Developing metal coated mesh filters for mid-infrared astronomy

A metal mesh filter is appropriate to a band-pass filter for astronomy in the long mid-infrared between 25 and 40 win, where most of optical materials are opaque. The mesh filter does not require transparent dielectric materials unlike interference filters because the transmission characteristics bare determined by surface plasmon-polariton (SPP) resonances excited on a metal surface with a periodic structure. In this study, we have developed the mesh filters optimized to atmospheric windows at 31.8 and 37.5 mu m accessible from the Chajnantor site of 5,640 m altitude. First, mesh filters made of a gold film of 2 jam thickness have been fabricated. Four identical film-type filters are stacked incoherently to suppress leakages at stop-bands The transmissions of the stacked filters have been measured to be 0.8 at the peaks and below 1 x 10(-3) at the stop-bands at 4 K. The ground-based mid-infrared camera MAX38 has been equipped with the stacked filters and successfully obtained diffraction-limited stellar images at the Chajnantor site. The film-type mesh filter does not have sufficient mechanical strength for a larger aperture and for use in space. We have developed mesh filters with higher strength by applying the membrane technology for X-ray optics. The membrane-type mesh filter is made of SiC and coated with a thin gold layer. The optical performance of the mesh filter is independent of internal materials in principle because the SPP resonances are excited only on the metal surface. The fabricated membrane-type mesh filter has been confirmed to provide comparable optical performance to the film-type mesh filter.