High-efficiency unidirectional wavefront manipulation for broadband airborne sound with a planar device

In the past decade, one-way manipulation of sound has attracted rapidly growing attention with application potentials in a plethora of scenarios ranging from ultrasound imaging to noise control. Here we propose a design of a planar device capable of unidirectionally harnessing the transmitted wavefront for broadband airborne sound. Our mechanism is to use the broken spatial symmetry to give rise to different critical angles for plane waves incident along opposite directions.Along the positive direction, the incoming sound is allowed to pass with high efficiency and be arbitrarily molded into the desired shape while any reversed wave undergoes a total reflection. We analytically derive the working bandwidth and incident angle range, and present a practical implementation of our strategy. The performance of our proposed device is demonstrated both theoretically and numerically via distinct examples of production of broadband anomalous refraction,acoustic focusing and non-diffractive beams for forward transmitted wave while virtually blocking the reversed waves.Bearing advantages of simple design, planar profile, broad bandwidth and high efficiency, our design opens the possibility for novel one-way acoustic device and may have important impact on diverse applications in need of special control of airborne sound.

Metalenses: from design principles to functional applications

Lens is a basic optical element that is widely used in daily life,such as in cameras,glasses,and microscopes.Conventional lenses are designed based on the classical refractive optics,which results in inevitable imaging aberrations,such as chromatic aberration,spherical aberration and coma.To solve these problems,conventional imaging systems impose multiple curved lenses with different thicknesses and materials to eliminate these aberrations.As a unique photonic technology,metasurfaces can accurately manipulate the wavefront of light to produce fascinating and peculiar optical phenomena,which has stimulated researchers9 extensive interests in the field of planar optics.Starting from the introduction of phase modulation methods,this review summarizes the design principles and characteristics of metalenses.Although the imaging quality of existing metalenses is not necessarily better than that of conventional lenses,the multi-dimensional and multi-degree-of-freedom control of metasurfaces provides metalenses with novel functions that are extremely challenging or impossible to achieve with conventional lenses.

Fano-resonance-assisted metasurface for color routing

Controlling the phase of an electromagnetic field using plasmonic nanostructures provides a versatile way to manipulate light at the nanoscale.Broadband phase modulation has been demonstrated using inhomogeneous metasurfaces with different geometries;however,for many applications such as filtering,hyperspectral imaging and color holography,narrowband frequency selectivity is a key functionality.In this work,we demonstrate,both theoretically and experimentally,a narrowband metasurface that relies on Fano resonances to control the propagation of light.By geometrically tuning the sub-radiant modes with respect to a fixed super-radiant resonance,we can create a phase modulation along the surface within a narrow spectral range.The resulting anomalous reflection measured for such a Fano-resonant metasurface exhibits a 100 nm bandwidth and a color routing efficiency of up to 81%at a central wavelength ofλ=750 nm.The design flexibility provided by this Fano-assisted metasurface for colorselective light manipulation is further illustrated by demonstrating a highly directional color-routing effect between two channels,atλ=532 and 660 nm,without any crosstalk.

Wavevector Selective Metasurfaces and Tunnel Vision Filters

Metasurfaces offer unprecedented flexibility in the design and control of light propagation,replacing bulk optical components and exhibiting exotic optical effects.One of the basic properties of the metasurfaces,which renders them as frequency selective surfaces,is the ability to transmit or reflect radiation within a narrow frequency band that can be engineered on demand.Here we introduce and demonstrate experimentally in the THz domain the concept of wavevector selective surfaces–metasurfaces transparent only within a narrow range of light propagation directions operating effectively as tunnel vision filters.Practical implementations of the new concept include applications in wavefront manipulation,observational instruments,vision and free-space communication in light-scattering environments.

Optical fiber meta-tips

We report on the first demonstration of a proof-of-principle optical fiber‘meta-tip’,which integrates a phase-gradient plasmonic metasurface on the fiber tip.For illustration and validation purposes,we present numerical and experimental results pertaining to various prototypes implementing generalized forms of the Snell’s transmission/reflection laws at near-infrared wavelengths.In particular,we demonstrate several examples of beam steering and coupling with surface waves,in fairly good agreement with theory.Our results constitute a first step toward the integration of unprecedented(metasurface-enabled)light-manipulation capabilities in optical-fiber technology.By further enriching the emergent‘lab-on-fiber’framework,this may pave the way for the widespread diffusion of optical metasurfaces in real-world applications to communications,signal processing,imaging and sensing.

Tunable vector vortex beam generation using phase change metasurfaces [Invited]

Vector vortex beams(VVBs), novel structured optical fields that combine the polarization properties of vector beams and phase characteristics of vortex beams, have garnered widespread attention in the photonics community. Capitalizing on recently developed metasurfaces, miniaturized VVB generators with advanced properties have been implemented. However,metasurface-empowered VVB generators remain static and can only generate one pre-designed structured light. Here, we propose a kind of phase change metasurface for tunable vector beam generation by utilizing anisotropic Ge2Sb2Se4Te1(GSST) unit cells with tunable phase retardation when GSST transits between two different phase states. By properly rotating the orientations of the tunable GSST unit structures that transit between quarter-wave plates and half-wave plates, we can effectively transform incident plane waves into vector beams with distinct topological charges and polarization states.When GSST is in the amorphous state, the designed metasurface can transmit circularly polarized light into VVBs. In the crystalline state, the same GSST metasurface converts linearly polarized light into second-order radially polarized(RP) and azimuthally polarized(AP) beams. Our phase-change metasurface paves the way for precise control over the polarization patterns and vortex characteristics of beams, thereby enabling the exact manipulation of beam structures through the alteration of their phase states.