Proceedings Volume 12130

Metamaterials XIII

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Proceedings Volume 12130

Metamaterials XIII

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Volume Details

Date Published: 7 June 2022
Contents: 9 Sessions, 20 Papers, 11 Presentations
Conference: SPIE Photonics Europe 2022
Volume Number: 12130

Table of Contents

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Table of Contents

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  • Front Matter: Volume 12130
  • Analytical and Numerical Modelling
  • Tunable, Switchable, Reconfigurable, and Programmable Metamaterials I
  • Chiral, Bianisotropic, and Hyperbolic Metamaterials
  • Subwavelength Concentration of Light and Imaging
  • Machine Learning in Metamaterial Design and Application
  • Novel Effects in Metamaterials: PT-Symmetry, Quantum and Topological Phenomena
  • Metamaterial Multiphysics and Integration
  • Poster Session
Front Matter: Volume 12130
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Front Matter: Volume 12130
This PDF file contains the front matter associated with SPIE Proceedings Volume 12130, including the Title Page, Copyright information, Table of Contents, and Conference Committee listings.
Analytical and Numerical Modelling
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Multiple scattering analysis of resonant modes in quasi-periodic crystals
Marc Martí-Sabaté, Daniel Torrent
Finite clusters of scatterers attached to a thin elastic plate are analyzed by means of multiple scattering theory. Two quasi-periodic distributions are considered: quasi-periodic lines and twisted bilayers. The former consist in a periodic lattice where an incommensurate modulation is superimposed. The latter are formed by the superposition of two-dimensional periodic lattices with a relative angle between them. These structures show a great variety of modes, which can be thoroughly analyzed with multiple scattering theory. The quality factor of the found resonances will be discussed, and the influence of mirror symmetry and anisotropy factor will be analyzed. Results show the relevance of quasi-periodic structures as candidates for high quality wave-trapping devices.
Removing grazing incidence reflection with half-bound states and non-Hermitian systems
Dean A. Patient, Simon A. R. Horsley
Grazing incidence waves incident onto a surface will almost always be completely reflected. Here, we focus on removing reflection at grazing incidence, adopting the factorisation method from quantum mechanics and applying it to the Helmholtz equation that governs a single electromagnetic polarisation. We show that there are two approaches, the first is to require real dielectric profiles that support a half-bound state at grazing incidence. The second is to allow non-Hermitian dielectric profiles that exhibit PT symmetry, supporting waves with constant intensity throughout the profile.
Tunable, Switchable, Reconfigurable, and Programmable Metamaterials I
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Optically controllable coupling between edge and topological waveguide modes of certain graphene metasurfaces
In this paper, optically controllable and topologically protected plasmon transport is implemented via a topological nanohole plasmonic waveguide coupled to a standard edge mode of a graphene metasurface. By introducing nanoholes with different sizes in the unit cell, one breaks the spatial-inversion symmetry of a graphene metasurface in which the topological waveguide is constructed, leading to the emergence of topologically protected modes located in a nontrivial band-gap. Based on the strong Kerr effect and tunable optical properties of graphene, the coupling between the edge and topological interface modes can be efficiently controlled by optical means provided by an optical pump beam injected in a bulk mode. In particular, by tuning the power inserted in the bulk mode, one can control the difference between the wave-vectors of the topological and edge modes and consequently the optical power coupled in the topological mode. Our results show that when the pump power approaches a specific value, the edge and topological modes become phase-matched and the topological waveguide mode can be efficiently excited. Finally, we demonstrated that the optical coupling is strongly dependent on the group-velocity of the pump mode, a device feature that can be important in practical applications.
Chiral, Bianisotropic, and Hyperbolic Metamaterials
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Chirality enhancement in substrate supported nanostructures
Circular dichroism spectroscopy is an important tool for detecting chiral molecules. Chirality is a feature of many biologically active molecules and hence circular dichroism, which is the difference in absorption of leftand right-handed circularly polarized light beams, is a spectroscopic footprint of a given molecule configuration and potentially composition. At the same time, intrinsic molecular chirality is weak which complicates the application of CD spectroscopy. The CD signal can be substantially enhanced by using nanophotonic structures that provide strong electromagnetic field enhancement. It has been recently shown that optimal nanostructure for CD enhancement should be achiral and preserve helicity upon scattering. An example of such structure is a dielectric nanodisk that supports both electric and magnetic multipoles and enables optimal response by multipolar interference. This latter condition cannot be fulfilled in substrate supported nanostructures, which are however relatively simple to fabricate and hence are omnipresent in nanophotonics. In this work we study the optical chirality enhancement by substrate supported nanostructures. We provide a T-matrix method based framework to theoretically describe the optical response of chiral molecules coupled with substrate supported nanostructures. We utilize the Riemann-Silberstein transformation to obtain free space multipoles with well defined helicity. Then, we propagate the multipolar fields through the layer system to obtain their counterparts for substrate supported nanostructures. We study the helicity change resulting from off-substrate reflection and seek means to minimize it by multipolar interference and to maximize the chirality enhancement. Finally, we exemplify the obtained method and theoretical results by studying the archetypical example of a substrate supported dielectric nanodisk.
Tunable chiro-optical effects in self-assembled metasurfaces: experiments, simulations and perspectives
E. Petronijevic, A. Belardini, G. Leahu, et al.
The nanoscale community has proposed various nanostructures for the enhancement of near- and far-field chiro-optical effects. Here we study such effects in asymmetric metasurfaces which can be produced by means of nanosphere lithography (NSL). NSL, combined with tilted plasmonic deposition, is a versatile, self-assembling method for fabrication of different asymmetric nanogeometries. Polystyrene nanospheres (PSN) are first self-assembled on glass, then reduced in diameter, and subsequently covered with a plasmonic layer. By controlling fabrication parameters, we can obtain three types of samples. First sample is based on PSN asymmetrically covered by metal under 45deg. This sample has a high contribution of the plasmonic elliptical nanohole array on the glass. Second sample is a plasmonic elliptical nanohole array obtained by simply removing the PSN from the first sample. Third sample is obtained by increasing the metallic deposition angle to 60deg; this way, nanohole array contribution vanishes, and the metasurface is based on asymmetric plasmonic nanoshells. We report on numerical studies on these three samples, when excited by oblique left or right circular polarization in the near-infrared range. The simulations are in good agreement with previously obtained experimental results, which gives a route to possible optimization of fabrication parameters for different applications. Finally, we comment on the follow-up application for each geometry. We believe that this technique can be used to produce high quality and low-cost substrates for chiral sensing; moreover, with the inclusion of near-field light emitting layer, these metasurfaces could lead to tunable circularly polarized visible or near-infrared light emission.
Subwavelength Concentration of Light and Imaging
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Tunable directional emission from electrically-driven nanostrip tunnel junctions
Saurabh Kishen, Jinal Tapar, Naresh Kumar Emani
Electrically driven sources of photons offer a unique platform for realising applications such as beam steering and free-space optical interconnects. Metal-insulator-metal (MIM) tunnel junctions have been extensively used to electrically generate and manipulate light via inelastic electron tunneling. However, beam steering by dynamically switching the excitation source is still not shown. Here, we numerically demonstrate tunable directional emission of light from electrically-driven nanostrip tunnel junctions. Our device consists of an Ag-SiO2-Ag stack with the top Ag film milled into 16 nanostrips. Two nanostrips at the centre, labelled S1 and S2, act as individual sources with a resonance wavelength of ∼ 695 nm. We show that, by individually exciting S1 or S2, the light emission can be directed to spatially different channels, with an angle of emission depending on the periodicity of the passive elements. On applying a bias to source S2, the calculated far-field radiation pattern showed a highly directional beam with an emission angle of 30° and FWHM of < 12°. When the source is switched to S1, the emission pattern shifts to −30° with a similar FWHM, thereby paving the way towards practical, reconfigurable electrically-driven light sources.
Machine Learning in Metamaterial Design and Application
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Improved solutions to optical inverse problems by neural networks and prior assumptions
We use convolutional neural networks (CNN) to predict scattering geometry from the fields outside of the scatterer. While this problem is nonunique, we show that by training on specific datasets, the CNN learns the underlying structure of the scatterers. I.e., if there is prior knowledge of the expected structure or form of the scatterers, this can be used to obtain a much more accurate solution to the inverse scattering problem. We show that our method faithfully recovers the original geometry for highly specific classes of structures, while the more conventional method falls victim to the nonuniqueness and fails to recover plausible-looking geometries.
Forward modeling for metamaterial design using feature-based machine learning
Machine learning techniques have been proposed in the literature for the modeling of photonic devices. In this paper, a modeling technique based on system identification, feature extraction, and machine learning methods is proposed for the design of photonic devices. Design features of interest are extracted based on a system identification step that uses a few samples of the electromagnetic device response. This system identification step allows saving computational resources significantly while collecting the data needed for the further machine learning step. Modeling design features instead of the wavelength-dependent device response as a function of the design parameters allows compacting the output space of interest in neural networks and reducing related model complexity issues. These features can be modeled as a function of design parameters by means of neural networks. The generated neural networks are of very limited complexity. Design features represent very valuable and meaningful information for designers. Numerical results successfully validate the proposed technique.
Novel Effects in Metamaterials: PT-Symmetry, Quantum and Topological Phenomena
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Exceptional surface and compound waves
Guided by the planar interface of two dissimilar partnering mediums, a surface wave can be exceptional if at least one of the partnering mediums is anisotropic. Exceptional surface waves propagate in isolated directions parallel to the interfacial plane, whereas unexceptional (i.e., garden-variety) surface waves propagate for a non-degenerate angular interval of directions parallel to the interfacial plane. Also, exceptional surface waves have localization characteristics different from those of unexceptional surface waves: the decay of fields for an exceptional surface wave has a combined linear-exponential dependency on distance from the interface in an anisotropic partnering medium, whereas the decay is purely exponential for an unexceptional surface wave. In order for exceptional surface waves to exist, the constitutive parameters of that anisotropic partnering medium must satisfy certain constraints. Exceptional surface waves of different types have been reported: (a) If both partnering mediums are dielectric, at least one of the two is anisotropic, then exceptional surface waves classified as Dyakonov-Voigt surface waves can exist, whether or not the partnering mediums are dissipative. Whereas the planar interface of an isotropic dielectric medium and a uniaxial dielectric medium can guide one exceptional surface wave in each quadrant of the interface plane, the planar interface of an isotropic dielectric medium and a biaxial dielectric medium can guide two exceptional surface waves in each quadrant of the interface plane. Furthermore, doubly exceptional Dyakonov-Voigt surface waves, which exhibit a combined linear-exponential dependency on distance from the interface on both sides of the interface, have been reported on for the planar interface of a biaxial dielectric medium and a uniaxial dielectric medium. (b) If one of the partnering mediums is metallic and the other is dielectric, and at least one of them is anisotropic, then exceptional surface waves classified as surface plasmon-polariton-Voigt waves can exist. The notion of exceptional surface waves guided by a single planar interface has been extended to compound waves that are guided by a pair of parallel planar interfaces, provided that the distance between the two interfaces is not too great. For example, if a thin film of metal is embedded within an anisotropic dielectric medium, the two planar interfaces can guide exceptional compound-plasmon-polariton waves.
Inverse-design of non-Hermitian potentials for light management
Muriel Botey, Waqas W. Ahmed, Ramon Herrero, et al.
In this paper, we propose a general inverse-design strategy based on genetic algorithm optimization to achieve ‘on demand’ manipulation of light in one-dimensional (1D) and two-dimensional (2D) non-Hermitian systems. The optimization process faithfully creates non-Hermitian potentials from any given arbitrary real (or imaginary) permittivity distribution for the desired frequency selective and broadband asymmetric response in 1D multilayer structures. As a demonstration in 2D, we design periodic and aperiodic complex permittivity spatial distributions to create "sink-type" concentrators of light around a desired area. The proposed inverse-design approach to generate non-Hermitian potentials represents an alternative to the Hilbert Transform (HT) generalizing the Kramers Kronig relations in space, additionally being selective in spectrum.
Supertoroidal skyrmions and space-time superoscillation of light
Yijie Shen, Nikitas Papasimakis, Nikolay I. Zheludev
Topological complex electromagnetic waves give access to nontrivial light-matter interactions and provide additional degrees of freedom for information transfer. An important example of such electromagnetic excitations are space-time non-separable single-cycle pulses of toroidal topology. Here we introduce an extended family of super-toroidal electromagnetic excitation, which exhibit skyrmionic structure of the electromagnetic fields, multiple singularities, and fractal-like energy backflow. By further introducing bandlimited effect into super-toroidal pulses, we show that spacetime non-separable band-limited light fields can exhibit superoscillations simultaneously in the spatial and temporal domains, i.e. can oscillate faster that the highest harmonics of their spectra. The super-toroidal pulses with space-time superoscillation are of interest for transient light-matter interactions, ultrafast optics, spectroscopy, and toroidal electrodynamics.
Metamaterial Multiphysics and Integration
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Cloaking of bulk and surface mechanical waves
S. Brûlé, S. Enoch, S. Guenneau
Ten years ago, two large scale experiments on seismic metamaterials demonstrated a cloaking effect for surface Rayleigh waves generated by a time-harmonic source at 50 Hertz in a sedimentary soil structured with boreholes 0.3m in diameter [1] and a lensing effect via negative refraction at 10 Hertz for surface Rayleigh waves generated by a multi-frequency source in a sedimentary soil structured with boreholes 2m in diameter [2]. These experiments have fueled the interest in large scale mechanical metamaterials for applications in civil engineering. Here, we propose that some experiments on broadband cloaking of spoof plasmon polaritons on metal surfaces structured with TiO2 [3] could be translated to the realm of seismic metamaterials. We point out that drawing analogies between surface Rayleigh waves in geophysics and spoof plasmon polariton in plasmonics, makes it possible to envision seismic cloaks and carpets at the decameter and kilometer scales. Research advances in photonics and plasmonics in the past twenty years might lead to a paradigm shift in earthquake engineering in the near future.
Poster Session
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Modal theory for twisted waveguides
Twisted waveguides are promising building blocks for broadband polarization rotation in integrated photonics. They may find applications in polarization-encoded telecommunications and quantum-optical systems. In our work, we develop a rigorous modal theory for such waveguides. To this end, we define an eigenmode of a twisted waveguide as a natural generalization of the eigenmode of a straight waveguide. Using covariant approach for expressing Maxwell’s equations in helical reference frame, we obtain the eigenmode equation which appears to be nonlinear with respect to the eigenvalue, i.e. propagation constant. By analyzing the obtained equations we establish fundamental properties of the eigenmodes and prove their orthogonality. We develop a finite-difference full-vectorial scheme for solving the eigenmode equation and solve it using two approaches: with perturbation theory and using routines for nonlinear eigenvalue problems. By analyzing the obtained propagation constants and modal fields we explain the modal mechanism of polarization rotation in twisted waveguides and explain qualitatively polarization conversion efficiency dependence on twist length. Although photonic applications are of our primary concern, our results are general and apply to twisted waveguides of arbitrary architecture.
Using optical potentials with gain-loss to generate structural colors
We study the possibility to obtain structural colors through the use of supersymmetric transformations in optics such as the Darboux transform. Structural colors were originally discovered by studying the interference of light with natural photonics structures giving rise to vivid and spectacular tonalities. They differ fundamentally from ordinary colors based on light absorption at particular wavelengths, as they result from light interference only. To treat interference analytically, we make use of the Darboux transform to define materials with continuously varying spatial distributions of the refractive index that are exactly solvable for the electric field. Consequently, it is possible to calculate analytically the Transfer Matrix linked to the definition of the transmission and reflection coefficients. Interestingly, by using gain, anomalous transmission/ reflection (i.e. larger than one) can be obtained, the physical system being open towards the external environment (the system is using external energy in order to increase both transmission and reflection). The generated active optical cavity can thus be used to amplify the incoming light in the desired spectral-angular region. The calculated refractive index distributions can be realized in practice as 1D multi-layered structures corresponding to optical filters in the visible.
Bound states in the continuum in the multipole approximation
Here we develop a theory of bound states in the continuum (BICs) in multipolar lattices – periodic arrays of resonant multipoles. We show that off-Γ BIC can be pinned in the k-space in this multipole approximation. The developed approach set a direct relation between the topological charge of BIC and the asymptotic behavior of Q-factor of the radiative modes in its vicinity.
Designing metasurfaces to manipulate antenna radiation
James R. Capers, Stephen J. Boyes, Alastair P. Hibbins, et al.
Designer manipulation of light at the nanoscale is key to several next–generation technologies, from sensing to optical computing. One way to manipulate light is to design a material structured at the sub–wavelength scale, a metamaterial, to have some desired scattering effect. Metamaterials typically have a very large number of geometric parameters than can be tuned, making the design process difficult. Existing design paradigms either neglect degrees of freedom or rely on numerically expensive full–wave simulations. In this work, we derive a simple semi–analytic method for designing metamaterials built from sub–wavelength elements with electric and magnetic dipole resonances. This is relevant to several experimentally accessible regimes. To demonstrate the versatility of our method, we apply it to three problems: the manipulation of the coupling between nearby emitters, focusing a plane wave to a single point and designing a dielectric antenna with a particular radiation pattern.
Graphene/liquid crystal-based multifunctional structure for tunable metasurfaces
Optical devices requiring a compact and intelligent design find multifunctionality and reconfigurability to be of paramount importance. In this manuscript, we present a numerical investigation of a new design exhibiting these features. The proposed structure relies on the joint paradigm of 1D photonic crystals (PhC) and reconfigurable materials (graphene and liquid crystals). This composite structure can perform several reconfigurable narrowband optical functions such as notch filtering, amplitude modulation, and phase shifting. The effects on the behavior of this structure of monolayer graphene and liquid crystal polarization are independent of each other and allow control of absorbance intensity, phase shift action, and spectral position of resonance are detailed. This structure may find use in the design of smart reconfigurable metasurfaces, for optical modulators and beam-steering systems.
2D hybrid epsilon near-zero platform for nanophotonics
In this work we, propose a tunable 2D-hydrid epsilon-near-zero (ENZ) platform in telecom windows. Taking advantage to the intrinsically ENZ of the Indium-thin-oxide (ITO) and exploiting the graphene capability to dynamically tune the plasmon polaritons we were able to adjust the cross-over frequency, where the epsilon vanishes, in four telecom bandwidth windows. Additionally, tunabilty can be achieved via electrical gating of the ITO leading to an interplay modulation of the surface plasmon polaritons at the graphene-ITO interface. Furthermore, a giant Purcell factor (PF) was observed at ENZ regimes. These results show how 2D-hybrid ENZ materials potentially find applications in multifunctional nonlinear nanophotonic systems such as ultrafast modulators, data processing and photonic quantum computers (QPCs).
Optical properties of hyperbolic nanoresonators
Maria Bancerek, Krzysztof M. Czajkowski, A. Korneluk, et al.
Hyperbolic metamaterials (HMMs) gained a lot of interest amongst researchers in recent years due to their novel properties stemming from hyperbolic dispersion, such as anomalous scattering, subwavelength confinement of light or enhancing far-field radiation. In our work, we investigate optical properties of nanostructured HMMs in a form of spherical nanoresonators composed of stacked alternating metal-dielectric layers, which is one way to realize hyperbolic dispersion. Using T-matrix analysis, combined with a quasistatic approach, we explore their unique spectral response to unravel fundamental electromagnetic properties of hyperbolic nanoresonators. The modal structure of hyperbolic nanospheres (HNSs) is richer than those of conventional nanoantennas and can be tuned either with material properties or incident light conditions. We show that, depending on the direction and polarization of incident light, such nanoresonators can exhibit a plasmonic-like response or one with an atypical modal order, with electric and magnetic modes of higher orders appearing at energies below those of lower order modes. We underline how constructive or destructive cross coupling between various electric and magnetic multipoles, determined by the hyperbolic dispersion, influences the overall optical response and enables phenomena absent in the isotropic medium. Such an example is a negative contribution of a given mode to the extinction cross-section, stemming from destructive cross coupling and is an indicator of energy transfer between modes. We show how mode cross coupling (and thus HNS optical properties) change with material properties – varying the metal fill-factor allows for significant tunability from a uniaxial dielectric through a type I or II hyperbolic material to a uniaxial metal. By applying the quasistatic approach we also analyse the origin of the dipolar modes and obtain material-dependent resonance conditions for both electric and magnetic mode. Furthermore, we conclude that magnetic dipolar resonance presence is determined by the hyperbolic disperison, i.e. opposite signs of the ordinary and extraordinary permittivities.