The amount of force produced in an optical trap depends on the electromagnetic interaction of the focused beam with the particle. This interaction is a complicated function of numerical aperture, beam power, wavelength, particle geometry, and refractive index. Previous attempts to quantify trapping forces have been limited by simplifying assumptions-namely that the particles are homogeneous, spherical objects. This assumption allows the forces to be solved analytically, but does not provide useful information for nonspherical particles, such as chromosomes. The primary difficulty in predicting the trapping forces for nonspherical particles lies in the calculation of the scattered electromagnetic fields. If the fields are known, the trapping forces can be quantified. Aside from a few selected geometries, MaxwellUs equations must be solved numerically to compute the fields. In this work the finite-difference time-domain(FDTD) technique will be used to quantify the fields for objects of arbitrary shape. Although the FDTD method is computationally intensive, it provides a way to compute the fields under a variety of conditions. The calculated forces will then be compared with measured forces.
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