Geometric control over surface plasmon polariton out-coupling pathways in metal-insulator-metal tunnel junctions

Abstract

Metal-insulator-metal tunnel junctions (MIM-TJs) can electrically excite surface plasmon polaritons (SPPs) well below the diffraction limit. When inelastically tunneling electrons traverse the tunnel barrier under applied external voltage, a highly confined cavity mode (MIM-SPP) is excited, which further out-couples from the MIM-TJ to photons and single-interface SPPs via multiple pathways. In this work we control the out-coupling pathways of the MIM-SPP mode by engineering the geometry of the MIM-TJ. We fabricated MIM-TJs with tunneling directions oriented vertical or lateral with respect to the directly integrated plasmonic strip waveguides. With control over the tunneling direction, preferential out-coupling of the MIM-SPP mode to SPPs or photons is achieved. Based on the wavevector distribution of the single-interface SPPs or photons in the far-field emission intensity obtained from back focal plane (BFP) imaging, we estimate the out-coupling efficiency of the MIM-SPP mode to multiple out-coupling pathways. We show that in the vertical-MIM-TJs the MIM-SPP mode preferentially out-couples to single-interface SPPs along the strip waveguides while in the lateral-MIM-TJs photon out-coupling to the far-field is more efficient.

Coherence Between Different Propagating Surface Plasmon Polariton Modes Excited by Quantum Mechanical Tunnel Junctions

“…As S 1 is blocked, the far-field interference between S 1 and the forward scattering SPPs is completely absent and thereby the fringe patterns are not observed in the BFP which only shows the SPP scattering from the stripe-end. We contrast this with our recent work, [39] where S 1 is inherently absent due to the geometric orientation of the TJ (in-plane, vertical to the glass substrate) and therefore fails to generate distinct fringes in the BFP. Similarly, we blocked S 2 and recorded the BFP image, which only shows the light scattering off the surface roughness in the MIM-TJ area, with no distinct fringes as the far-field interference is absent in this case as well (see Section S3 and Figure S6 in the Supporting Information for the recorded images).…”

“…[36] It is of fundamental interest to investigate whether this initial coherence, originating from the tunneling event, is preserved as the MIM-SPP outcouples to all available modes (SPP and photons) leading to a mutual correlation between different SPP modes excited by the MIM-TJ. Even though previous works have clarified the outcoupling mechanisms of the MIM-SPP mode, [37][38][39] the correlation between the MIM-SPP and the outcoupling channels (SPPs and photonic modes) for biased MIM-TJs still remains elusive. Here we show that the coherence of the MIM-SPP mode with its outcoupling pathways (single interface SPPs and photo ns) is preserved and leads to strong interference in the far-field.…”

Geometric Control Over the Edge Diffraction of Electrically Excited Surface Plasmon Polaritons by Tunnel Junctions

“…However, the edge diffraction of SPPs, which provides a complementary way of far-field SPP characterization, as it can readily be identified through BFP imaging as a straight-line feature adjacent to the leakage of the propagating SPPs ( k SPP ), ,, has not been examined in detail before. Compared to optical excitation schemes, where the position of the plasmonic waveguide within the laser waist beam has to be precisely controlled in order to excite SPPs close to the edges of the waveguide, ,,− in MIM-TJs connected to plasmonic waveguides, SPPs can easily be excited across the entire width of the waveguides and the edge diffraction can easily be obtained as previously observed. ,,, Additionally, the edge diffraction clearly separates the metal–air leaky SPP contributions from those of the metal–glass SPPs, which can only be detected via scattering at the end of the waveguide due to the bound nature of the modes. , Hence, an integrated structure where the MIM-TJ is in-plane with a plasmonic stripe waveguide offers an efficient platform for investigating the SPP edge diffraction. Moreover, even though the interaction of SPPs with the edges has been discussed in previous works, , no control over the edge diffraction process has been demonstrated so far.…”

“…Compared to optical excitation schemes, where the position of the plasmonic waveguide within the laser waist beam has to be precisely controlled in order to excite SPPs close to the edges of the waveguide, 30,35,42−44 in MIM-TJs connected to plasmonic waveguides, SPPs can easily be excited across the entire width of the waveguides and the edge diffraction can easily be obtained as previously observed. 38,39,41,45 Additionally, the edge diffraction clearly separates the metal−air leaky SPP contributions from those of the metal−glass SPPs, which can only be detected via scattering at the end of the waveguide due to the bound nature of the modes. 38,46 Hence, an integrated structure where the MIM-TJ is in-plane with a plasmonic stripe waveguide offers an efficient platform for investigating the SPP edge diffraction.…”

“…Recently, for MIM-TJs with integrated SPP stripe waveguides, various SPP outcoupling pathways were identified, control over the outcoupling pathways was demonstrated by controlling the tunneling direction, and plasmonic antennas were used to enhance the SPP outcoupling . It was also demonstrated that the outcoupling pathways can be selectively turned on and off by changing the electrode thickness .…”