The following example shows how to calculate the optical force on a spherical particle illuminated by a plane wave. We expect the force to exhibit a 1/w^4 frequency dependence where w is the angular frequency.
A TFSF source is used to simulate a plane wave incidence upon the sphere. The wavelength range is 400-1500 nm. The dielectric sphere has an index of 1.1 and a radius of 40 nm. The background index is 1. A mesh override region forces a 10 nm mesh around the sphere, monitors and source. In this simulation, we use the volumetric optical force analysis object because the forces generated in this simulation are near the limit of the MST technique. The optical_force analysis objects can be inserted from the Object library under the Advanced analysis section.
Force on a particle
When the simulation is finished, you can right-click on the optical_force_volumetric analysis group and visualize the F_total result to generate a plot of the total force in Newtons on the particle. In the following figure, the frequency (f) parameter has been selected to be plotted along the x-axis with units of terahertz.
From the symmetry of the system, we expect that Fx and Fy (the force in the X and Y directions) is zero. The force should only be in the + Z direction, and it should have a strong frequency dependence (1/w^4). The simulation confirms these points. Use the associated script file to confirm the w^4 dependence. The script file will also plot the force as calculated with the MST analysis object. The result is clearly less linear. This discrepancy is due to the fact that the force generated in this example is near the numerical limit of the MST technique. For situations with larger forces (ie. a larger particle, or larger index contrast), the volumetric and MST techniques should agree.