Permittivity-Driven Photonic Bound States in the Continuum for Ultrafast Optical Resonance Control

Dielectric metasurfaces supporting photonic bound states in the continuum (BICs) offer an exceptional platform for light confinement, yet conventional implementations almost universally rely on static geometric symmetry breaking, inherently limiting dynamic applications. This talk will present a departure from static designs by introducing resonance control mechanisms based on dynamic permittivity modulation and temporal symmetry breaking. Central to these approaches is the concept of permittivity-asymmetric quasi-BICs (ε-qBICs), which achieve resonance control by modulating material permittivity within a geometrically fixed unit cell. I will first discuss the theoretical framework of ε-qBICs, showing their equivalence to geometric asymmetry, followed by their experimental realization in the near-infrared using multi-material resonators. Extending this to dynamic systems, electrically reconfigurable ε-qBICs via environmental symmetry breaking will be demonstrated, showing robust resonance switching in the 10-20 millisecond range. Pushing into the ultrafast regime, I will describe the all-optical generation of ε-qBICs in featureless semiconductor films using transient permittivity gratings, enabling picosecond dynamics and spectrally selective enhancement of third-harmonic generation. Finally, I will introduce a new paradigm for all-optical resonance control based on temporal symmetry breaking in a "restored" symmetry-protected BIC (RSP-BIC): a system that is geometrically asymmetric but is engineered with cancelling dipoles to exhibit perfect photonic symmetry (i.e., a dark state). I will show how selective ultrafast pumping instantaneously breaks this restored symmetry, granting sub-picosecond control over radiative loss for dynamic resonance creation, annihilation, broadening, and sharpening at will. Together, these concepts demonstrate a powerful shift from fixed geometric asymmetry to dynamic permittivity and ultrafast resonance control for active nanophotonics.