Nanostructured Metamaterials Research Articles

Investigating spin-orbit interaction of light in micro- and nanophotonic systems using polarization Mueller matrix

Coupling between the spin and orbital angular momentum of light has led to a variety of exotic spin–orbit photonic effects, establishing a paradigm of nanophotonic meta-devices to control and manipulate light at the nanometer length scale. For the advancement of spin–orbit photonic technologies, quantitative characterization and unambiguous physical interpretation of the various spin–orbit interaction (SOI) effects exhibited in complex nanophotonic systems are of utmost importance. This Perspective addresses some of these outstanding challenges in the domain of spin–orbit photonics, focusing on the simultaneous manifestation of multiple SOI effects in hybridized nanophotonic systems and the role of polarization-based techniques in their characterization and quantification. After introducing the fundamentals and physical origins of SOI effects, we present a polarization Mueller matrix technique as a powerful tool to probe, decouple, and independently quantify these effects in a unified experimental framework, demonstrated with hybridized waveguided plasmonic crystals. We further discuss the potential of structured light and tailored polarization to enable unconventional SOI phenomena in simple nanostructured metamaterials and metasurfaces. These advancements not only enhance our fundamental understanding of SOI phenomena across micro- and nanoscale optical systems but also pave the way for the development of multifunctional and tunable spin–orbit photonic devices.

Swastika-shaped rotationally symmetric quadruple-structured near-infrared metamaterial absorber for absorbing solar energy

In this work, silicon dioxide based four-fold swastika shaped metamaterial nano structure absorber for solar radiation absorption in the near infrared (NIR) regime is proposed. The dimension of the unit cell is 648.50 × 648.50 nm2 which is designed on dielectric silicon dioxide and metallic conductor aluminum. The proposed structure is investigated by the finite-deference time domain (FDTD) method based software CST microwave studio. The metamaterial absorber (MMA) unit cell is simulated within the frequency range (200–398) THz that yielded dual resonance peaks at 260 THz and 378.0 THz with 95.50 % and 97.32 % of absorbance of solar energy, respectively. Additionally, the proposed nano design structure of unit cell was boosted up to attain the dual absorption band by engineering the design parameters. The electromagnetic response of the MMA was theoretically inspected and the excellent properties of the proposed designed absorber have promising applications in plasma-enhanced photovoltaic, optical absorption switching and infrared modulator optical communication.

Utilizing Mixed Training and Multi-Head Attention to Address Data Shift in AI-Based Electromagnetic Solvers for Nano-Structured Metamaterials.

When designing nano-structured metamaterials with an iterative optimization method, a fast deep learning solver is desirable to replace a time-consuming numerical solver, and the related issue of data shift is a subtle yet easily overlooked challenge. In this work, we explore the data shift challenge in an AI-based electromagnetic solver and present innovative solutions. Using a one-dimensional grating coupler as a case study, we demonstrate the presence of data shift through the probability density method and principal component analysis, and show the degradation of neural network performance through experiments dealing with data affected by data shift. We propose three effective strategies to mitigate the effects of data shift: mixed training, adding multi-head attention, and a comprehensive approach that combines both. The experimental results validate the efficacy of these approaches in addressing data shift. Specifically, the combination of mixed training and multi-head attention significantly reduces the mean absolute error, by approximately 36%, when applied to data affected by data shift. Our work provides crucial insights and guidance for AI-based electromagnetic solvers in the optimal design of nano-structured metamaterials.

High-Performance Metamaterial Light Absorption from Visible to Near-Infrared Assisted by Anti-Reflection Coating

This study experimentally demonstrates two types of ultra-broadband metamaterial absorbers with high performance in the visible-to-near-infrared range by using different anti-reflection coatings (i.e., SiO2 and Si3N4) and a multi-subcell Ti-SiO2-Ti metasurface. Compared to the bare metamaterial nanostructure, the absorption bandwidth of the coated metasurfaces exhibit increases of 594 nm and 1093 nm, respectively. Such improvements benefit from nearly perfect impedance matching to the free space enhanced by the anti-reflection coating, thin film interference, and excitations of different surface plasmon resonances. As a result, the absorber with SiO2 coating exhibits a measured bandwidth with an absorption of 0.9 ranging from 502 nm to 1892 nm, while the absorber with Si3N4 coating further broadens the bandwidth from 561 nm to 2450 nm. The measured average absorptions for both cases remain above 95% and 87%, respectively. Moreover, both nanostructures are robust to large incident angles of up to 60° for both TE and TM modes. Our findings highlight the promising potential of these absorbers for various applications, including solar energy harvesting, thermal emitters, and photodetectors.

Compressive properties of parametrically optimised mechanical metamaterials based on 3D projections of 4D geometries

The design process of 3D mechanical metamaterials is still an emerging field and in this paper, we propose for the first time, a new design and optimisation approach based on 3D projections of 4D geometries (4-polytopes) and evolutionary algorithms. We find that through iterative parametric optimisation, 4-polytope projected mechanical metamaterials can be optimised to achieve both high specific stiffness and high specific yield strengths. Samples manufactured using a low-stereolithography method were tested in compression. We find that optimised tesseracts (8-cell structures) had a higher specific yield strength (22.8 kN m/kg) than that of honeycomb structures tested out-of-plane (19.4 kN m/kg) and a specific stiffness of (0.68 MN m/kg) which is more than 3-fold that of gyroid structures. The compressive strength to solid-modulus ratio of the 8-cell tesseract is very high (3×10−3), exceeding that of out-of-plane honeycombs, which are themselves closer in value to 5-cell pentatopes (2×10−3). 8-cell and 5-cell structures are in the region of one order of magnitude higher than 16-cell and 24-cell structures (∼2×10−4–8×10−4) and are hence comparable to nanostructured metamaterials. The 8-cell tesseracts are 18% stiffer, 43% stronger, and 19% tougher in compression than out-of-plane honeycomb structures, but unlike honeycombs, 8-cell tesseracts are 3D structures with cubic symmetry. Architecture has a profound effect on the relative consistency of properties with cubically symmetric structures displaying the greatest levels of consistency in terms of both strength and stiffness reduction as a function of pore space. The results presented in this paper showcase the potential of this new class of mechanical metamaterial based on 3D projected 4-polytopes.

Photonic metamaterial analogue of a continuous time crystal

Time crystals are an eagerly sought phase of matter with broken time-translation symmetry. Quantum time crystals with discretely broken time-translation symmetry have been demonstrated in trapped ions, atoms and spins whereas continuously broken time-translation symmetry has been observed in an atomic condensate inside an optical cavity. Here we report that a classical metamaterial nanostructure, a two-dimensional array of plasmonic metamolecules supported on flexible nanowires, can be driven to a state possessing all of the key features of a continuous time crystal: continuous coherent illumination by light resonant with the metamolecules’ plasmonic mode triggers a spontaneous phase transition to a superradiant-like state of transmissivity oscillations, resulting from many-body interactions among the metamolecules, characterized by long-range order in space and time. The phenomenon is of interest to the study of dynamic classical many-body states in the strongly correlated regime and applications in all-optical modulation, frequency conversion and timing.

UV–visible broadband polarization-independent metamaterial absorber based on two-dimensional Au grating

A broadband metamaterial absorber (BMA) based on a thin metamaterial nanostructure consisting of 2D Au square grating on graphene-SiO2 film and Au substrate is proposed and designed. The optimized structure can achieve nearly perfect absorption with absorption of exceeding 90 % spanning a broad range from deep UV light to visible light (100 nm−700 nm). The excitation of Rayleigh anomaly (RA), cavity resonance (CR), surface plasmon polaritons (SPPs) and magnetic polaritons (MPs) are responsible for nearly perfect broadband absorption, which have been analysed clearly by finite-difference time-domain (FDTD) method and the LC circuit model. Furthermore, the BMA is polarization and angle insensitive for incident light even at large angle. This proposed BMA not only has a simple geometry and ordinary metal-based material composition, but also shows a high performance in both bandwidth and absorptivity, suggesting that great potential in solar cell and photodetector applications.

Enhanced Broadband Metamaterial Absorber Using Plasmonic Nanorods and Muti-Dielectric Layers Based on ZnO Substrate in the Frequency Range from 100 GHz to 1000 GHz

A broadband thin film plasmonic metamaterial absorber nanostructure that operates in the frequency range from 100 GHz to 1000 GHz is introduced and analyzed in this paper. The structure consists of three layers: a 200 nm thick gold layer that represents the ground plate (back reflector), a dielectric substrate, and an array of metallic nanorods. A parametric study is conducted to optimize the structure based on its absorption property using different materials, gold (Au), aluminum (Al), and combined Au, and Al for the nanorods. The effect of different dielectric substrates on the absorption is examined using silicon dioxide (SiO2), aluminum oxide (Al2O3), titanium dioxide (TiO2), and a combination of these three materials. This was followed by the analysis of the effect of the distribution of Al, and Au nanorods and their dimensions on the absorption. The zinc oxide (ZnO) layer is added as a substrate on top of the Au layer to enhance the absorption in the microwave range. The optimized structure achieved more than 80% absorption in the ranges 100–280 GHz, 530–740 GHz and 800–1000 GHz. The minimum optimized absorption is more than 65% in the range 100 GHz to 1000 GHz.

Bulk Plasmon Polariton Modes in Hyperbolic Metamaterials for Giant Enhancement of the Transverse Magneto-Optical Kerr Effect.

We demonstrate a concept for the giant enhancement of the transverse magneto-optical Kerr effect (TMOKE) using bulk plasmon polariton (BPP) modes in non-magnetic multilayer hyperbolic metamaterials (HMMs). Since the BPP modes are excited through the attenuated total reflection (ATR) mechanism, using a Si-based prism-coupler, we considered a single dielectric magneto-optical (MO) spacer between the prism and the HMM. The working wavelength was estimated, using the effective medium approach for a semi-infinite dielectric-plasmonic multilayer, considering the region where the system exhibits type II HMM dispersion relations. Analytical results, by means of the scattering matrix method (SMM), were used to explain the physical principle behind our concept. Numerical results for giant TMOKE values (close to their maximum theoretical values, ) were obtained using the finite element method (FEM), applying the commercial software COMSOL Multiphysics. Our proposal comprises a simple and experimentally feasible structure that enables the study of MO phenomena in HMMs, which may find application in future nanostructured magnetoplasmonic metamaterials for active nanophotonic devices.

Tuning the Electronic Properties of Mesocrystals

Colloidal crystals are arguably one of the most promising candidates when it comes to the fabrication of nanostructured metamaterials. Especially mesocrystals show exciting new properties that emerge from their inherent directional oriented assembly. With this work, the electrical conductivity of well‐defined micrometer‐sized platinum nanocube‐based mesocrystals is demonstrated and tuned through the variation of different capping agents. Herein, a method is presented to reproducibly quantify the intrinsic resistance of individual mesocrystals through electrical nanoprobing and focused ion beam deposition contacting. A thermally activated tunneling mechanism is identified as the main effect for electron propagation. In addition, the mesocrystals are altered through organically linking and mineral bridging the individual nanoparticles. This results in an increase in mesocrystal rigidity and, more importantly, conductivity by seven orders of magnitude while retaining shape, structure, and composition. In addition, these observations are transferred onto multicomponent superstructures in the form of binary mesocrystals. There, it is demonstrated that the electrical properties could be tuned through the ratio of nanoparticles incorporated into a mesocrystalline host system while simultaneously maintaining potential catalytic or superparamagnetic features of the guest particles.

Fast design and optimization method for an ultra-wideband perfect absorber based on artificial neural network acceleration

In recent years, ultra-wideband perfect absorbers (UWPAs) based on metamaterial nanostructures have been widely studied due to their excellent performance. However, designing and optimizing the absorber quickly and accurately remains challenging in the broad-band range. In this work, by adopting the genetic algorithm combined with artificial neural network (ANN) acceleration, an effective method with the global search ability and extremely fast optimization speed is developed, which only requires an accurate forward ANN model and offers tremendous controls to the designer. In the context of the optimization of the nano-multilayers absorber, the most of mean absolute errors (MAEs) of ANN between the actual spectral absorptance and the predicted spectral absorptance are less than 1E-2, demonstrating the outstanding consistency. What's more, the optimization speed is four orders of magnitude faster than conventional electromagnetic simulation software, 1.2 × 105 search calculations take only 573 s. Meanwhile, we also show that the hybridization between multilayer cavity plasmonic resonant modes is the main cause of the ultra-wideband absorptance. This method significantly increases the viability of more complex ultra-wideband perfect absorber designs and provides opportunities to manipulate light-matter interactions in the future.

Gas-Sensing Properties of a Carbyne-Enriched Nanocoating Deposited onto Surface Acoustic Wave Composite Substrates with Various Electrode Topologies

The application of carbyne-enriched nanomaterials opens unique possibilities for enhancing the functional properties of several nanomaterials and unlocking their full potential for practical applications in high-end devices. We studied the ethanol-vapor-sensing performance of a carbyne-enriched nanocoating deposited onto surface acoustic wave (SAW) composite substrates with various electrode topologies. The carbyne-enriched nanocoating was grown using the ion-assisted pulse-plasma deposition technique. Such carbon nanostructured metamaterials were named 2D-ordered linear-chain carbon, where they represented a two-dimensionally packed hexagonal array of carbon chains held by the van der Waals forces, with the interchain spacing approximately being between 4.8 and 5.03 Å. The main characteristics of the sensing device, such as dynamic range, linearity, sensitivity, and response and recovery times, were measured as a function of the ethanol concentration. To the authors’ knowledge, this was the first time demonstration of the detection ability of carbyne-enriched material to ethanol vapors. The results may pave the path for optimization of these sensor architectures for the precise detection of volatile organic compounds, with applications in the fields of medicine, healthcare, and air composition monitoring.

Tuning the Spatially Controlled Growth, Structural Self-Organizing and Cluster-Assembling of the Carbyne-Enriched Nano-Matrix during Ion-Assisted Pulse-Plasma Deposition

Revise and shorten the abstract as follows: Carbyne-enriched nanomaterials are of current interest in nanotechnology-related applications. The properties of these nanomaterials greatly depend on their production process. In particular, structural self-organization and auto-synchronization of nanostructures are typical phenomena observed during the growth and heteroatom-doping of carbyne-enriched nanostructured metamaterials by the ion-assisted pulse-plasma deposition method. Accordingly, fine tuning of these processes may be seen as the key step to the predictive designing of carbyne-enriched nano-matrices with improved properties. In particular, we propose an innovative concept, connected with application of the vibrational-acoustic effects and based on universal Cymatics mechanisms. These effects are used to induce vibration-assisted self-organized wave patterns together with the simultaneous manipulation of their properties through an electric field. Interaction between the inhomogeneous electric field distribution generated on the vibrating layer and the plasma ions serves as the additional energizing factor controlling the local pattern formation and self-organization of the nano-structures.

Polarization independent 2×2 multimode interference coupler with bricked subwavelength metamaterial

The silicon-on-insulator (SOI) platform enables high integration density in photonic integrated circuits while maintaining compatibility with CMOS fabrication processes. Nevertheless, its inherently high modal birefringence hinders the development of polarization-insensitive devices. The dispersion and anisotropy engineering leveraging subwavelength grating (SWG) metamaterials makes possible the development of polarization agnostic waveguide components. In this work we build upon the bricked SWG metamaterial nanostructures to design a polarization independent 2×2 multimode interference (MMI) coupler for the 220 nm SOI platform, operating in the telecom O-band. The designed device exhibits a 160 nm bandwidth with excess loss, polarization dependent loss and imbalance below 1 dB and phase error lower than 5°.

Broadband absorber using ultra-thin plasmonic metamaterials nanostructure in the visible and near-infrared regions

In this work, an ultra-thin plasmonic metamaterial nanostructure absorber is simulated using the finite difference time domain method in the visible and near-infrared regions. A periodic square titanium-silica cap coated with glass medium is mounted on top of a silver substrate. The presence of the glass host enhances the absorption bandwidth by 276%. With an almost perfect metal–insulator-metal absorber, over 90% absorbance has been obtained for wavelengths from 440 to 1850 nm, producing an absorption bandwidth of 1410 nm. The impact of changing dimensions and using different materials on the absorption spectra has been investigated in both visible and near-infrared regimes. The considered metals for the top layer are titanium, nickel, silver, aluminum, and gold; however, the insulators are silica, quartz, vanadium dioxide, methyl methacrylate, and aluminum dioxide. In addition, aluminum, silver, copper, and gold are then simulated as a substrate. The optimum structure, which produces the maximum absorber bandwidth, 1410 nm, with a higher absorption, over 90%, is Glass-Ti–SiO2–Ag. The finding illustrates that the optimum dimensions of the Ti–SiO2 cab and the square base unit cell of the silver substrate are 250 nm and 200 nm, respectively. Finally, the absorption bandwidth is calculated using different polarization angles ranges from 10° to 70° with a step10°.

Absorption Enhancement in Hyperbolic Metamaterials by Means of Magnetic Plasma

The main features of surface plasmon polaritons (SPPs) that can propagate in a metamaterial–magnetic plasma structure are studied from theoretical perspectives. Both the conventional and imaginary parts of the dispersion relation of SPPs are demonstrated considering transverse magnetic (TM) polarization. We examine and discuss the influence of the external magnetic field. The results demonstrate that this factor dramatically alters the nature of SPPs. It is concluded that the positions and propagation lengths of SPPs can be engineered. Moreover, we present an approach allowing for an absorption enhancement that is a pivotal factor in antenna design. A unified insight into the practical methods aiming to attain hyperbolic dispersion by means of nanostructured and nanowire metamaterials is demonstrated.

Optical vector field rotation and switching with near-unity transmission by fully developed chiral photonic crystals

State-of-the-art nanostructured chiral photonic crystals (CPCs), metamaterials, and metasurfaces have shown giant optical rotatory power but are generally passive and beset with large optical losses and with inadequate performance due to limited size/interaction length and narrow operation bandwidth. In this work, we demonstrate by detailed theoretical modeling and experiments that a fully developed CPC, one for which the number of unit cells N is high enough that it acquires the full potentials of an ideal (N → ∞) crystal, will overcome the aforementioned limitations, leading to a new generation of versatile high-performance polarization manipulation optics. Such high-N CPCs are realized by field-assisted self-assembly of cholesteric liquid crystals to unprecedented thicknesses not possible with any other means. Characterization studies show that high-N CPCs exhibit broad transmission maxima accompanied by giant rotatory power, thereby enabling large (>π) polarization rotation with near-unity transmission over a large operation bandwidth. Polarization rotation is demonstrated to be independent of input polarization orientation and applies equally well on continuous-wave or ultrafast (picosecond to femtosecond) pulsed lasers of simple or complex (radial, azimuthal) vector fields. Liquid crystal-based CPCs also allow very wide tuning of the operation spectral range and dynamic polarization switching and control possibilities by virtue of several stimuli-induced index or birefringence changing mechanisms.

Metascintillators for Ultrafast Gamma Detectors: A Review of Current State and Future Perspectives

Scintillation detector development is an active field of research, especially for its application to the medical imaging field and in particular to the positron emission tomography (PET). Effective sensitivity and signal-to-noise ratio in PET are greatly enhanced when improving detector timing capabilities: the availability to provide time-of-flight (TOF) information. However, physical barriers related to the characteristics of available organic and inorganic scintillators create a tradeoff between photon kinetics and gamma detection efficiency. We introduce the novel concept of metascintillators, composite topologies comprising of multiple scintillating and light-guiding materials functioning in synergy, that break this compromise. We provide an overview of published, ongoing and upcoming developments within this framework. Unconventional topologies, such as the multiple slabs approach comprising of a high-Z host and a fast emitter; materials such as CdSe/CdS nanoplatelets; and treatments related to nanostructured metamaterials and photonic interactions, are reviewed and complemented with new, unpublished advances in simulations and analysis. Future perspectives are further presented, encompassing developments in signal analysis and system integration. Within this concept, an improved generation of detectors and PET scanners with unprecedented time resolution is researched, paving the way toward the 10-ps TOF PET challenge for the advancement of PET and improvement of public health.

Retracted] The Study of Midinfrared Extremely Narrow Dual‐Band Absorber Based on a Novel Metamaterial Structure

Due to the problems that the metal pattern layer on the top of the traditional metamaterial structure is easy to be oxidized and easy to fall off, in this paper, a novel semiconductor metamaterial nanostructure composed of a periodic array of GaAs‐SiO2 cubes and a gold (Au) film has been proposed. Using FDTD solutions software to prove this metamaterial structure can achieve ultranarrow dual‐band, nearly perfect absorption with a maximum absorbance of 99% and a full‐width at half‐maximum (FWHM) value that is less than 20 nm in the midinfrared region. The refractive index sensitivity is demonstrated by changing the background index and analyzing the absorption performance. It had been proved that this absorber has high sensitivity (2000/RIU and 1300/RIU). Using semiconductor material instead of the metal material of the top pattern layer can effectively inhibit the performance failure of the metamaterial structure caused by metal oxidation. The proposed narrow, dual‐band metamaterial absorber shows promising prospects in applications such as infrared detection and imaging.

Electrogyration in Metamaterials: Chirality and Polarization Rotatory Power that Depend on Applied Electric Field

AbstractOne of the most fascinating properties of chiral molecules is their ability to rotate the polarization of light. Since Faraday's experiments in 1845, it has been known that nonreciprocal polarization rotatory power can be induced by a magnetic field. But can reciprocal polarization rotation in chiral molecules be influenced by an electric field? In the 1960s, Aizu and Zheludev introduced the phenomenon of electrogyration. While the linear (Pockels) and quadratic (Kerr) electro‐optical effects describe how an external electric field changes linear birefringence and dichroism, electrogyration describes how a field changes the circular birefringence and dichroism of a medium. Electrogyration is observed in dielectrics, semiconductors, and ferroelectrics, but the effect is small. This work demonstrates a nanostructured photonic metamaterial that exhibits quadratic electrogyration—proportional to the square of the applied electric field—six orders of magnitude stronger than in any natural medium. Giant quadratic electrogyration emerges as electrostatic forces acting against forces of elasticity change the chiral configuration of the metamaterial's nanoscale building blocks and consequently its polarization rotatory power. This observation of giant electrogyration alters the perception of the effect from that of an esoteric phenomenon into a functional part of the electro‐optic toolkit with application potential.