Broad Range Of Incident Angles Research Articles

Broadband and broad-angle asymmetric acoustic transmission by unbalanced excitation of surface evanescent waves based on single-layer metasurface

Currently, complicated structure, incident-angle selectivity, and narrow frequency band are the key drawbacks of the asymmetric acoustic transmission (AAT) devices. Here we tackle these problems by proposing a class of single-layer lossy acoustic metasurfaces. The broadband AAT performance is realized in a broad range of incident angles. When the incident angle is in the range between two critical values, which are derived in this paper, an external sound wave can be converted into an evanescent mode, and the total internal reflection occurs for backward sound. The incident sound wave can be negatively refracted for forward sound if the evanescent mode conversion condition is broken, representing the realization of the AAT. However, the AAT phenomenon cannot be observed outside of the range defined above. The proposed design of highly efficient broad-angle AAT can find applications in sound sensing and noise control.

Tunable and broadband coherent perfect absorption by ultrathin black phosphorus metasurfaces

Black phosphorus (BP), a relative new plasmonic two-dimensional (2D) material, offers unique photonic and electronic properties. In this work, we propose a new tunable and broadband ultrathin coherent perfect absorber (CPA) device operating in the terahertz (THz) frequency range. It is based on a bifacial metasurface made of BP patch periodic arrays separated by a thin dielectric layer. Broadband CPA bandwidth is realized due to the ultrathin thickness of the proposed device and the extraordinary properties of BP. In addition, a substantial modulation between CPA and complete transparency is achieved by adjusting the phase difference between the two counter-propagating incident waves. The CPA performance can be tuned by dynamically changing the electron doping level of BP. The CPA response under normal and oblique transverse magnetic (TM) and electric (TE) polarized incident waves is investigated. It is derived that CPA can be achieved under both incident polarizations and across a broad range of incident angles. The presented CPA device can be used in the design of tunable planar THz modulators, all-optical switches, detectors, and signal processors.

Guaranteed global optimization of thin-film optical systems

A parallel deterministic global optimization algorithm for thin-film multilayer optical coatings is developed. This algorithm enables locating a global solution to an optimization problem in this class to within a user-specified tolerance. More specifically, the algorithm is a parallel branch-and-bound method with applicable bounds on the merit function computed using Taylor models. This study is the first one, to the best of our knowledge, to attempt guaranteed global optimization of this important class of problems, thereby providing an overview and an assessment of the current state of such techniques in this domain. As a proof of concept on a small scale, the method is illustrated numerically and experimentally in the context of antireflection coatings for silicon solar cells—we design and fabricate a three-layer dielectric stack on silicon that exhibits an average reflectance of (2.53 ± 0.10)%, weighted over a broad range of incident angles and the solar spectrum. The practicality of our approach is assessed by comparing its computational cost relative to traditional stochastic global optimization techniques which provide no guarantees on their solutions. While our method is observed to be significantly more computationally expensive, we demonstrate via our proof of concept that it is already feasible to optimize sufficiently simple practical problems at a reasonable cost, given the current accessibility of cloud computing resources. Ongoing advances in distributed computing are likely to bring more design problems within the reach of deterministic global optimization methods, yielding rigorous guaranteed solutions in the presence of practical manufacturing constraints.

Flexible broadband polarization converter based on metasurface at microwave band**Project supported by the Fundamental Research Funds for the Central Universities, China (Grant No. kfjj20180401), the National Natural Science Foundation of China (Grant No. 61471368), the Aeronautical Science Foundation of China (Grant No. 20161852016), the China Postdoctoral Science Foundation (Grant No. 2016M601802), and Jiangsu Planned Projects for Postdoctoral Research Funds, China

A flexible broadband linear polarization converter is proposed based on the metasurface operating at microwave band. To achieve bandwidth extension property, long and short metallic arc wires, as well as the metallic disks placed over a ground plane, are combined into the polarizer, which can generate three neighboring resonances. Due to the combination of the first two resonances and the optimized size and thickness of the unit cell, the polarization converter can have a weak incident angle dependence. Both simulated and measured results confirm that the average polarization conversion ratio is over 85% from 11.3 GHz to 20.2 GHz within a broad range of incident angle from 0° to 45°. Moreover, the proposed polarization converter based on flexible substrates can be applied for conformal design. The simulation and experiment results demonstrate that our designed polarizer still keeps high polarization conversion efficiency, even when it adheres to convex cylindrical surfaces. The periodic metallic structure of the designed polarizer has great potential application values in the microwave, terahertz, and optic regimes.

Facile Film-Nanoctahedron Assembly Route to Plasmonic Metamaterial Absorbers at Visible Frequencies.

Plasmonic metamaterial absorbers (MAs) with broadband and near-perfect absorption properties in the visible region were successfully fabricated via a facile film-colloidal nanoparticle (NP) assembly method. In this approach, colloidal octahedral Au NPs were employed as the surface meta-atoms of MAs, whereas nanoscale-thick SiO2 and Al films were used as the dielectric spacer and reflector, respectively. It is worth noting that the Au nanoctahedra were randomly assembled onto the Al-SiO2 films, and no effort was made to precisely control their spatial arrangements. The optical characterization showed that the as-prepared MAs exhibited broadband high absorption (average absorptivity above 85%) within the whole visible spectrum for a broad range of incident angles (0°-60°). In particular, two polarization-independent near-perfect absorption peaks (absorptance above 99%) were recorded near 540 and 727 nm, respectively. Moreover, the absorption properties of the MAs can be effectively controlled and tailored by varying the geometry (the thickness of the dielectric spacer and the surface coverages of the Au nanoctahedra). Electromagnetic simulations further demonstrated that enhanced Mie resonances and strong plasmonic coupling effects were critical for the designed MAs. This work here may provide an efficient and alternative route for the design of scalable visible light absorbers for applications such as solar cells, photothermalvoltaics, and biochemical sensors.

Design of Polarization-Independent and Wide-Angle Broadband Absorbers for Highly Efficient Reflective Structural Color Filters.

We propose a design of angle-insensitive and polarization-independent reflective color filters with high efficiency (>80%) based on broad resonance in a Fabry–Pérot cavity where asymmetric metal-dielectric-metal planar structures are employed. Broadband absorption properties allow the resonance in the visible range to remain nearly constant over a broad range of incident angles of up to 40° for both s- and p-polarizations. Effects of the angles of incidence and polarization state of incident light on the purity of the resulting colors are examined on the CIE 1931 chromaticity diagram. In addition, higher-order resonances of the proposed color filters and their electric field distributions are investigated for improved color purity. Lastly, the spectral properties of the proposed structures with different metallic layers are studied. The simple strategy described in this work could be adopted in a variety of research areas, such as color decoration devices, microscopy, and colorimetric sensors.

Enhanced spectral response of CIGS solar cells with anti-reflective subwavelength structures and quantum dots

A cover glass integrated with subwavelength structures (SWSs) and quantum dots (QDs) was realized to enhance the spectral response of Cu(In1-xGax)Se2 (CIGS) thin film solar cells. The nanoscale SWSs was produced on the front and backside of cover glass using the self-masked etching method. The fabricated SWSs-integrated glass (SIG) exhibited superior anti-reflective properties over a broad wavelength range and a broad range of incident angle. The average transmittance of the single and double-sided SIG was increased from 92.4% to 95.2% and 97.8% in the wavelength range between 300 and 1200 nm, as compared to that of a flat surface glass. QDs were formed between the fabricated SIG and ZnO layer to act as a luminescent down-shifting (LDS) layer. A large amount of light can pass through the SIG, enhancing the efficiency of the LDS by improving light absorption and lowering reflection. After successful integration of the SIG with QDs (QD/SIG) on top of the CIGS devices, photovoltaic performance was significantly improved. The power conversion efficiency of the CIGS devices integrated with QD/double-sided SIG was improved by 7.41%, compared with that of a reference device (without QD/cover glass). The efficiency improvement is attributed to the enhanced solar energy harvesting by the efficient LDS and the minimized surface reflection loss.

Flexible omnidirectional and polarisation-insensitive broadband plasmon-enhanced absorber

We demonstrate a facile method to fabricate Ag-Sb2S3 nanoporous plasmonic absorbers on plastic, crystalline, and glassy substrates. Our lithography-free method exploits the immiscibility of Ag and Sb2S3 to control the surface morphology of the absorbing structure. We experimentally and theoretically show that the nanoporous structure exhibits a high absorption across the visible and near infrared spectrum, which is achieved by the coalescence of surface/bulk plasmons and the Sb2S3 semiconductor bandgap absorption. The absorptance is independent of polarisation over a broad range of incident angles. We found that the Ag-Sb2S3 nanoporous structure is an effective absorber on both flexible and non-flexible substrates, and the simple deposition of the Ag-Sb2S3 nanoporous structure facilitates fabrication over a 100 mm diameter wafer. Our work shows that plasmonic nanostructures can be fabricated at room temperature on an industrial scale using sputtering, which is already used to commercially produce photovoltaic cells. We foresee these inexpensive absorbers being applied to wearable light-harvesting systems, bio-sensing, and display technologies.

Transmodal elastic metasurface for broad angle total mode conversion

It has been long believed that a total mode conversion between longitudinal and shear elastic waves can only be achieved at a certain incidence angle. Here, we show that a total mode conversion can be achieved for a broad range of incidence angles by a specially designed elastic metasurface, namely, transmodal metasurface. From the generalized reflection law, we found that the incident longitudinal wave can be totally converted to a reflected shear wave over a broad range of incidence angles if a sufficiently large phase gradient is introduced at the boundary. Numerical and experimental investigations with a specially engineered transmodal metasurface showed that the total mode conversion can be achieved for wide incidence angles from 19° to 90°, which was impossible to be achieved previously. The proposed idea of the transmodal metasurface can open up an advanced avenue for tailoring elastic wave modes as an outstanding alternative to generating shear waves.

The effect of incidence angle on the reflectance of solar mirrors

Solar reflectors for Concentrating Solar Technologies require a high reflectance in the terrestrial solar spectrum (280–4000nm). Besides the wavelength, the reflectance of solar mirror materials is also dependent on the incidence angle of the incoming sunlight. The commonly used measurement equipment measures the reflectance at fixed near-normal incidence angles, typically between 8° and 15°. In this work, the annual incidence angle frequency distribution has been calculated for a LS3/Eurotrough-type parabolic-trough collector located at different sites, and for the heliostat field of the solar tower system CESA-1 located at the Plataforma Solar de Almería in Tabernas, southern Spain. It was found that the most frequent incidence angles registered in the solar field are quite higher than the ones at which reflectance is measured with state of the art instruments, obtaining mean incidence angles in the range of 28–35° depending on the type and location of the solar field.A methodology to predict the off-normal reflectance of silvered-glass mirrors based on near-normal reflectance and transmittance measurements of the uncoated glass is presented. The complex refractive index of 2, 4 and 5mm thick solar glass and the deposited silver was determined and used to model the solar weighted reflectance of silvered-glass mirrors at different incidence angles. The model was compared to experimental measurements. For this purpose, the Spectral Specular Reflectometer (S2R) has been improved and updated with a polarizer crystal to measure reflectance at perpendicular (s-pol) and parallel (p-pol) polarizations up to incidence angles of θ = 70°.Eight solar mirror materials (three silvered-glass mirrors of different glass thicknesses, two anti-soiling coated glass mirrors, two enhanced aluminum reflectors and a silvered polymer film) have been measured over a broad range of incidence angles and the results have been weighted with the annual incidence angle frequency distribution. The obtained incidence angle-weighted reflectance is a suited parameter to compare the efficiency of solar mirror materials taking into account their use in a specific collector type and location.

Highly antireflective AlGaN/GaN ultraviolet photodetectors using ZnO nanorod arrays on inverted pyramidal surfaces

Highly antireflective heterostructured aluminum gallium nitride (AlGaN)/GaN ultraviolet (UV) photodetectors were demonstrated using a combination of inverted pyramidal surfaces and zinc oxide nanorod arrays (i.e., antireflective surface modification) to enhance the optical sensitivity. The microfabricated hierarchical surfaces significantly reduced the average surface reflectance to less than 0.3% in the UV region and less than 1% in the visible light region, allowing near-perfect absorption of incident light regardless of the angle of incidence (5–80°). As a result, the photodetectors fabricated on highly antireflective AlGaN/GaN surfaces showed higher sensitivity and responsivity over a broad range of incidence angles compared to photodetectors on planar AlGaN/GaN surfaces, supporting the use of a hierarchically modified sensing surface for omnidirectional UV monitoring with higher sensitivity.

Tunable, omnidirectional structural color on reflection based on metal-SiOx-metal structure

An omnidirectional structural color based on the metal-SiOx-metal stack structure is proposed, which can present the same perceived color for a broad range of incidence angles. The tunable structural color can be obtained with adjustable intermediate dielectric layer by simply adjusting the deposition condition, especially the oxygen flow rate during the deposition processes. The resonance condition can be satisfied across the whole visible light region with this special dielectric. The strong absorption caused by the resonance within the metal-SiOx-metal structure accounts for the efficient spectral filtering feature, and the constant phase shift within the dielectric layer leads to angle insensitivity of this color filter. This simple color tuning method for omnidirectional structural colors can have a great potential in various applications such as displaying, imaging, colorful decoration, anti-counterfeiting and so forth.

Broadband, Polarization-Insensitive, and Wide-Angle Optical Absorber Based on Fractal Plasmonics

In this letter, a plasmonic absorber consisting of a metal–dielectric–metal stack with a top layer of Sierpinski nanocarpet is theoretically investigated. Such a compact absorber depicts broadband angle-independent behavior over a wide optical wavelength range (400–700 nm) and a broad range of incidence angles (0°–80°). Including several feature sizes, such a fractal-like structure shows an average simulated extinction response of 0.85 for either transverse electric or magnetic polarization states under normal incidence. Underlying mechanisms of absorbance due to excited surface plasmon modes as well as electric/magnetic dipole resonances are well revealed by investigating electric field, magnetic field, and current distributions. The proposed absorber opens a path to realize high-performance ultrathin light trapping devices.

Fabrication of high aspect ratio broadband antireflection porous nano-network on glass using candle soot as a sacrificial layer for solar energy utilization

In this paper, we report the fabrication of broadband antireflection porous nano-network on the glass substrate using the combination of candle soot and HF-based vapor phase etching method. Candle soot layer plays a key role to control the pore size during the etching process. Field emission electron microscopy results showed that the pores have the tapering profile, and the pores size was restricted to the sub-wavelength dimension. Therefore, an excellent broadband antireflection with an enhancement of ∼7% in the maximum total transmittance as compared to plain glass has been achieved. Moreover, reflectance from the etched surface remains quite low (<4%) over a broad range of incident angles up to 58°. The broadband antireflection property was further examined by finite difference time domain simulation. Further, we demonstrate a relative improvement of ∼6% in the Jsc (ΔJsc/Jsc) of solar cell covered with the etched glass.

Angle Robust Reflection/Transmission Plasmonic Filters Using Ultrathin Metal Patch Array

Angle robust reflection/transmission plasmonic filters using ultrathin metal patch array are proposed to achieve angle insensitive color filtering for a broad range of incidence angles up to 60°. This property is attributed to the localized surface plasmonic resonance of each metal patch. Further, an array with as few as two periods is sufficient to demonstrate the color filtering effect. As a service to our authors and readers, this journal provides supporting information supplied by the authors. Such materials are peer reviewed and may be re-organized for online delivery, but are not copy-edited or typeset. Technical support issues arising from supporting information (other than missing files) should be addressed to the authors. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.

Gyroid Optical Metamaterials: Calculating the Effective Permittivity of Multidomain Samples

Gold gyroid optical metamaterials are known to possess a reduced plasma frequency and linear dichroism imparted by their intricate subwavelength single gyroid morphology. The anisotropic optical properties are, however, only evident when a large individual gyroid domain is investigated. Multidomain gyroid metamaterials, fabricated using a polyisoprene-b-polystyrene-b-poly(ethylene oxide) triblock terpolymer and consisting of multiple small gyroid domains with random orientation and handedness, instead exhibit isotropic optical properties. Comparing three effective medium models, we here show that the specular reflectance spectra of such multidomain gyroid optical metamaterials can be accurately modeled over a broad range of incident angles by a Bruggeman effective medium consisting of a random wire array. This model accurately reproduces previously published results tracking the variation in normal incidence reflectance spectra of gold gyroid optical metamaterials as a function of host refractive index and volume fill fraction of gold. The effective permittivity derived from this theory confirms the change in sign of the real part of the permittivity in the visible spectral region (so, that gold gyroid metamaterials exhibit both dielectric and metallic behavior at optical wavelengths). That a Bruggeman effective medium can accurately model the experimental reflectance spectra implies that small multidomain gold gyroid optical metamaterials behave both qualitatively and quantitatively as an amorphous composite of gold and air (i.e., nanoporous gold) and that coherent electromagnetic contributions arising from the subwavelength gyroid symmetry are not dominant.

Angular Response of Logically Combined Photonic Crystals

When we logically combine two one-dimensional photonic crystals of slightly different periodicities, the resulting doubly resonant optical structure possesses dense number of localized modes along the axis of periodicity (on-axis). This paper investigates the angular response of such logically combined photonic crystals (LCPCs), and shows that the on-axis localized modes of the LCPCs can be excited using an off-axis light, over a broad range of incident angles. This enables a greater flexibility in the excitation of the localized modes, and consequently broadens the applications of the LCPCs. We present the frequency dispersion and the tunability of the quality factors of the localized modes, with respect to the off-axis angle, polarization, and the background medium's refractive index.

Plasmon-Enhanced Performance of an Array of Core-Shell Silver Nanoparticles Photodeposited onto a Titanium Dioxide Layer

A plasmonic photocatalytic reactor which composed of a periodic array of core-shell silver nanoparticles (CSSNPs) photodeposited onto a Titanium Dioxide (TiO2) thin film in a multilayer rotating-disk system was numerically investigated by using three-dimensional finite element method. Results show that the proposed structure can exhibit much higher photocalytic activity in a broad range of incident angle of light that are not observed for the same counterpart without the CSSNPs on the SiO2 surface. The enhanced electric field distribution and intensity could be expanded as the inner diameter of CSSNPs and incident angle of light increased.

Terahertz carpet cloak based on a ring resonator metasurface

In this work we present the concept and design of an ultrathin (\ensuremath{\lambda}/22) terahertz (THz) unidirectional carpet cloak based on the local phase compensation approach enabled by gradient metasurfaces. A triangular surface bump with center height of 4.1 mm (1.1\ensuremath{\lambda}) and tilt angle of 20\ifmmode^\circ\else\textdegree\fi{} is covered with a metasurface composed of an array of suitably designed closed ring resonators with a transverse gradient of surface impedance. The ring resonators provide a wide range of control for the reflection phase with small absorption losses, enabling efficient phase manipulation along the edge of the bump. Our numerical results demonstrate a good performance of the designed cloak in both near field and far field, and the cloaked object mimics a flat ground plane within a broad range of incidence angles, over 35\ifmmode^\circ\else\textdegree\fi{} angular spectrum centered at 45\ifmmode^\circ\else\textdegree\fi{}. The presented cloak design can be applied in radar and antenna systems as a thin, lightweight, and easy to fabricate solution for radio and THz frequencies.

Antireflection Coatings on Au Plasmonic Gratings for Infrared Photodetection

Based on analytic formulas and numerical simulations, we investigate the enhancement effect of dielectric antireflection coatings on the performance of quantum-well infrared photodetectors (QWIP) with Au grating couplers and operating at 15 μm. It is found that a simple analytic formula can well estimate the optimal coating thickness. By using a GaAs-coated Au grating with optimal parameters, the absorption can be enhanced by 13 times in the QW active region compared with the reference structure (without Au grating and with an isotropic active region). This enhancement is about three times of that without a dielectric coating. For s-polarized light, a high enhancement (>5) can occur in a broad range of incident angle (|𝜃|<60∘).