Optoelectromechanic effects in nanocontacts: Electric-field enhancement and mechanically driven conductance changes

本项目旨在研究脉冲激光照射下纳米结构的相互作用。一方面是纳米结构周围电磁场的分布，另一方面是电磁场对纳米结构性质的影响，特别是金属纳米结构之间通过原子尺寸的接触和尺寸可控的纳米间隙的电子传输。由于金属尖端的场增强--取决于入射波的极化--可能非常显著，因此在直流L-V特性是非线性的那些区域，预计会对通过结的电流产生很大的影响。一个需要解决的问题是，从低频非线性结的交流测量中获知的整流效应是否也在光学频率下持续存在。有待研究的其他可能效应包括表面等离子体的产生、电子的光电流或强烈的非平衡能量分布，以及纳米接触中的光学控制隧道电流。为了量化热膨胀的影响，将研究不同的样品几何形状，如机械可控断开结几何中的悬浮纳米桥以及蝴蝶结几何中的膜和纳米缝隙上的纳米结构。预期的结果是建立一个模型系统来研究纳米光机电效应，并探索其是否适合作为一种通过光机械驱动控制电子传输的工具。利用该模型系统研究了等离子体增强因子的偏振、强度和波长依赖关系及其对电子输运的影响。此外，还将在截面积为数十纳米的体积内进行等离子体介导的光化学反应(案例研究：光聚合)。

This project aims at the investigation of nanostructures interacting with pulsed laser irradiation. One aspect is the distribution of electromagnetic fields around nanostructures, another one the influence of these fields on the properties of the nanostructures, in particular the electron transport through atomic-size contacts and nanogaps of controllable size between metallic nanostructures. Since the field enhancement at metallic tips - depending on the polarization of the incident wave - can be very pronounced, a large influence on the current through the junction is to be expected in those regimes where the dc l-V characteristics are nonlinear. A question to be addressed is whether the rectification effect known from ac measurements with nonlinear junctions at low frequencies also persists at optical frequencies. Other possible effects to be studied include the creation of surface plasmons, photo-currents or a strong non-equilibrium energy distribution of the electrons, and optically controlled tunneling currents in nanocontacts. In order to quantify effects of thermal expansion, different sample geometries like suspended nanobridges in the mechanically controllable break-junction geometry as well as nanostructures on membranes and nanogaps of bowtie geometry will be investigated. The expected outcome is a model system to study nano-opto-electromechanic effects and to explore its suitability as a tool for controlling electronic transport by optomechanical drive. This model system with electrically biased nanogaps between plasmonic nanoparticles made with precision of 1-10 nm will be used to investigate the polarization, intensity, and wavelength dependence of the plasmon enhancement factor and its effects on the electron transport in the mechanically controlled gaps. Also, plasmon-mediated photochemical reactions in the volumes with cross-sections of tens-of-nm will be carried out (a case study: photopolymerization).