Twist Polarizer Research Articles

ПОЛЯРИЗАЦІЙНО ТА ЧАСТОТНО СЕЛЕКТИВНІ ХАРАКТЕРИСТИКИ ХІРАЛЬНОЇ ПОВЕРХНІ, ЩО СКЛАДАЄТЬСЯ З ПЕРІОДИЧНО РОЗТАШОВАНИХ ДІЕЛЕКТРИЧНИХ КВАДРАТНИХ СПІРАЛЕЙ

Subject and Purpose. The present study is concerned with the linearly polarized electromagnetic wave transmission through a chiral metasurface composed of periodically assembled square dielectric helices. We expect that the metasurface of the kind has a wider range of functional capabilities to transform a polarized wave into a cross-polarized one when compared to a similar metasurface composed of metal helices. Methods and Methodology. To find the scattering coefficients of the considered structure, the well-established method of integral functionals is followed. A set of volume integral equations in the vectorial form is solved for the equivalent electric and magnetic polarization currents of the analyzed periodic layer. A distinctive feature of the method is that the internal electromagnetic fields of the structure are initially found, whence the fields scattered by this structure are sought. The equations are discretized in terms of integral functionals related to the polarization currents and through the use of the double Floquet–Fourier series expansion technique. Results. It has been found that the metasurface transmission coefficients depend critically on the number of bars making the square helical particle. In the case of an even bar number, the chiral metasurface exhibits the same transmission coefficients for co- polarized field components in the event that linearly x- and y-polarized waves are incident. For cross-polarized field components, the transmission coefficients differ and can reach peak values at different frequencies. Finally, transmission coefficients of these polarizers have been investigated versus dimensions of helice-making bars. Conclusion. A wide variety of transmission properties observed in the metasurfaces make them particularly attractive for use in polarization converting and separating devices. The metasurface can feature dichroic asymmetric transmission and be used as a dichroic filter with polarization transformation. It can be put to use in differential phase sections and, also, as an effective dichroic cross-polarization converter (twist polarizer)

Chiral metasurfaces formed by 3D-printed square helices: A flexible tool to manipulate wave polarization

The transmission of linearly and circularly polarized waves is studied both theoretically and experimentally for chiral metasurfaces formed by arrays of metallic square helices. The helical particles of the metasurfaces are constructed of rectangular bars manufactured by direct three-dimensional printing in solid metals. The transmittance of the metasurface is found to depend critically on the number of bars forming the square helical particles. In the case of an even number of bars, the chiral metasurface exhibits identical co-polarized transmittance of orthogonal linearly polarized waves, which are characterized by a dual-band asymmetric transmission. For an odd number of bars, the metasurface provides the same cross-polarization conversion for any polarization orientation of the incident field and thus serves as a polarization-independent twist polarizer. Finally, the transmittance of this polarizer is investigated with respect to the dimensions of the square helices. The investigated chiral metasurfaces are characterized by strong broadband circular dichroism regardless of the number of bars in the helical particles. The wide variety of transmission properties observed in the metasurfaces makes them particularly attractive for use in polarization conversion and separation devices.

Highly transparent twist polarizer metasurface

A twist polarizer metasurface for polarization rotation by an angle of 90° is proposed and realized at microwave frequencies. The metasurface consists of sub-wavelength metallic helices arranged periodically in a single layer and operates in transmission geometry with a nearly unity cross-polarization conversion coefficient at resonance. The structure exhibits low reflectivity R<0.06 within a decade-spanning frequency range of 0.1–5.5 GHz and is insensitive to the polarization orientation of the incident wave. Moreover, it can operate with high efficiency at oblique incidence angles of up to 35°. Such twist polarizer metasurfaces are potentially applicable as electromagnetic/optical isolators and frequency-selective polarization antennas.

Flexible twist polarizer based on ultrathin multi‐layered polymer‐carbon nanotubes composite films

AbstractWe present an ultrathin and flexible microwave broadband polarization transformer concept based on oriented carbon nanotubes dispersed in a polycarbonate matrix. Stacking anisotropic composite films with a controlled angle mismatch between successive layers produces a twist polarizer able to change the orientation of the electric field over broad frequency range in the microwave domain. The smart organization confers to the multilayer a high efficiency measured as a degree of twist, which reaches 75.1% at GHz frequencies for a 420 μm thick seven‐layer stack.

Twisted optical-fiber-based acousto-optic tunable filter controlled by the flexural acoustic polarization

The spectral characteristics of twisted fiber-based acousto-optic filters are theoretically investigated. The influences of three types of flexural acoustic polarization states — linear, circular, and elliptical polarizations — on filter spectra are studied under realistic experimental conditions: a fiber length of 5 – 20 cm and a circumferential fiber twist angle of 80 nm for a twist angle of 12π. We will describe how the transmission of each resonance peak can also be selectively tuned by adjusting the ellipticity of the input acoustic polarization from linear to circular. The results illustrate that our approach exploiting a combination of the fiber twist and acoustic polarization management offers an excellent route to the spectral shaping of all-fiber acousto-optic devices in that the transmission of multiple resonances, as well as their spectral positions, are readily and individually controllable in a single device configuration. In addition, we also propose a novel cosine apodization method to suppress the undesirable sidelobe spectra occurring between the dual resonance peaks. The technique is based on a cosine modulation of the AO coupling strength along the fiber, which is achieved by using a combination of the fiber’s circumferential twist and the linear acoustic polarization. The proposed scheme is useful to minimize the crosstalk occurring between adjacent resonance peaks. We highlight that our approach is directly applicable to matched filtering as robust, adaptable, stable, and versatile optical filters.

Synthesis of Polarization Transformers

We study the possibility of analytically synthesizing different polarization-transforming devices realized as arrays of small particles. The proposed method is based on general relations between the reflection and transmission coefficients and the polarizabilities of arbitrary bi-anisotropic particles. As an example, we reveal all possible types of inclusions which can be used to realize twist polarizers, select one of them and synthesize a novel twist polarizer. The synthesized twist polarizer is then fitted on a standard printed circuit board, optimized numerically, and finally manufactured and measured. The experimental results for the twist polarizer show good correspondence with the simulations and the new method is found to be a useful tool for developing any polarization-transforming devices. As one more example, a novel circular polarization selective surface is synthesized.

Orthogonal polarization mode coupling for pure twisted polarization maintaining fiber Bragg gratings

Spectral characteristics of orthogonal polarization mode coupling for pure twisted polarization maintaining fiber Bragg gratings (PM-FBG) are proposed and analyzed experimentally and theoretically. Different from the polarization mode coupling in PM-FBG due to side pressure, a resonant peak at the middle of two orthogonal polarization modes is found when the PM-FBG is twisted purely which is attributed to the cross coupling of polarization modes. Its intensity increases with the twisting rate. A new coupled mode equation is built to describe the pure twist polarization mode coupling, in which both the normal strain induced by strain-applied parts and the tangential strain induced by twisting are taken into consideration and expressed in a unified coordinate. The novel phenomenon and its explanation are believed to be helpful for PM-FBG applications in fiber sensor and laser technologies.

RCS control using cascaded circularly polarized frequency selective surfaces and an AMC structure as a switchable twist polarizer

AbstractThis article presents a cascaded arrangement comprising a double‐layer frequency selective surface circularly polarizing (CPFSS) and a second screen that can be switched between artificial magnetic conductor (AMC), or perfect electric conducting, (PEC), states.The CPFSS consists of two stacked aluminum sheets patterned with periodic split ring structures. While the AMC is a PCB sheet patterned with metallic squares interconnected by links. By either open or short circuiting these links it is shown that the cascade of screens can be made to twist, or not to twist, an incident 45° linearly polarized signal through 90° upon reflection from the assembly. The system was designed and optimized using CST software and predictions were validated experimentally at X‐band. The results obtained show good agreement between experimental and measured monostatic reflection loss results. © 2010 Wiley Periodicals, Inc. Microwave Opt Technol Lett 52: 577–580, 2010; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.24979

Design of doubly periodic filter and polarizer structures using a hybridized genetic algorithm

A hybrid genetic algorithm/local optimization technique for designing cascaded planar periodic structures such as frequency selective surfaces is developed. The genetic algorithm optimizes the shape and size of the periodic metallization in each of the screens as well as the thickness and dielectric constant of intervening dielectric layers in order to achieve a desired frequency response characteristic. Careful local optimization to fine‐tune the dielectric layer thicknesses is shown to accelerate both the spectral analysis of the cascades and the convergence of the stochastic search. Results are presented for the design of a satellite subreflector with a 3:1 ratio of stop band center to passband center, a circular polarizer with an axial ratio of less than 1 dB between 47 and 64 GHz, and a twist polarizer with a better than −1‐dB reflection into the cross‐polarized field component for 47–65 GHz.

Liquid-crystalline properties of some chiral benzoate and phenylpropiolate esters with optically active centres derived from (S,S)-isoleucine and (S,R)-allo-isoleucine

Four chiral mesogenic materials based on benzoate and phenylpropiolate core systems have been synthesized. The optically active moieties of the four materials were derived from the α-amino acids Isoleucine and allo-Isoleucine in order to facilitate the preparation of two compounds with an S,S configuration of sequential chiral centres and two compounds with an S,R configuration of sequential chiral centres in the terminal chain. In all four materials, the chiral centres located closest to the core of the mesogen were identical in terms of structure and absolute configuration. The absolute configuration of the second chiral centre was dependent on whether the material was derived from Isoleucine or allo-Isoleucine leading to both S and R forms being available. The study of these materials enabled a close study of the effects that the second chiral centre can have on mesogenic properties. The materials were analysed in terms of their phase behaviour, and in particular the incidence of frustrated mesophases. In addition, some of the properties of the smectic C* phase (helical twist sense and polarization direction) were determined for each material and the results analysed in terms of the predictions expected for the chiral centres when treated independently. Computer modelling of these systems enabled an explanation of the physical results to be formulated.

Analysis of a strip-grating twist reflector

We present a rigorous analysis of scattering from a twist reflector composed of a strip grating placed on a dielectric slab and backed by a conducting metallic sheet. The obliquely incident, arbitrarily polarized plane wave is decomposed into fast- and slow-polarized modes, and a complete and accurate solution of the total diffracted fields is derived. The solution procedure used is an extension of our solution procedure developed for the analysis of scattering from a strip grating. We calculate the optimum height w, in order for the fast- and slow-polarized diffracted fields to be in phase, for various angles of incidence and different periods of the strip grating. With air as the dielectric between the strip grating and the ground plane, it is shown that a strip-grating twist polarizer behaves similarly to the corrugated-surface twist polarizer.

A spectral-iteration technique for analyzing a corrugated-surface twist polarizer for scanning reflector antennas

An analysis of the corrugated-surface twist polarizer which finds application in the design of scanning reflector antennas is presented. The spectraI-interation technique, a novel procedure which combines the use of the Fourier transform method with an iterative procedure is employed. The first step in the spectral-iteration method is the conversion of the original integral equation for the interface field into a form which is suitable for iteration using a method developed previously. An important feature of the technique is that it takes advantage of the discrete Fourier transform (DFT) type of kernel of the integral equation and evaluates the integral operators efficiently using the fast Fourier transform (FFT) algorithm. Thus, in contrast to the conventional techniques, e.g., the moment method, the spectral-iteration approach requires no matrix inversion and is capable of handling a large number of unknowns. Furthermore the method has a built-in check on the satisfaction of the boundary conditions at each iteration.