Multiplexed Metasurfaces for High-Capacity Printing Imaging

We successfully overcome the problem of cross-talk in multiplexed metasurface design and realize the multiplexed metasurface with five printing images in both theoretical and experimental aspects,by employing the coherent pixel design considering coherent superposition of all the sub-elements.Compared with most previous studies where the integrated printing images were usually no more than three,our study shows obvious improvement.More importantly,in our approach all the sub-elements,which were crystalline silicon nanobricks with the size of 320×80×230 nm^3,were arranged in a square space of 1.45×1.45μm^2 following the closest packing way,enabling our multiplexed metasurface to have a potential of effective physical information capacity of printing image reaching the optical diffraction limit.Our study not only enlarges the information capacity of metasurfaces by expanding the integrated number of printing image in one metasurface,but also can promote metasurface applications in various fields such as information storage and encoding.

Facial optical flow estimation via neural non-rigid registration

Optical flow estimation in human facial video,which provides 2D correspondences between adjacent frames,is a fundamental pre-processing step for many applications,like facial expression capture and recognition.However,it is quite challenging as human facial images contain large areas of similar textures,rich expressions,and large rotations.These characteristics also result in the scarcity of large,annotated realworld datasets.We propose a robust and accurate method to learn facial optical flow in a self-supervised manner.Specifically,we utilize various shape priors,including face depth,landmarks,and parsing,to guide the self-supervised learning task via a differentiable nonrigid registration framework.Extensive experiments demonstrate that our method achieves remarkable improvements for facial optical flow estimation in the presence of significant expressions and large rotations.

Full-colour nanoprint-hologram synchronous metasurface with arbitrary hue-saturationbrightness control

The colour gamut,a two-dimensional(2D)colour space primarily comprising hue and saturation(HS),lays the most important foundation for the colour display and printing industries.Recently,the metasurface has been considered a promising paradigm for nanoprinting and holographic imaging,demonstrating a subwavelength image resolution,a flat profile,high durability,and multi-functionalities.Much effort has been devoted to broaden the 2D HS plane,also known as the CIE map.However,the brightness(B),as the carrier of chiaroscuro information,has long been neglected in metasurface-based nanoprinting or holograms due to the challenge in realising arbitrary and simultaneous control of full-colour HSB tuning in a passive device.Here,we report a dielectric metasurface made of crystal silicon nanoblocks,which achieves not only tailorable coverage of the primary colours red,green and blue(RGB)but also intensity control of the individual colours.The colour gamut is hence extruded from the 2D CIE to a complete 3D HSB space.Moreover,thanks to the independent control of the RGB intensity and phase,we further show that a singlelayer silicon metasurface could simultaneously exhibit arbitrary HSB colour nanoprinting and a full-colour hologram image.Our findings open up possibilities for high-resolution and high-fidelity optical security devices as well as advanced cryptographic approaches.

Giant enhancements of high-order upconversion luminescence enabled by multiresonant hyperbolic metamaterials

Photonic nanostructures with resonant modes that can generate large electric field(EF) enhancements are applied to enhance light-matter interactions in nanoscale, bringing about great advances in both fundamental and applied science. However, a small hot spot(i.e., the regions with strong EF enhancements) and highly inhomogeneous EF distribution of the resonant modes usually hinder the enhancements of light-matter interactions in a large spatial scale. Additionally, it is a severe challenge to simultaneously generate multiple resonant modes with strong EF enhancements in a broadband spectral range, which greatly limits the capacity of a photonic nanostructure in boosting optical responses including nonlinear conversion, photoluminescence, etc. In order to overcome these challenges, we presented an arrayed hyperbolic metamaterial(AHMM). This AHMM structure is applied to simultaneously enhance the three-photon and four-photon luminescence of upconversion nanoparticles. Excitingly, the enhancement of the three-photon process is 1 order of magnitude larger than previous records, and for the enhancing four-photon process, we achieve an enhancement of 3350 times, greatly beneficial for overcoming the crucial problem of low efficiency in near infrared light upconversion. Our results demonstrated a promising platform for realizing giant enhancements of light-matter interactions, holding potential in constructing various photonics applications such as the nonlinear light sources.

Integrating lattice and gap plasmonic modes to construct dual-mode metasurfaces for enhancing light–matter interaction

Photonic structures with optical resonances beyond a single controllable mode are strongly desired for enhancing light±matter interactions and bringing about advanced photonic devices. However, the realization of effective multimodal photonic structures has been restricted by the limited tunable range of mode manipulation, the spatial dispersions of electric fields or the polarization-dependent excitations. To overcome these limitations, we create a dualmode metasurface by integrating the plasmonic surface lattice resonance and the gap plasmonic modes;this metasurface offers a widely tunable spectral range, good overlap in the spatial distribution of electric fields, and polarization independence of excitation light. To show that such dual-mode metasurfaces are versatile platforms for enhancing light±matter interactions, we experimentally demonstrate a significant enhancement of second-harmonic generation using our design, with a conversion efficiency of 1±3 orders of magnitude larger than those previously obtained in plasmonic systems. These results may inspire new designs for functional multimodal photonic structures.

Microscopic pump-probe optical technique to characterize the defect of monolayer transition metal dichalcogenides

Monolayer transition metal dichalcogenides(TMDs)are ideal materials for atomically thin,flexible optoelec.tronic and catalytic devices.However,their optoelectrical performance such as quantum yield and carrier mobility often shows below theoretical expectations due to the existence of defects.For monolayer TMD-based devices,finding a low-cost,time-efficient, and nondestructive technique to visualize the change of defect distribution in the space domain and the defect-induced change of the carrier's lifetime is vital for optimizing their optoelectronic properties.Here, we propose a microscopic pump-probe technique to map the defect distribution of monolayer TMDs.It is found that there is a linear relationship between transient differential reflection intensity and defect density,suggesting that this technique not only realizes the visualization of the defect distribution but also achieves the quantitative estimation of defect density.Moreover,the carrier lifetime at each point can also be obtained by the technique. The technique used here provides a new route to characterize the defect of monolayer TMDs on the micro-zone, which will hopefiilly guide the fabrication of high-quality two-dimensional (2D) materials and the promotion of optoelectrical performance.