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Plasmonic nanostructures, which support highly localized and enhanced electromagnetic field are now actively researched as a means for efficient trapping of nanoscale objects, not addressable by conventional diffraction-limited optical tweezers. An issue of critical concern is how to efficiently transport and deliver the suspended particles to the illuminated plasmonic nanostructure. There are primarily two main approaches that researchers employ for trapping of particles with plasmonic nanostructure(s) on a substrate. The first approach involves illuminating arrays of closely-spaced plasmonic nanostructures. However resonant illumination of the nanostructures results in collective heating and this produces strong fluid convection that exerts drag forces on the particles. Elucidating the roles of these heating-induced forces and optical gradient forces arising from plasmonic field enhancement have so far remained elusive. The other scheme involves illuminating a single plasmonic nanostructure. However, due to the absence of thermoplasmonic convection in this case, the dynamics of the suspended particle to be trapped becomes dictated by Brownian motion- an inherently slow process. We will discuss a new fluid flow mechanism, which we have termed electrothermoplasmonic (ETP) flow to resolve this dilemma. ETP flow harnesses intrinsic plasmonic heating combined with AC electric field to generate on-demand fluid and particle transport, which means that particles could be rapidly transported for trapping in sub-wavelength plasmonic hotspots only when desired, and without any competition between heating-induced forces and optical gradient forces. These new capabilities certainly provide new directions for research in the field of plasmon-assisted optical trapping, which will be discussed.
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