Probing nanoscale light-matter interactions in photonic and plasmonic nanostructures
thesisposted on 06.05.2020 by Harsha Vardhana Eragam Reddy
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This thesis describes the development of experimental methods to probe the nanoscale light-matter interactions in photonic and plasmonic nanostructures. The first part of this thesis presents the experimental findings on the temperature evolution of optical properties in important plasmonic materials. Understanding the influence of temperature on the optical properties of thin metal films - the material platforms for plasmonics - is crucial for the design and development of practical devices for high temperature applications in a variety of research avenues, including plasmonics, novel energy conversion technologies and near-field radiative heat transfer. We will first introduce a custom built experimental platform comprising a heating stage integrated into a spectroscopic ellipsometer setup that enables the determination of optical properties in the wavelength range from 370 nm to 2000 nm at elevated temperatures, from room temperature to 900 oC. Subsequently, the temperature dependent complex dielectric functions of gold, silver and titanium nitride thin films that were obtained using the above described experimental platform will be presented. Furthermore, the underlying microscopic physical processes governing the temperature evolution and the role of film thickness and crystallinity will be discussed. Finally, using extensive numerical simulations we will demonstrate the importance of incorporating the temperature induced deviations into numerical models for accurate multiphysics modeling of practical high temperature nanophotonic applications.
The second part of this thesis focuses on the development of experimental techniques to quantify the nanoscale steady-state energy distributions of plasmonic hot-carriers. Such hot-carriers have drawn significant research interest in recent times due to their potential in a number of applications including catalysis and novel photodetection schemes circumventing bandgap. However, direct experimental quantification of steady-state energy distributions of hot-carriers in nanostructures, which is critical for systemic progress, has not been possible. Here, we show that transport measurements from suitably chosen single molecular junctions can enable the quantification of plasmonic hot-carrier distributions generated via plasmon decay. The key idea is to create single molecule junctions - using carefully chosen molecules featuring sharp molecular resonances - between a plasmonic nanostructure and the gold tip of a scanning tunneling microscope, and quantify the hot-carrier distributions form the current flowing through the molecular junctions with and without plasmonic excitation at various voltage biases. Using this approach, we reveal the fundamental role of surface-scattering assisted absorption - Landau damping - and the contributions of different plasmonic modes towards hot-carrier generation in tightly confined nanostructures. The approach pioneered in this work can potentially enable nanoscale experimental quantification of plasmonic hot-carriers in key nanophotonic and plasmonic systems.