Incident Circular Polarization Research Articles

Terahertz metasurface absorbed by both linearly and circularly polarized waves

In recent years, the development of ultrafast laser technology has provided a stable and reliable terahertz source for generating terahertz wave pulses, and the great research progress of terahertz wave has been made. As a new type of two-dimensional artificial metamaterial, metasurface can effectively control the transmission, reflection and polarization of electromagnetic waves, which has attracted the extensive attention. Most of the reported terahertz absorbers so far are based on metasurfaces with linear polarization incidence, and few studies have been conducted on terahertz metasurfaces that can produce efficient absorption at both linear and circular polarization incidence, which limits the practical application areas. Therefore, it is necessary to explore an efficient absorber which can realize both linear polarization and circular polarization. We propose a vanadium dioxide composite metasurface structure. The vanadium dioxide is a typical temperature-controlled phase change material, and its conductivity will undergo a huge mutation in the phase change process. When the temperature is lower than the critical temperature (68 ℃), the vanadium dioxide has high resistivity and good insulation performance. When the temperature is higher than the critical temperature, the resistance changes from high resistance state to low resistance state, showing metal characteristics. By changing the external temperature, the phase of vanadium dioxide is changed, the free switching frequency is achieved and both the linear polarization and circular polarization incident efficient absorption are realized. When the vanadium dioxide is insulated, its conductivity is 0 S/m, the metasurface can absorb left-handed circularly polarized wave at 1.30 THz and reflect the incident right-handed circularly polarized wave, and the circular dichroism is 0.85. When the vanadium dioxide is metallic, its conductivity is 2×10<sup>5</sup> S/m and it possesses linearly polarized incident metasurface, the absorption rate of TE linearly polarized incident wave by metasurface reaches 98.5% at 1.95 THz, and the perfect absorption of terahertz wave is realized. The structure has good wide-angle absorption performance for both TE polarization wave and left-handed circularly polarized wave. This composite metasurface structure can achieve the good absorption effect of terahertz waves with different frequencies and different polarization states. Therefore, the design concept of the composite metasurface structure can be used for designing other metasurface terahertz devices, and also for implementing the terahertz imaging and sensing systems due to different response characteristics to different polarization signals.

All-optical controlled-NOT logic gate achieving directional asymmetric transmission based on metasurface doublet

Optical logic gates play important roles in all-optical logic circuits, which lie at the heart of the next-generation optical computing technology. However, the intrinsic contradiction between compactness and robustness hinders the development in this field. Here, we propose a simple design principle that can possess multiple-input-output states according to the incident circular polarization and direction based on the metasurface doublet, which enables controlled-NOT logic gates in infrared region. Therefore, the directional asymmetric electromagnetic transmission can be achieved. As a proof of concept, a spin-dependent Janus metasurface is designed and experimentally verified that four distinct images corresponding to four input states can be captured in the far-field. In addition, since the design method is derived from geometric optics, it can be easily applied to other spectra. We believe that the proposed metasurface doublet may empower many potential applications in chiral imaging, chiroptical spectroscopy and optical computing.

Broadband and thermally switchable reflective metasurface based on Z-shape InSb for terahertz vortex beam generation

In this paper, a broadband and thermally switchable reflective metasurface based on InSb Z-shape resonator (ZSR) structure was proposed for the vortex beam generation in terahertz (THz) region. Numerical simulation results indicate that the operation frequency range and efficiency will be basically unchanged when the external environment temperature increases from 300 K to 340 K, but gradually decreased when the temperature decreases from 300 K to 240 K. More specifically, the proposed InSb metasurface can transform the normal incident circular-polarization (CP) wave to its orthogonal component with conversion coefficient over 0.8 from 0.65 THz to 1.66 THz (relative bandwidth of 87.4%) after reflection when the temperature is changed from 300 K to 340 K. However, the bandwidth will be narrowed and conversion coefficient will be limited with the temperature decreases from 300 K to 240 K. The full 2π phase shift could be obtained at operation frequency by rotating the InSb ZSR structure along the wave propagation direction. Taking advantages of the above characteristics, the reflective THz vortex beams carrying orbital angular momentum (OAM) with topological charge of l = ±1, ±2, and ±3 at the temperature of 300 K over a broadband range can be achieved. In addition, thermal switchable effect of the vortex beams with topological charge of l = 2 at three different frequencies also can be observed. The further numerical results also indicate that the generated reflective THz vortex beams have high mode purity by the designed InSb metasurface. Our work provides a potentially efficient method for the development and integration of the broadband and switchable wavefront controlling devices.

Autofocusing and Self-Healing Optical Vortices Realized via Circular Cubic Phase Modulation

Optical vortices have drawn extensive research interests due to their widespread applications in various fields. Therefore, it is of great significance to modulate optical vortices to endow them with more properties. Herein, the autofocusing and self-healing properties are introduced to optical vortices via implementing circular cubic phase modulation. The propagation dynamics of the modulated optical vortex is analyzed, and the experimental results match well with the simulations. Moreover, the autodefocusing optical vortices can also be generated, and the flexible switching between the autofocusing state and autodefocusing state can be easily realized by adjusting the helicity of the incident circular polarization. Besides, the topological charges of the two states are also experimentally verified. Our study provides a novel way to modulate optical vortices, which may enrich their applications in optics and photonics.

Polarization-Selective Bifunctional Metasurface for High-Efficiency Millimeter-Wave Folded Transmitarray Antenna With Circular Polarization

A novel high-efficiency polarization-selective bifunctional metasurface (MS) is proposed, which can realize cross-polarized focusing transmission for left-handed circular polarization (LHCP) incidence and co-polarized specular reflection for right-handed circular polarization (RHCP) incidence. As an example of its practical application, the designed MS is applied to form an RHCP folded transmitarray antenna (FTA) whose feed source (FS) is integrated with the bottom metallic surface. To prove the concept, a prototype of millimeter-wave (MMW) FTA based on the proposed MS is numerically simulated, physically implemented, and experimentally measured. The measured results coincide well with the simulated ones, showing a peak gain of 23.6 dBi at 27.7 GHz with the peak aperture efficiency of 43.1%, a 3 dB gain bandwidth of 15.8% (25–29.3 GHz), and a 3 dB axial-ratio (AR) bandwidth over 29.6%. The proposed high-performance antenna has the advantages of low profile, high efficiency, integration design, and broad bandwidth, which may have promising applications in the fifth-generation MMW communication system. The proposed MS and its application to high-gain antenna may promote the uses of MS to real-world applications in satellite communication, radar detection, and wireless communication.

Ultra-broadband high-efficiency circular polarization conversion and terahertz wavefront manipulation based on an all-metallic reflective metasurface.

In this paper, an all-metal metasurface (MS), which can achieve high-efficient reflective circular-polarization conversion and multifunctional terahertz (THz) wavefront manipulation in an ultra-broadband frequency range, is proposed and investigated theoretically. The proposed all-metal MS consists of the periodic array of a gold vertical-split-ring (VSR) structure adhered on gold substrate. Numerical simulation results indicate that the proposed MS structure can convert the incident circular-polarization (CP) wave into its orthogonal component after reflection with a conversion coefficient over 95% from 0.8 to 1.65 THz (relative bandwidth of 68.3%). The full 2π phase shift of the proposed MS in this frequency range can be obtained by changing the rotation angle of the VSR structure along the wave propagation direction. As proof of concept for the multifunctional wavefront manipulation, anomalous reflection, reflective planar focusing, and vortex beam generation are numerically demonstrated based on the Pancharatnam-Berry (PB) phase principle. Our work can provide an effective method of enhancing the performance of reflective-type all-metal MS and show endless potential in wavefront manipulation and communication applications in THz and even optical region.

Near-infrared doublet zoom lens based on Ge2Sb2Te5 metasurface

Although numerous studies showcase the capability of metalenses to serve as crucial elements of miniaturized optical systems, tunable focusing remains an ongoing challenge. Here, we propose a doublet zoom lens based on a Ge2Sb2Te5 (GST) metasurface at a wavelength of 1.55 μm. Multistep zooming is realized by the joint control of incident circular polarization inversion and GST phase transition. Three tight focal spots, 0.91 λ, 1.09 λ and 0.89 λ, are achieved from the composite lens. The devised highly-integrated doublet lens, with advantages such as as being multi-foci, easily operated and responding quickly, can find applications in fields that require ultracompact zoom imaging and beam focusing.

Ultra‐Wideband and Polarization‐Independent RCS Reduction Based on Polarization Conversion Metasurface

AbstractIn this work, an ultra‐wideband polarization conversion metasurface (PCM) is proposed. Because it is an anisotropic structure with a pair of mutually perpendicular symmetric axes u and v, and the phase difference between the reflection coefficients under u‐ and v‐polarized incidences is close to 180° in an ultra‐wide frequency range from 9.5 to 29.5 GHz, it can realize ultra‐wideband circular polarization (CP) maintaining reflection and make its co‐polarized reflection coefficient under CP incidence close to 1.0 in the ultra‐wide band 9.2–29.7 GHz. In addition, for any metasurface, when its unit cell structure is rotated by an angle ψ, almost ±2ψ Pancharatnam‐Berry (PB) phase will be generated in co‐polarized reflection coefficient under CP incidence. Thus, based on the proposed PCM, an ultra‐wideband Pancharatnam‐Berry (PB) 2‐dit coding metasurface is conveniently proposed. The simulation and experimental results show that the coding metasurface can achieve more than 10 dB radar cross section (RCS) reduction under normal incidence with arbitrary polarizations in the ultra‐wide frequency band 8.7–29.3 GHz with a relative bandwidth of 108.4%, in addition, when the incident angle is increased to 30°, the RCS reduction performance can be kept well, which shows that the proposed coding metasurface has good stealth performance under the detection of airborne radar, fire control radar and imaging radar working in X, Ku, and K bands, it is very practical.

Ultrathin Dual‐Band Wide‐Angle Beam Scanning Metalens Based on High‐Efficiency Meta‐Atom

Metasurfaces composed of planar subwavelength‐scaled meta‐atoms demonstrate unprecedented wavefront manipulation capabilities. However, the wavelength‐dependent property of the metasurface could severely reduce the design freedom, and most metasurfaces require multilayer structure for high efficiency. Herein, a single‐layered‐substrate meta‐atom is proposed to achieve independent phase manipulations as well as high transmission efficiencies at two frequencies. Specifically, the full 2π phase coverages can be individually realized through rotating the corresponding resonators. As a proof‐of‐concept demonstration, a dual‐band high‐efficiency metalens with wide field‐of‐view (FoV) under the circular polarization incidence is simulated, fabricated and measured at Ka‐band for satellite communications. Both simulation and experimental results agree well with each other, demonstrating that wide scanning coverages of ±60° can be achieved at both 20 and 30 GHz. The proposed method can be a competitive candidate for designing dual‐band high‐efficiency meta‐devices.

Multifocal flat lens using thin anisotropic surface in visible light spectrum

In this study, a novel design methodology of multifocal flat lens using a thin anisotropic surface is proposed and investigated in visible light spectrum. The unit cell of the anisotropic surface consists of two identical electric field coupled resonators (ELCRs) separated by a dielectric substrate. When the wave-front of transmitted wave through the anisotropic surface has a shape of parabola, the focusing spot can be generated at focal point. The transverse and longitudinal multi-focusing can be achieved by a separated mode and a chessboard alternating mode, respectively. The feasibility of the methodology is confirmed by both analytic and simulated results for circular polarization incidence with wavelength of 600 nm.

Broad-band and broad-angle linear and circular polarization converting metasurface

This paper presents a broad-band and broad-angle anisotropic reflective polarization converting metasurface (PCMS) for C- and X-band applications, which is composed of square split ring resonators. Such PCMS can be used in a wide range of applications such as satellite antennas, navigation systems, and biomedical devices, to name but a few. The performance of the presented PCMS is analyzed theoretically, numerically, and experimentally. The results at normal and oblique incidences demonstrate that the PCMS achieves broad-band linear polarization (LP) transformation within an operating bandwidth of 5.3 GHz (5.3–10.6 GHz), covering the C-band and a portion of the X-band. The broad-band performance of the PCMS sustains from to . Moreover, the PCMS achieves polarization conversion for circularly polarized (CP) waves in the same bands. The polarization conversion ratio is more than for both LP and CP incident waves.

Dual-frequency multi-function switchable metasurface

Most metasurfaces can manipulate electromagnetic waves in the deep sub-wavelength range, but often have the disadvantages of low modulation depth and single-frequency operation. Here, a vanadium dioxide composite aluminum antenna (VCAA) for dual-frequency switchable function is proposed. After the vanadium dioxide (VO2) phase transition, the VCAA can achieve a high reflection coefficient (about 0.78) and 2π phase coverage independently at different frequencies under circular polarization (CP) incidence by rotating double C-type resonator and double D-type resonator. In order to better understand this characteristic, the surface electric field distribution of the metasurface is calculated and analyzed. Three temperature-switchable metasurfaces with beam splitting, focusing (divergence) and vortex beams functions at different frequencies are designed by encoding different arrangements of particles and convolution operations. And these functions are not influenced by the temperature at 0.52 THz, but have the effect of temperature-switchable at 0.98 THz. The functions of these three metasurfaces are demonstrated and verified from the perspective of electric field and phase. This work provides a new method for the dynamic regulation of terahertz waves and the design of multi-frequency devices, and accelerates the development of terahertz communications and information encryption.

An ultra-wideband polarization conversion metasurface for RCS reduction

In this paper, a polarization conversion metasurface is proposed, which can realized ultra-wideband circular polarization (CP) maintaining reflection in the frequency range 7.3–28.7 GHz; in addition, when its unit cell structure is rotated by an angle ψ, almost ±2ψ Pancharatnam-Berry (PB) phase will be generated in its co-polarized reflection coefficient under CP incidence. Thus, base on the polarization conversion metasurface, an ultra-wideband PB 2-dit coding metasurface is further proposed for Radar Cross Section (RCS) reduction. The simulation and experiment results show that the coding metasurface can achieve more than 10 dB RCS reduction in the ultra-wide frequency band 7.2–28.6 GHz under normal incidence with arbitrary polarizations; in addition, when the incident angle is increased to 45°, the ultra-wideband RCS reduction can still be realized, which shows that the coding metasurface has good stealth performance in X, Ku and K bands, and it is very practical for most airborne radar, fire control radar and imaging radar working in these bands.

Revealing the Chiroptical Response of Plasmonic Nanostructures at the Nanofemto Scale.

The spatiotemporal origin of plasmonic chiroptical responses in nanostructures remains unexplored and unclear. Here, two orthogonally oriented Au nanorods as a prototype were investigated, with a giant chiroptical response caused by antisymmetric and symmetric mode excitations for obliquely incident left-handed circular polarization (LCP) and right-handed circular polarization (RCP) light. Time-resolved photoemission electron microscopy (PEEM) was employed to measure the near-field spatial distributions, spectra, and spatiotemporal dynamics of plasmonic modes associated with the chiroptical responses at the nanofemto scale, verifying the characteristic near-field distributions at the resonant wavelengths of the two modes and a very large spectral dichroism for LCP and RCP. More importantly, eigenmode excitations and their contributions to the ultrafast plasmonic chiroptical response in the space-time domain were directly revealed, promoting a full understanding of the ultrafast chiral origin in complex nanostructures. These findings open a way to design chiroptical nanophotonic devices for spatiotemporal control of chiral light-matter interactions.

Polarization-dependent spatial channel multiplexing dynamic hologram in the visible band.

In this work, we propose dynamic holography based on metasurfaces combining spatial channel multiplexing and polarization multiplexing. In this design, spatial channels can provide up to 3N holographic frames, which not only increase the possibility of dynamic control but also increase the privacy of the holographic display. This design is also sensitive to polarization, so it further expands the spatial channel capacity. For the left and right circular polarization incident light, there are different dynamic pixel schemes. Therefore, this approach holds promise in the holographic display, optical storage, optics communication, optical encryption, and information processing.

Fine manipulation of terahertz waves via all-silicon metasurfaces with an independent amplitude and phase.

Integrating independent wavefront controls into one device can meet the increasing demand for high-capacity flat electromagnetic devices. Simultaneously and independently controlling the amplitude and phase is pivotal for completely manipulating the propagation of electromagnetic waves. Here, we propose several all-silicon metasurfaces to achieve multifunctional designs and simultaneous modulation of amplitude and phase profiles in the terahertz (THz) band. These metasurfaces integrate two degrees of freedom of the propagation phase and Pancharatnam-Berry (PB) phase. To illustrate the feasibility of this design, three schematic functions are shown below: a three-channel vortex beam generator, a controllable intensity ratio of co- and cross-polarizations corresponding to the incident circular polarization (CP), and a bifocal metasurface that is capable of generating two off-axis vortices with controllable power allocation. A sample is fabricated to specifically verify the amplitude and phase modulation of this design. The experimental results agree well with the simulations and validate the good performances of our proposals. This approach for directly generating an editable amplitude and phase may provide a new choice to design ultra-thin photonic devices.

Multifunctional terahertz metasurfaces for polarization transformation and wavefront manipulation.

Conventionally, the realization of polarization transformation and wavefront manipulation in metasurfaces relies on the Pancharatnam-Berry (PB) phase together with the dynamic phase. However, the reported polarization transformation and wavefront manipulation were limited to spin-dependent wavefront manipulation for circular polarization (CP). To obtain more abundant functions, we propose a novel technology that relies on the dynamic phase with a spatial interleaving unit arrangement. With the functions of a quarter wave plate, the metasurfaces we designed can achieve multiple wavefront manipulations which are not only for the spin polarization transformation but also for the linear polarization transformation. Specifically, we design a bifocal metasurface, which can focus on one circularly polarized component as a point and spin-opposite component as a vortex under the linearly polarized (LP) incidence. With the further adjustment of the unit arrangement, the left-hand circularly polarized (LCP) and right-hand circularly polarized (RCP) components under the LP incidence can be refocused on the same point and then composited, resulting in a new LP exit wave. Furthermore, we prove theoretically that the desired x-LP component and y-LP component under the arbitrary CP incidence can also be manipulated independently. We believe that the versatility of this method will provide a novel platform for the development of terahertz integrated photonics.

Recent progresses of circular polarization multiplexing engineered multi-functional metasurfaces

In this paper, the recent progresses of circular polarization (CP) multiplexing engineered multi-functional metasurfaces is reviewed. Firstly, in order to break through the physical limitation introduced by the conjugate symmetric geometric phases responses under orthogonal CP incidences, the equivalent Jones matrix of metasurface model is derived and the independent manipulation method of two cross-polarized phase responses is proposed. Furthermore, the three-channel wave-front integration method based on metasurface is proposed based on amplitude and phase relationship between co- and cross-polarized output components, which achieves the full manipulation of transmitted CP electromagnetic energy. On these basis, the concept of chirality-assisted phase and the wave-front manipulation method for all full CP channels is proposed, which enhances the utilization efficiency of CP electromagnetic energy. The results proposed in this paper can pave the solid theoretical foundation for increasing the capability of information channels and transmission rate.

Polarization‐Engineered Noninterleaved Metasurface for Integer and Fractional Orbital Angular Momentum Multiplexing

AbstractVortex beam carrying orbital angular momentum (OAM) has been widely explored in optical and microwave regions attributed to its potential characteristics in communication systems. For circular polarization incidence, Pancharatnam–Berry (PB) phase is a direct resource to generate phase gradient along the azimuthal direction required by specific OAM mode. The main drawback of PB phase is that it only affects cross‐polarized fields and keeps the copolarized part unmodulated. Here, a paradigm‐shift perspective of noninterleaved metasurfaces is proposed, which can simultaneously generate separate multiple integer and fractional OAM modes occupying both copolarized and cross‐polarized output channels. The scheme is validated by a series of experimental demonstrations in the microwave regime. By adjusting the polarization states of both input and receiving ends, different integer and fractional OAM modes are demonstrated in the full transmission channels. The results offer a unique recipe to enhance information capacity of metasurfaces and trigger versatile electromagnetic (EM) wave function integrations for advanced compact systems. A variety of applications can be readily expected in spin‐selective optics, spin‐Hall metadevices, and multitask metasurfaces operating in both reflection and transmission modes.

High numerical aperture metalens to generate an energy backflow

Using electronic beam lithography and reactive ion beam etching, a metalens is manufactured in a thin layer of amorphous silicon of a 130-nm depth, a 30-µm diameter, and a 633-nm focal length (equal to the illumination wavelength). The metalens is composed of 16 sectored subwavelength binary gratings with a 220-nm period. The uniqueness of this metalens is that when illuminated by left-handed circularly polarized light, it is capable of generating a left-handed circularly polarized vortex beam with a topological charge of 2, generating a second-order cylindrical vector beam when illuminated by linearly polarized light. Both for linear and circular incident polarization, an energy backflow is found to be generated in the vicinity of the tight focus. Transverse intensity distributions measured with a scanning near-field optical microscope near the focus of the metalens are in qualitative agreement with the intensity distributions calculated by the FDTD method. This confirms that a backward energy flow takes place at the focus of the metalens. A metalens generating an energy backflow near its focus is fabricated and characterized for the first time.