摘要：
The spin–orbit interaction (SOI) of light manifests as the generation of spin-dependent vortex beams when a spin-polarized beam strikes an optical interface normally. However, the spin-momentum nature of this SOI process remains elusive, which impedes further manipulation. Here, we systematically investigate the spin-momentum properties of the transmitted beam in this SOI process using a full-wave theory. The transmitted beam has three components, a spin-maintained normal mode, a spin-reversed abnormal mode, and a longitudinal component. By decomposing the total spin angular momentum (SAM) into the transverse SAM (T-SAM) and the helicity dependent longitudinal SAM (L-SAM), we demonstrate that the L-SAM dominates the total SAM of the normal mode, while the T-SAM dictates that of the abnormal mode. The underlying physics is that the normal mode exhibits a much larger weight than the longitudinal field, while the abnormal mode has a weight comparable to the longitudinal field. This study enriches the understanding of the spin-momentum nature of light's SOI and offers new opportunities for manipulating light's angular momentum.
摘要：
Photonic spin Hall effect (PSHE) in chiral PT-symmetric systems exhibits many exotic features, but the underlying physical mechanism has not been well elucidated. Here, through rigorous calculations based on full-wave theory, we reveal the physical mechanism of the exotic PSHE and identify a chirality-enabled topological phase transition. When circularly polarized light is incident on a chiral PT-symmetric system, the transmitted beam contains two components: a spin-flipped abnormal mode that acquires a geometric phase (exhibiting a vortex or a spin-Hall shift), and a spin-maintained normal mode that does not exhibit such a phase. If the phase difference between the cross-polarized Fresnel coefficients cannot be ignored, it results in a chirality-enabled phase and intensity distribution in the abnormal mode, which induces an exotic PSHE. Consequently, as the incident angle increases, a chirality-induced topological phase transition occurs, namely the transition from the vortex generation to the exotic PSHE. Finally, we confirm that the asymmetric and periodic PSHE in the chiral slab is also related to the phase difference between the cross-polarized Fresnel coefficients. These concepts and findings also provide an opportunity for unifying the phenomena of topological phase transitions in various spin-orbit photonic systems.
通讯机构：
[Xiaohui Ling] L;Laboratory for Spin-Orbit Photonics, College of Physics and Electronic Engineering, Hengyang Normal University, Hengyang 421002, People's Republic of China
关键词：
Hall effect;Slab mills;Vortex flow;Beam shift;Mode decomposition;Orbital angular momentum;Orbitals;Photonic orbital hall effect;Physical mechanism;Thin slab;Vortex beams;Vortex mode;Vortex phase;Angular momentum
摘要：
<jats:title>Abstract</jats:title>
<jats:p>The photonic orbital Hall effect (POHE) refers to the vortex-dependent beam shifts, which is generally believed to result from the conversion of intrinsic orbital angular momentum (IOAM) to extrinsic orbital angular momentum (EOAM). However, the physical mechanism of the POHE, such as how the IOAM is converted to the EOAM, remains further elucidation. In this paper, we re-examine the POHE of a vortex beam with additional IOAM illuminating at an optically thin slab by means of vortex mode decomposition. By considering the competition and coupling between the radial and azimuthal vortex harmonics of the abnormal mode in the transmitted beam, it is found that the underlying mechanism of the POHE is in fact a spin-to-orbital angular momentum (OAM) conversion process. And the IOAM carried by the incident beam is directly superimposed on the OAM obtained during the conversion. Our findings not only offer an alternative perspective for understanding the POHE, but also exhibit application potential in orbit–orbit and spin–orbit optical components.</jats:p>
摘要：
Kirigami-inspired metasurface has attracted great attention in electromagnetic (EM) wave manipulations, due to its unprecedented and tailorable structural transformations via mechanical approaches. However, it is still challenging for wavefront control because of its unfeasibility in forming a phase gradient at different folding angle beta. Here, we report a strategy of kirigami-inspired reconfigurable phase gradient metasurfaces to efficiently control beam steering of scattering wave through mechanically changing beta, where the control range of beta is theoretically derived based on EM diffraction and generalized Snell's law. For demonstration, reconfigurable anomalous reflectors with high efficiency are demonstrated in both dual-element and tri-element tunable met-adevices. In former case, the reflection angle varies within 37 degrees-47 degrees under polarization along y axis by changing beta within the range of 10 degrees-40 degrees, whereas it varies within 36 degrees-50 degrees under that along x axis when beta is changed within 10 degrees-39 degrees, and numerical and experimental results show measured efficiency of beam steering over 78.6%. In latter case, the reconfigurable tri-element metadevice with various folding states are also experimentally characterized to further verify our strategy. Compared with available metasurfaces, our reconfigurable strategy using maneuverable structures to control EM wave is more versatile and shows great potential in engineering applications.
摘要：
The topological phase transitions (TPT) of light refers to a topological evolution from one type of spin-orbit interaction to another, which has been recently found in beam scattering at optical interfaces and propagation in uniaxial crystals. In this work, the focusing of off-axis and partially masked circular-polarization Gaussian beams are investigated by using of a full-wave theory. Moreover, two different types of spin-orbit interactions (i.e., spin-dependent vortex generation and photonic spin-Hall effect) in the focusing system are unified from the perspective of TPT. It is demonstrated that as the off-axis distance or the masked area increases, a TPT phenomenon in the focused optical field takes place, evolving from the spin-dependent vortex generation to the spin-Hall shift of the beam centroids. The intrinsic mechanism is attributed to the cylindrical symmetry-breaking of the system. This symmetry-breaking induced TPT based on the method of vortex mode decomposition is further examined. The main difference between the TPT phenomenon observed here and that trigged by oblique incidence at optical interfaces or oblique propagation in uniaxial crystals is also uncovered. Our findings provide fruitful insights for understanding the spin-orbit interactions in optics, providing an opportunity for unifying the TPT phenomena in various spin-orbit photonics systems.
摘要：
Achieving highly directive radiation with broadband operation, low scattering, and thin profile for a circularly polarized (CP) antenna is particularly challenging and yet rarely reported. Here, we propose a strategy of a CP Cassegrain meta-antenna by combining a planar helical antenna, a metasurface main reflector, and a metamaterial subreflector. The main reflector is designed to achieve focusing for CP waves at 13 GHz. The subreflector is chessboard-configured chiral metamaterial slab composed of two different types of chiral meta-atoms, aiming to achieve spin- and direction-selective CP transmissions and reflections. The distance between two reflectors is half of focal length, which enables our antenna to be dubbed as a folded reflectarray. The low radar cross section (RCS) is achieved based on scattering cancellation technique by realizing near 180 degrees reflection phase difference between two neighboring chessboard submeta-atoms. Thanks to the architecture of the two reflectors, the proposed antenna exhibits high gain and low profile simultaneously according to image theory. For verification, a planar CP Cassegrain antenna, excited by a left-handed CP (LCP) planar helical antenna, is numerically studied, fabricated, and experimentally measured. Numerical results are in good agreement with the experimental ones, showing a peak right-handed CP (RCP) gain of 26.6 dBi at 12.6 GHz. Furthermore, the backward monostatic RCS of the antenna is dramatically reduced over -10 dB in a broad bandwidth from 8.4 to 15.7 GHz when it is illuminated by an LCP planar wave. Our proposed Cassegrain antenna features simultaneously broadband, high gain, low profile, and low RCS, providing a new avenue to low-profile CP reflectarrays with invisibility.
摘要：
Dynamical controls on terahertz (THz) wavefronts are crucial for many applications, but available mechanism requests tunable elements with sub-micrometer sizes that are difficult to find in the THz regime. Here, different from the local-tuning mechanism, we propose an alternative approach to construct wavefront-control meta-devices combining specifically designed metasurfaces and globally tuned graphene layers. Coupled-mode-theory (CMT) analyses reveal that graphene serves as a tunable loss to drive the whole meta-device to transit from one functional phase to another passing through an intermediate regime, exhibiting distinct far-field (FF) reflection wavefronts. As a proof of concept, we design/fabricate a graphene meta-device and experimentally demonstrate that it can reflect normally incident THz wave to pre-designed directions with different polarizations under appropriate gating voltages. We finally design a graphene meta-device and numerically demonstrate that it can generate vectorial THz beams with continuously varying polarization distributions upon gating. These findings pave the road to realizing a wide range of THz applications, such as sensing, imaging, and wireless communications.
摘要：
The optical spin-orbit interaction (SOI) caused by momentum-dependent Pancharatnam-Berry phase (PB) provides new opportunities in the development of spin-optical devices, but the relatively low conversion efficiency limits its application. Here, through rigorous full-wave analyses on it in a parity-time (PT) symmetric system with thickness less than a wavelength, we find that the conversion efficiency of the SOI can be enhanced in both transmission and reflection in a wide range of incidence angles. When the parameters of the PT symmetric system meet the requirement of coherent perfect absorbers-laser mode, the effective anisotropy between the TM and TE components (e.g., a difference of their Fresnel coefficients) within the beam will be amplified dramatically, which results in significantly enhanced conversion efficiency of SOIs (up to 10(6)). These findings offer an effective way to modulate the SOIs with an ultra-thin PT symmetric system, and may exhibit applications in spin-orbit optical devices. (C) 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
摘要：
Several works on optical higher-order differential operations have recently attracted attention, particularly in image processing for edge detection. However, the inefficient differential operation leads to barriers to practical applications. Here, we report an anisotropic epsilon-near-zero slab to significantly enhance the transmission efficiency of second-order differentiators and discuss the Berry phase mechanism of this optical calculation process. Through a rigorous full-wave analysis of the process, we find that the conversion efficiency of differential operation depends on the spin-orbit interactions. Our scheme can strengthen the spin-orbit interaction by introducing anisotropy, which significantly enhances the transmission efficiency. We finally give transfer functions to reveal how to improve the efficiency and compare the quadratic coefficient among different systems. This highly efficient differentiation operation may develop significant applications in fast, compatible, and power-efficient ultrathin devices for data processing and biological imaging.
摘要：
Optical spin-Hall effect (SHE) exhibits many intriguing features as a linearly polarized (LP) light beam strikes an interface at incident angles around the Brewster angle, but the underlying physics remains obscure. Here, we elucidate the physics through reanalyzing this problem employing rigorous calculations and the Berry phase concept. As a circularly polarized (CP) light beam strikes an optical interface, the reflected light beam contains two components, a spin-flipped abnormal mode acquiring geometric phases (thus exhibiting a spin-Hall shift) and a spin-maintained normal mode without such phases. Strengths of these two modes are determined by the incident angle and the optical properties of the interface. Under the LP incidence, however, a spin component inside the reflected light beam must be the sum of normal and abnormal components of reflected light beams corresponding to CP incidences with different helicity, which thus sensitively depends on the incident angle. In particular, at incident angles near the Brewster one, reflection coefficients for two CP components exhibit opposite signs, leading to significant destructive interferences between normal and abnormal modes, finally generating highly deformed reflected light patterns with anomalously enhanced spin-Hall shifts. These findings can be extended to both reflected and transmitted cases with Brewster-like behaviors. Our analyses reinterpret previously discovered effects, providing an alternative understanding on the SHE of light.
摘要：
Dynamically controlling terahertz (THz) wavefronts in a designable fashion is highly desired in practice. However, available methods working at microwave frequencies do not work well in the THz regime due to lacking suitable tunable elements with submicrometer sizes. Here, instead of locally controlling individual meta-atoms in a THz metasurface, we show that rotating different layers (each exhibiting a particular phase profile) in a cascaded metadevice at different speeds can dynamically change the effective Jones-matrix property of the whole device, thus enabling extraordinary manipulations on the wavefront and polarization characteristics of a THz beam impinging on the device. After illustrating our strategy based on model calculations, we experimentally demonstrate two proof-of-concept metadevices, each consisting of two carefully designed all-silicon transmissive metasurfaces exhibiting different phase profiles. Rotating two metasurfaces inside the fabricated devices at different speeds, we experimentally demonstrate that the first metadevice can efficiently redirect a normally incident THz beam to scan over a wide solid-angle range, while the second one can dynamically manipulate both the wavefront and polarization of a THz beam. Our results pave the way to achieving dynamic control of THz beams, which is useful in many applications, such as THz radar, and bio- and chemical sensing and imaging.
期刊：
Proceedings of SPIE - The International Society for Optical Engineering,2021年11903 ISSN：0277-786X
通讯作者：
Ling, Xiaohui(xhling@hynu.edu.cn)
作者机构：
[Xiao, Weilai; Zhang, Zan; Ling, Xiaohui] College of Physics and Electronic Engineering, Hengyang Normal University, Hengyang;421002, China;[Xiao, Weilai; Zhang, Zan; Ling, Xiaohui] 421002, China
作者机构：
[Wang, Yanzhao; Xu, He-Xiu; Wang, Chaohui] Air Force Engn Univ, Air & Missile Def Coll, Xian 710051, Peoples R China.;[Xu, He-Xiu; Huang, Wei] Northwestern Polytech Univ, Inst Flexible Elect, Xian 710072, Peoples R China.;[Xu, He-Xiu; Ling, Xiaohui] Hengyang Normal Univ, Coll Phys & Elect Engn, Hengyang 421002, Peoples R China.;[Qiu, Cheng-Wei; Hu, Guangwei] Natl Univ Singapore, Dept Elect & Comp Engn, Singapore 117583, Singapore.;[Tang, Shiwei] Ningbo Univ, Fac Sci, Dept Phys, Ningbo 315211, Peoples R China.
通讯机构：
[He-Xiu Xu] A;[Wei Huang] I;[Cheng-Wei Qiu] D;Department of Electrical and Computer Engineering, National University of Singapore, Singapore, 117583 Singapore<&wdkj&>Air and Missile Defense College, Air force Engineering University, Xi'an, 710051 China<&wdkj&>Institute of Flexible Electronics, Northwestern Polytechnical University, Xi'an, 710072 China<&wdkj&>College of Physics and Electronic Engineering, Hengyang Normal University, Hengyang, 421002 China<&wdkj&>Institute of Flexible Electronics, Northwestern Polytechnical University, Xi'an, 710072 China
