Electron-Spin-Dependent Terahertz Light Transport in Spintronic-Plasmonic Media

In this Letter, we demonstrate that electron spin can influence near-field mediated light propagation through a dense ensemble of subwavelength bimetallic ferromagnetic/nonmagnetic microparticles. In particular, we show that ferromagnetic particles coated with nonmagnetic metal nanolayers exhibit an enhanced magnetic field controlled attenuation of the electromagnetic field propagated through the sample. The mechanism is related to dynamic, electromagnetically induced electron spin accumulation in the nonmagnet. The discovery of an electron spin phenomenon in the light interaction with metallic particles opens the door to the marriage of spintronic and plasmonic technologies and could pave the way for the development of light-based devices that exploit the electron spin state.

Figure 1

(a) Illustration of near-field dipole-dipole coupled transport across a chain of $\mathrm{Co}/\mathrm{Au}$ bimetallic particles. Spin accumulation at the $F/N$ bimetallic interfaces gives rise to a magnetically modulated interface resistance. (b) Time-domain THz transmission signals through 3 mm thick particle ensembles having varying Au coverage, in zero field.

Figure 2

Time-domain THz transmission through (a) Co (0% Au) ($B_{\perp }$), (b) Co (0% Au) (${\mathrm{B}}_{\parallel }$), (c) $\mathrm{Co}/\mathrm{Au}$ 35% ($B_{\perp }$), (d) $\mathrm{Co}/\mathrm{Au}$ 35% (${\mathrm{B}}_{\parallel }$), (e) $\mathrm{Co}/\mathrm{Au}$ 42% ($B_{\perp }$), and (f) $\mathrm{Co}/\mathrm{Au}$ 42% (${\mathrm{B}}_{\parallel }$) particle ensembles for $|B|=0\mathrm{T}$ (dark traces) and $|B|=150\mathrm{mT}$ (light traces). The diagrams depict the orientation of the $B$ field (arrow) relative to the electric field polarization. (g) Normalized transmission amplitude through 3 mm thick Co (open circle) and $\mathrm{Co}/\mathrm{Au}$ (solid circle) particle samples versus the field angle relative to the incident polarization, where $B=160\mathrm{mT}$. The transmission amplitudes for both samples have been normalized to their respective zero-field amplitudes, and the data have been fitted with ${cos}^2\theta$ functions shown by the solid lines. Solid squares represent the offset between the curves for the Co and $\mathrm{Co}/\mathrm{Au}$ samples. Within error, the offset is independent of the $B$ orientation.

Figure 3

(a) Normalized THz electric field amplitude transmitted through Co (0% Au) (diamonds), $\mathrm{Co}/\mathrm{Au}$ 35% (circles) and $\mathrm{Co}/\mathrm{Au}$ 42% (squares) particle ensembles versus magnetic field strength in the ${\mathrm{B}}_{\parallel }$ configuration. The $\mathrm{Co}/\mathrm{Au}$ 42% samples have about an order of magnitude more intensity attenuation than uncoated Co particles in a magnetic field of ${\mathrm{B}}_{\parallel }=150\mathrm{mT}$. (b) Delay of $E_t(t)$ transmitted through Co (0% Au) (diamonds), $\mathrm{Co}/\mathrm{Au}$ 35% (circles) and $\mathrm{Co}/\mathrm{Au}$ 42% (squares) particle ensembles versus magnetic field strength for the $B_{\parallel }$ configuration. (c) Normalized THz electric field amplitude transmitted through Co (0% Au) (diamonds), $\mathrm{Co}/\mathrm{Au}$ 35% (circles), and $\mathrm{Co}/\mathrm{Au}$ 42% (squares) particle ensembles versus magnetic field strength in the $B_{\perp }$ configuration. The normalized transmitted electric field amplitude versus $B_{\perp }$ for the $\mathrm{Co}/\mathrm{Au}$ 35% and $\mathrm{Co}/\mathrm{Au}$ 42% particle ensembles are shown in (d) and (e), respectively. The diagrams in the inset of (a) and (c) depict the $B$ orientation (arrow) relative to the electric field polarization. The inset in (e) shows magnetization measurements of the Co particles up to $\pm 1.0\mathrm{T}$. (f) Normalized, time-averaged electric field amplitude (in an applied field of $B_{\perp }=160\mathrm{mT}$) transmitted through 3 mm thick samples of $\mathrm{Co}/\mathrm{Au}$ particles versus the Au film thickness, $d$. The upper inset in (f) depicts an illustration of spin accumulation in the Au layer. The lower inset in (f) shows the resistivity measured using a four-point probe technique on witness Au films deposited on glass slides.