UW-Madison researchers have developed an uncooled material structure that enables ultra-narrowband and directional absorption in the infrared that can be fabricated with standard CMOS processes. By tuning the thicknesses, doping, and composition of the different layers, sharp absorption peaks from wavelengths of 2 to 15 µm could be achieved. The light is absorbed almost entirely in the middle high-index layer, which means this structure could be used in form factors and fabrication processes similar to microbolometers. This layer could also be integrated with other sensing materials and structures, such as plasmonic or 2D materials, to enable other sensing modalities. Finally, because of the reciprocity between absorption and thermal emission, this structure could enable narrowband and directional thermal emission, which would be useful in gas or chemical sensing applications.
The structure is a vertical planar stack of a metal back reflector, a dielectric or air spacer, a high-index absorber layer (doped silicon or a similar material), a dielectric or air spacer, and a top high-index non-absorbing layer (such as intrinsic silicon). The two quarter-wavelength spacers above the original stack act as an impedance transformer and match the effective optical impedance of the structure to that of free space, further enhancing absorption.