Efficient Surface Plasmon Excitation in Resonant Structures via Inelastic Electron Tunneling

表面等离激元（SPPs）是一种基元激发，表现出电子密度振荡和表面束缚局域光子之间的混合特性。因此，只要能量和动量守恒，SPP预计将与光子和电子强烈相互作用。然而，有趣的是，目前在等离子体激元领域的研究几乎完全集中在通过传播或局部光子场激发和检测SPP上，而SPP与带电粒子（如电子）的相互作用则很少被研究。在这里，我们的目的是在一个系统的研究，通过电子隧穿到共振等离子体结构与强场本地化的激发局部表面等离子体激元（LSP），以评估潜在的优化等离子体激元生成的效率。我们的方法的关键要素，使我们超越了以前的研究，是精确控制的隧穿势垒与使用原子平坦的单晶金纳米结构的组合，这些纳米结构表现出可以在整个可见/红外光谱中调谐的明确的共振。期望谐振天线状结构表现出高得多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的热点中的强烈增强的局部态密度。在这里，我们建议调查和利用共振等离子体激元结构中的等离子体激元产生在三个很大程度上互补的方式：（a）通过空间选择性电子注入使用扫描隧道显微镜的提示，（B）通过间隙依赖的垂直隧穿在包括绝缘体薄层的ESTA型结构和（c）通过隧穿在二氧化硅表面制备的电连接的天线结构的横向间隙。虽然第一几何形状将允许我们改变注入位置，但是对于第二和第三几何形状，隧道电流本质上与相应结构的高近场强度区域共定位。为了实现这一研究计划，该项目被设计为两个小组的共同努力：维尔茨堡大学的Hecht教授小组和德累斯顿工业大学的Eng教授小组，他们一方面致力于将单晶金片纳米结构化为电连接的谐振光学天线，另一方面，隧道间隙的精确主动控制以及SPP电激励的跟踪记录。总之，我们认为等离子体激元的电激发具有巨大的应用潜力。特别是，我们感兴趣的是提高效率的强大和无背景的单等离子体/光子源，并转换成传播SPP的电驱动局部等离子体。此外，我们将控制等离子体辐射图案以及进行实验，以验证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.