Dynamic Trapping and Printing of Plasmonic Dimers with Optical Vortex Beams

In this work, we analyze the motion of gold nanospheres in orbital angular momentum (OAM)-carrying optical vortex traps in real time using darkfield microscopy and high-speed video analysis. Notably, we observe that optical binding between gold nanoparticles within the ring-shaped laser trap leads to increased orbiting speeds at a lower focal plane for gold nanoparticle dimers compared to monomers. This behavior is attributed to stronger optical scattering forces acting on the dimers driven by the emergence of a coupled plasmon mode. As the particles move closer together, this mode red-shifts, becoming more resonant with the laser wavelength, eventually causing the system to transition from optical trapping to optical printing. This finding suggests a general mechanism for one-step dimer printing based on plasmonic coupling in vortex beams by adjusting the laser wavelength or modifying the dielectric environment of the nanoparticles via a molecular coating. The feasibility of this approach is demonstrated for optical printing and subsequent surface-enhanced Raman scattering (SERS) spectroscopy on gold nanoparticle dimers coated with 4-nitrothiophenol.