Fundamentals and applications of spin-decoupled Pancharatnam-Berry metasurfaces

Manipulating terahertz guided wave excitation with Fabry-Perot cavity-assisted metasurfaces

Metasurfaces are emerging as powerful tools for manipulating complex light fields, offering enhanced control in free space and on-chip waveguide applications. Their ability to customize refractive indices and dispersion properties opens up new possibilities in light guiding, yet their efficiency in exciting guided waves, particularly through metallic structures, is not fully explored. Here, we present a new method for exciting terahertz (THz) guided waves using Fabry-Perot (FP) cavity-assisted metasurfaces that enable spin-selective directional coupling and mode selection. Our design uses a substrate-free ridge silicon THz waveguide with air cladding and a supporting slab, incorporating placed metallic metasurfaces to exploit their unique interaction with the guided waves. With the silicon thin layer and air serving as an FP cavity, THz waves enter from the bottom of the device, thereby intensifying the impact of the metasurfaces. The inverse-structured complementary metasurface could enhance excitation performance. We demonstrate selective excitation of TE00 and TE10 modes with directional control, confirmed through simulations and experimental validations using a THz vector network analyzer (VNA) system. This work broadens the potential of metasurfaces for advanced THz waveguide technologies.

Recent advances in metasurface design and quantum optics applications with machine learning, physics-informed neural networks, and topology optimization methods

As a two-dimensional planar material with low depth profile, a metasurface can generate non-classical phase distributions for the transmitted and reflected electromagnetic waves at its interface. Thus, it offers more flexibility to control the wave front. A traditional metasurface design process mainly adopts the forward prediction algorithm, such as Finite Difference Time Domain, combined with manual parameter optimization. However, such methods are time-consuming, and it is difficult to keep the practical meta-atom spectrum being consistent with the ideal one. In addition, since the periodic boundary condition is used in the meta-atom design process, while the aperiodic condition is used in the array simulation, the coupling between neighboring meta-atoms leads to inevitable inaccuracy. In this review, representative intelligent methods for metasurface design are introduced and discussed, including machine learning, physics-information neural network, and topology optimization method. We elaborate on the principle of each approach, analyze their advantages and limitations, and discuss their potential applications. We also summarize recent advances in enabled metasurfaces for quantum optics applications. In short, this paper highlights a promising direction for intelligent metasurface designs and applications for future quantum optics research and serves as an up-to-date reference for researchers in the metasurface and metamaterial fields.

Achieving Photonic Spin Hall Effect, Spin-Selective Absorption, and Beam Deflection with a Vanadium Dioxide Metasurface

Metasurface-based research with phase-change materials has been a prominent and rapidly developing research field that has drawn considerable attention in recent years. In this paper, we proposed a kind of tunable metasurface based on the simplest metal-insulator-metal structure, which can be realized by the mutual transformation of insulating and metallic states of vanadium dioxide (VO₂) and can realize the functional switching of photonic spin Hall effect (PSHE), absorption and beam deflection at the same terahertz frequency. When VO₂ is insulating, combined with the geometric phase, the metasurface can realize PSHE. A normal incident linear polarized wave will be split into two spin-polarized reflection beams traveling in two off-normal directions. When VO₂ is in the metal state, the designed metasurface can be used as a wave absorber and a deflector, which will completely absorb LCP waves, while the reflected amplitude of RCP waves is 0.828 and deflects. Our design only consists of one layer of artificial structure with two materials and is easy to realize in the experiment compared with the metasurface of a multi-layer structure, which can provide new ideas for the research of tunable multifunctional metasurface.

Multifunctional Metasurfaces Enabled by Multifold Geometric Phase Interference

Geometric phase is frequently used in artificially designed metasurfaces; it is typically used only once in reported works, leading to conjugate responses of two spins. Supercells containing multiple nanoantennas can break this limitation by introducing more degrees of freedom to generate new modulation capabilities. Here, we provide a method for constructing supercells for geometric phases using triple rotations, each of which achieves a specific modulation function. The physical meaning of each rotation is revealed by stepwise superposition. Based on this idea, spin-selective holography, nanoprinting, and their hybrid displays are demonstrated. As a typical application, we have designed a metalens that enables spin-selective transmission, allowing for high-quality imaging with only one spin state, which can serve as a plug-and-play chiral detection device. Finally, we analyzed how the size of supercells and the phase distribution inside it can affect the higher order diffraction, which may help in designing supercells for different scenarios.

Generation of scalar/vectorial vortex beams by using the plasmonic metasurfaces

Scalar and vector vortex beams are characterized of a helical wavefront but different polarized states, which result in different applications. In this paper, we design and fabricate a plasmonic metasurface based on the geometric phase principle. The designed metasurfaces are capable of generating a scalar vortex beam with a topological charge of ±2 and a vectorial vortex beam with a topological charge of ±1 in the near-infrared band. The experimental results are in good agreement with the simulation results, and our work provides a new idea for the development of a multivortex beam converter.

Coexistence of circular dichroism and asymmetric transmission in Babinet-complementary metamaterials

Chiral metamaterials with circular dichroism (CD) or asymmetric transmission (AT) draw enormous attention for their attractive applications in polarization transformers, circular polarizers, and biosensing. In this study, a feasible trilayer chiral metamaterials (TCM) is designed and investigated in theory and simulation. The proposed TCM is composed of a nanoslit layer and a Babinet-complementary nanorod layer separated by a nanoslit spacer. Owing to symmetry breaking by the tilted nanoslit in metal film, the TCM shows simultaneous CD and AT effects in the near-infrared region. The simulated electric charge distributions prove that the chirality arises from the excitation of asymmetric electric dipole resonant modes due to the coupling of adjacent unit cells. Moreover, CD and AT can be tuned by the tilted angle of the nanoslit and the thickness of the spacer, the fitting functions of which are consistent with the theoretical formulas based on transmittance matrix analysis. The proposed nanostructure offers a potential strategy for manipulating metamaterials with simultaneous CD and AT effects, allowing a multitude of exciting applications such as ultra-sensitive polarization transformer and biosensor.

Functional Metasurface Quarter-Wave Plates for Simultaneous Polarization Conversion and Beam Steering

The capability to manipulate the polarization state of light at the nanoscale is of paramount importance in many emerging research areas ranging from optical communication to quantum information processing. Gap-surface plasmon (GSP) metasurfaces, which provide advantages and abilities of molding reflected fields, have been demonstrated excellently suited for integration of multifunctional polarization optics into a single device. Here, we establish a versatile GSP metasurface platform based on nanoscale quarter-wave plates (nano-QWPs) that enable efficient circular-to-linear polarization conversion along with the complete phase control over reflected fields. Capitalizing on the nano-QWP design, we demonstrate, both theoretically and experimentally, how resonance and geometric phases can be used in concert to achieve independent and simultaneous phase modulation of both co- and cross-polarized circularly polarized (CP) waves by realizing arbitrary beam steering of co- and cross-polarized CP channels in a broadband near-infrared range. The GSP metasurface platform established in our work provides versatile and flexible solutions to enrich multiple functionalities for diversified metasurface-based polarization optics exploited in modern integrated photonic devices and systems.