Suppression of photothermal convection of microparticles in two dimensional nanoplasmonic optical lattice

Photothermal convection has been a major obstacle for stable particle trapping in plasmonic optical

tweezer at high optical power. Here, we demonstrate a strategy to suppress the plasmonic

photothermal convection by using vanishingly small thermal expansion coefficient of water at low

temperature. A simple square nanoplasmonic array is illuminated with a loosely Gaussian beam to

produce a two dimensional optical lattice for trapping of micro particles. We observe stable

particle trapping due to near-field optical gradient forces at elevated optical power at low temperature.

In contrast, for the same optical power at room temperature, the particles are convected away

from the center of the optical lattice without their accumulation. This technique will greatly

increase usable optical power and enhance the trapping capability of plasmonic optical tweezer.