Efficient Surface Plasmon Excitation in Resonant Structures via Inelastic Electron Tunneling

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Surface-plasmon polaritons (SPPs) are elementary excitations that exhibit a hybrid character between electron-density oscillations and surface-bound localized photons. SPPs are therefore expected to strongly interact with both photons and electrons as long as energy and momentum are conserved. It is interesting to note, though, that current research in the vivid field of plasmonics nearly exclusively focuses on the excitation and detection of SPPs via propagating or localized photonic fields, while the interaction of SPPs with charged particles such as electrons is much less frequently examined. Here we aim at a systematic study of the excitation of localized surface plasmons (LSPs) via electron tunneling into resonant plasmonic structures with strong field localization in order to assess the potential for optimizing the efficiency of the plasmon generation. The key elements of our approach, taking us beyond previous studies, is the combination of precisely controlled tunneling barriers with the use of atomically-flat monocrystalline gold nanostructures exhibiting well-defined resonances that can be tuned throughout the visible/IR spectrum. Among the reasons why resonant antenna-like structures are expected to exhibit a much higher electron to plasmon conversion efficiency is the strongly enhanced local density of states in the hot spots of plasmonic resonators. Here we suggest investigating and harnessing plasmon generation in resonant plasmonic structures in three largely complementary ways: (a) via spatially selective electron injection using scanning tunneling microscopy tips, (b) by gap-dependent vertical tunneling in sandwich-type structures involving thin layers of insulators and (c) by tunneling over lateral gaps in electrically connected antenna structures prepared at a silica surface. While the first geometry will allow us to vary the injection position, for the second and third geometry the tunnel current is intrinsically colocalized with high-near-field intensity regions of the respective structures. In order to realize this research program, the project is designed as a combined effort of two groups: Prof. Hecht's group at the University of Würzburg and Prof. Eng's group at the TU Dresden, who contribute with their ability in nanostructuring of monocrystalline gold flakes into electrically connected resonant optical antennas on the one hand, and precise active control of tunneling gaps as well as a track record in electrical excitation of SPPs, on the other hand. Overall, we believe that the electrical excitation of plasmons bears great potential for applications. In particular we are interested in improving the efficiency of robust and background-free single plasmon/photon sources and to convert electrically driven localized plasmons into propagating SPPs. Furthermore we will control the plasmonic radiation pattern as well as perform experiments to verify the electrical amplification of SPP.