Dielectric Spacer Layer Research Articles

Genetic optimization of terahertz metamaterial absorber for improved absorption and quality factor

Terahertz metamaterial absorbers are typically comprised of an array of sub-wavelength meta-atoms, a dielectric spacer layer and a ground plane. The absorption and quality factor are two of the most important performance metrics for various advanced applications, such as THz communication, sensing, and imaging. However, a large number of structural parameters in the meta-atom make the design approach based on physical intuition and parameter sweep impractical and difficult. In this paper, we report an intelligent design methodology based on the genetic algorithm that facilitates the optimization of an air-spaced terahertz metamaterial absorber to improve its absorption and quality factor. The presented approach starts by randomly generating the size parameters of the meta-atom, and it iteratively obtains the optimized design of the dual narrow band terahertz metamaterial perfect absorber. The quality factor for the two near unity-absorption peaks are 23.8 at 0.5 THz and 60.3 at 0.7 THz, respectively. The device was fabricated using the surface and bulk-micromachining processes, and the measured absorption spectra agrees well with the simulated results. Not limited to this proof-of-concept demonstration, this methodology can be applied to all metamaterial-based photonic systems achieving efficient forward optimization design.

A near infrared MIM metamaterial absorber using SiC

In this study, we introduce a metamaterial absorber operating in the near infrared region. The current metamaterial absorber (MMA) comprising Silver-SiC-Gold demonstrates an absorptivity exceeding 80% within the wavelength range of 770 nm to 1150 nm. The current metamaterials absorber exhibits independence from both polarization and incident angle. Additionally, we made comparison between the proposed Metal-Insulator-Metal (MIM) based MMA with other previously reported MMA using Silicon Carbide (SiC) and another dielectric spacer layer.

Wide-angle high-efficiency absorption of graphene empowered by an angle-insensitive Tamm plasmon polariton.

In recent years, researchers utilized Tamm plasmon polaritons (TPPs) in conventional heterostructures composed of a metal layer, a dielectric spacer layer and an all-dielectric one-dimensional (1-D) photonic crystal (PhC) to achieve high-efficiency absorption of graphene. According to the Bragg scattering theory, photonic bandgaps (PBGs) in all-dielectric 1-D PhC strongly shift toward shorter wavelengths (i.e., blueshift) as the incident angle increases. Therefore, TPPs in conventional heterostructures also show strongly blueshift property. Such strongly blueshift property of TPPs greatly limits the operating angle range of the high-efficiency absorption of graphene. Herein, we realize an angle-insensitive TPP in a heterostructure composed of a metal layer, a dielectric spacer layer and a 1-D PhC containing hyperbolic metamaterial layers. Empowered by the angle-insensitive property of the TPP, we achieve wide-angle high-efficiency absorption of graphene. The operating angle range (A > 80%) reaches 41.8 degrees, which is much larger than those in the reported works based on TPPs and defect modes. Our work provides a viable route to designing cloaking devices and photodetectors.

A dual-band hydrogen sensor based on Tamm plasmon polaritons.

Optical hydrogen sensors possess significant potential in various fields, including aerospace and fuel cell applications, which is due to their compact design and immunity to electromagnetic interference. However, commonly used sensors mostly use single-band sensing, which increases the risk of inaccurate measurements due to environmental interference or operational errors. To address this issue, this study proposes a dual-band hydrogen sensor comprising a Pd metal layer, a dielectric spacer layer, a defect layer, and a photonic crystal. By leveraging the interaction between the defect mode in the excitonic microcavity structure and the Tamm plasmon polaritons (TPPs) and Fabry-Perot (FP) resonances, the structure simultaneously generates two near-zero resonance valleys in the visible wavelength range. By adjusting the thickness of the defect layer, the coupling effect of the defect mode and TPPs together with FP resonance respectively is optimized. When the thickness is 0.27 μm, the sensitivities of the Tamm resonance band and FP resonance band are 239 and 21 RIU-1, respectively. Compared with the common sensors with a single band, its low-sensitivity wavelength can be used as a reference to assist the high-sensitivity wavelength for sensing. In addition, we find that the proposed sensor, through calculation, has good fault tolerance for both the thickness of the defect layer and the incident light angle. This study demonstrates a dual-band hydrogen sensor with TPPs, which is important for exploring new optical hydrogen sensors.

Simulation Design and Characteristic Research of Electromagnetic Metasurface Absorber

Electromagnetic metasurface technology is one of the most promising technologies for electromagnetic energy regulation. It is formed by an artificial periodic structure of discrete subwavelengths at the interface, allowing wavefront control and polarization control at the subwavelength scale, exhibiting a variety of special electromagnetic response capabilities, including abnormal beam bending, polarization transformation, electromagnetic wave absorption, etc. We propose a wide-angle, polarization-insensitive infrared perfect metasurface absorber made of an opaque gold ground layer, a dielectric spacer layer, and a hole-shaped gold patch on the spacer layer. The absorption spectrum of the metasurface absorber is simulated by the finite element method, and the results show that the absorber produces an absorption peak with a maximum absorption rate of 99.91% and a half-height and full width of 0.52um in the visible light band.

Nonreciprocal transmission of electromagnetic waves with nonlinear active plasmonic metasurfaces

Nonreciprocity is important for optical information processing, full-duplex communications, and protection of sensitive laser equipment from unwanted reflections. However, it is very challenging to obtain strong nonreciprocal response in optical frequencies, especially when nanoscale configurations are considered. In this work, we solve this problem by demonstrating a compact bifacial plasmonic metasurface that acts as an ultrathin nonreciprocal transmission filter at near-infrared frequencies. The proposed nanostructure is simple to be practically implemented, since it is composed of two silver nanostripes with different dimensions placed on both sides of an ultrathin active dielectric spacing layer. The introduced gain leads to an exceptional point formation in the linear operation regime, where unidirectional perfect transmission due to loss compensation is achieved from the presented non-Hermitian parity-time symmetric nanoscale system. The demonstrated plasmonic metasurface breaks the Lorentz reciprocity law at the nanoscale when its nonlinear response is considered, mainly due to its spatially asymmetric geometry combined with enhanced nonlinearity. Strong and broadband nonreciprocity with unity transmission contrast from opposite directions is realized under relatively low-input laser intensity values. Furthermore, asymmetric nonlinear third-harmonic generation with a pronounced contrast is also illustrated by the same nonreciprocal metasurface design. The findings of this work can lead to the realization of various self-induced nonreciprocal nanophotonic configurations, such as integrated ultrafast switches, ultrathin protective coatings, and asymmetric directional imaging devices.

Dispersion-engineered broadband diffractive optical elements with multilayer subwavelength structures.

Diffractive optical elements (DOEs) play an important role in modern optical applications such as spectral and imaging systems, but it is challenging to balance the diffraction efficiency with the working bandwidth. The core issue is controlling the broadband dispersion of all phase units to achieve achromatic 2π-phase modulation in the broadband domain. Here, we demonstrate broadband DOEs utilizing multilayer subwavelength structures with different materials, making it possible to freely control the phase and phase dispersion of the structural units on a much larger scale than monolayer structures. The desired dispersion-control abilities arose due to a dispersion-cooperation mechanism and vertical mode-coupling effects between the top and bottom layers. An infrared design comprised of two vertically concatenated T i O 2 and Si nanoantennas separated by a S i O 2 dielectric spacer layer was demonstrated. It showed an average efficiency of over 70% in the three-octave bandwidth. This work shows enormous value for broadband optical systems with DOEs such as spectral imaging and augmented reality.

Reciprocal polariton-induced transparency in alpha-molybdenum trioxide-graphene heteronanostructures

An ultrathin heterogeneous nanostructure consisting of periodic orthorhombic-phase molybdenum trioxide (α-MoO3), graphene nanoribbons, and a dielectric spacer layer has been proposed. Coupling between a hyperbolic phonon polariton from α-MoO3 and a surface plasmon polariton from graphene leads to the presence of reciprocal polariton-induced transparency (RPoIT). The near-field coupling is further evaluated by combining the current distribution values of the multipole decomposition. The RPoIT can be controlled by changing the structural parameters and tuning the chemical potential of graphene. Effects of the incidence angle and polarization angle have been investigated thoroughly from the finite-difference time-domain calculations. The potential applications in the refractive index sensor and mid-infrared (MIR) absorbers have also been investigated. This research provides a clear understanding and practical guidance for the realization of tunable RPoIT, which is desirable from the development of nano-devices and multifunctional hybrid polarization devices in the MIR region.

Tailor-made mesoporous SiO2/Au thin film with a substitutable interface for highly sensitive and selective room-temperature gas detection of VOCs

Trace amounts of volatile organic compounds (VOCs) were detected in Au chip interfaces modified with mesoporous SiO2 nanomaterials by using a surface plasmon resonance (SPR) technique. With Kretchmann-configuration SPR, it was possible to detect interactive and specific signal changes between a SiO2/Au gas-sensor surface and acetone, formaldehyde, and methane gases at room temperature. A dielectric spacing layer on Au was expanded and SPR signals were amplified by the mesoporous SiO2 nanoparticles with a large surface area and surface properties that were easily modified with specific ligands and became highly sensitive to, and selective for, the target gas molecules. A phosphonitrilic chloride trimer (PNCT), a universal self-assembled monolayer (SAM), and post-treatment for simple substitutions of ligands equipped the surfaces of SiO2 nanoparticles with targeted receptors, which resulted in the sensitive and selective detection of acetone to CH3-, formaldehyde to NH2-, and methane to Cl-functional groups. The chips modified with each functional group had a response time of about 2 s for concentrations of each gas that ranged from 0.2 to 3.0 ppm. The sensors modified with the functional groups showed limit of detection (LOD) values of 0.3, 0.6, and 0.5 ppm, respectively, which exceeded the Occupational Safety and Health Administration (OSHA) regulations, and limit of quantification (LOQ) values of 0.9, 1.8, and 1.6 ppm. Relative humidity of 45 ∼ 84 % under ambient air had negligible influence on the baseline signals in this SPR gas-sensing system. These sensors have the potential to monitor VOCs in both environmental and indoor settings.

Precise spectrophotometric method for semitransparent metallic thin-film index determination using interference enhancement

A precise spectrophotometric method to determine the refractive index of a semitransparent metallic thin film is presented. This method relies on interference enhancement of the measured spectra, employing an opaque substrate with a dielectric spacer layer beneath the absorbing layer of interest to create interference fringes.The resulting spectral oscillations of the stack are highly sensitive to the properties of the top absorbing layer, allowing precise determination of the refractive index via fitting. The performance of this method is verified using simulations in comparison to the typical method of depositing the absorbing thin film directly onto a transparent substrate. An experimental demonstration is made for titanium thin films over the visible range (370-835 nm). The refractive index of these films is extracted from experimental data using a combination of the Modified Drude and Forouhi-Bloomer models. This method showed high repeatability and precision, and is verified for Ti films between 6-70 nm thickness.

Phase-change-mediated coupling between phononic and photonic mode in the mid-infrared region

By coupling the surface phonon polaritons (SPhPs) of the aluminum nitride (AIN) layer with the germanium antimony tellurium alloy (Ge2Sb2Te5, GST) planar waveguide (PWG) modes, phonon-induced transparency (PhIT) and Fano resonance (FR) can be achieved. The Kretschmann configuration consists of a high-index prism, AIN layer, dielectric spacing layer and phase change materials (PCMs). During the phase change of Ge2Sb2Te5, the interactions between SPhP and PWG modes can be dramatically tuned, resulting in hybrid phonon-photon coupling and consequently splitting the resonance. FR can be excited and its characteristics strongly depend on the thickness of the spacing and WG layer. Moreover, the model of classical harmonic oscillator (CHO) is used to explain FR, and the mechanical calculations agree well with numerical electromagnetic (EM) numerical results.

A novel frequency selective structure with steady pass-band and broadband absorption properties

Design on integrated thin frequency selective surface (FSS) with broadband absorptive and transmissive characteristics is presented. The structure consists of two FSS layers (FSS I, FSS II), two dielectric spacer layers and a resistor FSS. The structure has the characteristics of broadband absorption, and low transmissive loss. In the case of normal incidence, the overall structure with 5.1 mm thickness exhibits a broad absorption band (reflection coefficient below −20 dB) in the frequency range 8.6–15.1 GHz, and the transmission band appears at 3.0 GHz with an insertion loss 0.71 dB. In the case of oblique incidence, it has a stable response till 45° angle, and the absorption rate is more than 90%. The proposed structure is considered as a good absorptive and transmissive material.

Optimisation of dielectric spacer layer thickness in Ag nanospheres/ITO/c-Si structure for plasmonic solar cells using FDTD simulation

ABSTRACT The thickness of dielectric spacer layer (DSL) plays an important role in performance of plasmonic solar cells. In this work, effect of thickness variation of ITO (indium tin oxide) DSL in silver (Ag) nanospheres/ITO/crystalline silicon(c-Si) structure on forward and backward scattering efficiencies has been investigated. Simulations were carried out using the open-source software MEEP via FDTD method for Ag nanospheres of sizes 50nm, 80nm and 100nm. Maximum forward scattering was observed with 80nm thickness of ITO DSL for all sizes of Ag nanospheres. Transmittance at ITO/c-Si interface and spatial distribution of electric field have been investigated for optimised thickness of ITO DSL. In visible to near infra-red region, maximum transmittance was exhibited by 100nm Ag nanosphere. Enhanced electric field has been observed with increasing size of nanosphere. This study provides us an optimum value of ITO DSL thickness in Ag nanospheres/ITO/c-Si structure to fabricate a photovoltaic device with upgraded efficiency.

Ultra-Thin Broadband Transmission FSS for Linear Polarization Rotation

Frequency-selective surfaces (FSSs), have various applications in microwave electromagnetics. This paper reports a solution to the current FSS challenges of flexibly, low profile, simple fabrication and polarization control using a novel structure operating across X and Ku frequency bands where a linearly polarized wave is rotated by 90°. The FSSs were fabricated by laser engraving a thin layer of $5\mu \text{m}$ aluminum on a $65~\mu \text{m}$ Mylar substrate with a relative permittivity of 2.7, and separated by a dielectric spacing layer of 0.9 mm polypropylene substrate, with a relative permittivity of 3. The co and cross-polarized reflection and transmission response of the structure was investigated using numerical modeling and was measured experimentally. A parametric study was also conducted focusing on key performance indicators, and specifically the bandwidth of the structure. The novelty of this polarization rotation structure lies in its ultra-thin profile ( $0.034~\lambda _{0}$ ), flexibility and significant transmission bandwidth. The fabricated prototypes experimental results were in good agreement with the simulated results, with a simulated −6 dB bandwidth of 61% and a measured −6dB bandwidth of 60%. Applications include antenna radomes where polarization is particularly important, as well as other polarization filtering applications which require a conformal low profile structure.

Design, simulation, and analysis of an ultra-broadband polarization-insensitive terahertz metamaterial absorber

A novel ultra-broadband polarization-insensitive metamaterial absorber is presented in this paper, which can work in the terahertz (THz) regime. By stacking two layers with a certain proportion separated by dielectric spacing layers, on which six nested rings of different sizes and other metal resonators are arranged, the metamaterial absorber for a TE wave can achieve absorption of over 90% at 2.28–5.58 THz, and that for the TM wave is located at 2.3–5.82 THz, whose relative bandwidths (RBWs) are 82.5% and 88%, respectively, which is shown in the simulation results. Moreover, the absorption performance of such an absorber remains stable at 2.3–5.58 THz, whose RBW is 82% when the wave is incident with different polarization angles, which implies that the metamaterial absorber has insensitivity towards polarization. The absorption mechanism for this absorber originates from the Fabry–Perot resonances among different layers and the magnetic resonances among the metal resonators, based on analysis of the distributions of the power loss density, surface current, and magnetic fields. The metamaterial absorber presented in this work is promising for application owing to its broad absorption band and insensitivity towards polarization.

Performance enhancement due to a top dielectric coating on a metamaterial perfect absorber.

A tri-layer metamaterial structure with enhanced absorption is demonstrated at infrared wavelengths by coating the top surface of the metamaterial absorber with an additional thin layer of dielectric material. The metamaterial absorber, which consists of a micrometer-sized metallic circular patch separated from a metal ground plane by a dielectric spacer layer, when coated with a supplementary protective dielectric layer on the top, shows a spectral red shift of the peak absorption along with a change in the absorption amplitude. The increase or decrease in absorption arises basically from an interference phenomenon of light reflected from the surface of the protective dielectric and the surface of metamaterial structures, and is highly dependent on the thickness of the top dielectric layer. The protective dielectric coatings provide an alternative way to modify and optimize the absorption in a metamaterial absorber along with a robustness that protects metamaterial structures from environmental and mechanical degradation.

Critical role of device geometry for the phase diagram of twisted bilayer graphene

The effective interaction between electrons in two-dimensional materials can be modified by their environment, enabling control of electronic correlations and phases. Here, we study the dependence of electronic correlations in twisted bilayer graphene (tBLG) on the separation to the metallic gate(s) in two device configurations. Using an atomistic tight-binding model, we determine the Hubbard parameters of the flat bands as a function of gate separation, taking into account the screening from the metallic gate(s), the dielectric spacer layers and the tBLG itself. We determine the critical gate separation at which the Hubbard parameters become smaller than the critical value required for a transition from a correlated insulator state to a (semi-)metallic phase. We show how this critical gate separation depends on twist angle, doping and the device configuration. These calculations may help rationalise the reported differences between recent measurements of tBLG's phase diagram and suggests that correlated insulator states can be screened out in devices with thin dielectric layers.

Moth-eye shaped on-demand broadband and switchable perfect absorbers based on vanadium dioxide

Two biomimetic, moth-eye structure, perfect absorbers in the visible and near infrared regions are introduced and investigated. The moth-eye structure is made up of vanadium oxide (VO2), which is a phase change material that changes from an insulator state to a metallic state at around 85 °C. The VO2 structure sits on top of a sapphire (Al2O3) dielectric spacer layer, above a gold (Au) back reflector. Two perfect absorbers are designed, one with perfect absorption over an ultra-broadband range between 400 and 1,600 nm, for both the insulating and metallic phases, while the second can switch between being a perfect absorber or not in the range 1,000 and 1,600 nm. The absorption profiles and electric and magnetic fields are examined and discussed to provide insight into how absorbers function in the four different situations.

Multi-resonance and ultra-wideband terahertz metasurface absorber based on micro-template-assisted self-assembly method.

As a promising platform for multi-functional terahertz devices, metasurface absorbers have received widespread attention in recent years. However, due to the existence of manufacturing difficulties, high cost, fragility, single or narrow absorption and other disadvantages, their application ranges are severely limited. Therefore, to effectively solve these problems, we have designed a flexible and high-precision terahertz metasurface absorber based on the micro-template assisted self-assembly method. Free from high cost, complicated process and time-consumption, the sandwich structure terahertz metasurface absorber consisting of a ceramic microspheres layer, a dielectric spacer layer, and a metal copper film is fabricated economically. On the one hand, through assembling the microspheres on the dielectric spacer in a periodic pattern arrangement, multiple resonances can be observed with a maximum absorption rate of up to 92.5% at 0.745 THz and are insensitive to the polarization of incident light. On the other hand, by attaching the microspheres to the dielectric layer in a compact configuration, 90% absorption bandwidth beyond 1.2 THz can be observed with a central frequency of 1.8 THz. The theoretical model of multiple reflection and interference is employed to explain these absorption characteristics. Considering the flexible design and high-throughput manufacturing processes, this work provides a promising platform for the development of high-efficiency and multi-functional terahertz devices.

Strong coupling between metasurface based Tamm plasmon microcavity and exciton

In this paper, the Tamm plasmon and its interaction with excitons in a plasmon microcavity consisting of metasurface, dielectric spacer, distributed Bragg reflector (DBR) are studied. The reflection phase of light on the surface can be controlled by changing the structure parameters in the metasurface. When the thickness of the dielectric spacer layer of the microcavity structure keeps unchanged, the resonance position of the Tamm plasmon mode supported by the microcavity structure can be adjusted by varying the structure parameters of the metasurface, and thus providing more degrees of freedom for regulating the Tamm plasmon mode. In addition, by comparing the traditional metal thin film-dielectric spacer-DBR structure, we find that the introduction of metasurface and its regulation of reflection phase can make the metasurface-dielectric spacer-DBR structure support the Tamm plasmon mode resonance at the same wavelength under a smaller thickness of spacer. And combining the local characteristics of the super-surface field, the model volume of Tamm plasmon can be reduced effectively. On this basis, we compare the interaction of traditional and metasurface-based Tamm plasmon with single-layer tungsten disulfide (WS<sub>2</sub>), and find that metasurface-based Tamm plasmon can produce stronger photon-exciton coupling and obtain larger Rabi splitting.