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