Radiation Pressure Cooling With Localized Surface Plasmon

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We have theoretically and experimentally investigated radiation pressure induced by localized surface plasmon (LSP) of metal nano-gap structures, and analyzed the strength and spatial distribution of the optical force exerted on nanometer-sized particles and molecules under thermal motions. We showed that the nanoscale spatial profiles of the LSP fields within the nano-gap exhibit complicated fine structures, rather than single peaks. The nanopatterns are formed by interferences of dipolar, quadrupolar, and higher-order multipolar plasmonic modes. In addition, we developed the measurement system for quantitatively analyzing the radiation pressure exerted on nanoparticles. The experimental results demonstrated that the force strength is enhance by 3 order of magnitude compared to that with conventional far-filed trapping. We also succeeded in observation of two-dimensional trapping potentials, which is the first demonstration of super-resolution optical trapping.