Total optical torque and angular momentum conservation in dye-doped liquid crystal droplets spun by circularly polarized light

Micrometric droplets of nematic liquid crystal dispersed in water were trapped and made to spin in the focus of a circularly polarized laser beam. The measured rotation speed is directly proportional to the angular momentum exchanged per unit time by the droplet with the electromagnetic field, that is to the total optical torque acting on the droplet. We searched for a possible enhancement of this total torque when the liquid crystal of the droplets was doped with a light-absorbing dye. In contrast to the local optical torque acting on the molecular director, which is known to be strongly enhanced by the presence of the dye, the total torque acting on the droplet as a whole did not show any enhancement. This null result is in agreement with the leading theory of the dye-assisted (or photoinduced) optical torque, which postulates that this torque is associated with an internal angular momentum exchange between different material degrees of freedom, namely orientational and translational. Moreover, our findings confirm the predicted existence of a photoinduced stress-tensor acting on the dye-doped liquid crystal.