  Spectroscopic gas sensing is both a commercial success and a rapidly
advancing scientific field. Throughout the years, massive efforts have been
directed towards improving detection limits by achieving long interaction
pathlengths. Prominent examples include the use of conventional multipass gas
cells, sophisticated high-finesse cavities, gas-filled holey fibers,
integrating spheres, and diffusive reflectors. Despite this rich flora of
approaches, there is a continuous struggle to reduce size, gas volume, cost and
alignment complexity. Here, we show that extreme light scattering in porous
materials can be used to realise miniature gas cells. Near-infrared
transmission through a 7 mm zirconia (ZrO2) sample with a 49% porosity and
subwavelength pore structure (on the order of 100 nm) gives rise to an
effective gas interaction pathlength above 5 meters, an enhancement
corresponding to 750 passes through a conventional multipass cell. This
essentially different approach to pathlength enhancement opens a new route to
compact, alignment-free and low-cost optical sensor systems.
