Stokes meta-hologram toward optical cryptography.

Optical cryptography manifests itself a powerful platform for information security, which involves encrypting secret images into visual patterns. Recently, encryption schemes demonstrated on metasurface platform have revolutionized optical cryptography, as the versatile design concept allows for unrestrained creativity. Despite rapid progresses, most efforts focus on the functionalities of cryptography rather than addressing performance issues, such as deep security, information capacity, and reconstruction quality. Here, we develop an optical encryption scheme by integrating visual cryptography with metasurface-assisted pattern masking, referred to as Stokes meta-hologram. Based on spatially structured polarization pattern masking, Stokes meta-hologram allows multichannel vectorial encryption to mask multiple secret images into unrecognizable visual patterns, and retrieve them following Stokes vector analysis. Further, an asymmetric encryption scheme based on Stokes vector rotation transformation is proposed to settle the inherent problem of the need to share the key in symmetric encryption. Our results show that Stokes meta-hologram can achieve optical cryptography with effectively improved security, and thereby paves a promising pathway toward optical and quantum security, optical communications, and anticounterfeiting.

Flexible trajectory control of Bessel beams with pure phase modulation.

Spatial phase modulation has become an important method for the design of new self-accelerating light beams. Based on the transverse-longitudinal mapping of Bessel beam, we propose a method of pure phase modulation to directly convert a zero-order Bessel beam into a self-accelerating beam, of which the propagation trajectories can be flexibly predesigned. We experimentally demonstrate three typical types of curves that the modulated Bessel beam propagates along, and the parabolic, spiral, and teleporting self-accelarating beams are realized. The experimental results match the expected trajectory well. This method is simple to operate, and imposes fewer restrictions on the beam trajectory.

Controllable oscillated spin Hall effect of Bessel beam realized by liquid crystal Pancharatnam-Berry phase elements.

Pancharatnam-Berry (PB) phase has become an effective tool to realize the photonic spin Hall effect (PSHE) in recent years, due to its capacity of enhancing the spin-orbit interaction. Various forms of PSHEs have been proposed by tailoring the PB phase of light, however, the propagation trajectory control of the separated spin states has not been reported. In this paper, we realize the oscillated spin-dependent separation by using the well-designed PB phase optical elements based on the transverse-to-longitudinal mapping of Bessel beams. Two typical oscillated PSHEs, i.e., the spin states are circulated and reversed periodically, are experimentally demonstrated with two PB phase elements fabricated with liquid crystal. The displacements and periods of these oscillations can be controlled by changing the transverse vector of the input Bessel beam. The proposed method offers a new degree of freedom to manipulate the spin-dependent separation, and provides technical supports for the application in spin photonics.

Femtosecond laser plasmonic nano-printing metasurfaces for multiple-dimensional manipulation of light fields.

Metasurfaces enable the multidimensional manipulation of light fields in a subwavelength scale. However, the low-cost preparation of large-area metasurfaces is still a challenge. In this Letter, we first, to the best of our knowledge, use the laser plasmonic nano-printing technique to efficiently manufacture metasurfaces with multidimensional manipulation capability. By utilizing a phase-polarization mapping method, we fabricated a silicon-based metasurface for color display, and indium tin oxide-based metasurfaces for decoupled near- and far-field holographic displays. This flexible and efficient laser plasmonic nano-printing method has great potential in the preparation of large-area metasurfaces, and is of great significance to promote the practical application of metasurfaces.

Metasurface-assisted multidimensional manipulation of a light wave based on spin-decoupled complex amplitude modulation.

Achieving arbitrary manipulation of the fundamental properties of a light wave with a metasurface is highly desirable and has been extensively developed in recent years. However, common approaches are typically targeted to manipulate only one dimension of light wave (amplitude, phase, or polarization), which is not quite sufficient for the acquisition of integrated multifunctional devices. Here, we propose a strategy to design single-layer dielectric metasurfaces that can achieve multidimensional modulation of a light wave. The critical point of this strategy is spin-decoupled complex amplitude modulation, which is realized by combining propagation and geometric phases with polarization-dependent interference. As proofs of concept, perfect vector vortex beams and polarization-switchable stereoscopic holographic scenes are experimentally demonstrated to exhibit the capability of multidimensional light wave manipulation, which unlocks a flexible approach for the multidimensional manipulation of a light wave such as complex light-wave control and vectorial holography in integrated optics and polarization-oriented applications.

Full-Color Holographic Display and Encryption with Full-Polarization Degree of Freedom.

Metasurfaces provide a compact and powerful platform for manipulating the fundamental properties of light, and have shown unprecedented capabilities in both optical holographic display and information encryption. For increasing information display/storage capacity, metasurfaces with more polarization manipulation channel and full-color holographic functionality are now an urgent requirement. Here, a minimalist dielectric metasurface with the capability of full-color holography encoded with arbitrary polarization is proposed and experimentally demonstrated. Without the daunting exploratory and computational problem in nanostructure searching, full-color holographic images can be multiplexed into arbitrary polarization channels through vectorial ptychography and k-space ptychography based on tetratomic macropixel geometric phase metasurfaces. Thanks to the full degree of freedom tuning in polarization and color spaces, the application scenarios such as holographic 3D imaging and information encryption are realized. The strategy exhibits promising potential in applications of 3Dl display, augmented/virtual reality, high-density data storage, and encryption.

Compact polarization-resolved common-path digital holography based on the Pancharatnam-Berry phase.

We propose a compact polarization-resolved common-path digital holography for measuring the polarization distribution of a light field dynamically with high temporal stability. The designed experimental setup allows simultaneously recording, in a common-path manner, two holograms carrying the complex amplitude information of two orthogonal polarization components of the light field. Based on the theory of the Pancharatnam-Berry phase to retrieve the full Stokes parameters of the light field, we demonstrate the experiments with polarized optical elements, stressed glass plate, and micrometer-sized liquid crystal droplet. The measurement results verify the method's high accuracy and stability, and the capability of measuring light fields with sizes ranging from centimeters to micrometers. Owing to the stable and compact optical path structure, this method is conducive to instrumentation and is expected to find wide applications in many fields.

Observation of optical vortex knots and links associated with topological charge.

Knots and links, as three-dimensional topologies, have played a fundamental role in many physical fields. Despite knotted vortex loops having been shown to exist in the light field, the three-dimensional configuration of vortex loop is fixed due to their topological robustness, making the fields with different topologies independent of each other. In this work, we established the mapping between the torus knots/links and the integer topological charge of the optical vortex, and demonstrated the change of the intermediate state with fractional charges. Furthermore, we experimentally observed the transformation process of the three-dimensional topological structure by only changing the topological charge. Remarkably, we revealed two different reconnection mechanisms associated with the odd or even index of the torus topology. We hope these results may provide new insight for the study of singular optics and evolution in other physical fields.

Femtosecond laser-induced spatial-frequency-shifted nanostructures by polarization ellipticity modulation.

We demonstrate a prominent spatial frequency shift (SFS) for the femtosecond laser-induced periodic structures by only changing the polarization ellipticity of the working laser. The nanostructures are fabricated on the surfaces of silicon (Si) and zinc selenide (ZnSe) using elliptically polarized femtosecond laser pulses, with the pulse duration of 35 fs, the central wavelength of 800nm, and the repetition rate of 1kHz. The experimental results show that the red- and blue-shift trends of the SFS are individually represented on silicon and zinc selenide with the increased polarization ellipticity, where low- and high-spatial-frequency nano-ripples are fabricated, respectively. These unique phenomena are explained by using the laser-surface plasmon polariton interference mechanism and the effective medium theory. The proposed nanostructures with regulatable period are further used for creating nano-gratings on silicon which perform chirped characteristics.

Ferroelectric liquid crystal Pancharatnam-Berry lens with a fast control of output light's polarization-handedness.

We report the ferroelectric liquid crystal (FLC) Pancharatnam-Berry lenses (PBLs) with rapid transmittance tunability. The FLC PBLs were fabricated using a single-step holographic exposure system based on a spatial light modulator working as numerous polarization retarders, providing a simple way to fabricate FLC continuous aligning structures. A state-selection sector containing a binary FLC switch was utilized for fast changing input light's polarization handedness. Thus, when light passes through a FLC PBL, the output light's polarization handedness can be switched accordingly. In this case, FLC PBLs can function as concave/convex lenses with rapidly switching speed. Photo sensitive azo-dye material was used as the aligning layer for both FLC PBLs and FLC switches. The fabricated FLC PBLs and the FLC switches show fast switching-on times of 150µs and 50µs respectively. The FLC PBLs combining with the state-selection sector can have potential applications on varies displays and augmented reality.

Tightly focused light field with controllable pure transverse polarization state at the focus.

We report on a facile and flexible scheme for producing the controllable pure transverse polarization state at the focus within a tightly focused field. Toward this aim, a special type of hybrid vector beam exhibiting unusual "8-type" mapping tracks of azimuthal polarization states on the Poincaré sphere is employed. Due to the peculiar polarization structures, at the focus, there is only the transverse component, while the longitudinal component is zero for any 8-type vector beam. More strikingly, the transverse polarization state at the focus is exactly the same as that of the cross point of the 8-type mapping track. Benefiting from this appealing polarization relationship, an arbitrary transverse polarization state can be easily achieved at the focus via altering the mapping track of incident vector beams. These results may have potential applications in nano and spin photonics.

Autofocusing of ring Airy beams embedded with off-axial vortex singularities.

We report the autofocusing behaviors of ring Airy beams (RABs) embedded with two kinds of off-axial vortex singularities. The influences of embedded positions and topological charges of point and r vortices on the autofocusing dynamic are numerically and experimentally investigated. The results show that, for the first-order vortex, the embedded position significantly affects the focal field, and once the singularity is located on the main ring of RAB, the symmetric Bessel profile of the focal field will be broken, otherwise the Bessel-like focus can self-heal at the focal plane. However, for the higher-order vortex embedded near the main ring, it will split into several fundamental vortices and then separate with each other along the radial direction under the interaction with the RAB background. Our results hold potential for the practical application of RABs in the atmosphere and other propagation systems with perturbation and even singularities.

Accurate and rapid measurement of optical vortex links and knots.

Optical vortices can evolve in light fields, of which the singularity evolution forms dark lines with complex topological structures, knotted or linked. We propose a method to more accurately and rapidly measure the topology of optical vortex fields. To accurately locate the phase singular points, phase measurement based on digital holography and, further, a numerical search algorithm, are utilized. A motor-driven right-angle prism enables the implementation of a single exposure of hologram for each measurement along the propagation direction, greatly improving the measurement speed. The three-dimensional (3D) spatial distributions of several typical vortex links and knots are experimentally reconstructed. The proposed method is expected to rapidly observe the 3D evolution of other complicated, or even vector, fields.

Auto-transition of vortex- to vector-Airy beams via liquid crystal q-Airy-plates.

We propose the auto-transition of vortex-Airy to vector-Airy beams realized via a liquid crystal q-Airy-plate, whose director distribution is the integration of a q-plate and a polarization Airy mask. The polarization, phase, intensity distributions of the vortex-vector-Airy beams (VVABs) during the transition process and individual trajectories of the vortex beam, vector beam and Airy beam components are both theoretically and experimentally investigated. Interesting findings show that the pair of vortex components firstly experience transverse deflection with a smaller acceleration than the Airy components and then automatically evolve into a vector component propagating in a straight path. The polarization mode of the VVABs can be easily switched by tuning the incident polarization direction. Meanwhile, the Airy component still maintains its intrinsic self-accelerating and self-healing properties. The asymmetric intensity distribution and variation of VVABs are revealed, and the energy flows are simulated to better illustrate the interaction of the Airy, vortex and vector components. This work provides an approach for the manipulation of the spatially structured light beams, which may inspire their potential applications in optics, photonics and multidisciplinary fields.

Creation of independently controllable multiple focal spots from segmented Pancharatnam-Berry phases.

Recently, based on space-variant Pancharatnam-Berry (PB) phases, various flat devices allowing abrupt changes of beam parameters have been predicted and demonstrated to implement intriguing manipulation on spin states in three dimensions, including the efficient generation of vector beams, spin Hall effect of light and light-guiding confinement, and so on. Here, we report on the construction of independently controllable multiple focal spots with different inhomogeneous polarization states by utilizing segmented PB phases. Combining the phase shift approach with PB phases, we engineer fan-shaped segmented PB phases and encode them onto two spin components that compose a hybrid polarized vector beam in a modified common-path interferometer system. Experimental results demonstrate that the fan-shaped segmented PB phase enables the flexible manipulation of focal number, array structure and polarization state of each focal spot. Furthermore, we demonstrate that this fan-shaped approach enables to flexibly tailor the polarization state and the spin angular momentum distribution of a tightly focused field, which have potential applications in optical manipulation, tailored optical response and imaging etc.