Negative refraction at visible frequencies

Electrochemically grown porous platinum for electrocatalysis and optical applications

Localized electrochemically grown porous platinum layers on 2D and 3D microstructured materials enable a wide range of applications from electrocatalysis to optoelectronics. These layers exhibit a thickness gradient and surface corner overloading due to electric charge accumulation at the sharp corners. On one hand, these effects can be applied to create ultra-large surface area catalysts or electrocatalysts. On the other hand, they can be mitigated by guiding the electric field at the nanoscale. Here, we show that porous platinum grown on rough conductive silicon synergistically catalyses the electroreduction of CO2 in a humid gaseous atmosphere, overcoming the disadvantage of CO2´s low water solubility. In addition, using template-directed growth of porous platinum, we tuned the optical response of an infrared (IR) metamaterial fabricated by micropatterning on Si/NiCr/Ti substrates and constructed a broad absorber on potential IR-functional microcomponents.

Design and development of a compact, wide-angle metamaterial electromagnetic energy harvester with multiband functionality and polarization-insensitive features

This article presents a compact, wide-angle, polarization-insensitive metamaterial harvester that can efficiently harvest electromagnetic (EM) energy in the S, C, X, and Ku bands. The harvester's unit cell consists of a split ring resonator, two strip lines, and two split strip lines, giving it a total size of (10 × 10) mm2. Each split gap is filled with a 50 Ω resistive load. The input impedance of the harvester is precisely designed to match that of free space, allowing for efficient absorption of EM power and appropriate redirection towards the resistive loads. The harvester's performance is also evaluated for various polarization and incident angles, considering the Transverse Electric and Transverse Magnetic modes. The simulation results reveal that the proposed harvester exhibits a notably greater conversion efficiency of around > 95%. The simulation outcomes were carefully validated through experimental tests conducted in an anechoic chamber using a 3 × 3 cell array of the proposed design. This ensured the accuracy and reliability of the results. The strong correlation between the experimental data and the full-wave simulations strongly supports the effectiveness of the proposed harvester. Therefore, the demonstrated efficiency and compact size make it a perfect fit for energy harvesting systems in wireless sensor networks.

Superwavelength self-healing of spoof surface sonic Airy-Talbot waves

Self-imaging phenomena for nonperiodic waves along a parabolic trajectory encompass both the Talbot effect and the accelerating Airy beams. Beyond the ability to guide waves along a bent trajectory, the self-imaging component offers invaluable advantages to lensless imaging comprising periodic repetition of planar field distributions. In order to circumvent thermoviscous and diffraction effects, we structure subwavelength resonators in an acoustically impenetrable surface supporting spoof surface acoustic waves (SSAWs) to provide highly confined Airy-Talbot effect, extending Talbot distances along the propagation path and compressing subwavelength lobes in the perpendicular direction. From a linear array of loudspeakers, we judiciously control the amplitude and phase of the SSAWs above the structured surface and quantitatively evaluate the self-healing performance of the Airy-Talbot effect by demonstrating how the distinctive scattering patterns remain largely unaffected against superwavelength obstacles. Furthermore, we introduce a new mechanism utilizing subwavelength Airy beam as a coding/decoding degree of freedom for acoustic communication with high information density comprising robust transport of encoded signals.

Metamaterial-enabled asymmetric negative refraction of GHz mechanical waves

Wave refraction at an interface between different materials is a basic yet fundamental phenomenon, transversal to several scientific realms - electromagnetism, gas and fluid acoustics, solid mechanics, and possibly also matter waves. Under specific circumstances, mostly enabled by structuration below the wavelength scale, i.e., through the metamaterial approach, waves undergo negative refraction, eventually enabling superlensing and transformation optics. However, presently known negative refraction systems are symmetric, in that they cannot distinguish between positive and negative angles of incidence. Exploiting a metamaterial with an asymmetric unit cell, we demonstrate that the aforementioned symmetry can be broken, ultimately relying on the specific shape of the Bloch mode isofrequency curves. Our study specialized upon a mechanical metamaterial operating at GHz frequency, which is by itself a building block for advanced technologies such as chip-scale hybrid optomechanical and electromechanical devices. However, the phenomenon is based on general wave theory concepts, and it applies to any frequency and time scale for any kind of linear waves, provided that a suitable shaping of the isofrequency contours is implemented.

All-angle reflectionless negative refraction with ideal photonic Weyl metamaterials

Negative refraction, an unnatural optical phenomenon in which the incident and the refracted waves reside on the same side of the surface normal, has been demonstrated with the invention of negative index media based on artificially engineered photonic structures called metamaterials. It has received wide attention due to its potential applications in imaging, nonlinear optics, and electromagnetic cloaking. However, it is highly challenging to realize negative refraction operating at all angles and with the perfect transmission. In this work, leveraging the recent development in topological photonics, we propose to realize reflectionless negative refraction for all incident angles with a topological metamaterial. The proposed metamaterial possesses two Weyl points of opposite topological charges. By interfacing the metamaterial with a perfect electric conductor (PEC) or a perfect magnetic conductor (PMC), the Fermi arc connecting the two Weyl points can take the form of a half-circle possessing a positive or a negative refractive index. Importantly, due to the topological protection, there is no reflection at the interface between the PEC and PMC covered areas, leading to the observation of all-angle negative refraction without reflection at the boundary. Our work provides a new platform for manipulating the propagation of surface waves, which may find applications in the construction of integrated photonic devices.

Relaxed Phase-Matching Constraints in Zero-Index Waveguides

The utility of all parametric nonlinear optical processes is hampered by phase-matching requirements. Quasi-phase-matching, birefringent phase matching, and higher-order-mode phase matching have all been developed to address this constraint, but the methods demonstrated to date suffer from the inconvenience of only being phase matched for a single, specific arrangement of beams, typically copropagating, resulting in cumbersome experimental configurations and large footprints for integrated devices. Here, we experimentally demonstrate that these phase-matching requirements may be satisfied in a parametric nonlinear optical process for multiple, if not all, configurations of input and output beams when using low-index media. Our measurement constitutes the first experimental observation of direction-independent phase matching for a medium sufficiently long for phase matching to be relevant. We demonstrate four-wave mixing from spectrally distinct co- and counterpropagating pump and probe beams, the backward generation of a nonlinear signal, and excitation by an out-of-plane probe beam. These results explicitly show that the unique properties of low-index media relax traditional phase-matching constraints, which can be exploited to facilitate nonlinear interactions and miniaturize nonlinear devices, thus adding to the established exceptional properties of low-index materials.

Observation of optical gyromagnetic properties in a magneto-plasmonic metamaterial

Metamaterials with artificial optical properties have attracted significant research interest. In particular, artificial magnetic resonances with non-unity permeability tensor at optical frequencies in metamaterials have been reported. However, only non-unity diagonal elements of the permeability tensor have been demonstrated to date. A gyromagnetic permeability tensor with non-zero off-diagonal elements has not been observed at the optical frequencies. Here we report the observation of gyromagnetic properties in the near-infrared wavelength range in a magneto-plasmonic metamaterial. The non-zero off-diagonal permeability tensor element causes the transverse magneto-optical Kerr effect under s-polarized incidence that otherwise vanishes if the permeability tensor is not gyromagnetic. By retrieving the permeability tensor elements from reflection, transmission, and transverse magneto-optical Kerr effect spectra, we show that the effective off-diagonal permeability tensor elements reach 10⁻³ level at the resonance wavelength (~900 nm) of the split-ring resonators, which is at least two orders of magnitude higher than magneto-optical materials at the same wavelength. The artificial gyromagnetic permeability is attributed to the change in the local electric field direction modulated by the split-ring resonators. Our study demonstrates the possibility of engineering the permeability and permittivity tensors in metamaterials at arbitrary frequencies, thereby promising a variety of applications of next-generation nonreciprocal photonic devices, magneto-plasmonic sensors, and active metamaterials.

Optical gyromagnetic properties are not observed in natural or metamaterials to date. Here, the authors experimentally demonstrated optical gyromagnetic properties in a magneto-plasmonic metamaterial, realizing the long-sought bi-gyrotropic medium.

Demonstration of negative refraction induced by synthetic gauge fields

Negative refraction is a counterintuitive wave phenomenon that has inspired the development of metamaterials and metasurfaces with negative refractive indices and surface phase discontinuities, respectively. Recent theories have proposed an alternative mechanism for negative refraction: Synthetic gauge fields, induced by either dynamical modulation or motion, can shift a material’s dispersion in momentum space, forcing a positive refractive index medium to exhibit negative refraction above a certain threshold. However, this phenomenon has not previously been observed. Here, we report on the experimental demonstration of gauge field–induced negative refraction in a twisted bilayer acoustic metamaterial. The synthetic gauge fields arise in a projected two-dimensional geometry and can be continuously tuned by varying the wave number along the third dimension. Gauge field–induced waveguiding with backward-propagating modes is also demonstrated in a trilayer configuration. These results introduce a mechanism for performing wave manipulation in artificially engineered materials.

Transformation Metamaterials

Based on the form-invariance of Maxwell's equations under coordinate transformations, mathematically smooth deformation of space can be physically equivalent to inhomogeneous and anisotropic electromagnetic (EM) medium (called a transformation medium). It provides a geometric recipe to control EM waves at will. A series of examples of achieving transformation media by artificially structured units from conventional materials is summarized here. Such concepts are firstly implemented for EM waves, and then extended to other wave dynamics, such as elastic waves, acoustic waves, surface water waves, and even stationary fields. These shall be cataloged as transformation metamaterials. In addition, it might be conceptually attractive and practically useful to control diverse waves for multi-physics designs.

Nanoantennas Inversely Designed to Couple Free Space and a Metal-Insulator-Metal Waveguide

The metal-insulator-metal (MIM) waveguide, which can directly couple free space photons, acts as an important interface between conventional optics and subwavelength photoelectrons. The reason for the difficulty of this optical coupling is the mismatch between the large wave vector of the MIM plasmon mode and photons. With the increase in the wave vector, there is an increase in the field and Ohmic losses of the metal layer, and the strength of the MIM mode decreases accordingly. To solve those problems, this paper reports on inversely designed nanoantennas that can couple the free space and MIM waveguide and efficiently excite the MIM plasmon modes at multiple wavelengths and under oblique angles. This was achieved by implementing an inverse design procedure using a topology optimization approach. Simulation analysis shows that the coupling efficiency is enhanced 9.47-fold by the nanoantenna at the incident wavelength of 1338 nm. The topology optimization problem of the nanoantennas was analyzed by using a continuous adjoint method. The nanoantennas can be inversely designed with decreased dependence on the wavelength and oblique angle of the incident waves. A nanostructured interface on the subwavelength scale can be configured in order to control the refraction of a photonic wave, where the periodic unit of the interface is composed of two inversely designed nanoantennas that are decoupled and connected by an MIM waveguide.

Self-Assembled BaTiO3-AuxAg1-x Low-Loss Hybrid Plasmonic Metamaterials with an Ordered "Nano-Domino-like" Microstructure

Metallic plasmonic hybrid nanostructures have attracted enormous research interest due to the combined physical properties coming from different material components and the broad range of applications in nanophotonic and electronic devices. However, the high loss and narrow range of property tunability of the metallic hybrid materials have limited their practical applications. Here, a metallic alloy-based self-assembled plasmonic hybrid nanostructure, i.e., a BaTiO₃-Au_xAg_1-x (BTO) vertically aligned nanocomposite, has been integrated by a templated growth method for low-loss plasmonic systems. Comprehensive microstructural characterizations including high-resolution scanning transmission electron microscopy (HRSTEM), energy-dispersive X-ray spectroscopy (EDS), and three-dimensional (3D) electron tomography demonstrate the formation of an ordered "nano-domino-like" morphology with Au_0.4Ag_0.6 nanopillars as cylindrical cores and BTO as square shells. By comparing with the BTO-Au hybrid thin film, the BTO-Au_0.4Ag_0.6 alloyed film exhibits much broader plasmon resonance, hyperbolic dispersion, low-loss, and thermally robust features in the UV-vis-NIR wavelength region. This study provides a feasible platform for a complex alloyed plasmonic hybrid material design with low-loss and highly tunable optical properties toward all-optical integrated devices.

Control of Light Transmission in a Plasmonic Liquid Metacrystal

In this study, we experimentally demonstrated the control of light transmission through a slab of plasmonic liquid metacrystal by an external electric field. By applying the external static field, we were able to induce macroscopic anisotropy, which caused the polarization-dependent suppression of transmission at resonant frequencies. Such behavior indicates the selective plasmon excitation governed by the orientation of the meta-atoms with respect to the polarization of the electromagnetic wave. The problem of light transmission through a plasmonic liquid metacrystal was analyzed theoretically from first principles, and the obtained results were compared with the experimental data.

Near field excited state imaging via stimulated electron energy gain spectroscopy of localized surface plasmon resonances in plasmonic nanorod antennas

Continuous wave (cw) photon stimulated electron energy loss and gain spectroscopy (sEELS and sEEGS) is used to image the near field of optically stimulated localized surface plasmon resonance (LSPR) modes in nanorod antennas. An optical delivery system equipped with a nanomanipulator and a fiber-coupled laser diode is used to simultaneously irradiate plasmonic nanostructures in a (scanning) transmission electron microscope. The nanorod length is varied such that the m = 1, 2, and 3 LSPR modes are resonant with the laser energy and the optically stimulated near field spectra and images of these modes are measured. Various nanorod orientations are also investigated to explore retardation effects. Optical and electron beam simulations are used to rationalize the observed patterns. As expected, the odd modes are optically bright and result in observed sEEG responses. The m = 2 dark mode does not produce a sEEG response, however, when tilted such that retardation effects are operative, the sEEG signal emerges. Thus, we demonstrate that cw sEEGS is an effective tool in imaging the near field of the full set of nanorod plasmon modes of either parity.

Plasmonic color filter based on a hetero-metal-insulator-metal grating

Plasmonic color filters are expected to be candidates for application to complementary metal-oxide-semiconductor (CMOS) image sensor arrays with reduced pixel size, owing to the subwavelength mode volume of plasmons. Designs of metallic gratings based on the guided-mode resonance effect suffer from the sideband transmission issue due to high-order diffraction. Here, we propose a plasmonic color filter structure based on a hetero-metal-insulator-metal grating. The guided mode, in resonance with the second-order diffraction, is highly attenuated by the forbidden band, such that the sideband transmission can be suppressed. As calculated by using the transfer matrix method and the finite-difference time-domain method, the Al-ZnO-Ag waveguide-based structure presents a color filter characteristic with the peak transmittance greater than 70% and the peak wavelength tunable in the visible light band. It may find application in displays, image sensors, and biomedical imaging technologies.

A Perfect Absorber Based on Similar Fabry-Perot Four-Band in the Visible Range

A simple metamaterial absorber is proposed to achieve near-perfect absorption in visible and near-infrared wavelengths. The absorber is composed of metal-dielectric-metal (MIM) three-layer structure. The materials of these three-layer structures are Au, SiO₂, and Au. The top metal structure of the absorber is composed of hollow three-dimensional metal rings regularly arranged periodically. The results show that the high absorption efficiency at a specific wavelength is mainly due to the resonance of the Fabry-Perot effect (FP) in the intermediate layer of the dielectric medium, resulting in the resonance light being trapped in the middle layer, thus improving the absorption efficiency. The almost perfect multiband absorption, which is independent of polarization angle and insensitivity of incident angle, lends the absorber great application prospects for filtering and optoelectronics.

Multicomponent Plasmonic Nanoparticles: From Heterostructured Nanoparticles to Colloidal Composite Nanostructures

Plasmonic nanostructures possessing unique and versatile optoelectronic properties have been vastly investigated over the past decade. However, the full potential of plasmonic nanostructure has not yet been fully exploited, particularly with single-component homogeneous structures with monotonic properties, and the addition of new components for making multicomponent nanoparticles may lead to new-yet-unexpected or improved properties. Here we define the term "multi-component nanoparticles" as hybrid structures composed of two or more condensed nanoscale domains with distinctive material compositions, shapes, or sizes. We reviewed and discussed the designing principles and synthetic strategies to efficiently combine multiple components to form hybrid nanoparticles with a new or improved plasmonic functionality. In particular, it has been quite challenging to precisely synthesize widely diverse multicomponent plasmonic structures, limiting realization of the full potential of plasmonic heterostructures. To address this challenge, several synthetic approaches have been reported to form a variety of different multicomponent plasmonic nanoparticles, mainly based on heterogeneous nucleation, atomic replacements, adsorption on supports, and biomolecule-mediated assemblies. In addition, the unique and synergistic features of multicomponent plasmonic nanoparticles, such as combination of pristine material properties, finely tuned plasmon resonance and coupling, enhanced light-matter interactions, geometry-induced polarization, and plasmon-induced energy and charge transfer across the heterointerface, were reported. In this review, we comprehensively summarize the latest advances on state-of-art synthetic strategies, unique properties, and promising applications of multicomponent plasmonic nanoparticles. These plasmonic nanoparticles including heterostructured nanoparticles and composite nanostructures are prepared by direct synthesis and physical force- or biomolecule-mediated assembly, which hold tremendous potential for plasmon-mediated energy transfer, magnetic plasmonics, metamolecules, and nanobiotechnology.

Hybrid phonon-polaritons at atomically-thin van der Waals heterointerfaces for infrared optical modulation

Surface phonon polaritons (SPhPs) in polar dielectrics are potential candidates for infrared nanophotonics due to their low optical loss and long phonon life-time. However, the small confinement factors of bulk SPhPs, limits their applications that require small footprint and strong light-matter interaction. Here, we report that ultrathin van der Waals dielectrics (e.g., MoS₂ and h-BN) on Silicon Carbide enable ultra-confined dielectric tailored surface phonon polaritons (d-SPhPs) where the confinement factor can exceed 100. By creating a heterostructure of these vdW dielectrics with graphene, the d-SPhPs can hybridize with graphene plasmons which can be electrically tuned. By subwavelength patterning of the vdW dielectrics, these hybrid polaritons can be localized into ultra-small antenna volumes (λ0³/v_antenna ³~100³) with high-quality factor resonances (Q~85). Further, electric gating of graphene enables active tunability of these localized resonances which results in an electro-optic modulator with modulation depth exceeding 95%. Our report of manipulating and controlling ultra-confined SPhPs in van der Waals heterostructures, serves as a possible route for non-plasmonic platforms for infrared photodetectors, modulators and sensors.

Gate-tunable polariton superlens in 2D/3D heterostructures

Polaritons in polar-dielectrics and van der Waals (vdW) materials provide a channel for strong photon confinement. Precise control of their propagation could lead to deep sub-wavelength photonic devices. Here, we report negative refraction of hybrid surface phonon-hyperbolic polaritons (SPh-HP) at the interface of two-dimensional (2D) van der Waals layers such as hexagonal boron nitride (h-BN) and 3D semiconductors such as germanium and silicon carbide. These hybrid polariton modes have naturally negative group velocity arising from the intrinsic Type-I hyperbolicity of h-BN resulting in negative refraction at interfaces with positive group velocity. Using this phenomenon, we demonstrate an in-plane superlensing effect in an ultrathin (~10 nm) slab with spatial confinement of long Infrared wavelengths to below 200 nm focal spots. We further demonstrate electrical tunability of the superlens by controlling the Fermi level of graphene, thereby offering potential for miniaturized infrared to THz modulators, photodetectors as well as logic switches.

Nanostructured metals for light-based technologies

The basic theoretical understanding of light interacting with nanostructured metals that has existed since the early 1900s has become more relevant in the last two decades, largely because of new approaches to structure metals down to the nanometer scale or smaller. Here, a broad overview of the concepts and applications of nanostructuring metals for light-based technologies is given. The theory of the response of metals to an applied oscillating field is given, including a discussion of nonlocal, nonlinear and quantum effects. Using this metal response, the guiding of electromagnetic (light) waves using metals is given, with a particular emphasis on the impact of nanostructured metals for tighter confinement and slower propagation. Similarly, the influence of metal nanostructures on light scattering by isolated metal structures, like nanoparticles and nanoantennas, is described, with basic results presented including plasmonic/circuit resonances, the single channel limit, directivity enhancement, the maximum power transfer theorem, limits on the magnetic response from kinetic inductance and the scaling of gap plasmons to the nanometer scale and smaller. A brief overview of nanofabrication approaches to creating metal nanostructures is given. Finally, existing and emerging light-based applications are presented, including those for sensing, spectroscopy (including local refractive index, Raman, IR absorption), detection (including Schottky detectors), switching (including terahertz photoconductive antennas), modulation, energy harvesting and photocatalysis, light emission (including lasers and tunneling based light emission), optical tweezing, nonlinear optics, subwavelength imaging and lithography and high density data storage.

A Semi-Classical View on Epsilon-Near-Zero Resonant Tunneling Modes in Metal/Insulator/Metal Nanocavities

Metal/Insulator/Metal nanocavities (MIMs) are highly versatile systems for nanometric light confinement and waveguiding, and their optical properties are mostly interpreted in terms of surface plasmon polaritons. Although classic electromagnetic theory accurately describes their behavior, it often lacks physical insight, leaving some fundamental aspects of light interaction with these structures unexplored. In this work, we elaborate a quantum mechanical description of the MIM cavity as a double barrier quantum well. We identify the square of the imaginary part κ of the refractive index ñ of the metal as the optical potential and find that MIM cavity resonances are suppressed if the ratio n/κ exceeds a certain limit, which shows that low n and high κ values are desired for strong and sharp cavity resonances. Interestingly, the spectral regions of cavity mode suppression correspond to the interband transitions of the metals, where the optical processes are intrinsically non-Hermitian. The quantum treatment allows to describe the tunnel effect for photons and reveals that the MIM cavity resonances can be excited by resonant tunneling via illumination through the metal, without the need of momentum matching techniques such as prisms or grating couplers. By combining this analysis with spectroscopic ellipsometry on experimental MIM structures and by developing a simple harmonic oscillator model of the MIM for the calculation of its effective permittivity, we show that the cavity eigenmodes coincide with low-loss zeros of the effective permittivity. Therefore, the MIM resonances correspond to epsilon-near-zero (ENZ) eigenmodes that can be excited via resonant tunneling. Our approach provides a toolbox for the engineering of ENZ resonances throughout the entire visible range, which we demonstrate experimentally and theoretically. In particular, we apply our quantum mechanical approach to asymmetric MIM superabsorbers and use it for configuring broadly tunable refractive index sensors. Our work elucidates the role of MIM cavities as photonic analogues to tunnel diodes and opens new perspectives for metamaterials with designed ENZ response.

Manipulating the flow of light using Dirac-cone zero-index metamaterials

Metamaterials with a refractive index of zero exhibit properties that are important for integrated optics. Possessing an infinite effective wavelength and zero spatial phase change, zero-index metamaterials may be especially useful for routing on-chip photonic processes and reducing the footprint of nonlinear interactions. Zero-index has only been achieved recently in an integrated platform through a Dirac-cone dispersion, enabling some of these more exciting applications in an integrated platform. This paper presents an overview of Dirac-cone zero-index metamaterials, including the fundamental physics, history and demonstration in the optical regime, as well as current challenges and future directions.

Experimental demonstration of high sensitivity refractive index sensing based on magnetic plasmons in a simple metallic deep nanogroove array

A high-performance wide-angle refractive index sensor based on a simple one-dimensional (1D) metallic deep nanogroove array with a high aspect ratio is experimentally fabricated and demonstrated. The 1D deep groove array is featured by the excitation of magnetic plasmon (MP), referring to an effective coupling of incident electromagnetic waves with a strong magnetic response induced inside the deep grooves. Utilizing the MP resonances that are extremely sensitive to the surrounding dielectric medium, we successfully achieve a refractive index sensitivity (RIS) up to ∼1300 nm/RIU, which is higher than that of most experimentally designed plasmonic sensors in the infrared region. Importantly, benefiting from angle-independent MP resonances with strong confinement of the magnetic field inside the deep grooves and strong electric field localization at the groove openings, we demonstrate wide-angle sensing capability valid in a broadband infrared region with an excellent linear dependence on the change of refractive index. Such a MP-based sensor, together with its simple 1D flat nature and ease of fabrication, has great potential for the practical design of high sensitive, cost-effective and compact sensing devices.

Broadband Negative Refraction of Highly Squeezed Hyperbolic Polaritons in 2D Materials

Negative refraction of highly squeezed polaritons is a fundamental building block for nanophotonics, since it can enable many unique applications, such as deep-subwavelength imaging. However, the phenomenon of all-angle negative refraction of highly squeezed polaritons, such as graphene plasmons with their wavelength squeezed by a factor over 100 compared to free-space photons, was reported to work only within a narrow bandwidth (<1 THz). Demonstrating this phenomenon within a broad frequency range remains a challenge that is highly sought after due to its importance for the manipulation of light at the extreme nanoscale. Here we show the broadband all-angle negative refraction of highly squeezed hyperbolic polaritons in 2D materials in the infrared regime, by utilizing the naturally hyperbolic 2D materials or the hyperbolic metasurfaces based on nanostructured 2D materials (e.g., graphene). The working bandwidth can vary from several tens of THz to over a hundred of THz by tuning the chemical potential of 2D materials.

Reconfigurable phase-change meta-absorbers with on-demand quality factor control

Perfect absorber type devices are well-suited to many applications, such as solar cells, spatial light modulators, bio-sensors, and highly-sensitive photo-detectors. In such applications, a method for the design and fabrication of devices in a simple and efficient way, while at the same time maintaining design control over the key performance characteristics of resonant frequency, reflection coefficient at resonance and quality factor, would be particularly advantageous. In this work we develop such a method, based on eigenmode analysis and critical coupling theory, and apply it to the design of reconfigurable phase-change metasurface absorber devices. To validate the method, the design and fabrication of a family of absorbers was carried out with a range of 'on-demand' quality factors, all operating at the same resonant frequency and able to be fabricated simply and simultaneously on the same chip. Furthermore, by switching the phase-change layer between its amorphous and crystalline states, we show that our devices can provide an active or reconfigurable functionality.

Morphology dependent optical response tuning in planar square-shaped array of sodium nanoparticles

Among the plasmonic nanostructures, ordered arrangement of metal nanoparticles with inter-particle gap distances in the nanometer scale is becoming increasingly important due to their ability to confine huge electromagnetic fields and tunable optical properties. Using time dependent density functional theory calculations, we study the optical response evolution in a planar square-shaped array of Sodium nanoparticles via morphology deformation. To this aim, we vary the inter-particle gap distance in the range of 2 to 30 Å separately along one and two directions. We compare and cross-examine the optical response evolution for both deformation process, and we find that the interaction between sodium nanoparticles in an ordered arrangement can be controlled to a large extent by simple deformation process. We believe that our theoretical results will be useful for designing ultra-small and tunable plasmonic devices that utilize quantum effects.

Optical Chirality in Dispersive and Lossy Media

Several dynamical properties of electromagnetic waves such as energy, momentum, angular momentum, and optical helicity have been recently reexamined in dispersive and lossless media. Here, we address an alternative derivation for the optical chirality, extending it so as to include dissipative effects as well. To this end, we first elaborate on the most complete form of the conservation law for the optical chirality, without any restrictions on the nature of the medium. As a result we find a general expression for the optical chirality density both in lossless and lossy dispersive media. Our definition is perfectly consistent with that originally introduced for electromagnetic fields in free space, and is applicable to any material system, including dielectrics, plasmonic nanostructures, and left-handed metamaterials.

An eigenvalue approach to quantum plasmonics based on a self-consistent hydrodynamics method

Plasmonics has attracted much attention not only because it has useful properties such as strong field enhancement, but also because it reveals the quantum nature of matter. To handle quantum plasmonics effects, ab initio packages or empirical Feibelman d-parameters have been used to explore the quantum correction of plasmonic resonances. However, most of these methods are formulated within the quasi-static framework. The self-consistent hydrodynamics model offers a reliable approach to study quantum plasmonics because it can incorporate the quantum effect of the electron gas into classical electrodynamics in a consistent manner. Instead of the standard scattering method, we formulate the self-consistent hydrodynamics method as an eigenvalue problem to study quantum plasmonics with electrons and photons treated on the same footing. We find that the eigenvalue approach must involve a global operator, which originates from the energy functional of the electron gas. This manifests the intrinsic nonlocality of the response of quantum plasmonic resonances. Our model gives the analytical forms of quantum corrections to plasmonic modes, incorporating quantum electron spill-out effects and electrodynamical retardation. We apply our method to study the quantum surface plasmon polariton for a single flat interface.

Optical magnetism in planar metamaterial heterostructures

Harnessing artificial optical magnetism has previously required complex two- and three-dimensional structures, such as nanoparticle arrays and split-ring metamaterials. By contrast, planar structures, and in particular dielectric/metal multilayer metamaterials, have been generally considered non-magnetic. Although the hyperbolic and plasmonic properties of these systems have been extensively investigated, their assumed non-magnetic response limits their performance to transverse magnetic (TM) polarization. We propose and experimentally validate a mechanism for artificial magnetism in planar multilayer metamaterials. We also demonstrate that the magnetic properties of high-index dielectric/metal hyperbolic metamaterials can be anisotropic, leading to magnetic hyperbolic dispersion in certain frequency regimes. We show that such systems can support transverse electric polarized interface-bound waves, analogous to their TM counterparts, surface plasmon polaritons. Our results open a route for tailoring optical artificial magnetism in lithography-free layered systems and enable us to generalize the plasmonic and hyperbolic properties to encompass both linear polarizations.

Most natural materials do not have a magnetic response at optical frequencies and inducing optical magnetism by metamaterials typically requires complex nanostructures. Here, Papadakis et al. show that artificial optical magnetism can also be achieved with planar multilayer metamaterials.

Flexible manipulation of terahertz wave reflection using polarization insensitive coding metasurfaces

In order to extend to 3-bit encoding, we propose notched-wheel structures as polarization insensitive coding metasurfaces to control terahertz wave reflection and suppress backward scattering. By using a coding sequence of "00110011…" along x-axis direction and 16 × 16 random coding sequence, we investigate the polarization insensitive properties of the coding metasurfaces. By designing the coding sequences of the basic coding elements, the terahertz wave reflection can be flexibly manipulated. Additionally, radar cross section (RCS) reduction in the backward direction is less than -10dB in a wide band. The present approach can offer application for novel terahertz manipulation devices.

Harvesting the Energy of Multi-Polarized Electromagnetic Waves

We present the idea and design of a dual polarized metasurface for electromagnetic energy harvesting. A 4 × 4 super cell with alternating vias between adjacent cells was designed to allow for capturing the energy from various incident angles at an operating frequency of 2.4 GHz. The collected energy is then channeled to a feeding network that collects the AC power and feeds it to a rectification circuitry. The simulation results yielded a radiation to AC and an AC to DC conversion efficiencies of around 90% and 80%, respectively. As a proof of concept, an array consisting of 9 super cells was fabricated and measured. The experimental results show that the proposed energy harvester is capable of capturing up to 70% of the energy from a planewave having various polarizations and converting it to usable DC power.

Forward/Backward Switching of Plasmonic Wave Propagation Using Sign-Reversal Coupling

Backward waves with wave-front propagation opposite in direction to that of energy flow have attracted considerable interest in the context of photonic metamaterials. However, switching between forward and backward waves in the same frequency range has remained a challenge. Here, on a platform of coupled designer surface plasmon resonators in the microwave regime, multiband forward/backward switching of plasmonic wave propagation is demonstrated. This approach makes use of sign-reversal coupling that occurs when switching the coupling configuration between tightly confined photonic modes. Direct experimental measurements of plasmon dispersion curves confirm the forward/backward propagation of plasmonic waves in the same frequency range. This study provides a solution to forward/backward switching of subwavelength plasmonic wave propagation, and may find potential applications in photonic integrated systems.

All-angle negative refraction of highly squeezed plasmon and phonon polaritons in graphene-boron nitride heterostructures

A fundamental building block for nanophotonics is the ability to achieve negative refraction of polaritons, because this could enable the demonstration of many unique nanoscale applications such as deep-subwavelength imaging, superlens, and novel guiding. However, to achieve negative refraction of highly squeezed polaritons, such as plasmon polaritons in graphene and phonon polaritons in boron nitride (BN) with their wavelengths squeezed by a factor over 100, requires the ability to flip the sign of their group velocity at will, which is challenging. Here we reveal that the strong coupling between plasmon and phonon polaritons in graphene-BN heterostructures can be used to flip the sign of the group velocity of the resulting hybrid (plasmon-phonon-polariton) modes. We predict all-angle negative refraction between plasmon and phonon polaritons and, even more surprisingly, between hybrid graphene plasmons and between hybrid phonon polaritons. Graphene-BN heterostructures thus provide a versatile platform for the design of nanometasurfaces and nanoimaging elements.

Mode properties in metallic and non-metallic plasmonic waveguides

Non-metallic plasmonic materials have recently attracted research interest due to their adjustable plasmonic material properties and the potential low loss, which is important to plasmonic waveguides with ultrahigh mode confinement. In this paper, we analyzed the mode properties of four types of plasmonic waveguides based on noble metals, aluminum-zinc-oxide (AZO), and TiN, where the propagation length and mode size are chosen to compare the figures of merit. It is found that AZO has the smallest imaginary part of permittivity in the near-infrared region, while AZO waveguides have propagation lengths comparable to those of Cu waveguides but shorter than those of Au and Ag waveguides. Furthermore, due to the larger real part of permittivities, the mode sizes of the AZO and TiN waveguides are smaller than those of the metal waveguides, in particular, for the insulator-metal-insulator waveguide and dielectric-loaded plasmonic waveguide. AZO/ZnO films with tunable carrier density between 1.8×10¹⁷/cm³ and 8.6×10²⁰/cm³ were grown by pulsed-laser deposition. Metal-like properties, i.e., negative real part of permittivity around 1550 nm, were observed, predicting an interesting candidate in the plasmonic optical interconnect.

Meta-Chirality: Fundamentals, Construction and Applications

Chiral metamaterials represent a special type of artificial structures that cannot be superposed to their mirror images. Due to the lack of mirror symmetry, cross-coupling between electric and magnetic fields exist in chiral mediums and present unique electromagnetic characters of circular dichroism and optical activity, which provide a new opportunity to tune polarization and realize negative refractive index. Chiral metamaterials have attracted great attentions in recent years and have given rise to a series of applications in polarization manipulation, imaging, chemical and biological detection, and nonlinear optics. Here we review the fundamental theory of chiral media and analyze the construction principles of some typical chiral metamaterials. Then, the progress in extrinsic chiral metamaterials, absorbing chiral metamaterials, and reconfigurable chiral metamaterials are summarized. In the last section, future trends in chiral metamaterials and application in nonlinear optics are introduced.

Efficient finite element resolution of gyromagnetic and gyroelectric nonreciprocal electromagnetic problems

We present a rigorous derivation of the weak-form formulation of the Helmholtz equation for electromagnetic structures incorporating general nonreciprocal, dispersive materials such as magnetized ferrites or magnetized free-carrier plasmas. This formulation allows an efficient self-consistent treatment using finite elements of a variety of problems involving magnetic or magneto-optical materials biased by an external DC field where the eigenmodes become nonreciprocal or even unidirectional. The possibilities of this method are illustrated with several examples of TE-polarized modes at microwave frequencies and TM-polarized modes at optical and infrared wavelengths.

High-Performance Doped Silver Films: Overcoming Fundamental Material Limits for Nanophotonic Applications

The field of nanophotonics has ushered in a new paradigm of light manipulation by enabling deep subdiffraction confinement assisted by metallic nanostructures. However, a key limitation which has stunted a full development of high-performance nanophotonic devices is the typical large losses associated with the constituent metals. Although silver has long been known as the highest quality plasmonic material for visible and near infrared applications, its usage has been limited due to practical issues of continuous thin film formation, stability, adhesion, and surface roughness. Recently, a solution is proposed to the above issues by doping a proper amount of aluminum during silver deposition. In this work, the potential of doped silver for nanophotonic applications is presented by demonstrating several high-performance key nanophotonic devices. First, long-range surface plasmon polariton waveguides show propagation distances of a few centimeters. Second, hyperbolic metamaterials consisting of ultrathin Al-doped Ag films are attained having a homogeneous and low-loss response, and supporting a broad range of high-k modes. Finally, transparent conductors based on Al-doped Ag possess both a high and flat transmittance over the visible and near-IR range.

Utilizing the power of Cerenkov light with nanotechnology

The characteristic blue glow of Cerenkov luminescence (CL) arises from the interaction between a charged particle travelling faster than the phase velocity of light and a dielectric medium, such as water or tissue. As CL emanates from a variety of sources, such as cosmic events, particle accelerators, nuclear reactors and clinical radionuclides, it has been used in applications such as particle detection, dosimetry, and medical imaging and therapy. The combination of CL and nanoparticles for biomedicine has improved diagnosis and therapy, especially in oncological research. Although radioactive decay itself cannot be easily modulated, the associated CL can be through the use of nanoparticles, thus offering new applications in biomedical research. Advances in nanoparticles, metamaterials and photonic crystals have also yielded new behaviours of CL. Here, we review the physics behind Cerenkov luminescence and associated applications in biomedicine. We also show that by combining advances in nanotechnology and materials science with CL, new avenues for basic and applied sciences have opened.

Tunable plasmon lensing in graphene-based structure exhibiting negative refraction

We propose a novel method to achieve tunable plasmon focusing in graphene/photonic-crystal hybrid structure exhibiting all-angle negative refraction at terahertz frequencies. A two-dimensional photonic crystal composed of a square lattice of dielectric rods is constructed on the substrate of a graphene sheet to provide the hyperbolic dispersion relations of the graphene plasmon, giving rise to the all-angle plasmonic negative refraction. Plasmon lensing induced from the negative refraction is observed. We show that the ultracompact graphene-based system can produce sub-diffraction-limited images with the resolution significant smaller than the wavelength of the incident terahertz wave. Moreover, by adjusting the Fermi energy of the graphene, the imaging performance of the proposed system can remain almost invariant for different frequencies. Our results may find applications in diverse fields such as subwavelength spatial light manipulation, biological imaging, and so forth.

Broadband and Broad-angle Polarization-independent Metasurface for Radar Cross Section Reduction

In this work, a broadband and broad-angle polarization-independent random coding metasurface structure is proposed for radar cross section (RCS) reduction. An efficient genetic algorithm is utilized to obtain the optimal layout of the unit cells of the metasurface to get a uniform backscattering under normal incidence. Excellent agreement between the simulation and experimental results show that the proposed metasurface structure can significantly reduce the radar cross section more than 10 dB from 17 GHz to 42 GHz when the angle of incident waves varies from 10° to 50°. The proposed coding metasurface provides an efficient scheme to reduce the scattering of the electromagnetic waves.

Dielectric-Like Behavior of Graphene in Au Plasmon Resonator

Graphene has proven to be a promising conductive layer in fabricating optical plasmon resonators on insulator substrate using electron beam lithography and has the potential to construct electrically controlled active plasmon resonators. In this study, we investigate the effect of graphene on plasmon resonance using graphene and Au plasmon resonator system as a model at visible and near-infrared wavelength. Our experiment data show that the presence of graphene does not weaken and annihilate the plasmon resonance peaks, instead it predominantly makes the peaks redshift, which is similar to the behavior of depositing SiO₂ film on Au plasmon resonators. This fact indicates that graphene predominantly exhibits dielectric-like behavior at visible and near-infrared wavelength, which can be attributed to the low carrier density in graphene compared with metals.

Fresnel Refraction and Diffraction of Surface Plasmon Polaritons in Two-Dimensional Conducting Sheets

The propagation of surface plasmon polaritons (SPPs) along two-dimensional (2D) materials, such as graphene, is a complex phenomenon linking the microscale electronic properties to macroscale optical properties. Complex geometries increase the complexity of understanding the nature and performance of optoelectronic devices based on surface wave propagation. Here, we demonstrate that under a proper design of macroscopic conductivity profile, the propagation characteristics of SPPs in 2D materials can be made analogous to the propagation of plane waves in homogeneous layers with minimal out-of-plane scattering. Such a direct resemblance enables prediction, design, and calculation of SPP propagation through advanced geometries using fundamental laws of optics. We demonstrate that the propagation of surface waves can be manipulated in-plane using reflection, refraction, diffraction, and also generalized refraction laws analogous to plane waves. We present simple mathematical models to calculate the scattered electromagnetic fields of SPP waves based on Fresnel equations. The presented formulation could facilitate the transfer of many existing plane wave based optical phenomenon to a surface wave based integrated optoelectronic devices.

Wide band dispersionless slow light in hetero-MIM plasmonic waveguide

A flat slow-light band over a wide frequency range is obtained in the hetero-MIM (metal-insulator-metal) waveguide with zero group velocity dispersion (GVD). The zero GVD originates from dispersion compensation by the photonic mode and the plasmonic mode, the mechanism of which does not exist in the homo-MIM structure. By changing dielectric permittivity of the insulator or the difference of two different metallic plasma frequencies, the group index and the bandwidth can be tuned. The dispersionless slow light characteristic in the hetero-MIM waveguide may be useful in the new design of plasmonic devices.

Plasmonic coupled modes in metal-dielectric multilayer structures: Fano resonance and giant field enhancement

We provide an overview of Fano resonance and plasmon induced transparency (PIT) as well as on plasmons coupling in planar structures, and we discuss their application in sensing and enhanced spectroscopy. Metal-insulator-metal (MIM) structures, which are known to support symmetric and anti-symmetric surface plasmon polaritons (SPPs) arising from the coupling between two SPPs at the metal-insulator interfaces, exhibit anticrossing behavior of the dispersion relations arising from the coupling of the symmetric SPP and the metal/air SPP. Multilayer structures, formed by a metal film and a high-index dielectric waveguide (WG), separated by a low-index dielectric spacer layer, give narrow resonances of PIT and Fano line shapes. An optimized Fano structure shows a giant field intensity enhancement value of 10⁶ in air at the surface of the high-index dielectric WG. The calculated field enhancement factor and the figure of merit for the sensitivity of the Fano structure in air can be 10⁴ times as large as those of the conventional surface plasmon resonance and WG sensors.

Modal evolution in asymmetric three- and four-layer plasmonic waveguides

Through the employment of a novel approach in solving the dispersion for the three-layer plasmonic waveguides, considering lossy metals, we demonstrate that, besides well-known modes, the complete dispersion always contains high-lossy periodic solutions. Consideration of these solutions is shown to be crucial for the understanding of every aspect of dispersion evolution at broad spectral range when the thickness of the middle layer is varied. In particular, we show that generally considered modes of the three-layer waveguide transform into the single interface modes via interaction with high-lossy periodic solutions. Furthermore, the negative index mode is shown to experience a transition between low- and high-lossy regimes depending on the waveguide's thickness. Our results, avoiding complicated analytical analysis, perfectly integrate and importantly complement past theoretical works.

Plasmonic Structures, Materials and Lenses for Optical Lithography beyond the Diffraction Limit: A Review

The rapid development of nanotechnologies and sciences has led to the great demand for novel lithography methods allowing large area, low cost and high resolution nano fabrications. Characterized by unique sub-diffraction optical features like propagation with an ultra-short wavelength and great field enhancement in subwavelength regions, surface plasmon polaritons (SPPs), including surface plasmon waves, bulk plasmon polaritons (BPPs) and localized surface plasmons (LSPs), have become potentially promising candidates for nano lithography. In this paper, investigations into plasmonic lithography in the manner of point-to-point writing, interference and imaging were reviewed in detail. Theoretical simulations and experiments have demonstrated plasmonic lithography resolution far beyond the conventional diffraction limit, even with ultraviolet light sources and single exposure performances. Half-pitch resolution as high as 22 nm (~1/17 light wavelength) was observed in plasmonic lens imaging lithography. Moreover, not only the overview of state-of-the-art results, but also the physics behind them and future research suggestions are discussed as well.

Direct observation of negative-index microwave surface waves

Waves propagating in a negative-index material have wave-front propagation (wavevector, k) opposite in direction to that of energy flow (Poynting vector, S). Here we present an experimental realisation at microwave frequencies of an analogous surface wave phenomenon whereby a metasurface supports a surface mode that has two possible wavevector eigenstates within a narrow band of frequencies: one that supports surface waves with positive mode index, and another that supports surface waves with negative mode index. Phase sensitive measurements of the near-field of surface waves across the metasurface show the contrasting spatial evolution of the two eigenstates, providing a unique opportunity to directly observe the negative-index phenomenon.