Linear Polarization States Research Articles

Spectral Response and Wavefront Control of a C-Shaped Fractal Cadmium Telluride/Silicon Carbide Metasurface in the THz Bandgap.

We report theoretical investigations on the spectral behavior of two fractal metasurfaces, performed in the 3–6 THz frequency window (5–10 m equivalent wavelength window), under illumination with both linear and circular polarization state fields. Both metasurfaces stem from the same tree-like structure, based on C-shaped elements, made of cadmium telluride (CdTe), and deposited on silicon carbide (SiC) substrates, the main difference between them being the level of structural complexity. The simulated spectral behavior of both structures indicates the tunability of the reflection spectrum by varying the complexity of the tree-like structure.

Generation of arbitrary higher-order Poincaré sphere beam from a ring fiber laser with cascaded Q-plates

In this paper, vector vortex beams at arbitrary position on the higher-order Poincaré (HOP) sphere were directly generated by using a ring Ytterbium-doped fiber laser with cascaded Q-plates in the laser cavity. Here, two identical Q-plates were placed in cascade and utilized to achieve the intracavitary mode transformation between the LP01 mode and vector vortex modes. The change of the polarization state in the cavity was analyzed by using the Jones matrix, and the polarization distribution of arbitrary HOP sphere beam generated by this fiber laser was verified by the Stokes vector, theoretically. Then, it was confirmed experimentally that arbitrary HOP sphere beam (e.g., cylindrical vector beams, elliptically polarized cylindrical vector beams, and circularly polarized vortex beams) can be manipulated by changing the linear, elliptically, and circular polarization state in the cavity.

Self-temperature compensation approach for fiber specklegram magnetic field sensor based on polarization specklegram analysis

A self-temperature compensation method based on polarization specklegram analysis and magnetic birefringence effect of magnetic fluid (MF) is proposed for a fiber speckle sensor (FSS). The temperature compensation is achieved by measuring the variation difference of the two specklegrams with vertical linear polarization state. A fiber specklegram magnetic field sensor has been designed with this method and the magnetic-birefringence effect of MF. The measurement of magnetic field with high stability and good repeatability has been successfully achieved, which demonstrates the effectiveness of the proposed method. Due to the merits of low cost, ease of implement and system simplicity, the proposed method can promote the practical use of the FSS in long term monitoring.

Millimeter-Wave Slot-Based Cavity Antennas With Flexibly-Chosen Linear Polarization

Slot-based cavity antennas are hailed as promising candidates for millimeter-wave applications. Nevertheless, the linear-polarization (LP) angle of their broadside main beam is limited by the slots etched on the cavity’s top surface. In this work, an innovative technique is developed to significantly improve the selection flexibility of their LP inclination angle. It is attained by an integration of a single-layer, closely spaced C-shaped patch surface. A TE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">710</sub> -mode slot-based cavity antenna is employed as the base configuration, which radiates a broadside beam with its LP along <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\phi = 90^{\circ }$ </tex-math></inline-formula> . To effectively predict and monitor the polarization conversion of the surface-integrated TE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">710</sub> -mode cavity antenna, an analysis method using a unit cavity extracted from its original cavity antenna is presented. A subsequent surface-integrated system with the specified 45° LP was then simulated, fabricated, and measured. The measured results validate that a 45° LP state is achieved with an operating bandwidth from 33.3 to 36.5 GHz. Further investigation is conducted to flexibly choose the LP direction from <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\phi = 15^{\circ }$ </tex-math></inline-formula> to 165°. Two more examples with the fabricated antenna prototypes successfully radiate the specified <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\phi = 15^{\circ }$ </tex-math></inline-formula> and 75° LP beams, respectively. This near-field polarization conversion surface can be generalized to cavities with different resonant modes.

Design and characterization of polarization reconfigurable heart shape monopole antenna for 2.4 GHz application

&lt;span&gt;This article represented a heart shape reconfigurable monopole antenna with polarization diversity. The proposed antenna is fed by a 50 Ω microstrip feed line that is printed on a flexible FR-4 (εr=4.4) substrate. The antenna comprises a ring-slot, a cross slot and four positive-intrinsic-negative (PIN) diodes that are soldered on ring slot. Four PIN diodes act as a switch and by controlling these PIN diodes effective current direction is changed hence four various states of polarization are achieved. Four states of polarization such as horizontal linear polarization (H-LP), vertical linear polarization (V-LP), right-hand circular polarization (RHCP) and left-hand circular polarization (LHCP) can be switched easily with the help of these PIN diodes and achieved an efficiency of more than 90%. Proposed antenna shows voltage standing wave ratio (VSWR)&amp;lt;2 at all working frequency and -10 dB reflection coefficients (RC) bandwidths (BW) (i.e., S11≤-10 dB) about 32.86% for linear polarization (LP) states while RHCP and LHCP states possess BW of about 31.61% and 31.67% respectively. It also shows axial ratio (AR) BW of 3.41% and 2.44% for RHCP and LHCP, respectively. Besides, the antenna has a well-suited omnidirectional pattern with a positive gain of all working frequency of interest where cross-polarization level is much lower than that of antenna gain.&lt;/span&gt;

Configuring Polarization Singularity Array Composed of C-Point Pairs

The generation and topological configuration of double-layered polarization singularity array (PSA) having paired C-points of opposite topological index through superposing three Laguerre-Gaussian (LG) component beams with orthogonal linear polarization states are demonstrated. The generated PSA beam exhibits diversity in topological geometry and intensity pattern. The topological rule that the zero net topological charge and helicity conservation hold for the entire PSA is also confirmed. Based on our experiment setup consisting of a computer-controlled spatial light modulator (SLM), the PSA morphology can be flexibly customized and dynamically regulated by modulating the parameters of constituent beams. Our scheme provides a convenient device for manipulating polarization singularities and can facilitate the potential applications of singular optics.

Equal-efficiency diffraction of unpolarized wideband light with acousto-optic filters

We present the diffraction of unpolarized light over a broad wavelength range using paratellurite Acousto-Optic Tuneable Filters. The diffraction of unpolarized light usually relies on a specific operating point that is sensitive to the wavelength and angle of incidence, resulting in an operating ability over a narrow bandwidth. We present a detuning strategy to eliminate the angular dependence and obtain equal diffraction efficiency for both the linear polarization states over a broad bandwidth. We explain how to find the optimum operating frequency and present its dependency as a function of the wavelength. With the quasi-phase matching condition presented, the diffraction of unpolarized light over the 400 nm to700 nm band is possible with an efficiency of over 80%.

Complete analysis of beam analyzing powers in at near threshold energies

Focusing attention on the photon spin in at near threshold energies of interest to Big Bang Nucleosynthesis, a complete analysis of beam analyzing powers in reaction is carried out. A complete analysis of the reaction needs not only measurements using one state of linear polarization of photon but also measurements using another state of linear polarization inclined to the first at π/4 and the two states of circular polarization of the photon. A discussion on the complete characterization of the states of photon polarization is presented. The beam analyzing powers with respect to photon polarization are discussed theoretically, using model independent irreducible tensor formalism.

A Dipole with Reflector-Backed Active Metasurface for Linear-to-Circular Polarization Reconfigurability.

In recent years, significant advances have been made in diversifying the capabilities of communication systems by using reconfigurable antennas. There are many types of reconfigurable antennas—to achieve pattern, frequency, or polarization reconfigurability. These antennas are reconfigured either by the mechanical rotation of surfaces or by enabling or disabling specific sections of the structure using electrical switches. This paper focuses on the concept of a polarization reconfigurable antenna based on an active reflector-backed metasurface. An antenna system based on an active reflector-backed metasurface combined with a planar dipole is designed to achieve reconfigurable polarization. The polarization of the designed antenna can be switched between linear and circular polarization states using positive-intrinsic-negative diodes located in the unit cell elements of the metasurface. The measured results correlate well with the simulated results. The antenna has a physical size of 308 × 162 × 35 mm3 with an impedance bandwidth of 4.5% in the linear state and 7% in the circular state, as well as an axial ratio bandwidth larger than 8.3%.

Polarization-dependent photonic crystal fiber optical filters enabled by asymmetric metasurfaces

Abstract We demonstrate in-fiber polarization-dependent optical filter by nanopatterning an asymmetric metallic metasurface array on the end-facet of polarization-maintaining photonic-crystal fibers. The asymmetric cross-typed nanoslit metasurface arrays are fabricated on the core of the optical fiber using the focused ion beam milling technique. Highly polarization- and wavelength-dependent transmission with transmission efficiency of ∼70% in the telecommunication wavelength was observed by launching two orthogonal linear-polarization states of light into the fiber. Full-wave electromagnetic simulations are in good agreement with the experimental results. These advanced meta-structured optical fibers can potentially be used as novel ultracompact in-fiber filters, splitters, and polarization converters.

Terahertz Metagrating Emitters with Beam Steering and Full Linear Polarization Control.

We report the realization of broadband THz plasmonic metagrating emitters for simultaneous beam steering and all-optical linear polarization control. Two types of metagratings are designed and experimentally demonstrated. First, the plasmonic meta-atoms are arranged in a metagrating with a binary phase modulation which results in the nonlinear generation of THz waves to the ±1 diffraction orders, with complete suppression of the zeroth order. Complete tunability of the diffracted THz linear polarization direction is demonstrated through simple rotation of the pump polarization. Then, the concept of lateral phase shift is introduced into the design of the metagratings using interlaced phase gradients. By controlling the spatial shift of the submetagrating, we are able to continuously control the linear polarization states of the generated THz waves. This method results in a higher nonlinear diffraction efficiency relative to binary phase modulation. These functional THz metagratings show exciting promise to meet the challenges associated with the current diverse array of applications utilizing THz technology.

Pushing the Limits of Functionality-Multiplexing Capability in Metasurface Design Based on Statistical Machine Learning.

As 2D metamaterials, metasurfaces provide an unprecedented means to manipulate light with the ability to multiplex different functionalities in a single planar device. Currently, most pursuits of multifunctional metasurfaces resort to empirically accommodating more functionalities at the cost of increasing structural complexity, with little effort to investigate the intrinsic restrictions of given meta-atoms and thus the ultimate limits in the design. In this work, it is proposed to embed machine-learning models in both gradient-based and nongradient optimization loops for the automatic implementation of multifunctional metasurfaces. Fundamentally different from the traditional two-step approach that separates phase retrieval and meta-atom structural design, the proposed end-to-end framework facilitates full exploitation of the prescribed design space and pushes the multifunctional design capacity to its physical limit. With a single-layer structure that can be readily fabricated, metasurface focusing lenses and holograms are experimentally demonstrated in the near-infrared region. They show up to eight controllable responses subjected to different combinations of working frequencies and linear polarization states, which are unachievable by the conventional physics-guided approaches. These results manifest the superior capability of the data-driven scheme for photonic design, and will accelerate the development of complex devices and systems for optical display, communication, and computing.

Polarization-Insensitive Metalens and Its Applications to Reflectarrays With Polarization Diversity

A synthesized strategy for achieving polarization-insensitive metalens is proposed by imposing identical parabolic phase patterns individually on two decoupled left-handed (spin up, <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\sigma +$ </tex-math></inline-formula> ) and right-handed (spin down, <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\sigma -$ </tex-math></inline-formula> ) circular polarization (CP) channels of an anisotropic scheme. This is enabled by synergizing dynamic phases and local geometric phases within 0° and 90° rotations. As a consequence, similar focusing behavior is expected when it is illuminated by a plane wave of arbitrary polarization since any incoming wave is a superposition of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\sigma +$ </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">$\sigma -$ </tex-math></inline-formula> wave. Such an intriguing feature can be utilized to engineer a polarization-insensitive reflectarray/transmitarray in a shared aperture by placing a feeding source of different polarizations at its focal spot without altering the array aperture. Therein, the polarization of the meta-array is determined by its feeding which is readily to be changed in practice. For demonstration and possible applications, two reflectarrays composed of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$50\times50$ </tex-math></inline-formula> elements at Ku band with one in normal and the other in offset configuration were designed and experimentally characterized. Numerical and experimental results illustrate that both reflectarrays are able to work in Dual linear polarization (LP) and Dual CP states by utilizing LP and CP horns of different orientations. Moreover, the versatile polarization-agile property does not pose a significant penalty on its radiation performance and both reflectarrays exhibit a measured peak aperture efficiency of 63.8% and 39.6%, respectively, near 14 GHz, and a 1 dB gain bandwidth more than 11.8%.

Design and research of long-infrared dual-wavelength confocal metalens

Multi-wavelength confocal lens is an indispensable part of optical system, the traditional optical confocal system is often added by a certain number of optical lenses, or uses a different combination of optical lenses of different materials to implement multi-wavelength co-focusing,making the system possess a larger volume and weight, which, however, is difficult to meet the requirements for high integration and miniaturization of the system. As an optical element composed of two-dimensional planar subwavelength micro-element structure, the metalens has the advantages of flatness, ultra-thinness and regulating light waves, and has great potential applications in highly integrated and miniaturized optical confocal systems. According to relevant research reports, it is known that the existing research schemes of multi-wavelength confocal metalens have some shortcomings, such as relatively complex structure and relatively low focusing efficiency. In this work, a kind of metalens composed of simple micro-element structure is proposed and designed, which can simultaneously realize the long infrared dual wavelength confocal function. Based on the generalized Snell's law and the transmission phase modulation mechanism, a scientific evaluation function is established to select the optimal array of micro-elements structure to form a metalens. With the elliptical nano silicon column in a simple micro-element structure, the wavefront phase of the long infrared dual wavelength in the orthogonal linear polarization state can be adjusted independently and efficiently , while reducing the wavelength crosstalk and improving the focusing efficiency. The design results show that the proposed metalens achieves dual wavelength co-focused with a wavelength of 10.6 and 9.3 μm, and has a high focusing efficiency, The focusing spot is close to the diffraction limit. The quantitative analysis of the redundancy of the structural parameters of the metalens micro-element structure is made, and the trend of its influence on the focusing efficiency and the allowable deviation range of the micro-element structure parameters are obtained, which provides a theoretical basis for further precisely controlling the device fabrication. The matalens designed in this work is expected to meet the requirements for integration and miniaturization of long infrared optical confocal system, and has important applications in laser surgery, industrial cutting and welding and other fields.

Polarization-switchable plasmonic emitters based on laser-induced bubbles

Owing to weak light-matter interactions in natural materials, it is difficult to dynamically tune and switch emission polarization states of plasmonic emitters (or antennas) at nanometer scales. Here, by using a control laser beam to induce a bubble (<i>n</i>=1.0) in water (<i>n</i>=1.333) to obtain a large index variation as high as |Δ<i>n</i>|=0.333, the emission polarization of an ultra-small plasmonic emitter (~0.4<i>λ</i>²) is experimentally switched at nanometer scales. The plasmonic emitter consists of two orthogonal subwavelength metallic nanogroove antennas on a metal surface, and the separation of the two antennas is only <i>s</i><i>_x</i>=120 nm. The emission polarization state of the plasmonic emitter is related to the phase difference between the emission light from the two antennas. Because of a large refractive index variation (|Δ<i>n</i>|=0.333), the phase difference is greatly changed when a microbubble emerges in water under a low-intensity control laser. As a result, the emission polarization of the ultra-small plasmonic emitter is dynamically switched from an elliptical polarization state to a linear polarization state, and the change of the degree of linear polarization is as high as Δ<i>γ</i>≈0.66.

Polarization-State Modulation in Fano Resonant Graphene Metasurface Reflector

Although Polarization Modulation offers a high spectral efficiency, it has been not exploited much in modern communication systems because of the involvement of high switching speeds and complexity of associated hardware. We propose a dynamically controllable graphene metasurface capable to switch the polarization state of incident THz waves in real-time. The metasurface is designed by patterning two-dimensional graphene sheets with Fano-resonant chiral unit cells. Since the reflectance spectrum is characterized with co and cross polarized fields that bear asymmetric Fano line-shapes, several combinations of linear and elliptical polarization states are observed in a narrowband spectrum. At a fixed frequency, the electrostatic tunability property of graphene allows to switch between different polarization states by varying the chemical potential between 500 and 700 meV. The polarization state modulation with the proposed chiral graphene based metasurface is applied to implement a quaternary modulated digital communication system. Full-wave simulation results show the strong possibility of polarization modulation in THz communication systems. Graphene metasurfaces on account of the high electron mobility and discrete Fermi levels offer high switching speeds and seamless integration with digital systems.

Control of phase, polarization, and amplitude based on geometric phase in a racemic helix array

The Pancharatnam–Berry geometric phase has attracted great interest due to the elegant phase control strategy via geometric transformation of optical elements. The commonly used geometric phase is associated with circular polarization states. Here, we show that by exploiting the geometric phase associated with the two elliptical eigen-polarization states in a racemic metallic helix array, exotic features including full range phase modulation for linear polarization states, diverse polarization conversion, and full complex amplitude modulation can be obtained with rotation of the helices. As a proof of concept, several devices for implementing polarization conversion, vortex beam generating, and lateral dual focusing are built with a racemic helix array in the microwave regime. The calculated and experimental results validate our proposals, which can stimulate various advanced metadevices.

Linear-polarized terahertz isolator by breaking the gyro-mirror symmetry in cascaded magneto-optical metagrating

Abstract To realize nonreciprocal transmission, it is necessary to break the time-reversal symmetry of the transmission system, but it is very challenging to keep the linear polarized (LP) input and output unchanged in the free space transmission system. Magnetized semiconductor InSb can realize terahertz (THz) nonreciprocal transmission for the two conjugated photonic spin states, but it cannot realize efficient one-way transmission of LP state due to gyro-mirror symmetry. In this work, by introducing a pair of orthogonal uniaxial anisotropies from the meta-gratings on both sides of InSb, both the gyro-mirror and time-reversal symmetries are broken for the LP state, thus making this cascaded grating–InSb–grating structure serves as a high-performance isolator for the LP light. The experiment results indicate isolation of 50 dB at 0.4 THz for the same LP input and output under a weak biased magnetic field of 0.17 T. Moreover, we further illustrate the factors affecting the isolation bandwidth of the device, also demonstrated another broadband structure with the 10 dB isolation bandwidth from 0.2–0.7 THz, and the relative bandwidth achieves 110%. The mechanisms of THz nonreciprocal transmission and polarization manipulation proposed in this work will contribute to the development of efficient THz magneto-optical devices.

Two-dimensional birefringence measurement technique using a polarization camera.

A two-dimensional birefringence measurement method is described. In this procedure, we send light through a rotatable linear polarizer and subsequently through a transparent and birefringent specimen. A polarization camera measures the state of linear polarization from which the relative position of the refractive index axes and the relative phase difference is determined. The measurement range of the phase difference is up to π rad, which corresponds to an optical retardation of half the wavelength of the light. The imaging system can measure a large sample area within one measurement cycle. Measurement performance is demonstrated with a quarter-wave plate, and an exemplary test case is shown.

Normal-dispersion CS2-filled silica fiber with broadband single-polarization property

The CS2-filled silica fiber with two air holes in the cladding can offer high nonlinearity, high birefringence and normal group velocity dispersion (GVD) profiles for both two orthogonally polarized fundamental modes (FMs) by an appropriate choice of the fiber parameters. Furthermore, the highly birefringent CS2-filled fiber assisted by two subwavelength-diameter gold wires can exhibit broadband single-polarization (SP) property because that, only the y-polarized FM guided in the CS2-core can couple to the fundamental surface plasmon polariton (SPP) modes with the field vectors along the radial direction, and then has a much larger losses than the x-polarized FM in the wavelength region beyond 1500 nm. The numerical results show that when the 25-kW 0.1-ps pulse at 1920 nm is coupled into the 0.035-m fiber and linearly polarized along the fast axis, the output pulse exhibits over one-octave supercontinuum (SC) spectrum linearly polarized in the fast axis, and can maintain a linear polarization state across the whole SC spectrum even though the input polarization angle increases to 15° from the fast axis.