Department of Physics; Institute of Computational and Theoretical Studies
Light carries angular momentum, and as such it can exert torques on material objects. Applications of these opto-mechanical effects were limited initially due to their smallness in magnitude, but later becomes powerful and versatile after the invention of laser. Novel and practical approaches for harvesting light for particle rotation have since been demonstrated, where the structure is always subjected to a positive optical torque along a certain axis if the incident angular momentum has a positive projection on the same axis. We report here an interesting phenomenon of "negative optical torque", meaning that incoming photons carrying angular momentum rotate an object in the opposite sense. Surprisingly this can be realized quite straightforwardly in simple planar structures. Field retardation is a necessary condition and discrete rotational symmetry of material object plays an important role. The optimal conditions are explored and explained.
Source Publication Title
Nature Publishing Group
This work is licensed under a Creative Commons Attribution 4.0 International License.
This work is supported by Hong Kong RGC through HKUST2/CRF/11G and AoE/P-02/12. J.N. is also supported by RGC through HKBU604011 and HKBU603312. Z.L. is supported by NSFC through 11174059. J.C. is supported by Shanxi Science and Technology Department through 2014011008-1 and NSFC through 11404201.
Link to Publisher's Edition
Chen, Jun, Jack Ng, Kun Ding, Kin Hung Fung, Zhifang Lin, and C. T. Chan. "Negative optical torque." Scientific Reports 4 (2014): 6386.