Optical trapping of micron-size dielectric microspheres using a single beam gradient force [Fig. 1(a)] was first demonstrated by Ashkin1 in 1986. Since then, extensive research and development of this technique have turned it into a practical device (known as optical tweezers), which has been used in a wide variety of biological and biomedical applications.2'3 Based on the ray-optics analysis of various force components, it is widely accepted that for microspheres in the Mie scattering regime (i.e., diameter >> wavelength) to be trapped by a single-beam trap, a strongly focused beam with a numerical aperture (NA) of 0.5 or larger is required.4 In research as well as in commercial devices, a water-immersed or oil- immersed microscope objective lens with a high numerical aperture (NA ~ 1.25) is usually used to achieve a sufficiently strong axial trapping force. This imposes some restriction on the geometry of the sample cell, because the working distance of a high NA objective lens is limited to a few tens of microns below a cover glass, which is matched to the objective lens. Recently, we have observed optical trapping of micron-size dielectric particles using a laser beam (514.5 nm, ~5 to 10 mW) focused by an objective lens with NA ranging from 0.1 to 0.85, including NA of 0.1, 0.25, and 0.4. Whereas the theoretical explanation of single-beam trapping using a focused laser beam with NA much small than 0.5 requires further study (and is beyond the scope of this paper), the possibility of using a lower NA (and smaller magnification power) allows experimental investigations in a different regime where other optical techniques can be incorporated. In this paper, we report the first experimental observation, we believe, of trapping and manipulation of dielectric particles by a set of two-beam interference fringes [Fig. 1(b)] using a 20× objective lens (NA = 0.4).

© 1996 Optical Society of America

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