Spatial and temporal symmetry-breaking in bound states in the continuum metasurfaces

Aigner, Andreas (2025): Spatial and temporal symmetry-breaking in bound states in the continuum metasurfaces. Dissertation, LMU München: Fakultät für Physik

Abstract

Controlling and enhancing light-matter interactions at the nanoscale is fundamental to modern photonics. It drives advances in solar energy harvesting, optical communications, and quantum technologies. Metasurfaces, engineered two-dimensional arrays of subwavelength resonators, have transformed this field by offering compact and tunable platforms for manipulating light at the nanoscale. Recently, a new class of high quality factor (Q-factor) resonances in metasurfaces have emerged, called symmetry-protected bound state in the continuum (SP-BIC). These modes enable strong light confinement while remaining tunable via geometric parameters. However, key challenges remain: SP-BIC metasurfaces are mostly limited to low-loss dielectrics, they require large arrays of resonators to sustain collective modes, and they lack effective active tunability. This thesis advances SP-BIC metasurfaces in multiple aspects: expanding material choices, overcoming array size constraints, and enhancing active control. First, plasmonic SP-BICs are introduced, leveraging both novel three-dimensional nanoprinted geometries and conventional metal-insulator-metal (MIM) designs. These structures combine the strong field enhancement of plasmonics with the high Q-factors of SP-BICs. Thus, they achieve perfect absorbance over broad spectral ranges and provide high sensitivity for molecular sensing. By measuring the near-fields of SP-BIC metasurfaces, new insights into their mode formation were obtained. The results indicate that the mode fully develops in significantly smaller arrays than previously expected and exhibits greater stability against perturbations. These findings motivated the development of dual-gradient metasurfaces, a new platform that continuously encodes both spectral and coupling parameters within a single structured area. Instead of relying on thousands of identical resonators to sustain one mode, smoothly varying unit cells enable the creation of 27,500 distinct modes in a single metasurface. These dual gradients were later employed for molecular sensing and investigations of ultra strong coupling (USC) in epsilon near zero (ENZ) materials. Finally, new active tuning strategies for SP-BICs are demonstrated. Incorporating the phase-change material VO2 allows for loss-driven quenching of SP-BIC modes with minimal off-resonance switching. Finally, the new concept of restored SP-BICs is introduced, which allows ultrafast radiative loss control using photoexcited charge carriers in silicon. In pump-probe experiments, the SP-BIC mode can be coupled or decoupled from the far-field on femtosecond timescales. The results presented in this thesis provide the foundation for next-generation nanophotonic devices with unprecedented material versatility, scalability, and tunability.

• Dokumententyp: Dissertationen (Dissertation, LMU München)
• Themengebiete: 500 Naturwissenschaften und Mathematik ; 500 Naturwissenschaften und Mathematik > 530 Physik
• Fakultäten: Fakultät für Physik
• Sprache der Hochschulschrift: Englisch
• Datum der mündlichen Prüfung: 1. April 2025
• 1. Bericht­erstatter:in: Tittl, Andreas