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Circularly polarized optical excitation of plasmonic nanostructures causes coherent circulating motion of their electrons, which in turn, gives rise to strong optically induced magnetization - a phenomenon known as the inverse Faraday effect (IFE). In this study we report how the IFE also significantly decreases plasmon damping. By modulating the optical polarization state incident on achiral plasmonic nanostructures from linear to circular, we observe reversible increases of reflectance by 78% as well as simultaneous increases of optical field concentration by 35.7% under 10^9 W/m^2 continuous wave (CW) optical excitation. These signatures of decreased plasmon damping were also monitored in the presence of an externally applied magnetic field (0.2 T). The combined interactions allow an estimate of the light-induced magnetization, which corresponds to an effective magnetic field of ~1.3 T during circularly polarized CW excitation (10^9 W/m^2). We rationalize the observed decreases in plasmon damping in terms of the Lorentz forces acting on the circulating electron trajectories. Our results outline strategies for actively modulating intrinsic losses in the metal, and thereby, the optical mode quality and field concentration via opto-magnetic effects encoded in the polarization state of incident light.

Recent studies have established that the anti-Stokes Raman signal from plasmonic metal nanostructures can be used to determine the two separate temperatures that characterize carriers inside the metal -- the temperature of photoexcited "hot carriers" and carriers that are thermalized with the metal lattice. However, the related signal in the Stokes spectral region has historically impeded surface enhanced Raman spectroscopy (SERS), as the vibrational peaks of adsorbed molecules are always accompanied by the broad background of the metal substrate. The fundamental source of the metal signal, and hence its contribution to the spectrum, has been unclear. Here, we outline a unified theoretical model that describes both the temperature-dependent behavior and the broad spectral distribution. We suggest that the majority of the Raman signal is from inelastic scattering directly with non-thermal carriers that have been excited via damping of the surface plasmon. In addition, a significant spectral component (~ 1%) is due to a sub-population of hot carriers in an elevated thermal distribution. We have performed temperature and power-dependent Raman experiments to show how a simple fitting procedure reveals the plasmon dephasing time, as well as the temperatures of the hot carriers and the metal lattice, in order to correlate these parameters with quantitative Raman analysis of chemical species adsorbed on metal surface.

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., Comment: 13 pages, 5 figures, supplementary information 6 pages. Submitted