Nanoscale pillar hypersonic surface phononic crystals

We report on nanoscale pillar-based hypersonic phononic crystals in single crystal Z-cut lithium niobate. The phononic crystal is formed by a two-dimensional periodic array of nearly cylindrical nanopillars 240 nm in diameter and 225 nm in height, arranged in a triangular lattice with a 300-nm lattice constant. The nanopillars are fabricated by the recently introduced nanodomain engineering via laser irradiation of patterned chrome followed by wet etching. Numerical simulations and direct measurements using Brillouin light scattering confirm the simultaneous existence of nonradiative complete surface phononic band gaps. The band gaps are found below the sound line at hypersonic frequencies in the range 2-7 GHz, formed from local resonances and Bragg scattering. These hypersonic structures are realized directly in the piezoelectric material lithium niobate enabling phonon manipulation at significantly higher frequencies than previously possible with this platform, opening new opportunities for many applications in plasmonic, optomechanic, microfluidic, and thermal engineering.Nanodomain engineering in this work was carried out at the Melbourne Centre for Nanofabrication (MCN) and the Micro-Nano Research Facility (MNRF) at RMIT Univeristy. The technique for nanodomain engineering was a scientific output of the ARC Centre of Excellence program (CE110001018). B.G., F.A., and C.M.S.T. acknowledge the partial support of the Spanish national projects PHENTOM (FIS2015-70862-P) and the Severo Ochoa program (Grant No. SEV-2013-0295) and the EU projects MERGING (Grant No. 309150).Peer Reviewe