Plasmonic Nano-Candle Platform for Phase-Resolved Optical Tracking of Pd Hydrogenation with Structure-Sensitive Readout

Hydrogen detection requires platforms that are not only highly sensitive but also capable of tracking inherent gas–matter dynamics. This study reports a Λ-shaped Pd/Au nano-candle array that leverages reverse-tapered nano-slits to amplify light–matter interactions and provide dual-mode spectral readouts through extremely strong light localization and enhancement. Resonance shifts were interpreted as being dominated by dielectric variations during Pd→PdH transformation, whereas transmittance changes were used as a structure-sensitive readout to monitor phase-associated gap evolution under ambient conditions. Comparison between the optical measurements and finite-difference time-domain simulations enabled a model-assisted interpretation of phase-dependent optical responses and effective gap evolution during Pd hydrogenation. The optimized sensing platforms with Pd/Au achieved rapid response (< 5 s) and a low detection limit of 0.05% H₂ with high reversibility, outperforming the corresponding Pd-only design. Beyond hydrogen monitoring, this dual-mode nanophotonic strategy offers a broadly applicable framework for analyzing coupled optical and structural responses in functional materials, enabling new opportunities in catalytic sensing, reconfigurable meta-surfaces, and adaptive photonic devices.