Self-Assisted Optothermal Trapping of Gold Nanorods Under Two-Photon Excitation

Abstract

We report self-assisted optothermal trapping and patterning of gold nanorods (GNRs) on glass surfaces with a femtosecond laser. We show that GNRs are not only the trapping targets, but also can induce convective flows that drive more particles toward the trap. The trapping phenomenon is the net result of thermophoresis and convective flow caused by localized heating, which is due to the conversion of absorbed photons into heat at GNR's longitudinal surface plasmon resonance wavelength. We investigated the optothermal trapping of GNRs at the glass surface which can be obtained with laser power as low as 0.5 mW at 840 nm. The attraction of particles toward the central hot spot can be larger than the typical field of view, e.g. attraction toward the trap was observed from a range of 210 µm × 210 µm. By moving the laser focus away from the glass surface, at certain distances, ring patterns of GNRs on the glass surface can be obtained visualizing the regions of flow. These patterns could be controlled by the laser power and the numerical aperture of the microscope objective. Moreover, we examined the spectral emission of GNRs under different trapping conditions using the spectral phasor approach to reveal the temperature and association status of GNRs. Our study will help understanding manipulation of flows in solution and in biological systems which could arise in future applications of GNRs induced heating and flows. Work done in part with funds from NIH P41-GM103540 and NIH P50-GM076516.