Design Principles for Sensitivity Optimization in Plasmonic Hydrogen Sensors.

Palladium nanoparticles have proven to be exceptionally suitable materials for the optical detection of hydrogen gas due to the dielectric function that changes with the hydrogen concentration. The development of a reliable, low-cost, and widely applicable hydrogen detector requires a simple optical readout mechanism and an optimization of the lowest detectable hydrogen concentration. The so-called "perfect absorber"-type structures, consisting of a layer of plasmonic palladium nanoantennas suspended above a metallic mirror layer, are a promising approach to realizing such sensors. The absorption of hydrogen by palladium leads to a shift of the plasmon resonance and, thus, to a change in the far-field reflectance spectrum. The spectral change can be analyzed in detail using spectroscopic measurements, while the reflectance change at a specific wavelength can be detected with a simple photometric system of a photodiode and a monochromatic light source. Here, we systematically investigate the geometry of cavity-coupled palladium nanostructures as well as the optical system concept, which enables us to formulate a set of design rules for optimizing the hydrogen sensitivity. Employing these principles, we demonstrate the robust detection of hydrogen at concentrations down to 100 ppm. Our results are not limited to hydrogen sensing but can be applied to any type of plasmonic sensor.