Polarization Purity Research Articles

(Invited) Proximity Induced Chiral Quantum Light Generation in Strain-Engineered WSe2/NiPS3 Heterostructures

Quantum light emitters (QEs) capable of generating single photons of well-defined circular polarization could enable non-reciprocal single photon devices and deterministic spin-photon interfaces critical for realizing complex quantum networks. To date, emission of such chiral quantum light has been achieved via the application of intense external magnetic fields electrical/optical injection of spin polarized carriers/excitons, or coupling with complex photonic/meta-structures. Here we report free-space generation of highly chiral single photons from QEs created via nanoindentation of monolayer WSe2 - NiPS3 heterostructures at zero external magnetic field. These QEs emit in the 760-800 nm range with a degree of circular polarization and single photon purity as high as 0.89 and 80% respectively, independent of pump laser polarization. Scanning diamond nitrogen vacancy microscopy and temperature dependent magneto-photoluminescence studies reveal that the chiral quantum light emission arises from magnetic proximity interactions between localized excitons in the WSe2 monolayer and the out-of-plane magnetization of defects in antiferromagnetic (AFM) order of NiPS3, both of which are co-localized by the strain field associated with the nanoscale indentations.

Proximity-induced chiral quantum light generation in strain-engineered WSe2/NiPS3 heterostructures.

Quantum light emitters capable of generating single photons with circular polarization and non-classical statistics could enable non-reciprocal single-photon devices and deterministic spin-photon interfaces for quantum networks. To date, the emission of such chiral quantum light relies on the application of intense external magnetic fields, electrical/optical injection of spin-polarized carriers/excitons or coupling with complex photonic metastructures. Here we report the creation of free-space chiral quantum light emitters via the nanoindentation of monolayer WSe2/NiPS3 heterostructures at zero external magnetic field. These quantum light emitters emit with a high degree of circular polarization (0.89) and single-photon purity (95%), independent of pump laser polarization. Scanning diamond nitrogen-vacancy microscopy and temperature-dependent magneto-photoluminescence studies reveal that the chiral quantum light emission arises from magnetic proximity interactions between localized excitons in the WSe2 monolayer and the out-of-plane magnetization of defects in the antiferromagnetic order of NiPS3, both of which are co-localized by strain fields associated with the nanoscale indentations.

Independently Polarization Manipulable Liquid- Crystal -Based Reflective Metasurface for 5G Reflectarray and Reconfigurable Intelligent Surface

This study presents a new class of independently polarization manipulable liquid-crystal (LC)-based reflective metasurface antenna. To the best of the authors’ knowledge, this study first introduces independent polarization controllability of the LC-based reflective metasurface in mmWave. Two LC layers are embedded into the proposed structure for independent manipulation of the polarization. Isolation topologies, such as rectangular patch shape and metal strip ground, are utilized to enhance polarization purity, achieving compact bias configuration. The independent polarization controllability and antenna performance, such as aperture efficiency, sidelobe level, and beam coverage, are validated as a reflectarray. The proposed antenna has a maximum aperture efficiency of 22.5% and beam coverage of over 120°. In addition, the proposed metasurface is employed to achieve link connectivity as a reconfigurable intelligent surface (RIS) at millimeter-wave 5G. The demonstration confirms a small aperture RIS concept, considering spillover and taper efficiencies.

Design of LP and CP microstrip antennas covered by lossy thermal protection system

Low-temperature antenna system that covered by a dedicated thermal protection system (TPS) is an indispensable component of hypersonic vehicle. However, electromagnetic interactions between the installed antennas and the TPS tend to impair the antenna's performance, such as the resonant frequency, radiation efficiency, and polarization purity. To mitigate these negative effects of placing a thick, lossy TPS in close proximity to the microstrip patch antennas, this article proposes a concept of near-field matching technique. The magnetic field distribution of the installed antenna is calculated in the near field, followed by adopting a surface fitting method to determine the optimal shape of a modified TPS. Two patch antennas with linear polarization (LP) and circular polarization (CP) are analyzed by this method, respectively. It is discovered that different TPS shapes suit LP and CP antennas, respectively. Both experimental and numerical results show that the proposed TPS structure improves the impedance matching and radiation properties of the microstrip antennas simultaneously. The proposed design achieves a harmonious balance between the electrical and thermal protection performance, surpassing that of a traditional flat TPS. To verify the design, a prototype CP antenna is fabricated and measured.

Efficient Synthesis of Concentric-Rings Plane Wave Generators

In this paper, we present an efficient method for synthesizing sparse plane wave generators (PWGs) based concentric ring arrays. In particular, we introduce a novel circularly-polarized ring source whose field on a near-field surface can be represented using only three scalar functions. Using this representation, we can simplify the synthesis, reducing substantially its computational complexity, which can be easily handled using an ordinary office PC. After using the ring sources to identify the position and excitation of the PWG elements, we can employ discrete linearly polarized sources as feeds or use different kinds of radiators. Some numerical simulations validate the proposed approach, which can obtain planar and volumetric quiet zones with good polarization purity.

Polarization-mismatching transmissive metasurface for independent amplitude and phase control of circular polarization.

This work presents a strategy for independent control of the amplitude and phase of transmissive circular-polarization (CP) waves. The designed meta-atom consists of an elliptical-polarization receiver and a CP transmitter. By changing the axial ratio (AR) and polarization of the receiver, amplitude modulation can be realized based on polarization mismatching theory, with negligible cumbrous components. While by rotating the element, a full phase coverage enabled by the geometric phase is achieved. Subsequently, a CP transmitarray antenna (TA) with high gain and low side-lobe level (SLL) is implemented to experimentally validate our strategy, and the tested results match well with the simulated ones. During the operating band from 9.6 to 10.4 GHz, the proposed TA obtains an average SLL of -24.5 dB, a lowest SLL of -27.7 dB at 9.9 GHz, and a maximum gain of 19 dBi at 10.3 GHz, with the measured AR lower than 1 dB, which mainly benefits from high polarization purity (HPP) of the proposed elements. The proposed strategy for full amplitude-phase manipulation of CP waves together with HPP paves a way for complicated field manipulations and indicates a promising candidate in antenna applications, such as anti-jamming systems and wireless communications.

Patch antenna with low cross‐polarization using loading technique

AbstractIn this article, a low cross‐polarized patch antenna is proposed with simple loading techniques. A metal block is set next to the patch and a coupling fringing field generated by the block compensates for the fringing field distribution asymmetry of the patch, resulting in a lower cross‐polarization. In addition, a shorting pin is loaded to further improve polarization purity. To demonstrate the effectiveness of the proposed scheme, prototypes of the proposed antenna with specific parameters were fabricated and tested. The measured cross‐polarized level can be reduced to below −30 dB, which is greatly lower than that of the traditional patch.

Antennas for low-frequency radio telescope of SKA

The low-frequency radio telescope of the Square Kilometre Array (SKA) is being built by the international radio astronomical community to (i) have orders of magnitude higher sensitivity and (ii) be able to map the sky several hundred times faster, than any other existing facilities over the frequency range of 50–350 MHz. The sensitivity of a radio telescope array is in general, dependent upon the number of electromagnetic sensors used to receive the sky signal. The total number of them is further constrained by the effects of mutual coupling between the sensor elements, allowable grating lobes in their radiation patterns, etc. The operating frequency band is governed by the desired spatial and spectral responses, acceptable sidelobe and backlobe levels, radiation efficiency, polarization purity and calibratability of sensors’ response. This paper presents a brief review of several broadband antennas considered as potential candidates by various engineering groups across the globe, for the low-frequency radio telescope of SKA covering the frequency range of 50–350 MHz, on the basis of their suitability for conducting primary scientific objectives.

Wideband Circularly Polarized mm-Wave Array Antenna Using H-Shaped Low-Axial-Ratio Apertures

An H-shaped all-metal radiating element comprising an active and a coupled slot has been proposed to design wideband high-efficiency circularly-polarized antenna arrays. The proper excitation of several modes on the coupled slot enables a wide axial ratio bandwidth with high circular polarization purity. In order to feed an array of such radiating elements, a simple and scalable two-layer corporate network, implemented in groove gap waveguide technology, is conceived. Those good frequency properties have been confirmed by the fabrication of a prototype, whose experimental results report a 1-dB axial ratio bandwidth of 22%. In addition, a peak radiation efficiency of 97% with good pattern stability, and an impedance bandwidth of 21% with high return loss, have been measured. The high circular polarization purity demonstrated by these wideband apertures makes them particularly suitable for Ka-band satellite communications antennas.

A 3-D Metal-Printed Dual-Polarized Ridged Waveguide Slot Array Antenna for X-Band Applications

A compact three-dimensional (3-D) metal-printed ridged waveguide slot array antenna with wideband, dual linear polarization and low cross-polarization is presented for X-band applications. The proposed design achieves a compact structure due to the method of using a ridged waveguide (RWG). A vertically linear polarized (VP) array is realized through eight longitudinal shunt slots etched on the non-ridged broad wall of the RWG, while a horizontally linear polarized (HP) array is realized through eight V-shaped slots cut on the ridged broad wall of the RWG. To extend the operating bandwidth, each 1 × 8-element linear array is separated into two 1 × 4-element sub-arrays through a central feeding technique. The two sub-arrays of the VP array are excited with equal amplitude and are 180° out-of-phase through a T-junction power divider, which connects the coaxial feed and the radiated waveguide, while the two HP subarrays are directly excited by a coaxial feed with equal amplitude and remain in phase. The feed structure uses coaxial probes directly attached to the RWG to provide good mechanical stability and low voltage around the feed. An optimized 2 × 8-element dual-polarized array is fabricated using 3-D metal-printing technology for experimental verification. The measured −10 dB impedance bandwidth of the VP array is 1.2 GHz (12.2%) and, of the HP array, is 1.1 GHz (11.2%) with maximum gains of 14.6 and 15.5 dBi, respectively. There is good polarization purity over the entire operating bandwidth with a cross-polarization level better than −40 dB in the main beam direction. The isolation between two orthogonal polarized arrays is greater than 40 dB over the whole band.

Broadband 2D phase-gradient metasurface for linearly-polarized waves by suppressing Lorentz resonance of meta-atoms.

Metasurfaces have exhibited versatile capacities of controlling electromagnetic (EM) waves due to the high degree of freedom of designing artificially engineered meta-atoms. For circular polarization (CP), broadband phase gradient metasurfaces (PGMs) can be realized based on P-B geometric phase by rotating meta-atoms; while for linear polarization (LP), realization of broadband phase gradients has to resort to P-B geometric phase during polarization conversion and polarization purity has to be sacrificed for broadband properties. It is still challenging to obtain broadband PGMs for LP waves without polarization conversion. In this paper, we propose the design of 2D PGMs by combining the inherently wideband geometric phases and non-resonant phases of meta-atom, under the philosophy of suppressing Lorentz resonances that usually bring about abrupt phase changes. To this end, an anisotropic meta-atom is devised which can suppress abrupt Lorentz resonances in 2D for both x- and y-polarized waves. For y-polarized waves, the central straight wire is in perpendicular to electric vector Ein of incident waves, Lorentz resonance cannot be excited although the electrical length approaches or even exceeds half a wavelength. For x-polarized waves, the central straight wire is in parallel with Ein, a split gap is opened on the center of the straight wire so as to avoid Lorentz resonance. In this way, the abrupt Lorentz resonances are suppressed in 2D and the wideband geometric phase and the gradual non-resonant phase are left for broadband PGM design. As a proof of concept, a 2D PGM prototype for LP waves was designed, fabricated and measured in microwave regime. Both simulated and measured results show that the PGM can achieve broadband beam deflection for reflected waves for both x- and y-polarized waves in broadband, without changing the LP state. This work provides a broadband route to 2D PGMs for LP waves and can be readily extended to higher frequencies such as terahertz and infrared regimes.

Two-Port Dual-Band Filtering Network and Its Application on Filtering Antennas

This letter presents a two-port dual-band filtering network, which can be integrated with antennas for dual-band filtering antenna applications in sub-6 GHz systems. The proposed network consists of a slotline resonator and two stub groups, featuring second- and third-order resonant characteristics at the lower and upper frequencies. Analysis by establishing equivalent circuits is carried out to verify the performance of the network. Moreover, the proposed filtering network is integrated with a 2 × 2 antenna array. Notice that the 2 × 2 antenna array serves as the final resonator of the filtering network during the integration. A prototype working at 3.5 and 4.9 GHz is fabricated for verification purposes. The filtering antenna array features sharp roll-off at the edges, with low insertion loss, high polarization purity, and wide fractional bandwidth of 11.7% at 3.5 GHz and 13.5% at 4.9 GHz.

A Wideband Cavity-Backed Dual-Polarized Antenna for X-Band Applications

In this letter, a cavity-backed dual-polarized antenna operating in X-Band (8–12 GHz) is presented. The antenna is composed of a metal cavity with a circular slot, a dual-polarized feeding balun structure printed on a single-layer substrate, and a loaded dielectric block. The units of the antenna are fixed by screws, and no additional welding assembly is required. By setting notches structure on the dual-polarized planar feeding probes, the port isolation and polarization purity of the proposed antenna have been significantly improved. A prototype with the compact size of 0.44 × 0.44 × 0.39 λ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> (λ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> is the wavelength at 10 GHz) is fabricated and measured. Measured results demonstrate the return losses are lower than 10 dB and the isolations are greater than 23 dB from 8 to 12 GHz, the measured bore-sight gains vary from 4.5 to 5.8 dBi, and the cross-polarization levels less than −20 dB are obtained within the operational band.

Silicon metasurface-based infrared polarizing beam splitter with on-demand deflection angles.

Polarizing beam splitters (PBSs) play an important role in applications requiring polarization multiplexing or high polarization purity. Traditional prism-based PBSs usually have large volumes, which hampers their further applications in ultracompact integrated optical systems. Here, we demonstrate a single-layer silicon metasurface-based PBS with the ability to deflect two orthogonally linearly polarized infrared light beams to on-demand angles. The metasurface consists of silicon anisotropic microstructures, which can provide different phase profiles for the two orthogonal polarization states. In experiments, two metasurfaces designed with arbitrary deflection angles for x- and y-polarized light exhibit good splitting performance at an infrared wavelength of 10 μm. We envision that this type of planar and thin PBS can be used in a series of compact thermal infrared systems.

Producing slow light in warm alkali vapor using electromagnetically induced transparency

We present undergraduate-friendly instructions on how to produce light pulses propagating through warm Rubidium vapor with speeds less than 400 m/s, i.e., nearly a million times slower than c. We elucidate the role played by electromagnetically induced transparency (EIT) in producing slow light pulses and discuss how to achieve the required experimental conditions. The optical setup is presented, and details provided for preparation of pump, probe, and reference pulses of the required size, frequency, intensity, temporal width, and polarization purity. EIT-based slow light pulses provide the most widely studied architecture for creating quantum memories. Therefore, the basic concepts presented here are useful for physics and engineering majors who wish to get involved in the development of cutting-edge quantum technologies.

Linear‐Polarization‐Preserving Metasurfaces Based on Identically Spin‐Locked Geometric Phase

AbstractMetasurfaces have provided unprecedented degrees of freedom in controlling electromagnetic waves. By introducing geometric phase into metasurface, remarkable progresses have been made in manipulation circular polarized(CP) waves. However, for linearly polarized (LP) waves, polarization purity has to be sacrificed for wideband phase responses. Thus, it is desirable that wideband metasurfaces be explored for LP waves without polarization conversion. Herein, they propose a strategy of achieving wideband linear‐polarization‐preserving (LPP) metasurfaces based on identically‐spin‐locked (ISL) geometric phases. Different from the rotation‐induced oppositely‐spin‐locked (OSL) geometric phase (φL = −φR), the ISL geometric phase is the same for left‐ and right‐handed circularly polarization (LCP&amp;RCP) states (φL = φR). Since LP waves can be decomposed into LCP and RCP waves, the same phase can also be imparted to LP waves, without changing the polarization. Therefore, wideband LPP metasurfaces can be designed. As the proof‐of‐concept, the quadru‐arc structure (QAS) is proposed to construct the LPP metasurfaces. Two prototypes, one for vortex beam generation and the other for deflection‐absorption integration, are implemented. Both the simulated and measured results verify the LPP in wideband. This work opens a wideband gate towards wideband manipulation on LP waves and can be readily extended to higher frequency bands such as terahertz, infrared and optical regimes.

Wide-Band Wide-Beam Circularly-Polarized Slot-Coupled Antenna for Wide-Angle Beam Scanning Arrays.

The design of a wide-band wide-beam circularly-polarized slot-coupled (WWCS) radiating element for wide-angle scanning arrays (WASAs) is addressed. The WWCS radiator exploits a simple geometry composed of a primary (driven) and a secondary (passive) element to generate wide-beam patterns with rotational symmetry and high polarization purity. The synthesis was carried out by means of a customized version of the System-by-Design (SbD) method to derive a WWCS radiator with circular polarization (CP) and wide-band impedance matching. The results of the numerical assessment, along with a tolerance analysis, confirm that the synthesized WWCS radiating element is a competitive solution for the implementation of large WASAs. More specifically, a representative design working at f0=2.45 [GHz] is shown having fractional bandwidth FBW≃15%, half-power beam-width HPBWf0≃180 [deg] in all elevation planes, and high polarization purity with broadside axial ratio ARf0=3.2 [dB] and cross-polar discrimination XPDf0=15 [dB]. Finally, the experimental assessment, carried out on a PCB-manufactured prototype, verifies the wide-band and wide-beam features of the designed WWCS radiator.

Fano resonance-induced high-purity circularly polarized spectra for high-precision refractive index sensing from hybrid resonator-graphene meta-surfaces.

We demonstrate the perfect synthesis of terahertz circularly polarized Fano resonant reflecting spectra from hybrid resonator-graphene meta-surfaces for highly sensitive refractive index sensing of the biochemical analyte. Such a hybrid resonator-graphene meta-surface, consisting of periodic multi-node split ring resonators on the top of the grounded polyimide substrate inserted with a monolayer graphene sheet, can perfectly transform the linearly polarized electromagnetic fields into circularly polarized waves. Especially, the greatest polarization purity of the reflecting spectra can readily be obtained at the Fano resonance by tuning the Fermi level of the graphene, thus offering an alternative way to identify the difference between the given test specimens and other analytes with a very close refractive index on the basis of the polarization extinction ratio. The proposed methodology, capable of distinguishing the samples with a difference in the refractive index of ten thousandths, should pave the way for tangible applications of precision detections in biochemical assays with high accuracy.

Investigations on the Wideband Characteristics of a Cylindrical Conformal U-slot Loaded Microstrip Patch Antenna for X-Band Airborne Applications

A U-slot located symmetrically at the centre of the patch has been utilized to enhance the patch antenna’s operating bandwidth compared to the standard conventional patch. The operating frequency range of the circumferential patch antenna is from 8.61 to 11.43 GHz, while the axial patch covers the frequency range 8.71 to 11.33 GHz (covering X band). The antenna shows excellent gain with minimal variation in both E and H-planes across the operating frequency range. Moreover, the antenna achieves less than −10 dB sidelobe levels in both E and H-planes across all frequencies and configurations. The co and cross-polarization plots are also depicted to show the excellent polarization purity of the antenna for the planar and cylindrical conformal configurations. The antenna may be suitable for airborne applications where the conformability to the Arial host body is of prime importance.

A Circularly Polarized Parallel Plate Waveguide Lens-Like Multiple-Beam Linear Array Antenna for Satcom Applications

This work presents a full metal circularly-polarized lens-like antenna for Satcom applications at Ka-band. The antenna is composed of two continuous parallel plate waveguide (PPW) quasi-optical beamformers (QOBF), feeding an array of septum polarizers to generate circular polarization. The QOBFs operate over a wide band and provide a multi-beam coverage over a large field of view in the azimuthal plane, while maintaining a relatively simple mechanical design. The array of septum polarizers is based on a stepped profile to generate circular polarization with a good polarization purity in the uplink of Ka-band (27 - 31 GHz). The antenna is fully realized in aluminium. The antenna system provides 14 beams with alternating right/left handed circular polarization (RHCP/LHCP) with an axial ratio (AR) below 3 Db over an angular range of ±19° in the uplink of Ka-Band. The maximum gain in the azimuthal plane at 30 GHz is about 21 dB over the whole angular range, with a scan loss lower than 1.5 dB. The antenna efficiency is better than 78% for all beams in the operative band.