Orbital and spin photon angular momentum transfer in liquid crystals

All-optical angular control of the molecular alignment in liquid-crystal fi lms is demonstrated using a laser bean having an elliptically shaped intens ity profile. The material birefringence is unimportant, as proven by the fa ct that good alignment is obtained with unpolarized light. This raises the possibility of achieving optical angular control of transparent isotropic b odies. A general theoretical approach, based on light and matter angular mo mentum conservation, shows that the optical alignment is due to the interna l compensation between the transfer of the orbital and the spin part of ang ular momentum of the incident photons to the material.