Mechanically controllable strain junctions: targeting quantum effects and strong plasmonic coupling in ultra-narrow gaps

• Applicants: Dr. Kai Braun; Professorin Dr. Monika Fleischer
• Subject Area: Physical Chemistry of Molecules, Liquids and Interfaces, Biophysical Chemistry ; Experimental Condensed Matter Physics ; Optics, Quantum Optics and Physics of Atoms, Molecules and Plasmas
• Term: from 2015 to 2020
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 287911648

Within the last decade gap antennas have been widely studied and are commonly used due to the strongly enhanced coupled electrical fields within their gap, which can be spectrally tuned over a wide range. Many emerging nano-photonic technologies depend on the careful control of this plasmonic coupling, including optical nanoantennas for high-sensitivity sensors in chemical and biological applications and improved photovoltaic devices. Typically the distances between the metallic nanostructures range from several tens of nm down to a few nm. Only recently several groups managed to produce and measure sub-nanometer gaps, which show new phenomena such as coherent quantum tunnelling. These effects could become crucial in nanoscale optoelectronics and may pave the way to single molecule opto-electronics.Nevertheless, achieving reproducible and stable experimental conditions with sub-nanometer sized gaps remains a challenging task in high demand. In addition, the methods demonstrated so far are in many cases not suitable for preparing resonant plasmonic coupling with a preselected optical frequency, since their plasmonic properties are not very well-controllable due to a strong dependence on the random preparation processes. In the present work, the approach of using mechanically controllable break junctions (MCBJs) will be revised by developing mechanically controllable strain junctions (MCSJs). MCBJs have shown great performance in measuring tunneling effects and single molecule conductance even at ambient conditions, but unfortunately exhibit poor control of the formed electrode gap geometry itself. The shape of the plasmonic tips in the gap region of a MCSJ is lithographically pre-defined on a pre-stretched substrate, which afterwards will be controllably released to approach the tips.The initial part of this work is dedicated to test measurements on MCBJs. These experiments are used to validate the stretching control setup and establish a link to literature results. The objectives of this project are to develop a well-defined, tunable experimental setup for investigating the interplay between the optical and electronic properties of a nano-gap between metal antennas under wide parameter variation, both for pure gaps and for gaps bridged by molecules, with sub-nanometer control under ambient conditions. With this setup the regime of strong coupling and quantum plasmonics will be addressed. The results from luminescence, electronic transport and Raman studies, all on exactly the same system under variation of antenna geometry, gap size, bias voltage, and molecular bridging will be collected. The project thus aims at gaining new insight into the plasmonic mode distribution, role of the evanescent near-field, electrical biasing, and molecular conductivity in the strong coupling regime.

• DFG Programme: Research Grants