Method for GHz optical helicity modulation by 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 electrical generation of GHz membrane-like helical drum modes on a chip, and propose their application to the future generation of optical beams with tunable orbital angular momentum (OAM). Our concept relies on the strong dependence of the frequency spectrum of Lamb-like acoustic modes on the thickness of the propagating medium. 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 with radio-frequency spectroscopy and interferometric maps of the surface displacements. Furthermore, we show that an array of sector-shaped piezoelectric transducers powered with appropriate radio-frequency phases creates acoustic vortices with tunable OAM polarity, which we transfer 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. These acoustic drum modes provide a flexible platform for acousto-optical chiral functionalities in the GHz frequency range.