  Anisotropic nanostructured porous Pd films are fabricated using oblique angle
deposition in vacuum on a glass substrate. They display a dichroic response,
due to localised surface plasmon resonances (LSPR) within the nanoparticles
forming the film, dependent on the incident light polarisation. Ultrasensitive
hydrogen sensing is reached by using these films in conjunction with a
differential optical technique derived from the reflectance anisotropy
spectroscopy. The evolution of the samples optical responses is monitored
during the formation of Pd hydride in both the dilute alpha-phase and the dense
beta-phase, whilst the samples are exposed to different concentration of H2 in
Ar (from 100 percent H2 to a few ppm). The measurements are performed at a
single wavelength in the visible range and at 22 deg C. The results show that a
quantitative measurement of the hydrogen concentration in a carrier gas can be
measured throughout the concentration range. The limit of detection is 10 ppm
and the time for detecting the presence of H2 in the carrying gas is below one
second at concentration down to 0.25 percent of H2 in Ar. Furthermore, the
optical anisotropy of the samples and its evolution with exposure to H2 are
correctly reproduced with an effective medium theory.
