Optical Binding and Trapping

About Us

Our group, the Center for Electromagnetic Theory and Applications (CETA) at MIT, led by Prof. J. A. Kong, has an extensive background in theoretical and numerical electromagnetics, remote sensing, propagation of electromagnetic waves in complex media, and electromagnetic modeling in general. Recently, our activities have been concentrated on modeling optical tweezers and other known optical trapping systems for spherical and cylindrical dielectric particles. Although the experimental part of this work is being actively developed in various groups, the theoretical modeling of these systems still seems at its infancy.

Our recent activities include:

• Exact calculation of the forces on multiple particles. The method is based on an exact electromagnetic calculation of the field scattered by a collection of arbitrary size particles. To our knowledge, it is the first time that the force is computed exactly on a system of multiple Mie particles (i.e. not small compared to the wavelength, where approximations are usually done). More details on our work can be found in [2]. In particular, we have computed the dynamics of a system of 20 randomly distributed dielectric cylinders submitted to three incident plane waves each forming an angle of 120 degrees with each other (illustrated above). The problem therefore closely reproduces the experimental configuration found for example in the works of Prof. J.-M. Fournier. Our calculations show that positions assumed by the particles do not always align with the high field intensity of the incident field, which can be attributed to binding forces, as introduced by Prof. J.-M. Fournier and collegues. The theoretical predictions agree well with the experimental observations.
• Control of the binding force to realize customizable force field pattern. The binding forces refer to the forces induced by the interactions between multiple particles. Their existence has been shown experimentally multiple times already, but they have never been controlled to achieve desired properties. This is what we have reported in a recently published paper in the Physical Review Letters [1], where we have shown for the first time how to control the binding forces between a collection of particles to realize an optical trap.The advantage compared to holography for example is that the positions of the particles are reconfigurable, yielding a customizable force distribution in space.
• Calculation of the Lorentz force on dielectric and magnetic particles. The Lorentz force on bound electric and magnetic charges and currents is responsible for the force density distributed throughout particles in an electromagnetic field. This force density exists everywhere once the optical field is present and is responsible for observations of scattering, gradient, and binding forces. This result unifies the existing force predictions into one fundamental description in agreement with the computationally inexpensive stress tensor formalism [3].
• First principles study of radiation pressure. The radiation pressure on media is derived from first principles by applying either the Lorentz force directly to the material or by momentum conservation. The existing model for the Lorentz force on bound electric currents and charges is extended to bound magnetic currents and charges. The fundamental effects of radiation pressure such as zero tangential force on a boundary and the pulling of a dielectric interface toward an incident wave are derived [4].

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