Wavefront-Engineered Pd Metasurface for Spectrometer-Free Single-Point Photonic Hydrogen Readout

Palladium (Pd) hydride exhibits a reversible modulation of its plasmonic response under hydrogenation, but translating this material sensitivity into scalable photonic readout architectures remains challenging. Here we introduce a metasurface concept in which hydrogen-induced changes in Pd are directly mapped into far-field intensity variations through wavefront-engineered off-axis focusing. A nonlinear phase-encoding strategy enables deterministic conversion of the Pd hydrogenation state into spatially filtered focal signals, allowing single-point electrical readout without spectrometers or imaging systems. The resulting platform establishes a direct link between chemical-state modulation and photonic wavefront control, where hydrogenation-dependent plasmonic variations are transduced into monotonic intensity changes at a fixed detection point. Noise-limited analysis indicates that the encoded response remains resolvable under realistic detection conditions, while structural perturbations preserve the monotonic mapping. This work provides a general route toward chemical-state-to-intensity photonic transduction using metasurface wavefront engineering.