Polarization State Of Electromagnetic Waves Research Articles

Broadband polarizer using single-layer grating with ultra-high extinction ratio

Polarizers are an essential optical element for tailoring the polarization state of electromagnetic waves in a wide range of optical devices. Such polarizers, which exhibit a wide operating bandwidth and high performance, are attracting increasing attention, due to their extensive prospects for use in applications ranging from polarization imaging, to optical communications and detection, among others. However, achieving both broadband performance and ultra-high extinction ratio (ER), and that simultaneously, is still challenging in the design of effective polarizers. To tackle that demand, in this work, an Au-on-silica grating structure has been proposed as the basis of the design of a miniaturized high-efficiency polarizer that practically can cover the entire visible and near-infrared spectral ranges. The single-layer polarizer thus designed can show an ER of 60 dB in this spectral domain, and it has been shown that the geometrical parameters selected have a significant effect on the performance characteristics of the polarizer. Furthermore, an ER of ∼150 dB could be achieved merely by regulating the thickness of the grating to achieve the optimum performance. By integrating the high-performance polarizer proposed in this work with an optical fiber “meta-tip,” a refractive polarizer with a value of the ER of >45 dB, and that over the entire spectral domain considered, has been demonstrated. Such an approach offers an alternative route to achieving a broadband, powerful, and flexible processing polarizer design.

Electrically tunable THz graphene metasurface wave retarders

Abstract Anisotropic materials with chirality or birefringence can be used to manipulate the polarization states of electromagnetic waves. However, the comparatively low anisotropy of natural materials hinders the miniaturization of optical components and devices at terahertz frequencies. In this study, we experimentally demonstrate that the relative phase retardation of a THz wave can be electrically controlled by integrating patterned mono- and bilayer graphene onto an otherwise isotropic metasurface. Specifically, we show that a refractive index for one of the orthogonal polarization states can be electrically controlled by modulating graphene’s conductivity, thereby weakening the capacitive coupling between adjacent meta-atoms in an anisotropic manner. With monolayer graphene, phase retardation of 15° to 81° between two orthogonal polarization states can be achieved. Maximum phase retardation of 90° through a metasurface with bilayer graphene suggests its use as a tunable quarter-wave plate. Continuous control from linear- to circular-polarization states may provide a wide range of opportunities for the development of compact THz polarization devices and polarization-sensitive THz technology.

Multichannel Airy beam generator simultaneously for linear and circular polarized waves

The Airy beam has intrigued long-held research interests and developed various striking applications attributed to the unprecedented diffraction-free, self-bending and self-healing properties. Metasurfaces with superior design freedom and small structure size open a fire-new avenue for generating efficient and broadband Airy beam. Nevertheless, most existing Airy beam generators can only respond to one polarization state of electromagnetic waves and produce the single Airy beam, which immensely limit the serviceability and utilization of the Airy beam generators. Herein, an inspiring strategy of multichannel Airy beam generator based on the multichannel amplitude and phase controlled metasurface is proposed to independently generate the Airy beams for linear polarized waves in 20–24 GHz and orthogonal circular polarized waves in 12–14 GHz and 15–17 GHz. Notably, an appealing paradigm of multichannel beam focusing can be realized by the integrated multichannel Airy beam generator, which possesses strong ability to circumvent the obstacle owing to the self-healing feature of the Airy beam. A proof-of-concept prototype is constructed to verify the feasibility of our methodology, and multitudinous simulations and experiments are in excellent accordance with our theoretical predictions. This novel design of multichannel Airy beam generator will have tremendous application potential in sensing, communication and other multifunctional metadevices.

Polarization‐Multiplexing Bessel Vortex Beams for Polarization Detection of Continuous Terahertz Waves

AbstractDetermination of polarization states of electromagnetic waves plays a crucial role in photonic applications. However, it is very challenging to determine the polarization state of continuous terahertz (THz) waves. Here, a metasurface‐like metallic waveguide array (MWA) is proposed to address this by measuring both the phase difference and amplitude ratio of its two orthogonal components based on generation and interference of polarization‐multiplexing vortex beams. When a full‐polarized THz wave passes through the MWA, its two orthogonal components will be converted into +1st‐ and −1st‐order Bessel vortex beams, respectively. Their projection components in a proper polarized direction can interfere with each other, and their initial phase difference can be measured in real‐time owing to its linear relationship with the azimuth of the dark fringe in interference patterns. The two Bessel beams can be further collected separately using a linear polarizer to obtain their amplitude ratio. Therefore, the polarization state of incident waves can be determined with both the phase difference and amplitude ratio. The proposed method enables a convenient polarization determination of continuous THz waves and has great potential for developing polarization‐dependent investigations and applications in THz detection, communication, and sensing.

Full-polarization radar target feature modulation based on active polarization conversion metasurface

Electromagnetic (EM) metasurfaces comprising artificially designed subwavelength unit cells have drawn considerable attention due to the EM properties beyond the limits of natural materials. As one of the representative structures, active polarization conversion metasurface (APCM) is switchable by loading active components. It provides great freedom to manipulate the polarization state of EM waves. However, the current research mainly focused on the application of communication and paid less attention to the radar effect of APCM. APCM redistributes electromagnetic wave energy in multi-polarization channels, so it will have great application potential in polarimetric radar. Herein, based on the fully polarimetric radar one-dimensional high resolution range profile, the radar effect of time-modulated metasurface is studied. For this purpose, a method of target scattering mechanisms manipulation and a polarization-insensitive structure of APCM are proposed. The amplitude-phase joint modulation method is specifically analyzed in detail. The distance transformation and virtual multi-target phenomena are further discovered. Virtual targets along the distance dimension are generated in multi-polarization channels, while the scattering mechanisms of k-order targets are effectively manipulated. The relationship between the target scattering matrix and the modulation parameters is obtained. It may provide an effective method for the application of active metasurface in fully polarimetric radars.

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.

Dual tunable terahertz polarization conversion enabled by Double-Layer Graphene Metasurface

Tunable manipulation on the polarization states of electromagnetic waves is the greatly significant and largely desired fields including information encryption of electromagnetic waves, photodetection, and optical microscope. However, such tunability is usually limited by narrow-working bandwidth. In this paper, we propose a novel design of a dynamic tunable multi-function Double-Layer Graphene Metasurface (DLGM) in THz frequency range, assisted by the graphene plasmon resonance (GPR), which enables to realize dynamic tunable polarization states in the same frequency, and achieve polarization conversion in a broadband frequency range. Moreover, by tuning the Fermi level at different times in each layer of graphene, our design can be applied in time-domain encryption of electromagnetic waves since the frequency range of polarization conversion (circular polarization) as well as the polarization states at specific frequency can be controlled at different times corresponding to the Fermi level. Finally, the mechanism of the phase difference induced by GPR is revealed in our proposed design and our numerical calculation shows such polarization conversion can cover broader THz region compared with previous results.

Broadband and high-efficient reflective linear-to-circular polarizer with Wi-Fi shaped metasurface

Arbitrarily manipulating the polarization states of electromagnetic waves has gained interest due to its wide application in modern wireless communication systems. However, conventional polarizers face major bottlenecks such as bulky size, low efficiency and narrow frequency band. This paper proposes a reflective linear-to-circular polarizer using Wi-Fi shaped metasurface, and the designed polarizer can efficiently convert the linearly polarized incident waves into nearly perfect circularly polarized reflected waves in wideband frequency region of 12.1–20 GHz. Based on simulations, the polarizer has an axial ratio of ⩽3 dB at 12.1–20 GHz and a relative bandwidth of 49.2%. The polarization conversion rate is over 97% in 12.1–20 GHz. The physical analysis of the polarization conversion is based on the surface current distributions. To validate the simulations, microwave tests are carried out, and the theory, simulations, and experiments all accord quite well.

Design of metasurface-based polarization modulators by using Smith chart method

Polarization modulation (PoM) provides an additional degree of freedom for the modulation of carrier waves and may be an alternative to conventional wireless communication techniques that use amplitude, frequency, and phase modulations. Metasurfaces have provided unprecedented freedom for modulating the polarization state of electromagnetic waves. In this study, we introduce the Smith chart method to guide the design of metasurface-based modulator that transforms a linearly polarized wave into a circularly polarized wave. The proposed method bridges the metasurface pattern and its equivalent circuit impedance. We show that the parameter changes in the pattern can be directly displayed in the Smith chart, which provides clear physical image guidance to designers. According to the Smith chart analysis, we obtained a single-layer modulator with 50% efficiency and a three-layer modulator with 100% efficiency, all of which are verified using the full-wave simulation method. The Smith chart method will also be helpful in the design of other metasurface-based devices, such as metasurface absorbers.

Dynamic circular birefringence response with fractured geometric phase metasurface systems

Control over symmetry breaking in three-dimensional electromagnetic systems offers a pathway to tailoring their optical activity. We introduce fractured Pancharatnam–Berry-phase metasurface systems, in which a full-waveplate geometric phase metasurface is fractured into two half-waveplate-based metasurfaces and actively configured using shear displacement. Local relative rotations between stacked half-nanowaveplates within the metasurface system are transduced by shear displacement, leading to dynamic modulation of their collective geometric phase properties. We apply this concept to pairs of periodic Pancharatnam–Berry-phase metasurfaces and experimentally show that these systems support arbitrary and reconfigurable broadband circular birefringence response. High-speed circular birefringence modulation is demonstrated with modest shearing speeds, indicating the potential for these concepts to dynamically control polarization states with fast temporal responses. We anticipate that fractured geometric phase metasurface systems will serve as a nanophotonic platform that leverages systems-level symmetry breaking to enable active electromagnetic wave control.

Polarization manipulation associated with electromagnetically induced transparency based on metamaterials

The polarization of electromagnetic waves is a key feature in modern optics and information science research fields. How to effectively control the polarization state of electromagnetic waves in electromagnetically induced transparency (EIT) is still a challenge. Herein, the EIT behavior associated with transmissive linear-to-circular (LTC) polarization conversion is theoretically proposed in the terahertz (THz) regime by employing a metamaterial. Four asymmetric H-shaped resonators are arranged in a cross-shaped arrangement on the metamaterial. It has numerically demonstrated that the electric and magnetic resonances are mutually coupled, leading to the famous EIT phenomenon. Because of the strong dispersion properties and high transmission dominated by the EIT behavior and the anisotropy of metamaterial, the transmissive LTC polarization conversion is conspicuously realized while the x- and y-polarized waves are incidents. Furthermore, two new transparent windows are located in 1.396–1.654 THz and 1.258–1.721THz, which are higher than 0.9 can be gained from 1.409 (1.323) THz to 1.467 (1.594) THz, and the relative bandwidths are 4.0% and 18.6%, respectively. Moreover, the values of the maximum group delay and group index under the x- and y-polarized waves respectively reach 504 ps, 122 ps, and 18861, 4575, displaying a slow-light effect. In addition, the LTC polarization conversion is well obtained at 1.685 THz, where the transmission coefficient highly reaches 0.605. Based on the Stokes parameters, the corresponding ellipticity η, the band of the maximum 3 dB axial ratio, and the polarization azimuth ψ respectively are 0.961, 2.1%, and −42.5°. It is worth noting that not only the thickness of the reported metamaterial is subwavelength, but also the proposed structure with low loss is great transparency for the electromagnetic waves. Surpassing conventional EIT metamaterials and polarization converters, the proposed structure synchronously equipped with those two characteristics has numerous potential applications in the integration to the existing terahertz devices, future polarization controls, and the antenna fields.

Single-layer all-dielectric quarter-wave plate and half-wave plate metasurfaces for polarization conversion in the visible light region

Metasurfaces have the characteristics of ultra-thin volume, light weight, and planar structure, which can also effectively control the polarization state of electromagnetic waves. We propose single-layer all-dielectric quarter-wave plate (QWP) and half-wave plate (HWP) metasurfaces that work in the visible light region in transmission mode. The designed QWP can convert y-linearly polarized (YLP) light into left-handed circularly polarized light, whereas the HWP can convert YLP light into x-linearly polarized light. Moreover, the designed wave plates have a certain wide band working ability. When the incident angle is <30 deg for the QWP and <28 deg for the HWP, the polarization conversion performance shows strong robustness. The effects of fabrication tolerance, the height of elliptic-shaped pillar, and lattice constant on the polarization conversion performance are analyzed. In addition, the physical mechanism of polarization conversion is revealed by simulating the distribution of magnetic field and electric field inside unit structures. The results indicate that nanostructures of different sizes will excite different magnetic dipole resonance modes so as to realize the function of different polarization state conversions. It is believed that all-dielectric wave plate metasurfaces will become a good alternative for controlling the polarization state of electromagnetic waves.

Impact of Polarization Basis on Wind and Wave Parameters Estimation Using the Azimuth Cutoff From GF-3 SAR Imagery

The azimuth cutoff wavelength of SAR is an important parameter for retrieval of sea surface wind and wave. Earlier studies have fully demonstrated the substantial dependence of azimuth cutoff wavelength on polarization, but the present studies only focus on H-V linear polarization bases (HH, HV/VH, and VV) without considering the effects of other polarization bases (e.g., linear rotated, circular, and elliptical polarization). Benefiting from the quad-polarization advantage of GaoFen-3 SAR wave mode data and the support of polarization basis transformation theory, this study used 4,648 SAR data to study the correlation between cutoff wavelength and wind and wave parameters (e.g., significant wave height, and wind speed) under different polarization bases, and analyzed the variation of correlation coefficient caused by polarization basis change. Finally, the results were applied to evaluating the performance of wind and wave parameters retrieval. The results of the study show that the azimuth cutoff is strongly dependent on the polarization state of electromagnetic wave. The azimuth cutoff wavelength under the elliptical polarization bases has higher correlation with wind and wave than that under H-V linear, circular, and linear rotated polarization bases. Using the azimuth cutoff wavelength of the elliptical polarization bases can significantly improve the retrieval accuracy of wind and wave parameters. This study shall enhance the capabilities of polarized SAR systems to precisely derive more ocean surface properties. The result implies that polarization basis is an important factor that must be considered in future ocean SAR studies.

Transmission polarization converter based on V-shaped metasurface in terahertz region

Metasurfaces have attracted extensive attention due to their powerful functions, especially the manipulation of the polarization state of electromagnetic wave in many different areas, which have aroused a lot of research interest. In this work, a broadband transmission polarization converter based on V-shaped element array in terahertz band is designed and analyzed, which consists of grating-V-shaped metasurface-grating. The top layer and bottom layer form a pair of crossed gratings, and the middle layer is a V-shaped metasurface, and the layers are separated by polyimide. The structure parameters of the polarization converter are optimized by CST microwave studio, changes of which can result in narrow band or low transmission. Cross-polarization transmission rate and polarization conversion rate can reach more than 80% and 99%, respectively, in a frequency range from 0.35 THz to 1.11 THz. By studying the electric field distribution in the substrate under the V-shaped metasurface , it is found that the real part of the cross-polarization electric field between adjacent V-shaped metasurfaces presents similar values in a frequency range from 0.35 THz to 1.11 THz, resulting in high cross-polarization transmission. However, the real part of the cross-polarization electric field between adjacent V-shaped metasurfaces presents opposite values, resulting in low cross-polarization transmission at 1.40 THz. At the same time, the responses of the single layer structure of the V-shaped array and the bi-layer structure of the grating placed behind the V-shaped array to vertically incident x-polarized terahertz waves are investigated respectively, and the results show that the single-layer V-shaped array can convert part of linearly polarized incident light into cross-polarization light, however, in the bi-layer structure, Fabry-Perot cavity is formed between the V-shaped array and the grating, and the cross polarization transmission increases. This indicates that the V-shaped array provides the capability of polarization conversion, and the existence of the grating makes the F-P cavity inside the structure create the conditions for the back and forth reflection of terahertz waves. The combined action of the V-shaped metasurface and orthogonal grating results in a high polarization conversion rate.

Simultaneous Conversion of Polarization and Frequency via Time‐Division‐Multiplexing Metasurfaces

AbstractMetasurfaces are artificially engineered two‐dimensional materials composed of sub‐wavelength meta‐atoms, which have shown unprecedented capabilities in manipulating the amplitude, phase, frequency, and polarization states of electromagnetic waves. Specifically, polarization control can be attained via suitable anisotropic, linear, and time‐invariant designs, while frequency conversion is realized via nonlinear or time‐varying platforms. Simultaneous manipulations of polarization and frequency would be of considerable practical interest in many application scenarios, but remain unattainable with current approaches. Here, a time‐division‐multiplexing metasurface is proposed to realize the simultaneous conversion of polarization and frequency. The platform relies on time‐modulated polarization switches and, by varying the duty cycle and time delays of the polarization channels, can arbitrarily rotate the polarization at the central frequency of operation, and synthesize various polarization states at selected harmonic frequencies. Theoretical predictions are validated via measurements on a prototype operating at microwave frequencies, providing the first experimental evidence of simultaneous polarization and frequency conversions via time‐division‐multiplexing metasurfaces. The outcomes open a new pathway in manipulating the electromagnetic waves via time‐varying metasurfaces, and may be of interest for a broad variety of applications in scenarios ranging from polarization imaging to quantum optics.

Ultra-narrowband absorption filter based on a multilayer waveguide structure.

We propose a six-layer waveguide structure embedded in a single-layer grating based on guided-mode resonance (GMR), which can realize ultra-narrowband filtering with a tunable resonance wavelength. The filtering characteristics were analyzed and calculated by rigorous coupled-wave analysis (RCWA) and COMSOL Multiphysics. The narrowband resonance wavelength and absorption are tuned by changing the geometry and physical parameters of the structure such as the grating period and width, layer thickness, and materials. We designed and calculated the full width at half maximum (FWHM) and resonance absorption spectra in detail under different polarization states of electromagnetic waves. We obtained an absorption FWHM of 8.51e-5 nm for the transverse electric (TE) mode and 0.023 nm for the transverse magnetic (TM) mode, with the absorption coefficients having a value over 99.2%. The GMR filtering structure shows a good sensitivity and figure of merit (FOM) for refractive index sensing. For instance, a very high FOM of 17782.6/RIU for TM incidence is observed. These structures can have possible applications in optical information devices and sensors.

Broadband linear‐to‐circular polarization reflector using anisotropic metasurface

A high-performance polarization reflector is introduced based on anisotropic metasurface. This structure can convert a linearly polarized (LP) incident electromagnetic (EM) wave into a circularly polarized (CP) reflection wave. The unit cell of the proposed metasurface is made of metallic patch imprinted on the top surface of a metal-backed single-layer dielectric substrate. The shape of this metallic patch is a combination of a deformed Jerusalem cross and two deformed L. The simulation results show that the LP incident EM wave is converted to CP reflected wave from 7 to 19.4 GHz, which the bandwidth of axis ratio less than 3 dB is 12.4 GHz and the relative bandwidth is 94%. The current distributions and equivalent circuit model are given to analyze the generation of circularly polarized waves. To verify the metasurface structure performance, a sample consisting of 24 × 24 unit cells is fabricated and measured. The experimental and simulation 3-dB AR bandwidth are basically consistent with each other, which verify the properties of the design. The proposed metasurface structure has potential application in microwave sensors and antenna design to manipulate the polarization states of EM waves.

Dual-polarization programmable metasurface modulator for near-field information encoding and transmission

Controlling the polarization state of electromagnetic waves is an important topic in microwaves due to the enormous application potential in radar technology and mobile communications. Here, we propose a programmable metasurface based on single-pole double-throw switches to realize multifunctional polarization conversions. A structure of the double-sided metallic pattern is adopted in the metasurface, in which a novel double-pole double-throw hub is achieved to guide the energy direction. Such a mechanism successfully induces multiple transmission channels into the metasurface structure for functional design. By controlling the states of the switches with a field programmable gate array, the x- and y-polarizations of the incident waves can be efficiently modulated into linear co- and cross-polarizations of transmitted waves, suggesting a higher degree of freedom on wave manipulations. The proposed metasurface can be developed as a near-field information encoder to transmit binary coding sequence according to the energy distribution. Character transmissions are realized by programming binary ASCII codes on the transmitted fields. Nine supercells on the metasurface can encode 9-bit binary information in one frame of near-field imaging, which can be switched in real time with high speed. We envision that this work will develop digital coding applications to control the polarization information.

Design and experimental analysis of dual-band polarization converting metasurface for microwave applications

The manipulation of polarization state of electromagnetic waves is of great importance in many practical applications. In this paper, the reflection characteristics of a thin and dual-band metasurface are examined in the microwave frequency regime. The metasurface consists of a 22 × 22 element array of periodic unit cells. The geometry of the unit cell consists of three layers, including a 45° inclined dipole shape metal patch on top, which is backed by a 1.6 mm thick FR-4 substrate in the middle, and a fully reflective metallic mirror at the bottom. The proposed surface is exposed to horizontally (x) or vertically (y) polarized plane waves and the co and cross polarization reflection coefficients of the reflected waves are investigated experimentally in the 6–26 GHz frequency range. The metasurface is designed to convert incident waves of known polarization state (horizontal or vertical) to orthogonal polarization state (vertical and horizontal) in two distinct frequency bands, i.e. 7.1–8 GHz and 13.3–25.8 GHz. In these two frequency bands the simulated and experimental results are in good agreement. The polarization conversion ratio (PCR) of the surface is greater than 95% in the targeted frequency bands. A detailed parametric analysis of the metasurface is also discussed in this work and it has been estimated that the surface has the additional ability to convert linearly polarized waves to circularly polarized waves at several distinct frequencies. The proposed metasurface can be utilized in sensor applications, stealth technology, electromagnetic measurements, and antennas design.

Coherency vector formalism for polarimetric transformations

Despite the virtues of Jones and Mueller formalisms for the representation of the polarimetric properties, for some purposes in both Optics and SAR Polarimetry, the concept of coherency vector associated with a nondepolarizing medium has proven to be an useful mathematical structure that inherits certain symmetries underlying the nature of linear polarimetric transformations of the states of polarization of light caused by its interaction with material media. While the Jones and Mueller matrices of a serial combination of devices are given by the respective conventional matrix products, the composition of coherency vectors of such serial combinations requires a specific and unconventional mathematical rule. In this work, a vector product of coherency vectors is presented that satisfies, in a meaningful and consistent manner, the indicated requirements. As a result, a new algebraic formalism is built where the representation of polarization states of electromagnetic waves through Stokes vectors is preserved, while nondepolarizing media are represented by coherency vectors and general media are represented by coherency matrices generated by partially coherent compositions of the coherency vectors of the constituents.