Berlin 2015 – scientific programme

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O: Fachverband Oberflächenphysik

O 50: Coupled Nanostructures and Light Localization

O 50.6: Talk

Wednesday, March 18, 2015, 11:45–12:00, MA 043

Superluminescence from an Optically Pumped Molecular Tunneling Junction by Injection of Plasmon Induced Hot Electrons — •Kai Braun, Xiao Wang, Dai Zhang, Heiko Peisert, Thomas Chassé, and Alfred J. Meixner — Institute of Physical and Theoretical Chemistry, University of Tübingen, Auf der Morgenstelle 18, 72076 Tübingen, Germany

Here, we introduce and experimentally demonstrate amplification of the tip enhanced optical signal from a very low number of molecules enclosed in the optically pumped gap of a STM. The gap between a sharp gold tip and a flat gold substrate covered with a self-assembled monolayer of 5-chloro-2-mercaptobenzothiazole (Cl-MBT) can be used as an extremely small optical gain medium. When a bias-voltage is applied between tip and sample such that electrons tunnel from the Cl-MBT's highest occupied molecular orbital (HOMO) to the tip, holes are left behind in the molecules. These can be repopulated by hot electrons that are created by the laser-driven plasmon oscillation on the metal surfaces. Emission of photons occurs from the recombination of plasmon excited hot electrons with holes in the HOMO of surface bound molecules facing the tip apex. Varying the laser pump power or alternatively the applied bias voltage shows in both cases a distinct threshold above which amplification of the optical signal occurs. Solving the laser-rate equations for this system shows that the repopulation process can be efficiently stimulated by the gap modes near field. TERS scattering from neighboring molecules acts as an optical seed and feed-back is provided by the cavity plasmon-polariton.