  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.
