title: Negative optical torque.
abstract: 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.
authors.affiliation: 1] Department of Physics, The Hong Kong University of Science and Technology [2] Institute of Theoretical Physics and Department of Physics, Shanxi University [3] State Key Laboratory of Surface Physics, Key Laboratory of Micro and Nano Photonic Structures (MOE), and Department of physics, Fudan University [4].
authors.forename: Jun
authors.initials: J
authors.lastname: Chen
authors.affiliation: 1] Department of Physics and Institute of Computational and Theoretical Studies, Hong Kong Baptist University [2].
authors.forename: Jack
authors.initials: J
authors.lastname: Ng
authors.affiliation: 1] Department of Physics, The Hong Kong University of Science and Technology [2] Institute for Advanced Study, The Hong Kong University of Science and Technology.
authors.forename: Kun
authors.initials: K
authors.lastname: Ding
authors.affiliation: Department of Applied Physics, The Hong Kong Polytechnic University.
authors.forename: Kin Hung
authors.initials: KH
authors.lastname: Fung
authors.affiliation: State Key Laboratory of Surface Physics, Key Laboratory of Micro and Nano Photonic Structures (MOE), and Department of physics, Fudan University.
authors.forename: Zhifang
authors.initials: Z
authors.lastname: Lin
authors.affiliation: 1] Department of Physics, The Hong Kong University of Science and Technology [2] Institute for Advanced Study, The Hong Kong University of Science and Technology.
authors.forename: C T
authors.initials: CT
authors.lastname: Chan