DYNAMICS AND APPLICATIONS OF PLASMONICALLY INDUCED RESISTANCE IN METAL NANOGRATINGS
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This dissertation examines the mechanisms that enable the detection of surface plasmons using direct electronic measurements rather than established optical techniques, and considers potential applications that arise as a result. Surface plasmons are optically excited in metal thin-film nanogratings fabricated from gold, aluminum, or titanium. It is shown that surface plasmon excitation leads to a measurable increase in electronic resistance that depends on both the polarization and the power of incident light. Additionally, it is shown that the electronic resistance of these plasmonic structures shows strong dependence on the wavelength of the incident light. Both of these results are in agreement with known momentum matching requirements of surface plasmon excitation. The different metals used offer access to different scattering and absorption mechanisms. It is thus shown that the resistance induced by surface plasmon excitation is primarily dependent on electron-electron scattering. This is in contrast to DC resistance, which is dominated by electron-phonon scattering at room temperature. A model is established that takes into account the scattering of electrons excited from surface plasmons with electrons from an applied current. This model considers the asymmetry of the Fermi level under an applied bias and the resulting imbalance in the momentum contributions of plasmonically excited hot electrons in the system. This imbalance leads to a restoring of the overall system momentum that does not depend strongly on electron-phonon interactions. The intrinsic polarization sensitivity of these plasmonic detectors is leveraged to demonstrate direct polarization imaging using a single-pixel scanning technique. This offers potential applications in hyperspectral imaging and remote sensing where an integrated detector would be of value. We find from this data that the upper limit of angular resolution of polarization states using a single nanograting is 4◦. We investigate the dependence of the electronic response to the spatial distribution of illumination across a nanograting and show that not only can the active grating area be made smaller to improve image resolution for implementation as an arrayed sensor, but a smaller active area is likely preferable for detector uniformity. Since it is possible to distinguish different scattering mechanisms in time, we investigate the ultrafast dynamics of the electronic response in such a structure using a degenerate two-pulse resistance scheme. In this case, two ultrafast laser pulses impinge on the sample and the resulting induced voltage is measured as a function of the time delay between laser pulses. This gives a measure of the time resolved response of the detector, as well as provides some insight into the relaxation of plasmonically excited hot electrons on ultrafast timescales. In particular, the use of a DC current allows access to these processes beyond the skin depth, a region to which traditional optical pump-probe transient transmission/reflection spectroscopy is limited. We find from these measurements that this class of plasmonic detector has a resistance that depends on the time delay between laser pulses even when that delay is on the order of single picoseconds. In terms of dynamics, we see a large peak on timescales that correspond to electron-electron scattering, that is, on sub-picosecond timescales. On longer timescales, we see a fast increase in the measured resistance on the order of tens of picoseconds, followed by a slower rise in the resistance on the order of hundreds of picoseconds, after which they system response stabilizes. This ultrafast behavior of such plasmonic detectors allows a bandwidth near THz, and offers potential in optical communications applications, where detector response time is a limiting factor.