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Posted: May 29, 2013

Photonic wheels for drag racing at the nanoscale

(Nanowerk News) Optical tweezers and spanners are about to become more sophisticated. A group of physicists in Germany has just demonstrated, for the first time, the existence of a novel, transverse effect pertaining to light beams used for optical trapping, called photonic wheel.
This means that scientists will now have full rotational control over the micro- or nanoscale objects trapped in the tweezers’ optical beam. Peter Banzer and colleagues from the Max Planck Institute for the Science of Light, in Erlangen, Germany, just published their findings in the Journal of the European Optical Society Rapid Publications ("The photonic wheel - demonstration of a state of light with purely transverse angular momentum"). The authors speculate that, under favourable low viscosity condition, this approach could lead to spinning a trapped particle that will then start moving like a spinning top, as soon as the trapping laser beam is switched off, thus creating the conditions for a nano drag race (see figure).
Due to their lack of mass, photons do not behave intuitively. Rather, they have characteristics of their own. They can be circularly polarised, for example. This means their electric field spins around the propagation axis—a characteristic described as angular momentum, which is parallel to the direction in which the photon travels. This longitudinal angular momentum is akin to that of aircraft propellers, aligned with the direction in which the aircraft travels.
Now, the authors have shown that photons can display purely transverse angular momentum, at a right angle to the direction in which they move. This is similar to the angular momentum of the spinning wheel of a bicycle, whereby the rotational axis is transverse to the direction of movement.
Banzer and colleagues first predicted the new capability theoretically. They then confirmed it through experimental work, using a highly focused light beam with a special polarisation. They used a single metallic nanoparticle to probe the beam in the focal plane. Since there is a measurable deformation of the beam shape in that plane, it proves the existence of a purely transverse angular momentum in the beam for the investigated scheme.
Combining this newly discovered photonic wheel with conventional beams gives full rotational control when manipulating particles. This opens the possibility of new applications such as nanomixers and micromachines in addition to application in quantum optics and nano-optics.
Source: Max-Planck-Institut für die Physik des Lichts
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