Guo Ping Wang

Wide-angled off-axis achromatic metasurfaces for visible light

We present an approach to build multiwavelength achromatic metasurface that can work in off-axis configuration with an ultra-wide applicable incident angle range for visible light. The metasurface is constructed by combining multiple metallic nano-groove gratings, which support enhanced diffractions for transverse magnetic polarization in an ultrawide incident angle range from 10° to 80° due to the excitations of localized gap plasmon modes at different resonance wavelengths. To achieve the achromatic diffraction, the ratio between the resonance wavelength and the period of each elementary grating is fixed. Incident light at those multiple resonance wavelengths can be efficiently diffracted into the same direction with near-complete suppression of the specular reflection. Based on the similar approach, we also design a wide-angled off-axis achromatic flat lens for focusing light of different wavelengths into the same position. Our findings provide an alternative simple way to design various off-axis achromatic flat optical elements without stringent angle requirement for imaging and display applications.

Full controlling of Fano resonances in metal-slit superlattice

Controlling of the lineshape of Fano resonance attracts much attention recently due to its wide capabilities for lasing, biosensing, slow-light applications and so on. However, the controllable Fano resonance always requires stringent alignment of complex symmetry-breaking structures and thus the manipulation could only be performed with limited degrees of freedom and narrow tuning range. Furthermore, there is no report so far on independent controlling of both the bright and dark modes in a single structure. Here, we semi-analytically show that the spectral position and linewidth of both the bright and dark modes can be tuned independently and/or simultaneously in a simple and symmetric metal-slit superlattice, and thus allowing for a free and continuous controlling of the lineshape of both the single and multiple Fano resonances. The independent controlling scheme is applicable for an extremely large electromagnetic spectrum range from optical to microwave frequencies, which is demonstrated by the numerical simulations with real metal and a microwave experiment. Our findings may provide convenient and flexible strategies for future tunable electromagnetic devices.

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.

Trimeric metasurfaces for independent control of bright and dark modes of Fano resonances

In this paper, we present a simple trimeric metasurface consisting of three dipolar resonators in each unit cell, to achieve the independent controlling over both the broad bright mode and the sharp dark mode of Fano resonances. Through both the finite difference time domain simulation and microwave experiment, we find that spectral positions of the bright and dark modes are linearly dependent on, respectively, the global spacing between adjacent unit cells and the local spacing between adjacent dipoles within each unit cell. The dependence of the spectral position of bright (dark) mode on the global (local) spacing is independent without mutual influence, which provides a facile pathway to control the Fano resonance with large flexibility. Our proposed scheme to control Fano resonance is highly desired in various fields including lasing spaser and biosensing with improved performance.

Non-uniform annular rings-based metasurfaces for high-efficient and polarization-independent focusing

A reflection metasurface, composed of metallic annular rings, is presented for realizing high-efficient and polarization-independent focusing. By varying the inner and outer radii of the isotropic rings, we can achieve a full modulation on the phase from −180° to 180°. By properly arranging the annular rings, we design gradient metasurfaces for focusing without polarization sensitivity by finite element method simulations and further demonstrate the focusing effect with high-efficient and polarization-independent performance experimentally in the microwave domain. In addition, the structure is also proved applicable in the optical domain by simulations. This work expands the capabilities of metasurfaces to focusing and imaging applications without polarization limitations.

Fano Resonance in a Metasurface Composed of Graphene Ribbon Superlattice

We present a metasurface composed of graphene ribbon superlattice that supports plasmonic Fano resonance in a simple symmetric configuration. Without the necessity of changing the geometry size of graphene ribbons, we tune the Fano resonance of the metasurface containing identical graphene ribbons by simply changing the global or local periods of the superlattice. The increase of the global period of the superlattice leads to a blue-shift of the broad resonance of the bright mode, while the increase of the local period leads to simultaneous shifts of the broad resonance of the bright mode and the sharp resonance of the dark mode toward opposite direction with respect to each other. The resonance shift mechanism can be well explained by the restoring force model for longitudinal dipole arrays. In addition, the overall spectral position of the Fano resonance can be actively tuned by the fermi level of graphene ribbons. Our methods provide a simple and flexible pathway to tune the plasmonic Fano resonance, which holds great potentials for tunable biosensing and slow light applications with improved performance.

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.

Transmissive Refractive Index Sensing Based on Frequency-Sensitive Responses of Two-Dimensional Photonic Crystals

In this paper, we propose a new scheme of refractive index (RI) sensing, which utilizes frequency-sensitive responses of 2-D photonic crystals. Specifically, the 2-D photonics crystals consist of dielectric rods arranged in rectangular lattice and support frequency-sensitive supercollimation (SC). Small changes in ambient RI are sensed by measuring transmission rate of a narrow spectral source. This RI sensing scheme exploits the sensitive dispersion properties around the SC frequency: Both reflection and beam diffraction are enhanced in response to a slight increase of ambient RI. Operation and performance of the transmissive RI sensing are demonstrated by finite-difference time-domain (FDTD) simulations. The major advantage of our design is that all the essential components can be compactly integrated, which makes it attractive for a number of applications, such as hand-held equipment and distributed sensor networks.