The Quantum Regime in Tunneling Plasmonics
Prof. Dr. Javier Aizpurua
Center for Material Physics, San Sebastian, Spain
17:00 - 18:00 Tuesday 08 October 2013 KFU HS 5.01 Plasmonic nanostructures can be used as canonical building blocks to host and actively participate in a variety of complex physical phenomena such as in non-linear effects, in quantum tunneling or in photoemission processes, to cite a few. As the control of sub-nanometer separation distances becomes technological feasible, a classical description of the metal surface, based on the assumption of an abrupt change of the electron density at the surface of the metallic material, fails to correctly describe the optical response of a gap antenna. To account for the effect of the spill-out of the electrons at the surface of the metal as well as the coherent tunnelling that can be established across the gap, full quantum mechanical calculations are necessary. However, quantum-mechanical calculations can tackle a limited number of electrons effectively limiting the size of the nanostructures that can be addressed. Time-dependent density functional theory (TDDFT) has been successfully applied to obtain the response of a few thousand electrons, but the billions of electrons present in a realistic plasmonic structure exceeds the current capabilities of quantum frameworks.
We present a new method to calculate quantum effects in large plasmonic systems based on parametric inputs derived from simpler full mechanical calculations. With this quantum corrected model (QCM) [1], it is possible to obtain extinction cross sections as well as near-field enhancements for situations involving subnanometric interactions, thus bridging the gap between classical and quantum plasmonics.
Furthermore, the quantum-corrected model is applied to describe the tunneling regime in an experimental situation where two metallic particles are located in subnanometric proximity. A simultaneous measurement of the transport properties and the optical characterization by dark-field microscopy allows for capturing the quantum regime in tunneling plasmonics by means of a sudden blue-shift of the plasmonic modes as the gap distance is decreased, consistent with the results shown in the figure [2]. Classical descriptions fail to address the modal distribution and the field enhancement in plasmonic gaps separated by less than 5 Å (10 a.u.). The presence of quantum tunnelling screens the charge densities induced at the gap and reduces the field enhancement establishing a fundamental quantum limit to the minimum volume of a metallic cavity where light can be trapped.
[1] R. Esteban, A. Borissov, P. Nordlander and J. Aizpurua, Nature Communications 3, 825 (2012).
[2] K. Savage, M. Hawkeye, R. Esteban, A. Borissov, J. Aizpurua and J. J. Baumberg, Nature 491, 574 (2012).
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