Highly efficient and angle-tolerant mid-infrared filter based on a cascaded etalon resonator

Mid-infrared of the 3 - 5 μm spectral region has attracted considerable attention in various fields. We report a highly efficient and angle-tolerant mid-infrared filter based on a cascaded etalon resonator. Two hybrid etalon resonators are serially stacked to make a single peak resonance with a well-suppressed sideband. Each etalon resonator, which exploits a pair of cavities, includes not only a cavity featuring a resonance but also a functional cavity lowering the sideband of transmission spectrum. By taking advantage of double cavities, each etalon features a resonant characteristic with suppressed sideband. Consequently, the filter shows a peak transmission of 92% at resonance and well-suppressed sideband within the 3 - 5 μm spectral region. Transparent dielectric materials make the device highly efficient for both transmission and reflection. Furthermore, the filter utilizing a high-index cavity leads to an enhanced angular tolerance regardless of incident light polarization. The transmission of the filter exhibits center wavelength shifts of around 0.1 μm at an angle of 50° for both transverse electric and transverse magnetic modes, compared to normal incidence.