Resolving interparticle position and optical forces along the axial direction using optical coherence gating

In this paper, we demonstrate the use of coherence gating to resolve particle positions and forces in the axial direction. Through coherence gating, particle displacements and interparticle separations can be resolved with a high signal-to-noise ratio. We achieved both high depth resolvability (10-6 m) and weak optical force (10-15 N) measurements in an optical trapping system using a low coherence interferometry system. Trap stiffness as low as 1.46 fN μ m-1 was measured. This technique is well-suited for the direct visualization of interparticle optical-mechanical interactions.