Abstract
The term Poincaré beam, which describes the spacevariant 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 highquality 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 alldielectric 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 phaseonly modulation approach, a singlelayer spinmultiplexed metasurface is shown to achieve all the states of PPBs on the hybridorder Poincaré Sphere for visible light. Furthermore, as a proofofconcept 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.
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Introduction
Polarization and phase are two intrinsic properties of light^{1}. Light can possess a spin angular momentum (SAM) of σћ (σ = ± 1) per photon depending on the chirality of circular polarization^{2}, and an orbital angular momentum (OAM) of lћ (where l is the topological charge number) per photon depending on its helical phase structure^{3}. SAM and OAM constitute the total angular momentum (J = σћ + lћ) of a light beam^{4}. The hybridorder Poincaré Sphere (HyOPS)^{5}, which generalizes Poincaré Sphere (PS)^{6} and higherorder Poincaré Sphere (HOPS)^{7}, has been developed to describe the total angular momentum and the evolution of polarization and phase of light. The poles of the HyOPS denote two orthogonal, circularly polarized states of optical vortices^{8} with arbitrary topological charge numbers. Each point on the HyOPS describes the spacevariant polarization field of a light beam carrying OAM, which can be represented by a linear superposition of the two poles and is generally referred to as Poincaré beam^{9}. In particular, the optical vortices and cylindrical vector vortex beams^{10} described by the poles and equator of the HyOPS, respectively, have been demonstrated and applied in a number of applications^{11}, such as optical trapping, highresolution microscopy, optical communication, nonlinear optics, and optical encryption^{12}. However, the beam radii of these conventional Poincaré beams always depends on their topological charge number, hindering their applications in some cases, for e.g., coupling of multiple Poincaré beams into a single optical fiber for mode division multiplexing^{13}. In addition, it is difficult to generate a stable and highquality Poincaré beam by two orthogonally circularly polarized optical vortices with very different topological charge numbers, as the Poincaré beam formed in this way would easily collapse upon propagation^{14}.
Recently, the concept of perfect optical vortices (POVs) has been proposed to overcome the above limitations because their ring radii are independent of the topologic charges^{15}. Consequently, perfect Poincaré beams (PPBs), as the linear superposition of POVs, have been demonstrated using selective combinations of conventional optical components, such as axicons, spatial light modulators, qplates, and Fourier transform lenses^{16,17,18}. However, the complex light path and bulky footprint of optical components generally required in these approaches make them nonideal for integration into compact nanophotonic platforms. Furthermore, optical aberrations caused by misalignment between the different optical components can easily deteriorate the quality of PPBs. In addition, conventional spatial light modulators and qplates have micronscale sampling pixel sizes that significantly limits the spatial density of the generated PPBs array.
Metasurfaces, made up of nanoscale optical scatters, can arbitrarily modulate the phase, polarization, and amplitude of light with deep subwavelength spatial resolution, and thus provides an efficient, versatile, and integrationfriendly platform for compact planar optics^{19,20,21,22,23,24,25,26,27,28,29,30}. In particular, due to the flexible design freedom offered by them, transmissive metasurfaces have been applied to a wide variety of functional planar optics, such as imaging lenses^{31,32,33,34,35}, metaholograms^{36,37,38}, nonlinear optics^{39,40}, and structured beam generators^{41,42,43}. Recently, a plasmonic metasurface was shown to generate POVs^{44}; however, it suffers from low efficiency due to the large ohmic absorption caused by the constituent metallic nanostructures. A silicon metasurface has also been designed to generate PPBs^{45}; however, this design requires simultaneous phase and amplitude modulation and a complicated oblique incidence scheme, which further limit its efficiency and operating bandwidth.
In this work, based on an alldielectric metasurface, we propose and experimentally demonstrate a compact platform for broadband generation of generalized PPBs, including both elliptical and circular shapes. Through phaseonly modulation, a singlelayer metasurface composed of subwavelengthspaced titanium dioxide (TiO_{2}) nanopillars can straightforwardly achieve all the states of PPBs on the HyOPS for visible light, without the requirement of any additional optical elements. By combining both geometric phase and propagation phase modulation^{46}, the designed spinmultiplexed metasurface can provide two arbitrary, fully uncorrelated phase profiles for the two orthogonal circular polarizations of incident light. By changing the polarization states of incident light, all orders of PPBs with arbitrary phase and polarization distributions can be generated. Furthermore, as a proofofconcept demonstration, a metasurface encoding multidimensional SAM and OAM states in parallel channels for elliptical and circular PPBs is implemented for optical information encryption. We envision that this work will provide a platform for the efficient generation of PPBs for visible light and may promote their applications in optical communications, information encryption, optical data storage, and quantum information science. Furthermore, this method to achieve complete and independent phase control of orthogonal circular polarization states at the single pixel level also can be used to design vectorial metasurface to generate arbitrary 3D vectorial field distribution for 3D vectorial holography application^{47}. This vectorial metasurface may promote the development of compact and highlyintegrated optical system for polarization holography, Stokes holography, holographic trap display, multidimensional data storage, and optical microscopic imaging.
Results
Design of metasurface to generate generalized PPBs
The vectorial field of a monochromatic paraxial light beam can be represented by the superposition of scalar fields with orthogonal polarization states^{11}. Considering the orthogonal circular polarization basis and two POVs with same ring radius but different topological charges, a PPB can be described as:
where \({{\rm{POV}}}_{{\mathrm{R}}},{l}_{m}\rangle\) and \({{\rm{POV}}}_{{\mathrm{L}}},{l}_{n}\rangle\) denote the righthanded circularly polarized (RCP) and lefthanded circularly polarized (LCP) POV with same ellipticity, and different topological charge numbers of l_{m} and l_{n}, respectively. \(\cos (\frac{\alpha }{2})\) and \(\sin (\frac{\alpha }{2})\) represent the amplitude of RCP and LCP POV and β is the relatively phase difference between them, where α ∈ [0, π] and β ∈ [0, 2π]. \({U}_{N}\rangle \) is an arbitrary point on the surface of HyOPS with spherical coordinates (α, β). Figure 1a shows examples of various elliptical PPBs and their mapping points on the HyOPS. Each point on the surface of the HyOPS denotes a state of a PPB with spacevariant polarization and phase distribution. The polarization distribution of each state can be determined by polarization order \(p=({l}_{m}{l}_{n})/2\) and the HyOPS spherical coordinates (α, β). The phase distribution of the PPB is characterized by the topological Pancharatnam charge \({l}_{p}=({l}_{m}+{l}_{n})/2\)^{14,45}. Two poles of the HyOPS are represented by two elliptical POVs with same ellipticity factor γ = 1.2 and uniform circular polarization (depicted by red arrows) but different topological charge numbers l_{m} = 5 and l_{n} = 10. The annular intensity profiles at other points on the HyOPS are also of the same ring radius; and due to their anisotropic linear polarization distributions (depicted by red arrows), these elliptical hollow beams can be transformed to exhibit distinct intensity patterns by using a vertical linear polarizer (indicated by the white double arrow).
In principle, a POV can be generated by taking the Fourier transform of a higherorder Bessel beam^{15}. Due to the difficulty of experimentally generating an ideal Bessel beam, here we consider the Fourier transform of a higherorder BesselGaussian beam instead. To be specific, a Gaussian beam can be transformed into a POV by passing it through a spiral phase plate, an axicon, and a Fourier transform lens. In terms of the Fourier transform theory, the complex field amplitude of the POV with uniform polarization in the rear focal plane of a lens can be obtained as (see Supplementary Note 1 for details):
where (r, φ) are the polar coordinates, ω_{g} is the waist of the input Gaussian beam, ω_{γ} is the waist of the Gaussian beam in the rear focal plane, and l is the topological charge. \({R}_{\gamma }=\gamma {k}_{r}f/k\), where k_{r} is the radial wavevector relying on the numerical aperture (NA) of the axicon, \(k=2\pi /{\lambda }_{0}\) is the freespace wavevector, and f is the focal length of lens. It is obvious that the amplitude of the POV is shaped by a Gaussian function, which has a maximum value at r = R_{γ}. The radii of the POV along the vertical and horizontal directions can be expressed as R_{⊥} = fNA and R_{} = γfNA, respectively, which are independent of topological charges. Therefore, depending on the values of three parameters (γ, f, NA), the intensity profile and radius along the major and minor axis of POV can be arbitrarily controlled. Therefore, customized shaping of the POV guarantees the generation of arbitrary annular intensity distribution of the PPB, providing sufficient variables for potential use in optical information encryption.
Due to the unique capability of flexible manipulation of the properties of light, a metasurface is able to integrate multiple functions of different optical elements, such as the spiral phase plate, axicon and Fourier transform lens as mentioned above, into a single monolithic device for PPB generation. For a polarized light beam normally incident onto the metasurface, it can be decomposed into LCP and RCP spin eigenstates corresponding to 2D Jones vectors: \({\rm{LCP}}\rangle =[\begin{array}{c}1\\ i\end{array}]\) and \({\rm{RCP}}\rangle =[\begin{array}{c}1\\ i\end{array}]\). In order to generate two completely different POVs, the spinmultiplexed metasurface is required to provide two independent spatial phase profiles \({\varphi }_{1}(x,y)\) and \({\varphi }_{2}(x,y)\), respectively corresponding to \({\rm{LCP}}\rangle \) and \({\rm{RCP}}\rangle \) state (see Supplementary Note 2 for details). That is, for LCP incident light, the metasurface implements the transformation: \({\rm{LCP}}\rangle \to {{\rm{POV}}}_{{\mathrm{R}}},{l}_{m}\rangle\) (Fig. 1b). The output beam \({{\rm{POV}}}_{{\mathrm{R}}},{l}_{m}\rangle\) has flipped handedness compared to the incident light. Similarly, for the RCP incident light, the metasurface transforms it to a different POV with LCP state and topological charge of l_{n}: \({\rm{RCP}}\rangle \to {{\rm{POV}}}_{{\mathrm{L}}},{l}_{n}\rangle\) (Fig. 1c). Therefore, the metasurface can be described by a Jones matrix J(x, y) which simultaneously satisfies \(J(x,\,y){\rm{LCP}}\rangle =\exp [i{\varphi }_{1}(x,y)]{\rm{RCP}}\rangle \) and \(J(x,y){\rm{RCP}}\rangle =\exp [i{\varphi }_{2}(x,y)]{\rm{LCP}}\rangle \). The required Jones matrix takes the form:
Due to the symmetric and unitary conditions, J(x, y) can be written in a standard form J(x, y) = RΛR^{−1}, where R is a real unitary matrix and Λ is a diagonal matrix. For birefringent optical elements constituting the metasurface, the diagonal matrix Λ determines their phase shifts δ_{x} and δ_{y} along the two perpendicular symmetry axes, while the matrix R determines the rotation angle θ of their fast axes relative to the reference coordinate in the x–y plane. (δ_{x}, δ_{y}), and θ determine the propagation phase and geometric phase imposed on the transmitted light, respectively. For the given spinmultiplexed phase profiles φ_{1}(x, y) and φ_{2}(x, y), the required phase shifts and rotation angle are calculated as (see Supplementary Note 3 for details):
Therefore, in order to implement J(x, y), a series of subwavelength nanostructures are designed to provide the required phase shifts (δ_{x}, δ_{y}) covering the entire 2π phase range and satisfy the orientation angle θ at any point (x, y). The metasurface design flow is summarized in Fig. 2a. The propagation phase shifts δ_{x} and δ_{y} imparted by an individual nanopillar acting as a halfwave plate can be determined by its inplane dimensions along the two perpendicular symmetry axes, and the geometric phase is controlled by orientation angle θ of the nanopillar. Combining them, the inplane dimension and orientation of the nanopillars, of fixed height, constituting a vectorial metasurface can be determined at any point (x, y) on the metasurface plane.
Figure 2b shows a schematic diagram of metasurface composed of rectangular TiO_{2} nanopillars on a fusedsilica substrate with a subwavelength square lattice constant U. The nanopillars are of uniform heights H while their inplane dimensions (D_{x}, D_{y}), and orientation angles θ vary spatially. In order to investigate the transmission properties of rectangular TiO_{2} nanopillars, fullwave simulations are performed using the finitedifference timedomain (FDTD) technique. The height H of the TiO_{2} nanopillars is set as 600 nm to achieve the desired 2π phase coverage. The lattice constant U is chosen to be 450 nm, which guarantees that the nanopillars array act as a zerothorder grating^{22} with relatively high transmission in the visible range. Supplementary Fig. 1 shows the simulated phase shifts and transmission coefficients of x and ypolarized light for the rectangular nanopillars as a function of diameters (D_{x}, D_{y}) at a wavelength of λ_{0} = 530 nm. Based on these simulation results, polarization conversion efficiencies of TiO_{2} nanopillars are calculated and shown in Fig. 2c (see Supplementary Note 4 for details). A set of eight nanopillars, including four basic structures and four mirror structures are selected to provide eight phase levels covering the 2π phase range for δ_{x} and δ_{y} (Supplementary Fig. 2). The power transmission efficiency (T_{x} and T_{y}) and phase shifts (P_{x} and P_{y}) of eight nanopillars across the visible region are shown in Supplementary Fig. 3. As shown in Fig. 2d, these geometrical parameters (Supplementary Table 1) are optimized such that the nanopillars’ polarization conversion efficiencies are relatively high across the entire visible spectral range, which ensures broadband operation and efficient generation of POVs and PPBs. In addition, thanks to the high refractive index of the constituent material TiO_{2}, the optical fields at different wavelengths are all confined within individual nanopillars (Supplementary Fig. 4). This makes the TiO_{2} nanopillar behave as a weakly coupled lowquality factor resonator, and thus validates the approximation of each nanopillar to be the local pixel of Jones matrix.
Generalized PPBs generated by the metasurfaces
Based on the above principle, two TiO_{2} nanopillar metasurfaces (namely, MF1 and MF2) of area 90 μm × 90 μm are designed to generate two kinds of PPBs (PPB1 and PPB2). These two metasurfaces share the same device parameters: NA = 0.1, f = 200 μm, and γ = 1.2 at the design wavelength of λ_{0} = 530 nm. PPB1 generated by MF1 is the superposition of two POVs with topological charge numbers (l_{m},_{1} = 1, l_{n, 1} = 5) expressed by \(\cos \,\frac{\alpha }{2}{e}^{i\beta /2}{{\rm{POV}}}_{{\mathrm{R}}},l=1\rangle +\,\sin \,\frac{\alpha }{2}{e}^{i\beta /2}{{\rm{POV}}}_{{\mathrm{L}}},l=5\rangle\), while PPB2 generated by MF2 is the superposition of two POVs with topological charge numbers (\({l}_{m,2}=5,{l}_{n,2}=10\)) expressed by \(\cos \,\frac{\alpha }{2}{e}^{i\beta /2}{{\rm{POV}}}_{{\mathrm{R}}},l=5\rangle +\,\sin \,\frac{\alpha }{2}{e}^{i\beta /2}{{\rm{POV}}}_{{\mathrm{L}}},l=10\rangle\). The independent spatial phase profiles for LCP and RCP light encoded onto the metasurface are shown in Supplementary Fig. 5. According to these spatial phase profiles, the phase shifts (δ_{x}, δ_{y}) and rotation angle θ of the birefringent TiO_{2} nanopillars as a function of the spatial coordinates in the metasurface (MF1 and MF2) plane can be obtained and shown in Fig. 2e.
Figure 2f shows the optical microscopy image and scanning electron microscopy (SEM) images of MF1 (Top) and MF2 (Bottom). The metasurface fabrication details are given in the “Methods” section. Before the generation of PPBs, we first characterize the constituent spinmultiplexed optical vortices generated by the metasurfaces. By illuminating with RCP and LCP light generated from a supercontinuum laser attached to an acoustooptic tunable filter (AOTF) system, the inplane (x–y plane) annular intensity distributions of the four generated optical vortices from MF1 \(({{\rm{POV}}}_{{\mathrm{R}}},\,1\rangle ,{{\rm{POV}}}_{{\mathrm{L}}},\,5\rangle )\) and MF2 \(({{\rm{POV}}}_{{\mathrm{R}}},5\rangle ,{{\rm{POV}}}_{{\mathrm{L}}},10\rangle )\) are captured at a step of 1 μm along the light propagation direction (zaxis). The top panel of Fig. 3a shows the spatial intensities in the x–z plane of the generated optical vortices at the wavelength of λ_{0} = 530 nm by stitching all the captured inplane images together. It can be clearly seen that although the incident spin states and topological charges are all different for these two metasurfaces, the spatial patterns of the generated optical vortices along the propagation direction look very similar. The bottom panel of Fig. 3a shows the measured annular intensity profiles of four optical vortices at the designed focal position z = 200 μm. As expected, four inplane elliptical intensity profiles are almost identical. To better quantitively evaluate the quality of the generated optical vortices, Fig. 3b shows the cross sections of inplane intensity profiles extracted from Fig. 3a along the x and the ydirection, respectively depicted by the white dashed and solid lines. At the designed wavelength of λ_{0} = 530 nm, the vertical radius R_{⊥}, horizontal radius R_{} and ellipticity factor γ = R_{}/R_{⊥} of the generated optical vortices are measured as following: for MF1, R_{} = 24.3 μm, R_{⊥} = 19.9 μm, γ = 1.22 for RCP and R_{}= 25.1 μm, R_{⊥} = 20.9 μm, γ = 1.20 for LCP; For MF2, R_{}= 25.2 μm, R_{⊥} = 20.8 μm, γ = 1.21 for RCP and R_{}= 25.7 μm, R_{⊥} = 21.2 μm, γ = 1.21 for LCP. The measured results are very close to the theoretical values of R_{} = 24 μm, R_{⊥} = 20 μm, γ = 1.20. These results explicitly imply that the intensity profiles of the optical vortices generated by the metasurfaces are insensitive to the topological charge numbers, proving that they are POVs and thus can be used for the generation of PPBs. The experimentally measured generation efficiencies, defined as the ratio of the optical intensity of generated POVs to the intensity of the incident light, range between 50 and 54% for both MF1 and MF2 (see Supplementary Table 2).
In addition to the designed wavelength of 530 nm, thanks to the relatively high polarization conversion efficiency of the optimized nanopillars across the entire visible region, the metasurfaces exhibit a broadband response (see Supplementary Note 5 for details of broadband operation principle) and can also operate efficiently at other wavelengths. Figure 3c, d show the experimentally captured intensity profiles of the optical vortices generated by MF1 and MF2 at wavelengths of 480 nm (blue), 580 nm (yellow) and 630 nm (red). From the intensity profiles in the x–z plane (top panel of Fig. 3c, d), as expected the focal length and NA of the metasurface are wavelengthdependent due to chromatic dispersion, however, the optical vortices still exhibit very consistent spatial intensity distributions at these different wavelengths. At positions z = 230 μm, 195 μm and 180 μm, respectively corresponding to blue, yellow and red light, the inplane annular intensity patterns (bottom panel of Fig. 3c, d) are almost identical to the one measured in green (Fig. 3a). Therefore, the metasurfaces are still suitable for the generation of PBBs at other visible wavelengths. The cross sections of inplane annular intensity profiles along x and ydirection are given in Supplementary Fig. 11. The measured conversion efficiencies of these two metasurfaces for POVs at multiple wavelengths ranges from 31 to 45% (see Supplementary Table 2).
Next, to characterize these two metasurfaces for the generation of different PPBs, six points on the HyOPS are selected and their coordinates are given in Fig. 4a. The corresponding polarization states of light (Fig. 4b) incident on the metasurfaces are selected by rotating a quarter wave plate and a linear polarizer. The schematic of the experimental setup is illustrated in Supplementary Fig. 12. According to the topological charges of the constituent spinmultiplexed POVs, PPB1 and PPB2, generated from MF1 and MF2, respectively, have polarization order p and topological Pancharatnam charge l_{p} with \({p}_{1}=({l}_{m,1}{l}_{n,1})/2=2,{l}_{p,1}=({l}_{m,1}+{l}_{n,1})/2=3\) for PPB1, and \({p}_{2}=({l}_{m,2}{l}_{n,2})/2=2.5,{l}_{p,2}=({l}_{m,2}+{l}_{n,2})/2=7.5\) for PPB2. Figure 4c shows the experimentally measured intensity patterns of PPB1 and PPB2 in the x–y plane at the focal position z = 200 μm at a freespace wavelength of 530 nm. All the annular intensity patterns captured through a vertical linear polarizer for the two metasurfaces are of same contours, indicating that the radii of the generated PPBs are independent of sphere coordinate, polarization order and topological Pancharatnam charge. As the polarization order p determines the number of polarization rotations per round trip, the lobe number of the PPB pattern can be derived as 2p from the anisotropic polarization distribution through a vertical polarizer. As shown in the second column to fifth column (Fig. 4c), PPB1 and PPB2 are split into four and five lobes, respectively, which match well with the theoretical predictions of p_{1} = −2 and p_{2} = −2.5. In addition, we further confirm the anisotropic polarization distribution by Stokes polarimetry (see Supplementary Note 6 for details). The calculated and measured polarization distributions of the selected states corresponding to point II in Fig. 4a for the two kinds of PPBs are shown in Fig. 4d. It is clear that the polarization orientations rotate 4π and 5π per round trip for PPB1 and PPB2, respectively. The experimental results of these two PPBs agree well with the theoretical calculations (see Supplementary Note 7 for details and Supplementary Fig. 15 for calculated intensity patterns), except for a small rotation of the measured intensity and polarization patterns induced by the Gouy phase. In principle, the Gouy phase is dependent on the topological charge and can make the annular intensity pattern and polarization orientation distribution rotate by a small angle^{48}. As a consequence, this simple and efficient method to generate PPBs also offers a potential solution to visualize the Gouy phase experimentally.
Besides the operation wavelength of 530 nm, Fig. 4e shows the experimental results for MF1 and MF2 illuminated at three other wavelengths across the visible region (480, 580, and 630 nm). The intensity patterns for these illumination wavelengths are captured at z = 230, 195, and 180 μm, respectively. As expected, these lobed patterns exhibit very similar morphological characteristics as the one at a wavelength of 530 nm. Therefore, these results explicitly show that the metasurfaces can achieve efficient and broadband generation of PPBs in the visible region.
Optical information encryption
In the previous section, using a dielectric metasurface we have demonstrated the generation of PPBs that are independent of the topological charge number and polarization order. In principle, this approach encoding SAM and OAM states of light into PPBs in parallel channels can be employed for alloptical information encryption. For instance, various distinguishable PPBs with different polarization orders and ellipticities can be used to encode different doubledigit hexadecimal numbers. For e.g., if the absolute value of polarization order p and ellipticity factor γ of the intensity pattern respectively denote the first and second digit of a doubledigit hexadecimal number, and then a PPB represents a byte of data. As a result, the combination of 16 different values of p and γ for the PPBs can represent 256 hexadecimal numbers from 00 to FF. In our design, polarization order p ranging from 0 to 7.5 with an interval of 0.5 corresponds to the first hexadecimal digit from 0 to F, while ellipticity factor γ ranging from 0.5 to 2.0 with an interval of 0.1 represents the second hexadecimal digit from 0 to F. This code chart is given in Supplementary Table 3 and can be used to denote the characters of various encoding schemes.
During the encryption, two states of perfect Poincaré beams’ annular intensity images are combined to represent a doubledigit hexadecimal number, which does not directly expose the encrypted information and increases the security. Anyone can access the original information only when he or she simultaneously possess all four indispensable hardware and software contents including metasurface device (ciphertext) as the encrypted information carrier, customized keys for the acquisition of beam intensities, code chart for decrypting numbers and character encoding system for mapping numbers to plaintext composed of various characters. Although someone may steal the metasurface device and keys and capture the two kind of intensity patterns, they cannot decode the correct hexadecimal numbers without a code chart. Crucially, the code chart and character encoding system can be selfdefined and not fixed. Therefore, it is almost impossible to access the information without accurate onetoone mapping relationship. As a proofofconcept demonstration, here we use American Standard Code for Information Interchange (ASCII) to implement the optical encoding and decoding for information encryption. User 1 wants to send a set of highsecurity account number and password to User 2 and translates the plaintext into the combinations of ASCII hexadecimal numbers as illustrated in Fig. 5a. According to the hexadecimal numbers, 25 PPBs with the same f = 200 μm and NA = 0.1 but different values of p and γ are designed and encrypted on the metasurface, termed ciphertext (Fig. 5b). Then User 1 sends the metasurface sample and a customized key to User 2. With the illumination of horizontal linearly polarized (LP) light at a wavelength of 530 nm on the metasurface, an image of PPBs array is captured with the filtration of a vertical linear polarizer (right panel of Fig. 5c). Based on the code chart, the first hexadecimal digit can be determined by the lobe number of annular patterns. For instance, the first and last hexadecimal number in the text chart are respectively identified as “A” and “0”, on the basis of the fact that light beam is spilt into 10 lobes on the top left corner and completely blocked on the bottom right corner. On the other side, when the metasurface is illuminated with LCP or RCP light (left panel of Fig. 5c), another image of PPBs array without the filtration of a linear polarizer can be captured and the second hexadecimal digit can be determined by the ellipticity of annular patterns. For example, the first and last hexadecimal number in the text chart are respectively identified as “7” and “3” based on the annular beam with γ = 1.2 on the top left corner and another annular beam with γ = 0.8 on the bottom right corner. On the basis of this decoding rule, 25 doubledigit hexadecimal numbers can be easily decrypted by the image identification. According to the ASCII (Supplementary Table 4), a customized program then converts the decrypted numbers into a string of ASCII characters including the account number and password information for User 2 to login on a computer (Fig. 5d). In addition, this metasurface device also operates over a broad spectral range in the visible (see Supplementary Fig. 16) as evident from the intensity of PPB array at wavelengths of 480, 580, and 630 nm which show similar patterns as the ones for the designed wavelength of 530 nm.
Discussion
In conclusion, we have proposed and experimentally demonstrated broadband generation of perfect Poincaré beams via a singlelayer dielectric metasurface for visible light. By changing the spin angular momentum of incident light, the metasurface device with high spatial sampling resolution can generate spinmultiplexed perfect optical vortices with arbitrary orbital angular momentum. Via spatial superposition of two perfect optical vortices, different perfect Poincaré beams, whose total angular momenta are described by the hybridorder Poincaré sphere, can be generated. Furthermore, based on the angular momenta states encoded within the generated perfect Poincaré beams, a proofofconcept experimental demonstration for optical information encryption is implemented. We envision that this work will inspire creation of ultracompact flat nanophotonic elements for efficient generation and control of structured beams and further promote their practical applications, such as optical communication, optical encryption, optical data storage, and quantum information sciences.
Methods
Numerical simulations
Fullwave numerical simulations are performed using the finitedifference timedomain (FDTD) technique. Rectangular TiO_{2} nanopillars with a fixed height of 600 nm are sitting on a fusedsilica substrate with a lattice constant of 450 nm. The incident planewave is polarized along x or y axes and illuminates the nanopillars from the substrate side. Along x and y axes, periodic boundary conditions are applied and perfectly matched layer (PML) boundary condition is used in the z direction. The phase shifts (P_{x} and P_{y}) and power transmission (T_{x} and T_{y}) (see Supplementary Fig. 1) are obtained by parameter sweeping of the inplane dimensions (D_{x} and D_{y}) of the nanopillars by varying them between 50 nm and 350 nm at an interval of 5 nm.
Sample fabrication
Fusedsilica substrates were coated with a monolayer of hexamethyldisilazane and a layer of 600 nm thick positivetone electron beam (ebeam) resist. In order to eliminate the charging effect during ebeam lithography, a layer of 10 nm thick aluminum (Al) was deposited on the samples by thermal evaporation. Then ebeam lithography was performed at an accelerating voltage of 100 kV and beam current of 2 nA. Afterward, the samples were developed in hexylacetate for 120 s and then coated with TiO_{2} using atomic layer deposition (ALD) at a low temperature of 90 °C. The overcoated TiO_{2} layer was etched by the inductivelycoupledplasma reactive ion etching. Finally, the samples were exposed to UV irradiation and followed by soaking in nmethyl2pyrrolidone.
Data availability
The data that support the findings of this study are available from the corresponding author upon reasonable request.
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Acknowledgements
The work is supported by the Key Research and Development Program from Ministry of Science and Technology of China (2017YFA0303700 and 2016YFA0202100) and National Natural Science Foundation of China (11774163). W.Z., L.C., and A.A. acknowledge support under the Cooperative Research Agreement between the University of Maryland and the National Institute of Standards and Technology Center for Nanoscale Science and Technology, Award #70NANB14H209.
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M.L., P.H., S.Z., M.S., S.Z., Q.Z., and L.C. performed the theoretical design, numerical simulations, and experimental measurements. W.Z. and C.Z. performed the sample fabrication. M.L., P.H., W.Z., C.Z., H.L., A.A., Y.L., and T.X. contributed to the interpretation of results and participated in manuscript preparation. Y.L. and T.X. directed the project.
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Liu, M., Huo, P., Zhu, W. et al. Broadband generation of perfect Poincaré beams via dielectric spinmultiplexed metasurface. Nat Commun 12, 2230 (2021). https://doi.org/10.1038/s4146702122462z
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DOI: https://doi.org/10.1038/s4146702122462z
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