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Optically excited plasmonic nanostructures display remarkable electron dynamics in the form of coherent electron displacement motion, as well as efficient generation of non-thermal ‘hot electrons’ with large kinetic energy. Here, we provide a theoretical and experimental overview of our studies of photo-induced charge transport across plasmonic tunneling junctions composed of nanoscale metallic gaps, as a strategy for taking advantage of such electron motion for optoelectronic energy conversion.
In symmetric plasmonic tunneling gaps the energetic distribution of electrons due to photo-induced thermalization and hot electron generation is insufficient for significant electrical currents, either through excitation over the interface potential barrier, or via tunneling that exhibits a net preference for the direction of charge transfer. However, asymmetric resonant structures can provide optical absorption, photo-excitation and time-dependent electric fields that induce significant temperature gradients and local variations in the hot electron population. Such asymmetry can be used to promote unidirectional tunneling transport currents with significant enhancement compared with conventional photoelectron and thermionic emission (~ 10^15 enhancement), and thus comprises an intriguing mechanism for providing electrical work. This presentation will introduce the theoretical framework of tunneling phenomena associated with photon-induced hot electrons in plasmonic structures, including principles of electron distribution under photon excitation, strategies for amplifying hot electron generation (e.g. manipulating hot spots in nano-antennas) and provide a mechanistic quantum model of power conversion devices based on unidirectional electron tunneling across nanoscale plasmonic junctions. We also report on initial transport measurements of plasmonic tunnel junctions that exhibit optical power conversion by this method.
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