Titre de la dissertation: Resonant phenomena in photonic crystals with bound states in the continuum and Smith-Purcell radiation

Promoteur de thèse: Monsieur Bjorn Maes

Résumé de la dissertation: Photonic crystals and metasurfaces provide versatile platforms to control light through engineered guided resonances. In particular, Bound states In the Continuum (BICs), quasi-BICs, Unidirectional Guided Resonances (UGRs), and Smith-Purcell radiation enable high-Q resonances, directional emission, and tunable free-electron light sources. We use numerical simulations and semi-analytical models to study resonant phenomena in dielectric photonic structures, especially photonic crystals.

First, we investigate refractive-index sensing with quasi-BICs in a silicon metasurface made of asymmetric nanodisk dimers. Finite-element simulations, supported by experimental measurements, reveal two resonances with different electric and magnetic near-field characters. These differences explain their distinct sensitivities, sensing ranges, and robustness against refractive-index variations.

Then, we develop a multimodal interference model for BICs and UGRs. The model describes these resonances as resulting from the propagation, reflection, and interference of several Bloch modes guided transversely in the structure. It is applied to both two-dimensional asymmetric photonic crystals and the three-dimensional nanodisk metasurface used in the previous study, providing a compact interpretation of the resonances and an efficient way to explore the parameter space.

Additionally, we study Smith-Purcell radiation generated by an electron beam propagating between coupled high-index gratings. By laterally shifting the two gratings, top-down symmetry is broken and asymmetric emission is obtained. A coupled-mode theory model shows that the emission peaks arise from coupled guided Bloch modes.

Finally, we extend this concept to a moiré superlattice formed by two gratings with slightly different periods. This configuration preserves asymmetric emission while making the spectrum robust against lateral misalignment and enabling frequency-dependent control of the emission angle.