Lossy Layer Research Articles

Absorption and Scattering by a Temporally Switched Lossy Layer: Going beyond the Rozanov Bound

In this paper we study the electromagnetic scattering, absorption, and performance bounds for short time modulated pulses that impinge on a time-varying lossy layer that is sandwiched between vacuum and a perfect electric conductor. The electric characteristics of the layer, namely, the conductivity, permittivity, and permeability, are assumed to change abruptly or gradually in time. We demonstrate numerically that a time-varying absorbing layer that undergoes temporal switching of its permittivity and conductance can absorb the power of a modulated, ultrawideband, as well as a quasimonochromatic, pulsed wave beyond what is dictated by the time-invariant Rozanov bound when integrating over the whole frequency spectrum. We suggest and simulate a practical metamaterial realization that is constructed as a three-dimensional array of resistor loaded dipoles. By switching only the dipole's load resistance, desired effective media properties are obtained. Furthermore, we show that Rozanov's bound can be bypassed with abrupt and a more practical gradual, soft, switching, thus overcoming some possible causality issues in abrupt switching.

Wideband RCS Reduction of Slot Array Antenna Utilizing 3-D Absorptive Frequency-Selective Structure

A low radar cross-section (RCS) slot array antenna based on absorptive frequency-selective structure (AFSS) is presented in this letter. The AFSS unit cell is constructed by two lossy layers and four double-sided parallel strip lines (DSPSLs). The radiation performance of the antenna can be preserved due to the transmission window of the lossy layers. Wide absorption bands of the AFSS are achieved due to periodic impedance of the DSPSL and the combination of the lossy layers. The operating principle of the design is demonstrated, and a prototype is fabricated and measured to validate the design. The proposed design features both wide RCS reduction band of the antenna and wide transmission band of the AFSS.

Low-Profile and Dual-Polarization Water-Based Frequency Selective Rasorber With Ultrawideband Absorption

A low-profile and dual-polarization frequency selective rasorber (FSR) with an ultrawide absorption band is proposed. Based on the Mie resonance theory, water blocks with appropriate height are arranged as the lossy layer. The lossless layer is composed of a bandpass frequency-selective surface with Jerusalem-cross gaps. Utilizing the high loss of water, this rasorber can obtain ultrawideband absorption with a power absorptivity over 90% from 4.63 to 100 GHz, and the fractional bandwidth regarding the absorption band is over 182%. The insertion loss is 0.55 dB at 3 GHz under normal incidence and the thickness λ <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">_m</i> (λ <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">_m</i> being the free-space wavelength at the middle transmission frequency) is 0.077 λ <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">_m</i> . The FSR also shows favorable stability of the frequency response over a relatively broad range of oblique incidence angles of 30° for dual-polarization. Compared with formal water-based FSR, this letter places emphasis on the novelty of elimination of the transition band and extended length of absorption band. Ultimately, a low-cost prototype of the presented design is fabricated, simulated, and measured to verify the feasibility.

Low-RCS and Gain-Enhanced Antenna Using Absorptive/Transmissive Frequency Selective Structure

A gain-enhanced antenna consisting of a bandpass frequency selective surface (FSS) and a patch antenna is initially proposed in this communication. By utilizing the reflection characteristic of the FSS, a quasi-Fabry–Perot (F-P) cavity is formed between the FSS and antenna’s ground, which considerably improves the gain of the antenna. Meanwhile, the bandpass FSS has a little phase compensation for oblique transmitting waves, which also slightly enhances the gain. Then a low-radar cross section (RCS) and gain-enhanced antenna is proposed by using absorptive/transmissive frequency selective structure (ATFSS). It is implemented by mounting the ATFSS above a patch antenna. The ATFSS is constructed by stacking a lossy layer above a bandpass FSS. Both antennas are fabricated, assembled, and measured. They achieve a gain-enhancement of 5.27 and 3.84 dB, respectively. Besides, the low-RCS antenna achieves 10 dB monostatic RCS reduction from 5 to 5.33 GHz and from 9 to 13.4 GHz under the TE-polarization, and from 8.4 to 13.5 GHz under the TM-polarization, respectively. Simulated results show it also achieves a good improvement in bistatic RCS reduction as well.

Ultra-thin and near-unity selective emitter for efficient cooling.

For the efficient radiative cooling of objects, coolers should emit heat within atmospheric transparent window and block heat absorption from the surrounding environments. Thus, selective emitters enable highly efficient cooling via engineered photonic structures such as metamaterials and multi-stacking structures. However, these structures require sophisticated fabrication processes and large quantities of materials, which can restrict mass-production. This study introduces an ultra-thin (∼1 μm) and near-unity selective emitter (UNSE) within the atmospheric window, which can be fabricated using simple and affordable process. The combination of infrared (IR) lossy layers and high index lossless layer enhances the resonance in the structure thus, the emissivity in long wavelength IR region increases to near-unity within a thickness of ∼1 μm.

Design of a frequency selective rasorber with fast roll‐off and wide absorption/transmission band

A frequency selective rasorber (FSR) with fast roll-off and wide absorption/transmission band is introduced in this article, working as a band-pass filter from 1.7 to 3.1 GHz, while working as an absorber from 5.72 to 17 GHz. The proposed FSR is designed by the lossy layer at the top and the band-pass frequency selective surface (FSS) at the bottom. The lossy layer is constructed by two parts including a square loop and a series RLC structures to provide a wide absorption band. The band-pass FSS is a three-tier structure including two patch layers and one meander line layer in the middle. This multilayer FSS is used to generate a transmission window in S band and a wide reflection band in the high frequency. The working mechanism of the FSR is explained by the equivalent circuit model. Finally, the fabricated prototype and the measurement results are presented to verify the design of the FSR.

Broadband wide-angle multilayer absorber based on a broadband omnidirectional optical Tamm state.

Recently, broadband optical Tamm states (OTSs) in heterostructures composed of highly lossy metal layers and all-dielectric one-dimensional (1D) photonic crystals (PhCs) have been utilized to realize broadband absorption. However, as the incident angle increases, the broadband OTSs in such heterostructures shift towards shorter wavelengths along the PBGs in all-dielectric 1D PhCs, which strongly limits the bandwidths of wide-angle absorption. In this paper, we realize a broadband omnidirectional OTS in a heterostructure composed of a Cr layer and a 1D PhC containing layered hyperbolic metamaterials with an angle-insensitive photonic band gap. Assisted by the broadband omnidirectional OTS, broadband wide-angle absorption can be achieved. High absorptance (A > 0.85) can be remained when the wavelength ranges from 1612 nm to 2335 nm and the incident angle ranges from 0° to 70°. The bandwidth of wide-angle absorption (0°-70°) reaches 723 nm. The designed absorber is a lithography-free 1D structure, which can be easily fabricated under the current magnetron sputtering or electron-beam vacuum deposition technique. This broadband, wide-angle, and lithography-free absorber would possess potential applications in the design of photodetectors, solar thermophotovoltaic devices, gas analyzers, and cloaking devices.

On the instantaneous frequency and quality factor

SUMMARY We analyse the concepts of instantaneous frequency (IF) and quality factor (IQ). It is verified that the time-averaged IF is equal to the centroid of the signal energy of the spectrum and that the centroid of the signal spectrum is equal to the IF at the peak of the signal envelope. The latter property can be used to obtain the frequency shift required by tomographic methods. Then, we analyse the two-tone stationary Mandel signal in the lossless and lossy cases. The IQ is not infinite in the lossless case, although its reciprocal average vanishes, and the lossless and lossy IFs at the peak of the signal envelope are the same, whereas the IQ at this peak depends on the amplitudes and quality factors of the tones. The IQ of a propagating Ricker wavelet has a singularity at the peak of the envelope, which shows a shift in the lossy case, related to the velocity dispersion. We consider a lossy layer described by the Zener model. Varying its thickness implies a large variation in the IF, introducing unphysical spikes when the top and bottom reflections of the layer start to overlap. Finally, a practical application to real seismic data is presented.

A 2.5-D Miniaturized Frequency-Selective Rasorber With a Wide High-Transmission Passband

This letter presents a 2.5-D frequency-selective rasorber (FSR) with a wide high-transmission (|S <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">21</sub> | <; 1 dB) passband and two absorption bands. The proposed FSR consists of a 2.5-D lossy layer and a lossless layer, between which is an air gap. At the top layer, a 2.5-D convoluted resonator, which introduces large equivalent impedance to generate a wide high-transmission passband, is constructed in the center of lumped-resistor-loaded T-type metallic strips. A triple-layer bandpass frequency-selective surface is adopted as the lossless layer. Procedures to develop the accurate equivalent circuit model, aiming at providing physical insights into the operating mechanism of the proposed 2.5-D FSR, are discussed. The simulation results indicate that the proposed FSR has a 1 dB transmission band from 7.81 to 11.78 GHz with 40.53% relative bandwidth and 80% absorption bands covering 3.67-6.80 GHz and 13.74-15.31 GHz. A prototype of 2.5-D FSR is fabricated and measured to validate the design.

Low-Profile FSS-Based Polarization-Insensitive Rasorber With Switchable Transmission Band

In this letter, a new polarization-insensitive, low profile frequency-selective rasorber (FSR) is presented with a switchable transmission band. This FSR consists of a lossy layer, a bandpass layer, and a switchable layer where p-i-n diodes are mounted and parallel fed. The lossy layer is composed of compact fractal elements with lumped resistors, which miniaturizes the unit dimensions. The bandpass layer is composed of a grided square loop and the switchable layer is integrated by coaxial hybrid square loops with an embedded bias network, whose configuration reduces the impact of bias lines on high-frequency transmission and, consequently, achieves polarization insensitivity. A prototype was fabricated and measured, which verified that the 3 dB transmission window covers 8.9–9.8 GHz, and the −10 dB wideband absorption covers 2.3–8.2 GHz. Good agreement is obtained between the numerical simulation and measurement results.

Functionally Graded Tunable Microwave Absorber with Graphene-Augmented Alumina Nanofibers.

Graphene is currently attracting attention for radiation absorption particularly at gigahertz and terahertz frequencies. In this work, composites formed by graphene-augmented γ-Al2O3 nanofibers embedded into the α-Al2O3 matrix are tested for X-band absorption efficiency. Composites with 15 and 25 wt % of graphene fillers with shielding effectiveness (SE) of 38 and 45 dB, respectively, show a high reflection coefficient, while around the electrical percolation threshold (∼1 wt %), an SE of 10 dB was achieved. Furthermore, based on the dielectric data obtained for varying fractions of graphene-/γ-Al2O3-added fillers, a functionally graded multilayer is constructed to maximize the device efficiency. The fabricated multilayer offers the highest absorption efficiency of 99.99% at ∼9.6 GHz and a full X-band absorption of >90% employing five lossy layers of 1-3-5-15 and 25 wt % of graphene/γ-Al2O3 fillers. The results prove a remarkable potential of the fillers and various multilayer designs for broad-band and frequency-specific microwave absorbers.

Experimental and analytical investigations on a wide-angle, polarization-insensitive, and broadband water-based metamaterial absorber

In this work, a wide-angle, polarization-independent, and broadband superstrate-assisted water-based metamaterial absorber (SWMA) covering the whole X-band is theoretically and experimentally demonstrated. Our SWMA design is a copper-backed structure comprising a thin substrate of distilled water, an FR-4 lossy layer, and a magneto-electric anisotropic metamaterial to boost achieving broadband and wide-angle features. The absorptivity of the proposed SWMA has been elaborately assessed in a full analytical framework involving oblique illuminations and both transverse electric (TE) and transverse magnetic (TM) polarizations. Numerical results demonstrate that exploiting magneto-electric anisotropy, the impedance matching between air and SWMA has been remarkably improved for both major polarizations, especially at near grazing angles. Owing to the end-to-end analytical design, the designed SWMA does not suffer from the drawbacks associated with the traditional designs including intricate particle geometries and brute-force optimizations. As a proof of concept, the proposed SWMA is fabricated and its absorptivity is measured in an anechoic microwave chamber between 8 and 12 GHz. The experimental results depict good conformity with the numerical simulations and the theoretical predictions, elucidating that our design retains its strong absorptivity over the whole X-band and for a wide angular range up to near grazing angles for both TE- and TM-polarizations.

Multiband asymmetric sound absorber enabled by ultrasparse Mie resonators.

On the quest towards efficiently eliminating noises, the development of a subwavelength sound absorber with the capability of free ventilation remains challenging. Here, we theoretically propose and experimentally demonstrate an asymmetric metamaterial absorber constructed by tuned Mie resonators (MRs) with unbalanced intrinsic losses. The lossy MR layer is highly dissipative to consume the sound energy while the lossless one acts as an acoustically soft boundary. Thus, the absorber presents quasi-perfect absorption (95% in experiment) for sound waves incident from the port nearer the dissipative MR and large-amount reflection (71% in experiment) from the opposite port. Moreover, the fluid dynamics investigation confirms the superior character of free air circulation owing to the ultrasparsity (volume filling ratio as low as 5%) of the absorber and its robustness to the velocity of airflows. Due to the multiple-order resonant modes of MR, we further demonstrate the flexibility of a methodology to extend asymmetric absorptions into multibands. Coupled mode analysis is employed to reveal the physical mechanism and further indicates that sparsity can be tuned by attentively controlling the reference leakage factor and intrinsic loss.

Coherent perfect absorption in resonant materials

Coherent perfect absorption (CPA) is an interferometric effect that guarantees full absorption in a lossy layer independently of its intrinsic losses. To date, it has been observed only at a single wavelength or over narrow bandwidths, whereupon wavelength-dependent absorption can be ignored. Here we produce CPA over a bandwidth of ∼60 nm in a 2 µm thick polymer film with a low-doping concentration of an organic laser dye. A planar cavity is designed with a spectral ‘dip’ to accommodate the dye resonant linewidth, and CPA is thus achieved even at its absorption edges. This approach allows realizing strong absorption in laser dyes—and resonant materials in general—independently of the intrinsic absorption levels, with a flat spectral profile and without suffering absorption quenching due to high doping levels.

Closed-Form Analytical Design of Optically Transparent Wideband Absorbers for 5G Technology

The design of new transparent absorbers for the upcoming fifth-generation (5G) technology is investigated. The absorbers consist of a matching layer, a lossy sheet, a spacer, and a back conducting layer (BCL). Both the matching layer and the spacer are one-quarter-wavelength thick and made of polyethylene terephthalate (PET). The lossy sheet and the BCL consist of indium tin oxide (ITO) thin films or graphene-PET laminates in order to avoid the use of any metal and to guarantee the optical transmittance. An innovative closed-form formulation is developed with the aim of evaluating the optimal values of sheet resistances of the lossy layers through analytical expressions. The absorption performances are investigated considering impinging plane waves with an incidence angle between 0° and 30°. The computed absorption coefficients in transverse magnetic (TM) or transverse electric (TE) modes are greater than 0.8, and the transmission coefficient is lower than 0.1 in the 5G frequency range from 25 up to 47 GHz, assuming that the matching layer and the spacer are 1.3 mm thick. The optical transmittance of the absorbers is evaluated in the wavelength range from 400 up to 700 nm by means of an accurate matrix simulation model, considering both optical polarizations, S (i.e., TE) and P (i.e., TM) for different incidence angles. The computed average optical transmittances of the designed absorbers are greater than 80% at 550 nm, i.e., at the wavelength corresponding to the maximum eye sensitivity.

Switching between singular points in non-PT-symmetric multilayer structures using phase-change materials

We investigate the switching between singular points in non-parity-time-symmetric multilayer structures using phase-change materials at the optical communication wavelength. We first show that absorbing singularities can be switched to exceptional points (EPs) in a two-layer structure consisting of a phase-change material layer and a lossy layer by switching the phase-change material Ge2Sb2Te5 (GST) from its crystalline to its amorphous phase. We also show that spectral singularities (SSs) can be switched to EPs in a three-layer structure consisting of a lossless dielectric layer sandwiched between a GST layer and a gain layer by switching the GST from its crystalline to its amorphous phase. We then show that self-dual SSs can be switched to unidirectional spectral singularities in a three-layer structure consisting of a lossy layer sandwiched between a GST layer and a gain layer by switching the GST from its amorphous to its crystalline phase. In addition, at the unidirectional spectral singularity, zero reflection from one side and infinite reflection from the opposite side are simultaneously realized. We finally show that we can design an active device with large modulation depth achieved by a very small variation of the imaginary part of the refractive index of the active absorbing material in the lossy layer. Our results could potentially contribute to the development of a new generation of singularity-enhanced switchable optical devices.

High-Selectivity Frequency-Selective Rasorber Based on Low-Profile Bandpass Filter

A frequency-selective rasorber (FSR) with a high selectivity transmission passband and two absorption bands is proposed in this letter. The design method is introduced based on the equivalent circuit model (ECM). This structure consists of a lossy layer and a lossless layer. The lossy layer is composed of Jerusalem elements with a parallel resonance unit in center, and load with resistors on the metal branch. The lossless layer is integrated by a multilayer frequency selective surface to realize the high selectivity and wide transmission band. The surface current distributions and surface loss density were illustrated to explain the working principle. The simulated results show that the 1 dB transmission window is obtained from 7.9 to 9.0 GHz. The −10 dB absorption bands are over 3.3–7.1 GHz and 9.4–12.0 GHz. A prototype of the proposed FSR was fabricated and measured to verify our design; the measured results show a good agreement with simulated results.

Polarization‐independent reconfigurable frequency selective rasorber/absorber with low‐insertion loss

AbstractA polarization‐independent reconfigurable frequency selective rasorber (FSR)/absorber with low‐insertion loss (IL) based on diodes is proposed in this article. The presented structure consists of a lossy layer based on square loops and a bandpass frequency‐selective surface. These two layers are separated by an air layer. Each layer has an embedded bias network that provides the bias voltage to the diodes through metallic via. This configuration can avoid undesirable effects associated with the additional biasing wire. When the diodes are in off‐state, the structure is in FSR mode and exhibits a transmission window at 4.28 GHz with only 0.69 dB IL within the absorption bands. While diodes are in on‐state and the structure switches to absorber mode, it achieves perfect absorption with absorptivity of over 90% ranging from 2.8 to 5.2 GHz. An equivalent circuit model is developed to analyze the physical mechanism of the structure. A prototype of the proposed architecture is fabricated and measured, where reasonable agreements between simulations and measurements are observed, verifying the effectiveness of this design.

Ultrawideband Frequency-Selective Absorber Designed With an Adjustable and Highly Selective Notch

In this article, the working mechanism of a wideband absorber designed with an adjustable and highly selective notch band is studied, in which the narrow notch band is independently controlled by the lower lossless layer of the absorber, whereas the upper lossy layer loaded with lumped resistors realizes absorption. We present two instances with geometrically controlled and electrically controlled notch bands, respectively. Without decreasing absorption performance, the notch position can be flexibly adjusted throughout the entire frequency band by simply modifying the dimension of the lossless frequency-selective surface (FSS) or changing the capacitance of the varactor, i.e., using geometric control or electrical control. The narrow notch band allows two wide absorption bands to be retained on both sides; therefore, good stealth performance is still guaranteed. Equivalent circuit models (ECMs) are proposed to further explain the principle. The frequency-domain simulation, ECM, time-domain simulation, and experimental results are in good agreement and validate the adjustability and high selectivity of the notched absorbers. At the end of this article, an FSA-backed monopole antenna is simulated and measured, which clearly illustrates that these FSAs can serve as the ground plane for antennas and realize out-of-band RCS reduction.

Low-Loss Dual-Polarized Frequency-Selective Rasorber With Graphene-Based Planar Resistor

A novel dual-polarized frequency-selective rasorber (FSR) using an omnidirectional planar resistor based on graphene flake is presented in this article. The FSR exhibits a low-loss transmission window above an absorption band. It consists of a lossy layer and a lossless bandpass frequency-selective surface (FSS). In the lossy layer, a central symmetric graphene-metal hybrid structure is formed by connecting four metal strip-based resonant circuits with a square graphene flake. A low insertion loss transmission band is obtained by removing the resistance out of the parallel resonant circuits. Moreover, equivalent circuits are utilized to demonstrate the working mechanism. The proposed FSR exhibits a good absorption performance from 8.15 to 13.8 GHz, and a low insertion loss of 0.15 dB obtained at 16 GHz. Finally, a prototype FSR is fabricated and good agreement between the simulated and the measured results is obtained. The FSR has further advantages of avoiding the process of welding multiple lumped resistors per unit, thereby greatly reducing the number of lumped components and improving the planar integration of the structure.