Abstract
We present a new method of optical trapping based on the intensity gradient that is created by boundary diffraction of light at a metal thin-film edge. The structure consists of an optically thick metal-film step formed on a semi-transparent thin-film-metal-coated glass substrate. While the underlying thin layer of metal serves the purpose of suppressing the thermophoretic effect, the metal film step is found to induce a highly localized intensity distribution of light around the edge via self-interference of an incident wave and its boundary diffraction wave. Two-dimensional (2D) optical trapping of micron-sized dielectric particles is experimentally demonstrated with a 100-nm-thick Au film edge formed on a 10-nm-thick-Cr-coated glass slide. For a 2-µm polystyrene sphere, ∼2-pN trapping force is measured at 30-mW incident power of a 1064-nm laser beam. Not involving surface plasmon fields, this thin-film edge trapping is polarization independent and can be easily incorporated into an on-chip microfluidic configuration.
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