Optically induced rotation of dissymmetrically shaped fluorinated polyimide micro-objects in optical traps

Fluorinated polyimide micro-objects (6-7.5 μm cross-sectional radius) fabricated using reactive ion etching have been both optically trapped and simultaneously rotated in both high and low relative-refractive index surrounding media. Symmetrical micro-objects with a low relative-refractive index were optically trapped by exerting optical radiation pressure through their center openings by using a strongly focused trapping laser beam. Micro-objects were both trapped and rotated by the radiation pressure when the horizontal cross sections of these objects showed dissymmetry (that is, not bilateral but rotational symmetry). In the case of micro-objects with a high relative-refractive index, the pressure is exerted on the outer walls. For objects with a low relative-refractive index, the pressure is exerted on the inner walls. The rotation speed versus optical power (typically, 0.4-0.7 rpm/mW) and the axial position of the laser focal point were investigated for high relative-refractive index micro-objects. The optically induced torque generated by a TEM*₀₁ (doughnut)-mode laser beam was found to be greater than generated by a TEM₀₀-mode laser beam.