title: Determination of the force constant of a single-beam gradient trap by measurement of backscattered light.
abstract: A single-beam gradient trap could potentially be used to hold a stylus for scanning force microscopy. With a view to development of this technique, we modeled the optical trap as a harmonic oscillator and therefore characterized it by its force constant. We measured force constants and resonant frequencies for 1-4-μm-diameter polystyrene spheres in a single-beam gradient trap using measurements of backscattered light. Force constants were determined with both Gaussian and doughnut laser modes, with powers of 3 and 1 mW, respectively. Typical values for spring constants were measured to be between 10(-6) and 4 × 10(-6) N/m. The resonant frequencies of trapped particles were measured to be between 1 and 10 kHz, and the rms amplitudes of oscillations were estimated to be around 40 nm. Our results confirm that the use of the doughnut mode for single-beam trapping is more efficient in the axial direction.
authors.forename: M E
authors.initials: ME
authors.lastname: Friese
authors.forename: H
authors.initials: H
authors.lastname: Rubinsztein-Dunlop
authors.forename: N R
authors.initials: NR
authors.lastname: Heckenberg
authors.forename: E W
authors.initials: EW
authors.lastname: Dearden