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.