Improved modeling of optical forces for complex objective lens systems via vectorial ray tracing

We present a vectorial ray tracing-based model to compute optical forces on a spherical microscopic particle due to a tightly focused arbitrary input beam profile. Unlike models that describe the objective lens used to focus the trapping beam, by an overall parameter such as numerical aperture, the proposed model incorporates detailed specification of the constituent sub-elements of the objective lens system. The model traces each ray from a plane where the beam profile is generated till it intersects the particle surface and all along the trajectory the ray is also represented by the respective electric fields. Thus the model facilitates use of trapping beam with arbitrary polarization and complex amplitude profiles. Our results demonstrate that optical forces on a microscopic particle are highly sensitive to the objective lens design. We show that two different objective lenses sharing the same magnification and numerical aperture can give rise to two distinct optical force profiles.The proposed model therefore provides a simple yet more accurate scheme for optical force analysis on microscopic particle that may be useful in the design and optimization of advanced optical trapping systems.