Efficient Surface Plasmon Excitation in Resonant Structures via Inelastic Electron Tunneling

表面等离子激元（SPPs）是一种表现出电子密度振荡和表面束缚局域光子混合特性的基本激发。因此，只要能量和动量守恒，SPPs就有望与光子和电子发生强烈的相互作用。然而，值得注意的是，目前等离子体领域的研究几乎完全集中在通过传播或局域光子场激发和探测SPPs上，而SPPs与带电粒子（如电子）的相互作用则很少被研究。在这里，我们旨在系统地研究通过电子隧穿进入具有强场定位的共振等离子体结构来激发局域表面等离子体（LSPs），以评估优化等离子体产生效率的潜力。我们的方法的关键要素，使我们超越以往的研究，是精确控制的隧道障碍与使用原子平面单晶金纳米结构的结合，这些结构具有明确的共振，可以在整个可见/红外光谱中调谐。谐振天线样结构有望表现出更高的电子到等离子体激元转换效率的原因之一是等离子体激元谐振器热点处态的局部密度的强烈增强。在这里，我们建议以三种基本互补的方式研究和利用共振等离子体结构中的等离子体产生：(a)通过使用扫描隧道显微镜尖端的空间选择性电子注入，(b)通过涉及薄绝缘体层的三明治型结构中的间隙依赖垂直隧道，以及(c)通过在硅表面制备的电连接天线结构中的横向间隙上隧道。虽然第一种几何形状允许我们改变注入位置，但对于第二种和第三种几何形状，隧道电流本质上与各自结构的高近场强度区域共定位。为了实现这个研究项目,该项目被设计为两个群体的共同努力:赫克特集团维尔茨堡大学的教授和教授Eng集团你德累斯顿,贡献出他们的能力在纳米结构的单晶金片为电连接共振光学天线一方面,和精确的主动控制隧道的差距以及许可证的记录在电子激发,另一方面。总之，我们认为电激等离子体具有很大的应用潜力。我们特别感兴趣的是提高鲁棒和无背景的单等离子体/光子源的效率，并将电驱动的局部等离子体转化为可传播的spp。此外，我们将控制等离子体辐射模式，并进行实验来验证SPP的电放大。

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.