Helical acoustic fields open the way for several advanced functionalities including particle manipulation, the generation of optical beams with orbital angular momentum (OAM), polarization control of magnetic excitations as well as the exploitation of non-reciprocal and topological phenomena [1]. Helical acoustic fields have so far only been electrically excited at relatively low (i.e., sub-GHz) frequencies, which considerably limits their application scope.

In this contribution, we introduce a novel concept for the electrical excitation of laterally confined GHz helical acoustic modes in the form of surface acoustic (SAW) or Lamb bulk acoustic waves (plate modes, LBAW) on a chip [2,3]. The lateral confinement relies on the strong dependence of the frequency spectrum of the Lamb acoustic modes on the thickness of the propagating medium. As a result, waves generated by a piezoelectric resonator in a thicker (or thinner) region of the substrate remain confined via reflections at the lateral boundaries with thinner (thicker) substrate regions. If the generation area is disk-shaped, the lateral reflections form drum-like modes, which are like the low-frequency (sub-GHz) modes of a thin membrane but now oscillating at GHz frequencies. We experimentally confirm the lateral confinement by radio-frequency spectroscopy as well as by mapping of the surface displacements of the surface using scanning optical interferometry with a high spatial resolution of one m.

Helical drum modes with tunable helicity are generated by an array of sector-shaped piezoelectric transducers powered with appropriate radio-frequency phases. Design of these structures require careful consideration of the acoustic anisotropy of the substrate. Interferometric maps of the acoustic fields show that the OAM of the acoustic field can be transferred to an optical beam, thus providing a way to produce light with OAM modulated at GHz frequencies. Our finite-element and analytical models confirm that the confined Lamb modes with tunable helicity arise from the intrinsic elastic response of the structured sample and provide useful insights into the acousto-elastic coupling, thus guiding adaptation to other material systems.

The helical SAW and LBAW drum modes demonstrated here provide a flexible platform for acousto-optical chiral functionalities in the GHz frequency range. Applications for spin chiral control as well as for the efficient generation of optical beams with OAM will be discussed.

[1] X. Zhang et al., Comm. Phys. 1, https://doi.org/10.1038/s42005-018-0094-4 (2018)
[2] A. Pitanti et al., https://doi.org/10.48550/arXiv.2410.17877
[3] N. Ashuberkov et al., https://doi.org/10.48550/arXiv.2502.20899