Dispersion-engineered multilayer films for angle-insensitive narrowband filtering via material dispersion

Narrowband long-wave infrared (LWIR) optical filters are of great importance for applications in environmental monitoring, thermal imaging, and medical diagnostics. However, conventional all-dielectric interference filters suffer from intrinsic limitations, including strong sensitivity to the angle of incidence and a pronounced blue shift of the transmission peak with increasing angle, which severely degrades their performance. In this work, we systematically investigate the factors governing the angular sensitivity of multilayer narrowband filters and propose a novel angle-insensitive filtering strategy based on multilayer dispersion engineering. By introducing a highly dispersive material—germanium-doped layers—into a conventional Fabry–Pérot (F–P) cavity, the wavelength-dependent material dispersion is exploited to compensate for the phase shift induced by oblique incidence, thereby significantly suppressing angular sensitivity. At 11 μm, when the incident angle increases to 40°, the relative resonance drift of the dispersion-tailored filter is suppressed from 2.6% to 1.4%, representing a 185% improvement in angular robustness. Moreover, this approach preserves the advantages of high refractive index contrast while simultaneously enhancing the transmittance and narrowing the full width at half maximum, providing a simple and efficient route toward high-performance wide-angle infrared micro- and nanophotonic devices.