Conventional Circularly Polarized Research Articles

Programmable generation of counterrotating bicircular light pulses in the multi-terahertz frequency range

The manipulation of solid states using intense infrared or terahertz light fields is a pivotal area in contemporary ultrafast photonics research. While conventional circular polarization has been well explored, the potential of counterrotating bicircular light remains widely underexplored, despite growing interest in theory. In the mid-infrared or multi-terahertz region, experimental challenges lie in difficulties in stabilizing the relative phase between two-color lights and the lack of available polarization elements. Here, we successfully generated phase-stable counterrotating bicircular light pulses in the 14–39 THz frequency range circumventing the above problems. Employing spectral broadening, polarization pulse shaping with a spatial light modulator, and intra-pulse difference frequency generation leveraging a distinctive angular-momentum selection rule within the nonlinear crystal, we achieved direct conversion from near-infrared pulses into the designed counterrotating bicircular multi-terahertz pulses. Use of the spatial light modulator enables programmable control over the shape, orientation, rotational symmetry, and helicity of the bicircular light field trajectory. This advancement provides a novel pathway for the programmable manipulation of light fields, and marks a significant step toward understanding and harnessing the impact of tailored light fields on matter, particularly in the context of topological semimetals.

Lunar Impact Craters: New Perspectives From Full‐Polarimetric Analysis of Chandrayaan‐2 Dual‐Frequency SAR Data

AbstractWith the availability of the full‐polarimetry data from Chandrayaan‐2 Dual‐frequency Synthetic Aperture Radar (DFSAR) for the first time, the lunar impact craters are studied for comprehensive characterization, which was earlier limited due to hybrid‐polarimetry based observations. In conjunction with the conventional Circular Polarization Ratio (CPR), we explore the unique full‐polarimetric parameters such as Single‐bounce Eigenvalue Relative Difference and T‐Ratio which are sensitive to surface roughness and dielectric constant, respectively. We studied the polarimetric behavior of a total of 115 nos. of impact craters within a specific size range (∼1–25 km diameter) belonging to different categories viz. fresh, degraded, polar and non‐polar anomalous craters. Our results confirm that both kinds of non‐polar and polar anomalous craters overall hold centimeter‐decimeter scale surface roughness similar to the exterior of the fresh craters, which mainly leads to the anomalous nature of the craters. Our analysis indicates possible control of the crater degradation process on the change in the radar scattering behavior of the studied lunar craters. The crater degradation causes a reduction in the surface roughness and dielectric property (density) of the scatterers present in various craters leading to a change in the radar scattering behavior. Alongside, water ice, if at all present admixed into the regolith in the polar anomalous craters, is not in the form of dominant scatterers that can influence the radar signature. However, the occurrence of relatively higher CPR values in smooth low dielectric crater interiors supports the possibility of the presence of water ice in some of the studied polar anomalous craters belonging to Pre‐Imbrian to Imbrian lunar geological periods.

Wirelessly interfacing sensor-equipped implants and MRscanners for improved safety and imaging.

To investigate a novel reduced RF heating method for imaging in the presence of active implanted medical devices (AIMDs) which employs a sensor-equipped implant that provides wireless feedback. The implant, consisting of a generator case and a lead, measures RF-induced -fields at the implant tip using a simple sensor in the generator case and transmits these values wirelessly to the MR scanner. Based on the sensor signal alone, parallel transmission (pTx) excitation vectors were calculated to suppress tip heating and maintain image quality. A sensor-based imaging metric was introduced to assess the image quality. The methodology was studied at 7T in testbed experiments, and at a 3T scanner in an ASTM phantom containing AIMDs instrumented with six realistic deep brain stimulation (DBS) lead configurations adapted from patients. The implant successfully measured RF-induced -fields (Pearson correlation coefficient squared [R2 ] = 0.93) and temperature rises (R2 = 0.95) at the implant tip. The implant acquired the relevant data needed to calculate the pTx excitation vectors and transmitted them wirelessly to the MR scanner within a single shot RF sequence (<60 ms). Temperature rises for six realistic DBS lead configurations were reduced to 0.03-0.14 K for heating suppression modes compared to 0.52-3.33 K for the worst-case heating, while imaging quality remained comparable (five of six lead imaging scores were ≥0.80/1.00) to conventional circular polarization (CP) images. Implants with sensors that can communicate with an MR scanner can substantially improve safety for patients in a fast and automated manner, easing the current burden for MR personnel.

A Wideband Circularly Polarized Filtering Antenna Based on Slot-Patch Structure

This letter presents a wideband circularly polarized (CP) antenna with filtering radiation response based on a slot-patch structure. The proposed CP filtering antenna is composed of a top patch, a middle microstrip-to- (“+”-shape) slot structure, and a bottom metallic reflector. The middle layer not only functions as a slot-coupled feeding structure but also operates as a driven slot CP antenna. The top and bottom layers are used to reduce the slot antenna backside radiation and enhance the antenna axial ratio (AR) bandwidth. In addition, a diagonal slot etched on the patch and an additional E-shaped slot on the middle layer can generate the lower- and upper-band radiation zeros, respectively. For verification, the antenna is implemented. The measured results show that the overlapping bandwidth of the impedance and AR bandwidth is 46.9% (3.35-5.40 GHz). The measured average CP gain is about 7.6 dBic. High frequency selectivity and better than 10 dB out-of-band radiation rejection level is obtained. Compared to the conventional filtering CP design method which employs the filter or resonator feeding circuit, the proposed design features a simple structure, higher realized gain and wider AR/impedance overlapping bandwidth.

A Circularly Polarized Patch Antenna Array with Reduced Mutual Coupling Using the Aperture-Loading Decoupling Technique

An aperture-loaded decoupling strategy for 1 × 8 circularly polarized (CP) patch antenna array is presented in this article. By introducing an additional coupling path, the mutual coupling between adjacent antennas is cancelled. The result shows that more than 20-dB isolation enhancement is obtained by applying this strategy at the center frequency of 3.38 GHz. Mutual coupling between both adjacent and non-adjacent elements are suppressed to less than -25 dB. Moreover, the impedance bandwidth and axial ratio (AR) is also improved with decoupling. Compared with conventional CP antenna decoupling methods, the proposed approach has the characteristics of low profile, compact size, and low impact for the ground plane. It is shown that the AR bandwidth can be enhanced using the proposed decoupling method.

Bandwidth-Enhanced Circularly Polarized Slot Antenna and Array Under Two Pairs of Degenerate Modes in a Single Resonant Cavity

A novel design concept of bandwidth-enhanced full-metal circularly polarized (CP) cavity-backed slot antenna and array based on two pairs of degenerate modes in a single cavity is proposed in this paper. Conventional CP antenna based on one pair of degenerate modes only has one axial-ratio (AR) minima and causes a narrow AR bandwidth. Two pairs of degenerate modes in a proposed multimode cavity resonator can initially produce two separate CP operating bands. Then, a pair of metal prisms is introduced to allocate the two bands in proximity to each other, thus obtaining a wide operating bandwidth with two AR minima. As the radiation slots are directly fed by the electric field of the cavity modes, 2 × 2 cross slots are placed on the top wall so as to comprise a CP array. As such, a very simple wideband CP antenna array is realized by virtue of a single multiple-mode resonator without needing extra power dividers. Finally, the 2 × 2 wideband CP antenna array is fabricated and measured. Measurement shows that the proposed antenna array can achieve 15% bandwidth, a peak gain of 11.8 dBic, and a peak total efficiency of 96%.

A novel circularly polarized patch antenna based on polarization conversion metasurface for radiation and scattering performance improvement

A new design based on sequential-rotation arranged polarization conversion metasurfaces (PCMs) is proposed for reducing backscattering and improving radiation performance of circularly polarized (CP) patch antennas. The proposed antenna is composed of a conventional corner-truncated patch antenna loaded with a PCMs. The configuration of the PCMs increases the operating bandwidth and realized gain, as well as reduce radar cross section (RCS). The relative CP bandwidth (both |S11| < −10 dB and AR <3 dB) is improved to 13.65% due to achieving an additional mode by the PCMs. Also, the peak gain is enhanced by 6.6 dB. Moreover, an in-band and out-of-band RCS reduction is achieved compared with the conventional CP patch antenna due to the polarization conversion characteristic of the PCMs. The maximum RCS reduction is 27.5 dB within the band. This design enables an effective strategy for wideband, high-gain, and low-RCS CP patch antennas. A sample operating at C-band was fabricated and measured, demonstrating a satisfactory match between simulation and measurement results, so verifying the proposed idea.

Parallel transmit (pTx) with online pulse design for task-based fMRI at 7 T

PurposeParallel transmission (pTx) is an approach to improve image uniformity for ultra-high field imaging. In this study, we modified an echo planar imaging (EPI) sequence to design subject-specific pTx pulses online. We compared its performance against EPI with conventional circularly polarised (CP) pulses. MethodsWe compared the pTx-EPI and CP-EPI sequences in a short EPI acquisition protocol and for two different functional paradigms in six healthy volunteers (2 female, aged 23–36 years, mean age 29.2 years). We chose two paradigms that are typically affected by signal dropout at 7 T: a visual objects localiser to determine face/scene selective brain regions and a semantic-processing task. ResultsAcross all subjects, pTx-EPI improved whole-brain mean temporal signal-to-noise ratio (tSNR) by 11.0% compared to CP-EPI. We also compared the ability of pTx-EPI and CP-EPI to detect functional activation for three contrasts over the two paradigms: face > object and scene > object for the visual objects localiser and semantic association > pattern matching for the semantic-processing paradigm. Across all three contrasts, pTx-EPI showed higher median z-scores and detected more active voxels in relevant areas, as determined from previous 3 T studies. ConclusionWe have demonstrated a workflow for EPI acquisitions with online per-subject pulse calculations. We saw improved performance in both tSNR and functional acquisitions from pTx-EPI. Thus, we believe that online calculation pTx-EPI is robust enough for future fMRI studies, especially where activation is expected in brain areas liable to significant signal dropout.

Analysis and Design of Stable-Performance Circularly-Polarized Antennas Based on Coupled Radiators for Smart Watches

A singly fed circularly polarized (CP) antenna based on coupled radiator pair with stable CP performance is proposed for the accurate positioning of smart watches in this article. It is first demonstrated that two identical coupled monopoles sharing a finite ground can be equivalent as a second-order filter network consisting of composite J/K inverters. The coupled radiators will produce a pair of even and odd modes, whose resonant frequencies and bandwidths are determined by the coupling strength. After that, the characteristic mode analysis (CMA) is conducted to verify the circuit model, and it reveals that the even and odd modes are orthogonal to each other, which are candidates for CP radiation. In particular, the phase difference of the two modes is determined by the coupling coefficient, and it can be elaborately manipulated by properly changing the current distribution of characteristic modes through ground size, slot loading, and radiator length. At last, two antenna prototypes are designed, implemented, and measured to validate the working principle. Different from conventional CP antennas operating in degenerate modes, the proposed antennas are insensitive to perturbation segments and can maintain stable CP performance, which is highly demanded in smart watches under wearing conditions.

Total Angular Momentum Dichroism of the Terahertz Vortex Beams at the Antiferromagnetic Resonances.

Terahertz vortex beams with different superposition of the orbital angular momentum l=±1, ±2, ±3, and ±4 and spin angular momentum σ=±1 were used to study antiferromagnetic (AFM) resonances in TbFe_{3}(BO_{3})_{4} and Ni_{3}TeO_{6} single crystals. In both materials we observed a strong vortex beam dichroism for the AFM resonances that are split in external magnetic field. The magnitude of the vortex dichroism is comparable to that for conventional circular dichroism due to σ. The selection rules at the AFM resonances are governed by the total angular momentum of the vortex beam: j=σ+l. In particular, for l=±2, ±3, and ±4 the sign of l is shown to dominate over that for conventional circular polarization σ.

Size-Reduction of a Dual-Band Circularly Polarized Dielectric Resonator Antennas

In this paper, the size reduction of a dual-band circularly polarized (CP) Y-shaped dielectric resonator antenna (YDRA) is performed. Unlike the conventional CP DRAs, the circular polarization in the YDRA is achieved by the combination of linearly varying E-fields in the two arms of unequal length. Therefore, a PEC boundary condition based technique is applied to the long arm of the dielectric resonator (DR). As a result, a 33.18% reduction in the size is achieved after the optimization of the DR for the required frequency bands. A sample of the proposed design is fabricated to perform measurements. From the results, the antenna obtains a wide impedance bandwidth for $|S_{11}| \leq -10$ dB of 53.5% (2.3–3.98 GHz), 3 dB axial ratios bandwidths (ARBWs) of 4.08% (2.4–2.50 GHz) and 10.91% (3.38–3.77 GHz) with respective peak gains of 4.83 dBic and 4.81 dBic. The attained 3 dB ARBWs entirely cover the WLAN 2.4 GHz and WiMAX 3.5 GHz frequency bands.

Wideband Circularly Polarized Antenna Based on a Non-Uniform Metasurface

This paper presents a non-uniform metasurface (MS)-based circularly polarized (CP) antenna that is able to perform with a wideband operation characteristic. A squared patch with truncated corners was chosen as a radiating CP source. Then, unlike the conventional CP MS antennas with a uniform MS, the proposed design employed a non-uniform MS placed above the driven patch. Apart from increasing the impedance bandwidth, the non-uniform MS was also capable of generating two additional CP bands in the high-frequency region, which contributed to significantly increasing the antenna’s overall performance. For demonstration, an antenna prototype was fabricated and experimentally tested. The measured operating bandwidth of the fabricated antenna was about 30% and the broadside gain within this band was around 6.6 dBic. Compared to the other reported CP MS antennas in the open literature, the proposed design has the advantages of very wideband operation with a comparable size and gain.

Dielectric lens with stacked cone‐shaped cavity for broadside radiation enhancement of circularly polarised patch antenna

A dielectric lens is proposed here to broaden the half-power beamwidth (HPBW) of conventional circularly polarised (CP) patch antenna. The lens printed by the 3D technique is constructed by a dielectric cylinder with a stacked cone-shaped cavity. To illustrate the working principle, the energy distribution of the electromagnetic wave refracted by the lens is analysed. Besides, the design procedures of the lens are introduced in detail. The applicability of lens is also discussed including the changing of permittivity and dimensions. For verification, prototypes of the designed CP antenna with the lens are fabricated and measured. The performances of the CP antenna without lens are also provided for comparison. The measurements show that by using the proposed lens, the HPBW of the CP patch antenna is extended effectively from 69° to 170° while maintaining a wide 3-dB axial ratio beamwidth of >200°.

Pair of diagonal slots loaded low‐profile circularly polarised patch antenna with wide 3 dB axial ratio beamwidth

A low-profile circularly polarised (CP) antenna with wide 3 dB axial ratio beamwidth (ARBW) is proposed in this study. The possibilities of widening the 3 dB ARBW of single diagonal slot-loaded conventional CP antenna have been explored successfully by incorporating a pair of rectangular slots along the diagonal line of a square patch antenna. The design parameters and the symmetrical arrangement of this pair of slots across the centre of the patch are crucial to adjust and achieve the wide 3 dB ARBW with high CP purity. Coupling between the two diagonal slots provides nearly equal E θ and E φ radiation patterns over a large angular range, and hence the 3 dB ARBW has been enhanced by 85.71 and 55.4% for φ = 0° and φ = 90° planes, respectively, in comparison to the conventional slot-loaded CP antenna. The minimum 3 dB ARBW of 208° and a maximum of 230° have been achieved across different planes of radiation φ = 0°, 45°, 90°, and 135° with a minimum axial ratio of 0.20 dB only. The absolute planar antenna of size (0.29 × 0.29 × 0.013)λ0 3, where λ0 is the free-space wavelength corresponding to the CP frequency, has been fabricated and tested. The measured and simulated results show fair consistency to each other.

Miniaturized Low-Profile Circularly Polarized Metasurface Antenna Using Capacitive Loading

A miniaturized circularly polarized (CP) metasurface (MS) antenna using capacitive loading is investigated. First, one novel miniaturized MS unit cell is proposed. By introducing a pair of capacitive-loading strips inserted along the diagonal of a corner-truncated patch, 56% size reduction is achieved for this miniaturized unit cell. Accordingly, a compact CP MS antenna of a $4\,\,\times \,\,4$ array composed by the miniaturized unit cells is designed and fed by a slot structure. Compared with the conventional CP MS antenna without capacitive-loading strips, 63% overall size reduction can be achieved. Furthermore, the CP radiation mechanism of the CP MS antenna is revealed and explained clearly using the characteristic mode analysis (CMA). For demonstration, a prototype of the proposed CP antenna was fabricated and measured. The measured 3 dB axial ratio (AR) band is from 3.33 to 3.63 GHz, and the 10 dB impedance band is from 3.02 to 3.82 GHz, while 6.5 dBi gain is realized within the CP operating band.

3-D-Printed Dielectric Lens With Cone-Shaped Cavity for Axial Ratio Beamwidth Enhancement of Circularly Polarized Patch Antenna

A 3-D-printed dielectric lens is proposed in this paper to widen the 3-dB axial-ratio beamwidth (ARBW) of conventional circularly polarized (CP) patch antenna. The proposed lens is constructed by a solid dielectric cylinder with a cone-shaped air cavity. To illustrate the working principle, the geometric optics method is introduced, then refractions of the CP wave caused by the lens are theoretically analyzed and the related equations of axial ratio (AR) are derived. Besides, the key parameters that affect the ARBW of the CP antenna are adequately investigated, and the general rules are summarized to design it conveniently. To provide two orthogonal modes at center frequency, a dual-feed network is used to drive the patch antenna. To demonstrate, prototypes with and without lens are designed, fabricated, and measured. The experimental results show a good agreement with the simulated ones in terms of returned loss, radiation pattern, gain, and AR. The measured 15-dB impedance bandwidth is 20.4%, the 3-dB AR bandwidth is 17.5%, and the peak gain at main radiated direction is 5.5 dBic. Furthermore, the compared results in measurements show that by using the proposed lens, the 3-dB ARBW of CP antenna can be extended effectively from 82° to 162°, which is highly demanded in global navigation satellite system for reducing multipath interference. Finally, the applicability of the proposed lens is discussed including the available frequency bandwidth, the size of ground plane, and the feasibility for other types of CP patch antennas.

A Wideband Circularly Polarized Tightly Coupled Array

Tightly coupled arrays (TCAs) have received considerable interests recently. Although various TCAs have been reported, they are generally limited to single or dual linear polarizations. Considering the importance of circularly polarization in various wireless systems, it is meaningful to design a circularly polarized (CP) TCA with a simple configuration. This communication presents a CP tightly coupled crossed dipole array (CP-TCCDA) with intrinsic CP feeding structure, which achieves wide overlapped impedance bandwidth and axial ratio (AR) bandwidth. An analysis is given to explain the principles of AR bandwidth improvement and is validated by the comparison of radiated E-fields between a CP-TCCDA and a conventional CP crossed dipole array. To verify the design concept, a $4 \times 4$ CP-TCCDA with elaborate feeding network is fabricated and measured. The measured results confirm that the proposed array achieves VSWR < 3 bandwidth from 2.06 to 6.46 GHz (3.14:1) and 3 dB AR bandwidth from 2.35 to 5.6 GHz (2.38:1), which are much wider than the bandwidth of an isolated element and a conventional array using the same element.

Circularly polarised patch antenna using 3D‐printed asymmetric substrate

A microstrip patch antenna with circular polarisation is presented by using an intrinsically asymmetric 3D-printed substrate. Unlike conventional circularly polarised (CP) antenna, the perturbation for generating CP waves is caused by the asymmetry of substrate, where a pair of triangular air insertions is symmetrically etched in one of diagonals of the square substrate. Besides, a slot is etched on the centre of patch and parallel to its side for good impedance matching. Actually, in consideration of the fact that in 3D printing, the dimensional tolerance for dielectric is less than the one for metal, the presented approach has more superiority than the conventional methods. A prototype is fabricated and measured. Measured results are found in reasonably good agreement with the simulated ones. Stereo lithography apparatus technology is here employed in the 3D printing process.

Compact dual‐band circularly polarised antenna with omnidirectional and unidirectional properties

A novel dual-band circularly polarised (CP) antenna is presented in this study. The proposed antenna is composed of a U-slot patch and four sequentially rotated shorted monopoles. Different from conventional omnidirectional CP antennas with shorted monopolar patches, a novel capacitively coupled technique is proposed to achieve good impedance matching, which makes it possible to generate unidirectional CP patterns simultaneously. This antenna can generate omnidirectional CP radiation patterns in the global positioning system L1 band (1575 ± 5 MHz) for navigation application and unidirectional CP radiation patterns in the 2.4-GHz industrial, scientific, and medical band (2.40-2.48 GHz) for municipal wireless networks. The dual-band performance can be tuned separately. Experimental results confirm the good performances of a compact antenna with the size of 0.26 λ 0 × 0.26 λ 0 × 0.026 λ 0 . The obtained 10-dB impedance bandwidths can cover the two required bands. The 3-dB axial ratio (AR) bandwidths cover the lower band and the upper band from 2.425 to 2.46 GHz. In addition, a wide 3-dB AR beamwidth of 200° is obtained at 2.45 GHz. The simple structure and good performances make this antenna a good candidate for handheld devices and vehicular applications.

AR bandwidth and gain enhancements of patch antenna using single dielectric superstrate

A single-feed circularly polarised (CP) patch antenna loaded with a single dielectric superstrate for a wideband high-gain CP radiation is presented. Along with the gain enhancement, the emphasis is on extra CP radiation of the antenna caused by the superstrate, which broadens the axial-ratio (AR) bandwidth significantly. A prototype antenna was fabricated and measured at a frequency of 11 GHz. The antenna with overall size of 40 × 40 × 17.45 mm has a voltage standing wave ratio 26% (10.0-13.0 GHz), and a radiation efficiency of >90%. The presence of superstrate produces a ~350% increase in 3 dB AR bandwidth and a ~7 dB increase in the broadside gain relative to the conventional CP patch antenna.