# Suppressed Transmission of Long-Range Surface Plasmon Polariton by TE-Induced Edge Plasmon
## 2 Plasmonic Signal Copier and Long-Range Surface Plasmon Polariton  
Figure 1a illustrates the 3D schematic of a PSC. A PSC is composed of gapped-surface plasmon polariton waveguides (G-SPPWs) [39; 40] and a dielectric channel waveguide laid across the gap. The thickness of the gold SPP waveguides was 20 nm and low-loss polymers were used as the dielectric materials for the clad (n\({}_{\text{c}\text{d}\text{d}}=1.45\)) and for the 6 \(\times\) 6 \(\upmu\)m\({}^{2}\) core (n\({}_{\text{core}}=1.46\)) of the channel waveguide. Experiments were performed using 1.550 \(\upmu\)m laser diodes. One of them was used to excite LRSPPs with a 20 nm thick input surface plasmon polariton waveguide (SPPW) with the power of 0 dBm, and the other was used to excite polarized optical signals with a variable optical attenuator and a polarization controller. Details on the experimental setup and fabrication processes can be found in [38; 41]. To summarize, when LRSPPs and polarized optical signals intersected coincidently, a plasmonic signal, of which the binary state is inverted to the modulated optical signal, was detected for only the TE-polarized optical signal at the output SPPW rather than the TM-polarized optical signal. Using LRSPPs as carrier waves, a plasmonic signal is copied invertedly from the TE-polarized optical signal-like NOT gate (inverter). Figure 1: (**a**) 3D schematic of the plasmonic signal copier (PSC). (**b**) Induced current density (J\({}_{x}\)) by long-range surface plasmon polaritons (LRSPPs) in the propagation direction (unit: A/m\({}^{2}\)). (**c**) Comparison of the LRSPP transmission at the output surface plasmon polariton waveguide (SPPW) as a function of the gap size according to the type of excitation. The LRSPP excited in the input SPPW propagates along the \(x\) direction and carries an antisymmetric charge density distribution along the central horizontal plane of the waveguide. The excited LRSPP is one of the fundamental modes that is supported by a gold strip and has Gaussian-like field distribution. Figure 1b shows the surface plasmon wave of the LRSPP, characterized by the current density in the direction of propagation (J\({}_{x}\)). The current density is calculated using finite-difference time-domain (FDTD) simulations. The amounts of other components (J\({}_{y}\) and J\({}_{z}\)) are minute, due to the fact that J\({}_{y}\) contributes to the radiation in higher-order modes [42]; the normal component of the electric field (E\({}_{z}\)), proportional to J\({}_{z}\), is significantly small inside the SPPW, due to the electromagnetic boundary condition [8, 43]. At the end-facet of input SPPW, the guided LRSPP decouples to a TM-polarized lightwave (SPP-photon) due to the structural interruption that cannot allow the surface plasmon wave to be carried, and the Gaussian-like lightwave remains propagating along the direction of G-SPPWs. After the radiation in the gap, the lightwave re-excites the LRSPP (photon-SPP) at the front-facet of output SPPW through end-fire coupling [39, 40, 44, 45], which implies that LRSPPs tunnel the dielectric gap via the SPPP-photon-SPP conversion process. The transmission of LRSPPs decreases with the gap length, since the coupling loss at the front-facet of the output SPPW increases with the broadening of the Gaussian beam during the propagation in the gap, and the scattering also occurs at the edges of the channel waveguide. To investigate why the transmission is lowered by the broadening of the Gaussian beam, the tunneling of LRSPPs across the gap and the coupling of LRSPPs by a Gaussian beam are compared. Figure 1c shows the FDTD-calculated transmission of LRSPPs at the output SPPW in the absence of an input optical signal across the gap. The red dots represent the transmission of the LRSPP when excited in the input SPPW (SPP-photon-SPP), and the black squares represent the transmission of the LRSPP when excited in the output SPPW after the radiation of the Gaussian beam in the gap.