Despite a plethora of applications ranging from quantum memories to high-resolution lithography, the current technologies to generate vector vortex beams (VVBs) suffer from less efficient energy use poor resolution, low damage threshold, and bulky size, preventing further practical applications. We propose and experimentally demonstrate an approach to generate VVBs with a single metasurface by locally tailoring phase and transverse polarization distribution. This method features the spin-orbit coupling and the superposition of the converted part with an additional phase pickup and the residual part without a phase change. By maintaining the equal components for the converted part and the residual part, the cylindrically polarized vortex beams carrying orbital angular momentum are experimentally demonstrated based on a single metasurface at subwavelength scale. The proposed approach provides unprecedented freedom in engineering the properties of optical waves with the high-efficiency light utilization and a minimal footprint.

A metasurface can manipulate light in a desirable manner by imparting local and space-variant abrupt phase change. Benefiting from such an unprecedented capability, the conventional concept of what constitutes an optical lens continues to evolve. Ultrathin optical metasurface lenses have been demonstrated based on various nanoantennas such as V-shape structures, nanorods and nanoslits. A single device that can integrate two different types of lenses and polarities is desirable for system integration and device miniaturization. We experimentally demonstrate such an ultrathin metasurface lens that can function either as a circular lens or a cylindrical lens, depending on the helicity of the incident light. Helicity-controllable focal line and focal point in the real focal plane, as well as imaging and 1D/2D Fourier transforms, are observed on the same lens. Our work provides a unique tool for polarization imaging, image processing and particle trapping. 

Driven by miniaturization and system integration, ultrathin, multifunction optical elements are urgently needed. Traditional polarization-selective optical elements are mainly based on birefringence, which is realized by using the well-designed structure of each phase pixel. However, further reduction of the pixel size and improvement of the phase levels are hindered by the complicated fabrication process. An approach is proposed to realize a metasurface device that possesses two distinct functionalities. The designed metasurface device, consisting of gold nanorods with spatially varying orientation, has been experimentally demonstrated to function as either a lens or a hologram, depending on the helicity of the incident light. As the phase of the scattered light is controlled by the orientation of the nanorods, arbitrary phase levels and dispersionless phase profile can be realized through a much simpler fabrication process than the conventional device. This approach provides an unconventional alternative to realize multifunction optical element, dramatically increasing the functionality density of the optical systems.

Metasurfaces are engineered interfaces that contain a thin layer of plasmonic or dielectric nanostructures capable of manipulating light in a desirable manner. Advances in metasurfaces have led to various practical applications ranging from lensing to holography. Metasurface holograms that can be switched by the polarization state of incident light have been demonstrated for achieving polarization multiplexed functionalities. However, practical application of these devices has been limited by their capability for achieving high efficiency and high image quality. Here we experimentally demonstrate a helicity multiplexed metasurface hologram with high efficiency and good image fidelity over a broad range of frequencies. The metasurface hologram features the combination of two sets of hologram patterns operating with opposite incident helicities. Two symmetrically distributed off-axis images are interchangeable by controlling the helicity of the input light. The demonstrated helicity multiplexed metasurface hologram with its high performance opens avenues for future applications with functionality switchable optical devices.

We propose and experimentally demonstrate an ultrathin flat metasurface lens with longitudinal multiple foci for circularly polarized light. The designed metasurface consists of a monolayer of metallic nanorods with spatially varying orientation. Six focal points are observed for the linearly polarized light, while three focal points are observed for the circularly polarized light. Unlike the traditional multi-foci diffractive lenses, the position and the polarization of the focal points depend on the helicity of the incident light. The developed lenses may find application in simultaneously manipulating multiple particles for bioscience, imaging for integrated photonics and controlling orbital angular momentum for quantum information processing.

The miniaturization of measurement systems currently used to characterize the polarization state of light is limited by the bulky optical components used such as polarizers and waveplates. We propose and experimentally demonstrate a simple and compact approach to measure the ellipticity and handedness of the polarized light using an ultrathin (40 nm) gradient metasurface. A completely polarized light beam is decomposed into a left circularly polarized beam and a right circularly polarized beam, which are steered in two directions by the metasurface consisting of nanorods with spatially varying orientations. By measuring the intensities of the refracted light spots, the ellipticity and handedness of various incident polarization states are characterized at a range of wavelengths and used to determine the polarization information of the incident beam. To fully characterize the polarization state of light, an extra polarizer can be used to measure the polarization azimuth angle of the incident light.

Xianzhong Chen, Fuyong Yue, Dandan Wen, Jingtao Xin, Brian D. Gerardot, Jensen Li, Single Metasurface for vector vortex beam generation,  PIERS in St Petersburg, Russia, 22-25 May, 2017.

X. Chen, F. Yue, D. Wen, C. Zhang, B. D. Gerardot, W. Wang, S. Zhang, Metasurface for polarization-controllable multichannel superpositions of orbital angular momentum states, META'17, Seoul, Korea, 25-28 July, 2017.

[1] X. Chen, L. Zhang, C.-W. Qiu and S. Zhang, Chapter 3 Ultrathin Metalens and Three-Dimensional Optical Holography Using Metasurfaces, In Book Plasmonics and Super-Resolution Imaging, edited by Zhaowei Liu, Pages 91-126, published by Pan Stanford Publishing Pte Ltd, ISBN 9789814669917 (2017).

[2] X. Chen, D. Wen, F. Yue, "Metasurface and ultrathin optical devices", to be published by Taylor and Francis.

Copyright belongs to Xianzhong Chen's research group