Orbital Angular Momentum Reversal and Asymmetry in Acoustic Vortex Beam Reflection

We present acoustic modeling, measurements, and interpretation of the angular momentum carried in an ultrasonic vortex beam that is obliquely reflected off a flat water-air interface. The experimental measurements observe the theoretically predicted reversals of phase rotation, topological charge, and orbital angular momentum in a reflected vortex beam in direct analogy to optical phenomena. The spatial and temporal evolution of the linear and angular momentum during the reflection are determined by calculating the velocity field from two-dimensional scanned pressure fields. A conversion of the angular momentum indicates a radiation torque along the oblique reflecting surface. We understand this radiation torque originates from the break of rotational symmetry with respect to the incident plane for normal components of the energy flux and linear momentum density at the reflecting surface. Our study provides mechanical evidence on the effect of a flat surface on the reflection of vortex beams and gains insight into the underlying physics, impacting non-contact manipulation of objects and communication.