Mid-infrared resonant optical tunneling induced bandpass filter and sensor at room temperature

Abstract In this work, we present a mid-infrared (mid-IR) thin-film narrowband interference filter operated in transmission mode. The structure is comprised with an air gap symmetrically bounded by thin-film-stack-coated prisms. We utilize the coupled mode theory (CMT) and transfer matrix method (TMM) to theoretically analyze the resonant tunneling effect. When the corresponding parameter conditions are satisfied, a high-performance bandpass filter can be achieved at incidence angles greater than the critical angle. 
 The high-index-contrast thin film stacks (TFSs) can lead to strong rejection of one polarization state at different wavelength bands.
 By introducing a refractive index-adjustable tunneling layer, enabling passband tuning over a very large wavelength range, which allows us to turn the wavelength dependence into an advantage for sensing application. The strategy in manipulating selective transmission responses holds great potential in realizing optical filters in mid-IR spectroscopy.