Angle-Insensitive Broadband Absorption Enhancement of Graphene Using a Multi-Grooved Metasurface

Active control of resonant asymmetric transmission based on topological edge states in paired photonic crystals with a Ge2Sb2Te5 film

For many high-precision applications such as filtering, sensing, and photodetection, active control of resonant responses of metasurfaces is crucial. Herein, we demonstrate that active control of resonant asymmetric transmission can be realized based on the topological edge state (TES) of an ultra-thin G e 2 S b 2 T e 5 (GST) film in a photonic crystal grating (PCG). The PCG is composed of two pairs of one-dimensional photonic crystals (PCs) separated by a GST film. The phase change of the GST film re-distributes the field distributions of the PCG; thus active control of narrowband asymmetric transmission can be achieved due to the switch of the on-off state of the TES. According to multipole decompositions, the appearance and disappearance of the synergistically reduced dipole modes are responsible for the high-contrast asymmetric transmission of the PCG. In addition, the asymmetric transmission performances are robust to the variation of structural parameters, and good unidirectional transmission performances with a high peak transmission and high contrast ratio can be balanced, as the layer number of the two PCs is set as four. By changing the crystallization fraction of GST, the peak transmission and peak contrast ratio of asymmetric transmission can be flexibly tuned with the resonance locations kept almost the same.

Broadband and Efficient Metamaterial Absorber Design Based on Gold-MgF2-Tungsten Hybrid Structure for Solar Thermal Application

We have presented a solar absorber design with gold-MgF₂-tungsten materials. The solar absorber design is optimized with nonlinear optimization mathematical method to find and optimize geometrical parameters. The wideband absorber is made of a three-layer structure composed of tungsten, magnesium fluoride, and gold. This study analyzed the absorber's performance using numerical methods over the sun wavelength range of 0.25 μm to 3 μm. The solar AM 1.5 absorption spectrum is a benchmark against which the proposed structure's absorbing characteristics are evaluated and discussed. It is necessary to analyze the behavior of the absorber under a variety of various physical parameter conditions in order to determine the results and structural dimensions that are optimal. The nonlinear parametric optimization algorithm is applied to obtain the optimized solution. This structure can absorb more than 98% of light across the near-infrared and visible light spectrums. In addition, the structure has a high absorption efficiency for the far range of the infrared spectrum and the THz range. The absorber that has been presented is versatile enough to be used in a variety of solar applications, both narrowband and broadband. The design of the solar cell that has been presented will be of assistance in designing a solar cell that has high efficiency. The proposed optimized design with optimized parameters will help design solar thermal absorbers.

Tunable light trapping in the graphene metasurface

Graphene metasurfaces based on surface plasmon resonance can greatly enhance the interaction between light and matter at the nanoscale. At present, the resonance of graphene metasurfaces is widely used to enhance the absorption of atomic layer graphene, but little work has focused on the light field trapping capabilities it brings. In this paper, we numerically study the light trapping and manipulation of an asymmetric graphene metasurface. The designed device supports two resonant modes, and the multipole decomposition confirms that the electric dipole response dominates them. The calculated average electric field enhancement factor (EF) can reach 1206 and 1779, respectively. The near-field distribution indicates that the electric field is mainly localized in the graphene nanodisks. When the Fermi energy changes, the intensity and peak position of EF can be effectively regulated. In addition, when the polarization of the incident light is adjusted, the light field capture of the two modes is independently regulated. These results reveal that the graphene metasurface has significant light field capture and regulation ability, which provides a new idea for the realization of active regulation of high-performance low-dimensional optical devices.

Ultra-broadband, wide-angle plus-shape slotted metamaterial solar absorber design with absorption forecasting using machine learning

Energy utilization is increasing day by day and there is a need for highly efficient renewable energy sources. Solar absorbers with high efficiency can be used to meet these growing energy demands by transforming solar energy into thermal energy. Solar absorber design with highly efficient and Ultra-broadband response covering visible, ultraviolet, and near-infrared spectrum is proposed in this paper. The absorption response is observed for three metamaterial designs (plus-shape slotted design, plus-shape design, and square-shape design) and one optimized design is used for solar absorber design based on its high efficiency. The design results are compared with AM 1.5 spectral irradiance response. The electric field response of the plus-shape slotted metamaterial design is also presented which matches well with the absorption results of different solar spectrum regions. The results proved that the attained absorption response showing wide angle of incidence. Machine learning is also used to examine the design data in order to forecast absorption for various substrate thickness, metasurface thickness, and incidence angles. Regression and forecasting simulations based on machine learning are used to try to anticipate absorber behaviour at forthcoming and intermediate wavelengths. Simulation results prove that Machine Learning based methods can lessen the obligatory simulation resources, time and can be used as an effective tool while designing the absorber. The proposed highly efficient, wide-angle, ultra-broadband solar absorber design with its behavior prediction capability using machine learning can be utilized for solar thermal energy harvesting applications.

Lithography-free tunable absorber at visible region via one-dimensional photonic crystals consisting of an α-MoO3 layer

Flexible control of light absorption within the lithography-free nanostructure is crucial for many polarization-dependent optical devices. Herein, we demonstrated that the lithography-free tunable absorber (LTA) can be realized by using two one-dimensional (1D) photonic crystals (PCs) consisting of an α-MoO₃ layer at visible region. The two 1D PCs have different bulk band properties, and the topological interface state-induced light absorption enhancement of α-MoO₃ can be realized as the α-MoO₃ thin film is inserted at the interface between the two 1D PCs. The resonant cavity model is proposed to evaluate the anisotropic absorption performances of the LTA, and the results are in good agreement with those of the transfer matrix method (TMM). The absorption efficiency of the LTA can be tailored by the number of the period of the two PCs, and the larger peak absorption is the direct consequence of the larger field enhancement factor (FEF) within the α-MoO₃ layer. In addition, near-perfect absorption can be achieved as the LTA is operated at the over-coupled resonance. By varying the polarization angle, the absorption channels can be selected and the reflection response can be effectively modulated due to the excellent in-plane anisotropy of α-MoO₃.

Multiband infrared emissions limited in the grazing angle from metal-dielectric-metal metamaterials

Thermal radiation management remains a challenge because of the incoherent and isotropic nature of electromagnetic waves. In this study, a multiband and angular-selective infrared emitter, consisting of a simple one-dimensional (1D) metal-dielectric-metal metamaterial, is demonstrated. Although this structure has been well known as spectrally selective emitters, we analytically reveal that when the dielectric layer thickness is much smaller than the wavelength of interest (< 1/10), directive emission at nearly equal to the grazing angles (> 80°) can be obtained at multiple resonant wavelengths. As the absorption peaks can be entirely characterized by geometrical parameters, this angular selective technology offers flexible control of thermal radiation and can be adjusted to specific applications.

Graphene-based metasurface solar absorber design with absorption prediction using machine learning

Solar absorber is required to absorb most of the energy of the solar spectral irradiance. We propose a graphene-based solar absorber design with two different metasurfaces to improve this absorption and increase the efficiency of the solar absorber. The metasurfaces are selected based on their symmetrical/asymmetrical nature (O-shape and L-shape). The O-shape metasurface design is showing better performance over the L-shape metasurface design. The absorption performance is also compared with AM 1.5 solar spectral irradiance to show the effectiveness of the solar absorber. The absorption values are also enhanced by varying the parameters like resonator thickness and substrate thickness. The proposed solar absorber design gives maximum absorption in the ultraviolet and visible range. Furthermore, the design is also showing a high and similar absorption rate over a wide angle of incidence. The absorption of O-shape metasurface design is also predicted using machine learning. 1D-Convolutional Neural Network Regression is used to develop a Machine Learning model to determine absorption values of intermediate wavelength for assorted values of angle of incidence, resonator thickness, and substrate thickness. The results of experiments reveal that absorption values may be predicted with a high degree of accuracy. The proposed absorber with its high absorbing capacity can be applied for green energy applications.

Towards Perfect Absorption of Single Layer CVD Graphene in an Optical Resonant Cavity: Challenges and Experimental Achievements

Graphene is emerging as a promising material for the integration in the most common Si platform, capable to convey some of its unique properties to fabricate novel photonic and optoelectronic devices. For many real functions and devices however, graphene absorption is too low and must be enhanced. Among strategies, the use of an optical resonant cavity was recently proposed, and graphene absorption enhancement was demonstrated, both, by theoretical and experimental studies. This paper summarizes our recent progress in graphene absorption enhancement by means of Si/SiO2-based Fabry-Perot filters fabricated by radiofrequency sputtering. Simulations and experimental achievements carried out during more than two years of investigations are reported here, detailing the technical expedients that were necessary to increase the single layer CVD graphene absorption first to 39% and then up to 84%. Graphene absorption increased when an asymmetric Fabry-Perot filter was applied rather than a symmetric one, and a further absorption increase was obtained when graphene was embedded in a reflective rather than a transmissive Fabry-Perot filter. Moreover, the effect of the incident angle of the electromagnetic radiation and of the polarization of the light was investigated in the case of the optimized reflective Fabry-Perot filter. Experimental challenges and precautions to avoid evaporation or sputtering induced damage on the graphene layers are described as well, disclosing some experimental procedures that may help other researchers to embed graphene inside PVD grown materials with minimal alterations.

Ultrabroadband and ultrathin absorber based on an encapsulated T-shaped metasurface

Ultrabroadband absorbers are vital for applications such as solar energy harvesting and integrated optoelectronic devices. Herein, we design, fabricate and characterize a novel ultrabroadband and ultrathin absorber based on the encapsulated T-shaped metasurface (ETM). The ETM consists of a 20 nm Cr film and a Cr substrate sandwiched by the T-shaped polymethyl methacrylate (PMMA) arrays. The Cr film provides a robust absorptive surface with improved impedance matching, and ultrabroadband absorption can be achieved via the excitation of the localized surface plasmon (LSP) of this ultrathin film. The average absorption of simulated and experimental results of the ETM in the visible range of 400-800 nm for the TM (TE) polarization are 96.4% (96.3%) and 90.6% (89.4%), respectively. Three-dimensional (3D) power dissipation density distributions of the proposed structure have been investigated, which indicates that the synergistic absorption effect of different parts of the T-shaped ultrathin Cr film contributes to the major absorption enhancement. The absorption of the ETM is very robust to the changes of geometrical parameters and the symmetry of the structure, and it can be maintained almost the same even if T-shaped profiles are changed to L-shaped profiles. Moreover, the absorption performance of the ETM exhibits polarization-insensitive and wide-angle features, which has advantages for many potential applications.

Highly Efficient Light Absorption of Monolayer Graphene by Quasi-Bound State in the Continuum

Graphene is an ideal ultrathin material for various optoelectronic devices, but poor light–graphene interaction limits its further applications particularly in the visible (Vis) to near-infrared (NIR) region. Despite tremendous efforts to improve light absorption in graphene, achieving highly efficient light absorption of monolayer graphene within a comparatively simple architecture is still urgently needed. Here, we demonstrate the interesting attribute of bound state in the continuum (BIC) for highly efficient light absorption of graphene by using a simple Si-based photonic crystal slab (PCS) with a slit. Near-perfect absorption of monolayer graphene can be realized due to high confinement of light and near-field enhancement in the Si-based PCS, where BIC turns into quasi-BIC due to the symmetry-breaking of the structure. Theoretical analysis based on the coupled mode theory (CMT) is proposed to evaluate the absorption performances of monolayer graphene integrated with the symmetry-broken PCS, which indicates that high absorption of graphene is feasible at critical coupling based on the destructive interference of transmission light. Moreover, the absorption spectra of the monolayer graphene are stable to the variations of the structural parameters, and the angular tolerances of classical incidence can be effectively improved via full conical incidence. By using the full conical incidence, the angular bandwidths for the peak absorptivity and for the central wavelength of graphene absorption can be enhanced more than five times and 2.92 times, respectively. When the Si-based PCS with graphene is used in refractive index sensors, excellent sensing performances with sensitivity of 604 nm/RIU and figure of merit (FoM) of 151 can be achieved.

Imaging metasurfaces based on graphene-loaded slot antennas

Spectral imagers, the classic example being the color camera, are ubiquitous in everyday life. However, most such imagers rely on filter arrays that absorb light outside each spectral channel, yielding ∼1/N efficiency for an N-channel imager. This is especially undesirable in thermal infrared (IR) wavelengths, where sensor detectivities are low. We propose an efficient and compact thermal infrared spectral imager comprising a metasurface composed of sub-wavelength-spaced, differently-tuned slot antennas coupled to photosensitive elements. Here, we demonstrate this idea using graphene, which features a photoresponse up to thermal IR wavelengths. The combined antenna resonances yield broadband absorption in the graphene exceeding the 1/N efficiency limit. We establish a circuit model for the antennas' optical properties and demonstrate consistency with full-wave simulations. We also theoretically demonstrate ∼58% free space-to-graphene photodetector coupling efficiency, averaged over the 1050 cm^-1 to 1700 cm^-1 wavenumber range, for a four-spectral-channel gold metasurface with a 0.883 µm by 6.0 µm antenna pitch. This research paves the way towards compact CMOS-integrable thermal IR spectral imagers.

Ultrabroadband Absorption Enhancement via Hybridization of Localized and Propagating Surface Plasmons

Broadband metamaterial absorbers (MAs) are critical for applications of photonic and optoelectronic devices. Despite long-standing efforts on broadband MAs, it has been challenging to achieve ultrabroadband absorption with high absorptivity and omnidirectional characteristics within a comparatively simple and low-cost architecture. Here we design, fabricate, and characterize a novel compact Cr-based MA to achieve ultrabroadband absorption in the visible to near-infrared wavelength region. The Cr-based MA consists of Cr nanorods and Cr substrate sandwiched by three pairs of SiO₂/Cr stacks. Both simulated and experimental results show that an average absorption over 93.7% can be achieved in the range of 400–1000 nm. Specifically, the ultrabroadband features result from the co-excitations of localized surface plasmon (LSP) and propagating surface plasmon (PSP) and their synergistic absorption effects, where absorption in the shorter and longer wavelengths are mainly contributed bythe LSP and PSP modes, respectively. The Cr-based MA is very robust to variations of the geometrical parameters, and angle-and polarization-insensitive absorption can be operated well over a large range of anglesunder both transverse magnetic(TM)- and transverse electric (TE)-polarized light illumination.

Cascaded Nanorod Arrays for Ultrabroadband, Omnidirectional and Polarization-Insensitive Absorption

An ultrabroadband, omnidirectional, and polarization-insensitive absorber based on cascaded nanorod arrays (CNAs) is numerically demonstrated, and an average absorptivity of 98.2% with a relative absorption bandwidth (RAB) of 149.8% can be achieved in the 0.38–2.65 μm wavelength range. The proposed CNA-based absorber requires only several pairs of multilayers to achieve excellent absorption performance. More significantly, the physical mechanism for this intriguing ultrabroadband absorption results from the synergistic effect of localized surface plasmon (LSP) and plasmonic resonant cavity (PRC) modes, which is fundamentally different from the tapered metal/dielectric multilayer-based absorbers associated with the slow-light mode. We investigated the absorption properties of the CNA-based metasurface by using the impedance theory, which indicates that the impedance of the structure matches well with the impedance of the free space from the visible to near-infrared wavelength range. In addition, the absorption properties of the CNA-based metasurface are robust to the variation of the structural parameters and the metal/dielectric materials, and ultrabroadband absorption performance can be maintained within 0–60° for both TM and TE modes.

Symmetry-broken square silicon patches for ultra-narrowband light absorption

The effect of ultra-narrowband light absorption enhancement is presented by using metamaterials with symmetry-broken square silicon patches (SSPs). The symmetry of the SSP can be broken by introducing a narrow slit deviating from its center. By breaking the symmetry of the SSPs, slit resonance mode with standing wave patterns can be excited, and the locations of the absorption peaks can be well estimated by using the Fabry-Pérot (F-P) cavity model. Although there is no excitation of surface plasmon resonance, ultra-narrowband light absorption can be achieved by minimizing the reflectance through perfect impedance matching and simultaneously eliminating the transmittance by the metallic substrate. Good ultra-narrowband absorption features can be maintained as the parameters of the buffer layer and the SSPs are altered. When this type of symmetry-broken SSPs-based metamaterial is used in refractive-index sensors, it shows excellent sensing properties due to its stable ultra-narrowband absorption enhancement.

Multifunctional graphene metasurface to generate and steer vortex waves

Graphene, an innovated 2D material with atomic thickness, is a very promising candidate and has drawn great attentions in various applications. Graphene metasurface enables dynamic control of various wavefronts, achieving distinguished functionalities. The flexibility of graphene metasurface makes it possible to implement multifunctional devices with ease. In this work, a novel design of multifunctional graphene metasurface, which can combine the functionalities of generating and steering vortex waves, has been proposed. The multifunctional graphene metasurface consists of a large array of graphene reflective unit cells. Each unit cell is controlled independently by its size and external static gate voltage. By scrutinizing the reflective property of the graphene cell, the graphene metasurface is designed to realize multi-functionalities. Simulation results show that vortex wave can be generated and steered. This work can establish a methodology to design multifunctional graphene metasurfaces, and the tunability of graphene opens the gate to the design and fabrication of reconfigurable graphene devices.

Dual broadband infrared absorptance enhanced by magnetic polaritons using graphene-covered compound metal gratings

A dual broadband perfect absorber based on a graphene-covered compound silver (Ag) grating structure working in the infrared (0.8-2.1 µm) regime is proposed and investigated numerically. Two distinct absorption peaks approximately 1.0 are achieved by the excitation of magnetic polaritons over a large range of incident angles from 0 to 70 degrees. The physics underlying the structure is also explained by computing interactions of electromagnetic fields with the graphene and the Ag grating. In addition, it has shown that the absorption peaks can be tuned by changing geometric parameters of the structure; however, their spectral shape and absorption remain unchanged. Furthermore, the proposed compound grating with a graphene overlay provides potential applications for infrared absorbing devices.

Plasmonically induced perfect absorption in graphene/metal system

The constructive interference of bright and dark plasmonic modes results in plasmon-induced absorption (PIA) effect. Here, we theoretically investigate PIA effect, which is realized by the constructive interference between a Fabry-Perot (F-P) resonance mode and a graphene quasi-guided mode. Numerical simulation reveals at least three advantages of our structure over previous ones. First, the extinction ratio can reach ~ 99.999%, resulting in the ultra-high figure of merit* (FOM*) as high as 106. Second, the intensity of this pronounced PIA effect can be optimized by adjusting the coupling distance. Third, the resonance frequency can be easily tunable by tuning the graphene Fermi level. This system may have potential applications in dynamically optical switching and biochemical sensing.