Photonic crystal enhanced fluorescence using a hybrid hexagonal boron nitride spacer and plasmonic gold cryosoret cavity.

The introduction of photonic technologies in fluorescence-based detection platforms such as point-of-care diagnostics enables highly reliable quantitative and qualitative analysis. Photonic crystals (PCs) and plasmonic nanoparticles (NPs) have individually shown promise, and combining them with radiating dipoles is expected to yield synergistic effects. However, integration has been thus far hindered by severe fluorescence quenching as a result of metal-fluorophore proximity in the so-called 'zone of inactivity'. Here, we show that ultrathin hexagonal boron nitride (hBN) can serve as an active insulating spacer to suppress nonradiative quenching while maintaining strong coupling between the localized surface plasmon resonance of gold nanoparticles and the guided mode resonance of an underlying photonic crystal. Furthermore, we fabricate tunable gold cryosoret nanoassemblies atop the fluorophore layer, creating nanocavity architectures that concentrate and amplify electromagnetic fields at the infinitesimal gap of radiating dipoles. This hybrid platform comprising photonic crystal-guided mode resonances, localized surface plasmon resonance, and Bragg-Mie hybrid modes from cryosoret assemblies is found to produce 650-fold enhancement, which corresponds to the attomolar limit of detection of a fluorescent reporter. Our study provides a new design strategy that maximizes fluorescence output while preventing detrimental energy loss pathways, a critical step for future applications in ultrasensitive biomarker detection and related technologies.