Optical manipulation of ratio-designable Janus microspheres

• Yulu Chen, Cong Zhai, Xiaoqing Gao, Han Wang, Zuzeng Lin, Xiaowei Zhou, and Chunguang Hu
• received 01/04/2024; accepted 03/22/2024; posted 03/25/2024; Doc. ID 517601
• Abstract: Angular optical trapping based on Janus microspheres has been proven to be a novel method to achieve controllable rotation. In contrast to natural birefringent crystals, Janus microspheres are chemically synthesized of two compositions with different refractive indices. Thus, their structures can be artificially regulated, which brings excellent potential for fine and multi-degree-of-freedom manipulation in the optical field. However, it is a considerable challenge to model the interaction of heterogeneous particles with the optical field, and there has also been no experimental study on the optical manipulation of microspheres with such designable refractive index ratios. How the specific structure affects the kinematic properties of Janus microspheres remains unknown. Here, we report systematic research on the optical trapping and rotating of various ratio-designable Janus microspheres. We employ an efficient T-matrix method to rapidly calculate the optical force and torque on Janus microspheres to obtain their trapped postures and rotational characteristics in the optical field. We have developed a robust microfluidic-based scheme to prepare Janus microspheres. Our experimental results demonstrate the relationship between the microspheres’ kinematic characteristics and refractive index distributions. The trapped postures vary linearly within a specific ratio range, which is of great importance in guiding the design of Janus microspheres, and their orientations flip at a particular ratio, all consistent with the simulations. Our work provides a reliable theoretical analysis and experimental strategy for studying the interaction of heterogeneous particles with the optical field and further expands the diverse manipulation capabilities of optical tweezers.