PhD defense by Enok Johannes Haahr Skjølstrup
Theoretical light-matter interaction on nanoscale - Plasmonic properties of noble metal structures
02.03.2020 kl. 13.00 - 16.00
This thesis deals theoretically with the interaction between light and noble metal structures on nanoscale. This interaction can involve the excitation of a particular wave, called a surface plasmon polariton (SPP), which can be found in many different structures. The following structures are considered in this thesis: An array of ultrasharp or rectangular grooves in gold, a dielectric gap located between two parallel metal walls, and a metal slab sandwiched between different dielectrics.
First a classical model is applied to study the transition from one to multiple grooves in gold. Surprisingly, the optical cross sections are found to scale almost linearly with the number of grooves. Furthermore, when the incident field is a narrow Gaussian beam focused entirely with an array of 20 grooves, the reflectance becomes the same as for an infinite array illuminated by a plane wave.
Quantum effects are included, where the electron density is calculated using density-functional theory in the jellium model. The gap plasmon mode index converges to the refractive index of bulk gold in the limit of vanishing gap width, thereby restoring the correct physical behavior. The imaginary part of the slab plasmon mode index is significantly enhanced, and surprisingly, for wide slabs approaching bulk, the increase is 20 %. This is explained in terms of strong plasmonic absorption mostly taking place at narrow peaks a few Å outside the surface.
Finally, electron-energy loss spectroscopy (EELS) is studied, with an electron incident onto a thin metal film, thereby losing energy when exciting SPPs. Surprisingly, the energy losses calculated using classical and quantum models are in good agreement. This is explained in terms of electron motion in the metal, which mainly takes place in the direction parallel to the film, thus reducing the effects from quantum confinement.
Department of Materials and Production
Skjernvej 4 A, 9220 Aalborg East, room 5.018