Broadband generation of perfect Poincaré beams via dielectric spin-multiplexed metasurface

Abstract

The term Poincaré beam, which describes the space-variant polarization of a light beam carrying spin angular momentum (SAM) and orbital angular momentum (OAM), plays an important role in various optical applications. Since the radius of a Poincaré beam conventionally depends on the topological charge number, it is difficult to generate a stable and high-quality Poincaré beam by two optical vortices with different topological charge numbers, as the Poincaré beam formed in this way collapses upon propagation. Here, based on an all-dielectric metasurface platform, we experimentally demonstrate broadband generation of a generalized perfect Poincaré beam (PPB), whose radius is independent of the topological charge number. By utilizing a phase-only modulation approach, a single-layer spin-multiplexed metasurface is shown to achieve all the states of PPBs on the hybrid-order Poincaré Sphere for visible light. Furthermore, as a proof-of-concept demonstration, a metasurface encoding multidimensional SAM and OAM states in the parallel channels of elliptical and circular PPBs is implemented for optical information encryption. We envision that this work will provide a compact and efficient platform for generation of PPBs for visible light, and may promote their applications in optical communications, information encryption, optical data storage and quantum information sciences.

Fig. 1. Principle of generation of generalized perfect Poincaré beams (PPBs) via dielectric metasurface.

a A hybrid-order Poincaré Sphere (HyOPS) representation of various PPBs. As an example, the two poles are represented by two perfect vortices (POVs) with the same ellipticity and different topological charges l_m = 5 and l_n = 10. The annular intensity profiles of six PPBs (red arrows represent the polarization distributions) with different coordinates are of same size and these elliptical hollow beams are transformed into distinct patterns using a vertical linear polarizer depicted by the white double arrow. b, c Left: schematic illustration of the metasurface capable of providing two independent phase profiles φ₁ and φ₂ for LCP and RCP light, respectively. The output beam becomes a RCP (b) or LCP (c) POV with the topological charge of l_m (b) or l_n (c). Right: an example of the intensity (top) and phase (bottom) profiles of metasurface-generated POV with l_m = 5 (b) and l_n = 10 (c).

Fig. 2. Design of a single-layer metasurface.

a A general metasurface design method to generate arbitrary PPB. Given two arbitray phase map (φ₁, φ₂) for generation of different perfect vortices, the phase shifts (δ_x, δ_y) and rotation angle θ of metasurface pixels are calculated and utilized to design nanopillars with varying in-plane dimensions and orientation angle. b Left: schematic of the metasurface made up of TiO₂ rectangle nanopillars. Right: perspective view and top view of the unit cell arranged on a fused-silica substrate. c Calculated polarization conversion efficiency as a function of nanopillars’ in-plane dimensions at a design wavelength of 530 nm. The black dots denote the selected nanopillars in constructing MF1 and MF2. d Calculated polarization conversion efficiencies of the selected eight nanopillars across the visible band. e The phase shifts (δ_x, δ_y) and rotation angle θ of the birefringent TiO₂ nanopillars as a function of the spatial coordinates in the metasurface (MF1 and MF2) plane. f Left: optical microscope images of the fabricated metasurfaces: MF1 (top) and MF2 (bottom). Scale bar: 20 μm. Right: top view scanning electron microscopy (SEM) images of metasurfaces. Scale bar: 500 nm.

Fig. 3. Characterization of metasurfaces for the generation of arbitrary POVs.

a Top: measured intensity distributions of the four optical vortices with a linear scale in the y–z plane at a design wavelength of 530 nm: $\mid {\mathrm{POV}}_{\mathrm{R}},1⟩$ and$\mid {\mathrm{POV}}_{\mathrm{L}},5⟩$ corresponding to the metasurface MF1; $\mid {\mathrm{POV}}_{\mathrm{R}},5⟩$ and $\mid {\mathrm{POV}}_{\mathrm{L}},10⟩$ corresponding to the metasurface MF2. Bottom: measured annular intensity profiles of four optical vortices at the designed focal position z = 200 μm. Scale bar: 10 μm. b Normalized cross sections of the annular intensity profiles of the four optical vortices along the white dash and solid lines of a for MF1 (top) and MF2 (bottom). c, d Top: measured normalized intensity distributions of the optical vortices with a linear scale in the y–z plane at the wavelengths of 480 nm (blue), 580 nm (yellow) and 630 nm (red). Each wavelength corresponds to two orthogonally circular polarization optical vortices (c: $\mid {\mathrm{POV}}_{\mathrm{R}},1⟩$ and $\mid {\mathrm{POV}}_{\mathrm{L}},5⟩$ for metasurface MF1; d: $\mid {\mathrm{POV}}_{\mathrm{R}},5⟩$ and $\mid {\mathrm{POV}}_{\mathrm{L}},10⟩$ for metasurface MF2). Bottom: measured annular intensity profiles of optical vortices at the propagation distances z = 230 μm, 195 μm, and 180 μm corresponding to the wavelengths of 480 nm (blue), 580 nm (yellow) and 630 nm (red). Scale bar: 10 μm.

Fig. 4. Evolutions of the metasurface-generated PPBs corresponding to the points on HyOPS.

a The selected six points on HyOPS representing six states of PPBs generated successively by metasurface. b Six states of polarization (SOP) of the light incident on the metasurfaces are chosen to generate various states of PPBs corresponding to the coordinates in (a). c The measured annular intensity patterns of the output states corresponding to the points in a for PPB1 and PPB2 in the x–y plane after transmission through a vertical linear polarizer depicted by the white double arrow. These images are captured at the designed focal position z = 200 μm. Scale bar: 10 μm. d The calculated and measured polarization orientations and distributions of PPB1 and PPB2 corresponding to point II in (a). Note that the horizontal polarization orientation is defined as 0 rad. Scale bar: 10 μm. e Measured annular intensity patterns of the PPB1 and PPB2 in the x–y plane at the wavelengths of 480 nm (blue), 580 nm (yellow) and 630 nm (red) at a propagation distance of z = 230, 195, and 180  μm. These images are captured through a linear polarizer depicted by the white double arrow. Scale bar: 10 μm.

Fig. 5. Proof-of-concept experimental demonstration of optical information encryption.

a The plaintext message including a set of complex account number and password composed of different characters (left) are translated into the combinations of hexadecimal numbers (right) by User 1. b The design parameters of the 25 PPBs (left) and SEM image of a portion of the metasurface termed ciphertext (right). Scale bar: 500 nm. c User 2 captures two images with the two customized keys. According to the code chart, the first and second digits of these two-digit hexadecimal numbers are decrypted by User 2, respectively. Scale bar: 50 μm. d The hexadecimal number combination is decrypted as the plaintext message including the account number and password based on the ASCII.