Controlled motion of janus particles in point, circular and linear optical traps

Abstract

Optical tweezers have become an efficient tool to trap microscopic and nanoscopic objects and manipulate their mechanical motion via optical forces and torques [1–3]. In this talk we discuss our recent experimental and theoretical works on trapping and manipulation of micrometer-size Janus particle with half surface of polystyrene particle coated with nanometer-thick gold thin film in various optical tweezers, such as point, circular, and linear traps [4, 5]. As the Janus particle is asymmetric in geometry and physical properties, the focused laser beam, either in point, circular, or linear shape spot, not only exert an optical force that will push forward the particle along a revolution orbit surrounding the optical axis of the laser beam, but also an optical torque that will cause rotation of the particle around its own axis. The simultaneous revolution and rotation of the Janus particle can be controlled via several parameters, such as the power and the size of the optical tweezers. To fully understand the physics underlying the experimental observation of the complicated mechanical motions, we develop a ray-optics model to calculate the optical force and torque of Janus particle in point, circular, and linear traps. The calculation results quantitatively agree with experimental data. These studies will help to develop new optical tweezers to enable better control of the mechanical motion of various microscopic and nanoscopic particles of complicated geometric and physical configurations.