Dielectric Metamaterials Research Articles

Metasurface Resonator for 1.5 T MRI Based on BaTiO3 Host Material.

Magnetic resonance imaging (MRI) is a widely used clinical tool for medical diagnosis and therapy. Several research studies focus on passively improving MRI sensitivity using high dielectric constant (HDC) materials and metamaterials. In this work, we investigate a new metasurface resonator which can enhance local transmit and receive efficiency in 1.5T MRI. The metasurface has been realized with an array of non-magnetic rods embedded in two blocks of a BaTiO3 aqueous mixture. BaTiO3 when mixed with water exhibits high dielectric permittivity values in the 40-200 MHz range, allowing the design of a compact and safe device for practical use in an MRI scanner. Simulation results show 50% enhancement of the magnetic field in the region-of-interest. The resonance frequency of the metasurface is also validated experimentally with a small loop antenna and a vector network analyzer (VNA) in a laboratory-controlled environment.

Near-infrared TM-polarization ultra-narrowband absorber with dielectric metamaterials

Ultra-narrowband light absorption is desired for many applications. A near-infrared TM-polarization (magnetic field is parallel to grating grooves) ultra-narrowband absorber with dielectric metamaterials has been reported theoretically in this paper. The simulation results show that we can achieve an ultra-narrowband absorption at the incident wavelength of [Formula: see text] with the absorption bandwidth less than 0.9 nm and the absorption rate more than 0.99 for TM polarization. At the same time, we find that the high absorption rate can only remain up to [Formula: see text], which means our absorber has high directivity. In addition, the field distribution at the resonance wavelength shows that the ultra-narrowband absorption in our absorber has originated from magnetic resonance effect. Our near-infrared TM-polarization ultra-narrowband absorber is a good candidate for laser stealth.

Determining the complex refractive index of cellulose nanocrystals by combination of Beer-Lambert and immersion matching methods

Nanocelluloses have received significant interest due to their unique structural, mechanical, and optical properties. Nanocellulose refractive indices can be used to indicate many crucial characteristics, such as crystallinity, transparency, and purity. Thus, accurate measurement is important. This study describes a new method to determine the wavelength dependent complex refractive index of cellulose nanocrystals (CNCs) by the measurement of light transmittance with a spectrophotometer. The data analysis is based on a combination of the Beer-Lambert and immersion liquid matching equations. The immersion liquid method's main advantage is that it is independent of particle shape and size. Moreover, the measurement is easy and relatively quick to perform. The present procedure is not restricted to the nanocellulose and could potentially be applied to other nanomaterials, such as hyphenate nanoparticle-based, lignin nanoparticles, nanopigments, biological entities, structural elements of dielectric metamaterials, and nanoparticle-based composites.

Design of a high-performance tunable absorber based on composite dielectric layer metamaterial

A tunable metamaterial absorber based on hybrid structure design strategy is suggested and numerically verified. An absorption peak (82.5%, at resonance wavelength 1.52 μm) is achieved at room temperature without Fermi energy, which is derived from local surface plasma (LSP) modes excited on edges of metal particles. The adjustability of this structure is derived from two structural components: vanadium dioxide (VO2) particles are temperature sensitive and graphene films are sensitive to Fermi energy and Fermi energy difference. Therefore, the resonant properties of this structure can be modulated by varying the temperature, energy, or energy difference. Simulated results reveal that two new absorption peaks are achieved at the resonance wavelengths 1.82 and 2.09 μm, which are both derived from the LSP modes excited on edges of VO2 particles. Moreover, both new absorption peaks are enhanced with the simulated temperature increased. Three optimized values of the carrier mobility μ are achieved: VO2 particles support region 8200 cm2 / ( V · s ) , metal particle support region 8800 cm2 / ( V · s ) , and suspended region 230,000 cm2 / ( V · s ) . The absorption peak is enhanced and shifted to shorter wavelength with the Fermi energy level increasing, while the absorption peak is reduced and shifted to longer wavelength with the Fermi energy difference reducing.

ZnS nanoparticles-based tunable dielectric metamaterials

In recent years, increasing attention has been paid to Mie resonance-based dielectric metamaterials. Here, we report a ZnS nanoparticles-based dielectric metamaterials. The permittivity of ground state and excited state of ZnS were calculated by Materials Studio, and the obtained permittivity was used to simulate the light transmission characteristics of ZnS nanoparticles-based Mie metamaterials. ZnS nanoparticles were prepared by solid reaction process, which were dispersed into alcohol solution to get Mie resonance-based metamaterials. The transmission characteristics were measured by UV–Vis spectrophotometer. There is resonance peak both in simulation and experiment and the different electromagnetic response in ground state and excited state indicates that tunable Mie resonance-based metamaterials were fabricated. This work demonstrates the feasibility of fabrication of tunable dielectric metamaterials at optics frequencies which can be used as sensors with chemical methods.

Review: recent progress in metal-less metasurfaces and metamaterials

Plasmonic metamaterials (MMs) are based on light coupling to metal free electrons, which inherently accompanied with severe dissipation losses that usually limit device performance. Furthermore, the complexity in designing 3D plasmonic MMs have left them far beyond from being realized in the real market for on-chip nanophotonic applications. Dielectric MMs have alternatively served as an ultimate solution to tackle the low efficiency problem of metal-based MMs. In this review paper, we present optically thin dielectric-based metasurfaces (DMs) that allow for phase and polarization manipulation of light. DMs are essentially based on excitation of double Mie resonance effect, which inherits DMs the ability to modulate light with 2π phase modulation, thus, serving in variety of applications including metalens, beam steering, holograms and optical chirality. Two main classes of DMs are mainly covered which are: Huygens’s metaurfaces and Geometric phase metasurfaces. We first introduce the underlying physics of DMs and propose applications that have been realized with high efficiencies. In the second part, we demonstrate another class of MMs which is semiconductor based hyperbolic metamaterial (HMM). Semiconductor based HMM has two main advantages which are: their ease of fabrication in analogy with DMs and their ability to operate in the mid IR range. We outline HMMs’ applications including thermal harvesting, focusing and mid IR absorption. Finally, we overview the current challenges and future goals for the field of dielectric MMs in order to be realized for real market application.

Asymmetric Transmission in a Mie-Based Dielectric Metamaterial with Fano Resonance.

Chiral metamaterials with asymmetric transmission can be applied as polarization-controlled devices. Here, a Mie-based dielectric metamaterial with a spacer exhibiting asymmetric transmission of linearly polarized waves at microwave frequencies was designed and demonstrated numerically. The unidirectional characteristic is attributed to the chirality of the metamolecule and the mutual excitation of the Mie resonances. Field distributions are simulated to investigate the underlying physical mechanism. Fano-type resonances emerge near the Mie resonances of the constituents and come from the destructive interference inside the structure. The near-field coupling further contributes to the asymmetric transmission. The influences of the lattice constant and the spacer thickness on the asymmetric characteristics were also analyzed by parameter sweeps. The proposed Mie-based metamaterial is of a simple structure, and it has the potential for applications in dielectric metadevices, such as high-performance polarization rotators.

Second‐Harmonic Generation by 3D Laminate Metacrystals

Abstract3D dielectric woodpile photonic crystals, which are conformally coated with locally noninversion‐symmetric dielectric laminate metamaterials, are discussed. These “3D laminate metacrystals” are fabricated by 3D direct laser writing of an acrylate without photoinitiator to reduce two‐photon‐induced fluorescence, followed by atomic‐layer deposition of alternating layers of Al2O3 and ZnO. Resonant enhancements of second‐harmonic generation under normal incidence of light of up to two orders of magnitude with respect to the same laminate metamaterial on a glass substrate under oblique incidence of light are found experimentally. The nonlinear optical resonances in the range of 800–1020 nm fundamental wavelength are explained theoretically in terms of 3D photonic crystal slab guided resonances, leading to pronounced local‐field‐enhancement effects. Numerical calculations reproduce the salient features of the experiments and yield second‐harmonic enhancement factors yet larger than the experiment due to the absence of spectral and angular averaging.

Design and measure of a tunable reflection polarization converter based on hybrid dielectric layer metamaterials

We design and experimentally investigate of a temperature sensitive reflection polarization converter based on circular hole array metamaterial. This achieved converter consists of two metal layers (silver) and three dielectric layers (SU-8, polyimide, VO2). Measured results reveal that two polarization conversion ratio (PCR) peaks are obtained (99.1%, 99.3%) at two neighboring frequencies at normal temperature, which are derived from the cavity and electrical resonance modes, respectively. A PCR band is achieved based on the overlap of two PCR peaks, and the average conversion rate reaches 94.2%. When the measured temperature is increased, the average value of the PCR band is enhanced. Moreover, a new PCR peak is achieved with the measured temperature increasing, which is derived from a new electrical resonance mode excited between the bottom metal layer and the VO2 layer.

All-angle optical switch based on the zero reflection effect of graphene–dielectric hyperbolic metamaterials

We have studied a switchable hyperbolic metamaterial composed of a graphene–dielectric periodic structure. By tuning the chemical potential of all graphene sheets simultaneously, the isofrequency curve can switch between an ellipse and a hyperbola conveniently. In particular, a special hyperbolic isofrequency curve with its asymptote perpendicular to the interface is obtained and used to realize the zero reflection effect. Furthermore, a zero-reflection-based optical switch working in the terahertz spectrum is demonstrated. Its bandwidth can be efficiently adjusted by geometric parameters such as permittivity and period. Such an optical switch possesses the merits of low loss, high transmittance contrast, high response speed, compact size, high tolerance of chemical potential, and having all incident angles (0°–90°) simultaneously. Such an optical switch holds great potential in many fields, such as data storage, beam steering, and integrated photonic circuits.

Reconfigurable Ultraviolet and High-Energy Visible Dielectric Metamaterials.

Photonic materials with tunable and switchable ultraviolet (UV) to high-energy visible (HEV) optical properties may benefit applications such as sensing, high-density optical memory, beam-steering, adaptive optics, and light modulation. Here, for the first time we demonstrate a nonvolatile switchable dielectric metamaterial operating in the UV-HEV spectral range. Nanograting metamaterials in a layered composite of low-loss ZnS/SiO2 and the chalcogenide phase-change medium germanium-antimony-telluride (Ge2Sb2Te5 or GST) exhibit reflection resonances at UV-HEV wavelengths that are substantially modified by light-induced (amorphous-crystalline) phase transitions in the chalcogenide layer. Despite the presence of the lossy GST, resonance quality factors up to Q ∼ 15 are ensured by the transparency (low losses) of ZnS/SiO2 in the UV-HEV spectral range and values of Q increase as the refractive index of Ge2Sb2Te5 decreases, upon crystallization. Notably, however, this switching leaves resonance spectral positions unchanged.

All Dielectric Terahertz Left-Handed Metamaterial Based on Mie Resonance Coupling Effects

Precise control of a coupling effect is critical for space-limited communication system applications. Dielectric metamaterials, which derive their electromagnetic properties from subwavelength structures, have emerged as a promising way to tune coupling effect due to their complex resonant modes. However, it has not yet achieved quantitative control of coupling effects in metamaterial designs, which is important for advanced engineering applications such as artificial intelligence antenna and reconfigurable camouflage. In this paper, all dielectric left-handed metamaterial operating in THz band based on the deep coupling of higher mode Mie resonance is presented. Distinguished from those designs published before, this left-handed metamaterial is composed only by one same structure, not two separated parts providing negative permittivity and permeability, respectively. By utilizing different Mie resonant modes, negative permittivity and permeability are accomplished via high order Mie resonant modes and the coupling between cubes. This work opens a gate to highly control of electromagnetic wave in dielectric metamaterial by using the coupling between the unit cells.

Frequency band gaps in dielectric granular metamaterials modulated by electric field

Wave propagation in granular materials is known to be dispersive. Micromorphic continuum model based upon granular micromechanics (Misra, A. and P. Poorsolhjouy, Continuum Mech. Thermodyn, 2016. 28(1-2): p. 215-234.) has the ability to describe this dispersion behavior. In this paper we show that the dispersive behavior can be modulated by using electric field when the grains have dielectric properties. To this end, we apply the recently enhanced model that incorporates electro-elastic coupling by connecting microstrain to electric dipole and quadrupole densities due to bound charges in dielectric grains (Romeo, M., Mech. Res. Commun., 2018. 91: p. 33-38.). We particularly investigate the effect of induced polarization that arises due to an imposed electric field. Two cases of dielectric one dimensional infinite rods with the same micromorphic properties have been studied, where case 1 and 2 are in null and nonzero external electric fields, respectively. Parametric studies are performed to understand the contribution of the polarizability (dipole effect), intrinsic quadrupole density, and external electric field on the dispersive behavior of granular media. Results predict an acoustic and an optical branch in the dispersive curve. Polarizability and external electric field are mainly affecting small wavenumber behavior of the wave branches, while quadrupole density alters the behavior of the material at large wavenumbers. A possibility of altering the optical branch to an acoustic branch is also observed, for which instability or attenuation occurs depending upon the direction of the imposed electric field with respect to the wave propagation direction. We find that the location and the width of the frequency band gaps can be altered using external electric field. The possibility of creating or removing frequency band gaps is also shown to exist. The extended theory accounting for electro-elasticity can therefore be utilized as a tool to analyze existing granular media, or to design granular metamaterials, as it systematizes the design process and eliminates ad-hoc manners leading to large data libraries.

Direct Observation of Nanoscale Light Confinement without Metal.

Manipulation of light below the diffraction limit forms the basis of nanophotonics. Metals can confine light at the subwavelength scale but suffer from high loss of energy. Recent reports have theoretically demonstrated the possibility of light confinement below the diffraction limit using transparent dielectric metamaterials. Here, nanoscale light confinement (<λ/20) in transparent dielectric materials is shown experimentally through a luminescent nanosystem with rationally designed dielectric claddings. Theoretically, green light with a wavelength of 540 nm has a transmission of 98.8% when passing through an ultrathin NaYF4 /NaGdF4 superlattice cladding (thickness: 6.9 nm). Unexpectedly, the complete confinement of green emission (540 nm) by such an ultrathin dielectric cladding is directly observed. FDTD calculations are used to confirm that the ultrathin dielectric cladding has negligible influence on the transmission of propagating light, but extraordinary confinement of evanescent waves. This will provide new opportunities for nanophotonics by completely averting the use of metals.

Aperiodic order induced enhancement of weak nonlocality in multilayered dielectric metamaterials

Recent studies on fully dielectric multilayered metamaterials have shown that the negligibly small nonlocal effects (spatial dispersion) typically observed in the limit of deeply subwavelength layers may be significantly enhanced by peculiar boundary effects occurring in certain critical parameter regimes. These phenomena, observed so far in periodic and randomly disordered geometries, are manifested as strong differences between the exact optical response of finite-size metamaterial samples and the prediction from conventional effective-theory-medium models based on mixing formulas. Here, with specific focus on the Thue-Morse geometry, we make a first step toward extending the studies above to the middle ground of aperiodically ordered multilayers, lying in between perfect periodicity and disorder. We show that, also for these geometries, there exist critical parameter ranges that favor the buildup of boundary effects leading to strong enhancement of the (otherwise negligibly weak) nonlocality. However, the underlying mechanisms are fundamentally different from those observed in the periodic case and exhibit typical footprints (e.g., fractal gaps, quasilocalized states) that are distinctive of aperiodic order. The outcomes of our study indicate that aperiodic order plays a key role in the buildup of the aforementioned boundary effects and may also find potential applications to optical sensors, absorbers, and lasers.

Toward Silicon-Based Metamaterials

We study periodic lattices of silicon nanorods and introduce the concept of a phase diagram that characterizes a transition between the regimes of photonic crystals and the dielectric metamaterials when the lattice spacing and operational wavelength vary. We find the conditions when a hexagonal periodic lattice of silicon nanorods can operate as a metamaterial described by averaged parameters. In general, we reveal that the metamaterial regime can be achieved for dielectric permittivity exceeding the value e = 14, being commonly available for semiconductors in both visible and near-infrared frequency ranges. Thus, advanced semiconductor technologies can offer a versatile platform for novel designs of all-dielectric Mie-resonant metadevices.

Perfect Absorption in One–Dimensional Photonic Crystal with Graphene‐Dielectric Hyperbolic Metamaterials

The absorption characteristics of one‐dimensional photonic crystals (1DPC) consisting of an ordinary dielectric and graphene–dielectric hyperbolic metamaterials in terahertz (THz) are theoretically analyzed and numerically simulated using the transfer matrix method. The perfect absorption peaks can be achieved in the structure designed in our research. In the 1DPC, the dielectric layers are embedded into graphene layers and the optical axis of the graphene–dielectric hyperbolic metamaterials is normal to the graphene layers. The graphene–dielectric layers are seen as anisotropic effective medium and possess the properties of hyperbolic dispersion in THz frequency band. The locations of perfect absorptance peaks are dependent on chemical potential, the thicknesses of dielectric layers and the inclined angle. The positions of absorption peaks are blue‐shifted with increasing the chemical potential and red‐shifted by increasing the thicknesses of the dielectric layers. We testified that the orientation of the optical axis of the graphene–dielectric layers has prominent effect on the locations of the perfect absorption peaks. Moreover, multiband absorption peaks can be obtained by introducing more structure as given in our research with different parameters. The controllable perfect absorption achieved at the far infrared ray (FIR) frequencies could be applied in the design of multichannel perfect absorption filters.

Multipolar-interference-assisted terahertz waveplates via all-dielectric metamaterials

Polarization control via metamaterials boosts the design of polarimetric devices in the realm of terahertz technology for sensitive detection, bio-imaging, and wireless communication. Here, we propose all-dielectric metamaterials composed of silicon brick arrays that operate as terahertz quarter- and half-waveplates with close to unity transmission. Spherical multipole decomposition calculation indicates that the silicon brick can support multiple Mie-type resonances, such as electric dipole, electric quadrupole, magnetic dipole, and magnetic quadrupole modes. By tailoring the multipolar interference among these resonances, near unity transmission can be obtained with over π phase delay. We experimentally realize dielectric terahertz metamaterials that function as a quarter-wave plate at 0.79 THz and a half-wave plate at 0.91 THz with insertion losses of 0.54 and 1.25 dB, respectively. Such anisotropic dielectric metamaterials promise an exotic approach to engineer the interference among multipolar resonances and reveal the feasibility to realize functional, efficient, and compact terahertz meta-devices.

Surface enhanced perfect absorption in metamaterials with periodic dielectric nanostrips on silver film.

Integrated dielectric metamaterials with plasmonic structures can cause drastic optical resonances and strengthen the capacity of light absorption. Here, we describe the optical properties of silicon nanoarrays on a thin silver film for extreme light confinement at subwavelength nanoscales. We attain the nearly total absorption in silicon nanostrips, which support magnetic quadruple Mie-type resonances in the visible regions. The Mie resonant field of the dielectric nanostrip engages the screening response of the silver film, resulting in plasmon resonance configuration and thus achieving perfect light absorption in the dielectric nanostrip. Moreover, we can attain similar results in other nanostructures, such as silicon cylinder and rhombus column arrays. Because it can sustain hybridized plasmon modes and magnetic modes, the combined system will benefit the application of solar energy accumulation.

Dielectric metamaterials with electric response.

Dielectric metamaterials are usually studied as low-loss media with magnetic response. However, control over the electric response is more promising for applications in photonics. Here we report an all-dielectric metamaterial with electric response. The structure consists of high-index dielectric rods arranged in a square lattice. We present a phase diagram that includes regions of metamaterials with magnetic and electric response. A metamaterial behavior is demonstrated for homogeneous ϵ-near zero modes, which are observed regardless of a lattice orientation and a structure boundary. The ϵ-near zero modes make it possible to enhance electric field intensity by two orders of magnitude, which can be used for applications exploiting light-matter interactions.