Broad Range Of Incident Angles Research Articles

Single layer dual wideband linear to circular polarisation converter with integrated parasitic elements

This paper presents a novel single-layer, single-sided, dual-wideband linear-to-circular (LTC) polarsation converter. The unit cell of the polarisation converter comprises two identical diagonally oriented C-shaped metallic strips arranged symmetrically and integrated with four outwardly extending diagonal parasitic strips. The LTC polarisation converter is placed on the top side of an ultra-thin substrate layer with a profile size of 0.020 λo at 11.9 GHz and 0.05 λo at 30.7 GHz. The inclusion of parasitic strips in the unit cell design is pivotal, inducing a 90º phase shift between the orthogonal wave components, thus enhancing LTC polarisation conversion. The design achieved near-equal transmission amplitudes and maintained a stable phase difference of nearly 90° across two distinct frequency bands, enabling right-hand circular polarisation in the lower frequency band and left-hand circular polarisation in the higher frequency band. Numerical and experimental results showed that the polariser can realise a wideband LTC polarisation conversion for both x- and y-polarised electromagnetic wave incidences at two distinct frequency bands of 6.78–17.08 GHz (fractional bandwidth of 86.3%) and 25.09–36.40 GHz (36.8%). The converter maintained consistent polarisation conversion performance across a broad range of incidence angles and exhibited high and uniform total transmittance in both operational frequency bands.

Polarization-selective terahertz absorber based on square cyclic graphene surface

Graphene-based absorbers are highly valued in terahertz technology due to their efficient performance. In this study, we theoretically propose a tunable, high sensitivity, single-band terahertz perfect absorber with polarization coherence that exhibits stable performance across a broad range of incidence angles. Simulation results show that the absorber's sensitivity to both TM and TE waves can be maintained at about 2 THz/RIU, with optimal values above 13.3 RIU−1 for TM waves and 12.9 RIU−1 for TE waves, and all Q-factors above 52. Furthermore, the impedance matching approach is utilized to explain the mechanism underlying its efficient absorption, and the electric field distribution is used to analyze the polarization coherence phenomena. We also propose a polarization-insensitive hetero-geometric structure based on the original structure, which allows for the separation of the absorption peaks by adjusting the geometrical parameters. These results show that the structure has a wide range of applications in aerospace, communication engineering, and image processing.

Optical axis-driven field enhancement in a hyperbolic medium.

Field enhancement of applied electric field is the foundation for the variety of applied domains at a nanoscale level. Traditionally, efforts to achieve field enhancement have required the use of complicated metamaterial-based structures with a transition behavior. Here, the electromagnetic field solution of the TM-polarized wave that interacts with an optical-axis-driven hyperbolic medium with a transition behavior is established. Detailed calculations reveal that such field enhancement can be achieved over a broad range of incident angles (i.e., near critical angle). Definitely, such flexibility of the incident angle for achieving the field enhancement enriches the understanding and provides novel prospective toward its practical realization.

On-Chip Polarization Rotators Using Metasurface.

Metasurfaces, with their ability to finely manipulate light properties (e.g., polarization), represent a frontier in optical technology. Here, we report innovative gold-based metasurfaces that are adept at changing the light polarization across a broad range of incident angles. Their expansive acceptance angle facilitates seamless integration with silicon waveguides, culminating in the realization of a novel, compact, broadband, and low-loss on-chip polarization rotator. The demonstrated metasurface devices show polarization conversion efficiencies as high as 98.5% at 1550 nm in the free space measurements. Unlike conventional plasmonic metasurfaces, which lack broadband capabilities, our metasurface-based waveplates show extinction ratios of >15 dB for a 120 nm bandwidth. These metasurfaces are positioned on top of up-reflecting mirrors fabricated in a micrometer-scale silicon-on-insulator platform to demonstrate on-chip polarization rotation. The demonstrated polarization rotator shows an extinction ratio of more than 10 dB for a 100 nm bandwidth. This study marks the first successful demonstration of on-chip polarization manipulation utilizing metasurface integration, heralding significant potential impacts for enhancing the functionality and miniaturization of photonic integrated circuits.

Tailoring resonant modes in dual cavities for transmissive structural colors with high brightness and high purity.

We present quad-layered structural color filters producing transmissive red (R), green (G), and blue (B) colors with high brightness and high purity, where thicknesses of layers for the RGB colors are optimized by using a L-BFGS-B algorithm. To evaluate the performance of the proposed structural color filters, computer-based inverse designs based on meta-heuristic and reinforcement learning algorithms are employed, where the optical properties obtained from the inverse designs are comparable to those shown in our proposed design. A peak separation phenomenon in dual cavities is applied to make a spectral response rectangular, and also a resonance order is optimally tailored to maximize the transmittance at a resonant wavelength with the suppression of undesired higher-order resonances at the same time for achieving pure colors. Transmission efficiency over 75% and the full width at half-maximum (FWHM) less than 90 nm are achieved. Besides, selecting a cavity medium with a high refractive index allows the optical properties of the structural color filters to remain almost constant in wavelength over a broad range of incident angles up to 60°. Moreover, only a few deposition steps are necessary, thus leading to a much simple fabrication as compared to previous works that involve a series of complicated lithographic processes. The approach described in this study may provide new ways for achieving diverse applications, such as displays, imaging devices, decorations, and colored solar cells.

Data-driven modeling and fast adjustment for digital coded metasurfaces database: Application in adaptive electromagnetic energy harvesting

Metasurfaces (MSs) show great promise in efficient electromagnetic energy harvesting (EMEH) due to their compactness, high efficiency, and long-distance transmission capabilities. Nonetheless, the conventional iterative and time-consuming solving process of MSs significantly escalates computational demands. Furthermore, once processed, the MS shape remains fixed and cannot be adapted to changing requirements. Accordingly, a critical challenge is the development of a new efficient solver for MS real-time tuning. Here, we introduce a class of digital coded MS databases including multiple pre-defined resonant frequency MS. The combination of multiple MS base functions from the database enables swift resonance frequency adjustments to adapt to changing environmental conditions. A topology optimization method based on data-driven modeling is employed to rapidly acquire the optimal digital coding for the corresponding MS at various operating frequencies, facilitating the construction of a database. This approach integrates a convolutional neural network and genetic algorithm (CNNGA). It not only enables more accurate and expedited forward prediction of MSs' electromagnetic (EM) response but also facilitates inverse design based on specified requirements. We employ this method to design a MS that achieves perfect energy harvesting (EH) over a broad range of incident angles and polarization directions. In addition, a data-driven modeling is used to establish an EH efficiency predictive model corresponding to MS combination. This model serves as a guide for real-time MS adjustments as per changing requirements. Compared to previously designed MSs, this model achieves rapid design and adaptive adjustment capabilities. Through the incorporation of various functional MS base functions into the database, this method can be universally applied to MS combinations tailored to specific functions, including EM cloaking, ultra-thin flat lenses, and computational MSs.

Enhancing 5G propagation into vehicles and buildings using optically transparent and polarisation insensitive metasurfaces over wide-incidence angles

This article introduces two transmissive metasurfaces applied to normal windows, aiming to improve the 5G outdoor-to-indoor (O2I) coverage. These windows can be utilized in various settings, such as vehicles or buildings. The proposed unit cells, designed to be wide-incident angle and polarization insensitive, are implemented in both single-glazing and double-glazing glasses, arranged in a periodic structure to form the transmission surfaces. Both metasurfaces maintain optical transparency by incorporating Indium Tin Oxide (ITO) as the conductive element in each unit cell. These engineered transmission surfaces enhance the 5G signal indoor coverage at the 3.5 GHz band across a broad range of incident angles. While multi-layer structures typically exhibit heightened sensitivity to the angle of incidence, the proposed two-layered transmissive surfaces demonstrate substantial angular stability, reaching up to 65 and 75 degrees for double- and single-glazed glass, respectively. To achieve this wide and stable angular response, evolutionary optimization techniques were employed to fine-tune the proposed unit cells. Both designs exhibit a high transmission coefficient across the operating frequency for a variety of incident angles, surpassing those reported in the existing literature. Experimental evaluations of the fabricated prototypes indicate that both metasurfaces hold significant potential for enhancing signal propagation into buildings and vehicles.

Metasurfaces for sound absorption over a broad range of wave incidence angles

Control of reflected waves that meet surfaces from oblique directions is crucial, for instance, in closed spaces. Metasurfaces composed of Helmholtz resonators can be efficient and compact absorbers but have limited ability to achieve high absorption over a wide incidence angle range, especially when designed for high performance in the region approaching grazing incidence. In turn, sonic crystals can be used to manipulate wave propagation direction at low frequencies. We propose a type of absorber that combines a surface of 2D Helmholtz resonators and a 2D sonic crystal with cylindrical scatterers arranged in a hexagonal lattice. The combined effect of both structures yields a metasurface that can achieve high absorption over a broad range of incidence angles. Here, an analytic model to estimate the behavior of the absorbers for wavelengths that are much longer than the unit cell dimensions is presented. The model is used in combination with an optimization strategy to realize designs for single frequency and octave-band performance. The test cases show that surfaces with absorption coefficient values above 0.9 for the range of incidence angle extending from 0∘ until 83∘ can be realized. The performance of the absorbers is verified with a finite element model and experimentally.

Tunable broadband terahertz absorber based on graphene with bilayer hexagonal

An ultra-wideband absorber with an absorption rate of near unit is studied and numerically simulated in the terahertz frequency band. The proposed absorber is a stacked compact structure consisting of a double-layer patterned graphene embedded between two separated dielectric. Under normal incidence, the absorption rate exceeds 99% at 2.31–3.92 THz. The absorption bandwidth can reach 2.54 THz for absorptions greater than 90% (1.91–4.45 THz), and the relative bandwidth achieves 80%. Furthermore, the absorber is tunable, with absorption ranging from 14% to almost 99.9% by altering the Fermi energy of graphene from 0 to 1 eV. The phenomenon of ultra-wideband absorption is analyzed in relation to impedance matching and field distribution. Moreover, the absorber has high polarization-independence and can operate across a broad range of incidence angles. This tunable ultra-wideband absorber has promising functions in modulation, stealth, switching, and imaging technologies.

A tri-functional, independently tunable terahertz absorber based on a vanadium dioxide-graphene hybrid structure.

This paper proposes a simulated design for a versatile terahertz absorber that can be actively tuned. The absorber utilizes the unique tuning capabilities of graphene and vanadium dioxide, enabling it to alternate between ultra-broadband absorption, broadband absorption, and almost complete reflection. In the metallic phase of vanadium dioxide, coupled with a graphene Fermi level at 0 eV, the absorber achieves ultra-broadband absorption. This spans an extensive frequency range from 3.85 THz to 9.73 THz, exhibiting an absorption rate surpassing 90%. As we shift to the insulating phase of vanadium dioxide and adjust the graphene Fermi level to 1 eV, the absorber operates in a broadband absorption mode. This mode spans 2.98 THz to 4.63 THz, demonstrating an absorption rate exceeding 90%. In the insulating state of vanadium dioxide with a graphene Fermi level at 0 eV, the absorber metamorphoses into a nearly total reflector. Its maximum absorption rate is a mere 0.52%. The unique adjustability of vanadium dioxide and graphene independently enables the fine-tuning of absorption rates for both ultra-broadband and broadband absorption without encountering interference. Additionally, thanks to the central symmetry inherent in the proposed structure, the absorber exhibits insensitivity to alterations in polarization angles and remains stable under a broad range of incident angles. With these benefits, the absorber shows promising potential for applications in electromagnetic stealth, wireless communication, and so on.

Lattice constant and polarization-independent high transmission in tellurium-based dielectric metasurfaces

High-refractive-index dielectric metasurfaces give rise to unprecedented control of light manipulation, such as control of phase, polarization and amplitude, giving rise to interesting properties, such as directional beam steering, polarization detectors and sensors. Dielectric metasurfaces of sub-wavelength dimensions have tremendous applications in the field of optics, such as negative refractive index, cloaking, perfect absorbers and reflectors. The study of light–matter interactions in such materials has gained impetus due to the formation of novel states, such as anapoles, and transparent states obtained by interference between resonant electric, magnetic and higher-order modes. In this article, we investigate the light–matter interaction of an array of periodic dielectric metasurfaces made from high-refractive-index tellurium in cubic geometries and study its electromagnetic response as a function of the lattice constant, angle of incidence and angle of polarization. More specifically, we observe a non-resonant transparent state at 60.69 THz, which is independent of both the lattice constant and polarization of the input radiation. Moreover, this state shows high transmission for a broad range of incident angles with potential applications as optical filters. It also depends on the incident polarization, thus acting as a perfect polarization detector. Detailed investigations of scattering parameters, the spatial distribution of electric and magnetic fields in the near- and far-field regions and detailed multipole analysis are carried out to analyze the electromagnetic response of the metasurface.

Numerical Study of High Spectral Efficiency and High-Temperature Energy Harvesting Metamaterial Emitter to Improve Thermophotovoltaic Performance

Design work was done on a broadband and wide-angle selective pyramid metamaterial emitter. COMSOL Multiphysics software was used to study the emitter, which was made of tungsten and aluminum nitride. The width of the unit cell and the tungsten ground thickness were fixed while other geometric parameters, such as the base length of the pyramid, the height of the pyramid, and the thickness of the dielectric, were tuned to produce the emitter’s desired broadband emission. A high average emissivity over 0.96 below the cutoff wavelength (0.1 μm-2.2 μm) was seen in the numerical simulation. The developed metamaterial emitter also had good emissivity across a broad range of incidence angles, from 0° to 60°, and was polarization independent. In addition, the planned emitter for the InGaAs cell has a better spectral efficiency than the blackbody other designed emitter. In general, the planned selective nanopyramid emitter was realized with 75% spectrum efficiency for InGaAs band gap energy (0.55 eV) at 1200 K, which was greater than blackbody and prior papers.

Three-Dimensional Nanopillar Arrays-Based Efficient and Flexible Perovskite Solar Cells with Enhanced Stability.

Perovskite nanopillars (PNPs) are propitious candidates for solar irradiation harvesting and are potential alternatives to thin films in flexible photovoltaics. To realize efficient daily energy output, photovoltaics must absorb sunlight over a broad range of incident angles and wavelengths congruent with the solar spectrum. Herein, we report highly periodic three-dimensional (3D) PNP-based flexible photovoltaics possessing a core-shell structure. The vertically aligned PNP arrays demonstrate up to 95.70% and 75.10% absorption at peak and under an incident angle of 60°. The efficient absorption and the orthogonal carrier collection facilitate an external quantum efficiency of 84.0%-89.18% for broadband wavelength. PNPs have been successfully implemented in flexible solar cells. The porous alumina membrane protects PNPs against water and oxygen intrusion and thereby imparts robustness to photovoltaic devices. Meanwhile, the excellent tolerance to mechanical stress/strain enables our unique PNP-based device to provide efficient solar-to-electricity conversion while undergoing mechanical bending.

Broadband coherent perfect absorption employing an inverse-designed metasurface via genetic algorithm.

Coherent perfect absorption (CPA) possesses the unique characteristics of flexibly and actively molding the flow of light. However, restricted by the low design efficiency and limited geometry variety of metamaterial structures, the common CPA metamaterial absorbers based on artificial design show poor performance in bandwidth operation. Here, we proposed a tungsten-based metamaterial absorber to achieve broadband CPA via employing genetic algorithm inverse design. Under the irradiation of two coherent beams, the high coherent absorption (>90%) can be achieved within a wide range from 1.32 to 3.28 µm. By simply adjusting the relative intensity or phase difference of the two coherent beams, the absorption intensity can be continuously modulated to realize the transition between coherent perfect absorption and coherent perfect transparency. Moreover, the coherent absorption maintains greater than 90% over a broad range of incident angles for both TM and TE polarizations. The scattering matrix theorem is applied to explain the physical mechanism of CPA, and the analytical results exhibit good consistency with the numerical calculations. Such a tungsten-based CPA metamaterial absorber with broadband tunability and exceptional angular stability is expected to be utilized in optical signal processing chips, all-optical modulators, and optical switchers.

Broadband Negative Reflection of Underwater Acoustic Waves from a Simple Metagrating: Modeling and Experiment

Metagratings are periodic arrays of subwavelength scatterers, or atoms, engineered to refract or reflect waves toward anomalous directions with unit efficiency. Here, we design and build a metagrating to control the reflection direction of waterborne ultrasound waves impinging on a rigid or free surface. The grating and its atoms are designed to cancel the specular reflection and to redirect acoustic power toward a negative-reflection direction, through the first-negative-order Floquet mode. Despite a simple design, based on C-shaped brass particles acting as Helmholtz resonators, the grating is efficient ($>90\mathrm{%}$) over a relatively broad range of frequencies (74--103 kHz) for a broad range of incidence angles (${14}^{\ensuremath{\circ}}$--${54}^{\ensuremath{\circ}}$). This good performance is obtained by tuning the distance between the atoms and the reflective surface. A multiple-scattering analytical model is presented to explain the phenomenon and a finite-element model is developed to further investigate the performance of the proposed design. Predictions from the model are confirmed experimentally in a water tank. The simplicity, reconfigurability, and scalability of the design, as well as its high efficiency, broadband behavior, and robustness to the incidence angle are all features that make the grating potentially useful for various applications in underwater acoustics, such as telemetry, communication, or noise mitigation.

Colloidal self-assembly based all-metal metasurface absorbers to achieve broadband, polarization-independent light absorption at UV–Vis frequencies

Here, we demonstrate a straightforward colloidal self-assembly strategy to fabricate metasurface absorbers (MAs) with efficient broadband light absorption from ultraviolet (UV) to visible (Vis) spectral range. In this study, high-index faceted colloidal Au nanocrystals (NCs), i.e., Au icosidodecahedron with active surface plasmonic modes, are innovatively designed as the meta-atoms, while a facile electrostatic self-assembly progress is implemented to arrange these meta-atoms onto a gold reflecting film, resulting in MAs with all-metal structures. Optical characterizations show that the well-designed MAs with a 28–32% surface coverage of Au meta-atoms exhibit an average light absorption above 94% within the wavelengths ranging from 200 to 600 nm. Moreover, the particular absorption behaviors of the designed MAs are inherently polarization-independent for a broad range of incident angles (0–45°) owing to the ultrathin structure and topological symmetry of the devices. It has been inferred that the positive localized surface plasmon and gap plasmon resonances excited from the adjacent Au meta-atoms and gold film are critical for self-assembled MAs with excellent absorbing performances. Overall, this work provide an alternative and simple pathway for the construction of scalable and practical UV–Vis MAs, which may facilitate the high-frequency optical applications of thermophotovoltaics, photoelectric devices and photochemical systems.

Precision of Silicon Oxynitride Refractive-Index Profile Retrieval Using Optical Characterization

Layers with gradient refractive-index profile are an attractive alternative to conventional homogeneous stack coatings. However, the optical characterization and monitoring of the graded refractive-index profile is a complex issue, which has been typically solved by using a simplified model of mixed materials. Although this approach provides a solution to the problem, the precision, which can be expected from optical characterization of the refractive index gradient, remains unclear. In this work, we study optical characterization of SiO_xN_y layers deposited via reactive dual ion beam sputtering. To characterize the deposited layers, we use several methods including reflectance, and transmittance spectra at a broad range of incident angles, together with spectral ellipsometry. All the data were simultaneously fitted with a general profile of refractive index. The expected profile used in our fit was based on characterization of SiO_xN_y layers with a varying stoichiometry. By altering of the profile, we discussed sensitivity of such alternation on fit quality and we studied ambiguity of merit-function minimization. We demonstrate that while the scanning of particular parameters of the profile can be seemingly very precise, we obtain a very good agreement between the experimental data and model for a broad range of gradient shapes, where the refractive-index value on major part of the profile can differ as much as 0.02 from the mean value.

Cross-Shaped Titanium Resonators Based Metasurface for Ultra-Broadband Solar Absorption

We propose a scheme for ultra-broadband and polarization-insensitive metamaterial perfect absorber (PA) by utilizing a thin metal-insulator-metal (MIM) stack, which is comprised of an array of cross-shaped titanium resonators, a silica dielectric spacer, and an opaque titanium slab. The bandwidth of absorption (A) > 90% is up to 2100 nm, ranging from the visible to near infrared region. At the wavelength range of 400-2500 nm, this metamaterial absorber shows strong absorption of electromagnetic waves. The spectral average absorptivity reaches 93.8% and the maximal absorptivity is up to 99.8%. In addition, the simulations show that the absorption remains high over a broad range of incident angles. Additionally, we have investigated the influences of geometries, structural parameters and material features on the absorption properties. The utilization of titanium rather than the noble metals efficiently lowers the fabrication cost and enhances the thermal stability and biocompatibility, thus paving a way to numerous applications such as solar energy harvesting, imaging, infrared detection and bio-medical techniques.

Reflectivity reduction of nanopatterned c-Si solar cells with antireflective coatings exposed to a wide range of incidence angles

The scope of the present study is to assess through numerical calculations the effect of nanopatterning the photovoltaics (PV) and that of imposing additional superstrate layers to the efficiency of the PV’s, under normal and wide incidence angles. The numerical calculations were performed with the Rigorous Coupled-Wave Analysis (RCWA) method where we examined several types of Si wafers including additional Si3N4 and SiO2 superstrate layers, and periodic patterns of grooves affecting either the superstrates or both the superstrates and Si substrate. A very broad parameter space was investigated (thickness of the superstrates, and the depth, diameter and periodicity constant of hole and pillar-like patterns) from which we derived the optimal set of parameters for each wafer type. The performance of the optimized wafers was then evaluated over a broad range of incident angles. Both the deposition of the superstrates as well as the imposition of periodic patterns improve drastically the absorption efficiency of the wafers and they allow for maintaining the maximum efficiency across the high incidence angle regime.

Structural colors with angle-insensitive optical properties generated by Morpho-inspired 2PP structures

This paper describes how two-photon polymerization was used to generate biomimetic nanostructures with angle-insensitive coloration inspired by the blue butterflies of Morpho. Less angle dependence was achieved by engineering the structures with a certain degree of disorder, which delimited them from classical photonic crystals. Variations in the processing parameters enabled the color hue to be controlled. In this context, blue, green, yellow, and brown structures were demonstrated. Reflection spectra of the structures were simulated and studied experimentally in a broad range of incident angles. Additionally, a molding technique was performed as a potential scale-up strategy. The application of such biomimetic structures is discussed.