Metallic Nanoantennas Research Articles

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.

Arrays of Metal Nanostructures for Plasmon-enhanced Spectroscopy

We report on the study of localized surface plasmon resonances (LSPRs) arising in arrays of metal (Au) nanoclusters, dimers, and nanoantennas under the influence of external electromagnetic radiation. Using nanolithography, the plasmonic arrays with different morphologies including cylindrical nanoclusters and dimers, linear and H-type nanoantennas were fabricated. Their LSPR frequencies were determined from the analysis of optical reflection and IR spectra. LSPR frequency depends strongly on morphology of metal nanostructures and is varied from visible to terahertz spectral range with variation of the structures from cylindrical nanoclusters to the nanoantennas with a high aspect ratio. We established the interconnection between the structural parameters of the plasmonic arrays and their LSPR frequencies based on the results of 3D electrodynamic simulations.

Second-Harmonic Generation from a Quantum Emitter Coupled to a Metallic Nanoantenna

We use time-dependent density functional theory and a semiclassical model to study second-harmonic generation in a system comprising a quantum emitter and a spherical metallic nanoparticle, where t...

Geometrically Tunable Beamed Light Emission from a Quantum‐Dot Ensemble Near a Gradient Metasurface

AbstractOptical metasurfaces have been widely investigated in recent years as a means to tailor the wavefronts of externally incident light for passive device applications. At the same time, their use in active optoelectronic devices such as light emitters is far less established. This work explores their ability to control the radiation properties of a nearby continuous ensemble of randomly oriented incoherent dipole sources via near‐field interactions. Specifically, a film of colloidal quantum dots is deposited on a plasmonic metasurface consisting of a 1D array of metallic nanoantennas on a metal film. The array is designed to introduce a linear phase profile upon reflection, and a bi‐periodic nanoparticle arrangement is introduced to ensure adequate sampling of the desired phase gradient. Highly directional radiation patterns are correspondingly obtained from the quantum dots at an enhanced emission rate. The underlying radiation mechanism involves the near‐field excitation of surface plasmon polaritons at the metal film, and their selective diffractive scattering by the metasurface into well‐collimated beams along predetermined geometrically tunable directions. These results underscore the distinctive ability of metasurfaces to control radiation properties directly at the source level, which is technologically significant for the continued miniaturization and large‐scale integration of optoelectronic devices.

Hybrid KTP–Plasmonic Nanostructures for Enhanced Nonlinear Optics at the Nanoscale

The search for miniaturized components for nonlinear optical processes needs a way to overcome the efficiency loss due to the effective size reduction of the active medium. Here we investigate a combination of a nanosized nonlinear dielectric crystal and metallic nanoantennas that benefits from both the intrinsic nonlinear conversion efficiency of the nonlinear medium and the local field enhancement of plasmonic resonances in metallic nanostructures. Careful comparison between experiments and numerical simulations reveals that the observed 10- to 1000-fold enhancement in second harmonic generation intensity in the hybrid structure relative to the isolated elements is attributed unequivocally to the field enhancement effect of plasmonic resonances on the nonlinear crystal for gold–crystal structures, while the enhancement observed in aluminum-based hybrid structures can be attributed to the linear dielectric effect on the plasmonic antennas.

Enhancement of the second harmonic signal of nonlinear crystals by a single metal nanoantenna.

This work fundamentally investigates how the second harmonic generation (SHG) from commercial nonlinear crystals can be boosted by the addition of individual optical nanoantennas. Frequency conversion plays an important role in modern non-linear optics, and nonlinear crystals have become a widely used building block for non-linear processes. Still, SHG remains hampered by limited conversion efficiency. To strengthen SHG from the crystal surface, we investigate the interaction of LiNbO3 crystals with individual gold nanodiscs. The scattered intensities and resonance frequencies of the nanodiscs are analyzed by dark-field spectroscopy and simulations. Subsequently, the discs on LiNbO3 are excited by a pulsed femtosecond laser in a parabolic mirror setup. Comparing the SHG at the position of a single nanodisc at resonance on the crystal with that of the unstructured crystal and of gold nanodiscs on a reference substrate, local SHG enhancement of up to a factor of three was achieved in the focal volume through the presence of the antenna.

Nanoantennas with balanced gain and loss

Abstract The large cross sections and strong confinement provided by the plasmon resonances of metallic nanostructures make these systems an ideal platform to implement nanoantennas. Like their macroscopic counterparts, nanoantennas enhance the coupling between deep subwavelength emitters and free radiation, providing, at the same time, an increased directionality. Here, inspired by the recent works in parity-time symmetric plasmonics, we investigate how the combination of conventional plasmonic nanostructures with active materials, which display optical gain when externally pumped, can serve to enhance the performance of metallic nanoantennas. We find that the presence of gain, in addition to mitigating the losses and therefore increasing the power radiated or absorbed by an emitter, introduces a phase difference between the elements of the nanoantenna that makes the optical response of the system directional, even in the absence of geometrical asymmetry. Exploiting these properties, we analyse how a pair of nanoantennas with balanced gain and loss can enhance the far-field interaction between two dipole emitters. The results of this work provide valuable insight into the optical response of nanoantennas made of active and passive plasmonic nanostructures, with potential applications for the design of optical devices capable of actively controlling light at the nanoscale.

Anomalous plasmon hybridization in nanoantennas near interfaces.

We report on an anomalous plasmon hybridization in side-by-side coupled metallic nanoantennas on top of a silicon waveguide. Contrary to the conventional perception based on Coulomb coupling, the hybridized anti-symmetric mode in our structure possesses a higher resonance frequency than the symmetric mode. This unusual phenomenon reveals a new mechanism of plasmon hybridization, namely, coupling-induced charge redistribution. Our work includes numerical simulation, experimental validation, and theoretical analysis, emphasizing the importance of dielectric interfaces in coupled plasmonic structures, and offers new possibilities for non-Hermitian systems and integrated devices.

Ultrasensitive Transmissive Infrared Spectroscopy via Loss Engineering of Metallic Nanoantennas for Compact Devices.

Miniaturized infrared spectroscopy is highly desired for widespread applications, including environment monitoring, chemical analysis, and biosensing. Nanoantennas, as a promising approach, feature strong field enhancement and provide opportunities for ultrasensitive molecule detection even in the nanoscale range. However, current efforts for higher sensitivities by nanogaps usually suffer a trade-off between the performance and fabrication cost. Here, novel crooked nanoantennas are designed with a different paradigm based on loss engineering to overcome the above bottleneck. Compared to the commonly used straight nanoantennas, the crooked nanoantennas feature higher sensitivity and a better fabrication tolerance. Molecule signals are increased by 25 times, reaching an experimental enhancement factor of 2.8 × 104. The optimized structure enables a transmissive CO2 sensor with sensitivities up to 0.067% ppm-1. More importantly, such a performance is achieved without sub-100 nm structures, which are common in previous works, enabling compatibility with commercial optical lithography. The mechanism of our design can be explained by the interplay of radiative and absorptive losses of nanoantennas that obeys the coupled-mode theory. Leveraging the advantage of the transmission mode in an optical system, our work paves the way toward cheap, compact, and ultrasensitive infrared spectroscopy.

Formation of optical antennas interfaces by laser processing of thin metal coatings

The work is devoted to the problem of creating interfaces of metallic nanoantennas for optoplasmonics. The technology of micro- and nanoscale elements topology formation by the method of selective laser ablation of thin-film coatings is described. The possibility of using the femtosecond laser complex for rapid prototyping of various geometric configurations of optical antennas interfaces is shown.

Selective Autonomous Molecular Transport and Collection by Hydrogel-Embedded Supramolecular Chemical Gradients.

Selective transport and concentration of molecules to specified regions on a substrate both enhances the potential to detect such molecules and provides a path to spatially localize such molecules prior to initiation of subsequent chemical reactions. Here, we first embed radially symmetric α-, β-, and γ-cyclodextrin gradients in a hydrogel matrix. Driven by host-guest interactions between the cyclodextrins and the target molecule, we observe these gradients can serve to direct 2D molecular transport. Using xanthene dyes and organophosphates as target molecules, we found the transport metrics, e.g., selectivity, rate, and concentration limits, are strongly dependent on the specific cyclodextrin forming the gradient. In all cases, as the concentrating power of the gradient increased, the rate of target concentration slowed, which we hypothesize is because stronger interactions between the target and the cyclodextrin decrease the rate of target diffusion. The concentration enhancement for the nerve agent simulant 4-methylumbelliferyl phosphate (15.8) is the greatest when the gradient is formed using β-cyclodextrin while directed concentration of cyanomethyl phosphonate, a smaller non-aromatic organophosphate, is observed only for the smaller α-CD. To provide a near real-time read-out of the concentration of the analyte, we used an array of IR resonant metallic nanoantennas tuned to a specific IR absorption band of the analyte to enhance the IR signal generated by the analyte.

Selective Autonomous Molecular Transport and Collection by Hydrogel‐Embedded Supramolecular Chemical Gradients

AbstractSelective transport and concentration of molecules to specified regions on a substrate both enhances the potential to detect such molecules and provides a path to spatially localize such molecules prior to initiation of subsequent chemical reactions. Here, we first embed radially symmetric α‐, β‐, and γ‐cyclodextrin gradients in a hydrogel matrix. Driven by host‐guest interactions between the cyclodextrins and the target molecule, we observe these gradients can serve to direct 2D molecular transport. Using xanthene dyes and organophosphates as target molecules, we found the transport metrics, e.g., selectivity, rate, and concentration limits, are strongly dependent on the specific cyclodextrin forming the gradient. In all cases, as the concentrating power of the gradient increased, the rate of target concentration slowed, which we hypothesize is because stronger interactions between the target and the cyclodextrin decrease the rate of target diffusion. The concentration enhancement for the nerve agent simulant 4‐methylumbelliferyl phosphate (15.8) is the greatest when the gradient is formed using β‐cyclodextrin while directed concentration of cyanomethyl phosphonate, a smaller non‐aromatic organophosphate, is observed only for the smaller α‐CD. To provide a near real‐time read‐out of the concentration of the analyte, we used an array of IR resonant metallic nanoantennas tuned to a specific IR absorption band of the analyte to enhance the IR signal generated by the analyte.

Intervalley Quantum Coherence Transfer and Coherently-Induced Chiral Plasmon Fields in WS2–Metallic Nanoantenna Systems

We theoretically show that when a hybrid system consisting of a transition metal dichalcogenide monolayer (WS2) and an achiral metallic nanoantenna interacts with a circularly polarized laser field, plasmonically mediated coherently induced processes can transfer quantum coherence from one valley (K or K′) to another. This leads to spin-valley-dependent plasmon field enhancement and coherently driven energy transfer from the WS2 monolayer to the nanoantenna. For the case when the incident laser is right circularly polarized (σ+), we study intervalley transfer of excitation and polarization from K to K′ valleys while mapping the evolution of the Stokes vectors associated with the emission of the WS2 monolayer. This is done as we explore the impact of variation of the laser frequency and intensity on K–K′ coupling. Our results show a formation of collective quantum states and an abrupt transition between these states at a certain frequency. We demonstrate that, as such a transition happens, the Stokes vecto...

Simulation on near-field light generated by a semiconductor ring resonator with a metal nano-antenna for heat-assisted magnetic recording

Heat-assisted magnetic recording (HAMR) is a promising technology for achieving more than 1 Tbit/inch2 recording density. A near-field transducer (NFT), which forms a small light spot on a recording medium, is necessary in HAMR. However, the heat generated by the NFT would melt the NFT itself. To solve this problem, the authors have proposed a novel device, in which a metal nano-antenna is attached to a semiconductor ring resonator. In this paper, the relationship between the various design parameters of the device and the near-field light was investigated through a numerical simulation to optimize the structure of the device. The simulation was conducted using the finite element method based on a two-dimensional model. It was found that the electric field at the tip of the nano-antenna depends on the type of eigenmode, the length of the nano-antenna, and the distance between the ring resonator and the nano-antenna.

Circuit theory of metal-enhanced fluorescence

Metal-enhanced fluorescence (MEF) comprises several linear phenomena which can be successfully described either by a classical theory or by a quantum one. Usually different phenomena are described by different classical models. Recently, an analytical model for a metal nanoantenna coupled to a quantum emitter was suggested that grants an approximate solution covering all basic linear phenomena observed in MEF from the Purcell effect to the fluorescent quenching. In this paper, the further development of this model is presented in terms of the equivalent circuits. The circuit model allows us to express the non-radiative Purcell factor of a nanoantenna through the previously evaluated radiative Purcell factor, to find the threshold of the fluorescence quenching and to determine the conditions when a fluorescent nanostructure transforms into a surface-plasmon laser (spaser).

Pure magnetic-quadrupole scattering and efficient second-harmonic generation from plasmon-dielectric hybrid nano-antennas

We theoretically demonstrate that pure magnetic quadrupole (MQ) scattering is achieved via the excitation of anapole modes and Fano resonance in noble metal (Au or Ag) and high refractive index dielectric (AlGaAs) hybrid nano-antennas. In Au-AlGaAs hybrid nano-antennas, electric anapole and magnetic anapole modes are observed, leading to the suppressions of electric and magnetic dipoles. Introducing gain material to AlGaAs nanodisk to increase the strength of electric quadrupole (EQ) Fano resonance leads to the suppression of EQ scattering. Then, ideal MQ scattering is achieved at the wavelength of total scattering cross-section dip. The increase of signal-to-noise ratio of MQ results in the great enhancement of near-field inside AlGaAs nanodisk. Additionally, the strong MQ resonance exhibits great capability for boosting second-harmonic generation by proper mode matching. These findings achieved in subwavelength geometries have important implications for functional metamaterials and nonlinear photonic nanodevices.

Photonic and plasmonic coupling of metallic nanoantenna supercells: Plasmonic lattices with hybrid cavity modes

Using numerical and experimental techniques we study plasmonic and photonic coupling of plasmonic supercells formed in one dimensional arrays (chains) of flat metallic nanoantennas. These supercells support their own collective resonances and are associated with two different types of near field couplings of the nanoantennas. When the incident light is polarized along the long axes the nanoantennas, these cells are formed via plasmonic bonding (bonding supercells). For light polarization along the short axes of the nanoantennas, the supercells are generated via corner edge-mode coupling, generating a collective mode across the one-dimensional arrays of the nanoantennas (cavity supercells). We study plasmonic lattices wherein these supercells act as the units that are periodically repeated along one dimension. By changing the distances between these units (lattice constant), we study the optical responses of such lattices as the nature of interaction between supercells are changed from pure plasmonic (near field) to diffractive (far field) coupling. For lattices with bonding supercells, the former leads to strong suppression of the side modes. In the case of diffractive coupling, our results suggest formation surface lattice resonance via hybridization of resonances of the cavity supercells with Rayleigh anomaly.

Absorption Enhancement of GaAs Thin-Film Solar Cells Using Tapered Metal Nanoantenna Structures

This paper focuses on field absorption by metallic nanoantennas as a means to enhance light trapping into thin-film solar cells. It determines the optimum metal nanoantennas distribution for solar cell applications. The proposed structure takes into account the interactions between the nano-particles as well as including their interaction with the absorbing substrate. Tapered structure is proposed for absorption enhancement using different metal nanoantennas like gold, silver, copper and aluminium. The sizes of nanoantennas are tapered in one direction. The tapered structures are composed of an array of metal nanoantennas and the sizes are tapered using two different ways according to the tapering direction. The degree of tapering can be set by controlling a scaling element. All simulation data are obtained using the frequency domain finite element method. Larger than 20% increase in the absorption level can be achieved using the proposed structures.

Bull's eye grating integrated with optical nanoantennas for plasmonic enhancement of graphene long-wave infrared photodetectors

Two-dimensional (2D) materials have exhibited potential for infrared detection at room temperature, yet their low light absorption impedes their widespread application. In addition, micromechanical cleavage, which is the main method by which high-quality 2D layers are achieved, typically leads to small-area flakes, hampering their application as photodetectors. In this work, we designed a hybrid plasmonic structure, comprising a metallic bull's eye grating and optical nanoantennas, to collect and concentrate light into a piece of single-layer graphene with sub-wavelength lateral extent. This boosts the interaction between the graphene and light, thereby improving its photodetection performance in the technologically important long-wave infrared (LWIR) region. Finite-difference time-domain electromagnetic simulations were performed to this end. The plasmonic structure we present is predicted to enhance the absorption of light by the graphene by ∼558 times, which in turn is predicted to enhance the detectivity of the LWIR photodetector by ∼32 times.

Design and Analysis of Nanoantenna Arrays for Imaging and Sensing Applications at Optical Frequencies

We present computational analysis of nanoantenna arrays for imaging and sensing applications at optical frequencies. Arrays of metallic nanoantennas are considered in an accurate simulation environment based on surface integral equations and the multilevel fast multipole algorithm developed for plasmonic structures. Near-zone responses of the designed arrays to nearby nanoparticles are investigated in detail to demonstrate the feasibility of detection. We show that both metallic and dielectric nanoparticles, even with subwavelength dimensions, can be detected.