Optical trapping of coated microspheres

V. Bormuth , A. Jannasch , M. Ander , C.M. van Kats , A. van Blaaderen , J. Howard , E. Schaeffer

Bibtex , URL
Opt. Express, 16, 18
Published 01 Sep. 2008
DOI: 10.1364/OE.16.013831
ISSN: 1094-4087


In an optical trap, micron-sized dielectric particles are held by a tightly focused laser beam. The optical force on the particle is composed of an attractive gradient force and a destabilizing scattering force. We hypothesized that using anti-reflection-coated microspheres would reduce scattering and lead to stronger trapping. We found that homogeneous silica and polystyrene microspheres had a sharp maximum trap stiffness at a diameter of around 800 nm-the trapping laser wavelength in water-and that a silica coating on a polystyrene microsphere was a substantial improvement for larger diameters. In addition, we noticed that homogeneous spheres of a correct size demonstrated anti-reflective properties. Our results quantitatively agreed with Mie scattering calculations and serve as a proof of principle. We used a DNA stretching experiment to confirm the large linear range in detection and force of the coated microspheres and performed a high-force motor protein assay. These measurements show that the surfaces of the coated microspheres are compatible with biophysical assays. (c) 2008 Optical Society of America.