Optical trapping of low-refractive-index microfabricated objects using radiation pressure exerted on their inner walls

Optical trapping of transparent ring-shaped micrometer-sized objects with a refractive index lower than that of the surrounding medium has been demonstrated by use of a strongly focused TEM00-mode laser beam. Axial trapping of these objects is a result of the upward radiation pressure induced when the incident light strikes the inner wall and transmitted light leaves from the bottom. Transverse trapping of these objects occurs because the total repulsive radiation pressure exerted on the inner wall of the object is directed toward the laser beam axis. Three-dimensional manipulation of fluorinated polyimide micro-objects (refractive index n1 = 1.525, outer diameter 10 μm, inner diameter 5 μm, and thickness 5.7 μm) in a high-refractive-index liquid (n2 = 1.605) was experimentally shown to be possible by use of an objective lens with a wide range of numerical apertures from 1.25 to 0.5.