Role of Magnetic Induction Currents in Nanoslit Excitation of Surface Plasmon Polaritons

We present a method for exciting surface plasmon polaritons (SPPs) caused by a magnetic field component perpendicular to the direction of slit. The excitation mechanism is based on the spatially oscillating induced current along the edges of the slit under obliquely incident electromagnetic waves. Our finding distinguishes itself from previous mechanisms based on transverse electric fields and unveils the missing point of the SPP-excitation problem in a nanoslit. The use of a magnetic field for SPP excitation can be highly efficient and even comparable to that with an electric field, so that their composition can lead to selective unidirectional excitation.

Figure 1

(a) The configuration of the system studied. The SPP field distributions are illustrated on the metal surface. The incident wave vector of the plane wave is always laid on the $y\mathrm{\text{−}}z$ plane so it does not break symmetry with respect to the slit structure. The surface currents ($\overrightarrow{J}$), accumulated charges, and induced dipole moments ($\overrightarrow{m}$, $\overrightarrow{p}$) are illustrated for (b) TM wave and (c) TE wave incidence.

Figure 2

(a) Incident angle dependent ratios of induced charge densities (dotted line, theoretical) and $E_z$ fields (solid line, numerical). (b) The $E_z$ field profiles at an incident angle of 50° from numerical (lines) and analytical (Green’s dyadic function, markers) methods. The amplitudes of the data are normalized in order to achieve a clear comparison.

Figure 3

(a) Schematic diagram for the unidirectional launching of SPPs done by the composition of TM and TE incidences. Experimental CCD image for unidirectional condition: (b) right-handed (RCP) and (c) left-handed circular polarized (LCP) light incidence with ${\theta }_{\mathrm{inc}}=50°$ provides left and right side SPP excitation, respectively. Insets show the respective simulation results for the $E_z$ field profile along $x\mathrm{\text{−}}z$ plane at optimized unidirectional conditions.

Figure 4

Experimentally measured (marker) and numerically calculated (dotted blue line) left-to-right SPP launching ratios are compared with the theoretical plot (solid black line) varying with the incident polarization state. The polarization elipticity $\chi$ is changed along the path shown in the Poincaré sphere (inset).