Abstract
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
PDF ArticleMore Like This
N. Malagnino, G. Pesce, A. Sasso, and E. Arimondo
4434_150 European Conference on Biomedical Optics (ECBO) 2001
Qiwen Zhan
ThE2 Frontiers in Optics (FiO) 2003
M. J. Kendrick, D. H. McIntyre, and O. Ostroverkhova
JWA122 Conference on Lasers and Electro-Optics (CLEO:S&I) 2008