TRAPPING FORCE IN OPTICAL TWEEZERS
This web app illustrates the mechanism by which a restoring trapping force
arises from the redistribution of light's momentum produced by the reflection
and refraction of light rays on a dielectric sphere.
Light rays are s-polarized (linearly polarized in the direction orthogonal
to the screen). That is used for computing reflection and transmission
Each ray is propagated up to order O(r) (r is the reflectivity) meaning that
we completely neglect reflection accompanying the third refraction.
Each incident ray is split into three components emerging from the sphere
respectively after the 1st reflection, the 2nd and the 3rd refraction.
Blue arrows represent 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 bead around and play with different numerical aperture and refractive
index values (the refractive index of the surrounding medium is 1.33).
Observe that stable axial trapping is only achieved for large NA.
Forces of a single-beam gradient laser trap on a dielectric sphere in the ray optics regime
Biophys J., 61, 569, (1992)