Optically Trapping Large Metallic Particles in Air Using a 'Boat' Trap with Direct-Drawn Sidewalls

We present a method for achieving stable optical trapping of large metallic particles (>1.16 µm mean diameter solid gold particles, and >100 μm conductor-coated microspheres) using a focused beam, scanned by acousto-optic modulators to directly draw a parabolic trap cross-section in the air.  The result of this drawing action is the formation of a 'boat' trap, open on top, capable of catching, loading, and stably holding large metallic particles without hopping.  A high-power continuous-wave 532 nm laser is scanned in both the x- and y-directions, forming a boat-shaped intensity distribution optimized for trapping microspheres. The trapping duration is maximized by careful selection of drawing frequency to be fast enough to appear continuous relative to particle dynamics, but slow enough to avoid trap distortion. This protocol describes the experimental setup, including laser modulation, particle introduction techniques, and validation. Carbon-coated microspheres are used primarily to visualize and validate trap formation and stability, while solid gold nanoparticles serve as the target for plasmonic interaction studies. This method allows for any one of a number of cross-sections to be drawn on command, in real-time, to provide a robust, flexible approach for capturing, loading, and translating airborne particles over long timescales, with potential applications in plasmonic enhancement of levitated optical systems.