A single-beam gradient-force optical trap for dielectric particles, which relies solely on the radiation pressure force of a TEM(00)-mode laser light, is demonstrated in air for what is believed to be the first time. It was observed that micrometer-sized glass spheres with a refractive index of n=1.45 remained trapped in the focus region for more than 30 min, and we could transfer them three dimensionally by moving the beam focus and the microscope stage. A laser power of ~40 mW was sufficient to trap a 5- microm -diameter glass sphere. The present method has several distinct advantages over the conventional optical levitation method.

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 propose the use of egg-shaped asymmetric resonant cavities (ARCs), each of which consists of a half-circular part and a half-deformed part, as promising candidates in obtaining desirable whispering-gallery-mode resonances. According to numerical analysis based on a ray-optics model, more than an order-of-magnitude higher Q and more-concentrated emission from the tip of the egg region were obtained for egg-shaped ARCs than for the previously studied quadrupolar ARCs.

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.