Hydrogen absorption effects on the transmittance of sub-wavelength palladium hole arrays with different thicknesses

Abstract

The far-field extraordinary optical transmission (EOT) of palladium (Pd) sub-wavelength hole arrays in the infrared region is used to detect hydrogen near the lower flammability threshold in air. Upon exposure to hydrogen, the Pd layer of the hole array expands, causing changes in the hole structure, and the Pd permittivity decreases. These two effects shift the main EOT transmittance peak of the Pd hole array to longer wavelengths. In this report, the effect of the Pd layer thickness on the redshift is analyzed by the rigorous coupled wave analysis technique and experimental observation. Our computational and experimental results show that the hole structural effect on the peak shift is dominant in the opaque region of the Pd layer transmission, whereas the Pd permittivity effect is dominant in the semi-transparent region. The optimum Pd layer thickness for hydrogen sensing is found to be at the boundary between the semi-transparent and the opaque regions of the Pd layer.