Strong light–matter interactions in extreme plasmonic and Mie-resonant systems

Elli Stamatopoulou

Research output: ThesisPh.D. thesis

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Abstract

In this thesis we develop and apply analytic methods for analyzing the interaction of light and matter in extreme plasmonic and Mie-resonant systems. Over the past few decades, optical excitations in matter, either in the form of plasmons in metals or Mie resonances in high-index dielectrics,
have emerged as potential building blocks for novel photonic devices that require efficient light manipulation. Owing to their ability to strongly enhance and confine incident electromagnetic fields, plasmonic elements are widely employed in sensors, nanoantennas, or metasurfaces, for applications ranging from optical and quantum communications to (bio)sensing and polaritonic chemistry. In applications where the high Ohmic losses and the subsequent heat generation are detrimental, dielectrics are being explored as an alternative for low-loss resonators. Even though Mie-resonant systems do not achieve as high a field confinement as plasmonic ones typically do, they support modes of both electric and magnetic multipolar character, thus exhibiting a richer optical response. Through multipolar interference, dielectrics yield high quality factors and controllable radiation emission, making them eligible components for advanced nanolasers, topological photonics, and sensing.

The thesis addresses the optical excitations sustained in metallic and dielectric nanospheres and their interaction with light, quantum emitters, and fast electrons. First, we discuss the strong coupling and the resulting hybrid light--matter modes that form when certain configurations of core--shell nanoparticles are illuminated. The structures studied here consist of an open nanocavity, in the form of one or more nanoparticles supporting plasmonic or Mie modes, encapsulated in a shell that sustains excitons, i.e. electronic transitions such as the ones found in low-dimensional semiconductors or organic molecules. We then proceed to investigate the impact of this modal hybridization on the chiroptical response of chiral nanostructures, and find that circular dichroism can act as an imprint of strong coupling.
Secondly, we examine the weak coupling of quantum emitters featuring a single optical transition with metallic or dielectric nanocavities, and compare how the two types of materials modify the excitation and emission properties of the emitter when the system is excited by light. Following the direction of modern photonics towards the integration of ever smaller components, in both coupling regimes we explore systems of nanometric dimensions, where the limits of classical and local material-response descriptions are inevitably exposed. Rather than developing a radically new theory, we show how nonclassical effects in plasmonics, such as electron-density spill-out, screening, and enhanced Landau damping, can be incorporated into classical electrodynamics via nonlocal corrections, which apply mainly to the boundary conditions.

However, to study the properties of nanoscale structures, spectroscopy techniques of atomic resolution are required. To this end, we redirect our focus from light to electrons as sources for probing optical excitations in matter, and develop analytic tools for simulating and interpreting measurements in cathodoluminescence and electron energy-loss spectroscopy. We cast particular attention on metallic and dielectric nanospheres excited by penetrating electron beams, and examine the coexistence of the resonant modes of the structure together with other potential emission or electron energy-loss mechanisms. In this regard, we provide a detailed account of the interplay between Mie resonances and transition radiation in the cathodoluminescence spectra of silicon nanoparticles.
Original languageEnglish
Awarding Institution
  • University of Southern Denmark
Supervisors/Advisors
  • Tserkezis, Christos, Supervisor
  • Mortensen, N. Asger, Supervisor
  • Wolff, Christian, Supervisor
Date of defence14. Dec 2023
Publisher
DOIs
Publication statusPublished - 1. Dec 2023

Keywords

  • nanophotonics
  • plasmonics
  • Mie-resonant photonics
  • strong coupling
  • electron-beam spectroscopy
  • cathodoluminescence
  • electron energy-loss spectroscopy
  • fluorescence
  • mesoscopic plasmonics

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