The widespread use of multifunctional metasurfaces has started to revolutionize conventional electromagnetic devices due to their unprecedented capabilities and exceedingly low losses. Specifically, geometric metasurfaces that utilize spatially varied single-celled elements to impart arbitrary phase modulation under circularly polarized (CP) waves have attracted more attention. However, the geometric phase has intrinsically opposite signs for two spins, resulting in locked and mirrored functionalities for the right-handed and left-handed CP beams. Additionally, the demonstrated geometric metasurfaces so far have been limited to operating in either transmission or reflection modes at a single wavelength. Here, we propose a double-layered metasurface composed of complementary elliptical and reversal ring resonator structures to achieve simultaneous and independent control of the reflection and transmission of CP waves at two independent terahertz frequencies, which integrates three functions of reflected beam deflection, reflected Bessel beam generation, and transmitted beam focusing on the whole space. The high efficiency and simple design of our metasurface will open new avenues for integrated terahertz metadevices with advanced functionalities.

For a conventional cascaded metasurface, the combination channel and each single channel are mutually dependent because the phase modulation of a cascaded metasurface is the sum of each single one. Here we propose a cascaded metasurface that can independently encode information into multiple channels. Based on the orientation degeneracy of anisotropic metasurfaces, each single metasurface can produce a quick-response (QR) image in the near field, governed by the Malus law, while the combined channel can produce a holographic image in the far field, governed by geometric phase. The independent and physically separated trichannel design makes information encryption safer.

Metasurfaces, two-dimensional (2D) or quasi-2D arrays of dielectric or metallic meta-atoms, offer a compact and novel platform to manipulate the amplitude, phase, and polarization of incoming wavefronts in a desired manner by engineering the geometry of meta-atoms. In polarization control, spin-insensitive metasurfaces have attracted significant attention due to the robustness of circular polarization against the beam misalignment and multi-path effects. Till now, several efforts have been made to realize polarization-insensitive metasurfaces for circularly polarized (CP) wavefront manipulation; however, these metasurfaces only consider the cross-polarization channels and keep the co-polarization channels abandoned. Such metasurfaces cannot be considered truly spin-insensitive, as one has to carefully choose the analyzer at output. Here, by combining the polarization-insensitive geometric phase and engineered propagation phase, we propose a spin-insensitive design principle based on metasurfaces that can perform identical functionality (on co- and cross-polarization channels) irrespective of the handedness of incident/transmitted light. As a proof of concept, we design and numerically realize two types of spin-insensitive wavefront engineering devices: (1) spin-insensitive meta-hologram and (2) spin-insensitive beam deflector with power splitting functionality. The proposed work is expected to open up new avenues for developing spin-independent metasurfaces-based devices.

The two-photon state with spatial entanglement is an essential resource for testing fundamental laws of quantum mechanics and various quantum applications. Its creation typically relies on spontaneous parametric downconversion in bulky nonlinear crystals where the tunability of spatial entanglement is limited. Here, we predict that ultrathin nonlinear lithium niobate metasurfaces can generate and diversely tune spatially entangled photon pairs. The spatial properties of photons including the emission pattern, rate, and degree of spatial entanglement are analyzed theoretically with the coupled mode theory and Schmidt decomposition method. We show that by leveraging the strong angular dispersion of the metasurface, the degree of spatial entanglement quantified by the Schmidt number can be decreased or increased by changing the pump laser wavelength and a Gaussian beam size. This flexibility can facilitate diverse quantum applications of entangled photon states generated from nonlinear metasurfaces.

The lateral geometry and material property of plasmonic nanostructures are critical parameters for tailoring their optical resonance for sensing applications. While lateral geometry can be easily observed by a scanning electron microscope or an atomic force microscope, characterizing materials properties of plasmonic devices is not straightforward and requires delicate examination of material composition, cross-sectional thickness, and refractive index. In this study, a deep neural network is adopted to characterize these parameters of unknown plasmonic nanostructures through simple transmission spectra. The network architecture is established based on simulated data to achieve accurate identification of both geometric and material parameters. We then demonstrate that the network training by a mixture of simulated and experimental data can result in correct material property recognition. Our work may indicate a simple and intelligent characterization approach to plasmonic nanostructures by spectroscopic techniques.

The temporal coupled-mode theory (TCMT) has made significant progress in recent years, and is widely applied in explaining a variety of optical phenomena. In this paper, the optical characteristics of the metasurface composed of nano-bars and nano-rings are simulated. The simulation results are well explained by TCMT under the coupled basis vector. However, when the structural asymmetry is large, the fitting of results shows that the total radiation loss is not conservative, in contradiction to the requirement of traditional TCMT. We solved this inconsistency by introducing the propagation phase into the near-field coupling term of TCMT. The studies show that, unlike the local mode near the exceptional point which corresponds to the radiation loss of the bright mode, the global mode near the diabolic point is closely related to the propagation phase. Furthermore, the structure near the diabolic point shows characteristic cross-coupling with the change of period. This study proposes a new theoretical framework for comprehending the interaction of light and matter and offers some guiding implications for the application of TCMT to a variety of related domains.

Spatial terahertz wave modulators that can arbitrarily tailor the electromagnetic wavefront are in high demand in nondestructive inspections and high-capacity wireless communications. Here, we propose a liquid crystal integrated metadevice. It modulates the terahertz wave based on the adjustable electromagnetically induced transparency analog when spatially changing the environmental refractive index. The functions of the device can be arbitrarily programmed via photo-reorienting the directors of liquid crystals with a digital micromirror device-based exposing system. The thin liquid crystal layer can be further driven by an electric field, and thus the function can be rapidly switched. Amplitude modulation and the lens effect are demonstrated with modulation depths over 50% at 0.94 THz.