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

两种方法已经建立，以探索在纳米级，光发射扫描隧道显微镜（STM-LE）和近场光学显微镜（SNOM）的表面的光学性质。虽然前者探测用金属尖端进行电子表面刺激后的发射响应，但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的主要缺点是其在10 nm范围内的有限空间分辨率。通过用锥形光纤代替传统的STM针尖，所提出的新仪器结合了这两种方法的优点。光纤顶端的受控金属化产生高度局部化的等离子体激元源，以在表面上扫描。此外，它能够实现基于隧道的距离控制，从而允许原子尺度的空间分辨率。测试实验已经证明了新方法探测表面上的纳米级形态特征的能力。同时，它们的光学特征可以以亚nm的空间分辨率检测。该方法已成功地用于测量电介质和金属表面上单个Ag纳米颗粒的等离子体特性。有了这个提议，我们的原理验证装置将被转换成一个全尺寸的科学仪器，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，例如在其特定原子环境中的孤立缺陷，掺杂剂和色心或介电表面上的单个纳米颗粒和分子的光学特征。此外，光子介导的能量转移过程，例如在光敏分子的受控光学刺激之后，变得可用于实验。在未来，进行时间分辨光谱的仪器升级的设想。

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.