Optical tractor beams can sort small particles

Physicists in the Czech Republic have produced a simple example of an optical tractor beam using two laser beams [1]. Interestingly, they have also discovered a new technique for sorting microscopic particles.

We are not at the stage of lifting cows quite yet!

I posted a little about tractor beams before here, which employ a Bessel beam. Prof David Grier at New York University, published a paper last year announcing that they have experimentally demonstrated such a class of tractor beams [2].

What is new?
The work of the physicists at the Institute of Scientific Instruments of the Academy of Sciences of the Czech Republic shows that a simpler version of a tractor beam is possible. Their tractor beam comprises of two laser beams brought to a focus with a lens, which can easily be done using a standard commercial microscope system. They found that by focusing the light inwards they could generate the same effect as if they had employed a Bessel beam.

We are not the first to demonstrate experimentally the motion of particles in a beam where the intensity is homogeneous, but our method is a little bit simpler and more straightforward than previous efforts.

Pavel Zemánek

Particle sorting
An unexpected effect is that the particles are pushed or pulled by the laser light depending on both the size of the particles and the polarization of the light. This allowed the researchers to separate mixtures of two different sized particles by changing the polarisation of the light.

The fact that the effect of the polarisation of lights is so prominent is surprising and for sure will lead to further research.

Scotty would be proud … (Image courtesy of http://en.memory-alpha.org)

Optical tractor beam sorts tiny particles (Physics World)(Jan 24, 2013)

[1] O. Brzobohatý, V. Karásek, M. Šiler, L. Chvátal, T. Čižmár & P. Zemánek, Experimental demonstration of optical transport, sorting and self-arrangement using a ‘tractor beam’, Nature Photonics (2013) doi:10.1038/nphoton.2012.332 (Link)

[2] Optical conveyors: A class of active tractor beams, D. B. Ruffner and D. G. Grier, Phys. Rev. Lett. 109, 163903 (2012) [5 pages]