H. He et al., DIRECT OBSERVATION OF TRANSFER OF ANGULAR-MOMENTUM TO ABSORPTIVE PARTICLES FROM A LASER-BEAM WITH A PHASE SINGULARITY, Physical review letters, 75(5), 1995, pp. 826-829
Black or reflective particles can be trapped in the dark central minim
um of a doughnut laser beam produced using a high efficiency computer
generated hologram. Such beams carry angular momentum due to the helic
al wave-front structure associated with the central phase singularity
even when linearly polarized. Trapped absorptive particles spin due to
absorption of this angular momentum transferred from the singularity
beam. The direction of spin can be reversed by changing the sign of th
e singularity.