Direct Mechanical Detection and Measurement of Wave-Matter Orbital Angular Momentum Transfer by Nondissipative Vortex Mode Conversion

We quantitatively report on the rotational mechanical effect of wave orbital angular momentum on matter by nondissipative vortex mode conversion. Our experiments consist of ultrasonic waves reflected off freely spinning helical acoustic mirrors that are capillary trapped at a curved air-water interface. Considering helical mirrors with integer topological charges these results represent the demonstration of the experiment proposed by Allen et al. originally introduced in the optical domain [Phys. Rev. A 45, 8185 (1992)PLRAAN1050-294710.1103/PhysRevA.45.8185], whose quantitative implementation remains elusive to date whatever the nature of the wave. The study is further generalized to helical mirrors with fractional charges.