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Microacoustic metagratings at ultra-high frequencies fabricated by two-photon lithography

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$λ$ thick can allocate 90% of the transmitted energy in the $-$1st diffraction order.