Pushing the limits of complete omnidirectional bandgaps in 2D nonsymmorphic single-phase phononic crystals

We expand the limits of complete omnidirectional bandgaps (neither P- nor S-waves can propagate) for high symmetry two-dimensional phononic crystal (PnC) designs. We reveal an extremely large 124% complete omnidirectional bandgap and demonstrate the possibility of creating custom, mechanically robust PnCs with improved characteristics via simple geometric changes to known PnC designs. The findings are experimentally validated, proving that it is feasible to achieve extreme ultrasonic attenuation using the nonsymmorphic p4gm symmetry group design, for both P- and S-waves, which significantly outperforms symmorphic p4mm-group PnC designs. We shed light on the high attenuation properties of the p4gm PnC for S-waves (which is rarely explored experimentally), revealing differences between the mode types. Practical insight into the design of PnCs with improved acoustic properties for potential applications in the field of vibration isolation, most notably when S-wave elimination is vital, is discussed.