To observe the origin of the universe, researchers at Kavli Institute for the Physics and Mathematics of the Universe developed a filter made of alumina that has an anti-reflection coating. In Big Bang cosmology, the cosmic microwave background (CMB) is electromagnetic radiation. CMB allows us to look far back into the past, right after the universe was created. It tells us about the evolution of the universe from the beginning of time. They are a remnant from an early stage of the universe, also known as “relic radiation”. They are faint cosmic background radiation filling all space. Surface characteristics of metallic materials are evaluated in the optical profilometry lab or metrology testing labs.
CMB is the main source of information on the early universe. With a traditional optical telescope, the space between stars and galaxies (the background) is completely dark. A sufficiently sensitive radio telescope, however, shows a faint background noise, or glow, almost isotropic, that is not associated with any star, galaxy, or other objects. This glow is strongest in the microwave region of the radio spectrum. To detect CMB, a telescope must be tuned to wavelengths in which the CMB is most intense (around 1-3 mm). The telescopes must also sort out shorter wavelength radiation, coming from the atmosphere and Milky Way. One of the most effective optical elements that absorb the short wavelength radiation but lets the CMB pass through is alumina, a material made of aluminum and oxygen. Surface characteristics of metallic materials are evaluated in the optical profilometry lab or various metrology testing labs.
One of the problems in using alumina is that it also reflects almost 50% of the radiation impinging on it. To reduce reflection, the researchers came up with a new way to make anti-reflective structures. The researchers build the alumina with small pyramidal structures, which are about 1 mm tall and repeat across the 1 ft. diameter with a periodicity of just less than 1 mm. With the small pyramids, light enters and leaves the material slowly, leading to much lower reflection. It was known that incorporating such structures on the surface of materials reduces reflections. Matsumura and Hanany’s innovation is in the way they patterned the alumina, which is too hard to be machined with standard tools. A good metrology testing lab has all the techniques required for material and product evaluation.
It has long been known that incorporating such structures on the surfaces of materials reduces reflections. With the small pyramids, light enters and is leaving the material more gradually, leading to much lower reflection. Matsumura and Hanany’s innovation is in the way they patterned the alumina, which is too hard to be machined with standard tools. They used ultra-short pulsed lasers, with pulses few trillionths of a second long and reaching 100 megawatts each, to ablate the material away and to shape the surface relief to its optimal anti-reflective shape. Within about four days the laser process produced 320,000 pyramids on both sides of the alumina disc. The researchers measured the properties of the alumina sample in a metrology testing lab and showed that it reflects less than 1% of the incident radiation. This is the first time such an optical element has been fabricated and coupled to an operating instrument, and it is the largest sample of alumina to have been laser-ablated.
The researchers gave the alumina with the new anti-reflective structures to Mark Devlin and Simon Dicker from the University of Pennsylvania. Devlin and Dicker operate the MUSTANG2 instrument, which is now successfully observing the early universe. Innovations such as these give us an opportunity to look back at the origin of the universe.
Ryota Takaku, Qi Wen, Scott Cray, Mark Devlin, Simon Dicker, Shaul Hanany, Takashi Hasebe, Teruhito Iida, Nobuhiko Katayama, Kuniaki Konishi, Makoto Kuwata-Gonokami, Tomtoake Matsumura, Norikatsu Mio, Haruyuki Sakurai, Yuki Sakurai, Ryohei Yamada, Junji Yumoto. A Large Diameter Millimeter-Wave Low-Pass Filter Made of Alumina with Laser Ablated Anti-Reflection Coating. Optics Express, 2021; DOI: 10.1364/OE.444848
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