Plasmon blockade in nanostructured graphene

Nonlinear Tunable Vibrational Response in Hexagonal Boron Nitride

Nonlinear light-matter interactions in structured materials are the source of exciting properties and enable vanguard applications in photonics. However, the magnitude of nonlinear effects is generally small, thus requiring high optical intensities for their manifestation at the nanoscale. Here, we reveal a large nonlinear response of monolayer hexagonal boron nitride (hBN) in the mid-infrared phonon-polariton region, triggered by the strongly anharmonic potential associated with atomic vibrations in this material. We present robust first-principles theory predicting a threshold light field ∼24 MV/m to produce order-unity effects in Kerr nonlinearities and harmonic generation, which are made possible by a combination of the long lifetimes exhibited by optical phonons and the strongly asymmetric landscape of the configuration energy in hBN. We further foresee polariton blockade at the few-quanta level in nanometer-sized structures. In addition, by mixing static and optical fields, the strong nonlinear response of monolayer hBN gives rise to substantial frequency shifts of optical phonon modes, exceeding their spectral width for in-plane DC fields that are attainable using lateral gating technology. We therefore predict a practical scheme for electrical tunability of the vibrational modes with potential interest in mid-infrared optoelectronics. The strong nonlinear response, low damping, and robustness of hBN polaritons set the stage for the development of applications in light modulation, sensing, and metrology, while triggering the search for an intense vibrational nonlinear response in other ionic materials.

Recent Progress on Metal-Enhanced Photocatalysis: A Review on the Mechanism

Metal-enhanced photocatalysis has recently received increasing interest, mainly due to the ability of metal to directly or indirectly degrade pollutants. In this review, we briefly review the recent breakthroughs in metal-enhanced photocatalysis. We discussed the recent progress of surface plasmon resonance (SPR) effect and small size effect of metal nanoparticles on photocatalysis; in particular, we focus on elucidating the mechanism of energy transfer and hot electron injection/transfer effect of metal nanoparticles and clusters while as photocatalysts or as cophotocatalysts. Finally, we discuss the potential applications of metal-enhanced photocatalysis, and we also offer some perspectives for further investigations.

Recent Progress on Metal-Enhanced Photocatalysis: A Review on the Mechanism

Metal-enhanced photocatalysis has recently received increasing interest, mainly due to the ability of metal to directly or indirectly degrade pollutants. In this review, we briefly review the recent breakthroughs in metal-enhanced photocatalysis. We discussed the recent progress of surface plasmon resonance (SPR) effect and small size effect of metal nanoparticles on photocatalysis; in particular, we focus on elucidating the mechanism of energy transfer and hot electron injection/transfer effect of metal nanoparticles and clusters while as photocatalysts or as cophotocatalysts. Finally, we discuss the potential applications of metal-enhanced photocatalysis, and we also offer some perspectives for further investigations.

A Review on the Development of Tunable Graphene Nanoantennas for Terahertz Optoelectronic and Plasmonic Applications

Exceptional advancement has been made in the development of graphene optical nanoantennas. They are incorporated with optoelectronic devices for plasmonics application and have been an active research area across the globe. The interest in graphene plasmonic devices is driven by the different applications they have empowered, such as ultrafast nanodevices, photodetection, energy harvesting, biosensing, biomedical imaging and high-speed terahertz communications. In this article, the aim is to provide a detailed review of the essential explanation behind graphene nanoantennas experimental proofs for the developments of graphene-based plasmonics antennas, achieving enhanced light-matter interaction by exploiting graphene material conductivity and optical properties. First, the fundamental graphene nanoantennas and their tunable resonant behavior over THz frequencies are summarized. Furthermore, incorporating graphene-metal hybrid antennas with optoelectronic devices can prompt the acknowledgment of multi-platforms for photonics. More interestingly, various technical methods are critically studied for frequency tuning and active modulation of optical characteristics, through in situ modulations by applying an external electric field. Second, the various methods for radiation beam scanning and beam reconfigurability are discussed through reflectarray and leaky-wave graphene antennas. In particular, numerous graphene antenna photodetectors and graphene rectennas for energy harvesting are studied by giving a critical evaluation of antenna performances, enhanced photodetection, energy conversion efficiency and the significant problems that remain to be addressed. Finally, the potential developments in the synthesis of graphene material and technological methods involved in the fabrication of graphene-metal nanoantennas are discussed.

Nonlinear Graphene Nanoplasmonics

Nonlinear optics is limited by the weak nonlinear response of available materials, a problem that is generally circumvented by relying on macroscopic structures in which light propagates over many optical cycles, thus giving rise to accumulated unity-order nonlinear effects. While this strategy cannot be extended to subwavelength optics, such as in nanophotonic structures, one can alternatively use localized optical resonances with high quality factors to increase light-matter interaction times, although this approach is limited by inelastic losses partly associated with the nonlinear response. Plasmons-the collective oscillations of electrons in conducting media-offer the means to concentrate light into nanometric volumes, well below the light-wavelength-scale limit imposed by diffraction, amplifying the electromagnetic fields upon which nonlinear optical phenomena depend. Due to their abundant supply of free electrons, noble metals are the traditional material platform for plasmonics and have thus dominated research in nanophotonics over the past several decades, despite exhibiting large ohmic losses and inherent difficulties to actively modulate plasmon resonances, which are primarily determined by size, composition, and morphology. Highly doped graphene has recently emerged as an appealing platform for plasmonics due to its unique optoelectronic properties, which give rise to relatively long-lived, highly confined, and actively tunable plasmon resonances that mainly appear in the infrared and terahertz frequency regimes. Efforts to extend graphene plasmonics to the near-infrared and visible ranges involve patterning of graphene into nanostructured elements, thus facilitating the optical excitation of localized resonances that can be blue-shifted through geometrical confinement while maintaining electrical tunability. Besides these appealing plasmonic attributes, the conical electronic dispersion relation of graphene renders its charge carrier motion in response to light intrinsically anharmonic, resulting in a comparatively intense nonlinear optical response. The combined synergy of extreme plasmonic field enhancement and large intrinsic optical nonlinearity are now motivating intensive research efforts in nonlinear graphene plasmonics, the recent progress of which we discuss in this Account. We start with a description of the appealing properties of plasmons in graphene nanostructures down to molecular sizes, followed by a discussion of the unprecedented level of intrinsic optical nonlinearity in graphene, its enhancement by resonant coupling to its highly confined plasmons to yield intense high harmonic generation and Kerr nonlinearities, the extraordinary thermo-optical capabilities of this material enabling large nonlinear optical switching down to the single-photon level, and its strong interaction with quantum emitters.

Giant Self-Kerr Nonlinearity in the Metal Nanoparticles-Graphene Nanodisks-Quantum Dots Hybrid Systems Under Low-Intensity Light Irradiance

Hybrid nanocomposites can provide a promising platform for integrated optics. Optical nonlinearity can significantly widen the range of applications of such structures. In the present paper, a theoretical investigation is carried out by solving the density matrix equations derived for a metal nanoparticles-graphene nanodisks-quantum dots hybrid system interacting with weak probe and strong control fields, in the steady state. We derive analytical expressions for linear and third-order nonlinear susceptibilities of the probe field. A giant self-Kerr nonlinear index of refraction is obtained in the optical region with relatively low light intensity. The optical absorption spectrum of the system demonstrates electromagnetically induced transparency and amplification without population inversion in the linear optical response arising from the negative real part of the polarizabilities for the plasmonic components at the energy of the localized surface plasmon resonance of the graphene nanodisks induced by the probe field. We find that the self-Kerr nonlinear optical properties of the system can be controlled by the geometrical features of the system, the size of metal nanoparticles and the strength of the control field. The controllable self-Kerr nonlinearities of hybrid nanocomposites can be employed in many interesting applications of modern integrated optics devices allowing for high nonlinearity with relatively low light intensity.

Optical second harmonic generation from nanostructured graphene: a full wave approach

Optical second harmonic generation (SHG) from nanostructured graphene has been studied in the framework of classical electromagnetism using a surface integral equation method. Single disks and dimers are considered, demonstrating that the nonlinear conversion is enhanced when a localized surface plasmon resonance is excited at either the fundamental or second harmonic frequency. The proposed approach, beyond the electric dipole approximation used in the quantum description, reveals that SHG from graphene nanostructures with centrosymmetric shapes is possible when retardation effects and the excitation of high plasmonic modes at the second harmonic frequency are taken into account. Several SHG effects similar to those arising in metallic nanostructures, such as the silencing of the nonlinear emission and the design of double resonant nanostructures, are also reported. Finally, it is shown that the SHG from graphene disk dimers is very sensitive to a relative vertical displacement of the disks, opening new possibilities for the design of nonlinear plasmonic nanorulers.

Tunable metamaterial-induced transparency with gate-controlled on-chip graphene metasurface

We propose and numerically investigate a gate-controlled on-chip graphene metasurface consisting of a monolayer graphene sheet and silicon photonic crystal-like substrate, to achieve an electrically-tunable induced transparency. The operation mechanism of the induced transparency of the on-chip graphene metasurface is analyzed. The tunable optical properties with different gate-voltages and polarizations have been discussed. Additionally, the spectral feature of the on-chip graphene metasurface as a function of the refractive index of the local environment is also investigated. The result shows that the on-chip graphene metasurface as a refractive index sensor can achieve an overall figure of merit of 8.89 in infrared wavelength range. Our study suggests that the proposed structure is potentially attractive as optoelectronic modulators and refractive index sensors.

Visible quantum plasmonics from metallic nanodimers

We report theoretical evidence that bulk nonlinear materials weakly interacting with highly localized plasmonic modes in ultra-sub-wavelength metallic nanostructures can lead to nonlinear effects at the single plasmon level in the visible range. In particular, the two-plasmon interaction energy in such systems is numerically estimated to be comparable with the typical plasmon linewidths. Localized surface plasmons are thus predicted to exhibit a purely nonclassical behavior, which can be clearly identified by a sub-Poissonian second-order correlation in the signal scattered from the quantized plasmonic field under coherent electromagnetic excitation. We explicitly show that systems sensitive to single-plasmon scattering can be experimentally realized by combining electromagnetic confinement in the interstitial region of gold nanodimers with local infiltration or deposition of ordinary nonlinear materials. We also propose configurations that could allow to realistically detect such an effect with state-of-the-art technology, overcoming the limitations imposed by the short plasmonic lifetime.

Nonlinear Terahertz Absorption of Graphene Plasmons

Subwavelength graphene structures support localized plasmonic resonances in the terahertz and mid-infrared spectral regimes. The strong field confinement at the resonant frequency is predicted to significantly enhance the light-graphene interaction, which could enable nonlinear optics at low intensity in atomically thin, subwavelength devices. To date, the nonlinear response of graphene plasmons and their energy loss dynamics have not been experimentally studied. We measure and theoretically model the terahertz nonlinear response and energy relaxation dynamics of plasmons in graphene nanoribbons. We employ a terahertz pump-terahertz probe technique at the plasmon frequency and observe a strong saturation of plasmon absorption followed by a 10 ps relaxation time. The observed nonlinearity is enhanced by 2 orders of magnitude compared to unpatterned graphene with no plasmon resonance. We further present a thermal model for the nonlinear plasmonic absorption that supports the experimental results. The model shows that the observed strong linearity is caused by an unexpected red shift of plasmon resonance together with a broadening and weakening of the resonance caused by the transient increase in electron temperature. The model further predicts that even greater resonant enhancement of the nonlinear response can be expected in high-mobility graphene, suggesting that nonlinear graphene plasmonic devices could be promising candidates for nonlinear optical processing.

Unusual and tunable one-photon nonlinearity in gold-dye plexcitonic Fano systems

Recent studies of the coupling between the plasmonic excitations of metallic nanostructures with the excitonic excitations of molecular species have revealed a rich variety of emergent phenomena known as plexcitonics. Here, we use a combined experimental and theoretical approach to demonstrate new and intriguing aspects in the ultrafast nonlinear responses of strongly coupled hybrid Fano systems consisting of gold nanorods decorated with near-infrared dye molecules. We show that the severely suppressed linear absorption around the Fano dip significantly enhances the unidirectional energy transfer from the plasmons to the excitons and further allows one-photon nonlinearity to be drastically and reversibly tuned. These striking observations are interpreted within a microscopic model stressing on two competing processes: saturated plasmonic absorption and weakened destructive Fano interference from the bleached excitonic absorption. The unusually strong one-photon nonlinearity revealed here provides a promising strategy in fabricating nanoplasmonic devices with both pronounced nonlinearities and good figures of merit.

Plasmonics in atomically thin materials

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.

3D plasmonic nanoantennas integrated with MEA biosensors

Neuronal signaling in brain circuits occurs at multiple scales ranging from molecules and cells to large neuronal assemblies. However, current sensing neurotechnologies are not designed for parallel access of signals at multiple scales. With the aim of combining nanoscale molecular sensing with electrical neural activity recordings within large neuronal assemblies, in this work three-dimensional (3D) plasmonic nanoantennas are integrated with multielectrode arrays (MEA). Nanoantennas are fabricated by fast ion beam milling on optical resist; gold is deposited on the nanoantennas in order to connect them electrically to the MEA microelectrodes and to obtain plasmonic behavior. The optical properties of these 3D nanostructures are studied through finite elements method (FEM) simulations that show a high electromagnetic field enhancement. This plasmonic enhancement is confirmed by surface enhancement Raman spectroscopy of a dye performed in liquid, which presents an enhancement of almost 100 times the incident field amplitude at resonant excitation. Finally, the reported MEA devices are tested on cultured rat hippocampal neurons. Neurons develop by extending branches on the nanostructured electrodes and extracellular action potentials are recorded over multiple days in vitro. Raman spectra of living neurons cultured on the nanoantennas are also acquired. These results highlight that these nanostructures could be potential candidates for combining electrophysiological measures of large networks with simultaneous spectroscopic investigations at the molecular level.

Plasmon modes in graphene: status and prospect

Plasmons in graphene have unusual properties and offer promising prospects for plasmonic applications covering a wide frequency range, ranging from terahertz up to the visible. Plasmon modes have been recently studied in both free-standing and supported graphene. Here, we review plasmons in graphene with particular emphasis on plasmonic excitations in epitaxial graphene and on the influence of the underlying substrate on the screening processes. Although the theoretical comprehension of plasmons in supported graphene is still incomplete, several experimental results provide clues regarding the nature of plasmonic excitations in graphene on metals and semiconductors. Plasmon in graphene can be tuned by chemical doping and gating potentials. We show through selected examples that the adsorbates can be used to tune the plasmon frequency, while the intercalation of chemical species allows the decoupling of the graphene sheet from the substrate to recover the plasmon dispersion of pristine graphene. Finally, we also report intriguing effects due to many-body interaction, such as the excitations generated by electron-electron coupling (magnetoplasmons) and the composite modes arising from the coupling of plasmons with phonons and with charge carriers.

Investigation of plasmonic whispering-gallery mode characteristics for graphene monolayer coated dielectric nanodisks

In this Letter, we theoretically studied high-quality (Q) factor plasmonic whispering-gallery modes (WGMs) with ultrasmall mode volumes in graphene monolayer coated semiconductor nanodisks in the mid-infrared range. The influence of the chemical potential, the relaxation time of graphene, and the radius of the nanodisk on the cavity Q factor and the mode volume was numerically investigated. The numerical simulations showed that the plasmonic WGMs excited in this cavity had a deep subwavelength mode volume of 1.4×10(-5)(λ(0)/2n)(3), a cavity Q factor as high as 266 at a temperature lower than 250 K, and, consequently, a large Purcell factor of ∼1.2×10(7) when the chemical potential and relaxation time were assumed to be 0.9 eV and 1.4 ps, respectively. The results provide a possible application of plasmonic WGMs in the integration of nano-optoelectronic devices based on graphene.

Multiple excitation of confined graphene plasmons by single free electrons

We show that free electrons can efficiently excite plasmons in doped graphene with probabilities in the order of one per electron. More precisely, we predict multiple excitations of a single confined plasmon mode in graphene nanostructures. These unprecedentedly large electron-plasmon couplings are explained using a simple scaling law and further investigated through a general quantum description of the electron-plasmon interaction. From a fundamental viewpoint, multiple plasmon excitations by a single electron provide a unique platform for exploring the bosonic quantum nature of these collective modes. Not only does our study open a viable path toward multiple excitation of a single plasmon mode by a single electron, but it also reveals electron probes as ideal tools for producing, detecting, and manipulating plasmons in graphene nanostructures.

Enhanced light-matter interactions in graphene-covered gold nanovoid arrays

The combination of graphene with noble-metal nanostructures is currently being explored for strong light-graphene interactions enhanced by plasmons. We introduce a novel hybrid graphene-metal system for studying light-matter interactions with gold-void nanostructures exhibiting resonances in the visible range. Enhanced coupling of graphene to the plasmon modes of the nanovoid arrays results in significant frequency shifts of the underlying plasmon resonances, enabling 30% enhanced absolute light absorption by adding a monolayer graphene and up to 700-fold enhancement of the Raman response of the graphene. These new perspectives enable us to verify the presence of graphene on gold-void arrays, and the enhancement even allows us to accurately quantify the number of layers. Experimental observations are further supported by numerical simulations and perturbation-theory analysis. The graphene gold-void platform is beneficial for sensing of molecules and placing Rhodamine 6G (R6G) dye molecules on top of the graphene; we observe a strong enhancement of the R6G Raman fingerprints. These results pave the way toward advanced substrates for surface-enhanced Raman scattering (SERS) with potential for unambiguous single-molecule detection on the atomically well-defined layer of graphene.

Plasmonic control of nonlinear two-photon absorption in graphene nanocomposites

Nonlinear two-photon absorption in a quantum dot-graphene nanoflake nanocomposite system has been investigated. An external laser field is applied to the nanocomposite to simultaneously observe two-photon processes in the quantum dot and excite localized surface plasmons in the graphene nanodisk. This resonance condition can be achieved by tuning the plasmon resonance frequency in the graphene nanoflake via electrostatic gating. It is found that the strong local field of the graphene plasmons can enhance and control nonlinear optical processes in the quantum dot. Specifically, we show that the two-photon absorption coefficient in the quantum dot can be switched between single- and double-peaked spectra by modifying the graphene-quantum dot separation. Two-photon processes in the quantum dot can also be switched on or off by slightly changing the gate voltage applied to the graphene. Our findings indicate that this system can be used for nonlinear optical applications such as all-optical switching, biosensing and signal processing.

Scattering of graphene plasmons by defects in the graphene sheet

A theoretical study is presented on the scattering of graphene surface plasmons (GSPs) by defects in the graphene sheet they propagate in. These defects can be either natural (as domain boundaries, ripples, and cracks, among others) or induced by an external gate. The scattering is shown to be governed by an integral equation, derived from a plane wave expansion of the fields, which in general must be solved numerically, but it provides useful analytical results for small defects. Two main cases are considered: smooth variations of the graphene conductivity (characterized by a Gaussian conductivity profile) and sharp variations (represented by islands with different conductivity). In general, reflection largely dominates over radiation out of the graphene sheet. However, in the case of sharply defined conductivity islands, there are some values of island size and frequency where the reflectance vanishes and, correspondingly, the radiation out-of-plane is the main scattering process. For smooth defects, the reflectance spectra present a single maximum at the condition k(p)a ≈ √2, where k(p) is the GSP wavevector and a is the spatial width of the defect. In contrast, the reflectance spectra of sharp defects present periodic oscillations with period k(p)′a, where k(p)′ is the GSP wavelength inside the defect. Finally, the case of cracks (gaps in the graphene conductivity) is considered, showing that the reflectance is practically unity for gap widths larger than one-tenth of the GSP wavelength.

A perfect absorber made of a graphene micro-ribbon metamaterial

Metamaterial-based perfect absorbers promise many applications. Perfect absorption is characterized by the complete suppression of transmission and reflection and complete dissipation of the incident energy by the absorptive meta-atoms. A certain absorption spectrum is usually assigned to a bulk medium and serves as a signature of the respective material. Here we show how to use graphene flakes as building blocks for perfect absorbers. Then, an absorbing meta-atom only consists of a molecular monolayer placed at an appropriate distance from a metallic ground plate. We show that the functionality of such device is intuitively and correctly explained by a Fabry-Perot model.

Plasmon-induced doping of graphene

A metallic nanoantenna, under resonant illumination, injects nonequilibrium hot electrons into a nearby graphene structure, effectively doping the material. A prominent change in carrier density was observed for a plasmonic antenna-patterned graphene sheet following laser excitation, shifting the Dirac point, as determined from the gate-controlled transport characteristic. The effect is due to hot electron generation resulting from the decay of the nanoantenna plasmon following resonant excitation. The effect is highly tunable, depending on the resonant frequency of the plasmonic antenna, as well as on the incident laser power. Hot electron-doped graphene represents a new type of hybrid material that shows great promise for optoelectronic device applications.