Plasmon resonance-based optical trapping of single and multiple Au nanoparticles

The plasmon resonance-based optical trapping (PREBOT) method is used to achieve stable trapping of metallic nanoparticles of different shapes and composition, including Au bipyramids and Au/Ag core/shell nanorods. In all cases the longitudinal plasmon mode of these anisotropic particles is used to enhance the gradient force of an optical trap, thereby increasing the strength of the trap potential. Specifically, the trapping laser is slightly detuned to the long-wavelength side of the longitudinal plasmon resonance where the sign of the real component of the polarizability leads to an attractive gradient force. A second (femtosecond pulsed) laser is used to excite two-photon fluorescence for detection of the trapped nanoparticles. Two-photon fluorescence time trajectories are recorded for up to 20 minutes for single and multiple particles in the trap. In the latter case, a stepwise increase reflects sequential loading of single Au bipyramids. The nonlinearity of the amplitude and noise with step number are interpreted as arising from interactions or enhanced local fields amongst the trapped particles and fluctuations in the arrangements thereof.