3D characterization of the forces in optical traps based on counter-propagating beams shaped by a spatial light modulator

An experimental characterization of the 3D forces, acting on a trapped polystyrene bead in a counter-propagating beam geometry, is reported. Using a single optical trap with a large working distance (in the BioPhotonics Workstation), we simultaneously measure the transverse and longitudinal trapping force constants. Two different methods were used: The Drag force method and the Equipartition method. We show that the counter-propagating beams traps are simple harmonic for small displacements. The force constants reveal a transverse asymmetry as kappa(-) = 9.7 pN/mu m and kappa(+) = 11.3 pN/mu m (at a total laser power of 2x35 mW) for displacements in opposite directions. The Equipartition method is limited by mechanical noise and is shown to be applicable only when the total laser power in a single 10 mu m counter-propagating trap is below 2x20 mW.