Three-dimensional force calibration of a single-beam optical gradient trap

We have used a high-resolution back-focal-plane detector to make simultaneous measurements of the axial and lateral force constants of a micron-sized particle in a single-beam optical gradient trap. We measure the optical force applied to the trapped particle as a function of the relative index of refraction of particle and medium, the radius of the particle, and the laser beam power. Results are compared with recent theoretical predictions.