RDL The Trap Forces Applet


The Trap Forces Applet

This Java applet illustrates the trapping forces arising from the scattering of light rays intersecting a dielectric sphere. It is a physical raytracing where:
  • we choose a quite high ratio of sphere to surrounding medium refractive index (1.5 ~ silica in air) to enhance effects.
  • light rays are s-polarized (linearly polarized in the direction orthogonal to the screen). That is used for computing reflection and transmission coefficients.
  • each ray is propagated up to order O(r) (r is the reflectivity) that means we completely neglect reflection accompanying the third refraction
Each incident ray is split into three components emerging from the sphere after the 1st reflection, 2nd and 3rd refraction. Blue arrows represent the changes in light linear momentum corresponding to each of the three components. Gray dashed lines indicate the direction of incident rays. Red arrow is the resulting force on the particle calculated as the negative total change in light momentum flux (vectorial sum of blue arrows). Drag the particle around with the mouse and play with different NA (numerical aperture) values. Observe that stable axial trapping is only achieved for large NA.

Applet files are available for download here:
TrapForces.tgz (updated 04/01/09)
If you're using it, make sure you have the last one, I keep discovering small errors and bugs...

Forces of a single-beam gradient laser trap on a dielectric sphere in the ray optics regime
A. Ashkin
Biophys J., 61, 569, (1992)