Cross-polarized Wave Research Articles

Metasurface spatial filters for multiple harmonic signals

Abstract Nonlinear frequency mixings have shown an alternative way to create new electromagnetic sources in frequency ranges that are difficult to access with conventional techniques. To simultaneously use the fundamental frequency pump beam and multiple harmonic signals generated in the same channel, a device capable of separating each frequency component is required. Here, we propose and experimentally demonstrate metasurface-based spatial filters for the pump frequency and multiple harmonic frequencies. The metasurface was designed using eight different split ring resonator-based phase elements with 45° phase spacing, which allows wavefront shaping. The metasurface designed to have a one-dimensional gradient phase array produces cross-polarized reflection waves with different beam steering angles at the third- and fifth-harmonic frequencies (15 and 25 GHz) and operates as a metallic mirror at the fundamental frequency of 5 GHz. Our work suggests a new method to enable simultaneous use of broadband multi-frequency sources based on nonlinear frequency mixing.

Low-Scattering X-Band Planar Phased Vivaldi Array Antenna

A low-scattering X-band planar phased Vivaldi array antenna is proposed in this communication. To reduce the scattering cross section (SCS) of the antenna, two types of Vivaldi elements with the defected ground structures (DGSs) are designed, as well as an embedded absorbing array. Under the normal co-polarized (co-pol) incident waves, the reflections of the two Vivaldi elements are close in magnitude but out of phase. Hence, the scattering of the two elements cancels out in the normal direction. Besides, the problem of obvious bistatic SCS peaks, which always arise when utilizing the phase cancellation method to achieve SCS reduction (SCSR), is solved by the interlacing arrangement of the two elements. Under the cross-polarized (cr-pol) incident waves, the incident power is mostly received and absorbed by the absorbing array, which is composed of the dipole elements in the array center and the slot elements on two edges. According to the measured results, the proposed phased Vivaldi array antenna achieves notable cr-pol SCSR over 6–18 GHz, and the average in-band co-pol SCSR reaches 20 dB simultaneously, without any deterioration on the radiation and scanning performances compared to a referenced one.

A reconfigurable asymmetric-transmission metasurface for dynamic manipulation of transmission, reflection, and polarization

A broadband reconfigurable metasurface consisting of perpendicular metallic gratings and a PIN diode-driven split circular ring for polarization and propagation modes control is demonstrated. The designed metasurface possesses tunable properties in two propagation modes. In the “OFF” state, it can convert incident linear-polarized waves into transmitted cross-polarized waves with full transmission. In the “ON” state, it can reflect incident linear-polarized waves into co-polarized waves with full reflection. Without the increasing number of active elements commonly utilized in chirality-type metasurface polarizers, the equivalent transformation of the split circular ring can be achieved by switching the single PIN diode, and high polarization-conversion efficiency is guaranteed. A metasurface array with 10 × 10 unit cells is fabricated and measured. The experimental results show that the tunable transmission, reflection, and polarization properties of the proposed reconfigurable metasurface can be achieved from 4.9 to 7 GHz.

Metasurfaces Excited by an Evanescent Wave for Terahertz Beam Splitters with a Tunable Splitting Ratio

In terahertz (THz) photonics, a beam splitter is an important component. Although various THz beam-splitting devices using several principles have been proposed, the splitting ratio of existing designs is not adjustable. Here, a THz beam splitter with an adjustable splitting ratio is demonstrated using a metasurface integrated onto a prism. The metasurface excited by an evanescent wave can convert part of a linearly polarized incident wave into a cross-polarized wave and manipulate its phase simultaneously. As a result, the cross-polarized wave can pass through the interface, even if the incident angle is larger than the total internal reflection angle, while the co-polarized wave is reflected by the prism. The splitting ratio of the device can be adjusted from 4.56:1 to 0.63:1 by tuning the resonant response of the metasurface and varying the interval distance between the metasurface and the prism. The metasurface samples are fabricated using low-cost standard printed circuit technology, and the experimental results are consistent with the simulations. Therefore, the proposed beam splitter with a tunable splitting ratio is promising as a key component in the THz system.

High-Contrast Frontend for Petawatt-Scale Lasers Using an Optically Synchronized Picosecond Optical Parametric Chirped Pulse Amplification

We present a new scheme of picosecond optical parametric chirped pulse amplification (OPCPA) in which a Fourier-transform-limit 5.0 ps pulse is optically sheared from a single-longitudinal-mode 1064 nm CW laser. The pulse is amplified and frequency-doubled as the pump in order to maintain the pump narrow bandwidth and picosecond duration simultaneously, which is very important to ensure the high temporal contrast for an OPCPA amplifier. Combined with the cross-polarized wave generation (XPW), a compound frontend for the high-power femtosecond laser system that delivers a 1 Hz chirped pulse train is established. The experiments provide an output pulse energy of 17.1 mJ, a spectrum bandwidth 71 nm (FWHM), and a pulse duration 16.4 fs. The pulse contrast reaches 1:10−12 several picoseconds before the peak of the main pulse, which is the best value of the available measuring instruments.

A Low-RCS Folded Reflectarray Combining Dual-Metasurface and Rasorber

A strategy for folded reflectarray with low detectability and enhanced gain is presented. For this purpose, the folded reflectarray is engineered by combining a polarization-conversion focusing metasur face, a frequency selective rasorber (FSR), and a diffusion metasurface. The FSR is composed of a lossy layer and a single-polarized I-shaped slotted frequency selective surface (FSS) between which the diffusive metasurface is inserted for in-band and out-of-band cross-polarized low radar cross-sectional (RCS) reduction. Therein, the antenna exhibits a transmission window and two out-of-band absorptions due to the FSR for copolarized radiation. In contrast to detection, the system operated for both wave incidence. For cross-polarized wave incidence, a wideband RCS reduction is enabled due to the wave-diffusive function of the inserted metasurface. The half profile is guaranteed since the wave bounces two times between the FSS and focusing metasurface. The numerical and experimental results coincide well, both indicating that the low-RCS folded reflectarray achieves a peak gain of 23.5 dB at 10.2 GHz, corresponding to an aperture efficiency of 37.8%. Besides, the antenna achieves 10 dB monostatic RCS reduction within 4.6–8.8 GHz and 9.6–13.8 GHz under copolarization detection and a 10 dB monostatic RCS reduction from 8.4 to 19 GHz under cross-polarization detection. Moreover, good invisibility is also achieved for bistatic RCS reduction.

Bifunctional metasurface for cross-polarization conversion and ultra-broadband absorption in terahertz range

A bifunctional terahertz (THz) metasurface based on vanadium dioxide (VO2) was proposed, which can achieve near-perfect cross-polarization conversion (CPC) and ultra-broadband absorption. When VO2 is in insulator state, the metasurface acts as a cross-polarization converter, converting linearly polarized (LP) and circularly polarized (CP) waves into the corresponding cross-polarized waves. The polarization conversion ratio (PCR) exceeds 99.5% in the frequency range of 4.1∼6.7 THz at normal incidence, and can be maintained more than 98.3% even at an incident angle of 20°. When VO2 is in metal state, the metasurface becomes a THz broadband absorber with absorption efficiency above 90% in a wide frequency range of 2.9 THz∼8.4 THz. The absorption efficiency is almost unchanged when the incident angle increases from 0° to 15° for TE wave. In addition, the absorption efficiency can maintain above 90% at 4.0∼7.1 THz when the incident angle is up to 60° for TM wave. Moreover, the interference theory model was utilized for verifying theoretically the validity of the numerical simulation results of our proposed metasurface, and the theoretical calculation results are in good agreement with the simulation results. We believe that this proposed metasurface has potential applications in many fields, such as sensing, imaging, non-destructive detectors, super lenses, and telecommunications.

Frequency multiplexed metasurface for polarization conversion in full-space

In order to solve the problem of a single function in passive metasurface, a frequency multiplexed metasurface using full-space to realize different polarization conversion modes is proposed. On the basis of the fundamental theory of metasurface polarization transformation, a spliced coupling of multilayer structures was utilized. A compact architecture of five metallic layers and four dielectric layers was fabricated, and the principle analysis of polarization transformation was also performed. In the metallic part, two layers of oblique arrows and one layer of oblique rectangular patches constitute a low-frequency transmission structure, one layer of cross-shaped patches are constituted a high-frequency reflection structure, and one layer of rectangular patches constitutes an intermediate filter structure. Thus, it achieves a variable frequency polarization multiplexing function for transmission on one side and reflection on the opposite one. When the x- and y-polarized waves in the 6.3 GHz–8 GHz band are incident on the low-frequency unit side, they are converted into left- and right-hand circularly polarized transmitted waves, respectively. The axial ratio is less than 3 dB, and the transmission amplitude is up to 0.84 and 0.8. When 45° linearly polarization incident waves in 16.4 GHz–16.8 GHz and 16.9 GHz–18 GHz bands impinge on the high-frequency unit side, the cross-polarized wave reflection is realized. The measured and simulated results are in good agreement. The metasurface proposed in this paper has potential applications in communication frequency multiplexing and multifunctional integration.

Low Profile Reflective Polarization Conversion Metasurface With High Frequency Selectivity

In this article, a multilayer reflective polarization conversion metasurface with both high-frequency selectivity and low profile is proposed. The frequency selective polarization conversion metasurface (FSPCM) is composed of four metal layers separated by three substrates. The cross-shaped patch on the top layer is connected to the folded microstrip line on the bottom layer with two via holes through the circle slots inside two ground layers, which converts the linearly polarized electromagnetic incident wave into cross-polarized wave after reflection. The second-order response can extend to the third-order response by introducing a coupled half-wavelength microstrip resonator on the bottom metal layer, which achieves high-frequency selectivity with a transmission zero and high polarization conversion rate (PCR) without increasing the thickness. The equivalent circuit analysis, structure simulation, and experimental demonstration are conducted in detail; the frequency selectivity of the fabricated FSPCM at the lower and upper band edge is 94 and 192 dB/GHz, respectively; PCR is more than 97% from 4.85 to 5.12 GHz, and the thickness is only <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$0.028\lambda _{0}$ </tex-math></inline-formula> , where <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\lambda _{0}$ </tex-math></inline-formula> is the free space wavelength at the center operating frequency. Our findings pave a new way for enhancing the anti-interference capability and electromagnetic compatibility in applications of communication system, microwave imaging, and stealth technology.

A dual-broadband dual-function polarization converter based on reflective metasurface

In this paper, we introduce a dual-broadband dual-function polarization conversion metasurface, which can convert a linearly polarized (LP) incident electromagnetic wave into a cross-polarized LP wave and a circularly polarized wave, effectively, in different bands. The numerical simulations and experiments show that the metasurface can be used as a high-efficiency cross-polarization converter from 5.50 to 12.4 GHz, where the polarization conversion ratio is greater than 0.9 with a fractional bandwidth of 77.1%. Simultaneously, it can convert the LP incident wave into a right-handed circularly polarized wave from 13.50 to 19.75 GHz, where the axis ratio is lower than 3 dB and the fractional bandwidth is 37.4%. A honeycomb is used as a support layer instead of air gaps, which improves the efficiency of cross-polarization conversion and broadens the bandwidth of linear to circular conversion. To reveal the conversion mechanism of the proposed polarization converter, the surface current is analyzed. Finally, we fabricated and tested it, and the experimental results are in line with the simulation results.

Broadband Ultra-Thin High-Efficiency Linear Polarizer Based on Metasurfaces

In this paper, an ultra-wideband (UWB) polarizer with high performance based on the metasurface is designed and demonstrated, which is composed of a dielectric substrate with a double-gap circular pattern and metal film. Multiple strong resonance points enable the design to achieve the conversion from incident linearly polarized waves to cross-polarized waves at 6.49–11.64 GHz, with a fractional bandwidth of 56.8% and a corresponding polarization conversion rate (PCR) of 90%. The PCR remains above 90% at 6.49–11.52 GHz when the electromagnetic wave is obliquely incident between 0° and 30°. Furthermore, the surface current distribution of the polarizer is discussed to explain the physical mechanism. The sample is fabricated for microwave validation. Compared with previous reports, the proposed polarizer has a larger bandwidth and higher efficiency and is expected to be used in microwave communications, antennas, radar cross section reduction, and other fields.

Dispersion-Assisted Dual-Phase Hybrid Meta-Mirror for Dual-Band Independent Amplitude and Phase Controls

Frequency-multiplexing metasurfaces are typically attributed to the integration of multiple resonators, where mode coupling may significantly affect their performances. In this communication, we develop a dispersion-assisted dual-phase hybrid strategy for dual-frequency independent control of amplitude and phase with a single resonance. Therein, the frequency and phase decoupling can be achieved by precise dispersion control between two frequency states, while the amplitude distribution of each meta-atom can be controlled by properly allocating the energy of copolarized and cross-polarized waves. As a proof of concept, a bi-functional meta-mirror shaping multifocal points at two different frequencies have been experimentally demonstrated. The measured results are in good agreement with simulated results, showing the diffraction efficiencies (DEs) of 38% and 45%, and a signal-to-noise ratio (SNR) of 8.36 and 10.35 at <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$f_{1}$ </tex-math></inline-formula> and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$f_{2}$ </tex-math></inline-formula> , respectively. In addition, another three meta-mirrors with tailorable amplitudes at dual frequencies are designed to further verify our strategy. This method offers an alternative platform for designing high-performance devices with dual-frequency wavefront manipulations, which may find potential applications in microwave integrated systems and wireless communication systems.

Ultra-Thin/Wide-Band Polarization Conversion Metasurface and Its Applications in Anomalous Reflection and RCS Reduction

In this paper, the design of an ultra-wideband polarizer based on a metasurface with high-performance is reported and demonstrated. The polarizer is composed of a dielectric substrate with double semicircular gap patches and a metal film. Multiple strong resonance points enable the design to convert the incident linearly polarized waves into cross-polarized waves in the 14.8–28.0 GHz range, with a fractional bandwidth of 61.7% and a corresponding polarization conversion rate (PCR) above 95%. Further simulated results show that the PCR remains above 87% in the 14.37–24.75 GHz range when the incident angle of the electromagnetic (EM) waves is between 0–30°, and the physical mechanism is explained by the surface current distribution. In addition, the gradient metasurface is designed according to the Pancharatnam–Berry phase principle to achieve anomalous reflection, and the 1-bit metasurface is coded to reduce the Radar Cross Section (RCS). The EM waves reach an anomalous reflection of −23° at 15 GHz normal incidence, and the RCS is reduced by 10 dB in the range of 15.3–28.0 GHz. These findings have potential application value in stealth and antenna design.

VO2-assisted multifunctional metamaterial for polarization conversion and asymmetric transmission.

A VO2-assisted temperature-controlled multifunctional metamaterial polarization converter with large asymmetric transmission (AT) is proposed by introducing a gold-VO2 grating. The converter can be switched between reflection mode and transmission mode by controlling the phase transition. When VO2 is in the metallic state, the converter works in reflection mode, converting the incident forward/backward linearly/circularly polarized waves into the cross-polarized waves, and the broadband polarization conversion rates (PCRs) can reach 90% with relative bandwidth of up to 91.1% and 87.5%, respectively; when VO2 is in the insulating state, the converter shows giant AT effect for circularly polarized waves at 0.64 THz and 1.28 THz. The multifunctional polarization converter holds great potential in the fields of communication and imaging, which provides a new way to design optical devices such as polarizers, isolators.

Terahertz graphene metasurfaces for cross-polarized deflection, focusing, and orbital angular momentum.

Polarization is an important characteristic of electromagnetic wave. Due to novel optical properties, graphene-based anisotropic structure is widely used to control polarization state of electromagnetic wave. In this work, four graphene-based meta-atoms are designed to regulate polarization state of terahertz wave by changing Fermi energy level of graphene. When Fermi energy level is 0.01 eV, cross-polarized wave is emitted by four meta-atoms with phase difference of 90° at 1.18 THz, and the corresponding polarization conversion ratio reaches ∼90%. When Fermi energy level is adjusted to 0.70 eV, linear phase gradient will disappear, and cross-polarized wave almost disappears. Using four selected elements, three dynamic metasurfaces are designed for controlling wavefront of reflected beam, and they are gradient metasurface, metalens, and vortex beam generator. The designed metasurfaces successfully combine wavefront control and polarization manipulation, and greatly improve the ability to control electromagnetic wave. Our designs may have many potential applications, such as terahertz switching, imaging, and polarization beam splitter.

Programmable Manipulations of Terahertz Beams by Graphene-Based Metasurface With Both Amplitude and Phase Modulations

Terahertz metasurface with digital, programmable control recently gained considerable attention for its potential applications in high-speed imaging, nondestructive sensing, and wireless communication. With elaborate design, the metasurface can perform amplitude, phase, and polarization modulations of electromagnetic waves. Most digital programmable metasurfaces focus on only one of the three dimensions. Here, we propose a graphene-based THz metasurface with both phase and amplitude modulations, which consists of an artificially constructed metal-insulator-metal structure and two-dimensional graphene material. Each meta-atom of the metasurface is divided into two sub-atoms, and the two sub-atoms can reflect terahertz waves with a phase difference of 180°. Meanwhile, the amplitude of the sub-atom can be effectively modulated or even switched off by applying different gate voltages to the graphene. By independently controlling the amplitude response of the two sub-atoms, the whole meta-atom can dynamically control both amplitude and phase responses of the cross-polarization waves. By carefully designing the coding patterns, the digital metasurface can control both beam direction and intensity, which may lead to various advanced applications in beam shaping, radar detection systems, and high-quality holography.

Active wavefronts control with graphene-functionalized terahertz Metasurfaces

Metasurfaces, a new kind of artificial subwavelength structures, have exhibited unprecedented capabilities in wavefront control of electromagnetic waves. Currently, a series of active metasurfaces have been explored for engineering applications in anomalous refraction, ultrathin flat lens, holograms, and vortex beam generation, However, to date, most reported metasurfaces are mainly concentrated on active control on the polarization conversion and wavefront of circular polarized waves. Here, graphene-functionalized metasurfaces consisting of metallic double split ring resonators integrated with graphene (G-DSRRs) are proposed to actively control polarization conversion and wavefronts of linearly polarized waves at the terahertz region. By carefully designing spatially phase profile, anomalous reflection of linearly cross-polarized terahertz waves is demonstrated in a broadband range of frequencies. Moreover, the amplitude of the anomalous reflection terahertz wave can be dynamically tuned by varying Fermi energy via electrically doping graphene. Based on the tunable graphene-functionalized metasurface, several polarization conversion metasurface devices including vortex beam generators, invisibility cloaks, and metalens, are successfully designed and numerical results show that their amplitudes can be actively tuned to a significant extent by doping graphene. Therefore, such graphene-functionalized metasurfaces may have potential applications in THz telecommunications, high-resolution terahertz displays, and advanced THz imaging devices.

High-Gain Fabry–Pérot Antenna With Reconfigurable Scattering Patterns Based on Varactor Diodes

In this article, a low radar cross section (RCS) and high-gain Fabry–Pérot (F-P) antenna with reconfigurable scattering beams based on varactor diodes is proposed. The partially reflecting surface (PRS) of the proposed F-P antenna is composed of an anisotropic frequency-selective absorber (AFSA) and an active phase gradient metasurface (PGM) loaded with varactor diodes. The proposed AFSA can absorb the cross-polarized wave and make the co-polarized wave pass through. The in-band co-polarized scattering beams can be tuned continuously with stable radiation beams by changing the capacitance of the varactor diodes. Therefore, the proposed device can reduce the RCS for in-band co- and cross-polarized incident waves. Moreover, taking advantage of the path difference between the radiating and scattering waves, the gain enhancement and RCS reduction (RCSR) are realized simultaneously. Simulated results show that the scattering beam can be tuned dynamically from –40° to 40° with a maximum gain enhancement of 7.2 dB. The co-polarized in-band reconfigurable scattering beam and the stable gain enhancement are first implemented in a single antenna. A significant monostatic RCSR is obtained for the co- and cross-polarized wave in the frequency band of 5.5–7 GHz with a maximum reduction value of 20 dB. The experimental and simulated results are in good agreement.

High-efficiency terahertz full-space metasurface for the transmission linear and reflection circular polarization wavefront manipulation

In this work, a high-efficiency full-space metasurface (MS) is demonstrated numerically, which can independently manipulate the transmitted linear polarization (LP) and the reflected circular polarization (CP) wavefront based on the transmission and geometric phase at terahertz (THz) range. The unit-cell of the full-space MS is composed of a C-shaped slot structure sandwiched with two twisted outer single-split-rings (SSRs) and parallel inner double-split-rings (DSRs). The proposed MS structure can achieve cross-polarization conversion of LP waves after transmission at the lower frequency (f1=0.55 THz), and CP waves after reflection at the higher frequency (f2=1.37 THz). The full 2π phase shifts of the transmitted cross-polarization wave and reflected orthogonal CP wave can be obtained independently by adjusting the opening direction and size of the C-shaped slot structure based on the transmission phase, and the orientation angle of the parallel inner DSRs structure based on geometric phase, respectively. In particular, anomalous refraction and reflection for LP and CP wave, focusing lens and vortex beam generation are also demonstrated numerically. These findings are beneficial in expanding the scope of full-space MS for wavefront manipulations, providing new possibilities to develop multifunctional THz device for both LP and CP waves.

Reflective and transmissive cross-polarization converter for terahertz wave in a switchable metamaterial

Utilizing the phase transition characteristic of vanadium dioxide, we present a metamaterial configuration to achieve both reflective and transmissive cross-polarization converters. When vanadium dioxide is metal, the design behaves as a reflective cross-polarization converter. It consists of metallic grating, topas spacer, and vanadium dioxide film. Polarization conversion ratio is more than 90% in the frequency range from 4.80 THz to 13.13 THz. When vanadium dioxide is insulator, the design behaves as a transmissive cross-polarization converter using cascaded metallic gratings with rotation angle 45°. High-efficiency broadband cross-polarization wave conversion is achieved in the frequency band of 0.50–4.75 THz. Effect of oblique incidence is studied on polarization conversion. Results tell that cross-polarization conversion is better when incident angle is in the range of 0°–40°. The designed metamaterial may have a certain inspiration for the research of terahertz multifunctional polarization converter.