Metamaterial tiles boost sensitivity of large telescopes

Research on a low-cost, mass producible technology is poised to help the Simons Observatory uncover new insights into how the universe began.

a person wearing a hardhat looking at an apparatus with wires and tubing
Postdoc Zhilei Xu conducing thermal testing of novel metamaterial tiles in an advanced cryogenic facility at Penn. In a new study, Xu and a team of researchers demonstrate that these tiles could be effectively cooled to the cryogenic temperatures at which the Simons Observatory will operate. (Pre-pandemic image)

A new study published in Applied Optics describes novel metamaterial tiles that will help improve the sensitivity of telescopes being built at the Simons Observatory in Chile. Thanks to efforts by researchers from Penn, the University of Chicago, NASA’s Goddard Space Flight Center, the Massachusetts Institute of Technology, and other institutions, these new tiles have now 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, electromagnetic radiation left over from an early stage of the universe, using some of the world’s largest and most sophisticated ground-based telescopes. These measurements will help improve scientist’s 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,” says lead author and Penn postdoctoral researcher Zhilei Xu, who works in the laboratory of Mark Devlin. “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.”

a person in a hard hat sitting on the floor working with a ring of insulation behind a massive metal cyllinder, which a person is standing inside and working on in the background
As part of the largest ground-based cosmic microwave background (CMB) experiment ever built, twice as big as previous observatories, putting together the large aperture telescope receiver (right) for the Simons Observatory will be a multiyear endeavor for researchers in Mark Devlin’s lab. (Pre-pandemic image)

In their paper, the researchers show that the metamaterial microwave tiles they developed absorb more than 99% 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-moulding commercially-available materials, they are an economic, mass-producible, and easy-to-install solution to what has been a long-standing problem,” says 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.”

Working at low temperatures

Ground-based millimeter-wave telescopes use receivers that are cooled to cryogenic temperatures to reduce noise and 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.

a close up view of a black material next to a coin for scale and the same material arranged in a ring with a scale showing 30 cm
A close-up view of the metamaterial tiles that will improve the sensitivity of telescopes at the Simons Observatory by absorbing stray light. The top left photo shows one tile, with its anti-reflective surface shown in the insert. The bottom left photos show the back of the tile, and the right photo shows the assembly of 240 tiles installed on the wall of an optics tube. (Image: Zhilei Xu)

However, developing a material that can suppress stray light while operating at extremely low temperatures is 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, ones that are engineered to have 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.

Reducing reflection

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 moulding.

“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,” says Xu.

a laboratory setup showing the location of motor one on the right as a receiver, motor 2 on the left as a source, a sample in the middle, source on the bottom, and receiver below that, then a close up inset image showing the placement of the mirrors
University of Chicago graduate student Grace Chesmore developed the setup used to quantify the scattering and reflectance of millimeter-wave signals off the metamaterial tiles. (Image: Shreya Sutiraya)

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 (-458° F) and then measured their optical performance. “We developed a custom test facility to measure the performance of the tiles with high fidelity,” says Grace Chesmore, a graduate student at the University of Chicago who led the optical measurements of this research project. 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,” says Xu. “The successful combination of a metamaterial and injection molding manufacturing opens up many possibilities for millimeter-wave instrument scientific instrument design.”

This project is supported by the Simons Foundation and the Heising-Simons Foundation with additional support provided by the Dean’s Office of the School of Arts & Sciences at the University of Pennsylvania. A complete list of collaborating institutions can be found at the Simons Observatory website.

This study was the result of a collaboration by researchers from the University of Pennsylvania, the Massachusetts Institute of Technology, the University of Chicago, Kyoto University, the University of California, Berkeley, Princeton University, the University of Milano, Cornell University, Stockholm University, Tohoku University, the University of Tokyo, the Lawrence Berkeley National Laboratory, the University of California, San Diego, West Chester University of Pennsylvania, and the Goddard Space Flight Center.

Mark Devlin is the Reese W. Flower Professor of Astronomy and Astrophysics in the Department of Physics and Astronomy in the School of Arts & Sciences at the University of Pennsylvania.

Zhilei Xu is a post-doctoral researcher in the Department of Physics and Astronomy in Penn’s School of Arts & Sciences.