Active magneto-plasmonics in hybrid metal-ferromagnet structures

Surface-plasmon-mediated confinement of optical fields holds great promise for on-chip miniaturization of all-optical circuits(1-4). Following successful demonstrations of passive nanoplasmonic devices(5-7), active plasmonic systems have been designed to control plasmon propagation. This goal has been achieved either by coupling plasmons to optically active materials(8-13) or by making use of transient optical nonlinearities in metals via strong excitation with ultrashort laser pulses(14-17). Here, we present a new concept in which the active optical component is a metal-ferromagnet-metal structure. It is based on active magneto-plasmonic microinterferometry, where the surface plasmon wave vector in a gold-ferromagnet-gold trilayer system is controlled using a weak external magnetic field. Application of this new technique allows measurement of the electromagnetic field distribution inside a metal at optical frequencies and with nanometre depth resolution. Significant modulation depth combined with possible all-optical magnetization reversal induced by femtosecond light pulses(18) opens a route to ultrafast magneto-plasmonic switching.