Generation of Phonons with Out-of-Plane Angular Momentum by Superposition of Longitudinal Surface Acoustic Phonons

Circularly polarized phonons carry angular momentum, and surface acoustic waves (SAWs) offer an ideal platform for manipulating the electron spins in magnetic thin films. However, achieving phonons with out-of-plane angular momentum using easily controllable SAWs has been challenging. In this study, we demonstrate a device that superposes two longitudinal leaky SAWs (LL-SAWs) to generate spatially distributed out-of-plane angular momentum in surface acoustic phonons. By using an optical imaging setup, we measure the surface tilt associated with LL-SAWs and confirm the interference pattern resulting from their superposition. This approach to generating phonon angular momentum through superposition enables the creation of spatially distributed phonon angular momentum with easily switchable polarization, opening new avenues for phonon-based applications in spintronics.