We measure, by photonic torque microscopy, the nonconservative rotational motion arising from the transverse components of the radiation pressure on optically trapped, ultrathin silicon nanowires. Unlike spherical particles, we find that nonconservative effects have a significant influence on the nanowire dynamics in the trap. We show that the extreme shape of the trapped nanowires yields a transverse component of the radiation pressure that results in an orbital rotation of the nanowire about the trap axis. We study the resulting motion as a function of optical power and nanowire length, discussing its size-scaling behavior. These shape-dependent nonconservative effects have implications for optical force calibration and optomechanics with levitated nonspherical particles.

CNR IPCF Istituto per i Processi Chimico Fisici 1 98158 Messina Italy

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

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

Institute of Scientific Instruments of the CAS v v i Czech Academy of Sciences 612 64 Brno Czech Republic

MATIS CNR IMM and Dipartimento di Fisica e Astronomia Università di Catania 1 95123 Catania Italy

Scuola Superiore di Catania Università di Catania 1 95123 Catania Italy

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