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Broadband optical absorbers are increasingly in demand in various applications, including solar cells and radiative cooling systems. Among various types of structures, absorbers based on metamaterial structures have attracted much attention. However, they generally suffer from the issues of narrow bandwidth, high-cost fabrication, and high sensitivity to polarization changes. This paper presents a broadband, polarization-independent metamaterial absorber working in both infrared and visible frequency regimes. This structure is composed of a continuous phase-change material film ( Ge 2 Sb 2 Te 5 ) separated between two thin spacer layers of SiO 2 , and an array of amorphous Silicon particles located on the top of the structure. The proposed device was shown to exhibit remarkable absorptivity (more than 90 percent) within a broad range of frequencies starting from 250 to 1050 THz. In addition, thanks to the structural symmetry, the strong absorbance shows a considerable overlap between transverse magnetic (TM) and transverse electric (TE) modes over an extensive range of incident angles. The designing procedure for this absorber can be used as a guideline for designing similar metamaterial absorbers in any desirable frequency band. [ABSTRACT FROM AUTHOR]

Nanoparticle aggregate structures allow for efficient photon capture, and thus exhibit excellent optical absorption properties. In this study, a model of randomly distributed nanochain aggregates on silicon substrates is developed and analyzed. The Gaussian, uniform, and Cauchy spatial distribution functions are used to characterize the aggregate forms of the nanochains and their morphologies are realistically reconstructed. The relationships between the structural parameters (thickness and filling factor), equivalent physical parameters (density, heat capacity, and thermal conductivity), and visible absorptivity of the structures are established and analyzed. All the above-mentioned parameters exhibit extreme values, which maximize the visible-range absorption; these values are determined by the material properties and nanochain aggregate structure. Finally, Al nanochain aggregate samples are fabricated on Si substrates by reducing the kinetic energy of the metal vapor during deposition. The spectral reflection characteristics of the samples are studied experimentally. The Spearman correlation coefficients for the calculated spectral absorption curves and those measured experimentally are higher than 0.82, thus confirming that the model is accurate. The relative errors between the calculated visible-range absorptivities and the measured data are less than 0.3%, further confirming the accuracy of the model. [ABSTRACT FROM AUTHOR]

We demonstrate a concept and fabrication of lithography-free layered metal-SiO2 thin-film structures which have reduced reflectivity (black appearance), to as low as 0.9%, with 4.9% broadband reflectance (8.9% for soda lime) in the 500−1400 nm range. The multi-layered (four layers) thin-film metamaterial is designed so that optical impedance matching produces minimal reflectance and transmittance within the visible and infra-red (IR) spectral region for a range of incident angles. The structure has enhanced absorbance and is easily tuned for reduced minimal transmission and reflection. This approach should allow for novel anti-reflection surfaces by impedance matching to be realized.

Optical absorbers have been a topic of intense research due to their importance in many applications. In particular, multi-band and perfect absorption features in a desired frequency range are essential in broadband applications. In this work, we numerically studied the absorption properties of subwavelength metallic gratings coated with a dielectric layer. Here, the structure is considered to be an integration between a resonant cavity and a subwavelength metallic grating. Two appropriately designed structures can exhibit multi-band absorption properties. In addition to the numerical simulation results, we elaborate on determining the appropriate structural parameters that yield the desired spectral absorption profile in the visible range. We also numerically identify critical coupling conditions for perfect absorption. [ABSTRACT FROM AUTHOR]

A simplified and cost-effective optical absorber is designed by means of optimization on the basis of finite domain and time difference (FDTD) algorithm. It is suitable for fabrication and mass production in future industry. To investigate its absorbing performance and photo-thermal effect, experiments are carried out using focused ion beam nanofabrication, characterization of spectroscopy, and measurement of temperature rise. Our experimental results demonstrate that the absorber is possible to be used in photo-thermal therapy for tumor/cancer cells. [ABSTRACT FROM AUTHOR]

In order to achieve the enhancement and manipulation of light absorption in graphene monolayer within the visible (Vis) and near infrared (NIR) regions, a design of absorber inspired by contact coupled gratings with an absentee layer is demonstrated. It is proved that the absorptance of monolayer graphene can be greatly enhanced to near unity through rigorous coupled-wave analysis (RCWA) numerical calculation. The thickness of grating and homogeneous absentee layers can significantly change the linewidth and resonant mode position in absorption spectrum. Furthermore, the lateral shift of the contact coupled gratings changes the magnetic field distributions in the grating cavity and the surface-confined mode at the cover/grating interface, thus facilitating the dynamic control of the spectral bandwidth of the graphene absorber. The proposed devices could be efficiently exploited as tunable and selective absorbers, allowing to realize other two-dimensional (2D) materials-based selective photo-detectors. [ABSTRACT FROM AUTHOR]

Monolayer two-dimensional (2D) materials have promising optical properties and hold potential as solar absorbers for photovoltaic devices. Efficient solar absorbers should have high absorption especially for wavelengths slightly smaller than the semiconductor bandgap wavelength. In this spectrum, solar cells have higher internal quantum efficiency, lower energy loss to lattice vibration, and resultant higher power conversion efficiency. 2D materials have been integrated into nanophotonic structures to enhance their absorption, but fabrication challenges hindered practical applications of these structures. Here, we theoretically and experimentally demonstrated a simple methodology to design efficient 2D material, and more generally, sub-nanometer planar, optical absorbers by placing a sub-nanometer film either onto a transparent layer on a metallic film substrate (structure 1) or between the transparent layer and the substrate (structure 2) with oblique sunlight illumination. We show that there always exist a pair of transparent layer thickness and incident angle for which these structures achieve 100% absorption (broadband and angle-robust absorption for structure 1 and narrowband for structure 2). 92% absorption, which is 2.6 times greater than prior theoretical demonstrations, was achieved in monolayer MoS 2 at 660 nm near its band edge to demonstrate material versatility (even materials with very low losses, which is counter-intuitive to conventional absorber designs). 2D material solar cells were further designed with 4.4% power conversion efficiency, a four-fold increase compared to prior designs. Our proposed methodology is applicable to numerous materials with atomic-layer or sub-nanometer thicknesses and paves the way to efficient sub-nanometer-thick energy harvesting devices with simple planar structures. [ABSTRACT FROM AUTHOR]

By describing an idea based on binary matrices, a meta-surface design methodology has been introduced in the THz spectrum window. Independence of the numerical values of the geometrical parameters, the shape of graphene patterns, as well as the mode of the bias network for the patterns, have been investigated. The possibility of describing meta-surfaces by circuit equivalents has made it possible to predict the behavior of meta-surfaces at least in the THz band with acceptable accuracy by exploiting the idea of impedance matching. Considering the significant effect of changing graphene patterns and applying multiple biases, this work tries to optimize the pattern's shapes and external bias application. In this regard, two simple binary matrices are introduced which present pattern types and corresponding biasing schemes. Leveraging developed equivalent circuit models for known periodic graphene patterns, a systematic procedure is suggested to consider pattern diversity and simultaneous multiple biases for single layers. Exploiting patterns of graphene disks, rings, and ribbons besides a back reflector and polymer substrate, a reconfigurable and adjustable wave absorber is suggested. Two sets of external stimulations via chemical potentials force the device to show wide-band and multi-band absorption while appealing tuning capability is achieved for multi-band mode. Considering both operational modes, it seems that the proposed absorber can cover almost all of the THz spectrum with excellent shifting possibilities. The validity of impedance matching description is verified by full-wave simulations while ample analysis is shown to investigate reliability against oblique radiation. [Display omitted] • The idea behind the design is to involve topological and biasing features in design process. • According to the simulation results, the proposed absorber shows more than five peaks with absorption over 80%. • The peak frequencies are interestingly able to be shifted via potential chemical variations. • Proposed device is an appropriate choice for super lenses, solar cells, security, and indoor communication applications. [ABSTRACT FROM AUTHOR]

Optical absorbers have been a topic of intense research due to their importance in many applications. In particular, multi-band and perfect absorption features in a desired frequency range are essential in broadband applications. In this work, we numerically studied the absorption properties of subwavelength metallic gratings coated with a dielectric layer. Here, the structure is considered to be an integration between a resonant cavity and a subwavelength metallic grating. Two appropriately designed structures can exhibit multi-band absorption properties. In addition to the numerical simulation results, we elaborate on determining the appropriate structural parameters that yield the desired spectral absorption profile in the visible range. We also numerically identify critical coupling conditions for perfect absorption.

Broadband low reflectance materials have various applications in the field of optical energy management; however, materials with ultra-low reflectance (below 0.1%) have been considered as mechanically delicate. We have developed a novel hard-surface optical absorber with microstructured, diamond-like carbon coated ion tracks on CR-39 plastic substrate. The spectral reflectance of the first prototype was below 2% for wavelengths ranging from 400 nm to 1400 nm; moreover, the optical absorber had mechanically hard surface and exhibited temporal durability. Choosing the appropriate design of the surface structure and coating layer is likely to reduce the reflectance to the order of 0.1%. This technique yields easy-to-handle broadband ultra-low reflectance absorbers. [ABSTRACT FROM AUTHOR]

In this work, spectral absorbance of hybrid nanowire-hyperbolic metamaterial was investigated numerically by using transfer-matrix method. The hybrid nanowire-hyperbolic metamaterial is consisted of two-dimensional periodically arranged arrays of subwavelength-sized hybrid nanowires (with circular cross-section) embedded in silicon dioxide host matrix. Each hybrid nanowire is made by core-shell structure used silicon as a core and gold as a shell. The hybrid nanowire-hyperbolic metamaterial is used as a top layer of a broadband absorber. The top layer is over on a planar silicon dioxide layer, which is backed by a nano-sized layer of gold as a perfect reflector for guaranteeing high absorbance. When electromagnetic waves in the range of 400–1600 nm wavelength are obliquely incident on the top hybrid nanowire-hyperbolic metamaterial surface, the absorption spectra of the designed absorber structure are analyzed for different incident angles, and radii of hybrid nanowire components or fill-factor of the hyperbolic metamaterial layer. The multiple absorption peaks, which are continuously generated from multiple longitudinal and transverse surface plasmon modes and are supported by the hybrid nanowires, are observed directly. The absorption feature can be also tuned by various operational conditions. The proposed structure is made by in-complex nano-fabricating techniques comparing with the another and it provides acceptable broadband absorptivity. • HNW-HMM based broadband absorber exhibits high absorbance in visible to NIR regions. • HNW-HMM based broadband absorber is working in both TE- and TM-polarizations. • HNW supports multiple longitudinal and transverse surface-plasmon resonant modes. • HNW-HMM based broadband absorber can be made by uncomplicated nano-fabrications. [ABSTRACT FROM AUTHOR]

A hybrid plasmonic absorber consisting of graphene and silica layers coated on a metal film with concentric-rings grooves is presented and numerically investigated in this paper. The proposed structure provides two absorption peaks at near-infrared region. The absorptivity of each peak can be tuned independently, by controlling the chemical potential of the graphene layer. Modulation of absorption per graphene Fermi energy about 85 % /eV and 51.25 % /eV can be obtained for the first and the second peaks, respectively. Simulation results also indicate that the absorption magnitudes of the peaks are independent of the polarization of the incident light. Hence, the proposed device can be utilized as a polarization insensitive optical modulator with two independently tunable bands. It is also shown that by splitting the silica filled rings (in this case, some parts of the silica inside the grooves are replaced by silver.), more individual peaks are appeared in the absorption spectrum of the device, whose absorptivity are dependent directly upon the positions of the silver gaps. Hence, the proposed device is a tunable light absorber with fully controllable absorption spectrum that can be used in a wide range of applications from photo-detectors to multi spectral tunable reflectors [ABSTRACT FROM AUTHOR]

We demonstrate a concept and fabrication of lithography-free layered metal-SiO2 thin-film structures which have reduced reflectivity (black appearance), to as low as 0.9%, with 4.9% broadband reflectance (8.9% for soda lime) in the 500–1400 nm range. The multi-layered (four layers) thin-film metamaterial is designed so that optical impedance matching produces minimal reflectance and transmittance within the visible and infra-red (IR) spectral region for a range of incident angles. The structure has enhanced absorbance and is easily tuned for reduced minimal transmission and reflection. This approach should allow for novel anti-reflection surfaces by impedance matching to be realized. [ABSTRACT FROM AUTHOR]

We program the optical properties of colloidal Au nanocrystal (NC) assemblies via an unconventional ligand hybridization (LH) strategy to precisely engineer interparticle interactions and design materials with optical properties difficult or impossible to achieve in bulk form. Long-chain hydrocarbon ligands used in NC synthesis are partially exchanged, from 0% to 100%, with compact thiocyanate ligands by controlling the reaction time for exchange. The resulting NC assemblies show transmittance, reflectance, optical permittivity, and direct-current (DC) resistivity that continuously traverse a dielectric-metal transition, providing analog tuning of their physical properties, unlike the digital control realized by complete exchange with ligands of varying length. Exploiting this LH strategy, we create Au NC assemblies that are strong, ultrathin film optical absorbers, as seen by a 6× increase in the extinction of infrared light compared to that in bulk Au thin films and by a temperature rise of 20 °C upon illumination with 808 nm light. Our LH strategy may be applied to the design of materials constructed from NCs of different size, shape, and composition for specific applications.