Construction of a Fiber-Scanning Tunneling Microscope for an Optical Investigation of Surfaces at Atomic Length Scales

建立了两种研究纳米尺度表面光学性质的方法：光发射扫描隧道显微镜（STM-LE）和近场光学显微镜（SNOM）。前者探测的是金属尖端电子表面刺激后的发射响应，后者是一种全光学技术。STM-LE通常具有非常低的发射截面；SNOM的主要缺点是它在10nm范围内的空间分辨率有限。通过用锥形光纤代替传统的STM尖端，提出的新仪器结合了两种方法的优点。控制金属化的光纤顶端产生高度局部等离子体源扫描表面。此外，它可以实现基于隧道的距离控制，从而实现原子尺度的空间分辨率。一项测试实验证明了这种新方法探测表面纳米级形态特征的能力。同时，它们的光学特征可以在亚纳米的空间分辨率下检测到。该方法已成功地用于测量介电介质和金属表面上单个银纳米粒子的等离子体特性。有了这个建议，我们的原理验证装置将转化为一个完整的科学仪器，能够将表面的发射响应与局部结构参数相关联。新仪器将能够解决表面科学和化学领域的许多科学问题，例如在特定原子环境中孤立缺陷、掺杂剂和色心的光学特征，或介电表面上单个纳米粒子和分子的光学特征。此外，光子介导的能量传递过程，例如，在光活性分子的受控光刺激之后，变得易于实验。在未来，一个升级的仪器，以执行时间分辨光谱学设想。

Two methods have been established to explore the optical properties of surfaces at the nanoscale, light-emission scanning tunneling microscopy (STM-LE) and near-field optical microscopy (SNOM). While the former probes the emission response after electronic surface stimulation with a metal tip, the latter is an all-optical technique. STM-LE typically suffers from very low emission cross sections; the main drawback of SNOM is its limited spatial resolution in the 10-nm regime. By replacing the conventional STM tip with a tapered optical fiber, the proposed, new instrument combines advantages of both approaches. Controlled metallization of the fiber apex produces a highly localized plasmon source to be scanned over the surface. Moreover, it enables a tunneling-based distance control and thus allows for atomic-scale spatial resolution. A test experiment has demonstrated the ability of the new approach to probe nanoscale morphological features on surfaces. Simultaneously, their optical signature could be detected with sub-nm spatial resolution. The approach was successfully exploited to measure the plasmonic properties of single Ag nanoparticles on dielectric and metallic surfaces. With this proposal, our proof-of-principle setup shall be converted into a full-scale scientific instrument, capable of correlating the emission response of surfaces with local structural parameters. The new instrument will be able to address numerous scientific questions in the fields of surface science and chemistry, such as on the optical signature of isolated defects, dopants and color centers in their specific atomic environment or of single nanoparticles and molecules on dielectric surfaces. Moreover, photon-mediated energy transfer processes, e.g. after controlled optical stimulation of a photoactive molecule, become accessible to the experiment. In the future, an upgrade of the instruments to perform time resolved optical spectroscopy is envisioned.