Orthogonal Polarization Channels Research Articles

Spatial Varying Vectorial Holography and Polarization Encryption Based on Birefringent Metasurface

AbstractVectorial holography technology plays a fundamental role by exploring the vectorial nature of light and enriches disruptive applications in optical encryption, display, and manipulation. With its flexible control methods and high‐precision subwavelength design, metasurface possesses rich modulation flexibility that surpasses traditional optical modulation devices. Here, a vectorial holographic design is implemented that can cover various polarization information by using a birefringent metasurface. The design adopts a gradient descent optimization method to jointly optimize the phase information of the orthogonal polarization channels with the birefringent metasurface, for continuous variation of polarization information in the entire vector space on the Poincare Sphere with efficient information integration. Based on the detailed depiction of full vector space polarization texture, such efficient vectorial holographic optimization methods can expand the encryption channels and achieve high spatial resolution allowing large capacity multiplexing, data storage, pattern recognition, etc.

Neural‐Optic Co‐Designed Polarization‐Multiplexed Metalens for Compact Computational Spectral Imaging

AbstractIn the expanding fields of mobile technology and augmented reality, there is a growing demand for compact, high‐fidelity spectral imaging systems. Traditional spectral imaging techniques face limitations due to their size and complexity. Diffractive optical elements (DOEs), although helpful in reducing size, primarily modulate the phase of light. Here, an end‐to‐end computational spectral imaging framework based on polarization‐multiplexed metalens is introduced. A distinguishing feature of this approach lies in its capacity to simultaneously modulate orthogonal polarization channels. When harnessed in conjunction with a neural network, it facilitates the attainment of high‐fidelity spectral reconstruction. Importantly, the framework is intrinsically fully differentiable, a feature that permits the joint optimization of both the metalens structure and the parameters governing the neural network. The experimental results presented herein validate the exceptional spatial‐spectral reconstruction performance, underscoring the efficacy of this system in practical, real‐world scenarios. This innovative approach transcends the traditional boundaries separating hardware and software in the realm of computational imaging and holds the promise of substantially propelling the miniaturization of spectral imaging systems.

High-Resolution Metalens Imaging Polarimetry.

Imaging polarimeters find many critical applications in applications ranging from remote sensing to biological detection. Metasurfaces have been proposed as a compact approach for imaging polarimeters, but prior strategies suffer from low imaging resolution. Here, we propose an interleaved metalens configuration for polarization imaging where three-row metasurface units within a group individually interact with three pairs of orthogonal polarization channels. The optical paths between the object and adjacent three-row metasurfaces are nearly equal, allowing the construction of a metalens polarimeter with an unlimited numerical aperture (NA), which is beneficial for high-resolution polarization imaging. The metalens polarimeter fabricated by crystalline silicon nanostructures has a NA of 0.51 at 632.8 nm and achieves an imaging resolution of up to a 1.2-fold wavelength. Polarimetric microscopy experiments demonstrate that metalens polarimeters can realize high-resolution polarization imaging for various microscopic samples. This study offers a promising solution for high-resolution metasurface polarization imaging, with the potential for widespread applications.

Dual-Channel Switchable Metasurface Filters for Compact Spectral Imaging with Deep Compressive Reconstruction.

Spectral imaging technology, which aims to capture images across multiple spectral channels and create a spectral data cube, has been widely utilized in various fields. However, conventional spectral imaging systems face challenges, such as slow acquisition speed and large size. The rapid development of optical metasurfaces, capable of manipulating light fields versatilely and miniaturizing optical components into ultrathin planar devices, offers a promising solution for compact hyperspectral imaging (HSI). This study proposes a compact snapshot compressive spectral imaging (SCSI) system by leveraging the spectral modulations of metasurfaces with dual-channel switchable metasurface filters and employing a deep-learning-based reconstruction algorithm. To achieve compactness, the proposed system integrates dual-channel switchable metasurface filters using twisted nematic liquid crystals (TNLCs) and anisotropic titanium dioxide (TiO2) nanostructures. These thin metasurface filters are closely attached to the image sensor, resulting in a compact system. The TNLCs possess a broadband linear polarization conversion ability, enabling the rapid switching of the incidence polarization state between x-polarization and y-polarization by applying different voltages. This polarization conversion facilitates the generation of two groups of transmittance spectra for wavelength-encoding, providing richer information for spectral data cube reconstruction compared to that of other snapshot compressive spectral imaging techniques. In addition, instead of employing classic iterative compressive sensing (CS) algorithms, an end-to-end residual neural network (ResNet) is utilized to reconstruct the spectral data cube. This neural network leverages the 2-frame snapshot measurements of orthogonal polarization channels. The proposed hyperspectral imaging technology demonstrates superior reconstruction quality and speed compared to those of the traditional compressive hyperspectral image recovery methods. As a result, it is expected that this technology will have substantial implications in various domains, including but not limited to object detection, face recognition, food safety, biomedical imaging, agriculture surveillance, and so on.

Flexible terahertz phase modulation profile via all-silicon metasurfaces

Metasurfaces represent a group of artificially designed structures capable of efficiently manipulating the transmission properties of incident waves, have attracted considerable attention in terahertz (THz) radiation, and gradually overcome the complexity of bulky optical systems. Although the introduction of meta-optics has made great progress in full-span phase modulation schemes within orthogonal polarization channels, flexible phase functions can still effectively alleviate the design complexity of wavefront engineering. Here, we experimentally demonstrate several designs of general-purpose all-silicon THz metasurfaces applied in different scenarios. By combining geometric phase and propagation phase, switchable multiple phase modulation in orthogonal circularly polarized (CP) channels can be realized by switching the handedness of the incident wave. The advantage of the proposed scheme is that not only a locked phase modulation profile can be generated within the cross-polarized channels with opposite handedness, but also the phase distribution within each channel can be arbitrarily tailored to break the locking, demonstrating the flexible wavefront manipulation capability of the spin-decoupled phase control. Two different metalens are experimentally demonstrated to verify the feasibility of the multiple phase modulation scheme, mainly exploiting the topological charge and type of the focused beams. This work allows the metasurface to show a higher degree of freedom in wavefront manipulation by using a simpler phase-composite algorithm.

Longitudinal evolution from scalar to vector beams assembled from all-dielectric metasurfaces.

Vector vortex beams (VVBs) with non-uniform polarization states have a wide range of applications, from particle capture to quantum information. Here, we theoretically demonstrate a generic design for all-dielectric metasurfaces operating in the terahertz (THz) band, characterized as a longitudinal evolution from scalar vortices carrying homogeneous polarization states to inhomogeneous vector vortices with polarization singularities. The order of the converted VVBs can be arbitrarily tailored by manipulating the topological charge embedded in two orthogonal circular polarization channels. The introduction of the extended focal length and the initial phase difference effectively guarantees the smoothness of the longitudinal switchable behavior. A generic design approach based on vector-generated metasurfaces can assist in the exploration of new singular properties of THz optical fields.

Graphene-enabled metasurface with independent amplitude and frequency controls in orthogonal polarization channels

Graphene-based metasurfaces offer a novel approach to the dynamic manipulation of electromagnetic waves. Until now most of active graphene-based metasurfaces operating with microwaves have been able to control only one wave parameter, e.g, amplitude or frequency, while the realization of independent dual parameter control is rare and remains challenging. Here we report on the design, modelling, fabrication, and characterization of a graphene-based absorbing metasurface capable of independent dynamic control of reflection amplitude in one polarization channel and resonance frequency in another. Numerical simulations show that varying the resistance of the graphene sheet in the designed metasurface allows one to change the amplitude of the reflected x-polarized wave by more than 13 dB and shift the resonance frequency of the absorption spectrum of the y-polarized wave from 3.35 to 4.60 GHz. The dual tunability feature is analysed using the transmission line theory and the lumped-element model of the metasurface and experimentally demonstrated via the fabrication and testing of the metasurface. This work brings a new degree of freedom to tunable graphene-based metasurfaces for controlling microwaves with potential applications in communications, wireless transmission, and sensing.

Improved Detection of Buried Elongated Targets by Dual-Polarization GPR

Ground-penetrating radar (GPR) has been widely applied to the detection and delineation of buried targets in the subsurface. Compared with conventional single-channel GPR, polarimetric GPR has been proven to possess an improved ability to detect and characterize an elongated object in the subsurface. This letter proves that the scattering signals from a cylinder in two orthogonal polarization channels have a phase difference of about 90° when its diameter-to-wavelength ratio is about 0.05–0.33. Consequently, a polarization-difference imaging method, which shifts the VV component by −90° and subtracts it from the HH component, is proposed for the improved detection and imaging of a subsurface elongated object. Its effectiveness is verified by numerical, laboratory, and field tests on buried rebars and pipes. The signal-to-clutter ratios of the reconstructed GPR images can be improved by up to 4.5 dB by considering the phase difference between the dual-polarization components.

Tunable Polarization-Multiplexed Achromatic Dielectric Metalens.

Tunable metasurfaces provide a compact and efficient strategy for optical active wavefront shaping. Varifocal metalens is one of the most important applications. However, the existing tunable metalens rarely serves broadband wavelengths restricting their applications in broadband imaging and color display due to chromatic aberration. Herein, an electrically tunable polarization-multiplexed achromatic metalens integrated with twisted nematic liquid crystals (TNLCs) in the visible region is demonstrated. The phase profiles at different wavelengths under two orthogonal polarization channels are customized by the particle swarm optimization algorithm and matched with the dielectric metaunits database to achieve polarization-multiplexed achromatic performance. By combining the broadband linear polarization conversion ability of TNLC, the tunability of varifocal achromatic metalens is realized by applying different voltages. Further, the electrically tunable customized dispersion-manipulated metalens and switchable color metaholograms are demonstrated. The proposed devices will accelerate the application of metasurfaces in broadband zoom imaging, AR/VR displays and spectral detection.

Improving the phase reconstruction accuracy of simultaneous phase-shifted lateral shearing interferometry using a polarization redundant sub-region interpolation method.

In a simultaneous phase-shifted lateral shearing interferometry, a division of focal plane polarization camera is generally used as the phase-shifting device. However, acquiring simultaneous phase-shift interferograms in a single frame suffers from a lack of spatial resolution, significantly affecting the phase reconstruction accuracy. A polarization redundant sub-region interpolation (PRSI) method is proposed to solve this problem. This interpolation method distinguishes smooth regions from stripe fringe regions by calculating the polarization redundancy error of the synchronous phase shift interferogram. After sub-regional processing, resolution reconstruction is performed in the smoothed area using a fast convolutional bilinear interpolation method. In the streak detail region, the resolution reconstruction is performed based on the strength of the correlation between the orthogonal and non-orthogonal polarization channels crossing the streak region. The PRSI method can quickly reconstruct the lost pixels and accurately recover the stripe detail information. Experiment results show that the proposed interpolation method outperforms the existing dominant methods in terms of visual reconstruction effect and quantitative index of phase reconstruction.

A Dual-Polarized Reconfigurable Reflectarray Antenna Based on Dual-Channel Programmable Metasurface

The concept of dual-polarized reconfigurable reflectarray (RRA) featuring dual-polarized beam control is proposed and demonstrated using a dual-channel programmable metasurface. The p-i-n diodes are incorporated into each metasurface element as the active components to provide independent phase control in two orthogonal polarization channels. By altering the working states of each p-i-n diode, well-defined beam scanning can be obtained. Element design, focal diameter ratio selection, and discretized phase distributions are carefully optimized to ensure stable radiation performance of the proposed RRA. As verification, a prototype consisting of 20 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\times20$ </tex-math></inline-formula> elements is fabricated and measured, which is controlled by a field programmable gate array-based controller. Measured results show that wide-angle beam scanning up to ±40° can be obtained around 7.45 GHz, with peak gains of 21.13 and 20.89 dBi in dual channels, corresponding to aperture efficiencies of 16.35% and 15.47%. The proposed dual-polarized RRA brings a new freedom in polarization manipulations for versatile beam scanning, which provides promising applications in wireless communication and tracking/detecting systems.

Stokes Parameters Retrieval and Calibration of Hybrid Compact Polarimetric GNSS-R Signals

Global Navigation Satellite System &#x2013; Reflectometry (GNSS-R) is a promising field with a diverse set of applications. Polarimetric GNSS-R has been shown to be sensitive to different land parameters, such as freeze/thaw, crop growth, or soil moisture. In this manuscript, the polarimetric scenario of a GNSS reflection is defined, and the theoretical basis and the algorithm to retrieve the Stokes parameters of a GNSS-R signal is proposed. The presented algorithms are validated using data collected by the two linear orthogonal polarization channels of the SMAP radar antenna working as GNSS-R, known as SMAP-Reflectometry (SMAP-R). The Stokes parameter retrieval is presented for the SMAP-R case, and the receiver phase offset and intensity are calibrated. The same calibration procedure proposed by the CYclone GNSS (CYGNSS) mission is applied to the SMAP-R first Stokes parameter, allowing the retrieval of the normalized bistatic radar cross section of such measurement. Finally, the SMAP-R data is compared to the CYGNSS Normalized Bistatic Radar Cross Section (NBRCS) over the ocean, showing an unbiased root-mean-square error of 3.3 dB, and a small bias.

Anisotropic Temporal Metasurfaces for Tunable Ultrafast Photoactive Switching Dynamics (Laser Photonics Rev. 15(10)/2021)

Switchable Metamaterial Devices In article number 2100244, Tian Jiang and co-workers report a novel polarization-coding all-optical metadevice with a temporal dynamic modulation feature. The switching dynamic can be alternated between a quasi-steady state with a recovery time larger than 2 ns and an ultrafast transient state with a recovery time less than 25 ps in two orthogonal polarization channels, which is believed to be a substantial advance in extending polarization multiplexing metadevices.

Broadband Polarization‐Switchable Multi‐Focal Noninterleaved Metalenses in the Visible

AbstractMulti‐focal lenses are widely applied in zoom imaging, parallel laser processing, 3D displays, among others. However, conventional refractive or diffractive elements cannot realize independently switchable multi‐focal points at arbitrary spatial positions. The ultra‐thin metasurface provides a compact solution while the commonly used interleaved meta‐units scheme deteriorates the performance of the meta‐devices. Here the authors propose all‐dielectric noninterleaved metalenses with polarization‐switchable multi‐focal points. By fully using the degrees of freedom of the single rectangular nanopillar, up to three independently switchable focal points at any spatial position can be realized in a metalens with a single‐structure lattice. With the near‐zero crosstalk between orthogonal polarization channels, polarization detection, zoom imaging, variable‐parameter phase detection, and parallel laser lithography are demonstrated. The multichannel scheme by fully utilizing the polarization freedom paves the way for various multifunctional components.

Polarization‐Assisted Visual Secret Sharing Encryption in Metasurface Hologram

Secret sharing is an advanced encryption method that distributes the secret information into multiple participants and only the predesigned subsets of authorized participants can recover the secret by gathering their independent subinformation. Herein, the security level of secret sharing using electromagnetic manners is further expanded and a simple platform for the realization and direct observation of secret sharing encryption using metasurface hologram is presented. As a proof of concept, a polarization‐assisted secret sharing phase encoding method by integrating visual secret sharing scheme with phase encoding technology using orthogonally polarization‐multiplexing metasurface is experimentally presented. The method achieves its encryption security and decryption simplicity by hiding the secret information into a pair of orthogonal polarization channels ensuring that the secret information is direct decryption as the two channels participate simultaneously while fully concealed in single channel. This scheme may pave a new route for future information encryption, hiding, and authentication.

A ultra wideband dual-polarized antenna with high isolation degree for passive radar application

ABSTRACT This paper presents an ultra wideband (UWB) dual polarization antenna with high polarization isolation for passive radar system The antenna is composed of a modified Vivaldi antenna and a folded dipole log periodic antenna. Based on the respective structural characteristics of the printed Vivaldi antenna and the folded dipole log periodic antenna, they are placed orthogonally without touching each other, which form two orthogonal polarization channels and can achieve high polarization isolation between the channels. The radiator shape of traditional Vivaldi antenna is modified and simplified to further reduce the electromagnetic coupling between two polarized radiators. The planar folded dipole log periodic antenna is fed by UWB Microstrip tapered line Balun, and a vertical through hole is introduced to connect the parallel double line of Balun and the coplanar stripline (CPS) at the input terminal of the folded dipole log periodic antenna. The two polarization ports of the antenna are welded by coaxial line and microstrip line. The dual polarization antenna is designed, optimized, fabricated and tested within the range of 3 GHz ~ 8 GHz. In the investigated frequency range, the polarization isolation of the simulated dual polarization antenna is greater than 30 dB, and the average polarization isolation is about –35 dB.The gain of Vivaldi antenna polarization port increases from 2.82dBi to 9.49 dBi, and the gain of folded dipole log periodic antenna polarization port increases from 5.21 dBi to 8.56dBi. A large number of experimental tests and analysis are carried out on the fabricated dual polarization antenna, and the test results are close to the simulation results, which prove the effectiveness of the proposed dual polarization antenna. In view of the printed circuit implementation method and structure characteristics of the dual polarization antenna designed in this paper, the scheme has the advantages of low cost, simple design and good polarization characteristics, so it is suitable for the application of anti radiation missile guidance radar and electronic reconnaissance.

Anisotropic Temporal Metasurfaces for Tunable Ultrafast Photoactive Switching Dynamics

AbstractRecent progress in miniaturized photonic devices has promoted the development of metasurfaces for multidimensional manipulation of optical light fields. One significant example is the polarization multiplexing metadevice, which has earned considerable attention because of its low crosstalk and tremendous polarization‐controlled functionalities for numerous applications, such as wavefront steering, holography, and display. Here, a novel metasurface platform is demonstrated for photoactive terahertz switching by encoding the temporal modulation dynamic into the polarization channels. Coupled plasmonic resonators with four‐fold symmetry are utilized to exhibit an isotropic plasmon‐induced transparency analog resonance. Unlike typical optically tunable metamaterials that are limited to a single switching speed, this temporal modulation is multiplexed into two sets of polarization profiles. Specifically, the switching dynamic can be alternated between a quasi‐steady state with a recovery time larger than 2 ns and an ultrafast transient state with a recovery time of &lt;25 ps in the orthogonal polarization channels. This extraordinary performance is attributed to the high contrast in crystallinity between two photoactive islands embedded in one‐unit cell. The long‐standing bottleneck of invariant switching dynamics in all‐optical metadevices is addressed, which provides a new paradigm for the design of future switchable metamaterial devices.

Birefringent transmissive metalens with an ultradeep depth of focus and high resolution

Depth of focus (DOF) and transverse resolution define the longitudinal range and definition of the focusing lens. Although metasurface axilenses and light-sword metalenses with radial and angular modulations can elongate the DOF, these approaches have inherent limitations in being reliable only for small numerical aperture (NA) cases, which in turn compromises the transverse resolution for the given aperture dimension. To conquer this limitation, we propose and experimentally demonstrate a birefringent metalens, achieving an ultradeep DOF of 41 λ in terms of the total scattered field, corresponding to a record-breaking wide NA range from 0.14 to 0.7. Meanwhile, the diffraction limited focal spot size in this NA range can guarantee acquisition of images with high resolution. A hybrid methodology is proposed that utilizes both the accuracy of holography in electromagnetic field reconstruction and the polarization multiplexing to double the DOF. A stratified transmissive meta-atom is utilized to encode a pair of independent phase profiles in two orthogonal polarization channels. Furthermore, we combine the generalized scattering matrix with the multipole expansion theory for the first time to elucidate the mechanism of maintaining high transmittance and widening the transmission phase coverage by using the multilayered structure. The proposed metalens provides a competitive platform for devising high-resolution deep DOF systems for imaging and detection applications.

Dielectric metasurface based polarization and orbital angular momentum demultiplexer

The development of high-speed optical communication calls for compatible multiplexing dimensions to enlarge transmission capacity and process optical information. Although vortex beam multiplexing has been previously demonstrated by exploiting the orthogonality of orbital angular momentum (OAM) modes, the multidimensional demultiplexer remains elusive due to the limited phase and polarization manipulation capability. We introduce dielectric propagation phase metasurface based demultiplexers and investigate their application in simultaneously demultiplexing OAM and orthogonal linear polarization (OLP) channels. Utilizing the size-dependent anisotropy of dielectric rectangular nano-pillars, we reveal that the multimode fork grating phase masks with different off-axis phase items can be integrated on the same substrate with polarization selectivity, which allows simultaneously demultiplexing OAM and OLP channels. The demultiplexed position and the number of channels depend on its off-axis phase items, and a 16-channel (8-OAM modes and 2-polarization states) demultiplexer is demonstrated. The vortex beams with different OAM modes and OLP states are successfully demultiplexed with the maximum crosstalk is below −9.2 dB. Changing the number of superimposed complex amplitudes and the off-axis phase items in multimode fork grating phase masks, we further present two 12-channel demultiplexers with different channel distributions. Our results provide an approach that breaks the polarization-insensitive in manipulating OAM modes, introducing demultiplexers that apply to OAM mode and OLP states simultaneously.

Single-pixel, single-input-state polarization-sensitive wavefront imaging.

In this Letter, we describe a single-pixel polarization-sensitive imaging technique, capable of generating the birefringence map of a thin specimen by using single-pixel detectors. Spatially modulated light is circularly polarized to illuminate the specimen. The transmitted light through the specimen is then focused via a lens and measured by position-sensitive detectors in two orthogonal polarization channels. The measurement of the irradiance and centroid position of the optical focus and subsequent computations enable the production of polarization-dependent wavefront maps, which can then be utilized to reconstruct sample birefringence information. We demonstrate the feasibility of our method by measuring distribution of optic-axis orientation and phase retardation of various birefringent samples.