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：
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：
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.
作者机构：
[Zhang, Zan; Mei, Wang; Cheng, Jiahao; Tan, Yawei; Dai, Zhiping; Ling, Xiaohui] Laboratory for Spin-Orbit Photonics, College of Physics and Electronic Engineering, Hengyang Normal University, Hengyang 421002, China
摘要：
The generation of vortices in the focusing of circularly polarized light beams results from the spin-orbit interaction (SOI), which is very similar to the generation of vortices when a light beam normally passes through an optically thin slab. However, its physical origin and the intrinsic relationship between both systems are still not well elucidated. Here, we establish a unified full-wave theory to revisit this problem. It is revealed that the origin of vortex generation in the transverse fields of the focusing system is the momentum-dependent Pancharatnam-Berry (PB) phase, which shares the same physics as the vortex generation in the slab case. In particular, we find that, under normal incidence, the beam transmitted through an optically thin slab has a longitudinal field and also exhibits a vortex with a topological charge of ±1 owing to the Berry-phase mechanism, akin to the focusing case. These findings not only unify the SOI-induced vortex generation in the optically thin slab and focusing systems within a single framework, uncovering the physical origin behind the SOI-induced vortex generation in optics, but also may shed light on understanding the SOIs in other fields.
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
