Towards high-performance resonant filter for the long-wave infrared band: fabrication and characterization of notch filters based on the guided-mode resonance effect

The long-wave infrared (LWIR) spectroscopy has emerged as a promising technique for applications ranging from medical diagnosis to satellite imaging and terrestrial imaging. However, traditional optical elements are not realized well for the LWIR region. In this work, notch filters for LWIR spectral range (8 ~ 12 μm) based on the guided mode resonance (GMR) effect were designed, fabricated, and characterized with Germanium (Ge) thin film on Zinc Selenide (ZnSe) substrates. In contrast to the typical photolithography process, which faces challenge of resolution limit, we used e-beam lithography process to pattern the grating. By using a reactive ion etching process with a mixture of etching and passivation gases, we fabricated grating with well-defined vertical sidewalls. Finite-difference time-domain (FDTD) method was used to calculate the optical responses and model the geometry of the notch filters, and the optical transmittance of fabricated filters agrees well with the calculations. Moreover, we demonstrate the improved notch filtering response with reduced Fabry-Perot noise by introducing anti-reflection layer on the bottom of the ZnSe substrate. Therefore, findings of this work will be useful for various filter fabrications that prefer high spatial resolution in the LWIR spectral region.