Optical Metamaterials Research Articles

Non-Hermitian photonic lattices: tutorial

Non-Hermitian photonic lattices combine the peculiar consequences of energy non-conservation with the physics of bandstructures, giving rise to a variety of exotic properties not found in conventional materials or photonic metamaterials. In this tutorial, we introduce the key concepts in the design and implementation of non-Hermitian photonic lattices, including the general features of non-Hermitian lattice Hamiltonians and their bandstructures, the role of non-Hermitian lattice symmetries, and the topological chracterization of non-Hermitian bandstructures. We survey several important non-Hermitian lattice designs, as well as the photonics platforms on which they can be realized. Finally, we discuss the possibilities for future developments in the field.

Flexible but Refractory Single-Crystalline Hyperbolic Metamaterials

The fabrication of flexible single-crystalline plasmonic or photonic components in a scalable way is fundamentally important to flexible electronic and photonic devices with high speed, high energy efficiency, and high reliability. However, it remains a challenge. Here, we have successfully synthesized flexible single-crystalline optical hyperbolic metamaterials by directly depositing refractory nitride superlattices on flexible fluorophlogopite-mica substrates with magnetron sputtering. Interestingly, these flexible hyperbolic metamaterials show dual-band hyperbolic dispersion of dielectric constants with small dielectric losses and high figures of merit in the visible to near-infrared ranges. More importantly, the optical properties of these nitride-based flexible hyperbolic metamaterials show remarkable stability during 1000 °C heating or after being bent 1000 times. Therefore, the strategy developed in this work offers an easy and scalable route for fabricating flexible, high-performance, and refractory plasmonic or photonic components, which can significantly expand the applications of current electronic and photonic devices.

Electrochemical photonics: a pathway towards electrovariable optical metamaterials.

This review article focuses on the latest achievements in the creation of a class of electrotuneable optical metamaterials for switchable mirrors/windows, variable colour mirrors, optical filters, and SERS sensors, based on the voltage-controlled self-assembly of plasmonic nanoparticles at liquid/liquid or solid/liquid electrochemical interfaces. Practically, these experimental systems were navigated by physical theory, the role of which was pivotal in defining the optimal conditions for their operation, but which itself was advanced in feedback with experiments. Progress and problems in the realisation of the demonstrated effects for building the corresponding devices are discussed. To put the main topic of the review in a wider perspective, the article also discusses a few other types of electrovariable metamaterials, as well as some of those that are controlled by chemistry.

Magnetron sputter deposition of Ta2O5-SiO2 quantized nanolaminates.

Quantized nanolaminates are a type of optical metamaterials, which were discovered only recently. Their feasibility was demonstrated by atomic layer deposition and ion beam sputtering so far. In this paper, we will report on the successful magnetron sputter deposition of quantized nanolaminates based on Ta2O5-SiO2. We will describe the deposition process, show results and material characterization of films deposited in a very wide parameter range. Furthermore, we will show how quantized nanolaminates deposited by magnetron sputtering were used in optical interference coatings such as antireflection and mirror coatings.

Optical Processes behind Plasmonic Applications.

Plasmonics is a revolutionary concept in nanophotonics that combines the properties of both photonics and electronics by confining light energy to a nanometer-scale oscillating field of free electrons, known as a surface plasmon. Generation, processing, routing, and amplification of optical signals at the nanoscale hold promise for optical communications, biophotonics, sensing, chemistry, and medical applications. Surface plasmons manifest themselves as confined oscillations, allowing for optical nanoantennas, ultra-compact optical detectors, state-of-the-art sensors, data storage, and energy harvesting designs. Surface plasmons facilitate both resonant characteristics of nanostructures and guiding and controlling light at the nanoscale. Plasmonics and metamaterials enable the advancement of many photonic designs with unparalleled capabilities, including subwavelength waveguides, optical nanoresonators, super- and hyper-lenses, and light concentrators. Alternative plasmonic materials have been developed to be incorporated in the nanostructures for low losses and controlled optical characteristics along with semiconductor-process compatibility. This review describes optical processes behind a range of plasmonic applications. It pays special attention to the topics of field enhancement and collective effects in nanostructures. The advances in these research topics are expected to transform the domain of nanoscale photonics, optical metamaterials, and their various applications.

Transverse Hypercrystals Formed by Periodically Modulated Phonon Polaritons

Photonic crystals and metamaterials are two overarching paradigms for manipulating light. By combining these approaches, hypercrystals can be created, which are hyperbolic dispersion metamaterials that undergo periodic modulation and mix photonic-crystal-like aspects with hyperbolic dispersion physics. Despite several attempts, there has been limited experimental realization of hypercrystals due to technical and design constraints. In this work, hypercrystals with nanoscale lattice constants ranging from 25 to 160 nm were created. The Bloch modes of these crystals were then measured directly using scattering near-field microscopy. The dispersion of the Bloch modes was extracted from the frequency dependence of the Bloch modes, revealing a clear switch from positive to negative group velocity. Furthermore, spectral features specific to hypercrystals were observed in the form of sharp density of states peaks, which are a result of intermodal coupling and should not appear in ordinary polaritonic crystals with an equivalent geometry. These findings are in agreement with theoretical predictions that even simple lattices can exhibit a rich hypercrystal bandstructure. This work is of both fundamental and practical interest, providing insight into nanoscale light-matter interactions and the potential to manipulate the optical density of states.

Discussion on localized pulses for third-order nonlinear Schrödinger equation and negative index material with quadratic cubic nonlinearity

In this paper, we study the third-order nonlinear Schrödinger equation (NLSE) and optical metamaterial (OM) with quadratic-cubic nonlinearity (OM-QCNL) to describe the properties and presence of propagating solitary waves (SW) in a fibre optic medium. We present an effective field amplitude ansatz that allows us to obtain SW using the governing model. More specifically, in the presence of all orders of dispersion and various nonlinear variables, we find SW solutions of dipole, bright, dark and other forms. The graphical representation of these waves will also be presented.

Active topological phase transitions in high-order elastic topological insulators driven by pneumatic methods and liquid metals

Active topological phase transitions widely occur in active matters and biological systems, such as developing embryos. Since the discovery of the intriguing bulk-boundary effects of topological insulators in Hermitian and non-Hermitian systems, various electric, optical, acoustic, and mechanical topological metamaterials with efficient energy transmission and robust defect-immunization have been designed. To date, however, it remains a challenge to precisely and fast manipulate the topological phase transitions in elastic topological insulators. In this paper, on the basis of theoretical analysis and numerical simulations, we propose an active strategy to achieve this aim through a combination of pneumatic actuation and liquid metals. The proposed method can precisely tune the connecting stiffness and vertex mass in the tight Su–Schrieffer–Heeger model. Thus, we realize the effective and fast control of topological phase transitions and elastic wave bandgap switching. We also uncover the active spinning bulk-boundary effects and higher-order topological states in the elastic topological insulators, demonstrating the high effectiveness and practicability of the proposed method. In addition, the differences between the 1D edge and 0D corner higher-order states are specified by information entropy theory. This work not only gains insights into the active manipulation of topological phase transitions but also inspires novel strategies to design active topological materials through untethered methods, e.g., magnetism or biological cells.

Broadband circular dichroism in chiral plasmonic woodpiles

The circular dichroism (CD) of a material is the difference in optical absorption under left- and right-circularly polarized illumination. It is crucial for a number of applications, from molecular sensing to the design of circularly polarized thermal light sources. The CD in natural materials is typically weak, leading to the exploitation of artificial chiral materials. Layered chiral woodpile structures are well known to boost chiro-optical effects when realized as a photonic crystal or an optical metamaterial. We here demonstrate that light scattering at a chiral plasmonic woodpile, which is structured on the order of the wavelength of the light, can be well understood by considering the fundamental evanescent Floquet states within the structure. In particular, we report a broadband circular polarization bandgap in the complex band structure of various plasmonic woodpiles that spans the optical transparency window of the atmosphere between 3 and 4 upmum and leads to an average CD of up to 90% within this spectral range. Our findings could pave the way for an ultra-broadband circularly polarized thermal source.

Fast tunable metamaterial liquid crystal achromatic waveplate.

Photonic metamaterials combined with liquid crystals (LCs) for tunability is a great niche for building miniature devices with high performance such as fast flat tunable lenses, tunable filters, and waveplates. Sub-wavelength or nano-grating surfaces are homogenized to uniaxial waveplates with negative birefringence of unique dispersion when the period is less than the wavelength by at least a few times. This uniaxial metasurface, combined with the LC layer, is shown to act as a tunable retardation achromatic waveplate with 8μm thick LC layer operating over wide spectral and angular ranges, as compared to using two nematic liquid crystal (NLC) retarders of thicknesses on the order of 30-60μm, when no metasurface is used. Hence the device becomes miniature and 50× faster due to the thinner liquid crystal layer. The silicon nano-grating of 351nm pitch and 0.282 fill factor is designed and fabricated to operate in the short-wave infrared range (SWIR). Switching between three achromatic retardation levels: full-, half-, and quarter-waveplates is accomplished by changing the applied voltages on the NLC cell with a switching time of a few milliseconds. This device has applications in fast broadband shutters, low coherence phase shift interferometry, ellipso-polarimetry, dynamic control of light intensity, and smart windows.

Square-Root Higher-Order Topology in Rectangular-Lattice Acoustic Metamaterials

Square-root topology offers a distinctive scheme towards topological phases with unconventional spectral properties. In many cases, the formation of square-root topology is governed by the inherent lattice symmetry, with honeycomb lattices being prominent examples. Here, we report on the experimental discovery of square-root higher-order topological insulator phases in an acoustic metamaterial with decorated rectangular lattice. Through acoustic pump-probe techniques, we explore the boundary-dependent topological edge and corner states in both the parent and square-root acoustic metamaterials. We further validate their higher-order topologies as well as their spectral connections via various simulations. Our work not only substantiates the square-root higher-order topology in rectangular-lattice systems from the experimental aspect, but also serves as a step towards versatile higher-order topological phenomena in photonic, acoustic, and mechanical metamaterials.

Anisotropic optical and magnetic response in self-assembled TiN–CoFe2 nanocomposites

Transition metal nitrides (e.g., TiN) have shown tremendous promise in optical metamaterials for nanophotonic devices due to their plasmonic properties comparable to noble metals and superior high temperature stability. Vertically aligned nanocomposites (VANs) offer a great platform for combining two dissimilar functional materials with a one-step deposition technique toward multifunctionality integration and strong structural/property anisotropy. Here, we report a two-phase nanocomposite design combining ferromagnetic CoFe2 nanosheets in the plasmonic TiN matrix as a new hybrid plasmonic metamaterial. The hybrid metamaterials exhibit anisotropic optical and magnetic responses, as well as a pronounced magneto-optical coupling response evidenced by Magneto-optic Kerr Effect measurement, owing to the novel vertically aligned structure. This work demonstrates a new TiN-based metamaterial with anisotropic properties and multifunctionality toward light polarization modulation, optical switching, and integrated optics.

Overview of Optical Biosensors for Early Cancer Detection: Fundamentals, Applications and Future Perspectives

Conventional cancer detection and treatment methodologies are based on surgical, chemical and radiational processes, which are expensive, time consuming and painful. Therefore, great interest has been directed toward developing sensitive, inexpensive and rapid techniques for early cancer detection. Optical biosensors have advantages in terms of high sensitivity and being label free with a compact size. In this review paper, the state of the art of optical biosensors for early cancer detection is presented in detail. The basic idea, sensitivity analysis, advantages and limitations of the optical biosensors are discussed. This includes optical biosensors based on plasmonic waveguides, photonic crystal fibers, slot waveguides and metamaterials. Further, the traditional optical methods, such as the colorimetric technique, optical coherence tomography, surface-enhanced Raman spectroscopy and reflectometric interference spectroscopy, are addressed.

Nanoscale Characterization of Individual Three-Dimensional Split Ring Resonator Systems

In this work, we investigate the hybridization of three-dimensional plasmonic split ring resonator (SRR) pairs using focused electron beam nanospectroscopy and model their combined electric and magnetic responses using electromagnetic theory and numerical calculations. Specifically, we fabricate three-dimensional (3D) SRR dimers with varying in-plane rotations and out-of-plane tilts and perform electron energy loss spectroscopy (EELS) measurements to elucidate the impact of their 3D electric and magnetic interactions on the EEL spectrum of each individual SRR dimer. On the basis of the competition between the SRR’s electric and magnetic interactions within our model, we find that varying the 3D tilt angle diminishes the magnetic coupling but increases the level of overall mode mixing. Through further modeling of the experimental data, we determine the system’s electric and magnetic coupling constants, as well as the overall effective coupling constant, a useful metric for characterizing the strength of light–matter interaction. We additionally explore the potential for geometric frustration in the magnetic mode ordering of a coupled SRR trimer using both EELS experiment and companion theoretical modeling. Taken together, the insight gained into the behavior of coupled 3D SRRs serves as a stepping stone to the rational design of 3D photonic metamaterials endowed with even richer optical functionality.

Evolution of the edge states and corner states in a multilayer honeycomb valley-Hall topological metamaterial

The valley-Hall effect provides topological protection to a broad class of defects in valley-Hall photonic topological metamaterials. Unveiling precisely how such protection is achieved and its implications in practical implementations is paramount to move from fundamental science to applications. To this end, we investigate a honeycomb valley-Hall topological metamaterial and monitor the evolution of the topological valley-Hall edge states and higher-order corner states under different perturbation $\ensuremath{\delta}R$. The evolutions of the edge states of the armchair and zigzag interfaces are demonstrated, respectively. By adjusting the geometric parameters and introducing disturbances to break the inversion symmetry, we achieve the edge states with different modes including the conventional crossed edge state and the specific gapped edge state. It is found that the edge states of topological valley kinking will gradually separate with the increase of $\ensuremath{\delta}R$, and finally a complete gap between the edge states appears. The gap has rarely been reported previously in topological materials fabricated by printed circuit board technology. In addition, the higher-order topological corner states can also be observed in the proposed topological metamaterial. The higher-order topological phase is theoretically characterized by nontrivial bulk polarization and the Wannier centers. Our results show that the corner state localization becomes stronger with the increase of $\ensuremath{\delta}R$. It is expected that our results will provide a platform for the realization of optical topological insulators.

Manipulation of Scattering Spectra with Topology of Light and Matter

AbstractStructured lights, including beams carrying spin and orbital angular momenta, radially and azimuthally polarized vector beams, as well as spatiotemporal optical vortices, have attracted significant interest due to their unique amplitude, phase front, polarization, and temporal structures, enabling a variety of applications in optical and quantum communications, micromanipulation, and super‐resolution imaging. In parallel, structured optical materials, metamaterials, and metasurfaces consisting of engineered unit cells—meta‐atoms, opened new avenues for manipulating the flow of light and optical sensing. While several studies explored structured light effects on the individual meta‐atoms, their shapes are largely limited to simple spherical geometries. However, the synergy of the structured light and complex‐shaped meta‐atoms has not been fully explored. In this paper, the role of the helical wavefront of Laguerre–Gaussian beams in the excitation and suppression of higher‐order resonant modes inside all‐dielectric meta‐atoms of various shapes, aspect ratios, and orientations, is demonstrated and the excitation of various multipolar moments that are not accessible via unstructured light illumination is predicted. The presented study elucidates the role of the complex phase distribution of the incident light in shape‐dependent resonant scattering, which is of utmost importance in a wide spectrum of applications ranging from remote sensing to spectroscopy.

Piezoelectric properties enhancement of perforated ferroelectric thin films during in-plane polarization reversal

In this work, we present a result of PFM measurements on perforated and nonperforated Ba0,8Sr0,2TiO3 thin epitaxial films. A huge enhancement of effective values for both lateral and vertical piezoelectric tensors during polarization reversal was experimentally detected. An assumption of short-range quadrant domain pattern in perforated ferroelectric structure was discussed. The obtained results are crucial for calculations of ferroelectric polarization in ferroelectric photonic crystals and metamaterials.

An ultra-narrow-band optical filter based on zero refractive index metamaterial

Owing to the photonic band gap effect and defect state effect, photonic metamaterials have received much attention in the design of narrow bandpass filters, which are the key devices of optical communication equipment such as wavelength division multiplexing devices. In this work, based on zero-index metamaterial (ZIM), a compact filter with both high peak transmission coefficient and ultra-narrow bandwidth is proposed. The photonic metamaterial with conical dispersion and Dirac-like point is achieved by optimizing the structure and material component parameters of dielectric rods with square lattice in air. It is demonstrated that a triply degenerate state can be realized at the Dirac-like point, which relates this metamaterial to a zero-index medium with effective permittivity and permeability equal to zero simultaneously. Electromagnetic (EM) wave can propagate without any phase delay at this frequency, and strong dispersion occurs in the adjacent frequency cone, leading to dramatic changes in optical properties. We introduce a ZIM into photonic metamaterial defect filter to compress the bandwidth to the realization of ultra-narrow bandpass filter. The ZIM is embedded into the resonant cavity of line defect filter, which is also composed of dielectric rods with square lattice in air. In order to increase the sensitivity of the phase change with frequency, the Dirac-like frequency is adjusted to match the resonant frequency of the filter. We study the transmission spectrum of the structure through the COMSOL Multiphysics simulation software, and find that the peak width at half-maximum of the filter decreases as the thickness of ZIM increases, and the peak transmittance is still high when bandwidth is greatly compressed. The zero phase delay inside the slab can be observed. Through field distribution analysis, the zero-phase delay and strong coupling characteristics of electromagnetic field are observed at peak frequency. The comparison with conventional photonic metamaterials filter is discussed. We believe that this work is helpful in investigating the realization of ultra-narrow bandpass filters.

ZnO-ferromagnetic metal vertically aligned nanocomposite thin films for magnetic, optical and acoustic metamaterials.

Magnetoacoustic waves generated in piezoelectric and ferromagnetic coupled nanocomposite films through magnetically driven surface acoustic waves present great promise of loss-less data transmission. In this work, ferromagnetic metals of Ni, Co and Co x Ni1-x are coupled with a piezoelectric ZnO matrix in a vertically-aligned nanocomposite (VAN) thin film platform. Oxidation was found to occur in the cases of ZnO-Co, forming a ZnO-CoO VAN, while only very minor oxidation was found in the case of ZnO-Ni VAN. An alloy approach of Co x Ni1-x has been explored to overcome the oxidation during growth. Detailed microstructural analysis reveals limited oxidation of both metals and distinct phase separation between the ZnO and the metallic phases. Highly anisotropic properties including anisotropic ferromagnetic properties and hyperbolic dielectric functions are found in the ZnO-Ni and ZnO-Co x Ni1-x systems. The magnetic metal-ZnO-based hybrid metamaterials in this report present great potential in coupling of optical, magnetic, and piezoelectric properties towards future magnetoacoustic wave devices.

Effect of geometry, anisotropy and composition on the third-order optical nonlinearities of multilayer hyperbolic metamaterials -INVITED

Quantum technologies require advanced optical metamaterials whose properties can be tailored and controlled as desired. Hyperbolic metamaterials have great potential for applications in nonlinear nanophoton-ics, such as all-optical switching, optical limiting, mode-locking and optical sensing. In this work, we show how to obtain strong third-order optical nonlinearities in hyperbolic multilayers exploiting the effect of bulk plas-mon polaritons. We demonstrate the tunability of these properties with angle and polarization, and we propose a model to predict them. We evidence the enhancement of the nonlinear response in low-loss metamaterials.