GHz helical acoustic drum modes on a chip

On-chip laterally confined GHz acoustic modes with tunable helicity open the way for advanced optomechanical functionalities. Here, we demonstrate a novel concept for the implementation of these functionalites through the electrical excitation of GHz membrane-like drum modes. Our concept relies on the strong dependence of the frequency spectrum of Lamb acoustic modes on the thickness of the propagating medium. Lamb modes generated by a piezoelectric resonator in a thicker (or thinner) substrate region remain confined in this region via reflections at the lateral boundaries with thinner (thicker) substrate regions. If the generation region is disk-shaped, the lateral reflections form drum-like modes, which we experimentally confirm by radio-frequency spectroscopy as well as by maps of the surface displacements. Furthermore, we show that an array of sector shaped piezoelectric transducers powered with appropriate radio-frequency phases creates helical modes with tunable helicity, which can then be transferred to an optical beam. Our analytical and finite-element models yield useful insights into the acoustic coupling of bulk and surface modes that can guide adaptation to other material systems. The acoustic drum modes thus provide a flexible platform for acousto-optical chiral functionalities in the GHz frequency range.