Metamaterials Boost Sensitivity of Large Telescopes

Thermal testing of the new metamaterial tiles in an advanced cryogenic facility showed that they could be effectively cooled to the cryogenic temperatures necessary. (Source: E. Sucar, Penn Today)

A multi-insti­tutional group of researchers has developed new meta­material tiles that will help improve the sensi­tivity of telescopes being built at the preeminent Simons Observatory in Chile. The tiles have been incorporated into receivers that will be deployed at the obser­vatory by 2022. The Simons Obser­vatory is the center of an ambitious effort to measure the cosmic microwave background using some of the world’s largest and most sophis­ticated ground-based telescopes. These measurements will help improve our under­standing of how the universe began, what it is made of and how it evolved into what it is today.

“The Simons Observatory tele­scopes will use a new ultra-sensitive millimeter-wave camera to measure the afterglow of the big bang with unpre­cedented sensi­tivity,” said Zhilei Xu from the Univer­sity of Pennsyl­vania. “We developed a new low-cost absorbing tile that will be used in the camera to absorb environmental emissions that can obscure the signals we want to measure.” Now, the researchers show that the meta­material microwave tiles they developed absorb more than 99 percent of millimeter wave radiation and retain their absorp­tive properties at the extremely low temperatures in which the millimeter-wave camera operates.

“Because the tiles can be made by injection molding commer­cially available materials, they are an economic, mass-producible and easy-to-install solution to what has been a long-standing problem,” said Xu. “With this technology, the Simons Obser­vatory will transform our under­standing of the universe from many aspects, including the beginning of the universe, the formation and evolution of the galaxies and the ignition of the first stars.”

Ground-based millimeter-wave tele­scopes use receivers that are cooled to cryogenic tempera­tures to reduce noise and thus boost sensitivity. Receiver technology has advanced to the point where any amount of stray light can degrade the image while also decreasing the sensitivity of the detector. A better way to suppress stray light within the receivers would further increase their sensi­tivity to the very faint signals coming from deep within space. However, developing a material that can suppress stray light while operating at such extremely low tempera­tures is quite challenging. Previous attempts resulted in materials that either couldn’t be cooled effec­tively to cryogenic tempera­tures or didn’t achieve the necessary combi­nation of low reflectance and high absorption. Other solutions have also tended to be difficult to install or challenging to mass produce.

To overcome these challenges, the researchers turned to meta­materials because they can be engi­neered to achieve specific properties that don’t occur in nature. After complex electro­magnetic simulation studies, the researchers designed meta­materials based on a material that combined carbon particles and plastic. Although the plastic composite exhibited high absorption in the desired microwave region of the electro­magnetic spectrum, the surface reflected a significant amount of radiation before it could get inside the material to be absorbed. To reduce the reflection, the researchers added an anti-reflective coating that was tailored using injection molding. “The low-reflectance surface combined with high-absorp­tion bulk material allowed the metamaterial absorber tiles to deliver excellent sup­pression of unwanted signals at cryogenic tempera­tures close to absolute zero,” said Xu.

After ensuring that tiles made of the new meta­material could mechanically survive thermal cycles from room tempera­tures to cryogenic tempera­tures, the researchers verified that they could be effectively cooled to -272° C and then measured their optical performance. “We developed a custom test facility to measure the performance of the tiles with high fidelity,” said Grace Chesmore, a graduate student at the University of Chicago who led the optical measure­ments of this research. The testing showed that the meta­material exhibited excellent reflec­tance properties with low scattering and that it absorbed almost all of the incoming photons. “As detector sensi­tivity continues to improve for millimeter-wave telescopes, it becomes crucial to control scattered photons,” said Xu. “The successful combination of a metamaterial and injection molding manu­facturing opens up many possi­bilities for millimeter-wave instru­ment scientific instrument design.” (Source: OSA)

Reference: Z. Xu et al.: The Simons Observatory: Metamaterial Microwave Absorber (MMA) and its Cryogenic Applications, Appl. Opt. 60, 864 (2021); DOI: 10.1364/AO.411711

Link: Dept. of Physics and Astronomy, University of Pennsylvania, Philadelphia, USA

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