Double Resonant Cavity of Oxides/Metal/Nitrides for Long Wavelength Infrared Sensing

Subwavelength double resonant cavity structures of oxides/metal/nitrides (OMN) for bolometric detection in the long wavelength infrared (LWIR) range are studied for the first time. An optimized OMN structure allows for a significant enhancement in effective absorbance (50–70%) with respect to that of the bare film on an amorphous Al2O3 substrate (1–30%) in the LWIR atmospheric transparent window (8–14 μm). Remarkable improvement in the response range and sensitivity has been achieved by designing the double cavity OMN structure, where the bottom cavity originates from the high reflectance between the transition metal oxide film and the Al2O3 substrate, while the silicon nitride film on the top serves as a quarter-wavelength antireflection cavity centered at about 8 μm. The OMN structured LWIR detector can be realized by cost-effective methods of chemical solution deposition (CSD) or radio frequency (RF) sputtering deposition. A detectivity of (0.18–0.2) × 108 cm·Hz0.5/W at 6–10 Hz was achieved by using the OMN structured detector. Good sensitivity and improved flatness in response spectra were realized over a broadband of 3–14 μm, which provides potentials for applications in infrared spectroscopy and thermal imaging.