Probing the interaction between two microspheres in a single Gaussian beam optical trap

Interactions between trapped microspheres have been studied in two geometries so far: (i) using line optical tweezers and (ii) in traps using two counter propagating laser beams. In both trap geometries, the stable inter bead separations have been attributed to optical binding. One could also trap two such beads in a single beam Gaussian laser trap. While there are reports that address this configuration through theoretical or simulation based treatments, there has so far been no detailed experimental work that measures the interactions. In this work, we have recorded simultaneously the fluctuation spectra of two beads trapped along the laser propagation direction in a single Gaussian beam trap by measuring the back scattered signal from the trapping and a tracking laser beam that are counter propagating . The backscattering from the trapping laser monitors the bead encountered earlier in the propagation path. The counter propagating tracking laser, on the other hand, is used to monitor the fluctuations of the second bead. Detection is by using quadrant photo detectors placed at either end. The autocorrelation functions of both beads reveal marked departures from that obtained when there is only one bead in the trap. Moreover, the fall-off profiles of the autocorrelation indicates the presence of more than one relaxation time. This indicates a method of detecting the presence of a second bead in a trap without directly carrying out measurements on it. Further, a careful analysis of the relaxation times could also reveal the nature of interactions between the beads. © (2016) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only

Trap Stiffness Modification of an Optically Trapped Microsphere through Directed Motion of Nanoparticles

We report an enhancement in the corner frequency of an optically trapped non-magnetic microsphere in the plane perpendicular to the laser propagation direction on addition of ferrofluid to the suspension medium. We conjecture that a directed motion of the nanoparticles toward the trap in this plane is responsible for the augmentation. Changes in the corner frequency in the presence of external magnetic field gradients lend credence to this conjecture. Corner frequency augmentation is also observed when zinc oxide nanoparticles are used. Here, however, no further changes are seen in the presence of magnetic field gradients

Reorientation Dynamics and Micromanipulation of Natural Microscopic Soft Matter in an Optical Trap with Varying Polarization of the Laser

We report here on the reorientation dynamics of the avian Red Blood Cell (aRBC), a structure ellipsoidal in shape and with a non-uniform distribution of birefringence across its diameter. We find that in linearly polarized light, an aRBC shows a dual reorientation behavior with the first reorientation about the major axis and the second about the minor axis so as to align its major axis along the laser propagation direction. We are able to explain the observed sequence of reorientation as a consequence of the minimization of work done in rotating the avian cell first about the major axis and then about the minor axis. These calculations are also used to also show why a single reorientation about the minor axis to reach the final equilibrium position is not seen experimentally. Further, in the case of elliptically polarized light, we see that the second reorientation process about the minor axis of the cell can be controlled by the ellipticity of the polarization of light. We explain this as a consequence of balance between the torques due to birefringence and reorientation