We have demonstrated optically induced rotation of microscopic dielectric
particles in air. The particles were anisotropically shaped, and were simultaneously
trapped three-dimensionally and rotated about the beam axis, which depended
solely on the radiation pressure of an extremely focused laser light. It was observed
that the rotational speed was linearly dependent on the irradiated beam power and
the slope of the fitting lines revealed up to 860 rpm/mW for 3.0 µm-diameter-particles
and 540 rpm/mW for 4.0 µm-diameter-particles, which was much higher than the
previously reported values measured in water. This technique will be useful for
micromotors and microfans assembled in microelectromechanical systems.
We developed a computational code to calculate the forces of a single-beam gradient-force optical trap exerted on dielectric ellipsoidal particles in the geometric-optics regime. Using this code, the axial and the transverse trapping stability of spheroidal particles, the semi-major axis of which is perpendicular (type-A) and parallel (type-B) to an incident beam axis, was evaluated and the effects of the nonspherical geometry of the particles were analyzed. As the fractional deformation ratio increased, the axial trapping stability improved for type-B particles, whereas it degraded for type-A particles. It can, therefore, be concluded that type-B particles can be trapped more stably than type-A particles. It was also observed that the axial trapping stability can be improved by the use of a TEM01
*-mode beam instead of a TEM00-mode beam.
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