Title: Full-angle Negative Reflection with An Ultrathin Acoustic Gradient Metasurface: Floquet-Bloch Modes Perspective and Experimental Verification,
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 full-angle incidence. The mode
expansion theory is developed to help understand how the gradient metasurface
tailors the incident beams, and the full-angle negative reflection occurs when
the first negative order Floquet-Bloch mode dominates. The coiling-up space
structures are utilized to build desired acoustic gradient metasurface and the
full-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 enables a new degree of the acoustic wave manipulating.