  Strategies for ultrafast optical control of magnetism have been a topic of
intense research for several decades because of the potential impact in
technologies such as magnetic memory, spintronics, and quantum computation, as
well as the opportunities for non-linear optical control and modulation in
applications such as optical isolation and non-reciprocity. Here we report the
first experimental quantification of optically induced magnetization in
plasmonic Au nanoparticles due to the inverse Faraday effect (IFE). The induced
magnetic moment in nanoparticles is found to be ~1,000x larger than that
observed in bulk Au, and ~20x larger than the magnetic moment from optimized
magnetic nanoparticle colloids such as magnetite. Furthermore, the
magnetization and demagnetization kinetics are instantaneous within the
sub-picosecond time resolution of our study, supporting a mechanism of coherent
transfer of angular momentum from the circularly polarized excitation to the
orbital angular momentum of the electron gas.
