Measurement of rotated Janus particle’s position and orientation in the optical trap

Abstract

Optical tweezers are precise tools for translating and rotating micrometer-sized particles and are used widely in biology and physics. In a linearly polarized optical trap, Janus particles comprising two hemispheres with different refractive indexes can be rotated about the beam axis following the polarization direction. In previous research, the position and orientation of a Janus particle relative to the polarization direction were simulated using the T-matrix method, whereas this paper is focused on accurate experimental measurements. The position and orientation of a Janus particle are extracted by image processing, and the polarization direction is controlled by a motor-driven half-wave plate (HWP). The particle’s orientation and the polarization direction are related via the HWP’s rotation angle. However, rotating the HWP not only rotates the polarization direction but also causes the trap center to fluctuate positionally because of the inevitable misalignment between the HWP’s rotation axis and the optical axis. Both the trap center and the polarization direction affect the particle’s position, but the influence of the former is removed with the aid of a homogeneous microsphere. The experimental results show that the Janus particle rotates following the polarization direction to keep the interface between its two hemispheres always parallel to the polarization direction. Also, the particle’s centroid moves in a circular orbit around the trap center to hold the trap center in the hemisphere with the higher refractive index. The experimental results agree with the previous simulation results and are of use for future applications.