Brownian ratchets are of fundamental interest in fields from statistical physics to molecular motors. The realization of Brownian ratchets in engineered systems opens up the potential to harness thermal energy for directed motion, with applications in transport and sorting of nanoparticles. Implementations based on optical traps provide a high degree of tunability along with precise spatiotemporal control. Near-field optical methods provide particular flexibility and ease of on-chip integration with other microfluidic components. Here, we demonstrate the first all-optical, near-field Brownian ratchet. Our approach uses an asymmetrically patterned photonic crystal and yields an ultra-stable trap stiffness of 253.6 pN/nm-W, 100x greater than conventional optical tweezers. By modulating the laser power, optical ratcheting with transport speed of ~1 micron/s can be achieved, allowing a variety of dynamical lab-on-a-chip applications. The resulting transport speed matches well with the theoretical prediction.
KEYWORDS: Gold, Nanoparticles, Optical tweezers, Photon transport, Monte Carlo methods, Particles, Video, Optical micromanipulation, Current controlled current source
We demonstrate optical trapping of a periodic array of closely spaced gold nanoparticles. To achieve the experimental result, we considered competition between optical gradient forces and strong interparticle interactions. We achieve control of the gradient forces using a photonic-crystal template designed to create a periodic optical trapping potential. We modeled the interparticle interactions using a kinetic Monte Carlo approach. The results predict the formation of different particle superstructures (such as chains or filled-in arrays) depending on lattice constant and symmetry. Using the model prediction, we designed and demonstrated a template that allows trapping of a regular periodic array.
We calculate optical forces on colloidal particles over a photonic crystal slab. We show numerically that exciting a
guided resonance mode of the slab yields a resonantly-enhanced, attractive optical force. Optical forces in the lateral
direction result in a two-dimensionally periodic pattern of stable trapping positions. Trapping patterns can be
reconfigured by changing the wavelength or polarization of incident light. We study the dependence of optical forces on
particle size, particle dielectric constant, and photonic-crystal slab parameters. Finally, we describe the fabrication and
measurement of a photonic crystal slab with a Q ~ 370.
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