Circular Polarization States Research Articles

A Compact Single-Feed Patch Antenna with Frequency and Polarization Diversity

A new compact single feed square ring patch antenna using meandered 4λ transformer is designed, for frequency and polarization diversity. The proposed antenna is constructed from a square ring patch antenna, and two orthogonal meandered cascaded 4λ transformer (OMCT), incorporated with six switches for frequency and polarization reconfiguration purposes. The OMCT with switches is necessary to get good impedance bandwidth (BW) and axial ratio bandwidth (ARBW) for circular polarization state. In addition, it utilized to excite the antenna at two orthogonal locations, with equal magnitude and quadratic in phase, for achieving circular polarization mode at resonant frequencies 2.44GHz, 4.7GHz, and 5.6GHz. Moreover, it can excite the antenna as non-orthogonal modes for various other frequency bands, such as 2.89 GHz, 3.49 GHz, 4.9 GHz, 5.2GHz, 5.49GHz, 6.16GHz and 3.1GHz as linear polarization (LP) state. The proposed antenna has a compact low profile planar structure with area equal to 23mm2. Simulation and measured results show that the proposed antenna demonstrates a reasonable impedance bandwidth, and axial ratio in the circularly polarized state. Simulation results have been obtained from commercial CST-2014 Microwave Studio. The proposed antenna is fabricated for simulation result verification, and the implemented antenna is tested using R&S ZVL13 Vector Network Analyzer. The experimental confirms the simulation results.

Exploiting the circular polarization of light to obtain a spiral energy flow at the subwavelength focus

Using Richards–Wolf formulas, we show analytically that when strongly focusing a circularly polarized axisymmetric non-vortex field (e.g., a Gaussian beam or zero-order Bessel–Gaussian beam), a spiral energy flow is produced around the sub-wavelength focal spot. This phenomenon can be explained by the conversion of spin angular momentum associated with the circular polarization state into the near-focus orbital angular momentum, with the on-axis angular momentum being strictly zero. It is also shown that a linearly polarized optical vortex with topological charge 2 produces an on-axis near-focus energy backflow whose amplitude (defined by the negative longitudinal component of the Poynting vector) is comparable with that of the incident beam.

A novel filter wheel for multi-channel switching and polarization rotation

This study proposes an innovative filter wheel for the multi-channel switching of polarization state. The proposed device enables switching between two orthogonal linear polarization states or two orthogonal circular polarization states to achieve either linear dichroism (LD) or circular dichroism (CD). The filter wheel enables channel switching and orientation rotation with high repeatability. This paper describes in detail the methods used in the alignment of multiple polarizers and quarter-waveplates. In addition, the extinction ratio and retardation, which can be used as indications of polarimetric performance, are measured to a high degree of precision. The apparatus was applied for 6-channel polarization state switching in which three channels were used for multi-spectral broadband polarimetric applications across a range of 400–1600 nm. The remaining three channels were used for multi-wavelength precision polarimetric applications at wavelengths of 457, 532, and 632 nm in the visible range. When installed in a microspectrophotometer, the proposed apparatus can be used in multi-spectral broadband polarimetric spectroscopy as well as multi-wavelength polarimetric imaging applications to achieve LD and CD effects at the micro scale.

Circularly Polarized States Spawning from Bound States in the Continuum.

Bound states in the continuum in periodic photonic systems like photonic crystal slabs are proved to be accompanied by vortex polarization singularities on the photonic bands in the momentum space. The winding structures of polarization states not only widen the field of topological physics but also show great potential that such systems could be applied in polarization manipulating. In this Letter, we report the phenomenon that by in-plane inversion (C_{2}) symmetry breaking, pairs of circularly polarized states could spawn from the eliminated bound states in the continuum. Along with the appearance of the circularly polarized states as the two poles of the Poincaré sphere together with linearly polarized states covering the equator, full coverage on the Poincaré sphere could be realized. As an application, ellipticity modulation of linear polarization is demonstrated in the visible frequency range. This phenomenon provides a new degree of freedom in modulating polarization. The C points could also find applications in light-matter interactions. Further studying and manipulating the reported polarization singularities may lead to novel phenomena and physics in radiation modulating and topological photonics.

Analysis of the retarders’ errors in the channeled spectropolarimeters

Channeled spectropolarimetry (CS) system is an attractive scheme with compact structure, which can acquire full spectrally resolved Stokes parameters without any internal moving or electrically controllable parts. For the CS system, retarders are the key components to extract the spectral polarization information of the targets. However, there always exist non-negligible errors brought by the retarders which would severely damage the measurement accuracy. In this paper, the effects of the retardance errors and orientation errors of the retarders on the measurement accuracy of three typical CS systems, including the dual high-order retarders system (DHRS), the single high-order retarder system (SHRS), and the linear spectropolarimetric system (LSPS), are theoretically derived and analyzed. A numerical simulation is then performed for verification. The analysis and simulation results show that the SHRS and DHRS are less sensitive to the retarders’ errors than the LSPS. Although the LSPS is only used for measurement of the linear polarization state, the measurement accuracy is affected by the circular polarization state of the input light.

Metasurface hologram for polarization measurement.

Polarization measurement is crucial for many optical applications in science and technology. Geometric metasurfaces have been used to develop polarization-sensitive holograms, providing a new opportunity for polarization measurement. We propose and experimentally demonstrate a hologram method to measure the polarization state of light. A reflective-type metasurface hologram is used to generate holographic images of graphene pattern. The ellipticity and helicity of the incident light are measured based on the intensities of the neighboring light spots, corresponding to two opposite circular polarization states. Benefiting from the advantages of reflective geometric metasurfaces, this device can operate in broadband.

Polarization-Reconfigurable Leaky-Wave Antenna With Continuous Beam Scanning Through Broadside

A simple single-layer reconfigurable leaky-wave antenna (LWA) is presented that has polarization agility and beam-scanning functionality. This LWA system realizes a scanned beam that can be switched between all of its linear polarization (LP) and circular polarization (CP) states using only one dc biasing source. A slot-loaded substrate-integrated waveguide (SIW)-based LWA is first explored to attain CP performance with continuous beam scanning through broadside. This CP LWA realizes a measured CP performance with a 3 dB gain variance within 2.75–3.35 GHz for scan angles ranging from −28.6° to +31.5°. A row of shorted stubs is then incorporated into the CP LWA to obtain similar LP performance. Finally, by introducing p-i-n diodes into this LP LWA configuration to facilitate reconfigurable connections between the main patch and the shorted stubs, the radiated fields can be switched between all of its CP and LP states. The measured results of all three antennas confirm their simulated performance. It is demonstrated that the main beam of the polarization-reconfigurable LWA can be scanned from −31.5° to +17.1° with gain variations between 9.5 and 12.8 dBic in its CP state and from −34.3° to +20° with them between 7.8 and 11.7 dBi in its LP state.

Compact, Wideband, Planar Filtenna With Reconfigurable Tri-Polarization Diversity

A compact, wideband, planar filtenna with reconfigurable tri-polarization diversity is presented. The operating mechanisms of its bandpass filter and its bandwidth enhancement are explained through a step-by-step design evolution. Its feed network consists of three J-shaped probes, a Wilkinson power divider, and phase shifters. Reconfigurability is achieved by integrating four pairs of p-i-n (PIN) diodes into it. Three polarization-reconfigurable states: linear polarization (LP), left-hand circular polarization (LHCP), and right-hand circular polarization (RHCP), are dynamically controlled by those PIN diode switches. A prototype was fabricated, assembled, and tested, which has a compact size ( $0.48 \lambda _{0} \times 0.45 \lambda _{0}$ ) and is low profile ( $0.096~\lambda _{0}$ ). The measured results, in good agreement with their simulated values, demonstrate that the filtenna has a wide −10-dB impedance bandwidth ( $\vert \text{S}_{11}\vert \le -10$ dB) of 15.6%, which completely encompasses the axial ratio (AR) bandwidths (AR ≤ 3 dB) of the two circular polarization (CP) states. The realized gain values over this bandwidth: 7.7 ± 0.5 dBi, are very stable for all three dynamical states. Furthermore, the filtenna exhibits good out-of-band rejection performance characteristics.

Polarization controllable generation of flat superimposed OAM states based on metasurface.

In this paper, two kind of metasurface-based flat orbital angular momentum (OAM) superposition-state generators are proposed, which can generate OAM superposition states possessing tens of OAM modes being evenly spaced by topological charge number 1. The power spectra of the generated OAM superposition states are flat. The power variation of the OAM modes of the OAM superposition states from one generator is less than 3 dB, and the power variation of the OAM modes of the OAM superposition states from the other one is less than 0.3 dB. By controlling the left-handed and right-handed circular polarization states of the incident light, the OAM spectra of the OAM superposition states generated in the two polarization cases are separated from each other, therefore, the proposed generators are light polarization controllable. In addition, the two generators can operate efficiently on a wide wavelength region ranging from 635nm to 730nm. Our work may have some potential applications, such as used for OAM multicasting, OAM based optical manipulation, or manufacturing integrated OAM-superposition-state generators and OAM modulation devices.

Second-harmonic optical vortex conversion from WS2 monolayer

Wavelength, polarization and orbital angular momentum of light are important degrees of freedom for processing and encoding information in optical communication. Over the years, the generation and conversion of orbital angular momentum in nonlinear optical media has found many novel applications in the context of optical communication and quantum information processing. With that hindsight, here orbital angular momentum conversion of optical vortices through second-harmonic generation from only one atomically thin WS2 monolayer is demonstrated at room temperature. Moreover, it is shown that the valley-contrasting physics associated with the nonlinear optical selection rule in WS2 monolayer precisely determines the output circular polarization state of the generated second-harmonic vortex. These results pave the way for building future miniaturized valleytronic devices with atomic-scale thickness for many applications such as chiral photon emission, nonlinear beam generation, optoelectronics, and quantum computing.

Polarimetric Fiber-Optic Current Sensor With Integrated-Optic Polarization Splitter

We report on a simple, metering class polarimetric fiber-optic current sensor (FOCS) for electric power transmission systems. The sensor uses a fiber coil operated in reflection mode and an integrated-optic polarization splitter (IOPS) for interrogation. At alternating currents, the sensor performs comparably to high-end interferometric FOCS that work with an integrated-optic phase modulator in a closed-loop detection circuit. Besides the passive interrogation scheme, a particular advantage of the sensor is the use of standard single-mode fiber leads to the sensor head instead of polarization maintaining fiber. The IOPS introduces a 90° phase bias between the interfering light waves and generates two anti-phase sensor signals. By various measures, we obtain repeatable splitter performance between −40 °C and 85 °C. Like for interferometric FOCS, we employ the fiber retarder that generates the left and right circular or elliptical polarization states in the coil to compensate for the temperature dependence of the Faraday effect. We show that there are substantial differences, however, between interferometric and polarimetric FOCS in this regard. Finally, we demonstrate sensor accuracy well within ±0.1% during more than 200 hours of temperature cycling between −45 °C and 85 °C, which is unprecedented for polarimetric sensors.

Polarization-Tailored Raman Frequency Conversion in Chiral Gas-Filled Hollow-Core Photonic Crystal Fibers.

Broadband-tunable sources of circularly polarized light are crucial in fields such as laser science, biomedicine, and spectroscopy. Conventional sources rely on nonlinear wavelength conversion and polarization control using standard optical components and are limited by the availability of suitably transparent crystals and glasses. Although a gas-filled hollow-core photonic crystal fiber provides pressure-tunable dispersion, long well-controlled optical path lengths, and high Raman conversion efficiency, it is unable to preserve a circular polarization state, typically exhibiting weak linear birefringence. Here we report a revolutionary approach based on a helically twisted hollow-core photonic crystal fiber, which displays circular birefringence, thus robustly maintaining a circular polarization state against external perturbations. This makes it possible to generate pure circularly polarized Stokes and anti-Stokes signals by rotational Raman scattering in hydrogen. The polarization state of the frequency-shifted Raman bands can be continuously varied by tuning the gas pressure in the vicinity of the gain-suppression point. The results pave the way to a new generation of compact and efficient fiber-based sources of broadband light with a fully controllable polarization state.

Temperature Compensation of Interferometric and Polarimetric Fiber-Optic Current Sensors With Spun Highly Birefringent Fiber

We theoretically and experimentally investigate intrinsic temperature compensation of interferometric and polarimetric fiber-optic current sensors with a coil of spun highly birefringent fiber operated in reflection mode. The interferometric sensor recovers the differential magneto-optic phase shift of the left- and right-handed circular (or slightly elliptical) polarization states in the fiber by means of non-reciprocal phase modulation, whereas the polarimetric sensor employs a simple passive polarization analyzer. We show that the two sensor types exhibit substantial differences in their response to temperature changes. The main parameters that determine the sensors' sensitivity to temperature are, besides the fiber's Verdet constant, the intrinsic birefringence of the spun fiber, the retardation of the fiber retarder at the coil entrance that generates the elliptical polarization states, and the angle α between the principal axes of the retarder and spun fiber. In particular, fringe contrast changes at varying fiber birefringence make the polarimetric sensor version significantly more sensitive to temperature. Furthermore, whereas in the interferometric sensor, the contribution of the fiber birefringence to the temperature dependence disappears for special fiber orientations (α = 0°, 90°), such an arrangement is not possible for the polarimetric sensor. Also, the response to changes in the retardation of the retarder shows different patterns for the two sensor types. In spite of the differences, we achieve intrinsic temperature compensation well within ±0.2% between -40 °C and 85 °C for both sensors by using the retarder contribution to balance the contributions from the Verdet constant, the fiber birefringence, and potential further effects of temperature.

Zeeman-Induced Valley-Sensitive Photocurrent in Monolayer MoS_{2}.

The control of the valley degree of freedom lies at the core of interest in monolayer transition metal dichalcogenides, where specific valley-spin excitation can be created using circularly polarized light. Measurement and manipulation of the valley index has also been achieved, but mainly with purely optical methods. Here, in monolayer MoS_{2}, we identify a response to the valley polarization of excitons in the longitudinal electrical transport when the valley degeneracy is broken by an out-of-plane magnetic field B_{z}. The spin information is also simultaneously determined with spin-sensitive contacts. In the presence of B_{z}, a significant modulation of the photocurrent is observed as a function of the circular polarization state of the excitation. We attribute this effect to unbalanced transport of valley-polarized trions induced by the opposite Zeeman shifts of two (K and K^{'}) valleys. Our interpretation is supported by the contrasting behavior in bilayer MoS_{2}, as well as the observed doping and spatial dependence of the valley photocurrent.

Chiral Excitonic Organic Photodiodes for Direct Detection of Circular Polarized Light

AbstractA facile route to soft matter self‐powered bulk heterojunction photodiode detectors sensitive to the circular polarization state of light is shown based on the intrinsic excitonic circular dichroism of the photoactive layer blend. As light detecting materials, enantiopure semiconducting small molecular squaraine derivates of opposite handedness are employed. Via Mueller matrix ellipsometry, the circular dichroism is proven to be of H‐type excitonic nature and not originating from mesoscopic structural ordering. Within the green spectral range, the photodiodes convert circular polarized light into a handedness‐dependent photocurrent with a maximum dissymmetry factor of ±0.1 corresponding to 5% overall efficiency for the polarization discrimination under short circuit conditions. On the basis of transfer matrix optical simulations, it is rationalized that the optical dissymmetry fully translates into a photocurrent dissymmetry for ease of device design. Thereby, the photodiode's ability to efficiently distinguish between left and right circularly polarized light without the use of external optical elements and voltage bias is demonstrated. This allows a straightforward and sustainable future design of flexible, lightweight, and compact integrated platforms for chiroptical imaging and sensing.

Reflected and transmitted powers of arbitrarily polarized plane waves at interface between chiral and achiral media**Project supported by the National Natural Science Foundation of China (Grant Nos. 61431010 and 61501350) and the National Key Basic Research Program of China (Grant No. 2014CB340203).

A study has been made on the reflected and transmitted powers generated when an arbitrarily polarized plane wave impinges at a planar interface separating an isotropic achiral medium (IAM) and an isotropic chiral medium (ICM). It follows that the theoretical formulas are derived for the normalized reflection and transmission powers from an IAM–ICM interface for an arbitrary polarized incident wave, and those at an ICM–IAM interface for right-hand circularly polarized (RCP) or left-hand circularly polarized (LCP) incident wave are also obtained. The behavior of the reflected and transmitted waves in different cases is studied, and the dependence of the reflection/transmission of the incident wave on the incident angle and the parameters of the media is investigated in detail. Our numerical results show that high transmission is exhibited under the impedance matching condition and the incident wave can be split into two waves of the same circular polarization state in the case of the ICM–IAM interface, which indicates that circular polarizing beam splitter is achieved.

Spin-Selective Transmission in Chiral Folded Metasurfaces.

Controlling the spin angular momentum of light (or circular polarization state) plays a crucial role in the modern photonic applications such as optical communication, circular dichroism spectroscopy, and quantum information processing. However, the conventional approaches to manipulate the spin of light require naturally occurring chiral or birefringent materials of bulky sizes due to the weak light-matter interactions. Here we experimentally demonstrate an approach to implement spin-selective transmission in the infrared region based on chiral folded metasurfaces that are capable of transmitting one spin state of light while largely prohibiting the other. Due to the intrinsic chirality of the folded metasurface, a remarkable circular dichroism as large as 0.7 with the maximum transmittance exceeding 92% is experimentally demonstrated. The giant circular dichroism is interpreted within the framework of charge-current multipole expansion. Moreover, the intrinsic chirality can be readily controlled by manipulating the folding angle of the metasurface with respect to the cardinal plane. Benefiting from its strong chirality and spin-dependent transmission characteristics, the proposed folded metasurface may be applied to a range of novel photon-spin selective devices for optical communication technologies and biophotonics.

Multi-Beam Metasurface Antenna by Combining Phase Gradients and Coding Sequences

In this paper, we propose a strategy to achieve a single-feed multi-beam metasurface antenna by combining phase gradient and coding sequence. Theoretically, the multi-beam antennas with various far-field patterns can be realized via superimposing different kinds of phase coding sequences. As three proof-of-concepts, the 0101.../0101...coding sequence, chessboard coding sequence, and 1-bit sector coding sequence are selected to design double-beam, four-beam, and eight-beam circular-polarized reflector antennas, respectively. Furthermore, the four-beam reflector antenna is fabricated and measured to demonstrate the feasibility of our strategy. The fabricated antenna can produce four perfect pencil-shaped beams in a wide frequency range from 11 to 13 GHz. At central frequency, the realized gain is about 19.9 dB and the axial ratio is lower than 3 dB. Moreover, the circular polarization state of the reflective beams is consistent with the polarization state of spot source, which differs from the reflector antennas based on the polarization-insensitive unit cells. This paper connects the multi-beam patterns with binary digits, which provide the potential for reconfiguring the multi-beam patterns by computer programs.

Method for Mueller matrix acquisition based on a division-of-aperture simultaneous polarimetric imaging technique

The Mueller matrix is an important parameter in the field of polarization optics, and how to measure it efficiently and accurately becomes considerably significant for its practical applications. In this paper, for the first time to our knowledge, we propose a new method for Mueller matrix acquisition based on a division-of-aperture simultaneous polarimetric imaging technique. Different from the traditional method where, for obtaining the 16 elements of the Mueller matrix, a 16-times-measurement must be conducted by jointly changing the states of polarization (SOPs) of the input and output lights, while in our method it is enough by just changing the SOPs of the input light 4 times. This time-saving and easy-calculating feature is owing to our specific polarimetric camera, where a full-Stokes vector is obtained easily since 3 linear SOPs (0°, 45°, 90°) and 1 circular SOP can be recorded simultaneously by spatially sharing the same detector. To simply verify the effectiveness of our method, a specific polarization device patched by 8 sub-polarizers is chosen as the sample to be measured. Experimental results show that they are consistent with the theoretical results, both in the Mueller matrix and the corresponding images. We predict that this method for Mueller matrix acquisition can get wide potential applications for rapid identifications in biomedicine and material science.

Information Encoding with Optical Dielectric Metasurface via Independent Multichannels

Information encryption and security is a prerequisite for information technology, which can be realized by an optical metasurface owing to its arbitrary manipulation over the wavelength, polarization, phase, and amplitude of light. So far information encoding can be implemented by the metasurface in one-dimensional (1D) mode (either wavelength or polarization) only with several combinations of independent channels. Herein, we design dielectric metasurfaces by multiplexing for information encoding in a two-dimensional (2D) mode of both wavelength and polarization. Sixty-three combinations made out of six independent channels by two circular polarization states (RCP and LCP) and three visible wavelengths (633, 532, and 473 nm) are experimentally demonstrated, in sharp contrast with seven combinations by three independent channels in 1D mode. This 2D mode encoding strategy enhances the encryption security dramatically and paves a novel pathway for escalating the security level of information in multichannel ...