Exploring the Decay Channels of Plasmons
Halas, Naomi J.
Doctor of Philosophy
When an incident electromagnetic wave at optical frequencies couples to a metallic nanostructure, it creates coherent resonances of free carriers in the metal known as plasmons. Plasmons can decay and generate carriers (hot electrons) that have higher energies than the Fermi level of the metal. Plasmon-induced hot electrons are important since they carry the energy signature of the absorbed incident photons. When a metallic nanostructure is patterned on a silicon substrate, hot electrons can be transported over the junction between the metal and the semiconductor, to be collected as a photocurrent. Plasmonic absorbers that generate hot electrons are tunable over much broader spectral regimes than semiconductor absorbers. However, the poor responsivity (~ 1mA/W) is the major drawback with these devices. Thus, increasing the photocurrent response is crucial for hot electron devices. In my research, we increase the current flow of hot electrons over a Schottky junction by modulating the Schottky barrier in reverse bias and we acquire a signal that is much greater than the original hot electron flow of the nanostructures. With this method we amplify the photocurrent signal using a CMOS compatible fabrication process on silicon substrates. Our approach to amplify the hot electron-based photocurrent opens up the possibility of making cheap plasmonic sensors with direct electrical readout, such as an on-chip plasmonic detector with tunable wavelength sensitivity that can operate beyond the conventional semiconductor photodetectors. Along with generating hot carriers in the absorptive decay channel, plasmons also lose some part of their energy to radiation. The plasmonic resonances of metallic nanoparticles concentrate electromagnetic fields into nanoscale regions. We used this property of plasmons to focus light on ultra-thin semiconductors (such as monolayers of MoS2) to increase the light absorption in the material and therefore increase the photocurrent signal. In this work, by tuning the plasmon resonances of gold nanoshells to the direct band gap of monolayer MoS2 and depositing them onto the surface of the material, we acquired a threefold increase in photocurrent and photoluminescence signal for the excitonic transitions of the monolayer. This finding provides a new mechanism toward increasing the quantum efficiency of ultra-thin semiconductors for opto-electronic applications.