High-bandwidth microrheology of complex fluids and biopolymer networks

Correlated motions of two micron‐sized particles can be conveniently detected using two optical traps with interferometric displacement detection. When the correlations become small, the elimination of crosstalk between the two beams becomes important. We have used dual optical traps created by either two orthogonally polarized laser beams derived from one laser source, or by two independent lasers of different wavelengths. High NA lenses (objective and condenser) in the optical path introduce depolarization, and thereby lead to optical cross‐talk. We have characterized the cross‐talk in this case and demonstrate that the use of two independent laser eliminates cross‐talk entirely. Part I: One‐ and Two‐Particle Microrheology with Optical Tweezers