Large Oblique Incidence Research Articles

Robust broadband dual-polarized rectenna with high conversion efficiency

In this paper, a robust wide-beam dual-polarized broadband rectenna with high conversion efficiency is proposed. First, a metal wall-loaded broadband cross-dipole antenna is designed as the receiving antenna. With the metal wall, the bandwidth of the receiving antenna is greatly improved. The measured results show that the −10 dB impedance bandwidth is 820 MHz from 1.65 to 2.47 GHz, and the 3-dB beamwidth is over 80° within the operating bandwidth. Besides, a novel impedance matching network is introduced to help the rectifier achieve high conversion efficiency in a wide bandwidth. The measured results show that the RF-DC conversion efficiency is maintained above 50 % with a maximum of 78 % within the bandwidth from 1.56 to 2.53 GHz when the load impedance is 840 Ω and the input power is 12.0 dBm. Finally, the receiving antenna and rectifier are integrated and measured. The results illustrate that the proposed design can be robustly against dynamic ambient power density, large oblique incidence angle, and arbitrarily polarized wave. Specifically, the measured conversion efficiency is over 49 % at an ambient power density of 200 μW/cm2 within the incident angle from −40° to 40° under arbitrary polarization. Hence, it could be a feasible solution to wireless energy harvesting (WEH) applications.

Oblique incidence-insensitive metamaterial absorber using strong magnetic coupling resonance for low-frequency microwave absorption

In this work, we presented an oblique incidence-insensitive metamaterial absorber (MA) based on the strong magnetic coupling resonance using the printed circuit board process, and the spacer is produced by a typical magnetic absorber material (MAM). As for reflection loss (RL) at oblique angles, it can achieve high-efficiency absorption for both transverse-electric (TE) and transverse-magnetic (TM) polarizations with a wide angular range from 0° to 60° in the low frequency of 2.33 GHz to 3.44 GHz. In addition, the magnetic coupling mechanism of the metamaterial absorber was deeply analyzed. Comparing the results of the simulation and experiment, it displays excellent absorption performance of the proposed absorber in TE and TM modes under large oblique incidence angles.

Thermally tunable vanadium-dioxide-based broadband metamaterial absorber with switchable functionality in the terahertz band

In this paper, a thermally tunable broadband metamaterial absorber, with switchable functionality in the terahertz band, consisted of periodically arranged vanadium dioxide (VO2) and a gold film separated by a layer of polyimide is reported, which is capable of switching from absorber to reflector through the phase change property of VO2. When VO2 is in the metallic state, three near-perfect absorption peaks localized at 3.48 THz, 5.09 THz and 7.05 THz are obtained, and the combination of them gives rise to a broadband absorption, more than 90% of absolute absorption bandwidth reaches 4.35 THz (3.1–7.45 THz) with a relative absorption bandwidth of 82.46%. When VO2 is in the dielectric state, it can switch from near-perfect broadband absorption to near-perfect reflection with the maximum intensity modulation of 92.4%. The broadband absorption is insensitive to polarization of incident beam due to symmetrical structure design and exhibits excellent tolerance for large oblique incidence angle. In addition, size changes of patterned VO2 array structure provides a large impact on the absorption performance of the thermally tunable device, especially the absorption bandwidth. Our proposed device is expected to have outstanding prospects in terahertz thermal imaging, communications, and temperature-controlled metasurface.

Synthesis of 3-D Polarization Rotation Transmissive Surface by Arbitrary Angle Based on Orthogonally Inserted Parallel-Couple Slotlines

In this article, a polarization rotation transmissive surface (PRTS) of arbitrary angle based on a three-dimensional (3-D) frequency selective structure is proposed. The PRTS unit cell is composed of three parallel-coupled slotline sections with open-short ends etched on vertically and horizontally inserted singlelayer printed circuit boards (PCBs). Under the incidence of vertically polarized waves, the spatial waves are converted into guided waves by the open-circuited end of one slotline section, and then the guided waves are coupled to two other slotline sections that can act as two independent coupling transmission paths in the orthogonal planes. This can be observed as a power divider, and the power division ratio can be fully controlled by virtue of the coupling strengths of the two coupling transmission paths. Thus, the polarization of incidence waves can be rotated at an arbitrary angle at the output port. To simplify the design procedure, a filtering power division theory and an equivalent circuit model are first used to obtain the theoretical response. Subsequently, a coupling matrix is employed to synthesize and design the proposed PRTS. To confirm the effectiveness of the proposed PRTS, two examples with different polarization rotation angles and fractional bandwidths at the same center frequency of 5 GHz and return loss of 20 dB are designed, fabricated, and measured. The measured results confirm our theoretical predictions and agree with the simulated results, thus demonstrating that the proposed PRTS exhibits stable frequency response under a large oblique incidence, in addition to their easiness in fabrication.

Independently tunable multi-band terahertz absorber based on graphene sheet and nanoribbons.

A multi-band terahertz (THz) absorber based on graphene sheet and nanoribbons is proposed and investigated. In the studied frequency range, five absorption peaks are observed, with four originate from lateral Fabry-Perot resonance (LFPR) and one originates from guided-mode resonance (GMR). The LFPR and GMR peaks behave differently when geometric parameters are adjusted, which makes independent tuning possible. When period increases, the GMR peak red shifts and the frequencies of LFPR peaks remain almost unchanged. On the contrary, as nanoribbon width increases, the frequency of GMR remains almost unchanged while that of LFPRs decrease significantly. With increasing top dielectric layer thickness, the LFPR peaks blue shift while the GMR peak red shifts. In addition, the absorber has the merit of multi-band high absorptivity and frequency stability under large angle oblique incidence. The proposed terahertz absorber may benefit the areas of medical imaging, sensing, non-destructive testing, THz communications and other applications.

Numerical study of an ultra-broadband, wide-angle, polarization-insensitive absorber in visible and infrared region

An ultra broadband high-performance metamaterials absorber is designed based on the Fabry-Perot resonance effect and hybrid coupling theory. In this paper, we use the numerical method to analyze the absorption characteristics of the absorber under the conditions of wide band, different polarization states and large angle oblique incidence. The results reveal that the absorption band of the absorber is 300 nm–3000 nm, and its average absorption rate exceeds 93%. In addition, the absorptivity of the absorber is less affected by the polarization angle and incident angle of the incident light. The physical mechanism of wide-band high-absorption is analyzed, which is mainly caused by Fabry-Perot resonance, surface plasmon resonance, local surface plasmon resonance and the hybrid coupling among them. The proposed metamaterial absorber is expected to play a role in invisible devices, solar cells and other fields owing to its wide absorption bandwidth, high absorption, independent to polarization and stable at wide-angle of incidence.

Design of a Single-Layer Frequency Selective Surface for 5G Shielding

In this letter, a single-layer bandpass frequency selective surface (FSS) working at 2.6 GHz is proposed to decrease the interference to communication systems and the cost of processing based on a multimode resonator. A metasurface unit cell is adopted to suppress the grating lobe and achieve miniaturization. Therefore, the performance of the proposed FSS can remain stable under a large oblique incidence. The surface current distribution and equivalent circuit model at each design step are extracted to explain its working principle. Good agreement between the experimental results and the simulated response verifies the feasibility of the proposed FSS.

Thermal driven wavelength-selective optical switch based on magnetic polaritons coupling

A wavelength-selective optical switch by thermally controlling which has higher transmittance at metallic phase of Vanadium dioxide (VO2) and lower transmittance at insulating phase is presented. By skillfully designing the structure, magnetic polaritons (MPs) are excited to enhance transmission to 0.79 indicating “on” mode with VO2 at metallic phase. The coupling between two magnetic polariton modes is excited at the same frequency to suppress the transmittance peak to 0.045 indicating “off” mode with VO2 at insulating phase, demonstrating tunability by over an order of magnitude during phase transition. Magnetic polariton coupling mode is proposed to explain the phenomenon of surpressed transmission and the electromagnetic field distributions confirm it. The proposed optical switch still works well at a large oblique incidence and the resonant frequency does not change with the incident angle. Our study may benefit the microelectronics and infrared active filtering applications.

Planar ultrathin omni-directional perfect absorber utilizing amorphous silicon for photovoltaics

Resonant plasmonic metasurfaces and thin film stacks have been extensively studied for spectral control and perfect absorption enhancement functionality. Essentially, the plasmonic nanostructures or metallic films enable the optical field resonant and confinement at the nanoscale, and thus yield the Ohmic heat absorption in the nanoscale metals. However, typical perfect absorbers based on film coatings are usually sensitive to the variation of large oblique incident angles, and mostly lack the capability for direct conversion to photocurrents and photovoltaics. Here, we proposed a lithography-free perfect absorber design consisting of metallic and amorphous silicon (α-Si) films with deep-subwavelength thickness (∼ λ/20 - λ/100). The perfect absorptivity spectrum enjoys Omni-directional optical characteristics, which remains the high absorption for the normal incidence to large oblique incidence angles of ± 60°. Due to the strongly trapped resonance in the Fabry-Perot cavity, the majority of light absorption (∼89%) takes place in the core α-Si layer, which could enable the potential optoelectronic conversion to photocurrents and photovoltaics. Our proposed perfect absorber based on ultrathin α-Si films enjoys the great simplicity of design and manufacturing and suggests a variety of promising applications, including photovoltaics, optical sensors, solar cells, photodetectors, thermal bolometers, nano-imaging devices, color filters, and thermal emitters, etc.

Selectively thermal radiation control in long-wavelength infrared with broadband all-dielectric absorber.

Artificial control of the thermal radiation is of growing importance to fundamental science and technological applications, ranging from waste heat recovery to thermophotovoltaics. Nanophotonics has been proven to be an efficient approach to manipulate the radiation. In comparison with structures utilizing planar subwavelength scale lithography, in this paper, we propose a cascaded all-dielectric multilayer structure to selectively manipulate the thermal radiation characteristics in long-wavelength infrared (LWIR). The broadband emissivity in non-atmospheric windows (6.3-7.5 µm) can reach 0.95 and the average absorption rate is below 3% in atmospheric windows (8-14 µm). The multilayer structure is insensitive to the polarization of the incident waves and maintains a good rectangular absorptivity curve even with large oblique incidence angle at 45 degrees. The outstanding properties of the nanostructures promise various applications in infrared sensing and thermal imaging.

Nonlinear seismic responses of the powerhouse of a hydropower station under near-fault plane P-wave oblique incidence

Near-fault ground motion has unique motion characteristics that are mainly manifested as long periodic velocity and large displacement pulses. Engineering structures in near-fault areas are frequently damaged severely. Most previous studies have assumed that the seismic waves on a site in a near-fault area are vertical or approximately vertical incidence that are prone to large oblique incidence angles. First, the displacement expression of free surface is derived on the basis of the boundary conditions of free surface. The theoretical displacement curves of free surface in the range of 0°–90° are drawn on the basis of displacement expression. Then, the most unfavorable incidence angle (65°) is obtained. Subsequently, the equivalent nodal force formula on the artificial boundary node of plane P-wave oblique incidence is derived on the basis of the dynamic viscoelastic artificial boundary. Numerical verification shows that the proposed method has good precision. Ten near-fault ground motions are selected from the strong earthquake database of the Pacific Earthquake Engineering Research Center (PEER), in which velocity and non-velocity pulse ground motions have five groups each. Finally, the nonlinear seismic responses of the ground powerhouse of a hydropower station are investigated when the incidence angles of a near-fault plane P-wave are 15°, 30°, and 65°. Results show that the damage of the powerhouse under pulse-like ground motions are more serious than that under non-pulse ground motions when the incidence angles of seismic wave are the same. The superstructural interlayer displacement angles of the powerhouse and the relative displacements in the water flow direction between the upstream and downstream cattle legs increase with the ratio of the peak ground velocity to the peak ground acceleration (PGV/PGA). In comparison with the smaller incidence angle of seismic wave, the PGV/PGA of ground motion has a more significant influence on the seismic response of the powerhouse at a larger incidence angle of seismic wave.

Perfect acoustic absorption by subwavelength metaporous composite

We present a design of a low-frequency perfect acoustic absorption metaporous composite with frequency tunability and the insensitivity to the incident angle based on a critical coupling mechanism. The metaporous composite is constructed by embedding an Archimedean spiral structure in traditional porous materials. We verified the tunable balance of the dissipation and leakage of the metaporous composite by adjusting the slit orientation of the embedded spiral and the width and depth of the porous materials. Our simulations show that the composite's thickness is 1/20th of the corresponding wavelength at the perfect absorption (PA). Experiment and complex frequency plane analysis confirm the perfect acoustic absorption. The simulations show that the acoustic absorption of the system is highly efficient (0.95), up to a large oblique incidence, e.g., 60°. We also achieve the dual-band PA by embedding two spirals or using the higher order mode of a single spiral. The proposed composite absorber provides promising potential for low-frequency noise reduction applications.

Necessary conditions for out-of-plane lattice plasmons in nanoparticle arrays

Out-of-plane lattice plasmons (OLPs) supported by metallic nanoparticle arrays are promising for diverse applications due to their remarkably narrow linewidths and significant enhancement of local fields. Here we investigate the necessary conditions for metallic nanoparticle arrays to achieve OLPs. Our results show that, besides the prerequisites of large height, oblique incidence, and TM polarization, which have been pointed out by the literature; the array period should be properly designed within a limited range, the dielectric environment should be homogeneous, and the height and the angle of incidence need to be optimized. We expect this work will advance the understanding and engineering of OLPs in metallic nanoparticle arrays and promote their applications in nanolasers, ultrasensitive biochemical sensors, and nonlinear optics.

A Thin and Broadband Linear-to-Circular Polarizer Based on Grid-Patch Structure for Ku-Band

A three-layer circular polarization selective surface operating in Ku-band is proposed in this communication. We design a thin grid-patch structure to achieve linear-to-circular polarization transformation over a wide frequency band. The periodic structure could be analyzed via equivalent circuit model theory. Numerical simulations have been carried out and the prototype structures have been fabricated and experimentally validated. The axial ratio (AR) of the transmitted field remained below 3 dB over the entire bandwidth, and the 3 dB AR bandwidth is approximately 44.6%. The design has a subwavelength thickness of 6 mm (0.28λc). Besides, this design could be also applied to beam scanning antennas because of its capability in condition of large oblique incidence angle (±50 deg).

In-situ impedance and absorption coefficient measurements using a double-layer microphone array

Acoustic impedance is typically measured using an impedance tube, which requires a material sample physically fitted to the tube. However, the impedance can vary greatly between the material mounted in the tube and the material located in a real environment, where the mounting conditions are likely to be different. Also, oblique incidence cannot be measured in an impedance tube. In this paper, we investigate the use of a double-layer microphone array for in-situ measurement of surface impedance and absorption coefficient. With the array positioned near the material surface, a source emits broad-band sound towards the array and the material. A measurement is taken, and the sound pressure and the surface-normal particle velocity at the material surface are calculated using Statistically Optimized Near-field Acoustical Holography (SONAH). From the surface pressure and velocity, the impedance across a selected area is calculated, and finally the absorption coefficient is calculated from the impedance. A set of tests has been performed on porous material samples in an anechoic chamber as well as in a fitted room. Different sample sizes and different sound incidence angles have been considered. The results show consistency between the measurements in the anechoic room and the ordinary room as well as good agreement with Miki’s model up to large oblique incidence angles.

Ultrawide Bandwidth Electromagnetic Wave Absorbers Composed of Double-Layer Frequency Selective Surfaces with Different Patterns

A novel design for an ultra-wide bandwidth and thin microwave absorber is introduced utilizing two frequency selective surfaces (FSSs) with different patterns of resonating frequencies. The circuit parameters, inductance and capacitance, of the three types of FSS (square loop, cross, square patch) were determined using an equivalent circuit and strip wire conductor model. The square loop FSS indicates a low frequency resonance (10 GHz) due to its high inductance and capacitance. On the other hand, the square patch of small inductance reveals a high resonating frequency (36 GHz). By optimizing the combination of the two FSSs, an ultra-wide absorption bandwidth (6.3–40.0 GHz for −10 dB reflection loss) was designed with a small total thickness of 5.5 mm, which is close to the theoretical limit. The free space measurement result with a test sample prepared by the screen printing method was in good agreement with the simulation result and verified the validity of the proposed design method. For these periodic array structures, however, the grating lobes were observed above the high frequency limit, and it needs to be emphasized that the further control of the unit cell periodicity is important, particularly for large oblique incidence angles.

Uniaxial Anisotropy in PEDOT:PSS Electrodes Enhances the Photocurrent at Oblique Incidence in Organic Solar Cells

PEDOT:PSS is a well studied organic conductor, commonly used as a transparent electrode material in printed organic devices such as organic solar cells. PEDOT:PSS thin films are known to be uniaxially anisotropic and exhibit a lower extinction coefficient and lower refractive index in the out of plane direction. To determine the maximum attainable photocurrent in thin film solar cells, the optical power dissipation can be calculated by the transfer matrix method. However, until now the anisotropic properties of PEDOT:PSS films have not been included in the model. In this work we have included an uniaxial anisotropic treatment of PEDOT:PSS films. We investigate reversed and semitransparent solar cells, with aluminum and PEDOT:PSS respectively as the second electrode and PEDOT:PSS as the top electrode, as compared to devices with isotropic PEDOT:PSS electrodes. For p-polarized light at large oblique incidence the inclusion of anisotropy shows a gain of over 7% for the maximum photocurrent in reversed solar cells. In semitransparent solar cells the photocurrent enhancement reaches 4–5% for p-polarized light. However, an enhancement of optical power dissipation and thus photocurrent generation of close to 40% is calculated for wavelengths close to the absorber bandgap. This work shows that for correct calculations of optical power dissipation in devices with PEDOT:PSS electrodes anisotropy should be included in the optical model. This will be especially important to determine the daily energy output of organic solar cells as their expected first markets are on building facades and indoor applications with larger fractions of diffuse light at large oblique incidence.

Metamaterial for polarization-incident angle independent broadband perfect absorption in the terahertz range

We propose a broadband perfect metamaterial absorber (MA) working at the terahertz (THz) frequencies based on the rectangular fish-scale structure. The averaged absorption of the MA is higher than 85% from about 0.85 to 1.95 THz with a maximum value of 99.5% at normal incidence. The broad and high absorption can also be obtained for the large oblique incidence of ±70°. The mixtures of the electric and the magnetic resonances contribute to the excellent performance of the MA. Furthermore, this MA is independent to the polarization states of the incident electromagnetic (EM) waves due to its four-folder rotational symmetry of the unit cell.

Frequency-tunable circular polarization beam splitter using a graphene-dielectric sub-wavelength film.

Manipulating the circular polarization of light is of great importance in chemistry and biology, as chiral molecules exhibit different physiological properties when exposed to different circularly polarized waves. Here we suggest a graphene/dielectric-stacked structure, which has both the properties of an epsilon-near-zero material and the high Hall conductivity of graphene. The proposed sub-wavelength structure demonstrates efficient manipulation of circular polarization properties of light. In a quite broad frequency range and at a large oblique incidence angle, the present magnetically active structure is transparent for one circularly polarized wave, and opaque for another. Such an effect can be further tuned by changing the magnitude of the applied magnetic field and chemical potential of graphene.

大角度斜入射情况下的猫眼效应激光反射特性

大角度斜入射情况下的猫眼效应激光反射特性