Guixin Li

Diatomic Metasurface for Vectorial Holography

The emerging metasurfaces with the exceptional capability of manipulating an arbitrary wavefront have revived the holography with unprecedented prospects. However, most of the reported metaholograms suffer from limited polarization controls for a restrained bandwidth in addition to their complicated meta-atom designs with spatially variant dimensions. Here, we demonstrate a new concept of vectorial holography based on diatomic metasurfaces consisting of metamolecules formed by two orthogonal meta-atoms. On the basis of a simply linear relationship between phase and polarization modulations with displacements and orientations of identical meta-atoms, active diffraction of multiple polarization states and reconstruction of holographic images are simultaneously achieved, which is robust against both incident angles and wavelengths. Leveraging this appealing feature, broadband vectorial holographic images with spatially varying polarization states and dual-way polarization switching functionalities have been demonstrated, suggesting a new route to achromatic diffractive elements, polarization optics, and ultrasecure anticounterfeiting.

Rotational Doppler shift induced by spin-orbit coupling of light at spinning metasurfaces

The interaction of light with spinning objects can lead to a frequency shift as a result of the transferred angular momentum. Subwavelength plasmonic nanostructures on an optical surface provide an efficient platform for local light manipulation and thus can be utilized for such momentum exchanges. Here we demonstrate reflective-type plasmonic metasurface q-plates based on the geometric Pancharatnam-Berry phase that are capable of changing the spin and orbital angular momentum of light. When these metasurfaces rotate at a constant angular speed, we observe a rotational Doppler frequency shift of the reflected light, which depends on the total angular momentum transfer. The flexibility in the design of the metasurfaces even enables complex reflective phase masks that can transfer different orbital angular momenta, at the same time, to the beam. Our experiments show that such complex metasurface phase masks can be used to obtain spatially variant Doppler shift in the reflected beam profile.

Imaging through Nonlinear Metalens Using Second Harmonic Generation

The abrupt phase change of light at metasurfaces provides high flexibility in wave manipulation without the need for accumulation of propagating phase through dispersive materials. In the linear optical regime, one important application field of metasurfaces is imaging by planar metalenses, which enables device miniaturization and aberration correction compared to conventional optical microlens systems. With the incorporation of nonlinear responses into passive metasurfaces, optical functionalities of metalenses are anticipated to be further enriched, leading to completely new application areas. Here, imaging with nonlinear metalenses that combine the function of an ultrathin planar lens with simultaneous frequency conversion is demonstrated. With such nonlinear metalenses, imaging of objects with near infrared light while the image appears in the second harmonic signal of visible frequency range is experimentally demonstrated. Furthermore, the functionality of these nonlinear metalenses can be modified by switching the handedness of the circularly polarized fundamental wave, leading to either real or virtual nonlinear image formation. Nonlinear metalenses not only enable infrared light imaging through a visible detector but also have the ability to modulate nonlinear optical responses through an ultrathin metasurface device while the fundamental wave remains unaffected, which offers the capability of nonlinear information processing with novel optoelectronic devices.

Metagrating holograms with ultra-wide incident angle tolerances and high diffraction efficiencies

High-performance meta-holograms are experimentally demonstrated in a near-unity-efficiency metagrating platform without any size or shape variation of the meta-atoms, which is robust against large incident angles, and sustains high diffraction efficiency in a broadband.

Facile metagrating holograms with broadband and extreme angle tolerance

The emerging meta-holograms rely on arrays of intractable meta-atoms with various geometries and sizes for customized phase profiles that can precisely modulate the phase of a wavefront at an optimal incident angle for given wavelengths. The stringent and band-limited angle tolerance remains a fundamental obstacle for their practical application, in addition to high fabrication precision demands. Utilizing a different design principle, we determined that facile metagrating holograms based on extraordinary optical diffraction can allow the molding of arbitrary wavefronts with extreme angle tolerances (near-grazing incidence) in the visible–near-infrared regime. By modulating the displacements between uniformly sized meta-atoms rather than the geometrical parameters, the metagratings produce a robust detour phase profile that is irrespective of the wavelength or incident angle. The demonstration of high-fidelity meta-holograms and in-site polarization multiplexing significantly simplifies the metasurface design and lowers the fabrication demand, thereby opening new routes for flat optics with high performances and improved practicality.Metasurfaces: Wide-Angle HologramsThe use of plasmonic metasurfaces has enabled the realization of broadband holograms that can operate at extreme angles approaching grazing incidence. Zi-Lan Deng and coworkers from Jinan University, Southern University of Science and Technology, Shenzhen University and Nankai University fabricated the holograms from an array of closely spaced, identical silver nanorods (90nm wide, 200nm long) on top of a silicon substrate coated with a thin layer of silver and then a layer of SiO2. The phenomenon of extraordinary optical diffraction allows highly efficient diffraction at very large angles and customizing the spacing of the nanorods allows the desired phase profile to be programmed, into the surface. The design, which operates in the visible and near-infrared range, may prove useful for various holographic applications including data encryption, anti-counterfeiting and 3D displays.

Optical image encryption with an ultrathin nonlinear metasurface

Recently, the concept of the geometric Pancharatnam-Berry (P-B) phase has been successfully extended to optical metasurfaces operating in the nonlinear regime1–3. It was shown that the nonlinear geometric P-B phase of each meta-atom is determined by the spin of the fundamental wave, the spin of the nonlinear signal, and the order of the nonlinear process1. By manipulating the interferences of SHG waves from two meta-atoms within one unit-cell, we demonstrate a grey-scale image encryption in the nonlinear signal. We successfully demonstrate spatially variant SHG for real space image generation directly from the metasurface interface by controlling the phase difference of the nonlinear polarization between neighboring meta-atoms.