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.
Deep Dive into 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 mode
All-angle Negative Reflection with An Ultrathin Acoustic Gradient
Metasurface: Floquet-Bloch Modes Perspective and Experimental
Verification
Bingyi Liu1, Jiajun Zhao2, Xiaodong Xu1, Wenyu Zhao1,3 and Yongyuan
Jiang1,4,5,6,*
1 Institute of Modern Optics, Department of Physics, Harbin Institute of Technology,
Harbin 150001, China
2 GOWell International LLC, Houston, Texas 77041, United States
3 Department of Physics, University of California at Berkeley, Berkeley, California
94720, United States
4 Key Lab of Micro-Optics and Photonic Technology of Heilongjiang Province,
Harbin 150001, China
5 Key Laboratory of Micro-Nano Optoelectronic Information System of Ministry of
Industry and Information Technology, Harbin 150001, China
6 Collaborative Innovation Center of Extreme Optics, Taiyuan 030006, Shanxi,
People’s Republic of China
*Corresponding author: jiangyy@hit.edu.cn
Abstract
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.
Metasurfaces, the quasi 2D metamaterials composed by elaborately arranged artificial
scatters of subwavelength geometrical size, have shown powerful wavefront
manipulation capabilities over the past few years. Based on the idea of sampling the
desired wavefront and replacing the pixels with appropriate nanostructures1,2, which
exactly behave like the secondary sources proposed in Huygens’ principle, optical
metasurfaces constructed with plasmonic nano-antenna of various geometrical shape or
low loss dielectric nano-post have been experimentally demonstrated to function as the
ultrathin planar lenses3,4, holograms5,6 and low profile conformal optical devices7-9. As
another important form of classical wave, acoustic wave can also be flexibly tailored
by deep subwavelength diffraction inclusions, which is known as the acoustic
metasurfaces10-12. Following the strategy that tunes the effective refractive index by
coiling–up spaces13-17, patterning the surface impedance profiles of metasurfaces18,19 or
taking advantage of low/high quality factor resonators20-22, numerous types of acoustic
metasurface structures have been utilized to build up functional acoustic lens10,16,23,24,
acoustic vortices generator25,26, acoustic wave absorber27,28 and ultrathin acoustic
ground cloak29,30.
Gradient metasurface composed by periodic supercells has been well studied for it
steers the incident waves in the anomalous way governed by the generalized Snell’s
law1,31-33. The nature of momentum matching dictates that the contribution of local
phase gradient cannot be ignored when the incident beam encounters the gradient
metasurface. Therefore, the incident beam would be deflected asymmetrically under the
all-angle illumination. Furthermore, when the beam is incident beyond the critical angle,
no free space scattered fields exist, and only the non-propagating evanescent field,
which is termed as the surface bounded waves12,31, can be obtained. Notwithstanding
that, recently, several research articles have reported that the apparent free-space
propagating scattered field, which is featured as negative reflection34,35 and negative
refraction12,36,37, can be observed even when the beam illuminates beyond the critical
angle. These intriguing phenomena indicate that the underlying physical mechanism of
the beam manipulation functionality of the acoustic gradient metasurface is still needed
to be studied.
Similar to the electromagnetic gradient metasurface31,38, in this work, we develop
the mode expansion theory to help study and understand the mechanism of the
extraordinary negative reflection behaviors of the acoustic gradient metasurface when
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