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

Abstract

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/cm(2)) 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.

Ultralow-Intensity Magneto-Optical and Mechanical Effects in Metal Nanocolloids

“…We claim that the DC magnetic dipoles that are non-negligible and capable of influencing nanocolloids optical properties using external magnetic fields. In prior work, we investigated the scattering and describe the low-intensity, nonlinear, broadband MO responses in aqueous gold nanospheres that occur on minute timescales [22].In this Letter, we demonstrate that both linear and nonlinear plasmon dynamics lead to MO responses. We distinguish characteristics associated with volume charge densities in the linear regime and surface charges associated with SPPs in the nonlinear regime that dominate the response above low-threshold magnetic fields.…”

“…Our theoretical results are demonstrated via numerical simulations, as well as via experiments. We observe increased absorption and scattering [22] of nanocolloid solutions generated from the coincident circularly-polarized light and application of DC magnetic fields.In the remainder of this Letter we provide analytical and numerical calculations that predict the formation of DC magnetic dipoles when elliptically-polarized light illuminates nanoparticles. We attribute the minute-time responses to the movement and alignment of the photoinduced magnetic dipoles generated on the nanoparticles with the external magnetic field.In the linear regime the volume charge density, within the quasi-static limit [30], is governed by the equations of motion: m * d 2 r dt 2 + m * γ d r dt = q E + q( d r dt × B) r − k r, where r denotes the spatial coordinate, m * is the effective mass of the electron, γ is the decay rate, q is the charge of an electron, and k is the force constant associated with the restoring force.…”

“…Our theoretical results are demonstrated via numerical simulations, as well as via experiments. We observe increased absorption and scattering [22] of nanocolloid solutions generated from the coincident circularly-polarized light and application of DC magnetic fields.…”

“…Moreover, we show that there exists a nonlinear response that is explained with the theoretical model of Ref.[19] when applied to nanostructures. DC magnetic dipoles arise from the coupling between scattered and incident fields [22][23][24] where vortex phases characterized by phase singularities are the signature of azimuthal power flow [25,26]. The vortex flows studied here differ distinctly in character from polar-coordinate whirlpool flows investigated in other work [27,28].…”

“…We claim that the DC magnetic dipoles that are non-negligible and capable of influencing nanocolloids optical properties using external magnetic fields. In prior work, we investigated the scattering and describe the low-intensity, nonlinear, broadband MO responses in aqueous gold nanospheres that occur on minute timescales [22].…”