  We have designed and experimentally realized an ultra-broadband acoustic
metasurface (UBAM) capable of going beyond the intrinsic limitation of
bandwidth in existing designs of optical/acoustical metasurfaces. Both the
numerical and experimental results demonstrate that the UBAM made of
subwavelength gratings can manipulate the reflected phase-front within a
bandwidth larger than 2 octaves. A simple physical model based on the phased
array theory is developed for interpreting this extraordinary phenomenon as
well as obtaining deeper insight to the underlying physics of our design. We
anticipate the UBAM to open new avenue to the design and application of
broadband acoustical devices.
