Optical tweezers have been widely used to manipulate biological and colloidal material, but the diffraction limit of far-field optics makes focused beams unsuitable for manipulating nanoscale objects with dimensions much smaller than the wavelength of light. While plasmonic structures have recently been successful in trapping nanoscale objects with high positioning accuracy, using such structures for manipulation over longer range has remained a significant challenge. In this work, we introduce a conveyor belt design based on a novel plasmonic structure, the resonant C-shaped engraving (CSE). We show how long-range manipulation is made possible by means of handoff between neighboring CSEs, and we present a simple technique for controlling handoff by rotating the polarization of laser illumination. We experimentally demonstrate handoff between a pair of CSEs for polystyrene spheres 200, 390, and 500 nm in diameter. We then extend this technique and demonstrate controlled particle transport down a 4.5 μm long "nano-optical conveyor belt."

We propose a method for peristaltic transport of nanoparticles using the optical force field over a nanostructured surface. Nanostructures may be designed to produce strong near-field hot spots when illuminated. The hot spots function as optical traps, separately addressable by their resonant wavelengths and polarizations. By activating closely packed traps sequentially, nanoparticles may be handed off between adjacent traps in a peristaltic fashion. A linear repeating structure of three separately addressable traps forms a "nano-optical conveyor belt"; a unit cell with four separately addressable traps permits controlled peristaltic transport in the plane. Using specifically designed activation sequences allows particle sorting.

We present a method for obtaining accurate numerical design sensitivities for metal-optical nanostructures. Adjoint design sensitivity analysis, long used in fluid mechanics and mechanical engineering for both optimization and structural analysis, is beginning to be used for nano-optics design, but it fails for sharp-cornered metal structures because the numerical error in electromagnetic simulations of metal structures is highest at sharp corners. These locations feature strong field enhancement and contribute strongly to design sensitivities. By using high-accuracy FEM calculations and rounding sharp features to a finite radius of curvature we obtain highly-accurate design sensitivities for 3D metal devices. To provide a bridge to the existing literature on adjoint methods in other fields, we derive the sensitivity equations for Maxwell's equations in the PDE framework widely used in fluid mechanics.

We consider the escape of ballistic trajectories from an open, vase-shaped cavity. Such a system serves as a model for microwaves escaping from a cavity or electrons escaping from a microjunction. Fixing the initial position of a particle and recording its escape time as a function of the initial launch direction, the resulting escape-time plot shows "epistrophic fractal" structure--repeated structure within structure at all levels of resolution with new features appearing in the fractal at longer time scales. By launching trajectories simultaneously in all directions (modeling an outgoing wave), a detector placed outside the cavity would measure a train of escaping pulses. We connect the structure of this chaotic pulse train with the fractal structure of the escape-time plot.