A Tunable Perfect THz Metamaterial Absorber with Three Absorption Peaks Based on Nonstructured Graphene

In this paper, a graphene-based tunable multi-band terahertz absorber is proposed and numerically investigated. The proposed absorber can achieve perfect absorption within both sharp and ultra-broadband absorption spectra. This wide range of absorption is gathered through a unique combination of periodically cross- and square-shaped dielectrics sandwiched between two graphene sheets; the latter enables it to offer more absorption in comparison with the traditional single-layer graphene structures. The aforementioned top layer is mounted on a gold plate separated by a Topas layer with zero volume loss. Furthermore, in our proposed approach, we investigated the possibility of changing the shapes and sizes of the dielectric layers instead of the geometry of the graphene layers to alleviate the edge effects and manufacturing complications. In numerical simulations, parameters, such as graphene Fermi energy and the dimensions of the proposed dielectric layout, have been optimally tuned to reach perfect absorption. We have verified that the performance of our dielectric layout called fishnet, with two widely investigated dielectric layouts in the literature (namely, cross-shaped and frame-and-square). Our results demonstrate two absorption bands with near-unity absorbance at frequencies of 1.6–2.3 and 4.2–4.9 THz, with absorption efficiency of 98% in 1.96 and 4.62 THz, respectively. Moreover, a broadband absorption in the 7.77–9.78 THz is observed with an absorption efficiency of 99.6% that was attained in 8.44–9.11 THz. Finally, the versatility provided by the tunability of three operation bands of the absorber makes it a great candidate for integration into terahertz optoelectronic devices.

     

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).     

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.

     

Numerical Study of an Au-ZnO-Al Perfect Absorber for a Color Filter with a High Quality Factor

In this research, a perfect absorber based on an Au-ZnO-Al structure was studied numerically. The wavelength-selective and angle-independent characteristics of the device were demonstrated by simulation. The roles of the top metallic layer and the middle dielectric layer in producing a wavelength-selective perfect absorber with a high quality factor were investigated. The direction for further improving the quality factor is also pointed out in this paper. The research will be helpful for understanding the origination of perfect absorption in these types of metal-insulator-metal structures and producing a color filter with a high quality factor.

     

Broadband Microwave Absorber with a Double-Split Ring Structure

Plasmonics - In this paper, we present a novel broadband microwave absorber. Each unit in the metamaterial consists of a layer of dielectric material sandwiched between two layers of metal patches....     

Broadband Ultrathin Absorber and Sensing Application Based on Hybrid Materials in Infrared Region

A broadband and ultra-thin absorber in the infrared region is proposed. The structure is composed of three layers, and the most remarkable difference is that two hybrid materials (Sn and InSb) are used in the top layer. The numerical results show that a broadband perfect absorption from 85.2 to 114.3 THz can be achieved for either transverse electric or magnetic polarization waves due to the effect of using hybrid materials. Moreover, the power loss and surface current distribution in the absorber are investigated to explain the physical mechanism of high absorption. The metamaterial absorber is ultra-thin, having total thickness of 0.3 μm, i.e.,λ/10 with respect to the center frequency of the high absorption bands. The proposed hybrid materials which are used in the same layer provides a useful way to realize a broadband perfect absorber in the infrared region and it is important for a variety of applications, such as solar energy harvest, sensors, and integrated photodetectors .     

An Ultra-Wideband Terahertz Metamaterial Absorber Utilizing Sinusoidal-Patterned Dielectric Loaded Graphene

In this paper, a non-structured graphene sheet loaded with a sinusoidal-patterned dielectric is introduced as an ultra-wideband metamaterial absorber in terahertz regime. Regardless of conventional structures with multilayered-graphene, a single layer sheet of non-structured graphene is used whereas the proposed structure benefits from dielectric width modulation and cavity method in order to excite continuous graphene plasmon resonances. The structure comprises four layers that two Fabry-Perot cavity mirrors are constructed by upper sinusoidal-patterned dielectric and a gold film. Full wave simulation results demonstrate that a broadband over 90% absorption with absolute bandwidth of 6.58 THz and central frequency of 3.97 THz is achieved under normal TE/TM incident plane wave. The designed structure yields 166% relative bandwidth. According to the symmetric configuration, the absorption spectra of mentioned polarizations are thoroughly close to each other resulting to a polarization insensitive structure. The stability of bandwidth and absorbance of the structure versus angle of incidence, θ, up to 35°/65° for TM/TE polarizations, respectively, and azimuth angle, φ, shows an interesting capability for utilization as detectors and sensors. The simple geometry of utilized graphene layer results in easy fabrication. The designed structure has wideband absorption in THz regime. Moreover, it is more compact than conventional broadband THz absorbers.

     

Plasma-Assisted Deposition of Au/SiO2 Multi-layers as Surface Plasmon Resonance-Based Red-Colored Coatings

In this work, the expanding thermal plasma chemical vapor deposition in combination with radio frequency magnetron sputtering is used to deposit dielectric/metal multi-layers with controlled size and density of nanoparticles. The multi-layer structure serves the purpose of increasing the nanoparticle number density, without changing the metal particle size, shape and the interparticle distance. The possibility of independently tuning and, therefore, controlling the nanoparticle size and number density allows developing surface plasmon resonance-based deep-colored coatings. The influence of the number of layers, metal surface area coverage, and thickness of the dielectric layer on the multi-layer nanostructure and on the developed color is presented here in detail. The nanoparticle size and distribution have been measured by transmission electron microscopy. Rutherford back-scattering and infra-red transmission spectroscopy have been used to determine the metal surface coverage and the film chemistry, respectively. Optical properties of the nano-composite layers have been investigated by UV-VIS spectroscopy and exhibit an increase in amplitude of the plasmon absorption spectra at a fixed plasmon resonance frequency with an increase in the number of layers.     

Wide-Angle Near-Perfect Absorber Based on Sub-Wavelength Dielectric Grating Covered by Continuous Thin Aluminum Film

We design and numerically investigate an optical absorber consisting of the sub-wavelength dielectric grating covered by continuous thin aluminum film. In this absorber, the aluminum film act as an efficient absorbing material because of the enhanced electric field in the air nano-grooves, and the absorption spect+rum can be manipulated by Fabry-Perot cavity mode resonance. According to the spectrum manipulation mechanism, the wavelength of absorption peak can be tuned by changing the heights and widths of the air nano-grooves. More importantly, the high absorption is very robust to the incident angle around the designed wavelength. From the nanofabrication point of view, the light absorber can be fabricated more easily without the need for ion or electrochemical etching of metal and it is easy to be integrated into complex photonic devices.     

Design Method of a Broadband Wide-Angle Plasmonic Absorber in the Visible Range

A new design method of a broadband wide-angle metal-dielectric-metal plasmonic absorber is presented based on the cavity mode theory. The broadband absorption is implemented by filling a unit cell with multi-size square metal patches resonant at adjacent wavelengths, with the widths of the patches and thickness of the dielectric layer optimized with the presented method. A broadband plasmonic absorber working in the visible range is designed, the absorption of which is insensitive to the azimuth angle of incident field and keeps over 0.7 at incident angle up to 60° for p polarization and above 0.6 at up to 40° for s polarization.     

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.

     

Single-Patterned Metamaterial Structure Enabling Multi-band Perfect Absorption

Multi-band or broadband perfect metamaterial absorbers, based on coplanar super-unit structure or multiple vertically stacked layers, have received intense attention because of their potential for practical applications. The resonance mechanism of them usually only utilizes the overlapping of the fundamental resonance of the different-sized patterns, and neglects the high-order resonance of the structure, and thus making the proposed structures quite troublesome to be fabricated and the mechanism of the current demonstrated absorbers lack of novelty. In this paper, a simple design of dual-band terahertz absorber consisted of only a traditional square metallic patch and a dielectric layer on top of a continuous ground plane is presented. Simulation results show that the single resonant structure has two resonance absorption peaks, which are both average over 99.5 %. The mechanism of the dual-band absorber is due to the overlapping of the fundamental mode and three-order response of the patterned structure, which is totally different from previous reports that only combining the fundamental resonances of the different-shaped complex structures to obtain the dual-band response. Furthermore, the proposed single-patterned structure can be used to extend the number of the absorption peaks (for example, triple-band absorber) by combining one more resonance (the five-order response). The proposed absorbers with the simple structure design have potential applications in many areas, such as detection, sensing, and selective thermal emitters.     

Resonant TE Transmission Through a Continuous Metal Film: Perspectives for Low-Loss Plasmonic Elements

Whereas resonant transverse magnetic transmission across an undulated continuous metal film is achieved with the mediation of plasmon modes excited by the undulation, it is shown here that transverse electric (TE) resonant transmission through a continuous metal film can also be achieved with the mediation of the second-order TE₁ mode of a dielectric slab waveguide having the metal film sandwiched at its middle. The demonstration is made by using the materials currently used in the domain of optical security and counterfeit deterrence: ZnS is shown to possibly be a lossless interface/adhesion layer between a polymer and a noble metal for plasmonic resonant elements.     

Optical Transmission Through Multilayered Ultra-Thin Metal Gratings

Optical transmission properties of multilayered ultra-thin metal gratings are numerically studied. The transmission spectrum has a broad stop-band with extremely low transmittance compared to that of a single-layer one for TM polarization. The stop-band is shown to be formed by multiple-interference tunneling and various plasmon resonance processes in ultra-thin-metal and dielectric multilayers. That is on the transmission background of non-apertured metal/dielectric multilayer structures that have low transmission in the long-wavelength range due to destructive multiple-interference tunneling, the transmission is further suppressed in the stop-band by plasmon resonances in the top metal/dielectric layers, e.g., the anti-symmetric bound surface plasmon mode in the ultra-thin metal layer and the gap surface plasmon mode in the metal-sandwiched dielectric layer. High transmission beyond the stop-band is due to coupled gap surface plasmon mode in the entire multilayer structures. Applications of the optical properties of the multilayered ultra-thin metal gratings are suggested for optical filtering (wavelength or polarization selective).     

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.     

Ultrafast Optical Properties of Dense Electron Gas in Silicon Nanostructures

We investigate the ultrafast dynamics of carriers in a silicon nanostructure by performing spectrally resolved femtosecond spectroscopy measurements with a supercontinuum probe. The nanostructure consists of a 158-nm-thick crystalline Si layer on top of which a SiO₂ passivation layer leads to a very high quality of the Si surface. In addition, a dielectric function approach, including contributions from a Drude part and interband transitions, combined with the Transition Matrix Approximation is used to model the photogenerated carrier dynamics. The spectrotemporal reflectivity reveals two mechanisms. First, an electron–hole plasma is created by the pump pulse and lasts for a few picoseconds. Importantly, its spectral signature is either a positive or a negative change of reflectivity, depending on the probe wavelength. This is complementary to the already reported results obtained with degenerate frequency measurements. The second mechanism is a thermal diffusion of carriers which occurs during several hundreds of picoseconds. The overall dynamics at short and long delays in the whole visible spectrum is well explained with our model which shows that the main contribution to the reflectivity dynamics is due to the Drude dielectric function. The observation of this predominance of free carriers requires both a long lived high density of carriers as well as a little influence of surface scattering as provided by our thin crystalline Si layer with passivated Si/SiO₂ interface.     

New Type Design of the Triple-Band and Five-Band Metamaterial Absorbers at Terahertz Frequency

A new scheme to achieve a simple design of triple-band metamaterial absorber at terahertz frequency is presented. In this scheme, we employ a traditional sandwich structure, which is consisted of a metallic resonator and an appropriate thickness of the dielectric layer backed with an opaque metallic board, as the research object. Three strong but discrete resonance peaks with the narrow bandwidths and high absorptivities are realized. The combination of the dipolar resonance, LC (inductor-capacitor circuit) resonance, and the surface resonance of the metallic resonator determines the triple-band absorption. Numerical results also show that the frequencies of the three absorption bands and the number of the resonance peaks can be effectively tuned by adjusting or changing the geometric parameters of the metallic resonator. Moreover, we present a simple design of five-band terahertz absorber by further optimizing the sizes of the metallic elements in the top layer of the metamaterial. The design of the unit structures will assist in designing innovative absorbing devices for spectroscopy imaging, detection, and sensing.     

Improvement of Transparent Metal Top Electrodes for Organic Solar Cells by Introducing a High Surface Energy Seed Layer

We present highly transparent and conductive silver thin films in a thermally evaporated dielectric/metal/dielectric (DMD) multilayer architecture as top electrode for efficient small molecule organic solar cells. DMD electrodes are frequently used for optoelectronic devices and exhibit excellent optical and electrical properties. Here, we show that ultrathin seed layers such as calcium, aluminum, and gold of only 1 nm thickness strongly influence the morphology of the subsequently deposited silver layer used as electrode. The wetting of silver on the substrate is significantly improved with increasing surface energy of the seed material resulting in enhanced optical and electrical properties. Typically thermally evaporated silver on a dielectric material forms rough and granular layers which are not closed and not conductive below thicknesses of 10 nm. With gold acting as seed layer, the silver electrode forms a continuous, smooth, conductive layer down to a silver thickness of 3 nm. At 7 nm silver thickness such an electrode exhibits a sheet resistance of 19 Ω/□ and a peak transmittance of 83% at 580 nm wavelength, both superior compared to silver electrodes without seed layer and even to indium tin oxide (ITO). Top‐illuminated solar cells using gold/silver double layer electrodes achieve power conversion efficiencies of 4.7%, which is equal to 4.6% observed in bottom‐illuminated reference devices employing conventional ITO. The top electrodes investigated here exhibit promising properties for semitransparent solar cells or devices fabricated on opaque substrates.     

Localization of Near-Field Resonances in Bowtie Antennae: Influence of Adhesion Layers

The near-field resonances of gold bowtie antennae are numerically modeled. Besides the short-range surface plasmon polariton (SR-SPP) mode along the main axis of the structure, a coupled SPP mode is also found in the gap region (G-SPP). The influence of adhesion layers is considered, which depends on the refractive index and the absorption of the adhesion material and whether it is continuous or etched. A high refractive index causes the peak of the SR-SPP to red-shift. High absorption quenches the intensity of the SR-SPP. The magnitude of influence depends on the overlap of the adhesion layer with the SR-SPP and G-SPP modes. The near-field resonance of the SPP mode on the top surface is also considered. An etched metal adhesion layer changes the near-field localization in the gap and causes the enhancement peaks at different heights within the gap to red-shift from top to bottom. A simple optimization method for the near-field localization by the combination of different top and bottom layers is demonstrated.     

Influences of Nanostructure Arrays on Light Absorption in Amorphous Silicon Thin-Film Solar Cells

By means of finite-difference time-domain (FDTD) numerical method, we investigate the possibility to enhance the light absorption in solar cells by employing different nanostructures. The solar cells are made of 100-nm-thick amorphous silicon (α-Si). The impacts of gold nanohole arrays, dielectric nanosphere arrays, and gold nanoparticle arrays on the light absorption are simulated, compared, and analyzed. The results show that gold nanohole arrays functioning as the back reflective layer, dielectric nanosphere arrays, and gold nanoparticle arrays can significantly enhance the light absorption for the solar cells, and the former two can increase the short-circuit current by more than 40 %, showing a great potential to improve the utilization efficiency of solar energy.