Optically excited plasmonic nanostructure display remarkable electron dynamics in the form of coherent electron displacement motion, as well as efficient generation of non-thermal ‘hot electrons’ at room temperature with kinetic energy substantially greater than kT. Here, we provide a theoretical framework of our studies of photo-enhanced 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 a symmetric plasmonic tunneling gap the redistribution of electrons due to photo-induced thermalization and hot electron generation is not sufficient to provide significant electrical currents, either through injection over the interface potential barrier, or via electron tunneling effect. However, asymmetric resonant structure can provide uneven optical absorption and photo-excitation across metallic tunneling junction 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, and thus comprises an intriguing mechanism for providing electrical work. We will introduce the theoretical frame work of tunneling phenomena associated with photo-excited hot electrons in plasmonic structures, including principles of hot 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 electron transport and power conversion based on unidirectional electron tunneling across nanoscale plasmonic junctions.