Electron Energy Loss Spectroscopy for Probing Nanophotonic Excitations at High Energy and Momentum

• Author / Creator

  Shekhar, Prashant

• Strong nanoscale light-matter interaction is often accompanied by ultra-confined photonic modes and large momentum polaritons existing far beyond the light cone. A direct probe of such phenomena is difficult due to the momentum mismatch of these modes with free space light however, fast electron probes can reveal the fundamental classical, quantum and spatially dispersive behavior of these excitations. In chapter 2, we use momentum-resolved electron energy loss spectroscopy (k-EELS) in a transmission electron microscope to explore the optical response of plasmonic thin films including momentum transfer up to wavevectors (k) significantly exceeding the light line wave vector. We show close agreement between experimental k-EELS maps, theoretical simulations of fast electrons passing through thin films and the momentum-resolved photonic density of states (k-PDOS) dispersion. Although a direct link between k-EELS and the k-PDOS exists for an infinite medium, here we show fundamental differences between k-EELS measurements and the k-PDOS that must be taken into consideration for realistic finite structures with no translational invariance along the direction of electron motion.Chapter 3 builds on the foundations of chapter 2 to probe silicon thin films and probe its properties in a completely new regime of the spectrum with k-EELS. Silicon is widely used as the material of choice for semiconductor and insulator applications in nano-electronics, MEMS, solar cells and on-chip photonics. In stark contrast, in this thesis, we explore silicon's metallic properties and show that it can support propagating surface plasmons, collective charge oscillations, in the extreme ultra-violet (EUV) energy regime not possible with other plasmonic materials such as aluminum, silver, or gold. This is fundamentally different from conventional approaches where doping semiconductors is considered necessary to observe plasmonic behavior. We experimentally map the photonic band structure of extreme ultra-violet (EUV) surface and bulk plasmons in silicon using k-EELS. The experimental observations are validated by macroscopic electrodynamic electron energy loss theory simulations as well as quantum density functional theory calculations. As an example of exploiting these EUV plasmons for applications, we propose a tunable and broadband thresholdless Cherenkov radiation source in the EUV using silicon plasmonic metamaterials.In chapter 4 we expand the use of k-EELS to probe more exotic nanophotonic structures in the form of Bi2Te3, a naturally occurring hyperbolic material. Hyperbolic materials, uniaxial structures with a metallic response along one direction and dielectric response along the orthogonal direction, support unique electromagnetic modes with a wide variety of deep subwavelength applications in waveguiding, imaging, sensing, quantum and thermal engineering beyond conventional devices. They derive their name from their unique hyperbolic isofrequency typology that can support photonic excitations at large wave-vectors (high-k modes) that would normally decay in conventional media. With k-EELS we perform the first measurements of the high-k modes in Bi2Te3 and confirm its natural hyperbolic character from the visible to the UV. k-EELS proves to be the ideal tool for probing hyperbolic media as the relativistic electrons have a high momentum that are able to couple to Bi2Te3's large momentum (high-k) states that are difficult to probe optically. Additionally, we expand on the theoretical ideas proposed in chapter 4 to perform the first measurement of hyperbolic Cherenkov radiation in a natural hyperbolic material and discuss its unique thresholdless Cherenkov radiation properties. The work in this thesis paves the way for using k-EELS as the preeminent tool for mapping the k-PDOS of exotic phenomena with large momenta (high-k) such as hyperbolic polaritons, Cherenkov radiation and spatially-dispersive plasmons. In addition k-EELS can also probe excitations at high energy that are difficult to probe optically. As a result, this work has laid the foundations for a focused application: a coherent, compact, tunable, and broadband EUV source.

• Subjects / Keywords

  • nanophotonics
  • extreme ultra-violet source
  • plasmonics
  • electron energy loss spectroscopy
  • metamaterials

• Graduation date

  Spring 2019

• Type of Item

  Thesis

• Degree

  Doctor of Philosophy

• DOI

  https://doi.org/10.7939/r3-dhnp-pr09

• License

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