Rotating Ag-Fe3O4-Au Nanograin by Optical Torque with a Monochromatic Light Beam

Optical torques of asymmetrical Ag–Fe3O4–Au nanograins were investigated by the method of discrete dipole approximation (DDA). The results show that surface plasmon resonance (SPR) causes the optical torques which can keep the nanograins rotating clockwise or counterclockwise. When the power density of optical radiation is I = 109 W/m2, the angular velocities of the hybrid sphere and cube heterotrimers can reach to about 104 rad/s in the ranges of 360–374 nm and 403–426 nm, which is ten times larger than that of Brownian rotation. The peak widths at half height of angular velocity curves for two kinds of grains are in the ranges of 31–47 nm and 54–70 nm, respectively. When light radiation force offers a regular driving force, such grains can serve as potential nanoscale optical wrench or microscopic mixers. In addition, the influences of Brownian rotation and photophoresis were discussed.