Reactive optical matter: Light-induced motion

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Newton's third law dictates that forces between interacting particles are equal and opposite for closed systems. In a non-equilibrium environment, the third law can be defied, giving rise to "nonreciprocal" forces. Theoretically, this was shown when dissimilar, optically trapped particles were mediated by an external field. In a recent study, Yuval Yifat and colleagues measured the net nonreciprocal forces in electrodynamically interacting, asymmetric nanoparticle dimers and nanoparticle aggregates. In the experiments, the nanoparticle structures were confined to pseudo one-dimensional geometries and illuminated by plane waves. The observed motion was due to the conservation of total momentum for particles and fields with broken mirror symmetry (represented by a changed direction of motion). The results are now published on Light: Science & Applications.

The ability to convert light energy into self-directed motion with light-driven nanomotors or micromachines has already attracted great interest. A variety of methods in optics can produce rotational motion or give rise to translational motion with photoreactive materials. The promise to engineer light-driven nanomotors arose from recent theoretical work, which predicted that dissimilar particles illuminated by an electromagnetic plane wave, will experience a nonreciprocal net force.

The predicted nonreciprocal forces were demonstrated with simulations to vary very little with interparticle separation. However, straightforward experimental evidence on the phenomenon was not presented thus far. Exploring the reactive optical effects can open new possibilities of self-assembling, light-driven micromachines to herald a new field in optics and photonics.

To fill the experimental gap, in the present study, Yifat et al. demonstrated self-motility using optically bound dimers of disproportionate metallic nanoparticles (NPs). The experimental findings were also supported by quantitative electrodynamic simulations. Aside from dimers, the scientists similarly generated and measured the motion of asymmetric nanoparticle clusters or assemblies. To perform the experiments, Yifat et al. used a standard optical trapping setup with a Ti:Sapphire laser operating at a wavelength of 790 nm. A tightly focused, circularly polarized spatially phase-modulated beam of light formed an optical ring trap.




Video of the silver (Ag) heterodimer in a ring trap – motion in a counter-clockwise direction. Credit: Light: Science & Applications, doi: https://doi.org/10.1038/s41377-018-0105-y

Read more at: https://phys.org/news/2018-12-reactive-optical-light-induced-motion.html#jCp


Video of gold (Au) nanoparticle clusters in the ring trap. Credit: Light: Science & Applications, doi: https://doi.org/10.1038/s41377-018-0105-y

Read more at: https://phys.org/news/2018-12-reactive-optical-light-induced-motion.html#jCp

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https://phys.org/news/2018-12-reactive-optical-light-induced-motion.html#nRlv