Holographic optical trapping of core-shell colloidal particles

We built and optimized a holographic optical tweezers setup for simultaneously trapping multiple micrometer-sized colloidal particles in focused laser beams in order to measure their interactions. In holographic optical tweezers, a spatial light modulator (SLM) modifies the incident laser beam, enabling the creation of multiple traps that need not lie in the same plane and the real-time manipulation of those traps. Our system consists of a 1064-nm-wavelength laser, a 60x, 1.2 NA microscope objective, and an SLM capable of updates at 60 Hz. We calibrated the trap stiffness by observing the Brownian fluctuations of optically-trapped particles. We successfully trapped two silica spheres in separate optical traps, each occupying a distinct position in all three dimensions. We then trapped 850-nm-diameter core-shell particles consisting of a polystyrene core and a transparent, thermoresponsive poly-N-isopropylacrylamide shell. Due to their weak scattering, we managed to hold two of these particles in separate traps by employing a time-sharing scheme in which we rapidly shifted the position of the laser between two locations.