2D nanosphere lithography using surface plasmon-enhanced optical trapping

A two-dimensional (2D) surface plasmon (SP)-enhanced optical trapping system based on a single high numerical aperture objective has been developed. The system can be utilized to trap dielectric particles and simultaneously provide imaging. The 40-fold electric field enhancement, and hence strong 2D trapping force distribution with SP excitation through a gold film with a thickness of 45 nm in the near infrared region, was analyzed. The strong trapping force and high-resolution trapping image of nanoparticles can be concurrently achieved via the same high NA objective. The developed SP-enhanced trapping system was successfully applied to efficiently trap dielectric particles with a size down to 350 nm on a cover slip surface and allows for real-time imaging observation. Also, in order to further increase the penetration depth and the electric field of the evanescent wave, a coupled-waveguide surface plasmon resonance configuration consisting of a five-layer structure of Bk7/Au/SiO₂/Au/H₂O for two-dimensional optical trapping has been developed. Theoretical analysis shows that the maximum enhancement of the local electric field intensity is about 60-fold while the penetration depth is about 1 μm at the resonance angle. The trapped and aligned dielectric single layer particles were spread over a large area with a reduction in feature size to form a hexagonally close-packed (HCP) pattern on a cover slip surface. The HCP pattern has the potential for well-ordered 2D nanosphere lithography.