Bingyi Liu

Full-color hologram using spatial multiplexing of dielectric metasurface

In this Letter, we demonstrate theoretically a full-color hologram using spatial multiplexing of dielectric metasurface for three primary colors, capable of reconstructing arbitrary RGB images. The discrete phase maps for the red, green, and blue components of the target image are extracted through a classical Gerchberg-Saxton algorithm and reside in the corresponding subcells of each pixel. Silicon nanobars supporting narrow spectral response at the wavelengths of the three primary colors are employed as the basic meta-atoms to imprint the Pancharatnam-Berry phase while maintaining minimum crosstalk between different colors. The reconstructed holographic images agree well with the target images making it promising for colorful display.

Dielectric Huygens’ Metasurface for High-Efficiency Hologram Operating in Transmission Mode

Conventional metasurface holograms relying on metal antennas for phase manipulation suffer from strong Ohmic loss and incomplete polarization conversion. The efficiency is limited to rather small values when operating in transmission mode. Here, we implement a high-efficiency transmissive metasurface hologram by leveraging the recently developed Huygens’ metasurface to construct an electric and magnetic sheet with a transmission efficiency up to 86% and optical efficiency of 23.6%. The high-efficiency originates from the simultaneous excitations of the Mie-type electric and magnetic dipole resonances in the meta-atoms composed of silicon nanodisks. Our hologram shows high fidelity over a wide spectral range and promises to be an outstanding alternative for display applications.

Apparent Negative Reflection with the Gradient Acoustic Metasurface by Integrating Supercell Periodicity into the Generalized Law of Reflection

As the two dimensional version of the functional wavefront manipulation metamaterial, metasurface has become a research hot spot for engineering the wavefront at will with a subwavelength thickness. The wave scattered by the gradient metasurface, which is composed by the periodic supercells, is governed by the generalized Snell’s law. However, the critical angle that derived from the generalized Snell’s law circles the domain of the incident angles that allow the occurrence of the anomalous reflection and refraction, and no free space scattering waves could exist when the incident angle is beyond the critical angle. Here we theoretically demonstrate that apparent negative reflection can be realized by a gradient acoustic metasurface when the incident angle is beyond the critical angle. The underlying mechanism of the apparent negative reflection is understood as the higher order diffraction arising from the interaction between the local phase modulation and the non-local effects introduced by the supercell periodicity. The apparent negative reflection phenomena has been perfectly verified by the calculated scattered acoustic waves of the reflected gradient acoustic metasurface. This work may provide new freedom in designing functional acoustic signal modulation devices, such as acoustic isolator and acoustic illusion device.

Full-angle negative reflection realized by a gradient acoustic metasurface

We theoretically demonstrate that full-angle negative reflection can be realized by the gradient acoustic metasurface with a specific surface phase gradient value. A straightforward physical picture is presented here to understand such anomalous phenomena by considering the influence of the non-local effect that originates from the supercell periodicity on the gradient metasurface. Basing on the generalized law of reflection which is modified by a reciprocal lattice vector term, the negative reflection that beyond the critical angle is possible. In this paper, we utilize the coiling-up space structures of deep subwavelength geometrical scale to construct the desired gradient acoustic metasurface and observe the apparent full-angle negative reflection phenomenon. The present work enriches the content of the generalized law of reflection and provide new design methodology for functional acoustic wave modulation devices, such like directional ground acoustic cloaking and acoustic isolation devices.

High-efficiency beam manipulation combining geometric phase with anisotropic Huygens surface

Conventional geometric metasurfaces relying on space-variant metal antennas for beam manipulation suffer from strong Ohmic loss and incomplete polarization conversion. The efficiency is often limited to rather small values, especially when operating in transmission mode. Here, we tackle this challenge by deliberately constructing an equivalent sheet with anisotropic surface electric and magnetic polarizabilities using cross-shaped dielectric antennas. An incident circularly polarized light can be almost fully converted to a transmitted light of opposite helicity with an unprecedented efficiency up to 98%. Such a transmissive metasurface possessing the merits of high-efficiency, non-dispersion, and robust against variations can serve as an outstanding candidate for flat optics, such as anomalous refraction and beam focusing.

All-angle Negative Reflection with An Ultrathin Acoustic Gradient Metasurface: Floquet-Bloch Modes Perspective and Experimental Verification

Metasurface with gradient phase response offers new alternative for steering the propagation of waves. Conventional Snell’s law has been revised by taking the contribution of local phase gradient into account. However, the requirement of momentum matching along the metasurface sets its nontrivial beam manipulation functionality within a limited-angle incidence. In this work, we theoretically and experimentally demonstrate that the acoustic gradient metasurface supports the negative reflection for all-angle incidence. The mode expansion theory is developed to help understand how the gradient metasurface tailors the incident beams, and the all-angle negative reflection occurs when the first negative order Floquet-Bloch mode dominates inside the metasurface slab. The coiling-up space structures are utilized to build desired acoustic gradient metasurface, and the all-angle negative reflections have been perfectly verified by experimental measurements. Our work offers the Floquet-Bloch modes perspective for qualitatively understanding the reflection behaviors of the acoustic gradient metasurface, and the all-angle negative reflection characteristic possessed by acoustic gradient metasurface could enable a new degree of the acoustic wave manipulating and be applied in the functional diffractive acoustic elements, such as the all-angle acoustic back reflector.

Experimental realization of all-angle negative refraction in acoustic gradient metasurface

In this work, we numerically and experimentally demonstrate that all-angle negative refraction can be obtained with the acoustic gradient metasurface of subwavelength thickness. The coiling labyrinthine structures are utilized to build the desired gradient metasurface, and the apparent negative refraction occurring beyond the critical incident angle has been validated by simulations and experimental measurements, which agrees well with the theoretical predictions given by the revised generalized law of refraction while taking the contribution of the Bragg scattering into account. This work provides the solution to manipulate the acoustic waves and shows good promise in building functional diffractive acoustic elements.

Fano resonance based optical modulator reaching 85% modulation depth

In this paper, we demonstrate the combination of nematic liquid crystal with a binary silicon nanohole array to realize a high performance Fano resonance based optical modulator. The simulations using a finite difference time domain method reveal that the sharp Fano profile in the binary array originates from the interaction of the in-phased and anti-phased lattice collective resonance hybridized through lattice coupling effects. Experimental results agree very well with the simulations and demonstrate the strong dependence of the Q factor and spectral contrast of the resonance on the radius difference of the two nanohole arrays. Infiltrated with nematic liquid crystal, E7, the Fano profile can be dynamically and continuously tuned by an applied voltage, and an unprecedented modulation depth up to 85% is achieved.

Controllable asymmetric transmission via gap-tunable acoustic metasurface

In this work, we utilize the acoustic gradient metasurface (AGM) of a bilayer configuration to realize the controllable asymmetric transmission. Relying on the adjustable gap between the two composing layers, the metasurface could switch from symmetric transmission to asymmetric transmission at a certain gap value. The underlying mechanism is attributed to the interference between the forward diffracted waves scattered by the surface bound waves at two air-AGM interfaces, which is apparently influenced by the interlayer distance. We further utilize the hybrid acoustic elements to construct the desired gradient metasurface with a tunable gap and validate the controllable asymmetric transmission with full-wave simulations. Our work provides the solution for actively controlling the transmission property of an acoustic element, which shows potential application in acoustic communication as a dynamic tunable acoustic diode.