Metasurface orbital angular momentum holography

ARTICLE https://doi.org/10.1038/s41467-019-11030-1 OPEN Metasurface orbital angular momentum holography 1234567890():,; Haoran Ren 1,2, Gauthier Briere3, Xinyuan Fang 1, Peinan Ni3, Rajath Sawant3, Sébastien Héron3, Sébastien Chenot3, Stéphane Vézian3, Benjamin Damilano3, Virginie Brändli3, Stefan A. Maier2 & Patrice Genevet 3 Allowing subwavelength-scale-digitization of optical wavefronts to achieve complete control of light at interfaces, metasurfaces are particularly suited for the realization of planar phaseholograms that promise new applications in high-capacity information technologies. Similarly, the use of orbital angular momentum of light as a new degree of freedom for information processing can further improve the bandwidth of optical communications. However, due to the lack of orbital angular momentum selectivity in the design of conventional holograms, their utilization as an information carrier for holography has never been implemented. Here we demonstrate metasurface orbital angular momentum holography by utilizing strong orbital angular momentum selectivity offered by meta-holograms consisting of GaN nanopillars with discrete spatial frequency distributions. The reported orbital angular momentummultiplexing allows lensless reconstruction of a range of distinctive orbital angular momentum-dependent holographic images. The results pave the way to the realization of ultrahigh-capacity holographic devices harnessing the previously inaccessible orbital angular momentum multiplexing. 1 Laboratory of Artificial-Intelligence Nanophotonics, School of Science, RMIT University, Melbourne, VIC 3001, Australia. 2 Chair in Hybrid Nanosystems, Nanoinstitute Munich, Faculty of Physics, Ludwig-Maximilians-University Munich, 80539 Munich, Germany. 3 Université Côte d’Azur, CNRS, CRHEA, rue B. Gregory, 06560 Valbonne, France. Correspondence and requests for materials should be addressed to H.R. (email: ) or to P.G. (email: ) NATURE COMMUNICATIONS | (2019)10:2986 | https://doi.org/10.1038/s41467-019-11030-1 | www.nature.com/naturecommunications 1 ARTICLE NATURE COMMUNICATIONS | https://doi.org/10.1038/s41467-019-11030-1 M etasurfaces, which allow the complete control of the wavefront of an electromagnetic wave with an ultrathin photonic device, have provided an indispensable platform for both fundamental studies of light-matter interactions1–5 and a diverse range of photonic applications in optical microscopy and imaging6–9, dispersion compensation10–12, skin cloak13, surface waves engineering14 and multiplexing15, and intelligent photonics16. Owing to the subwavelength nature of plasmonic1 and dielectric17–20 meta-atoms, high-resolution metasurfaces have revolutionized the photonic design of metaholograms for holographic displays21–24, optical encryption25,26, and nonlinear holography27. In this context, meta-holograms responsive to different physical properties of light including polarization28, helicity29, wavelength30, and incidence angle31 have recently been exploited to address independent information channels for high-capacity holographic multiplexing. Orbital angular momentum (OAM), manifested by a helical wavefront of light, has emerged as a new degree of freedom of light for boosting both optical32,33 and quantum34,35 information capacities. To date, however, OAM of light has not been implemented as an independent information carrier for optical holography, mainly due to the lack of OAM selectivity in conventional hologram design. Typically, a digital hologram with a quasicontinuous spatial frequency distribution destroys the extrinsic OAM of light36, completely losing the OAM physical property in the holographic reconstruction process. Despite the fact that generation and detection of multiple wavefronts carrying the OAM have been demonstrated through holographic optical elements with only a few diffraction orders37,38, implementing the OAM as an independent information carrier for optical holography remains elusive. More importantly, merging OAM holography with high-resolution metasurfaces could open up an unprecedented opportunity for ultrahigh-capacity holographic devices and systems, due to a physically unbounded set of OAM modes. Here we demonstrate an entirely new concept of metasurface OAM holography capable of reconstructing a range of distinctive OAM-dependent holographic images from a single metahologram. We adopted subwavelength Gallium Nitride (GaN) nanopillars on a transparent sapphire substrate to digitize designed meta-holograms at a visible wavelength of 632 nm. To this purpose, three types of meta-holograms with discrete spatial frequency distributions are designed, including OAM-conserving (Fig. 1a), -selective (Fig. 1b), and -multiplexing (Fig. 1c) metaholograms, respectively. Such a discrete spatial frequency distribution of a meta-hologram plays a key role to demonstrate the metasurface OAM holography, which preserves the OAM property in the holographic reconstruction process. In this context, an OAM-conserving meta-hologram with a discrete spatial frequency distribution is able to produce OAM-pixelated holographic images by preserving the OAM property of incident OAM beams in each pixel of reconstructed holographic images (Fig. 1a). Results OAM property preservation in metasurface holography. According to Fourier transform holography, the spatial frequency distribution of a hologram corresponds to the electric field distribution in the image plane. Applying an incident OAM beam for the holographic reconstruction, the reconstructed electric field distribution in the image plane can thus be expressed as a convolution between a holographic image and the Fourier transform of a helical wavefront (see Supplementary Note 1). In this case, the Fourier transform of a helical wavefront, which acts as the kernel function of the convolution, is simply copied in each pixel 2 of the holographic image. As such, to preserve the OAM property in each pixel of a reconstructed holographic image, it is necessary to spatially sample the holographic image by an OAM-dependent two-dimensional (2D) Dirac comb function to avoid spatial overlap of the helical wavefront kernel, i.e. creating OAMpixelated images. In this context, the constituent spatial frequencies (kg in the momentum space) of an OAM-conserving meta-hologram add a linear spatial frequency shift to an incident OAM beam (kin). As such, outgoing spatial frequencies leaving the meta-hologram (kout) possess a helical wavefront inherited from the incident OAM beam, which implies that the OAMconserving meta-hologram could create OAM-pixelated holographic images (see Supplementary Fig. 1A). In contrast, previous meta-holograms16–26 based on the conventional digital hologram design feature a quasi-continuous spatial frequency distribution that could completely destroy the helical wavefront and the associated OAM physical property of an incident OAM beam due to wave interference (see Supplementary Fig (...truncated)