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 ~300 nm, transmission efficiencies of 60-75% and narrowband FWHMs of 50-65 nm (Q-factor ~70-90). We further show multispectral thermal imaging and gas sensing. By matching the filter’s initial passband to a CO2 vibrational-absorption mode (~4.25 µm), tunable atmospheric CO2 sensing and dynamic plume visualization of added CO2 is realized.H.J.K and S.B acknowledge support from the NASA LaRC CIF/IRAD Program. C.W. acknowledges support from the Cancer Research UK Pioneer Award (C55962/A24669), Engineering and Physical Sciences Research Council (EP/R003599/1) and through the Wellcome Trust Interdisciplinary Fellowship