PhD defence by Benjamin Gøtzsche

Abstract

Since the invention of the single-beam optical tweezer, optical forces have been used for trapping particles of interest, and the optical tweezer has become an essential tool within biology, material science and basic physics. The optical tweezer is, however, bound by the diffraction limit, making direct trapping of small particles, such as single molecules, infeasible. By using nanophotonic resonators, inherently not bound by the diffraction limit, this limitation can be overcome.

Nanophotonic resonators confine light and may locally enhance the electromagnetic field. The local enhancement can be used to trap and manipulate single molecules or strongly localize a particle. Plasmonic resonators have been employed successfully for such purposes. A new generation of dielectric nanocavities with extreme dielectric confinement (EDC) has been experimentally shown to confine the optical mode deeply below the diffraction limit, a regime hitherto reserved for plasmonic resonators. With negligible Ohmic losses, such classes of dielectric nanocavities give access to an exciting parameter space for electromagnetic resonators.

This thesis investigates the use of dielectric nanocavities with EDC for nanophotonic trapping of single particles. Mapping the optical forces exerted on a particle present in a dielectric nanocavity is not straightforward, since the presence of the particle alters the nanophotonic environment. This is known as self-induced back-action, and the optical forces are typically calculated with computationally extensive full electromagnetic field simulations. We derive the optical forces exerted on a sphere in a first-principles framework, connecting the optical modes of a nanophotonic resonator to the optical force. Using the quasi-normal mode formalism, we propose two novel methods for evaluating the optical forces exerted on a spherical particle in a nanophotonic resonator without the use of fitting parameters.

Using a novel dielectric nanocavity for nanophotonic trapping, we compare the methods with reference calculations and find that the effect of self-induced back-action is captured for both dielectric and metallic nanoparticles. The proposed models can be useful for a clear interpretation of nanophotonic optical forces and lead to a reformulation of the standard optical trapping potential. Furthermore, the proposed methods are computationally inexpensive and reproducible. Spatial mapping the optical force exerted on the silica sphere in the resonator takes a few hours of computation time, contrary to the several weeks needed with full electromagnetic field simulations.

The proposed dielectric nanocavity may serve as a novel testbed for nanophotonic trapping with EDC, while the proposed methods constitute efficient tools for assessing and interpreting nanophotonic optical forces in a variety of nanophotonic resonators.

Supervisors

• Principal supervisor: Professor Martijn Wubs, DTU Electro, Denmark
• Co-supervisor: Senior Researcher Philip Trøst Kristensen, DTU Electro, DenmarkCo-supervisor: Professor Jesper Mørk, DTU Electro, Denmark

Evaluation Board

• Associate Professor Kirstine Beg-Sørensen, DTU Health, Denmark
• Professor Giovanni Volpe, University of Gothenburg, Sweden
• Senior Researcher Francesco Intravaia, Humboldt University of Berlin, Germany

Master of the Ceremony

• Associate Professor Thomas Christensen, DTU Electro, Denmark