Tracking of optic axis with an acousto-optically diffracted beam: generation of vector-vortex beam in triglycine sulfate crystals

We analyze acousto-optic interactions for the case when a divergent diffracted optical beam propagates along one of the optic axes in biaxial crystals. We demonstrate that the diffracted beam reveals a specific spatial distribution of polarization states and transfers an optical vector-vortex field. It can be referred to as an optical vector-vortex beam with inseparable polarization and spatial states. At least intra-system entanglement occurs between different degrees of freedom, which are represented by the spin and orbit angular momentums in our case. The spatial orientation of the beam can be controlled while changing the angle between the optic axes. This is achieved by tuning the wavelength of optical radiation and the acoustic wave frequency. We have performed basic calculations using a particular example of optically biaxial triglycine sulfate crystals and assuming that the initial acoustic wave frequency amounts to 50 MHz and the light wavelength changes from 400 to 675 nm. Then the resulting acoustic frequency can increase up to 133 MHz for the longitudinal acoustic mode and 93 MHz or 89 MHz for the two transverse modes. In other words, one can build a generator of optical vortices and operate their spatial orientation, using triglycine sulfate.