Hydrogen-tunable Fabry-Pérot resonator for high-response and visual hydrogen sensors

Hydrogen sensing is crucial for safely utilizing hydrogen gas (H₂). The soft substrate can effectively accommodate the expansion stress of palladium (Pd) film during hydrogenation, thereby transforming the gas-matter reaction into a light-matter interaction. This mechanism significantly enhances the optical response of H₂ absorption of the Pd film while enabling direct visualization of this process. Here, we introduce a hydrogen-tunable Fabry-Pérot (FP) resonator (HTFPR) comprising a PMMA/Pd bilayer on a soft spacer and a Pd mirror. The absorption/desorption of H₂ on the resonator exhibits repeatable and tunable modulation of FP resonance based on the surface wrinkling of the Pd film, leading to color variation. By optimizing the structural parameters, the resonator achieves an extremely high reflectance variation of approximately 43% with a response time of about 5 s in 4% H₂ mixed with air, which can be applied in visual hydrogen sensing. Furthermore, the resonator demonstrates great repeatability and selectivity towards H₂. These results indicate that the tunable Fabry-Pérot resonator is suitable for practical hydrogen detection.