Strategies for the 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 memory1, spintronics2 and quantum computation, as well as the opportunities for nonlinear optical control and modulation3 in applications such as optical isolation and non-reciprocity4. Here we report experimental quantification of optically induced magnetization in plasmonic gold nanoparticles due to the inverse Faraday effect. The induced magnetic moment is large under typical ultrafast pulse excitation (<1014 W m−2 peak intensity), with magnetization and demagnetization kinetics that are instantaneous within the subpicosecond time resolution of our study. Our results support a mechanism of coherent transfer of angular momentum from the optical field to the electron gas, and open the door to all-optical subwavelength strategies for optical isolation that do not require externally applied magnetic fields. Optically induced magnetization is experimentally demonstrated using gold nanoparticles. The inverse-Faraday-effect-enabled magnetization may lead to new types of compact optical isolator.

Light-induced magnetism in plasmonic gold nanoparticles

Magneto-plasmonics is a designation generally associated with ferromagnetic-plasmonic materials because such optical responses from nonmagnetic materials alone are considered weak. Here, we show that there exists a switching transition between linear and nonlinear magneto-optical behaviors in noble-metal nanocolloids that is observable at ultralow illumination intensities and direct current magnetic fields. The response is attributed to polarization-dependent nonzero-time-averaged plasmonic loops, vortex power flows, and nanoparticle magnetization. This work identifies significant mechanical effects that subsequently exist via magnetic-dipole interactions.

Ultralow-intensity magneto-optical and mechanical effects in metal nanocolloids.

The mechanical forces associated with surface currents are widely overlooked and point to a new family of plasmonically-driven processes. Here, we investigate the Lorentz forces acting on a free electron gas that is bound to the surface of a nanowire. We demonstrate that appreciable mechanical forces are produced by longer illumination wavelengths between longitudinal and transverse absorption resonances via the excitation of chiral hybrid plasmon modes. We are the first to associate plasmonic activity as the underlying mechanism for nanowire rotation, which explains prior experimental results. The presence of chiral hybrid plasmon modes yields the greatest net translation and torque forces. The asymmetric plasmon behavior subsequently affects the complex nonlinear dynamics of plasmonic nonspherical nanoparticles in fluids.

http://metaconferences.org/ocs/public/conferences/9/pdf/3003.pdf

Plasmon-induced Lorentz forces of nanowire chiral hybrid modes

We theoretically and numerically investigate the linear and nonlinear magneto-optical contributions to the refractive index of metal nanospheres. The analysis is in good agreement with the experimental extinction spectra of gold nanocolloid solutions, with threshold magnetic fields less than 1 mT when illuminated with light intensities less than 1 W/cm^2. Plasmonic current loops and vortex power flows provide a new framework for the dynamical interaction between material magnetization, light polarization and nano-surfaces. The photo-induced magneto-optical interaction of non-magnetic metal nanocolloids imparts significant, non-negligible torque forces in nanofluids.

Ultra-Low-Intensity Magneto-Optical and Mechanical Effects in Metal Nanocolloids

Pattern formation often reveals constituent nonlinear mechanisms of a complex system. Here, we study self-synchronizing, light-induced thermal cycles in plasmonically absorbing nanofluids, whose anomalous thermal, optomechanical, electrochemical, and hydrodynamic responses are not yet well understood. We show that the oscillatory behavior—caused by light grazing the nanofluid meniscus—exhibits a strong dependence on hydrogen bonding in the solvent environment and that there are low-intensity optical thresholds in alcohol–water binary-solvent nanofluids. Moreover, these thermal cycles occur with a periodic, vertically discharging heat-dissipation mechanism, which could be facilitated by nanobubbles or thermophoresis. We show that an incoherent white-light source, such as sunlight, will also induce self-synchronizing thermal cycles; in this demonstration, we illustrate new methods of energy storage, transfer, and harvesting that will not alter the natural carbon cycle of life.

Influence of solvent polarity on light-induced thermal cycles in plasmonic nanofluids

We experimentally, analytically, and numerically demonstrate the nonlinear photo-induced plasmon-assisted magnetic response that occurs with metallic nanoparticles in aqueous solution. We measure the scattered spectra from solutions of gold nanospheres (10−7 fill factor) and observe appreciable changes when simultaneously applying DC magnetic fields and illuminating samples with light. The magnetic response is achieved using light from a solar simulator at unprecedented low illumination intensities (< 1W/cm2) and is sustained when the magnetic field is removed. Distinctly different behavior is observed depending on the circular-polarization handedness given a fixed magnetic field. Nanoparticle aggregation is more likely to occur when the circular-polarization trajectory opposes the solenoid current that produces the magnetic field. Using Mie’s theoretical solution, we show how vortex orbital surface currents lead to an increased and anisotropic electrical conductivity, which shifts the scattered spectra in agreement with experimental results. The single-nanoparticle plasmon-induced magnetization, which couples the scattered and incident electric fields, changes sign with orthogonal circular-polarization handedness.

Anomalously-large photo-induced magnetic response of metallic nanocolloids in aqueous solution using a solar simulator.

We experimentally demonstrate and theoretically model a photo-induced plasmon-assisted magnetic response in non-magnetic nano-particulate solutions that occurs using linear, circular, and unpolarized light.

Manipulation of Gold and Silver Nanoparticles via Photo-induced Magnetism

We demonstrate for the first time a plasmon-assisted magnetic response that occurs with disperse gold nanoparticles in aqueous solution. We observe increased Fano-like resonances and show the nonlinear interaction that occurs with matrix vortices.

Anomalously-large photo-induced magnetic response of disperse metallic nanocolloids

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Anomalously-large photo-induced magnetic response of metallic nanocolloids in aqueous solution using a solar simulator.

Manipulation of Gold and Silver Nanoparticles via Photo-induced Magnetism

Anomalously-large photo-induced magnetic response of disperse metallic nanocolloids