Mechanism of angular momentum transfer from microwaves to a copper ring

International audience; In the exchange of orbital angular momentum between an electromagnetic wave and a copper ring we examine the origin of the Angular Momentum. We then investigate the transfer mechanism between the microwave and the object, and compare it with other mechanisms. We evidence a transfer mechanism based on the reflection of the electromagnetic field on the copper ring. In particular, at a microscopic scale, we show that the electromagnetic field induces alternative electric currents in the ring, with a small drift. Although little, the resistivity of copper leads to a force that rotates the ring. The estimation of the torque, which is of the order of 10 −8 Nm, is in good agreement with the experimental measurements. We also show that the transfer of electromagnetic orbital angular momentum to objects could be a way to measure the orbital angular momentum carried by electromagnetic fields, and we discuss possible applications. PACS. 41.20.Jb Electromagnetic wave propagation; radiowave propagation – 45.20.da Forces and torques – 84.40.Ba Antennas: theory, components and accessories – 42.50.Tx Optical angular momentum and its quantum aspects