Optically induced angular alignment of trapped birefringent micro-objects by linearly polarized light

Optically induced mechanical angular alignment of trapped birefringent micrometer-sized objects resulting from the transfer of angular momentum produced by birefringence using linearly polarized light has been experimentally demonstrated. Fluorinated polyimide (PMDA/TFDB) micro-objects having a large birefringence of $\ensuremath{\Delta}{n=n}_{\mathrm{slow}}\ensuremath{-}{n}_{\mathrm{fast}}=0.13$ (refractive indices ${n}_{\mathrm{slow}}=1.62,$ ${n}_{\mathrm{fast}}=1.49),$ which were fabricated by micromachining (reactive ion etching) and suspended in water $(n=1.33),$ were trapped and manipulated by radiation pressure from a single focused Gaussian beam (wavelength $\ensuremath{\lambda}=1.064\ensuremath{\mu}\mathrm{m},$ $\mathrm{power}g130\ensuremath{\mu}\mathrm{W}).$ The effect of the linearly polarized light on such micro-objects showed that they were angularly aligned about the laser beam axis and their angular position could be smoothly controlled by rotating the vibration plane of the electric field. We have shown that the retardation of the birefringent micro-object determines which axis (fast or slow) of the micro-object coincides with the vibration plane of the electric field of the incident light.