Numerical Study of an Ultra-Broadband and Polarization Independence Metamaterial Cross-Shaped Fractal Absorber

A three-dimensional cross-shaped fractal metamaterial absorber with ultra-wide wavelength band, polarization-independence and wide-angle, is numerically investigated by the finite-difference time-domain method. In this absorber, the solar energy is trapped by the cross-shaped fractal of the upper layer, and the Si-ring filled with iron in the middle layer and the wavelength band can be broadened by the self-similarity of fractal structure. The absorber exhibits absorptivity higher than 91% for the wavelengths from 400 to 2000 nm and an absorption bandwidth of about 133%. Furthermore, the proposed absorber realizes polarization independence, and the maximum incident angle is 76°. However, as the iron material applied in the nano-metamaterial absorber (NMA) can be easily oxidized and rusted, it is replaced by nickel with characteristics such as corrosion resistance and high-temperature resistance; thus, an improved NMA is obtained. The improved absorber not only eliminates the corrosion-prone defects of the above proposed structure but also maintains polarization independence and high absorption and widens the angle of incidence up to 79° and thereby can be applied in many areas, such as solar energy harvesting.

     

Dual-Band Infrared Near-Perfect Absorption by Fabry-Perot Resonances and Surface Phonons

In this article, we present a simple absorber design which enables dual-band near-perfect absorption at infrared (IR) frequencies. The absorber is an unpatterned hBN/dielectric/hBN triple layer, with a 1150-nm-thick hBN film as the top layer, a 850-nm-thick dielectric film as the middle layer, and a hBN substrate. Unlike the metal/dielectric/metal triple layer, it is found that the high efficiency absorption at specific wavelengths is mainly caused by two mechanisms: Fabry-Perot (FP) resonances and surface phonons. The absorption response is found sensitive to the top and middle layers. The two mechanisms can be coupled to affect the absorption spectra by choosing a proper thickness of the top and middle layers.     

Dual-Band Infrared Perfect Absorber Based on a Ag-Dielectric-Ag Multilayer Films with Nanoring Grooves Arrays

In this paper, we demonstrate a dual-band metamaterial perfect absorber based on a Ag-dielectric-Ag multilayer nanostructure. The structure of top metal film covers nanoring grooves array. A dielectric layer has a function of confining electromagnetic fields. Theoretical analysis shows that two absorption peaks (1059 nm and 1304 nm) with the absorption of 99.2% and 99.9% have been achieved, respectively. The physical origin of perfect absorption peaks are related to the Fabry-Perot resonance effect and localized surface plasmon resonance (LSPR) of the nanoring grooves. Its perfect absorption and resonance wavelength can be well regulated by adjusting the relevant structural parameters. Additionally, the absorber demonstrates good operation angle-polarization-tolerance at wide incident angles (0–60°). We believe that our design has a promising application in plasmon-enhanced photovoltaic, optical absorption switching, and modulator optical communications in the infrared regime.

     

Multi-layered Graphene Silica-Based Tunable Absorber for Infrared Wavelength Based on Circuit Theory Approach

Graphene can be utilized as a tunable material for a wide range of infrared wavelength regions due to its tunable conductivity property. In this paper, we use Y-shaped silver material resonator placed over the top of multiple graphene silica-layered structures to realize the perfect absorption over the infrared wavelength region. We propose four different designs by placing the graphene sheet over silica. The absorption and reflectance performance of the structures have been explored for 1500- to 1600-nm wavelength range. The proposed design also explores the absorption tunability of the structure for the different values of graphene chemical potential. We have reported the negative impedance for the perfect absorption for proposed metamaterial absorber structures. All the metamaterial absorbers have reported 99% of its absorption peaks in the infrared wavelength region. These designs can be used as a tunable absorber for narrowband and wideband applications. The proposed designs will become the basic building block of large photonics design which will be applicable for polariser, sensor, and solar applications.

     

A Two‐Resonance Tapping Cavity for an Optimal Light Trapping in Thin‐Film Solar Cells

An optimal photon absorption in thin film photovoltaic technologies can only be reached by effectively trapping the light in the absorber layer provided a considerable portion of the photons is rejected or scattered out of such layer. Here, a new optical cavity is proposed that can be made to have a resonant character at two different nonharmonic frequencies when adjusting the materials or geometry configurations of the partially transmitting cavity layers. Specific configurations are found where a reminiscence of such two fundamental resonances coexists leading to a broadband light trapping. When a PTB7‐Th:PC₇₁BM organic cell is integrated within such cavity, a power conversion efficiency of 11.1% is measured. This study also demonstrates that when materials alternative to organics are used in the photoactive cell layer, a similar cavity can be implemented to also obtain the largest light absorption possible. Indeed, when it is applied to perovskite cells, an external quantum efficiency is predicted that closely matches its corresponding internal one for a broad wavelength range.     

Wideband Graphene-Based Fractal Absorber and its Applications as Switch and Inverter

In this paper, the idea of square fractal geometry has been utilized to introduce a tunable wideband graphene-based perfect plasmonic absorber in the near-infrared region. It consists of a MgF₂ layer and an array of gold squares fractal loaded on a graphene layer. In the designed absorber a single layer of graphene has been used instead of multilayered graphene structures. The structure is polarization-insensitive under normal incidence due to the geometric symmetry. The absorption and bandwidth of the structure are almost insensitive to the incident angle up to 15° and 45° for TE and TM polarizations, respectively. Moreover, by choosing appropriate structural parameters, the resonance wavelength of the desired plasmonic absorber can be controlled. The absorption of the introduced structure can be tuned by changing the chemical potential of the graphene. Therefore, the proposed fractal absorber can act as switch and inverter at λ = 1995 nm. Furthermore, the equivalent circuit model of the absorber has been derived to confirm the validity of the simulation results. The superiorities of our fractal absorber are wide full-width at half-maximum of 406 nm, multi-applicant, perfect absorption, and fabrication feasibility due to the simple structure with the maximum absorption tolerance error of 5.12%.

     

Optimization Strategy Aided by an Interplay Between Plasmonic Material and Nanoarray Geometry for Light Trapping Application in Thin-Film Photovoltaics

In this article, we have developed an optimization strategy taking into consideration the interplay between the choice of plasmonic material and geometrical parameters that lead to enhanced photocurrent density. We have demonstrated this by computing the optical absorption, using finite difference time domain technique, due to front-end placed aluminum and silver nanosphere arrays on 1- μm-thick film of silicon. Results from this optimization procedure indicate that over a broad wavelength range (～600 nm onwards), absorption enhancement is primarily due to waveguiding effects and is independent of the plasmonic material. However, the significance of the plasmonic material becomes noticeable at lower wavelengths. The optimization yielded an inter-particle distance of 325 nm and nanosphere radius of 75 nm that corresponds to maximum photocurrent density for both aluminum and silver. Furthermore, it was noticed that the presence of a native oxide layer on aluminum does not deteriorate the enhancement significantly. In fact, the photocurrent density enhancement due to partially oxidized aluminum nanospheres is found to be better than using silver nanospheres.     

Tunable and Multichannel Terahertz Perfect Absorber Due to Tamm Plasmons with Topological Insulators

In this paper, Tamm plasmons with topological insulators in a composite structure consisting of Bi₂Se₃, spacer layer, and one-dimensional photonic crystal (1DPC) have been demonstrated theoretically. The perfect absorption has been realized in the terahertz regime because of the optical Tamm states (OTSs) excited at the interface between Bi₂Se₃ and 1DPC. The perfect absorption can be realized for both TE and TM waves, and it is noted that the perfect absorption can be obtained at any incident angle by simultaneously changing the wavelength of incident light for TE-polarizations. Moreover, the perfect absorption can be realized at different wavelengths with the change of the chemical potential and the thickness of Bi₂Se₃. The thickness and the dielectric constant of the spacer layer will also play a vital role in the performance of the perfect absorber. Especially, the multichannel perfect absorption phenomenon can be achieved by choosing the appropriate thickness of the spacer layer. This tunable and multichannel terahertz perfect absorber has great application potential in the solar energy, photodetection, and THz biosensor.

     

High Performance Infrared Plasmonic Metamaterial Absorbers and Their Applications to Thin-film Sensing

Plasmonic metamaterial absorbers (PMAs) have attracted considerable attention for developing various sensing devices. In this work, we design, fabricate and characterize PMAs of different geometrical shapes operating in mid-infrared frequencies, and explore the applications of the PMAs as sensor for thin films. The PMAs, consisting of metal-insulator-metal stacks with patterned gold nanostructured surfaces (resonators), demonstrated high absorption efficiency (87 to 98 %) of electromagnetic waves in the infrared regime. The position and efficiency of resonance absorption are dependent on the shape of the resonators. Furthermore, the resonance wavelength of PMAs was sensitive to the thin film coated on the surface of the PMAs, which was tested using aluminum oxide (Al₂O₃) as the film. With increase of the Al₂O₃ thickness, the position of resonance absorption shifted to longer wavelengths. The dependence of the resonant wavelength on thin film thickness makes PMAs a suitable candidate as a sensor for thin films. Using this sensing strategy, PMAs have potential as a new method for thin film detection and in situ monitoring of surface reactions.     

New method for spectrofluorometer monochromator wavelength calibration

A method is presented for wavelength calibration of spectrofluorometer monochromators. It is based on the distortion that the characteristic absorption bands of glass filters (holmium or didymium oxide), commonly used for calibration of spectrophotometers, introduce in the emitted fluorescence of fluorophores like indole, diphenyl hexatriene, xylene or rhodamine 6G. Those filters or a well characterized absorber with sharp bands like benzene vapor can be used for the same purpose. The wavelength calibration accuracy obtained with this method is better than 0.1 nm, and requires no modification in the geometry of the spectrofluorometer sample compartment.     

Photovoltaic Device with over 5% Efficiency Based on an n‐Type Ag2ZnSnSe4 Absorber

The kesterite material Cu₂ZnSn(S,Se)₄ (CZTSSe) is an attractive earth‐abundant semiconductor for photovoltaics. However, the power conversion efficiency is limited by a large density of I–II antisite defects, which cause severe band tailing and open‐circuit voltage loss. Ag₂ZnSnSe₄ (AZTSe) is a promising alternative to CZTSSe with a substantially lower I–II antisite defect density and smaller band tailing. AZTSe is weakly n‐type, and this study reports for the first time on how the carrier density is impacted by stoichiometry. This study presents the first‐ever photovoltaic device based on AZTSe, which exhibits an efficiency of 5.2%, which is the highest value reported for an n‐type thin‐film absorber. Due to the weakly n‐type nature of the absorber, a new architecture is employed (SnO:F/AZTSe/MoO₃/ITO) to replace conventional contacts and buffer materials. Using this platform, it is shown that the band tailing parameter in AZTSe more closely resembles that of CIGSe than CZTSSe, underscoring the strong promise of this absorber. In demonstrating the ability to collect photogenerated carriers from AZTSe, this study paves the way for novel thin‐film heterojunction architectures where light absorption in the n‐type device layer can supplement absorption in the p‐type layer as opposed to producing a net optical loss.     

1-D Metal-Dielectric-Metal Grating Structure as an Ultra-Narrowband Perfect Plasmonic Absorber in the Visible and Its Application in Glucose Detection

The need for an easy to fabricate perfect and narrowband light absorber in the visible range of electromagnetic (EM) spectrum has always been in demand for many scientific and device applications. Here, we propose a metal-dielectric-metal (MDM) 1-D grating plasmonic structure as a perfect narrow band light absorber in the visible and its application in glucose detection. The proposed structure consists of a 1- D grating of gold on the top of a dielectric layer on a gold film. Optimization for dielectric grating index (n), grating thickness (t), grating width (W), and grating period (P) has been done to improve the performance of plasmonic structure by calculating its quality factor and figure-of-merit (FOM). The optimized plasmonic structure behaves as a perfect narrowband light absorber. The flexibility to work at a specific wavelength is also offered by the proposed structure through an appropriate selection of the geometrical parameters and refractive index of the dielectric grating. The equivalent RC model is used to understand different components of the proposed structure on the optical response. The absorption response of the structure is invariant to the incident angle. Moreover, the calculated absorbance of the proposed plasmonic structure is ~ 100% with a narrow full-width half maxima (FWHM) of ~ 2.8 nm. We have numerically demonstrated a potential application of the proposed MDM absorber as a plasmonic glucose sensor in the visible range with detection sensitivity in the range of 140 to 195 nm/RIU.

     

Tunable Plasmonic Absorber Based on Propagating and Localized Surface Plasmons Using Metal-Dielectric-Metal Structure

We propose a metal-dielectric-metal super absorber based on propagating and localized surface plasmons which exhibits a near perfect absorption in the visible and near-infrared spectrum. The absorber consists of Ag/Al₂O₃/Al triple layers in which the top Al layer is a periodic nano disk array. The absorption spectrum can be easily controlled by adjusting the structure parameters including the period and radius of the nano disk and the maximal absorption can reach 99.62 %. We completely analyze the PSPs and LSPs modes supported by the MDM structure and their relationship with the ultrahigh absorption. Moreover, we propose a novel idea to further enhance the absorption by exciting the PSPs and high-order LSPs modes simultaneously, which is different from the previous works. This kind of absorber using stable inexpensive Al instead of noble metal Au or Ag is an appropriate candidate for photovoltaics, spectroscopy, photodetectors, sensing, and surface-enhanced Raman spectroscopy (SERS).     

Miniaturized Five-Band Perfect Metamaterial THz Absorber with Small Frequency Ratio

A five-band polarization-insensitive perfect metamaterial absorber (PMA) is reported in this paper for THz detection and sensing applications. The proposed absorber is constructed using interconnected circular ring elements enclosed by a square loop. The ring elements are interconnected using short strip lines which increases the electrical length to offer resonance at the lower frequencies of the THz regime without increasing the electrical length. The proposed absorber has a footprint of 0.12 λ_eff?×?0.12 λ_eff where λ_eff is the effective wavelength calculated at the lowest operating frequency. The absorber provides 92%, 84%, 90%, 100%, and 100% absorption at 0.24, 0.56, 0.65, 0.82, and 0.95 THz, respectively. The proposed structure offers structural symmetry, and hence, it is polarization-insensitive. The proposed five-band absorber has good angular stability consistent with many research works reported in the literature and has a small frequency ratio of 1:2.3:2.7:3.4:3.9. The proposed absorber can be used as a permittivity sensor and its sensitivity is estimated to vary from 5.8 GHz/permittivity unit (PU) to 23.56 GHz/PU.

     

Tunable Terahertz Plasmonic Perfect Absorber Based on T-Shaped InSb Array

High absorption efficiency is particularly desirable for various microtechnological applications. In this paper, a nearly perfect terahertz absorber for transverse magnetic (TM) polarization based on T-shaped InSb array is proposed and numerically investigated. Incident wave at the Fabry-Perot resonant frequency can be totally absorbed into the narrow grooves between the two adjacent T-shaped InSb arms. The absorption mechanism is theoretically and numerically studied by using the Fabry-Perot model and the finite element method (FEM), respectively. It is found that the proposed absorber has large angle tolerance. Moreover, the absorption peak can be controlled by varying the temperature. Furthermore, a new absorption peak will emerge while breaking the symmetry of the T-shaped InSb array. This tunable and angle-independent THz perfect absorber may find important applications in THz devices such as microbolometers, coherent thermal emitters, solar cells, photo detectors, and sensors.     

Design Method for Light Absorption Enhancement in Ultra-Thin Film Organic Solar Cells with the Metallic Nanoparticles

In this paper, a method is presented for designing the parameters of metallic nanoparticles introduced into ultra-thin film organic solar cells (OSCs) to improve the light absorption. On the basis of Mie theory, a relationship is setup between the scattering efficiency of localized surface plasmon resonance and the size parameter of metallic nanoparticles, by which metallic nanoparticles with optimal size can be designed to realize the highest ratio of resonant scattering to resonant absorption, thus light absorption enhancement of OSCs is maximized. By taking spherical Ag nanoparticles into an OSC system with an active layer of poly(3-hexylthiophene) and [6, 6]-phenyl-C61-butyric acid methyl ester as subject, light absorption increase of 26 % at an average wavelength of incident light is demonstrated. This design method is also applicable to other types of OSCs.     

Design of a Multi-functional Device Based on the Solid-state Plasma: Absorber and Splitter

Plasmonics - We propose a multi-functional device by using the solid-state plasma, which can be called a plasma metamaterial absorber (PMA). The absorber can get tunable absorption spectrum by...     

A Newfangled Terahertz Absorber Tuned Temper by Temperature Field Doped by the Liquid Metal

In this article, a terahertz absorber tuned by temperature field with a newfangled structure is presented, which comprises the mercury resonators. In this scheme, temperature (T) build-up will lead the mercury stored in the bottom slot to expand through the columniform hole and be full of the upper central cross container, which can transform the absorption bands of such an absorber. The simulated results manifest that when T is increased from 0 to 25 °C, the dual-frequency absorption points (2.59 THz, 3.03 THz) and a narrow absorption region over 90% (6.54–7.10 THz), whose relative bandwidth (RB) is 7.9%, will be tailored to a single-frequency point absorption (3.12 THz) and a broadband absorption area (6.00–7.21 THz, and RB = 18.3%). For figuring out the property of the absorber mentioned above, the impacts of incident and polarization angles along with some relevant parameters of the structure on the absorption property are investigated. In addition, for plainly expounding the physical mechanism of absorption, the distributions of the surface current diagrams of the presented absorber are calculated, as well as the electric field diagrams, the magnetic field diagrams, the power loss density diagrams, and the power flow density diagrams. The proffered scheme in this article may offer a novel idea for realizing the reconfigurable absorbers.

     

Optical Absorption of Thin Film Solar Cells with Hybrid Arranged Bottom Grating

Crystalline silicon thin film solar cells with hybrid arranged bottom grating are proposed. Optical absorption efficiency and photocurrent density are calculated to get optimized bottom grating parameters. Compared with mono arranged Ag grating or Al-doped zinc-oxide grating, hybrid arranged bottom grating could couple more near-infrared region lights into the active absorber layer. Optical absorption enhancement profiles are plotted for monolayer grating solar cells with four different bottom grating arrangements, which agree with dispersion characteristics well. The absorption enhancement profiles illustrate the mechanism of the more coupling of near-infrared lights. Electrical modeling is considered in the end and it is found that hybrid arranged bottom grating’s thin film solar cell outperforms the thin film solar cells with mono arranged bottom gratings evidently.     

Ultrabroadband Mid-infrared Light Absorption Based on a Multi-cavity Plasmonic Metamaterial Array

We present a broadband plasmonic metamaterial absorber in the infrared region based on localized surface plasmon polaritons (LSPPs). The unit cell of the proposed metamaterial absorber consists of a multi-cavity structure, in which absorption resonances can be tuned independently through the modification of the width and shift of metallic walls. In order to avoid the degeneration between two contiguous resonances, which dramatically reduces the bandwidth, we introduce a zigzag design rule to arrange the cavities within a compact unit. Thus, the possible number of resonances is greatly increased, enabling an ultrabroadband absorption. A broadband absorber is demonstrated with only a few-layer structure and it also has an incident-angle-insensitive feature. Our results have potential applications in photovoltaic devices, emitters, sensors, and camouflage systems.