Twin optical traps for two-particle cross-correlation measurements: Eliminating cross-talk

Abstract

The correlated motions of two micron-sized particles reflect the ͑micro-͒ rheological properties of a fluid and can be conveniently detected using two optical traps in combination with interferometric displacement detection. When the correlations become small, cross-talk 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 for microrheology experiments. High numerical aperture lenses ͑objective and condenser͒ in the optical path can introduce depolarization, and polarizing beam splitters are not perfect, both of which can lead to optical cross-talk. We have characterized the cross-talk in our setup and demonstrate that the use of two independent laser eliminates cross-talk entirely.

Light at work: The use of optical forces for particle manipulation, sorting, and analysis

“…The utility of microrheology to probe nanoscale structure and response in colloid-polymer suspensions was demonstrated by Chen et al who employed one-and two-point microrheology to study the depletion layer surrounding the probe particle [319]. In order to eliminate crosstalk between the trapping beams in two-bead microrheology, Atakhorrami et al employed two independent lasers of different wavelengths [320].A rich field of its own is the microrheology of living cells. The cell cytoskeleton is a highly dynamical system of biopolymers and auxiliary crosslinking proteins with rich kinetics of the crosslinkers and the polymerization-depolymerization cycles of the filaments themselves.…”

Measuring the accuracy of particle position and force in optical tweezers using high-speed video microscopy

“…However, it is difficult to completely isolate the trap from environmental sources of noise such as laser pointing stability and thermal drift of the microscope alignment. Various methods have been implemented to reduce such noise and include the development of dual trap techniques [12,13,14] where a second trapped particle is measured simultaneously. Since the drift affects both particles it can be eliminated from the data.…”

Scale-Dependent Nonaffine Elasticity of Semiflexible Polymer Networks

“…The squeezing flow leads to shear strain in the sample with an amplitude of ~10 -6 , ensuring linear elastic response. Microrheology measurements were performed with a custom-built dual optical tweezers set-up [25].…”