Metal-mesh linear variable bandpass filter for far-infrared wavelengths

Future far-infrared (IR) observatories require compact and cost efficient optical linear variable bandpass filters (LVBFs) to define their instrument spectral bands. We have designed novel far-IR LVBFs, to our knowledge, that consist of metal-mesh bandpass filters comprising a gold film with cross-slots of varying sizes along a silicon (Si) substrate with anti-reflection (AR) coatings. We present our work on the simulated and measured transmission of non-AR coated and AR coated LVBFs for bandpass peaks from wavelengths of 24 to 36 µm with a resolving power ( R = λ 0 /Δ λ ) of R ≈6 for non-AR coated LVBFs and R ≈4 for AR coated LVBFs. We also present a method to decrease the effects of out-of-band high-frequency transmission exhibited by metal-mesh filters by depositing a thin layer of hydrogenated amorphous silicon (a-Si:H) on the metal-mesh of the LVBF. We have fabricated and measured the LVBFs at room temperature and cryogenic temperatures (5 K). We measure a high peak transmission of ∼80%−90% for the AR coated LVBF at 5 K and demonstrate that the a-Si:H LVBF is a promising method to address out-of-band high-frequency transmission.