Extended Data Fig. 4
From: Coherent surface plasmon polariton amplification via free-electron pumping
SPPs’ magnetic-field dynamics analysis. a, Illustration of the magnetic field of the TM01 mode on a metallic wire structure. The wire is aligned along the z axis (perpendicular to the paper plane). Hence, only the \({B}_{x}=Bcos\varphi =Bcos(arctan(d/h))\) component of the magnetic field can be imprinted on the polarization changes of the probe light. b, The propagation velocity of the magnetic profiles on different TGG distances when compared to a spherical wavefront (solid line, predicted by \(z=\sqrt{{(0.3{\tau }_{d})}^{2}-{d}^{2}-{h}^{2})}\). Noteworthy, the superluminal propagation speeds result from the different projection angles of the SPPs when it encounters the TGG surface, giving rise to the large phase velocity on the TGG.
