Now Daniel Andrén and colleagues at Chalmers University of Technology, Sweden, Moscow Institute of Physics, Russia, and Gothenburg University, Sweden, have reported polarization-sensitive dielectric periodic arrays fabricated on the back of microparticles, enabling complex steering and even cargo transport (Nat. Nanotechnol. 16, 970–974; 2021). The underlying mechanism relies on transfer of momentum via asymmetric scattering, rather than more common gradient forces and thermal effects; it doesn’t require the translation of a beam or stage, only a change in polarization, and enables complex sequences of motion, as demonstrated in the image.
The microvehicle moves along the glass bottom of a container holding water, driven by a 1,064-nm-wavelength laser beam. The beam is 0.4 mm wide, which is effectively a plane wave relative to the vehicle’s proportions: 12 μm long, 10 μm wide and 1 μm thick. The structure is an array — with two-dimensional period of 600 nm and 950 nm — of dimer antennas, each consisting of two fins with a narrow gap between. The gaps are positioned such that each ‘half’ of the dimer antenna has a different length. For normal-incidence, parallel to the antenna’s long axis, 60% of light is deflected to the +1 diffraction order, with much less transmitted into the 0 and –1 orders, whereas light polarized across the other in-plane axis exhibits hardly any asymmetry. The overall result is that linear polarized light drives the vehicle forwards in a linear direction, and circularly polarized light propels the particle forwards in a curved trajectory, the direction of which depends on the handedness of the polarization. This is in contrast to symmetric antennas for which linear polarization does not drive motion and circularly polarized light produces on-the-spot rotation.
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