Circularly Polarized Research Articles

Properties of individual S-bursts observed in the frequency band of 10–32 MHz during the rising phase of 25-th solar cycle

Introduction: The properties of the S-bursts observed during the storm on 20–21 June 2022 in frequency band 10–32 MHz by the radio telescope URAN-2 are discussed in this paper. The storm was highly populated with other solar bursts, such as Type III bursts and drift pairs. The occurrence rate of S-bursts was very high reaching 60 bursts per minute. All observed S-bursts were characterized by low fluxes with respect to the background radio emission. Thus special processing methods are used to retrieve spectral properties of the bursts. Some individual “long” S-bursts covered the whole frequency band of the URAN-2 radio telescope from 10 to 32 MHz. Such extended in frequency S-bursts were recorded for the first time. 50 extended S-bursts were selected for the further analysis.Methods: The S-bursts dynamic spectra with time-frequency resolutions of 100 ms and 4 kHz as well as single-frequency profiles were used in the analysis. Due to low S-bursts intensities the drift rates were estimated from the time-differentiated dynamic spectra, highlighting the tracks of the bursts maxima. Polarization dynamic spectra were used for measuring the degree and sense of the S-bursts circular polarization. Individual S-bursts tracks were used for instant coronal inhomogeneities diagnostics. Mean S-bursts parameters retrieved from the statistical processing of the set of 50 bursts were compared with previously obtained ones.Results: We concluded that by the mean durations, drift rates, frequency extent and the polarization all observed S-bursts could be divided into two separate groups, the “short” and the “long” S-bursts. The power-law index of the drift rate-frequency dependence averaged over all 50 selected bursts was found to be 1.7. It was shown that sources of S-bursts most likely move through the Newkirk corona with the velocities of 0.06–0.08c. The power-law dependence of the “long” S-bursts durations on frequency in frequency band of 12–30 MHz was obtained. Its index equal to −0.61 appeared to be very close to that for Type III bursts. From this dependence the electron velocity dispersion in the beam, responsible for S-bursts generation was calculated. Its value of 0.02 indicates that the beams, responsible for S-bursts generation are almost monoenergetic.Discussion: It is assumed that non-monotonic appearance of individual S-bursts tracks on the dynamic spectrum reflects density inhomogeneities encountered by the sources on their paths. From the dynamic spectra of such S-bursts the characteristic size and amplitude of these coronal inhomogeneities were detected. From the S-bursts durations and the velocities of their sources the longitudinal sizes of the latter were estimated. It was then shown that the sizes of small-scale coronal inhomogeneities were comparable to those of “long” S-bursts sources. Thus we concluded that individual tracks of the “long” S-bursts can be used for fie diagnostics the coronal plasma at heliocentric heights range from 1.7 to 3.2 Rs, where Rs is the solar radius. On the other hand, these tracks being ensemble-averaged give the information about the long-term large scale properties of the corona.

Polarization-multiplexed three-channel dielectric metasurfaces integrated with liquid crystals for holographic displays

Multi-channel metasurface holography based on polarization multiplexing has received close attention for its application in increasing information capacity. However, most of the existing spin-selective metasurfaces focus on the utilization of the cross-polarized channels while neglecting the co-polarized channels. Moreover, the real-time dynamic switching of the incident polarization of the metasurface remains a great challenge. This work presents a dielectric metasurface that combines geometric and propagation phase modulations. It allows phase control of a pair of orthogonal circular polarizations. A liquid crystal (LC) film is placed at the bottom layer of the metasurface to achieve dynamic switching of the incident polarization of the metasurface. As a proof-of-concept, we design and simulate a dielectric metasurface integrated with liquid crystals (LCs) for three-channel holographic displays. The LCs modulate the incident linear polarization into two orthogonal circular polarizations and illuminate the metasurface. The metasurface displays one holographic image in the vertical direction from the co-polarized channel and two holographic images at certain angles from the cross-polarized channel. It means that three holographic images can be displayed simultaneously without overlapping for linearly polarized (LP) light. Our design and simulations could provide a new, to our knowledge, scheme for multi-channel holography and active tunable metasurfaces.

Complete analysis of differential cross section in 7 Li + γ→→ 6 Li + n at astrophysical energies

We have carried out complete analysis of differential cross section in 7 Li + γ → 6 Li + n using model-independent theoretical formalism. A complete analysis of the reaction involves measurements of not only one state of linear polarization of the photon but also another state of linear polarization inclined to the first at 45◦ and two states of circular polarization of the photon. An analytical study of the differential cross section including all the photon polarization states is carried out at near-threshold energies of interest to Big Bang Nucleosynthesis.

Terahertz reflective metasurfaces realize wavefront modulation of circular polarization channels

The emergence of many efficient optical field modulation methods and planar optical devices is attributed to the continuous research of geometric phase. Nevertheless, conjugate symmetry of the geometric phase limits the multiplexing of metasurfaces. To overcome this limitation, integrating the propagation phase and the geometric phase to achieve self-decouped metasurfaces can effectively double channel capacity. Herein, a more in-depth derivation of Jones matrices is conducted, leveraging two degrees of freedom offered by the propagation phase and the geometric phase. This approach enables complete modulation in circular polarization channels. By designing the phase difference between a fast axis and a slow axis, energy allocation between different channels can be controlled. It means independent complex amplitude modulation is achieved. On this basis, the geometric phase is introduced to realize tri-channel multiplexing metasurfaces. To verify the feasibility of this method, two metasurfaces are designed, including a bifocal metasurface with adjustable energy allocation and a tri-channel multiplexing metasurface. The proposed multifunctional metasurface offers new insights into wavefront multiplexing for communication systems.

Axially Chiral Multiple Resonance Thermally Activated Delayed Fluorescence Enantiomers for Efficient Circularly Polarized Electroluminescence

AbstractThe chiral materials and circularly polarized organic light‐emitting diodes (CP‐OLEDs) have potential applications in 3D display due to the generation of circularly polarized luminescence (CPL). However, achieving a combination of chirality, high efficiency, and narrowband emission for thermally activated delayed fluorescence (TADF) materials has proven to be a challenging endeavor, often involving complicated synthesis and chiral separation processes. In this study, a simple design approach is introduced by direct connection of the multiple resonance (MR) skeleton and chiral (R/S)‐1,1′‐binaphthyl units, obtaining two pairs of chiral MR‐TADF enantiomers (R/S)‐DtCzB‐OBN and (R/S)‐DtCzB‐BN without chiral resolution. These enantiomers exhibit blue‐green emissions with photoluminescence quantum efficiencies of up to 98% in doped films and full‐width at half‐maximum of 22 nm both in toluene and in the device. Furthermore, (R/S)‐DtCzB‐OBN and (R/S)‐DtCzB‐BN display symmetric CPL spectra with dissymmetry factors (gPL) of + 3.86( ± 0.317) × 10−4/‐3.37( ± 0.317) × 10−4 and + 7.09( ± 0.233) × 10−4/‐7.74(± 0.233) × 10−4, respectively. The CP‐OLEDs based on (R/S)‐DtCzB‐OBN and (R/S)‐DtCzB‐BN exhibit impressive maximum external quantum efficiencies of 30.0% and 33.9%, as well as gEL factors of +3.40( ± 0.362) × 10−4/‐3.11( ± 0.362) × 10−4 and + 7.99( ± 0.391) × 10−4/‐8.19( ± 0.391) × 10−4, respectively. This research proposes a strategy for directly constructing chiral MR‐TADF materials for high‐performance CP‐OLEDs, avoiding complicated chiral separation.

Polarization-multiplexing graphene-based coding metasurface for flexible terahertz wavefront control

In terahertz wireless communication systems, flexible wavefront control devices based on various structure metasurfaces have attracted enormous attention for next-generation communication. In general, tunable terahertz metasurfaces integrated with active materials or MEMS technologies are used for dynamic wavefront control. However, most existing metasurfaces suffer from various limitations, including intrinsic properties of active materials, low reliability of MEMS technologies, and single polarization mode of incident waves, which hinders their development and application. To address these challenges, herein, we design two types of reflective graphene-based coding metasurfaces for active wavefront control. The metasurface coding meta-atom is composed of a graphene split-ring resonator, a dielectric layer, and a metal ground plane. By simply rotating the coding meta-atom, independent 2π phase coverage for circularly polarized (CP) or linearly polarized (LP) illumination can be achieved, enabling polarization multiplexing. Thus, a metasurface (MS-1) is constructed based on the vortex phase profile to generate different wavefronts. Moreover, these wavefronts can be actively switched between a vortex beam, a multi-beam, and a specular reflection beam by altering the polarization mode of the incident waves and the Fermi level of the graphene coding regions Additionally, another metasurface (MS-2) is developed according to the parabolic phase profile to create a tunable metalens that allows active control over focal intensity and depth by adjusting the Fermi level of graphene. Such wavefront-controlled metasurfaces have high capacity and integration, making them very promising for potential applications in terahertz communication and imaging systems.

Self-Assembled Pure Covalent Tubes Exhibiting Circularly Polarized Luminescence.

Here, we successfully synthesized four structurally analogous, self-assembled chiral molecular tubes with relatively high yields. This achievement involved the condensation of six equivalents of enantiomerically pure trans-cyclohexane-1,2-diamine (trans-CHDA) and three equivalents of the corresponding tetraformyl precursor. Each precursor was equipped with a luminescent linker terminated by two m-phthalaldehyde units. Even though these tetraformyl precursors are barely soluble in almost all organic solvents, the molecular tubes are highly soluble in nonpolar solvents such as chloroform, allowing us to fully characterize them in solution. The stereo-chirality of the chiral bisamino building blocks endows the frameworks of molecular tubes with planar chirality. As a consequence, all of these molecular tubes exhibit circularly polarized luminescence (CPL) with relatively large dissymmetry values |glum| up to 7×10-3, providing an efficient method for synthesizing CPL-active materials.

Self‐Assembled Pure Covalent Tubes Exhibiting Circularly Polarized Luminescence

AbstractHere, we successfully synthesized four structurally analogous, self‐assembled chiral molecular tubes with relatively high yields. This achievement involved the condensation of six equivalents of enantiomerically pure trans‐cyclohexane‐1,2‐diamine (trans‐CHDA) and three equivalents of the corresponding tetraformyl precursor. Each precursor was equipped with a luminescent linker terminated by two m‐phthalaldehyde units. Even though these tetraformyl precursors are barely soluble in almost all organic solvents, the molecular tubes are highly soluble in nonpolar solvents such as chloroform, allowing us to fully characterize them in solution. The stereo‐chirality of the chiral bisamino building blocks endows the frameworks of molecular tubes with planar chirality. As a consequence, all of these molecular tubes exhibit circularly polarized luminescence (CPL) with relatively large dissymmetry values |glum| up to 7×10−3, providing an efficient method for synthesizing CPL‐active materials.

Reflective Surface for Linear-to-Linear and Linear-to-Circular Polarization Conversion at Multiple Frequencies

In this paper, a thin multiband reflective polarization converter surface is proposed that can reflect a circularly polarized (CP) wave in the following frequency bands: 7.8-8.10 GHz, 8.32-10.04 GHz, 10.80-13.75 GHz, 14.19-14.53 GHz, 16.22-16.52 GHz, and 16.77-17.18 GHz. It can also convert a linearly polarized (LP) incident electromagnetic wave into its orthogonal LP reflection wave in these bands: 8.14-8.25 GHz, 10.23-10.60 GHz, 13.90-14.08 GHz, and 16.59-16.71 GHz. With slotted semicircles supported by a metallic ground plane, the suggested unit cell of the periodic structure has significant stability in all frequency ranges, even at oblique incidence within a 75° angle limit. Furthermore, the suggested polarizer has an easy-to-manufacture construction that only needs one substrate layer and does not include any via holes. A sample prototype consisting of 30×30 unit cells was fabricated and tested, validating our design and simulations.

A broadband Millimeter-Wave rectenna using substrate Integrated Waveguide

In this paper, a circularly polarized rectenna is proposed for microwave energy transmission, which is composed of a broadband Substrate Integrated Waveguide (SIW) antenna and a rectifier circuit. A dielectric resonator is loaded on the SIW antenna, enhancing the impedance bandwidth. Circular polarization is achieved by chamfering the diagonal of the rectangular patch. Two sections of microstrip branches are employed for good impedance matching over the broadband operation. Measured results show that the proposed rectenna has a good impedance performance (|S11| < −10 dB) and high RF-DC conversion efficiency (>50 %) over a relative bandwidth of 22.9 % (29–36.5 GHz). The maximum conversion efficiency of 67.5 % is realized at the input power of 18 dBm.

Pyro‐Phototronic Circularly Polarized Light Detection Based on Deuterated L‐Alanine Doped Triglycine Sulfate's Bulk Photovoltaic Effect

Pyro‐Phototronic Circularly Polarized Light Detection Based on Deuterated L‐Alanine Doped Triglycine Sulfate's Bulk Photovoltaic Effect

One-shot self-calibrating phase-shifting interferometry by direct normalization of interferograms

Dynamic phase measurement is a valuable technique applied across various scientific and technological domains due to its high accuracy, non-contacting method, and numerous practical applications. In this manuscript, we present a phase retrieval method capable of measuring the object’s phase in real-time. Our proposal is based on a double aperture common-path interferometer adapted for polarization modulation in the input windows, enabling the superposition of two fields with orthogonal circular polarizations. Two interferograms and two intensities are captured in a single shot, showcasing the possibility of obtaining two normalized interferograms directly and straightforwardly. This allows for the derivation of expressions for phase recovery and, additionally, the determination of the arbitrary phase step. We demonstrate that this method is easy to implement, highly accurate, and sufficiently fast for real-time applications. To validate the effectiveness of the method, we conduct numerical error analysis and laboratory experiments.

High quality factor and unity circular dichroism enabled by dielectric-Weyl semimetal-dielectric stack

A mid-infrared (mid-IR) chiral absorber based on multilayer films of silicon (Si)/Weyl semimetal (WSM)/Si is proposed and numerically studied. The proposal exhibits high quality factor circular dichroism (CD) with different absorptance properties for right-hand circular polarization (RCP) and left-hand circular polarization (LCP) light. The absorption characteristics have been explored with the generalized 4 × 4 transfer matrix algorithm, and the effects of structural parameters on the circularly polarized (CP) absorptance are systematically investigated. The circular polarization efficiency (η) and CD are defined as crucial parameters for quantifying the chirality response. The results prove that the chiral absorber exhibits extraordinary efficiencies of 1 and 0 for LCP and RCP light at a wavelength of 11.5 μm, respectively. Thus a maximum CD in absorption of unity with a resonant quality factor (Q-factor) of 295 can be obtained. The performance of dual-band chiral responses can be engineered via the thickness of each layer. Moreover, it is identified that the design results in angle-independent performance, which is essential trait for an efficient chiral absorber.

Helical polyisocyanides with thermally activated delayed fluorescence pendants for efficient circularly polarized light emission and detection

Chiral organic polymeric semiconductors are widely regarded as promising candidates for circularly polarized light (CPL) detection due to their advantages of easy chemical modification, solution processing and low cost. However, traditional organic polymeric materials face low photoresponsivity and photocurrent asymmetry factor when constructing CPL detectors. To address this issue, we develope single-handed helical polyisocyanides with thermally activated delayed fluorescence (TADF) feature to fabricate a donor-acceptor heterojunction photodetector with C60, where efficient triplet exciton utilization enables a high photocurrent response while the static single-handed helical main chains of polyisocyanides ensure a high photocurrent asymmetry factor, simultaneously. Furthermore, the performance of TADF polyisocyanides is conveniencely optimized by copolymerizing the host. Benefiting from the comprehensive functionality of TADF polyisocyanides, the prepared photodetectors exhibit a high responsivity of 0.21 A W−1 and a very high photocurrent asymmetry factor of up to 0.12, which make it superior to the reported CPL photodetectors based on organic polymers. In addition, the detector has excellent reproducibility enabling no photocurrent roll-off after 1000 cycles. The long-term stability in ambient air also manifests its robustness. This work paves a new way for high-efficiency polymers based CPL detectors.

Circularly Polarized Luminescence Without External Magnetic Fields from Individual CsPbBr3 Perovskite Quantum Dots.

Lead halide perovskite quantum dots (QDs), the latest generation of the colloidal QD family, exhibit outstanding optical properties, which are now exploited as both classical and quantum light sources. Most of their rather exceptional properties are related to the peculiar exciton fine-structure of band-edge states, which can support unique bright triplet excitons. The degeneracy of the bright triplet excitons is lifted with energetic splitting in the order of millielectronvolts, which can be resolved by the photoluminescence (PL) measurements of single QDs at cryogenic temperatures. Each bright exciton fine-structure-state (FSS) exhibits a dominantly linear polarization, in line with several theoretical models based on the sole crystal field, exchange interaction, and shape anisotropy. Here, we show that in addition to a high degree of linear polarization, the individual exciton FSS can exhibit a non-negligible degree of circular polarization even without external magnetic fields by investigating the four Stokes parameters of the exciton fine-structure in individual CsPbBr3 QDs through Stokes polarimetric measurements. We observe a degree of circular polarization up to ∼38%, which could not be detected by using the conventional polarimetric technique. In addition, we found a consistent transition from left- to right-hand circular polarization within the fine-structure triplet manifold, which was observed in magnetic-field-dependent experiments. Our optical investigation provides deeper insights into the nature of the exciton fine structures and thereby drives the yet-incomplete understanding of the unique photophysical properties of this class of QDs for the benefit of future applications in chiral quantum optics.

Circularly polarized beam-scanning antenna with eliminated open-stopband

In the paper, a circularly polarized (CP) frequency beam-scanning antenna based on photonic crystal (PC) is proposed, which possesses the characteristics of eliminating open-stopband (OSB). In the microwave field, the PC is typically constructed by embedding the air or metal column into the material. However, it is difficult for the PC with perfect magnetic conductor walls to accomplish the degeneracy of eigenmodes at the Γ point. Additionally, antennas based on the PC with perfect electric conductor walls exhibit a narrow scanning angle. The proposed unit based on 1D PC possesses perfect electric and magnetic conductor walls. The unit can not only eliminate OSB but also has a wide scanning range. In addition, the unit breaks into the original symmetrical field distributions of 1D PC. Based on the perturbation theory, leaky-wave slots can be designed to generate electromagnetic waves with the CP characteristics. And a CP antenna based on the unit is designed. The antenna can achieve a 70° beam-scanning within the entire operating band (7.6–9.2 GHz), and the variation of the realized gains is below 1.0 dBi around the broadside frequency. Simultaneously, the antenna is fabricated and measured. The measured results are in accordance with the simulation, which validates the success of the design.

On the physical layer security of visible light communications empowered by gold nanoparticles

Visible light is a proper spectrum for secure wireless communications because of its high directivity and impermeability in indoor scenarios. However, if an eavesdropper is located very close to a legitimate receiver, secure communications become highly risky. In this paper, to further increase the level of security of visible light communication (VLC) and increase its resilience to malicious attacks, we propose to capitalize on the recently synthesized gold nanoparticles (GNPs) with chiroptical properties for circularly polarized light resulting in the phase retardation that interacts with the linear polarizer angle. GNP plates made by judiciously stacking many GNPs perform as physical secret keys. Transmitters send both the intended symbol and artificial noise to exploit the channel variation effect by the GNP plates, which is highly effective when an eavesdropper is located close to the legitimate receiver. A new, to our knowledge, VLC channel model is first developed by representing the effect of GNP plates and linear polarizers in the circular polarization domain. Based on the new channel model, the angles of linear polarizers at the transmitters and legitimate receiver are optimized considering the effect of GNP plates to increase the secrecy rate in wiretapping scenarios. Simulations verify that, when the transmitters are equipped with GNP plates, even if the eavesdropper is located right next to the legitimate receiver, insightful results on the physical layer security metrics are gained as follows: (1) the secrecy rate is significantly improved, and (2) the symbol error rate gap between the legitimate receiver and eavesdropper becomes much larger due to the chiroptical properties of GNP plates.

A dual-band dual-sense circularly polarized patch antenna using hybrid capacitive-inductive proximity-coupling feed

This paper presents a new proximity-coupling mechanism between a rectangular patch and a microstrip line with an open circuit terminal. The patch is located to cover both adjacent maximum-voltage (capacitive coupling) and maximum-current (inductive coupling) points of the standing wave on the feedline, and consequently, two orthogonal resonant modes with 90° phase difference are excited on the patch for generating a circularly polarized (CP) radiation. Based on this feeding mechanism, a dual-band dual-sense CP antenna is designed, which is composed of two linear rectangular patch arrays coupled to an open-ending microstrip line. It is realized on a single-layer substrate and its CP rotating sense in each band is opposite to each other. For verification, the antenna operating at 5.0 and 8.0 GHz are designed, fabricated, and tested. Noting that the design concept can be easily implemented to other frequency bands. The measurements result in an impedance-matching bandwidth of 4.72 – 5.10 GHz and 7.50 – 8.20 GHz. The antenna is left-hand CP for the lower band and right-hand CP for the upper band. In addition, the prototype shows a 3-dB axial ratio bandwidth of 4.93 – 5.02 GHz and 7.40 – 8.10 GHz and peak gains of 11.4 and 12.7 dBic at the lower and upper bands, respectively.

Design of a Compact Circularly Polarized Implantable Antenna for Capsule Endoscopy Systems.

This research proposes a miniature circular polarization antenna used in a wireless capsule endoscopy system at 2.45 GHz for industrial, scientific, and medical bands. We propose a method of cutting a chamfer rectangular slot on a circular radiation patch and introducing a curved radiation structure into the centerline position of the chamfer rectangular slot, while a short-circuit probe is added to achieve miniaturization. Therefore, we significantly reduced the size of the antenna and made it exhibit circularly polarized radiation characteristics. A cross-slot is cut in the GND to enable the antenna to better cover the operating band while being able to meet the complex human environment. The effective axis ratio bandwidth is 120 MHz (2.38-2.50 GHz). Its size is π × 0.032λ02 × 0.007λ0 (where λ0 is the free-space wavelength of at 2.4 GHz). In addition, the effect of different organs such as muscle, stomach, small intestine, and big intestine on the antenna when it was embedded into the wireless capsule endoscopy (WCE) system was further discussed, and the results proved that the WCE system has better robustness in different organs. The antenna's specific absorption rate can follow the IEEE Standard Safety Guidelines (IEEE C95.1-1999). A prototype is fabricated and measured. The experimental results are consistent with the simulation results.

Cyclic Azahelicene Dimers Showing Bright Circularly Polarized Luminescence and Selective Fluoride Recognition.

Although helicenes are promising molecules, the synthetic difficulty and tediousness have often been problems, and only small amounts of optically pure helicenes have been obtained by using chiral HPLC in most cases. Herein, aza[7]helicenes or closed-aza[7]helicenes with (1R)-menthyl substituents were selectively synthesized via the intramolecular Scholl reaction, and the diastereomeric pairs were separated by silica gel column chromatography. The optically pure helicenes were further transformed into the corresponding cyclic dimers, and the chiroptical properties were investigated. The rigid π-frameworks of the dimers led to the high molar extinction coefficients and fluorescence quantum yields, while the twisted helicene moieties induced clear Cotton effects and CPL in the visible region, and the high CPL brightness (BCPL) was achieved. Furthermore, the cyclic dimers were found to have the macrocyclic cavity with the two NH groups suitable for the selective binding of a fluoride anion, which induced significantly redshifted fluorescence and CPL in the red region.