Developing Metal Mesh Filters for Mid-Infrared Astronomy of 25 to 40 micron

We have developed bandpass filters for long mid-infrared astronomy in 25 to 40μm. Most of materials become opaque in wavelengths longer than 25μm. We have applied the metal mesh method to make filters of non-transparent materials. The mesh patterns are designed based on the FDTD calculations and fabricated by the photolithography method. Measured transmittances of the fabricated filters agree with model calculations. The mesh filter has leakage in wavelengths shorter than the peak wavelength in principle. The most effective way to achieve a high stopband rejection is to stack several identical mesh filters incoherently. A narrow bandwidth fitted to atmospheric windows is required in the ground-based 30μm observations. We have fabricated a thick mesh filter without dielectric substrate, which is main source of internal absorption. The thick mesh leads to narrowing of the bandwidth due to the waveguide effect. The fabricated non-coated thick mesh filter has a peak transmittance of 0.8 and a bandwidth of λ/dλ=8.3 at 4 K. When stacking four of these mesh filters, it is expected to achieve a stopband rejection over 50dB, a peak transmittance of 0.41, and a bandwidth of λ/dλ=17.5.