Autocalibrated colloidal interaction measurements with extended optical traps.

We describe an efficient technique for measuring the effective interaction potential for pairs of colloidal particles. The particles to be tested are confined in an extended optical trap, also known as a line tweezer, that is projected with the holographic optical trapping technique. Their diffusion along the line reflects not only their intrinsic interactions with each other, but also the influence of the line's potential energy landscape and interparticle interactions mediated by scattered light. We demonstrate that measurements of the particles' trajectories at just two laser powers can be used to correct explicitly for optically induced forces and that statistically optimal analysis for optically induced forces yields autocalibrated measurements of the particles' intrinsic interactions with remarkably few statistically independent measurements of the particles' separation.