Active Acoustic Metasurface: Complete Elimination of Grating Lobes for High-Quality Ultrasound Focusing and Controllable Steering

Eliminating the grating lobes is significant in ultrasonics. For example, highly efficient ultrasound focus and flexible focus steering without off-target heating for medical applications require the total rejection of grating lobes and side lobes. We investigate analytically, numerically, and experimentally the physical causes of grating lobes in ultrasound beam engineering. We find that the generation of grating lobes is attributed to two main factors, namely, the diffraction from the finite-sized source array and the different phase delays of the phased array. Based on these findings, we design and implement an active acoustic metasurface to completely eliminate the grating lobes. The lab-made metasurface comprises 16x16 square-lattice elements of subwavelength thickness, where each element is a supercell with 4x4 piezoelectric sheets in a matrix of epoxy resin. By tailoring the ratio of the supercell width over the lattice constant, the grating lobes due to the structure diffraction can be completely suppressed. With a programmable multichannel drive, we further impose Gaussian-shaped amplitude modulation on the metasurface to wipe out the grating lobes induced by inhomogeneous phase delays. Our experimental results convincingly demonstrate that the active metasurface with programmable phase and amplitude modulations shows a high performance in grating-lobe-proof and steerable ultrasound focusing.