Normal Incidence Research Articles

Theoretical and numerical investigation of an ultra-broadband near-perfect absorption in auxetic metamaterials

In this paper, we present and numerically investigate an auxetic metamaterial absorber based on vanadium dioxide (VO2), achieving more than 90% absorption of the incident terahertz (THz) waves between 4.15 THz to 8.43 THz with an average absorption of 98.4%. To our knowledge, the absorption bandwidth is higher than previously reported VO2-based absorbers. The proposed absorber contains two VO2 based resonator rings placed diagonally at the top of the dielectric substrate in such a way as to create an auxetic shape and provide more design space than the existing absorbers. Considering the vector nature of electromagnetic fields in three-dimensional space, numerical analysis is performed while keeping the phase-changing material VO2 in the metallic state. According to the full wave simulation, it is shown that under normal incidence, the proposed absorber provides almost flat and near-unity absorption, which covers from 4.82 THz to 7.53 THz. We describe the physical mechanism of the absorber through impedance matching theory, and the absorber performance is evaluated by observing the electric field distributions at various frequencies. The proposed structure also exhibits a satisfactory tunable range from 2% to 100%, which may satisfy the requirements of re-configurable metamaterial absorbers. We also show that the absorber provides better wide-angle absorption performance than the existing VO2-based models for both transverse polarization i.e., transverse electric (TE) and transverse magnetic (TM) modes. Owing to the higher absorption bandwidth and better tunable range, the proposed auxetic metamaterial has great potential applications in THz imaging, detectors, and sensing.

Sensitivity enhancement of guided mode resonance sensors under oblique incidence

The sensitivity of guided mode resonance (GMR) sensors is significantly enhanced under oblique incidence. Here in this work, we developed a simplified theoretical model to provide analytical solutions and reveal the mechanism of sensitivity enhancement. We found that the sensitivity under oblique incidence consists of two contributions, the grating sensitivity and waveguide sensitivity, while under normal incidence, only waveguide sensitivity exists. When the two contributions are constructively superposed, as in the case of positive first order diffraction of the grating, the total sensitivity is enhanced. On the other hand, when the two parts are destructively superposed, as in the case of negative first order diffraction, the total sensitivity decreases. The findings are further supported by FDTD numerical calculations and proof-of-concept experiments.

Low-voltage and high-contrast polymer dispersed liquid crystals enabled by ferroelectric fluoro-copolymer doped polyimide film

Smart windows incorporating polymer dispersed liquid crystal (PDLC) technology provide user-friendly operation and play a significant role in intelligent buildings and building energy conservation. Long standing issues of the PDLCs such as high driving voltage, low contrast ratio, and narrow viewing angle hinder their applications. Here PDLCs are sandwiched between the hybrid substrates composed of the ferroelectric polyvinylidene fluoride trifluoroethylene (P(VDF-TrFE)) copolymer and the homogeneous polyimide. The high dielectric constant and ion-trapping ability of ferroelectric copolymers increase the effective voltage drop in the PDLCs. Concurrently, the local electric field created by large dipole moment of ferroelectric copolymers triggers the alignment of liquid crystals (LCs) along the applied field. These synergistic effects contribute to a substantial decrease in the driving voltage of PDLCs. In the opaque state, the random dipole moments in ferroelectric copolymers disrupt the LC orientations near hybrid substrates, thereby increasing the light scattering and reducing the transmission of PDLCs. Consequently, the proposed PDLCs exhibit a contrast ratio ∼2× higher compared to the conventional PDLCs at normal incident. Moreover, the proposed PDLCs demonstrate a wider viewing angle in the transparent state compared to conventional PDLCs. This development promises energy-efficient, eco-friendly, and cost-effective smart windows.

Hierarchical meta-porous materials as sound absorbers

The absorption of sound has great significance in many scientific and engineering applications, from room acoustics to noise mitigation. In this context, porous materials have emerged as a viable solution towards high absorption performance and lightweight designs. However, their performance is somewhat limited in the low-frequency regime. Inspired by the concept of recursive patterns over multiple length scales, typical of many natural materials, here, we propose a hierarchical organization of multilayered porous media and investigate their performance in terms of sound absorption. Two types of designs are considered: a hierarchical periodic (HP) and a hierarchical gradient (HG). In both cases, it is found that in some frequency ranges the introduction of multiple levels of hierarchy simultaneously allows for: (i) an increase in the level of absorption compared with the corresponding bulk block of porous material (or to the previous hierarchical levels (HLs)), and (ii) a reduction in the quantity of porous material required. Another advantage of using this approach is on the fabrication, since only one porous material is required. The performances are examined for both normal and oblique incidences, as well as for different values of the static air-flow resistivity. The methodological approach is based on the transfer-matrix method, optimization algorithms such as the metaheuristic greedy randomized adaptive search procedure (GRASP), finite-element calculations and measurements performed in an impedance tube.

Tunable Graphene‐Based Absorber Using Nanoscale Grooved Metal Film at Telecommunication Wavelengths

Graphene‐based absorbers have various modern applications across industries due to their exceptional properties. Some common applications include: thermal management and energy storage. Herein, the design and simulation of a broadband tunable absorber based on graphene with perfect absorption spectra in the near‐infrared region are reported. The proposed structure consists of an MgF2 layer and golden disc surrounded by L‐shaped golden arms placed on single layer of graphene. The structure guarantees polarization‐insensitive (PI) performance under normal incident due to the symmetrical design. The investigation of the PI of the structure reveals almost similar absorption for oblique incident angles up to 55° for TM and up to 60° for TE polarization. The desirable resonance wavelength is achievable by tuning the geometrical parameters. By changing the chemical potential of graphene, the absorption and bandwidth of absorber are controllable. A full width at half maximum of 330 nm is another superiority of this absorber. These considerable aspects of the proposed structure make it practical for varieties of applications such as cloaking, sensing, switching, and so on.

Electromagnetic Response of a Uniformly Moving Resistive Sheet

ABSTRACTBy using the finite‐difference time‐domain (FDTD) method and a two‐port network model, this letter investigates the response of a uniformly moving resistive sheet to illuminating electromagnetic plane waves, at normal incidence. The numerical Doppler frequency shifts and scattering matrix are matched with curve‐fitting expressions. The effects of motion on the impedance matrix, the chain matrix, and the continuity equations are analyzed. Moreover, the total energy of the transferred and reflected waves are studied for the cases of a resistive sheet moving toward or away from the plane wave source.

Graphene pixel based broadband polarization insensitive THz absorber

Abstract Suppressing undesired radiation and eliminating stray radiation in a high-frequency circuit might be challenging. Ultra-high frequency (UHF) electromagnetic wave absorbers are finding new applications in both the military and civilian domains. Obtaining an ultra-wide absorption band in a thin, single-layer Terahertz absorber becomes difficult. Also, achieving perfect absorption bandwidth on transverse electric (TE) and transverse magnetic (TM) mode is very sensitive. This paper presents a polarization-independent graphene-based absorbers with a near unity absorption band of 2.32 THz and 3.78 THz in a lower mid-infrared regime. The structure’s unit cell consists of cross-slot circular patches with Au as a ground separated by SiO2. To improve the absorption band another graphene layer was placed between the top and dielectric material. The absorber is polarization independent under normal incidence because of the deposited graphene structure’s fourfold symmetry. Both TE and TM polarizations were investigated, with wide-band absorption observed up to 60° incident angles in both cases. In terms of the lowest frequency of absorption, the prototype is deemed to be as thin as ∼ 0.1 λ max for both absorbers. Similarly, the effective homogeneity criteria in the subwavelength region are supported by the unit cell’s period of ∼ 0.1 λ max . The proposed absorber-2 shows 10 % higher FBW than existing work.

Selective Plasmonic Responses of Chiral Metamirrors

The properties of circularly polarized light has recently been used to selectively reflect chiral metasurfaces. Here we report the more complete basic functionalities of reflectors and absorbers that display various optical phenomena under circularly polarized light at normal incidence as before. For the chiral metamirrors we designed, the circular dichroism in about 0.4 reflection is experimentally observed in visible wavelengths. The experimental results also show high reflectance for right-handed circular polarization with preserved handedness and strongly absorbed left-handed circular polarization at chiroptical resonant wavelengths. By combining a nanobrick and wire grating for our design, we find and offer a new structure to demonstrate the superposition concept of the phase in the same plane that is helpful in effectively designing chiral metamirrors, and could advance development of their ultracompact optical components.

A novel ultra-wideband rasorber based on triple lossy layers

Abstract By surpassing the limitations of transmission efficiency found in traditional double-lossy-layers rasorbers, a novel rasorber with three distinct lossy absorption layers is presented, offering low insertion loss and ultra-wideband bandwidth. Compared to conventional rasorbers which have non-negligible insertion loss and large thickness, our design extends the bandwidth in the higher frequency band without increasing thickness, while maintaining high transmission efficiency. To further broaden the absorption band, an additional lossy layer resonating at lower frequency band, with high transmission above the resonance frequency was incorporated into the previous design. The composite structure is developed by the equivalent circuit model (ECM) whose results are in accord well with the simulation results. The simulated results demonstrate that an insertion loss of 0.37 dB at 10 GHz, and a 159.5% 10 dB reflection reduction bandwidth from 2.04 to 18.05 GHz are attained under normal incidence. A manufactured specimen of the proposed structure is measured to validate our design.

Gradient metasurface covered with water-based channels for reconfigurable RCS reduction

A gradient metasurface covered with water-based channels (GMWC) is proposed for RCS (radar cross section) reduction reconfiguration. The amplitude and phase response of the metasurface unit covered by the water-based channel (WC) is altered, which results in a change in the monostatic scattering of the GMWC. Under the normal incidence of X-polarized plane waves, the RCS reduction of GMWC shows different levels when different water channels are activated. The simulated results show that the average RCS reduction of GMWC increases from 5 dB to 20 dB in 5 dB steps in the 6 GHz to 8.2 GHz frequency band under four different reconstruction states (10000;01000;00100;00001). Besides, GMWC has an average RCS reduction of 10 dB within UWB (4–18 GHz) under state 10000. Finally, GMWC is fabricated and measured, and the measured results are in agreement with the simulated results. The RCS reduction reconfiguration characteristic of GMWC has potential application in target camouflage.

Circular photocurrents in centrosymmetric semiconductors with hidden spin polarization

Centrosymmetric materials with site inversion asymmetries possess hidden spin polarization, which remains challenging to be converted into spin currents because the global inversion symmetry is still conserved. This study demonstrates the spin-polarized circular photocurrents in centrosymmetric transition metal dichalcogenide semiconductors at normal incidence without applying electric bias. The global inversion symmetry is broken by using a spatially-varying circularly polarized light beam, which could generate spin gradient owing to the hidden spin polarization. The dependence of the circular photocurrents on electrode configuration, illumination position, and beam spot size indicates an emergence of circulating electric current under spatially inhomogeneous light, which is associated with the deflection of spin-polarized current through the inverse spin Hall effect. The circular photocurrents is subsequently utilized to probe the spin polarization and the inverse spin Hall effect under different excitation wavelengths and temperatures. The results of this study demonstrate the feasibility of using centrosymmetric materials with hidden spin polarization and non-vanishing Berry curvature for spintronic device applications.

Holographic Multi-Notch Filters Recorded with Simultaneous Double-Exposure Contact Mirror-Based Method

This study presents a novel simultaneous double-exposure contact mirror-based method for fabricating holographic multi-notch filters with dual operational central wavelengths. The proposed method leverages coupled wave theory, the geometric relationships of K-vectors, and a reflection-type recording setup, incorporating additional reflecting mirrors to guide the recording beams. To validate the approach, a holographic notch filter was fabricated using photopolymer recording materials, resulting in operational wavelengths of 531.13 nm and 633.01 nm. The measured diffraction efficiencies at these wavelengths were ηs = 52.35% and ηp = 52.45% for 531.13 nm, and ηs = 67.30% and ηp = 67.40% for 633.01 nm. The component’s performance was analyzed using s- and p-polarized spectral transmission intensities at various reconstruction angles, revealing polarization-independent characteristics under normal incidence and polarization-dependent behavior under oblique incidence. The study also explored the relationships between recording parameters, such as incident angle, wavelength, emulsion expansion, and dispersion. The findings demonstrate that the first operational central wavelength is primarily influenced by the recording wavelength, while the second is primarily determined by the incident angle, covering a range from visible light to near-infrared. This method offers significant potential for cost-effective, mass-produced filters in optoelectronic applications.

Bifunctional terahertz metamaterial device with switchable properties between transmission and broadband absorption

A bifunctional terahertz metamaterial device with switchable properties between transmission and broadband absorption is proposed. The simulated results show that the switchable functional characteristics of the bifunctional terahertz metamaterial device can be achieved by taking advantage of the phase transition property of VO2. When VO2 is in the insulating state, there are transmission spectra with a maximum transmittance of 90 % and a minimum transmittance of 25 % in the frequency range from 1 THz to 10 THz. Meanwhile, transmission spectra be adjusted by controlling the Fermi level of graphene. When VO2 is in the fully metal state, the broadband absorptivity achieves over 90 % in the frequency range from 2.54 THz to 7.65 THz. Not only that, the absorption spectra can be continuously adjusted by controlling the conductivity of VO2 from 20 to 200000 S/m. Alternatively, the proposed bifunctional terahertz metamaterial device can work and show the same absorption spectra when the conductivity of VO2 is 200000 S/m under TM and TE polarized normal incidences. Our current research work has a potential to provide a valuable reference for the advancement of transmissive and broadband absorbent metamaterial devices in the terahertz range.

Design and architecting of a broadband bullet-diamond shaped integrated frequency selective meta-surface absorber for aero-stealth platform.

The work report on architecture of integrated frequency selective meta-surface (IFSMS) absorbers for aerospace stealth applications. Fabricated IFSMS comprised of a pattern metasurface integrated with dielectric interlayer and conducting ground. Initially, a supercell (2 × 2-unit cell: 24 × 24 mm2) was designed with a fourfold topological symmetry. Supercell produces impedances (R), inductances (L), and capacitances (C) in tune with design on its interaction with microwave. RC performance was tested at variable incident transverse electric/magnetic (TE/TM) modes over, Θ, 0°-60° and at the normal incidence (TE), against a planer, clockwise rotation over, Φ, 0°-90°. The mode stability and rotational invariance was analyzed for displacement current- and power-density distributions. The impedance behavior and phase reversal S11 reflection coefficient studies revealed the emergence of mid-band Fabry-Perot mode distinguishing LC behavior of the circuit. The meta-pattern was manufactured by mask lithography using a customized resistive micro-carbon ink and imprinted onto dielectric/ground tile (dimension: 30 × 30 cm2). Structure-property relationship of the ink material was investigated using SEM, XRD, FTIR, UV-visible spectroscopy to reveled surface properties of imprinted material. The absorber was subjected to the free space measurements over C (4-8), X (8-12), and Ku (12-18GHz) bands, including pristine interlayer dielectrics. The simulated and experimental RC data was found to be in excellent agreement. The proposed IFSMS design is a potential candidate for the stealth application.

Wide angle wide band polarization insensitive metamaterial absorber for C, X and Ku band application with hexagonal packaging

Abstract In this paper wide angle wide band polarization insensitive Metamaterial absorber (MA) has been proposed. The proposed unit cell structure consists of hexagonal concentric ring along with hexagonal packing. The structure is fabricated on FR4 substrate. The advantage of hexagonal packing is that it covers less area as compared to square packing. The unit cell structure is scaled w.r.t hexagonal ring and resistance of the substrate. The absorptivity achieved range from 6.54 to 14.85 GHz having Bandwidth (BW) of 8.31 GHz. The metamaterial behaviour of the structure is proven by plotting real and imaginary pat of permittivity and permeability. Absorption mechanism is shown by plotting surface current distribution at the top and bottom of the surface. The proposed MA structure is analysed for normal and oblique incidence. From the normal incidence it was found that structure has wide angle polarization insensitive. The absorptivity of the fabricated structure is measured inside the anechoic chamber. The simulated and measured plot is compared and found that both the results are very close to each other. At last, the proposed and already reported MAs are compared and from the observation it was found that proposed unit cell have larger BW compact in size, polarization insensitive and have hexagonal packing. The proposed MA covers partial C band, full X band and partial Ku band. The Proposed MA found practical applications for satellite communication, Wi-Fi devices, cordless telephone, military, air traffic control, defence tracking, vehicle speed detection.

Direct extraction of optical constants of organic semiconductors in ultrastrongly coupled microcavities and their application in antireflection design for polariton devices

The strongly bound Frenkel excitons in organic semiconductors enable strong or even ultrastrong exciton-photon coupling in room-temperature cavities, with the resulting polariton states typically resolved through reflectance measurements. This paper demonstrates that the distinct features of exciton and polariton modes in the reflectance spectra of strongly/ultrastrongly coupled organic microcavities can be effectively utilized to extract the optical constants and physical thickness of the embedded organic semiconductor. We investigate metal-clad microcavities based on two prototype conjugated polymers, poly[2-methoxy-5-(3,7-dimethyloctyloxy)-1,4-phenylenevinylene] (MDMO-PPV) and poly(3-hexylthiophene) (P3HT), both exhibiting ultrastrong coupling characteristics. The (n,k) spectra and thickness of these polymer films are determined by fitting the normal incidence reflectance spectra of organic microcavities, using Kramers-Kronig transformation and transfer-matrix calculations with varying optical and thickness parameters. We also examine the individual effects of the main fitting parameters on the spectrum, establishing a close correlation with the underlying polariton properties. Moreover, we analyze the optical admittance at exciton and polariton modes to understand reflectance variations with different parameters, which facilitates precise control of optical properties at specific modes through cavity design. Finally, using the extracted optical constants of MDMO-PPV and P3HT, we propose optimized microcavity designs that exhibit antireflection at the lower polariton mode for potential luminescence and photodetection device applications.

Experimental validity of using a ResNet to predict sound absorption coefficients of finite samples

The validity of using a neural network to predict sound absorption coefficients of finite porous materials is tested with experimental data with a flush-mounted glass wool sample on a baffle. The network is pre-trained and validated with numerical simulations of flushed-mounted finite absorbers using a Delany-Bazley-Miki model. The experimental setup consists of a 12 x 12 microphone array placed above the absorber and a sound source placed at angles of 0, 40, and 75 degrees with respect to the normal of the sample. The sound absorption coefficients predicted at normal incidence by the network are compared with the impedance tube method as a reference result.

A vibroacoustic model for low-frequency sound transmission loss in circular membranes with added strip mass

Noise control strategies involve physical barriers in the transmission path. Conventional barriers conform to the mass law which states an inverse relation between sound transmission, barrier mass, and incident frequency. Practical scenarios for low-frequency noise attenuation require prohibitively thick barriers making them infeasible. Mass-loaded membranes exhibit marked differences in their dynamic nature from that of bare membranes as they do not necessarily conform to the mass law. Being thin, compact, and lightweight makes them geometrically ideal for most applications. The paper presents an analytical model based on the Newtonian approach for the vibroacoustic behavior of a pre-stretched elastic circular membrane with rigidly attached strip mass under normal incidence. The point collocation approach distributes the effect of the strip mass on the membrane as a collection of discretized concentrated loads along the interfacial boundary resulting in a summation that is easier to solve. The peak and dip frequencies of sound transmission are determined for circular membranes with eccentric and central strip mass. The present study evaluates strip mass-loaded membranes' capability to attenuate noise at low frequencies as an unconventional physical barrier.

Investigating the acoustic performance of sustainable composites utilizing recycled polypropylene and denim shoddy: fabrication, characterization, and theoretical Modelling

Addressing the urgent need for sustainable materials, this study investigates the development of eco-friendly composites using Recycled Polypropylene (RPP) and Denim Shoddy (DS) for sound insulation. Two configurations were fabricated through compression molding: one with only RPP and the other incorporating a DS interlayer, repurposing waste materials such as post-consumer cosmetic containers and denim textile waste. The investigation evaluated the composites' density and sound transmission loss (STL). Theoretical calculations of STL in diffuse fields across various angles assessed the composites' real-world applicability. Incorporating a DS interlayer significantly improved sound insulation properties, with the composite exhibiting a higher density of 916 kg/m³ compared to 891 kg/m³ for Neat RPP. STL measurements showed that the composite achieved approximately average STL of 39.0 dB under normal incidence and 30.8 dB in the diffuse field, compared to 37.7 dB and 30.6 dB for Neat RPP, indicating a positive correlation between increased density and enhanced sound insulation. These findings highlight the potential of using recycled and waste materials for eco-friendly, sound-insulating material. The developed materials show promising applications in highway noise barriers, generator insulation, and office cabin partitions. This research contributes to sustainable materials science by demonstrating innovative sound-insulating composites' environmental and practical benefits.

Experimental observation of super-Klein tunneling in phononic crystals

We numerically and experimentally report the acoustic analogue of the super-Klein tunneling in a heterojunction of phononic crystals formed with Willis scatterers that exhibit pseudospin-1 Dirac cones. Pseudospin-1 Dirac cones are characterized by a triple states degeneracy in the band structure where two linear bands intersect with a flatband. The scattering properties of pseudospin-1 quasiparticles comprise an unity transmission through one potential barrier at its center frequency for all incident angles, which is called the super-Klein tunneling. In addition, the unity transmission does not depend on the barrier width. The Super-Klein tunneling differs from the conventional Klein tunneling, which consists of perfect transmission only under normal incidence through a potential barrier of any width. The Klein tunneling is predicted in pseudospin-1/2 systems like graphene, which are characterized by a two states degeneracy with two intersecting linear band. This direct observation in this acoustic analogue may have important implications in the exploration of the rich physics of pseudospin-1 quasiparticles.