Multiple scattering of light induces structured interactions, or optical binding forces, between collections of small particles. This has been extensively studied in the case of microspheres. However, binding forces are strongly shape dependent: here, we turn our attention to dielectric nanowires. Using a novel numerical model we uncover rich behavior. The extreme geometry of the nanowires produces a sequence of stationary and dynamic states. In linearly polarized light, thermally stable ladder-like structures emerge. Lower symmetry, sagittate arrangements can also arise, whose configurational asymmetry unbalances the optical forces leading to nonconservative, translational motion. Finally, the addition of circular polarization drives a variety of coordinated rotational states whose dynamics expose fundamental properties of optical spin. These results suggest that optical binding can provide an increased level of control over the positions and motions of nanoparticles, opening new possibilities for driven self-organization and heralding a new field of self-assembling optically driven micromachines.

CNR IPCF Istituto per i Processi Chimico Fisici Consiglio Nazionale delle Ricerche Viale F Stagno D'Alcontres 37 1 98158 Messina Italy

Department of Physics and Astronomy University College London Gower Street London WC1E 6BT U K

H H Wills Physics Laboratory University of Bristol Tyndall Avenue Bristol BS8 1TL U K

Institute of Scientific Instruments of the CAS Kràlovopolskà 147 612 64 Brno Czech Republic

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Coherent oscillations of a levitated birefringent microsphere in vacuum driven by nonconservative rotation-translation coupling