength in aqueous solution. Bulk refractive index sensitivity measurements indicated that the AuNR configuration had a strong effect on the sensing performance, and the corresponding wavelength-shift responses ranged from 125 to 290 nm per refractive index unit (RIU) depending on the APTES concentration used. Biosensing experiments involving protein detection and antigen-antibody interactions further demonstrated the high surface sensitivity of the optimized AuNR platform, especially in the low protein concentration range where the measurement shift was ~8-fold higher than that obtained with previously used sensing platforms.

Microwave Synthesized 2D Gold and Its 2D-2D Hybrids

Xenes, i.e., monoelemental two-dimensional atomic sheets, are promising for sensitive and ultrafast sensor applications owing to exceptional carrier mobility; however, most of them oxidize below 500 °C and therefore cannot be employed for high-temperature applications. 2D gold, an oxidation-resistant plasmonic Xene, is extremely promising. 2D gold was experimentally realized by both atomic layer deposition and chemical synthesis using sodium citrate. However, it is imperative to develop a new facile single-step method to synthesize 2D gold. Here, liquid-phase synthesis of 2D gold is demonstrated by microwave exposure to auric chloride dispersed in dimethylformamide. Microscopies (AFM and high-resolution TEM), spectroscopies (Raman, UV-vis, and X-ray photoelectron), and X-ray diffraction establish the formation of a hexagonal crystallographic phase for 2D gold. 2D-2D hybrids of 2D gold have also been synthesized and investigated for electronic/optoelectronic behaviors and SERS-based molecular sensing. DFT band structure calculation for 2D gold and its hybrids corroborates the experimental findings.

Effect of light polarization on plasmon-induced charge transfer

Plasmonic nanoclusters can strongly absorb light energy and generate hot carriers, which have great potentials in photovoltaic and photocatalytic applications. A vital step for those plasmonic applications is the charge transfer at the metal–semiconductor interface. The effect of the light polarization on the charge transfer has not been theoretically investigated so far. Here, we take the Ag–TiO2 system as a model system to study the polarization effect using time-dependent density functional theory simulations. We find that the charge transfer is sensitive to the light polarization, which has its origin in the polarization-dependent hot carrier distributions. For the linearly polarized light, it shows a sine-square dependence on the polar angle, indicating that the charge transfer response to the linear polarization can be decomposed into components perpendicular and parallel to the interface. We also find that there exists directional charge transfer with a circular light polarization. Our results demonstrate that the light polarization can significantly affect the charge transfer behavior and, thus, offer a new degree of freedom to manipulate the plasmonic applications.

Thickness-Dependent Drude Plasma Frequency in Transdimensional Plasmonic TiN

Plasmonic transdimensional materials (TDMs), which are atomically thin metals of precisely controlled thickness, are expected to exhibit large tailorability and dynamic tunability of their optical response as well as strong light confinement and nonlocal effects. Using spectroscopic ellipsometry, we characterize the complex permittivity of ultrathin films of passivated plasmonic titanium nitride with thicknesses ranging from 1 to 10 nm. By measuring passivated TiN, we experimentally distinguish between the contributions of an oxide layer and thickness to the optical properties. A decrease in the Drude plasma frequency and increase in the damping in thinner films is observed due to spatial confinement. We explain the experimental trends using a nonlocal Drude dielectric response theory based on the Keldysh-Rytova (KR) potential that predicts the thickness-dependent optical properties caused by electron confinement in plasmonic TDMs. Our experimental findings are consistent with the KR model and demonstrate quantum-confinement-induced optical properties in plasmonic transdimensional TiN.

Active control of dielectric nanoparticle optical resonance through electrical charging

A novel method for active control of resonance position of dielectric nanoparticles by increasing the excess charges carried by the nanoparticles is proposed in this paper. We show that as the excess charges carried by the particle increase, the oscillation frequency of excess charges will gradually increase, when it is equal to the incident frequency, resonance occurs due to resonant excitation of the excess charges. What is more, the formula of charges carried by an individual particle required to excite the resonance at any wavelength position is proposed. The resonance position can be directly controlled by means of particle charging, and the enhancement of resonance intensity is more obvious. This work has opened new avenues for the active control of plasmon resonances, which shows great promise for realizing tunable optical properties of dielectric nanoparticles.

Photo-modulated optical and electrical properties of graphene

Photo-modulation is a promising strategy for contactless and ultrafast control of optical and electrical properties of photoactive materials. Graphene is an attractive candidate material for photo-modulation due to its extraordinary physical properties and its relevance to a wide range of devices, from photodetectors to energy converters. In this review, we survey different strategies for photo-modulation of electrical and optical properties of graphene, including photogating, generation of hot carriers, and thermo-optical effects. We briefly discuss the role of nanophotonic strategies to maximize these effects and highlight promising fields for application of these techniques.

Atomic-Level Structure Determines Electron-Phonon Scattering Rates in 2-D Polar Metal Heterostructures

The electron dynamics of atomically thin 2-D polar metal heterostructures, which consisted of a few crystalline metal atomic layers intercalated between hexagonal silicon carbide and graphene grown from the silicon carbide, were studied using nearly degenerate transient absorption spectroscopy. Optical pumping created charge carriers in both the 2-D metals and graphene components. Wavelength-dependent probing suggests that graphene-to-metal carrier transfer occurred on a sub-picosecond time scale. Following rapid (<300 fs) carrier-carrier scattering, charge carriers monitored through the metal interband transition relaxed through several consecutive cooling mechanisms that included sub-picosecond carrier-phonon scattering and dissipation to the silicon carbide substrate over tens of picoseconds. By studying 2-D In, 2-D Ga, and a Ga/In alloy, we resolved accelerated electron-phonon scattering rates upon alloy formation as well as structural influences on the excitation of in-plane phonon shear modes. More rapid cooling in alloys is attributed to increased lattice disorder, which was observed through correlative polarization-resolved second harmonic generation and electron microscopy. This connection between the electronic relaxation rates, far-field optical responses, and metal lattice disorder is made possible by the intimate relation between nonlinear optical properties and atomic-level structure in these materials. These studies provided insights into electronic carrier dynamics in 2-D crystalline elemental metals, including resolving contributions from specific components of a 2-D metal-containing heterojunction. The correlative ultrafast spectroscopy and nonlinear microscopy results suggest that the energy dissipation rates can be tuned through atomic-level structures.

Nonlinear plasmonic response in atomically thin metal films

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.

Ultrathin Metals on a Transparent Seed and Application to Infrared Reflectors

Ultrathin metal films (UTMFs) are widely used in optoelectronic applications, from transparent conductors to photovoltaic cells, low emissivity windows, and plasmonic metasurfaces. During the initial deposition phase, many metals tend to form islands on the receiving substrate rather than a physically connected (percolated) network, which eventually evolves into continuous films as the thickness increases. For example, in the case of Ag and Au on dielectric surfaces, percolation begins when the thickness of the metal film is at least about 5 nm. It is known that the type of growth can be changed when a proper seed layer is used. Here, we show that a CuO layer directly deposited on a substrate can dramatically influence surface wetting and promote early percolation of polycrystalline Ag and Au UTMFs. We demonstrate that the proposed seed is effective even when its thickness is sub-nanometric, in this way maintaining the full transparency of the receiving substrate. The Ag and Au films seeded with CuO showed a percolation thickness close to 1 nm and were morphologically and optically characterized from an ultraviolet (λ = 300 nm) to a midinfrared (λ = 15 μm) wavelength. Infrared reflectors, a mirror and a resonant plasmonic structure, were also demonstrated and uniquely tuned by electrical gating, this being possible owing to the small thickness of the constituting Au UTMF.

Nontrivial, Unconventional Electrochromic Behaviors of Plasmonic Nanocubes

Plasmonic electrochromism, a change in the localized surface plasmon resonance (LSPR) with an applied electric potential, has been attracting increasing attention for the development of spectroscopic tools or optoelectronic systems. There is a consensus on the mechanism of plasmonic electrochromism based on the classical capacitor and the Drude model. However, the electrochromic behaviors of metallic nanoparticles in narrow optical windows have been demonstrated only with small monotonic LSPR shifts, which limits the use of the electrochromism. Here, we observed three distinct electrochromic behaviors of gold nanocubes with a wide potential range through in situ dark-field electrospectroscopy. Interestingly, the nanocubes show a faster frequency shift under the highly negative potential, and this opens the possibility of largely tunable electrochromic LSPR shifts. The reversibility of the electrochemical switching with these cubes are also shown. We attribute this unexpected change beyond classical understandings to the material-specific quantum mechanical electronic structures of the plasmonic materials.

Accessing Plasmonic Hotspots Using Nanoparticle-on-Foil Constructs

Metal-insulator-metal (MIM) nanogaps in the canonical nanoparticle-on-mirror geometry (NPoM) provide deep-subwavelength confinement of light with mode volumes smaller than V/V _λ < 10^-6. However, access to these hotspots is limited by the impendence mismatch between the high in-plane k _∥ of trapped light and free-space plane-waves, making the in- and out-coupling of light difficult. Here, by constructing a nanoparticle-on-foil (NPoF) system with thin metal films, we show the mixing of insulator-metal-insulator (IMI) modes and MIM gap modes results in MIMI modes. This mixing provides multichannel access to the plasmonic nanocavity through light incident from both sides of the metal film. The red-tuning and near-field strength of MIMI modes for thinner foils is measured experimentally with white-light scattering and surface-enhanced Raman scattering from individual NPoFs. We discuss further the utility of NPoF systems, since the geometry allows tightly confined light to be accessed simply through different ports.

Electrostatically Tunable Near-Infrared Plasmonic Resonances in Solution-Processed Atomically Thin NbSe2

In the broad spectral range, near-infrared (NIR) plasmonics find applications in telecommunication, energy harvesting, sensing, and more, all of which would benefit from an electrostatically controllable NIR plasmon source. However, it is difficult to control bulk NIR plasmonics directly with electrostatics because of the strong electric-field screening effect and high carrier concentration required to support NIR plasmons. Here, this constraint is overcome and the observation of NIR plasmonic resonances that can be modulated electrostatically over a range of ≈360 cm^-1 in few-layer NbSe₂ gratings is reported, thanks to the enhanced electrostatics of atomically thin 2D materials and the high-quality film produced by a solution method. NbSe₂ plasmons also render strong field confinement due to their atomic thickness and provide an extra degree of resonance frequency modulation from the layered structure. This study identifies metallic 2D materials as promising (easily produced and well-performing) candidates to extend electrostatically tunable plasmonics to the technologically important NIR range.

Electrochromic response and control of plasmonic metal nanoparticles

Plasmonic electrochromism, the dependence of the colour of plasmonic materials on the applied electrical potential, has been under the spotlight recently as a key element for the development of optoelectronic devices and spectroscopic tools. In this review, we focus on the electrochromic behaviour and underlying mechanistic principles of plasmonic metal nanoparticles, whose localised surface plasmon resonance occurs in the visible part of the electromagnetic spectrum, and present a comprehensive review on the recent progress in understanding and controlling plasmonic electrochromism. The mechanisms underlying the electrochromism of plasmonic metal nanoparticles could be divided into four categories, based on the origin of the LSPR shift: (1) capacitive charging model accompanying variation in the Fermi level, (2) faradaic reactions, (3) non-faradaic reactions, and (4) electrochemically active functional molecule-mediated mechanism. We also review recent attempts to synchronise the simulation with the experimental results and the strategies to overcome the intrinsically diminutive LSPR change of the plasmonic metal nanoparticles. A better understanding and controllability of plasmonic electrochromism provides new insights into and means of the connection between photoelectrochemistry and plasmonics as well as future directions for producing advanced optoelectronic materials and devices.

Anapole-assisted giant electric field enhancement for surface-enhanced coherent anti-Stokes Raman spectroscopy

The coherent anti-Stokes Raman spectroscopy (CARS) techniques are recognized for their ability to detect and identify vibrational coherent processes down to the single-molecular levels. Plasmonic oligomers supporting full-range Fano-like line profiles in their scattering spectrum are one of the most promising class of substrates in the context of surface-enhanced (SE) CARS application. In this work, an engineered assembly of metallic disk-shaped nanoparticles providing two Fano-like resonance modes is presented as a highly-efficient design of SECARS substrate. We show that the scattering dips corresponding to the double-Fano spectral line shapes are originated from the mutual interaction of electric and toroidal dipole moments, leading to the so-called non-trivial first- and second-order anapole states. The anapole modes, especially the higher-order ones, can result in huge near-field enhancement due to their light-trapping capability into the so-called "hot spots". In addition, independent spectral tunability of the second Fano line shape is exhibited by modulating the gap distance of the corner particles. This feature is closely related to the electric current loop associated with the corner particles in the second-order anapole state and provides a simple design procedure of an optimum SECARS substrate, where the electric field hot spots corresponding to three involved wavelengths, i.e., anti-Stokes, pump, and Stokes, are localized at the same spatial position. These findings yield valuable insight into the plasmonic substrate design for SECARS applications as well as for other nonlinear optical processes, such as four-wave mixing and multi-photon surface spectroscopy.

Enhancing Plasmonic Spectral Tunability with Anomalous Material Dispersion

The field confinement of plasmonic systems enables spectral tunability under structural variations or environmental perturbations, which is the principle for various applications including nanorulers, sensors, and color displays. Here, we propose and demonstrate that materials with anomalous dispersion, such as Ge in the visible, improve spectral tunability. We introduce our proposal with a semianalytical guided mode picture. Using Ge-based film (Ag/Au)-coupled gap plasmon resonators, we implement two architectures and demonstrate the improved tunability with single-particle dark-field scattering, ensemble reflection, and color generation. We observe three-fold enhancement of tunability with Ge nanodisks compared with that of Si, a normal-dispersion material in the visible. The structural color generation of large array systems, made of inversely fabricated Ge-Ag resonators, exhibits a wide gamut. Our results introduce anomalous material dispersion as an extra degree of freedom to engineer the spectral tunability of plasmonic systems, especially relevant for actively tunable plasmonics and metasurfaces.

Silicon carbide as a material-based high-impedance surface for enhanced absorption within ultra-thin metallic films

The absorption of infrared radiation within ultra-thin metallic films is technologically relevant for different thermal engineering applications and optoelectronic devices, as well as for fundamental research on sub-nanometer and atomically-thin materials. However, the maximal attainable absorption within an ultra-thin metallic film is intrinsically limited by both its geometry and material properties. Here, we demonstrate that material-based high-impedance surfaces enhance the absorptivity of the films, potentially leading to perfect absorption for optimal resistive layers, and a fourfold enhancement for films at deep nanometer scales. Moreover, material-based high-impedance surfaces do not suffer from spatial dispersion and the geometrical restrictions of their metamaterial counterparts. We provide a proof-of-concept experimental demonstration by using titanium nanofilms on top of a silicon carbide substrate.

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.

Two-Photon Spontaneous Emission in Atomically Thin Plasmonic Nanostructures

The ability to harness light-matter interactions at the few-photon level plays a pivotal role in quantum technologies. Single photons-the most elementary states of light-can be generated on demand in atomic and solid state emitters. Two-photon states are also key quantum assets, but achieving them in individual emitters is challenging because their generation rate is much slower than competing one-photon processes. We demonstrate that atomically thin plasmonic nanostructures can harness two-photon spontaneous emission, resulting in giant far field two-photon production, a wealth of resonant modes enabling tailored photonic and plasmonic entangled states, and plasmon-assisted single-photon creation orders of magnitude more efficient than standard one-photon emission. We unravel the two-photon spontaneous emission channels and show that their spectral line shapes emerge from an intricate interplay between Fano and Lorentzian resonances. Enhanced two-photon spontaneous emission in two-dimensional nanostructures paves the way to an alternative efficient source of light-matter entanglement for on-chip quantum information processing and free-space quantum communications.

Infrared Nanoimaging of Surface Plasmons in Type-II Dirac Semimetal PtTe2 Nanoribbons

Topological Dirac semimetals made of two-dimensional transition-metal dichalcogenides (TMDCs) have attracted enormous interest for use in electronic and optoelectronic devices because of their electron transport properties. As van der Waals materials with a strong interlayer interaction, these semimetals are expected to support layer-dependent plasmonic polaritons yet to be revealed experimentally. Here, we demonstrate the apparent retardation and attenuation of mid-infrared (MIR) plasmonic waves in type-II Dirac semimetal platinum tellurium (PtTe₂) nanoribbons and nanoflakes by near-field nanoimaging. The attenuated dispersion relations for the plasmonic modes in the PtTe₂ nanoribbons (15-25 nm thick) extracted from the near-field standing-wave patterns are applied for the fitting of PtTe₂ permittivity in the MIR regime, indicating that both free carriers and Dirac fermions are involved in MIR light-matter interaction in PtTe₂. The annihilation of plasmonic modes in the ultrathin (<10 nm) PtTe₂ is observed and analyzed, which manifests no near-field resonant pattern due to the intrinsic layer-dependent optoelectronic properties of PtTe₂. These results could pave a potential wave for MIR photodetection and modulation with TMDC semimetals.

Thermal manipulation of plasmons in atomically thin films

Nanoscale photothermal effects enable important applications in cancer therapy, imaging and catalysis. These effects also induce substantial changes in the optical response experienced by the probing light, thus suggesting their application in all-optical modulation. Here, we demonstrate the ability of graphene, thin metal films, and graphene/metal hybrid systems to undergo photothermal optical modulation with depths as large as >70% over a wide spectral range extending from the visible to the terahertz frequency domains. We envision the use of ultrafast pump laser pulses to raise the electron temperature of graphene during a picosecond timescale in which its mid-infrared plasmon resonances undergo dramatic shifts and broadenings, while visible and near-infrared plasmons in the neighboring metal films are severely attenuated by the presence of hot graphene electrons. Our study opens a promising avenue toward the active photothermal manipulation of the optical response in atomically thin materials with potential applications in ultrafast light modulation.

Functional Charge Transfer Plasmon Metadevices

Reducing the capacitive opening between subwavelength metallic objects down to atomic scales or bridging the gap by a conductive path reveals new plasmonic spectral features, known as charge transfer plasmon (CTP). We review the origin, properties, and trending applications of this modes and show how they can be well-understood by classical electrodynamics and quantum mechanics principles. Particularly important is the excitation mechanisms and practical approaches of such a unique resonance in tailoring high-response and efficient extreme-subwavelength hybrid nanophotonic devices. While the quantum tunneling-induced CTP mode possesses the ability to turn on and off the charge transition by varying the intensity of an external light source, the excited CTP in conductively bridged plasmonic systems suffers from the lack of tunability. To address this, the integration of bulk plasmonic nanostructures with optothermally and optoelectronically controllable components has been introduced as promising techniques for developing multifunctional and high-performance CTP-resonant tools. Ultimate tunable plasmonic devices such as metamodulators and metafilters are thus in prospect.

Plasmon-Induced Electron-Hole Separation at the Ag/TiO2(110) Interface

Plasmon-induced electron-hole separation at metal-semiconductor interfaces is an essential step in photovoltaics, photochemistry, and optoelectronics. Despite its importance in fundamental understandings and technological applications, the mechanism and dynamics of the charge separation under plasmon excitations have not been well understood. Here, the plasmon-induced charge separation between a Ag₂₀ nanocluster and a TiO₂(110) surface is investigated using time-dependent density functional theory simulations. It is found that the charge separation dynamics consists of two processes: during the first 10 fs an initial charge separation resulting from the plasmon-electron coupling at the interface and a subsequent charge redistribution governed by the sloshing motion of the charge-transfer plasmon. The interplay between the two processes determines the charge separation and leads to the inhomogeneous layer-dependent distribution of hot carriers. The hot electrons are more efficient than the hot holes in the charge injection, resulting in the charge separation. Over 40% of the hot electron-hole pairs are separated spatially from the interface. Finally, the second TiO₂ layer receives the most net charges from the Ag nanocluster rather than the interfacial layer. These results reveal the mechanism and dynamics of the charge separation driven by the surface plasmon excitation and have broad implications in plasmonic applications.

Giant near-field radiative heat transfer between ultrathin metallic films

Understanding energy transfer via near-field thermal radiation is essential for applications such as near-field imaging, thermophotovoltaics and thermal circuit devices. Evanescent waves and photon tunneling are responsible for the near-field energy transfer. In bulk noble metals, however, surface plasmons do not contribute efficiently to the near-field energy transfer because of the mismatch of wavelength. In this paper, a giant near-field radiative heat transfer rate that is orders-of-magnitude greater than the blackbody limit between two ultrathin metallic films is demonstrated at nanoscale separations. Moreover, different physical origins for near-field thermal radiation transfer for thick and thin metallic films are clarified, and the radiative heat transfer enhancement in ultrathin metallic films is proved to come from the excitation of surface plasmons. Meanwhile, because of the inevitable high sheet resistance of ultrathin metal films, the heat transfer coefficient is 4600 times greater than the Planckian limit for the separation of 10 nm in ultrathin metallic films, which is the same order or even greater than that in other 2D materials with low carrier density. Our work shows that ultrathin metallic films are excellent materials for radiative heat transfer, which may find promising applications in thermal nano-devices and thermal engineering.

Multi-frequency near-field enhancement with graphene-coated nano-disk homo-dimers

In this paper, a 3D sub-wavelength graphene-coated nano-disk dimer (GDD) is proposed for multi-frequency giant near-field enhancement. We observed that the dual-band operation originates from the excitation of hybridized localized surface plasmons on top and bottom faces of the disks along with the mutual coupling from the adjacent particle. Due to the sub-wavelength nature of the disks, the excited localized surface plasmons on the sidewalls are weak but they still can affect the dual operating bands. On the other hand, the strength and resonance frequency of the enhanced fields can be simply modulated by tuning the relative distances of 2D graphene disks on top and bottom faces. Adjustable dual-band performance is hardly attainable using simplified 2D graphene disks, however, it naturally comes out through modal hybridization in the subwavelength 3D structure containing multiple resonant units. Our suggested configuration has better optical properties than its noble metal counterparts because of its higher field enhancement and lower ohmic losses. Moreover, the electromagnetic response is reconfigurable by varying the bias voltage. The influence of graphene quality, chemical potential, and dimer gap size on the electric field enhancement and the resonance frequency of the surface plasmons are investigated, as well. To further improve its performance, a double negative metamaterial core is considered. This mechanism of the performance improvement by the core material is feasible thanks to the 3D nature of the structure. Two possible applications of the presented design are in Surface-Enhanced Raman Spectroscopy (SERS) and optical absorbers.

Circular Dichroism in Rotating Particles

Light interaction with rotating nanostructures gives rise to phenemona as varied as optical torques and quantum friction. Surprisingly, the most basic optical response function of nanostructures undergoing rotation has not been clearly addressed so far. Here we reveal that mechanical rotation results in circular dichroism in optically isotropic particles, which show an unexpectedly strong dependence on the particle internal geometry. More precisely, particles with one-dimensionally confined electron motion in the plane perpendicular to the rotation axis, such as nanorings and nanocrosses, exhibit a splitting of 2Ω in the particle optical resonances, while compact particles, such as nanodisks and nanospheres, display weak circular dichroism. We base our findings on a quantum-mechanical description of the polarizability of rotating particles, incorporating the mechanical rotation by populating the particle electronic states according to the principle that they are thermally equilibrated in the rotating frame. We further provide insight into the rotational superradience effect and the ensuing optical gain, originating in population inversion as regarded from the lab frame, in which the particle is out of equilibrium. Surprisingly, we find the optical frequency cutoff for superradiance to deviate from the rotation frequency Ω. Our results unveil a rich, unexplored phenomenology of light interaction with rotating objects, which might find applications in various fields, such as optical trapping and sensing.

Plasmonics in Atomically Thin Crystalline Silver Films

Light-matter interaction at the atomic scale rules fundamental phenomena such as photoemission and lasing while enabling basic everyday technologies, including photovoltaics and optical communications. In this context, plasmons, the collective electron oscillations in conducting materials, are important because they allow the manipulation of optical fields at the nanoscale. The advent of graphene and other two-dimensional crystals has pushed plasmons down to genuinely atomic dimensions, displaying appealing properties such as a large electrical tunability. However, plasmons in these materials are either too broad or lying at low frequencies, well below the technologically relevant near-infrared regime. Here, we demonstrate sharp near-infrared plasmons in lithographically patterned wafer-scale atomically thin silver crystalline films. Our measured optical spectra reveal narrow plasmons (quality factor of ∼4), further supported by a low sheet resistance comparable to bulk metal in few-atomic-layer silver films down to seven Ag(111) monolayers. Good crystal quality and plasmon narrowness are obtained despite the addition of a thin passivating dielectric, which renders our samples resilient to ambient conditions. The observation of spectrally sharp and strongly confined plasmons in atomically thin silver holds great potential for electro-optical modulation and optical sensing applications.

Anomalous phonon relaxation in Au333(SR)79 nanoparticles with nascent plasmons

Research on plasmons of gold nanoparticles has gained broad interest in nanoscience. However, ultrasmall sizes near the metal-to-nonmetal transition regime have not been explored until recently due to major synthetic difficulties. Herein, intriguing electron dynamics in this size regime is observed in atomically precise Au₃₃₃(SR)₇₉ nanoparticles. Femtosecond transient-absorption spectroscopy reveals an unprecedented relaxation process of 4-5 ps-a fast phonon-phonon relaxation process, together with electron-phonon coupling (∼1 ps) and normal phonon-phonon coupling (>100 ps) processes. Three types of -R capped Au₃₃₃(SR)₇₉ all exhibit two plasmon-bleaching signals independent of the -R group as well as solvent, indicating plasmon splitting and quantum effect in the ultrasmall core of Au₃₃₃(SR)₇₉ This work is expected to stimulate future work on the transition-size regime of nanometals and discovery of behavior of nascent plasmons.

Modulating phase by metasurfaces with gated ultra-thin TiN films

Active control over the flow of light is highly desirable because of its applicability to information processing, telecommunication, and spectroscopic imaging. In this paper, by employing the tunability of carrier density in a 1 nm titanium nitride (TiN) film, we numerically demonstrate deep phase modulation (PM) in an electrically tunable gold strip/TiN film hybrid metasurface. A 337° PM is achieved at 1.550 μm with a 3% carrier density change in the TiN film. We also demonstrate that a continuous 180° PM can be realized at 1.537 μm by applying a realistic experiment-based gate voltage bias and continuously changing the carrier density in the TiN film. The proposed design of active metasurfaces capable of deep PM near the wavelength of 1.550 μm has considerable potential in active beam steering, dynamic hologram generation, and flat photonic devices with reconfigurable functionalities.

Plasmons of hollow nanobar oligomers

Assembling metal nano-objects into well-defined configurations is an effective way to create hybrid plasmonic structures with unusual functionalities.

A quantum mechanical study of optical excitations in nanodisk plasmonic oligomers

Using state-of-the-art quantum-mechanical calculations, we investigate the optical excitations in plasmonic nanodisk oligomers.

Coherent perfect absorption and asymmetric interferometric light-light control in graphene with resonant dielectric nanostructures

Engineering light absorption in graphene has been the focus of intensive research in the past few years. In this paper, we show numerically that coherent perfect absorption can be realized in monolayer graphene in the near infrared range by harnessing resonances of dielectric nanostructures. The asymmetry of the structure results in different optical responses for light illuminated from the top side and the substrate side and enables asymmetric interferometric light-light control. The absorbed and scattered light exhibit interesting nonlinear behavior, allowing switching a strong optical signal output with a weak light. This work may stimulate potential applications including new types of sensors, coherent photodetectors and all-optical logical devices.

Plasmon-induced nonlinear response of silver atomic chains

Nonlinear response of a linear silver atomic chain upon ultrafast laser excitation has been studied in real time using the time-dependent density functional theory. We observe the presence of nonlinear responses up to the fifth order in tunneling current, which is ascribed to the excitation of high-energy electrons generated by Landau damping of plasmons. The nonlinear effect is enhanced after adsorption of polar molecules such as water due to the enhanced damping rates during plasmon decay. Increasing the length of atomic chains also increases the nonlinear response, favoring higher-order plasmon excitation. These findings offer new insights towards a complete understanding and ultimate control of plasmon-induced nonlinear phenomena to atomic precision.

Optical properties of magnesium nanorods using time dependent density functional theory calculations

Plasmonic nanostructures made of Earth-abundant and low-cost metals such as aluminum and magnesium have recently emerged as a potential alternative candidate to conventional plasmonic metals such as gold and silver.

High-Quality Ultrathin Gold Layers with an APTMS Adhesion for Optimal Performance of Surface Plasmon Polariton-Based Devices

A low-absorption adhesion layer plays a crucial role for both localized and propagating surface plasmons when ultrathin gold is used. To date, the most popular adhesion layers are metallic, namely, Cr and Ti. However, to the best of our knowledge, the influence of these adhesion layers on the behavior of propagating plasmon modes has not been thoroughly investigated nor reported in the literature. It is therefore important to study the effect of these few- to several-nanometers-thick adhesion layers on the propagating plasmons because it may affect the performance of plasmonic devices, in particular, when the Au layer is not much thicker than the adhesion layers. We experimentally compared the performances of the ultrathin gold films to show the pivotal influence of adhesion layers on highly confined propagating plasmonic modes, using Cr and 3-aminopropyl trimethoxysilane (APTMS) adhesion layers and without any adhesion layer. We show that the gold films with the APTMS adhesion layer have the lowest surface roughness and the short-range surface plasmon polaritons supported on the Au surface exhibit properties close to the theoretical calculations, considering an ideal gold film.

How To Identify Plasmons from the Optical Response of Nanostructures

A promising trend in plasmonics involves shrinking the size of plasmon-supporting structures down to a few nanometers, thus enabling control over light-matter interaction at extreme-subwavelength scales. In this limit, quantum mechanical effects, such as nonlocal screening and size quantization, strongly affect the plasmonic response, rendering it substantially different from classical predictions. For very small clusters and molecules, collective plasmonic modes are hard to distinguish from other excitations such as single-electron transitions. Using rigorous quantum mechanical computational techniques for a wide variety of physical systems, we describe how an optical resonance of a nanostructure can be classified as either plasmonic or nonplasmonic. More precisely, we define a universal metric for such classification, the generalized plasmonicity index (GPI), which can be straightforwardly implemented in any computational electronic-structure method or classical electromagnetic approach to discriminate plasmons from single-particle excitations and photonic modes. Using the GPI, we investigate the plasmonicity of optical resonances in a wide range of systems including: the emergence of plasmonic behavior in small jellium spheres as the size and the number of electrons increase; atomic-scale metallic clusters as a function of the number of atoms; and nanostructured graphene as a function of size and doping down to the molecular plasmons in polycyclic aromatic hydrocarbons. Our study provides a rigorous foundation for the further development of ultrasmall nanostructures based on molecular plasmonics.

Spectral Response of Plasmonic Gold Nanoparticles to Capacitive Charging: Morphology Effects

We report a study of the shape-dependent spectral response of the gold nanoparticle surface plasmon resonance at various electron densities to provide mechanistic insight into the role of capacitive charging, a topic of some debate. We demonstrate a morphology-dependent spectral response for gold nanoparticles due to capacitive charging using single-particle spectroscopy in an inert electrochemical environment. A decrease in plasmon energy and increase in spectral width for gold nanospheres and nanorods was observed as the electron density was tuned through a potential window of -0.3 to 0.1 V. The combined observations could not be explained by existing theories. A new quantum theory for charging based on the random phase approximation was developed. Additionally, the redox reaction of gold oxide formation was probed using single-particle plasmon voltammetry to reproduce the reduction peak from the bulk cyclic voltammetry. These results deepen our understanding of the relationship between optical and electronic properties in plasmonic nanoparticles and provide insight toward their potential applications in directed electrocatalysis.

Quantum effects in the plasmon response of bimetallic core-shell nanostructures

We report a quantum mechanical study of the plasmonic response of bimetallic spherical core/shell nanoparticles. The systems comprise up to 10⁴ electrons and their optical response is addressed with Time Dependent Density Functional Theory calculations. These quantum results are compared with classical electromagnetic calculations for core/shell systems formed by Al/Na, Al/Au and Ag/Na, as representative examples of bimetallic systems. We show that for shell widths in the nanometer range, the system cannot be described as a simple stack of two metals. The finite size effect and the transition layer formed between the core and the shell strongly modify the optical properties of the compound nanoparticle. In particular this configuration leads to a frequency shift of the plasmon resonance with shell character and an increased plasmon decay into electron-hole pairs which eventually quenches this resonance for very thin shells. This effect is difficult to capture with a classical theory even upon adjustment of the parameters of a combination of metallic dielectric functions.

Active modulation of visible light with graphene-loaded ultrathin metal plasmonic antennas

Electro-optical modulation of visible and near-infrared light is important for a wide variety of applications, ranging from communications to sensing and smart windows. However, currently available approaches result in rather bulky devices, suffer from low integrability, and can hardly operate at the low power consumption levels and fast switching rates required by microelectronic drivers. Here we show that planar nanostructures patterned in ultrathin metal-graphene hybrid films sustain highly tunable plasmons in the visible and near-infrared spectral regions. Strong variations in the reflection and absorption of incident light take place when the plasmons are tuned on- and off-resonance with respect to externally incident light. As a result, a remarkable modulation depth (i.e., the maximum relative variation with/without graphene doping) exceeding 90% in transmission and even more dramatic in reflection (>600%) is predicted for graphene-loaded silver films of 1–5 nm thickness and currently attainable lateral dimensions. These new structures hold great potential for fast low-power electro-optical modulation.