Angular-momentum exchange among acoustic and optical waves at the collinear acousto-optic diffraction

We consider collinear acousto-optic diffraction of circularly polarized optical waves at circularly polarized acoustic waves in the crystals that reveal both acoustic conical refraction and optical activity. We find that, due to the conical refraction of acoustic waves, these waves produce vector-vortex beam. In this process, the orbital angular momentum (OAM) of the conically refracted acoustic wave is transformed into the OAM of the diffracted optical wave. At the same time, the spin angular momentum of conically refracted phonons is transferred to diffracted photons. The possibility of phonon-photon entanglement is discussed in the work. (paper)