Polarization Insensitive Metasurface Research Articles

Nanoimprinted TiO2 Metasurfaces with Reduced Meta-Atom Aspect Ratio and Enhanced Performance for Holographic Imaging.

Metasurface holograms, with the capability to manipulate spatial light amplitudes and phases, are considered next-generation solutions for holographic imaging. However, conventional fabrication approaches for meta-atoms are heavily dependent on electron-beam lithography (EBL), a technique known for its expensive and time-consuming nature. In this paper, a polarization-insensitive metasurface hologram is proposed using a cost-effective and rapid nanoimprinting method with titanium dioxide (TiO2) nanoparticle loaded polymer (NLP). Based on a simulation, it has been found that, despite a reduction in the aspect ratio of meta-atoms of nearly 20%, which is beneficial to silicon master etching, NLP filling, and the mold release processes, imaging efficiency can go up to 54% at wavelength of 532 nm. In addition, it demonstrates acceptable imaging quality at wavelengths of 473 and 671 nm. Moreover, the influence of fabrication errors and nanoimprinting material degradation in terms of residual layer thickness, meta-atom loss or fracture, thermal-induced dimensional variation, non-uniform distribution of TiO2 particles, etc., on the performance is investigated. The simulation results indicate that the proposed device exhibits a high tolerance to these defects, proving its applicability and robustness in practice.

Flexible and Polarization-insensitive Metasurface Array for Microwave Power Transfer

Flexible and Polarization-insensitive Metasurface Array for Microwave Power Transfer

High-performance mid-infrared refractive index sensing all-dielectric metasurface with dual resonance excited by bound states in the continuum and toroidal dipole mode

Converting bound states in the continuum (BICs) domain to quasi-BICs regime effectively achieves high Q-factor excitation, enabling refractive index sensing. In this paper, we propose an all-dielectric metasurface consisting of four silicon nanopore cylinders. While maintaining a four-fold symmetry, an asymmetric distribution of nanopores is introduced, creating two resonances in the mid-infrared range. Through near-field analysis and multipole decomposition, it is demonstrated that magnetic dipoles (MD) and toroidal dipoles (TD) dominate the resonance modes, and the sensing performance of the structure is investigated. The quality factor (Q) and sensitivity reach 6688 and 1140nm/RIU, respectively. The sensing characteristics in different refractive index (RI) media are analyzed, and the results show that the refractive index sensing capability of the dielectric metasurface is influenced by its structural parameters and exhibits a significant linear response relationship. Furthermore, by varying the structural parameters of the dielectric metasurface, we also discovered the differences in sensing performance within different wavelength ranges. We study all polarization angle robust quasi-BICs in the mid-infrared spectral region, which can provide new references for polarization-insensitive metasurfaces with dual resonances. Polarization-insensitive technology can avoid the need for compensating the polarization state of the light signal in the sensor and improve the performance and reliability of the sensor.

Tunable ultra-wideband graphene metasurface absorber: A mode merger design approach for terahertz applications

This paper presents a systematic approach for designing an ultra-wideband (UWB) absorber for terahertz (THz) applications. The metasurface consists of top patterned graphene (Gpat) and bottom continuous graphene (Gcont), each having a thickness of 0.335 nm and separated by a 3 μm thick silicon dioxide (SiO2) dielectric. Multiple absorbing modes have been generated by engraving rectangular, star-shaped, and circular ring slots on the top Gpat resonating at 3.22, 4.52, and 0.32 THz, respectively. A circular ring slot works as a coupling controller and helps to combine different slot resonances. The mode merger technique, along with the high plasmonic coupling between Gpat and Gcont controlled by Gcont help to attain an absorption bandwidth (BW) of 5.9 THz (193.44%) with absorptivity ≥ 90% ranging from 0.1 to 6 THz. The periodicity and thickness of the proposed polarization-insensitive metasurface are found to be λ0/300 and λ0/996.68, respectively, where λ0 is free space wavelength computed at 0.1 THz. Absorptivity ≥ 80% for incidence angle up to 60∘ under both transverse electric (TE) and transverse magnetic (TM) polarization has also been achieved. The absorber provides excellent frequency tunable characteristics over a UWB band and shows blue-shift with increased chemical potential (μc).

Dispersion-engineered metasurfaces reaching broadband 90% relative diffraction efficiency

Dispersion results from the variation of index of refraction as well as electric field confinement in sub-wavelength structures. It usually results in efficiency decrease in metasurface components leading to troublesome scattering into unwanted directions. In this letter, by dispersion engineering, we report a set of eight nanostructures whose dispersion properties are nearly identical to each other while being capable of providing 0 to 2π full-phase coverage. Our nanostructure set enables broadband and polarization-insensitive metasurface components reaching 90% relative diffraction efficiency (normalized to the power of transmitted light) from 450 nm to 700 nm in wavelength. Relative diffraction efficiency is important at a system level – in addition to diffraction efficiency (normalized to the power of incident light) – as it considers only the transmitted optical power that can affect the signal to noise ratio. We first illustrate our design principle by a chromatic dispersion-engineered metasurface grating, then show that other metasurface components such as chromatic metalenses can also be implemented by the same set of nanostructures with significantly improved relative diffraction efficiency.

Optical Pulling Force in Non-Paraxial Bessel Tractor Beam Generated with Polarization-Insensitive Metasurface

Tractor beams, able to produce optical pulling forces (OPFs) on particles, are attracting increasing attention. Here, non-paraxial Bessel tractor beams are generated using polarization-insensitive metasurfaces. OPFs are found to exert on dielectric particles with specific radii at the axes of the beams. The strengths of the OPFs depend on the radii of the particles, which provides the possibility of sorting particles with different sizes. For the OPFs, the radius ranges of particles vary with the polarization states or topological charges of the incident beams. The change of polarizations can provide a switch between the pulling and pushing forces, which offers a new way to realize dynamic manipulation of particles. The change of topological charges leads to disjoint radii ranges for the OPFs exerting on particles, which provides the possibility of selective optical separation. Moreover, we study the behaviors of particles in the tractor beams. The simulation results reveal that linearly or circularly polarized tractor beams can pull particles a sufficient distance towards the light source, which verifies the feasibility of separating particles.

Design, development and characterization of resistive arm based planar and conformal metasurfaces for RCS reduction

This paper reports the design, development and characterization of a broadband, polarization insensitive metasurface absorber in planar, cylindrically bent and 90° dihedral surface geometry. Four metallic patches loaded with eight lumped resistors are used, which has been optimized numerically using CST MICROWAVE STUDIO, as a unit cell of developed metasurface absorber, to achieve 20 dB reflection reduction for 51.21% fractional bandwidth (13.42–22.66 GHz) under normal incidence with 0.12 {lambda }_{L} thickness (where {lambda }_{L} corresponds to lower operating frequency). The numerical findings are also verified analytically using equivalent circuit analysis, which exhibits very good agreement. Polarization-insensitive characteristics are achieved using fourfold rotation symmetry of the designed structure. The fabricated prototype of the designed absorber is experimentally characterized, using free space measurement method and ABmm vector network analyzer (VNA) system, and fairly good agreement with numerical-analytical findings are reported. The major novelty of this study is the design and development of a broadband (13.42–22.66 GHz), polarization insensitive metasurface absorber that provides 20 dB reflection reduction numerically as well as experimentally in the whole band, which to the author’s knowledge has not been observed till now. Also, keeping in mind the radar stealth applications, first time we have demonstrated both numerically and experimentally, different geometrical shapes of conformal metasurfaces that can be practically used in actual scenario.

Polarization-independent tetramer metasurface with multi-Fano resonances based on symmetry-protected bound states in the continuum

The bound state in the continuum (BIC) has drawn much attention in electromagnetics due to its infinite lifetime and quality factor. By the method of breaking the symmetry in the metasurface, the optically invisible BIC can be converted into visible quasi-BIC with ultra-high quality factor. However, symmetry-breaking metasurfaces often suffer from polarization sensitivity due to structural or field asymmetry. In this paper, we propose a metasurface consisting of nanodisk tetramer clusters to realize the transformation from BICs to quasi-BICs and support four resonances in the near-infrared range. The in-plane center square defect is introduced to provide symmetry-breaking while C4v symmetry is maintained. By performing the multipole decomposition and near-field analysis, these four modes can be demonstrated to be excited by electric quadrupole (EQ), magnetic quadrupole (MQ), magnetic dipole (MD) and toroidal dipole (TD) moments. We demonstrate the full angles polarization-independent characteristic of the metasurface and investigate the robustness by introducing off-center square defects. In addition, we also evaluate the sensing performance of the metasurface as a refractive index sensor. We believe that this work could provide new references for the polarization insensitive metasurface with multiple resonances and help to develop the applications such as biochemical sensing, low-threshold lasing and nonlinear devices.

Single-Step Fabricable Flexible Metadisplays for Sensitive Chemical/Biomedical Packaging Security and Beyond.

Secure packaging and transportation of light-sensitive chemical and biomedical test tubes are crucial for environmental protection and public health. Benefiting from the compact form factor and high efficiency of optical metasurfaces, we propose a broad-band polarization-insensitive flexible metasurface for the security of sensitive packages in the transport industry. We employ both the propagation and the geometric phase of novel TiO2 resin-based anisotropic nanoresonators to demonstrate a flexible and broad-band polarization-insensitive metasurface in the visible domain. The ultraviolet nanoimprint lithographic technique (UV-NIL) is used to fabricate high-index TiO2 nanoparticle-embedded-resin (nano-PER) structures that are patterned on a flexible substrate. This novel approach provides swift single-step fabrication without secondary fabrication steps such as deposition and etching. Moreover, replicating and transforming patterns over flexible substrates make the proposed technique highly suitable for large-throughput commercial manufacturing. As the proposed metahologram manifests high transmission efficiency in the visible domain, such flexible metaholographic platforms could find several exciting applications in bendable/curved displays, wearable devices, and holographic labeling for interactive displays.

A compact transparent polarization-insensitive metasurface with broadband monostatic and bistatic radar cross-section reduction of millimeter-waves

This paper proposes an optimized optically transparent metasurface (OTM) which achieves broadband monostatic and bistatic radar cross-section (RCS) reduction with polarization and angle insensitivity. Through employing (a) theoretical formulation involving both monostatic and bistatic RCS and (b) unit cell placement optimization using the particle swarm optimization approach, we achieve monostatic and bistatic RCS reduction over a broad bandwidth with a single-layer, ultra-thin metasurface featuring only two types of unit cells. The proposed metasurface has high optical transparency and electrically small size compared to counterparts with similar performances. Simulation and experimental measurement show that the metasurface achieves more than 10 dB monostatic and bistatic RCS reduction from 18 to 34 GHz, which completely covers the 5G mm-wave spectrum. The proposed optically transparent metasurface can find use in many areas, including mm-wave applications, invisible glass technology, and vehicle windshield systems.

Ultra-broadband and polarization-insensitive metasurface absorber with behavior prediction using machine learning

The solar spectrum energy absorption is very important for designing any solar absorber. The need for absorbing visible, infrared, and ultraviolet regions is increasing as most of the absorbers absorb visible regions. We propose a metasurface solar absorber based on Ge2Sb2Te5 (GST) substrate which increases the absorption in visible, infrared and ultraviolet regions. GST is a phase-changing material having two different phases amorphous (aGST) and crystalline (cGST). The absorber is also analyzed using machine learning algorithm to predict the absorption values for different wavelengths. The solar absorber is showing an ultra-broadband response covering a 0.2–1.5 µm wavelength. The absorption analysis for ultra-violet, visible, and near-infrared regions for aGST and cGST is presented. The absorption of aGST design is better compared to cGST design. Furthermore, the design is showing polarization insensitiveness. Experiments are performed to check the K-Nearest Neighbors (KNN)-Regressor model’s prediction efficiency for predicting missing/intermediate wavelengths values of absorption. Different values of K and test scenarios; C-30, C-50 are used to evaluate regressor models using adjusted R2 Score as an evaluation metric. It is detected from the experimental results that, high prediction proficiency (more than 0.9 adjusted R2 score) can be accomplished using a lower value of K in KNN-Regressor model. The design results are optimized for geometrical parameters like substrate thickness, metasurface thickness, and ground plane thickness. The proposed metasurface solar absorber is absorbing ultraviolet, visible, and near-infrared regions which will be used in solar thermal energy applications.

Ge2Sb2Se4Te1-based multifunctional metalenses for polarization-independent, switchable and dual-mode focusing in the mid-infrared region

Reconfigurable metasurfaces, whose optical responses can be tailored post-fabrication, hold great promise to enable multifunctional and versatile optical components. However, the singularity of adjustment, usually manifested as switching functions or wavelength-tunable features, hinders their further development in practice. Particularly, active polarization-insensitive metasurfaces usually appear to be forced by using the isotropic nanostructure as their constituent elements, rendering them to lose a degree of freedom in the design space and plaguing them in many practical scenarios. In this paper, we report a polarization-insensitive, switchable and dual-mode multifunctional metalens. By using anisotropic Ge2Sb2Se4Te1 nanofins as building blocks, we can accurately impart the phases to each atom in order to realize the full 2π coverage by restricting their rotation angle to either 0 or 90 degrees. This guarantees that the metalens can elegantly focus any incident polarized light at the operation wavelength of λ0 = 4200 nm. Meanwhile, the proposed metalens can also focus the oblique incident light into one spot within the incident angle range of [−2.4°, +2.4°], providing a field of view (FOV) of 4.8°. Moreover, the designed scheme supports a stable focusing efficiency (above 60%), a near-constant focal length (the focal length shifts less than 6%) and diffraction-limited focal spots over λ = 4000–4500 nm. The point to emphasize here is that when the amorphous Ge2Sb2Se4Te1 (aGSST) is converted into the crystalline state (cGSST), the designed metalens can be switched off in terms of focusing features at λ0 = 4200 nm, while the bright focal spot reappears on the focal plane at the wavelength of λ = 4700 nm unexpectedly. The cGSST-based metalens works well both in the reflection and transmission modes simultaneously at the wavelength of λ = 4000 nm owing to phase matching conditions. The generality of our polarization-insensitive, switchable and dual-mode design allows it to be implemented in a plethora of metasurface devices with promising applications, such as: optical imaging, optical encryption, augmented reality and sensing systems.

Full-color-sorting metalenses for high-sensitivity image sensors

Image sensors play a critical role in current technologies ranging from smartphones to autonomous vehicles. In these technologies, high-sensitivity image sensors are highly desired because they enable dark-scene/ultra-fast imaging. Unfortunately, a conventional sensor architecture that employs color filters on every pixel fundamentally limits the detected light power per pixel because of the filtering, which has been a long-standing barrier to sensitivity improvement. Here, we demonstrate polarization-insensitive metasurface lenses (metalenses) that sort primary colors on high-density pixels without the use of color filters. The metalenses simultaneously act as pixel-scale color splitters and lenses and are compatible with complementary metal–oxide-semiconductor sensor technology. An image sensor with such metalenses significantly enhances the detected light power, while affording high image quality, incident angle tolerance, and sub-micrometer spatial resolution. The demonstrated architecture opens the way to the development of high-sensitivity color image sensors that exceed current limits while maintaining consistency with state-of-the-art sensor technology.

Phase-only femtosecond optical pulse shaping based on an all-dielectric polarization-insensitive metasurface.

Recently, metasurfaces capable of manipulating the amplitude and the phase of an incident wave in a broad frequency band have been employed for femtosecond optical pulse shaping purposes. In this study, we introduce a phase-only pulse shaper based on an all-dielectric CMOS-compatible polarization-insensitive metasurface, composed of Si nano cylinders sitting on a fused silica substrate. The required phase profile of the metasurface for desired waveforms are calculated using an iterative Fourier transform algorithm, and the performance of the pulse shaper metasurface in implementing the phase masks was assessed using full-wave simulations. Such approach for realizing a polarization-insensitive metasurface-based phase-only pulse shaper has never been investigated to the best of our knowledge. It is demonstrated that the simulated results of the proposed metasurface-based pulse shaper is in great agreement with the results of the algorithm, while exhibiting a very high transmission efficiency. This work indicates yet another exciting but not fully examined application of meta-structures that is the optical pulse shaping.

Design, development and characterization of wide incidence angle and polarization insensitive metasurface absorber based on resistive-ink for X and Ku band RCS reduction

This article deals with the design, development and characterization of a broadband, wide incidence angle and polarization-insensitive metasurface, which consists of periodic resistive-ink pattern printed on top of FR-4 dielectric substrate as a RADAR absorber. Geometrical dimensions of the proposed unit cell have been optimized numerically using CST-Microwave Studio to achieve 99% absorptivity for X and Ku frequency spectrum (8–18 GHz) under the normal angle of incidence and to achieve almost 90% absorptivity under the oblique angle of incidence up to 50° within X-Band to Ku-Band spectrum. Furthermore, it is reported that by having fourfold rotation symmetry, the designed structure is insensitive to the polarization of the incident wave. An equivalent circuit analytical modeling has also been performed to understand and verify numerical findings that are nearly in agreement. The designed absorber is fabricated and characterized using ABmm VNA and free space measurement method, for both normal and oblique angle of incidence, and it is found that measured characteristics are in agreement with numerical-analytical findings. Furthermore, the monostatic and bistatic Radar Cross Section reduction capability of the designed absorbing structure for both planar and curved surface (cylinder) is studied extensively to assess its potential application in X-Band to Ku-Band Spectrum.

High-performance refractive index sensing system based on multiple Fano resonances in polarization-insensitive metasurface with nanorings.

An optical refractive index sensor is a detection device that can convert changes in the refractive index into detectable optical information. The combination of surface plasmon resonance (SPR) and Fano resonance can improve some key indicators, i. e., sensing sensitivity, figure of merit (FOM), band number, and polarization sensitivity, which are all related to the comprehensive performance for high-precision and multi-band sensing. In our manuscript, we proposed a refractive index sensor composed of a nanoring array and a Fabry-Pérot (F-P) resonant cavity. The coupling of the localized surface plasmon resonances (LSPR) of the nanoring array and the cavity mode of the F-P resonant cavity can produce double Fano resonances. The corresponding sensing sensitivities can reach 621.5 nm/RIU and 906.9 nm/RIU, and the corresponding FOMs can reach 119.7 and 119.0. Then we studied the influence of the structure parameters on the sensitivity and FOM of the sensor through simulation calculation and theoretical analysis and verified the insensitivity of the structure to the polarization of incident light. Our structure has high comprehensive performance, not only polarization insensitivity but also high sensing sensitivity and FOM in both bands, which is more suitable for practical applications.

Generation of needle beams through focusing of azimuthally polarized vortex beams by polarization-insensitive metasurfaces

Needle beams are widely used in advanced applications due to their high intensity, high resolution, and extended depth of focus. In this work, the generation of needle beams through transmitting azimuthally polarized beams to polarization-insensitive metasurfaces is presented. Defocus phase profiles and annular apertures were adopted to obtain needle beams, respectively. A relatively long depth of focus ( 16.9 λ ) with subdiffraction beam size ( 0.41 λ ) was obtained. The introduction of metasurfaces can significantly simplify an optical system compared with traditional methods utilizing complex optical elements. This study may contribute to photolithography, microscopy, particle manipulation, and acceleration.

Parallel Analog Computing Based on a 2×2 Multiple-Input Multiple-Output Metasurface Processor With Asymmetric Response

We present a polarization-insensitive metasurface processor to perform spatial asymmetric filtering of an incident optical beam, thereby allowing for real-time parallel optical processing. To enable massive parallel processing, we introduce a novel Multi Input-Multi Output (MIMO) computational metasurface with an asymmetric optical response that can perform spatial differentiation on two distinct input signals regardless of their polarization. In our scenario, two distinct signals set in x and y directions, parallel and perpendicular to the incident plane, illuminate simultaneously the metasurface processor, and the resulting differentiated signals are separated from each other via appropriate Spatial Low Pass Filters (SLPF). By leveraging Generalized Sheet Transition Conditions (GSTCs) and surface susceptibility tensors, we design an asymmetric meta-atom augmented with normal susceptibilitiesto reach asymmetric optical response at normal beam illumination. Proof-of-principle simulations are also reported along with the successful realization of signal processing functions. The proposed metasurface overcomes major shortcomings imposed by previous studies such as large architectures arising from the need of additional subblocks, slow responses, and most importantly, supporting only a single input with a given polarization. Our results set the path for future developments of material-based analog computing using efficient and easy-to-fabricate MIMO processors for compact, fast, and integrable computing elements without any Fourier lens.

The Design of a Switchable Infrared Hybrid Plasmonic Metasurface Absorber for Energy Harvesting Applications

A plasmonic switchable polarization-insensitive metasurface absorber is proposed. The design provides two modes of operation by employing phase-change material in semiconductor and metallic phases. In this paper, we study the switchable absorption behavior of the metasurface operating in a dual-band and single-band modes targeting the mid-infrared range suitable for energy harvesting applications such as thermophotovoltaics. The design is optimized using a global optimization technique.

Polarization-Insensitive Metasurface for Harvesting Electromagnetic Energy with High Efficiency and Frequency Stability over Wide Range of Incidence Angles

In this paper, a polarization-insensitive metasurface, harvesting electromagnetic (EM) energy with high efficiency and frequency stability over a wide range of incidence angles, is proposed. The previously reported metasurfaces suffer from their maximum efficiencies shifting with the frequency when the incidence angle increases. By introducing a square-shaped metal via ring around the elements, the mutual coupling among adjacent cells is reduced so that the proposed metasurface can maintain maximum efficiency at the fixed operation frequency over a wide range of incidence angles. Furthermore, with one single harvesting via in the proper position for the connection of a harvesting load, the metasurface can collect EM energy effectively with both transverse electric (TE) and transverse magnetic (TM) polarizations in one single harvesting load. Compared with the reported metasurfaces, this proposed metasurface has a higher efficiency and fixed operation frequency within a wide incidence range. The energy distribution, harvesting efficiency, and surface current are simulated to investigate the operation mechanism of the proposed metasurface. The simulation results show that the maximum harvesting efficiency is 91% at 5.8 GHz for both TE and TM polarizations at the normal incidence. When the incident angle increases to 75°, the maximum efficiency is achieved at 5.79 GHz (0.19% shift), and the maximum efficiencies of TM and TE polarizations are 91% and 68%, respectively. A 5 × 5 array is fabricated and tested. The experimental results are in good agreement with the simulated ones.