Development of an epoxy-based millimeter absorber with expanded polystyrenes and carbon black for an astronomical telescope.

We recently developed and characterized an absorber for millimeter wavelengths. To absorb a millimeter wave efficiently, we had to develop a low reflection and high absorption material. To meet these requirements, we added polystyrene beads in the epoxy for multiscattering in the absorber. The typical diameter of polystyrene beads corresponded to the scale of Mie scattering for photon multiscattering in the absorber. The absorber consists of epoxy, carbon black, and expanded polystyrene beads. The typical size of the expanded polystyrene beads is consistent with the peak of a cross-section of Mie scattering to increase the mean free path in the absorber. By applying this effect, we successfully improved the absorber's performance. In this paper, we measured the optical property of epoxy to calculate the Mie scattering effect. Based on the calculation results, we developed eight types of samples by changing the ratio in the absorber material. To compare the eight samples, we characterized the reflectance and transmittance of the absorber in a millimeter wavelength. The measured reflectance and transmittance of a 2 mm thick sample with optimized parameters are, respectively, less than 20% and 10%. We also measured the transmittance in a submillimeter wavelength. The measured transmittance is less than 1%. The shape of absorber can be modified for any shape, such as chip and pyramidal shapes. This absorber can be used to mitigate the stray light of a millimeter wave telescope with any shapes.

Wide field-of-view crossed Dragone optical system using anamorphic aspherical surfaces.

A side-fed crossed Dragone telescope provides a wide field of view. This type of telescope is commonly employed in the measurement of cosmic microwave background polarization, which requires an image-space telecentric telescope with a large focal plane over broadband coverage. We report the design of a wide field-of-view crossed Dragone optical system using anamorphic aspherical surfaces with correction terms up to the 10th order. We achieved a Strehl ratio larger than 0.95 over 32×18 square degrees at 150 GHz. This design is an image-space telecentric and fully diffraction-limited system below 400 GHz. We discuss the optical performance in the uniformity of the axially symmetric point-spread function and telecentricity over the field of view. We also address the analysis to evaluate the polarization properties, including the instrumental polarization, extinction rate, and polarization angle rotation. This work is a part of a program to design a compact multi-color wide-field-of-view telescope for LiteBIRD, which is a next-generation cosmic microwave background (CMB) polarization satellite.

Two-layer anti-reflection coating with mullite and polyimide foam for large-diameter cryogenic infrared filters.

We have developed a novel two-layer anti-reflection (AR) coating method for large-diameter infrared (IR) filters made of alumina, for use at cryogenic temperatures in millimeter wave measurements. Thermally sprayed mullite and polyimide foam (Skybond Foam) are used as the AR material. An advantage of the Skybond Foam is that the index of refraction is chosen between 1.1 and 1.7 by changing the filling factor. Combination with mullite is suitable for wide-band millimeter wave measurements with sufficient IR cutoff capability. We present the material properties, fabrication of a large-diameter IR filter made of alumina with this AR coating method, and characterizations at cryogenic temperatures. This technology can be applied to a low-temperature receiver system with a large-diameter focal plane for next-generation cosmic microwave background polarization measurements, such as POLARBEAR-2 (PB-2).

Millimeter-wave broadband antireflection coatings using laser ablation of subwavelength structures.

We report on the first use of laser ablation to make submillimeter, broadband, antireflection coatings (ARCs) based on subwavelength structures (SWSs) on alumina and sapphire. We used a 515 nm laser to produce pyramid-shaped structures with a pitch of about 320 µm and a total height of near 800 µm. Transmission measurements between 70 and 140 GHz are in agreement with simulations using electromagnetic propagation software. The simulations indicate that SWS-ARCs with the fabricated shape should have a fractional bandwidth response of Δν/νcenter=0.55 centered on 235 GHz for which reflections are below 3%. Extension of the bandwidth to both lower and higher frequencies, between a few tens of gigahertz and a few terahertz, should be straightforward with appropriate adjustment of laser ablation parameters.

Cryogenic infrared filter made of alumina for use at millimeter wavelength.

We propose a high-thermal-conductivity infrared filter using alumina for millimeter-wave detection systems. We constructed a prototype two-layer antireflection-coated alumina filter with a diameter of 100 mm and a thickness of 2 mm and characterized its thermal and optical properties. The transmittance of this filter at 95 and 150 GHz is 97% and 95%, respectively, while the estimated 3 dB cut-off frequency is at 450 GHz. The high thermal conductivity of alumina minimizes thermal gradients. We measure a differential temperature of only 0.21 K between the center and the edge of the filter when it is mounted on a thermal anchor of 77 K. We also constructed a thermal model based on the prototype filter and analyzed the scalability of the filter diameter. We conclude that the temperature increase at the center of the alumina IR filter is less than 6 K, even with a large diameter of 500 mm, when the temperature at the edge of the filter is 50 K. This is suitable for an application to a large-throughput next-generation cosmic-microwave-background polarization experiment such as POLARBEAR-2.