Metal-Mesh Linear Variable Filter for Far-Infrared Wavelengths

TL;DR

This paper introduces a novel metal-mesh linear variable filter for far-infrared wavelengths, demonstrating its design, fabrication, and performance improvements at cryogenic temperatures for future IR observatories.

Contribution

The work presents a new design of metal-mesh LVBFs with anti-reflection coatings and a method to reduce out-of-band transmission using a-Si:H layers, validated through simulations and measurements.

Findings

Achieved high peak transmission of 80-90% at 5 K

Demonstrated effective reduction of out-of-band transmission with a-Si:H coating

Validated filter performance at cryogenic temperatures

Abstract

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 that consist of metal-mesh bandpass filters comprised of 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 $\mu$m with a resolving power ($R=\lambda_0/\Delta\lambda$) of R$\approx$6 for non-AR coated LVBFs and R$\approx$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 $\sim$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.