Controlling Thermal Light-Matter Interactions

• Author / Creator

  Molesky, Sean J.

• Since the development of the fluctuation dissipation theorem for electromagnetics in 1956 by Landau, Lipschitz and Rytov, field correlations have gradually come to be understood as a consistent framework for treating all optical aspects of linear response. In turn, the theory of electromagnetic correlations acts as a central unifying thread running through a diverse collection of optical phenomena: ranging from the definition of a medium's relative permittivity and rate of spontaneous emission to its electromagnetic entropy, extractable energy density and thermal radiation characteristics. With the maturation of the field of nanophotonics, an array of techniques have recently emerged for controlling these correlations via resonant wavelength scale structuring and polaritonic excitations. These results challenge long held views of the equivalence between high temperature and incoherence, and through the discovery of modified scaling laws open a new landscape of possibilities for heat energy harvesting devices. This dissertation brings together our original results examining the role that permittivity properties play in shaping these correlations; addressing open problems in far-field radiative engineering, near-field energy harvesting, and the theory of hyperbolic media. Background on the field and motivation of our approach is provided in Chapter One. Chapters Two and Three are then dedicated to the control of radiative thermal emission for energy harvesting applications. Here, we begin by presenting a new perspective for understanding the far-field thermal radiation arising from any nanostructure through the use of effective medium parameters. This metamaterial approach to radiative emission control is then used to originate two classes of selective emitter designs to meet the engineering challenges of capturing latent heat energy. As a functional example, we conceive and demonstrate a refractory metamaterial using simple multilayer nanostructuring to regulate thermal emission in the near infrared. Mastery of thermal radiation in this spectral range is crucial to thermophotovoltaic energy harvesting technologies, and we analyze the usefulness of metamaterial concepts for this application in detail. In Chapter Four, we reveal the influence of electronic characteristics on near-field electromagnetic energy transfer. Approaching relative permittivity as a black box response function, subject only to the requirements of causality and bandgap absorption, we derive the ideal response characteristics for maximizing the magnitude and efficiency of electromagnetic energy transfer in the near-field. This analysis reveals that the traditional bulk semiconductors, considered in previous near-field thermophotovoltaic work, are ill-suited for this type of energy capture. Moreover, it also shows that the presence of van Hove singularities, seen in any semiconductor with a quantum-confined dimension, offer a clear path for improving future near-field devices. Chapter Five then develops a definitive, and first numerically predictive, framework for regularizing electromagnetic field fluctuations inside natural hyperbolic media based on the presence of previously overlooked charge oscillations. These media have long been considered one of the most promising directions for nanophotonics, but long standing divergence issues have left their fundamental electromagnetic correlation characteristics undefined. Our theory overcomes this hurdle, and places definite upper bounds on the enhancement features and thermal energy density of these exotic media. To showcase the flexibility of our results, concrete, experimentally verifiable, predictions of the enhancement properties of the naturally hyperbolic materials bismuth selenide and hexagonal boron nitride are given. Finally, in Chapter Six, we summarize our results, and provide a brief outlook of the field.

• Subjects / Keywords

  • Hyperbolic Media
  • Electromagnetics
  • Metamaterials
  • Near-field
  • Thermal

• Graduation date

  Fall 2017

• Type of Item

  Thesis

• Degree

  Doctor of Philosophy

• DOI

  https://doi.org/10.7939/R34B2XJ90

• License

  This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.