Light impinging onto a grating covering a slab waveguide can be coupled into it to give rise to a guided mode resonance (GMR), which leads to a low intensity of the transmitted light. Such type of resonance occurs at specific wavelengths and angles of incidence that are sensitive to a deformation of the structure. Therefore, a slab waveguide covered with a grating can be used to optically monitor changes in pressure, provided such changes deform the structure sufficiently. To produce efficient pressure sensors, the authors of this work have designed a grating-waveguide structure based on a free-standing low-density polyethylene slab. A grating is first imprinted onto the polymer and is then covered by a nearly conformal high-refractive index titanium dioxide film. The structure supports a GMR, which is monitored with monochromatic light in an environmental chamber while varying gas pressure below the polymer slab to bend it in a controlled way. The intensity of the transmitted light is measured as a function of this pressure. The observed changes in the GMR can reveal pressure variations as small as 0.01 mbar in a pressure range from 3 to 18 mbar. The specifications of this embodiment are compared with those of some previously reported sensors, pointing at a better limit of detection and a lower material cost.
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