  Optical nanosensors are promising for hydrogen sensing because they are
small, free from spark generation, and feasible for remote optical readout.
Conventional optical nanosensors require broadband excitation and
spectrometers, rendering the devices bulky and complex. An alternative is
spatial intensity-based optical sensing, which only requires an imaging system
and a smartly designed platform to report the spatial distribution of
analytical optical signals. Here, we present a spatial intensity-based hydrogen
sensing platform based on Fano-like spatial distribution of the transmission in
a Pd-Al2O3-Au metal-insulator-metal plasmonic Doppler grating (MIM-PDG). The
MIM-PDG manifests the Fano resonance as an asymmetric spatial transmission
intensity profile. The absorption of hydrogen changes the spatial Fano-like
transmission profiles, which can be analyzed with a "spatial" Fano-resonance
model and the extracted Fano resonance parameters can be used to establish
analytical calibration lines. While gratings sensitive to hydrogen absorption
are suitable for hydrogen sensing, we also found hydrogen insensitive gratings,
which provide an unperturbed reference signal and may find applications in
nanophotonic devices, that require a stable optical response under fluctuating
hydrogen atmosphere. The MIM-PDG platform is a spectrometer-free and
intensity-based optical sensor that requires only an imaging system, making it
promising for cellphone-based optical sensing applications.
