Modeling Optical Micromanipulation by Realistic Beams: Direct Comparison with Experiment

As pointed out by Ashkin, the significance of optical forces can be seen in a “back-of- envelope” calculation: by focusing a 0.1 W laser to a micron-sized particle and assuming a modest momentum absorption of ~10%, the resultant acceleration can reach 10^4 ms^- 2. This “astronomical” acceleration implies a great potential. Indeed, this technique is having major impacts on many important fields that involve small particles.

Our research is focused on optical micromanipulation – the manipulation of micron- sized particles by light. Optical micromanipulation has developed into a rapidly expanding branch of experimental science. However, the theoretical development is lagging significantly behind. For example, the modeling of the optical tweezers, invented 25 years ago, was not very successful until recent years. Indeed, the geometries and beams are difficult to model, and most theoretical works deal with over-simplified scenarios.

We seek to model realistic experiments, and compare our results directly with published data and with our collaborators’ measurements. We will use the computational tools that we have developed, which include the generalized Mie theory for multiple spheres, boundary element method, discrete dipole approximation, vector Debye integral, and Maxwell stress tensor. We will also generalize these tools for some specific needs in modeling certain problems. We will expand the capabilities of our numerical programs to incorporate various realistic beams and geometries. Our final model will help predict new exciting phenomena in optical micromanipulation not yet realized by current experiments. We will be working closely with the experimental group led by Prof. Ou-Yang of Lehigh University. We aim at explaining experimental results from Prof. Ou-Yang at a quantitative level, and Prof. Ou-Yang will, in turn, conduct experiments to verify our theoretical predictions. We plan to carry out more research of this kind in this proposed project, with a focus on widely employed geometries and beams.

Last but not least, the dynamics of light forces often give us new surprises, especially when complicated geometries or many particles are involved. As a complex system, multi-particle physics is always fascinating, but the non-conservative nature of light- induced forces adds to the complexity of dynamical phenomena, taking us to new frontiers that are not covered by the classical mechanics in standard texts. We seek to understand the complexity of having a non-conservative system, and to develop a comprehensive theory that is relevant to the light-matter interaction in the micron scale with forces induced by arbitrary geometries and beams.