# Optical trapping of non-spherical particles

1. Apr 5, 2014

### Gwinterz

Hey

I just have a kind of broad question, typical derivations of the optical trapping force involve the approximation that the particle is spherical.

I'm about to start looking into papers which discuss deviations from this but before I do I just wanted to see if I could reinforce some intuition to help better understand what I'm looking for.

As particles deviate from sphere's, the trapping force decreases right?

Thanks

2. Apr 5, 2014

### UltrafastPED

The theory of optical trapping was originally simplified for dielectric spheres. I don't think that there is a general theory for various geometric shapes ... though people can trap lots of small things with different shapes.

I don't work in this field so I'm not sure about the relative strength of trapping for various shapes.

3. Apr 11, 2014

### Andy Resnick

We have a paper currently in revision regarding this topic, relevant references provided below.

While the theory for trapping spherical particles is well-developed and has been extended to include anisotropic particles, nearly-spherical particles, arbitrary optical fields, etc; analytical results for things like cylinders and cubes are essentially non-existent. There are only a few numerical results for cubes and cylinders as well:

Ling, L., et al., Optical forces on arbitrary shaped particles in optical tweezers. Journal of Applied Physics, 2010. 108(7).
Nieminen, T.A., H. Rubinsztein-Dunlop, and N.R. Heckenberg, Calculation and optical measurement of laser trapping forces on non-spherical particles. Journal of Quantitative Spectroscopy & Radiative Transfer, 2001. 70(4-6): p. 627-637.
Bonessi, D., K. Bonin, and T. Walker, Optical forces on particles of arbitrary shape and size. Journal of Optics a-Pure and Applied Optics, 2007. 9(8): p. S228-S234.
Lock, J.A., Scattering of a diagonally incident focused Gaussian beam by an infinitely long homogeneous circular cylinder. Journal of the Optical Society of America a-Optics Image Science and Vision, 1997. 14(3): p. 640-652.
Grzegorczyk, T.M. and J.A. Kong, Analytical expression of the force due to multiple TM plane-wave incidences on an infinite lossless dielectric circular cylinder of arbitrary size. Journal of the Optical Society of America B-Optical Physics, 2007. 24(3): p. 644-652.
Kotlyar, V.V. and A.G. Nalimov, Analytical expression for radiation forces on a dielectric cylinder illuminated by a cylindrical Gaussian beam. Optics Express, 2006. 14(13): p. 6316-6321.
Rockstuhl, C. and H.P. Herzig, Rigorous diffraction theory applied to the analysis of the optical force on elliptical nano- and micro-cylinders. Journal of Optics a-Pure and Applied Optics, 2004. 6(10): p. 921-931.
Gauthier, R.C., Theoretical investigation of the optical trapping force and torque on cylindrical micro-objects. Journal of the Optical Society of America B-Optical Physics, 1997. 14(12): p. 3323-3333.