Plasmon Enhanced Optical Magnetism in Metal Nanostructures

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This research examines strategies for generating and controlling magnetic fields with light. The magnetism is the result of an interaction of light with gold metal. Although gold metal has no magnetic behavior in the dark, and light does not usually generate stable magnetic fields, two features of this system enable light-induced magnetism. First, the gold is prepared in the form of very small particles, called nanoparticles, that are much smaller than the wavelength of light. Because of their small size, gold nanoparticles have a unique ability to greatly concentrate and enhance the absorption of light. Second, the light is circularly polarized, meaning that the beam of light also causes rotational motion of electrons in the metal nanoparticles. These combined effects produce large circulating electric currents in the nanoparticles that, in turn, give rise to magnetic fields. Each nanoparticle behaves like a small, very strong magnet. The magnetic field is only present when the light is shining, so the magnetism can be switched on and off as quickly as the light can be turned on and off. With better understanding, this behavior can allow for even smaller and faster magnetic switching than current computer memory systems that store information in switchable magnets. This work also supports student training in scientific communication to non-technical audiences.This research analyzes the optical, magnetic, and thermal behavior that results from resonant enhancement of coherent, rotating charge displacement in plasmonic metal nanostructures during excitation with circularly polarized light. The generation of magnetic fields in matter due to the direct transfer of angular momentum from circularly polarized optical fields is termed the inverse Faraday effect, and is the central focus of study in these experiments. The inverse Faraday effect is analyzed using both time-resolved pump-probe spectroscopy and continuous wave spectroscopy on colloidal suspensions of gold nanoparticles that are designed to provide plasmonic enhancement of the incident optical field. Experiments are focused on distinguishing optically-induced magnetism from other non-linear optical phenomena, such as the optical Kerr effect, or non-coherent thermalization processes. The experiments also quantify the magnitude, frequency dependence, time response, and microscopic mechanism of the optically-induced magnetism, in order to elucidate new avenues of optoelectronic and opto-magnetic functionality in nanostructures enabled by ultrafast, optically modulated magnetic field generation.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.