PhD defence by Henrik Bødker Lassen

Abstract

Terahertz time-domain spectroscopy, combined with scattering-type scanning nearfield opticalmicroscopy, is pivotal in analysing materials' optical and electrical properties at the nanoscale. This thesis utilises this technique to investigate different materials and their properties. We study molybdenum disulfide nanoribbons in the terahertz range, extracting the real part of the permittivity as a two-dimensional map. Furthermore, we attempt to determine the full frequency resolved complex dielectric function for these nanoribbons in the range 0.5 to 1.5 THz. The second part investigates conductive materials at the nanoscale, challenging the theory of conductive surfaces as near-perfect reflectors. We demonstrate distinct conductivity contrasts in graphene layers and platinum thin films, supported by finite element method simulations, which align well with experimental data. This underscores the potential of terahertz techniques in analysing conductive surfaces' optical and electrical properties. Finally, we explore advanced research topics like resonant gold structures and magic-angle twisted bilayer graphene. The thesis underscores the breadth of terahertz spectroscopy's applications at the nanoscale, laying a foundation for future advancements.

Supervisors

• Principal supervisor: Associate professor Edmund Kelleher, Department of Electrical and Photonics Engineering, DTU
• Co-supervisor: Professor Peter Uhd Jepsen, Department of Electrical and Photonics Engineering, DTU

Evaluation Board

• Examiner: Professor, Rainer Hillenbrand, University of the Basque Country, Spain
• Examiner: Senior Lecturer, Jessica Boland, University of Manchester, UK

Chairman

• Professor Martijn de Wubs, Department of Electrical and Photonics Engineering, DTU

Master of the Ceremony

• Associate professor Binbin Zhou, Department of Electrical and Photonics Engineering, DTU