Merging Spintronics and Plasmonics: Evidence of Spinplasmonics

Photo: Prof. Abdulhakem Elezzabi, Professor & Canada Research Chair, Department of Electrical and Computer Engineering, University of Alberta, Canada

Researchers at the University of Alberta (Edmonton, Canada) and the Naval Research Laboratory (Washington, D.C., U.S.A.) have demonstrated a novel approach for the active control of terahertz plasmonic propagation. Using an ensemble of sub-wavelength size ferromagnetic/nonmagnetic spintronic structures, their experiments provide the first evidence of low frequency plasmonic conduction controlled via the electron-spin. Such phenomenon can be conceptualized as the photonic analog to the electrically-driven spin accumulation that serves as a basis for spintronic devices. The team is led by Prof. Elezzabi of University of Alberta.

In their experiments, the researchers employ a rudimentary plasmonic system consisting of ferromagnetic particles coated with nonmagnetic nano-layers. The excitation of the particles with a single-cycle, 1 picosecond wide electric field pulse induces nonresonant particle plasmons on the surface of the bimetallic particles. The dipolar electric fields associated with the particle plasmons on individual particles couple from particle to particle via nearest neighbor interaction and radiate into the far-field at the edge of the sample as coherent terahertz radiation.

When a magnetic field is applied to the sample, electron spin induced resistivity changes within the skin depth of the particles are mapped onto a modulation of the radiated electromagnetic fields. The researchers demonstrate that terahertz radiation propagated through the spintronic particles exhibits increased magnetic field dependent amplitude attenuation and phase modulation, nearly an order of magnitude larger than that of bare ferromagnetic particles.

The electron spin induced attenuation increases as the surface coverage of the nonmagnetic layer increases, showing that the striking enhancement of the magnetically dependent attenuation is attributed to the nonmagnetic layer.

The physical mechanism underlying the enhanced attenuation arises from non-equilibrium accumulation of electron spin electromagnetically driven from the ferromagnet into the nonmagnet (spin polarized surface currents). A quantitative measurement of the dependence of the attenuation on the nonmagnet layer thickness is in very good agreement with the spin diffusion length predicted by the spin accumulation model, as well as with other experimental measurements of this length scale.

Conceptual illustration of a nonresonant particle plasmon excited on a spintronic structure consisting of a sub-wavelength size ferromagnetic (Co) particle that has been coated with nonmagnetic (Au) layers. Shown below are the density of spin-up and spin-down electron states, N(E), in the ferromagnetic and nonmagnetic media. In an applied magnetic field, spin polarized electrons in the ferromagnet are electromagnetically driven into the nonmagnet layer, which results in excess interface resistance. The electron-spin induced resistivity change is mapped onto a modulation of the fields re-radiated from the non-resonant particle plasmon.

The demonstration of a spin-dependent photonic phenomenon opens up a novel avenue in both the fields of spintronics and photonics. The ability to magnetically manipulate near-field mediated light transport on metallic particles via electron spin promises another degree of freedom in the design of photonic devices. The researchers envision the development of solid-state, magnetically sensitive terahertz photonic switches, modulators, and band-pass filters based on electron spin.

Reference:
"Electron-Spin-Dependent Terahertz Light Transport in Spintronic-Plasmonic Media,”
by K. J. Chau, Mark Johnson, and A. Y. Elezzabi,
Physical Review Letters 98, 133901 (Link to Abstract)