Date/Time: 04-23-2019 - Tuesday - 05:00 PM - 07:00 PM
Ziqi Yu1 Ren Ren1 Jaeho Lee1

1, University of California, Irvine, Irvine, California, United States

Since the discovery of the Quantum Spin Hall Effect, electronic and photonic topological insulators have made substantial progress, but phononic topological insulators in solids have received relatively little attention due to challenges in realizing topological states without spin-like degrees of freedom and with transverse phonon polarizations. Here we present a holey silicon-based phononic topological insulator design, in which simple geometric control enables topologically protected in-plane elastic wave propagation up to GHz ranges when the unit cell reaches submicron scales. By integrating a hexagonal lattice of six small holes with one large hole in the center and by creating a hexagonal lattice by themselves, the six-petal holey silicon, which has C6 symmetry, induces zone folding to form a double Dirac cone. Based on the hole dimensions, breaking the discrete translational symmetry allows the six-petal holey silicon to achieve the topological phase transition, yielding two topologically distinct phononic crystals. Based on the unit cell periodicity, the transition readily shifts from low- to high-frequency ranges. Our numerical simulations confirm inverted band structures and show backscattering-immune elastic wave transmission up to 90 % at 14.83 GHz through defects including a cavity, a disorder, and sharp bends when the unit cell periodicity is 500√3 nm. The six-petal holey silicon design also offers robustness against geometric errors and potential fabrication issues such as over- or under-etching. The simulations of the six-petal holey silicon with the same periodicity show up to 90 % transmission of elastic waves at 13.8 and 15.37 GHz even when the holes are under-sized by 5 % or over-sized by 2.5 %, respectively, in which the shift of bandgap is led by the change of porosity. These findings provide a detailed understanding of the relationship between geometry and topological properties and pave the way for developing high-frequency phononic topological insulators and future phononic circuits.

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