Controlled transfer of transverse orbital angular momentum to optically trapped birefringent microparticles

The interaction between structured light beams possessing optical angular momentum and small particles promises new opportunities for optical manipulation, such as the generation of light-induced torque and rotation of objects. However, so far, studies have largely centred on nanoscale particles. Here we report the observation and measurement of the transfer of transverse angular momentum to birefringent spherical vaterite particles several wavelengths in size. We outline the physics behind the beam used to control the particles, perform quantitative measurements of the transverse spin angular momentum transfer and demonstrate the generation of fluid flow around multiple rotation axes. The findings show that light can impart controllable rotational degrees of freedom to microparticles. In the future, the approach may prove useful for investigating the dynamics of complex fluids in three dimensions, studying the shear force on cell monolayers or cooling an optically trapped particle to the quantum ground state. The angular momentum of light is shown to be able to impart light-induced transverse torque and rotation to microscale birefingent particles.