Ultrathin-film optical coating for angle-independent remote hydrogen sensing

We demonstrated an optically-active antireflection, light absorbing, optical coating as a hydrogen gas sensor. The optical coating consists of an ultrathin 20 nm thick palladium film on a 60 nm thick germanium layer. The ultrathin thickness of the Pd film (20 nm) mitigates mechanical deformation and leads to robust operation. The measurable quantities of the sensors are the shift in the reflection minimum and the change in the full width at half maximum of the reflection spectrum as a function of hydrogen gas concentration. At a hydrogen gas concentration of 4%, the reflection minimum shifted by ∼46 nm and the FWHM increased by ∼228 nm. The sensor showed excellent sensitivity, demonstrating a 6.5 nm wavelength shift for 0.7% hydrogen concentration, which is a significant improvement over other nanophotonic hydrogen sensing methods. Although the sensor's response showed hysteresis after cycling hydrogen exposure, the sensor is robust and showed no deterioration in its optical response after hydrogen deintercalation.