PhD defence by Johan Rosenkrantz Maack
Title: Nonlocal response in metals and Semiconductors
Supervisors:
Principal supervisor: Assoc. Prof. Martijn Wubs, DTU Fotonik
Co-supervisor: Professor N. Asger Mortensen, University of Southern Denmark
Evaluation Board:
Assoc. Professor Andrei Lavrinenko, DTU Fotonik
Assist. Prof. Antonio Fernández-Domínguez, Universidad Autónoma de Madrid, Spain
Assist. Prof. Marcello Ferrera, Heriot-Watt University, Edinburgh Campus, UK
Master of the Ceremony:
Dr. Radu Malureanu
Abstract:
In this thesis, the optical properties of the free-electron gas in metals and semiconductors are analysed theoretically using various nonlocal models. Nonlocal response is a phenomenon which is not accounted for in the classical theory for the electron gas, and it is a property that will become increasingly more significant as the structures approach the nanometer scale. In particular, the collective excitations of the electrons, known as plasmons, will depend on these size-dependent, nonlocal effects. We use the hydrodynamic Drude model (HDM) to analyse the optical response of spherical metal particles, in which it predicts a size-dependent resonance shift of the localized surface plasmon (LSP) that is not found in the classical theory. We also analyse the implications of nonlocal effects for an ensemble of particles with different sizes. We find that the combination of the size-dependent resonance shift and the distribution of particle sizes results in an effective broadening of the resonance peak. Semiconductors may also contain a free-electron gas from doping or from the thermal distribution of electrons in intrinsic semiconductors, and we adapt the HDM to these two scenarios. We find that the relative size-dependent resonance shift of the LSP is much larger than in metals, which opens up for new experimental investigations in nonlocal effects. Semiconductors are furthermore promising as plasmonic materials because they offer a tunability of the optical properties that is not possible in metals. This is also analysed with the HDM. Finally, an extended version of the HDM is developed to properly describe semiconductors with several different kinds of charge carriers, like electrons and holes. An extended version of the Mie theory is developed to account for the two kinds of charge carriers, and we use this to find the optical properties of semiconductor particles. We find that the two-fluid model predicts plasmon resonances that are completely absent in the single-fluid HDM.