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

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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.