Phys. Rev. Lett. 118, 138002 (2017)

Credit: APS

A scheme for realizing a reconfigurable, 2D lattice potential and that can perform directed transport of Brownian particles with a ratchet-like behaviour has been built by a Mexican–Czech collaboration. The set-up was shown to be able to control the propagation of 1.99-μm-diameter polystyrene microspheres immersed in water. Alejandro Arzola and co-workers from the Universidad Nacional Autonoma de Mexico and the Institute of Scientific Instruments of CAS used a spatial light modulator (SLM), programmed with a computer-generated hologram, to create a pattern of light spots in the form of a 2D lattice. Importantly, the beam leaving the SLM is split into two different polarizations that can have their relative intensities adjusted and their light spot pattern shifted with respect to one another to create an asymmetry in an arbitrary direction. As a result, an ensemble of particles can be made to move in a ratchet-type manner, as shown by the coloured paths in the image. Simulations with as many as 500 particles match well with the observed behaviour.