Controlling Ultrafast Photonic‐Plasmonic Coupling in a Hybrid Platform for Biosensing Applications

Photonic crystals (PhCs) are attractive for several applications due to their ability to confine light and enhance near-field interactions at low fabrication cost. Their optical response is characterized by the presence of a photonic bandgap (PBG). When combined with plasmonic nanostructures supporting localized surface plasmon resonances (LSPRs), they form metal–dielectric hybrids (MDHs) with enhanced and tunable optical functionalities. Here, we fabricate an MDH featuring gold nanocaps (NCs) on the surface of a colloidal PhC (opal) and characterize its optical response using static and femtosecond transient absorption (TA) spectroscopy. Static measurements reveal a broad plasmonic resonance, arising from the LSPRs sustained by the gold NCs. TA measurements uncover a strong interplay between the metal LSPRs and the opal PBG: the plasmon transient response is significantly enhanced by the PBG. For the first time, to the best of our knowledge, we demonstrate that this enhancement can be controlled by varying the excitation wavelength, owing to the slow-light regime, in agreement with the transfer matrix model predictions. Moreover, we show as proof-of-concept that the MDH ultrafast optical response is significantly modified by bacterial deposition, enabling ultrafast switching of the transient signal and underscoring the potential of these devices for biosensing applications.