Scanning-force microscope based on an optical trap

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Abstract
An optically trapped prolate glass stylus is the force-sensing element of a novel scanning-force microscope. Stylus displacement is detected with the use of the forward scatter of the trapping laser beam. Radiation forces owing to the three-dimensional intensity distribution near the focus permit the stylus to be both positioned with fine control and oriented along the z (optic) axis. Details of 20-nm size appear in traces recorded with a crude stylus in a trap formed with 1064-nm radiation. The spring constant of the optical-force transducer is below 10−4 N/m, which is to be compared with ∼0.1 N/m for typical mechanical cantilevers used in atomic-force microscopy. This gentler technique should improve the sensitivity of scanning-force microscopy for the imaging of soft samples in aqueous media.