  The recently proposed bianisotropic acoustic metagratings offer promising
opportunities for passive acoustic wavefront manipulation, which is of
particular interest in flat acoustic lenses and ultrasound imaging at
ultra-high frequency ultrasound. Despite this fact, acoustic metagratings have
never been scaled to MHz frequencies that are common in ultrasound imaging. One
of the greatest challenges is the production of complex structures of
microscopic size. Owing to two-photon polymerization, a novel fabrication
technique from the view of acoustic metamaterials, it is now possible to
precisely manufacture sub-wavelength structures in this frequency range.
However, shrinking in size poses another challenge; the increasing
thermoviscous effects lead to considerable losses, which must be taken into
account in the design. In this work we propose three microacoustic metagrating
designs refracting a normally incident wave towards -35$^{\circ}$ at 2 MHz. In
order to develop metaatoms insensitive to thermoviscous effects we use shape
optimization techniques incorporating the linearized Navier-Stokes equations
discretized with finite element method. We report for the first time
microscopic acoustic metamaterials manufactured using two-photon polymerization
and, subsequently, experimentally verify their effectively using a capacitive
micromachined ultrasonic transducer as source and an optical microphone as a
detector in a range from 1.8 MHz to 2.2 MHz. We demonstrate not just that a
microacoustic metagrating can effectively redirect the normally incident wave
despite the thermoviscous losses, but also that it being only 0.29$\lambda$
thick can allocate 90% of the transmitted energy in the $-$1st diffraction
order.

Applied Physics;Fluid Dynamics