Tunable mid-wave infrared Fabry-Perot bandpass filters using phase-change GeSbTe

We demonstrate spectrally-tunable Fabry-Perot bandpass filters operating across the MWIR by utilizing the phase-change material GeSbTe (GST) as a tunable cavity medium between two (Ge:Si) distributed Bragg reflectors. The induced refractive index modulation of GST increases the cavity's optical path length, red-shifting the passband. Our filters have spectral-tunability of similar to 300 nm, transmission efficiencies of 60-75% and narrowband FWHMs of 50-65 nm (Q-factor similar to 70-90). We further show multispectral thermal imaging and gas sensing. By matching the filter's initial passband to a CO2 vibrational-absorption mode (similar to 4.25 mu m), tunable atmospheric CO2 sensing and dynamic plume visualization of added CO2 is realized. Published by The Optical Society under the terms of the Creative Commons Attribution 4.0 License.