Millimeter and sub-millimeter characterization of polymers used for infrared filters in high-sensitivity cryogenic microwave telescopes

Deep in the cold silence of space, the universe whispers secrets in light that our eyes cannot see. This light, known as the Cosmic Microwave Background, is the oldest glow in existence, a faint afterimage of the Big Bang itself. To hear this whisper clearly, astronomers build telescopes that are not just powerful, but incredibly cold, cooled to temperatures near absolute zero to stop their own heat from drowning out the cosmic signal. However, these delicate instruments must still look out through a window to see the sky. That window, and the filters placed in front of it, must be made of materials that let this specific light pass through while blocking the warmth of the Earth and the atmosphere. For decades, scientists have relied on certain plastics for these windows, assuming they were transparent and harmless to the sensitive detectors inside. But a lingering doubt remained: could these materials be scattering tiny amounts of light in unexpected directions, adding a layer of noise that blurs the clearest images of the universe's birth?

A team of researchers at the University of Iceland and Stockholm University set out to answer this question by putting these common plastic materials through a rigorous test. They focused on four types of polymers often used in high-sensitivity telescopes: high-density polyethylene, which is used for lenses and windows; polytetrafluoroethylene, a material sometimes used for filters; nylon, a common choice for blocking infrared heat; and a specialized foam made from polyethylene. The scientists wanted to know not just how much light these materials let through, but what happened to the light that hit them. Specifically, they were looking for "scattering," a phenomenon where light bounces off tiny imperfections or structures inside the material and shoots off at wide angles, rather than traveling straight through. Even a tiny amount of this scattered light can be a problem, as it might hit the warm parts of the telescope and create a false signal that confuses the data.

To catch this elusive scattered light, the researchers built a unique testing station that functioned like a high-tech radar. They used a device called a vector network analyzer, which sends out radio waves in the millimeter and sub-millimeter range, the same frequencies used to study the cosmos. Instead of a human operator holding a detector, they used a sophisticated robot arm to move the receiver head around the sample in a precise arc, sweeping from side to side and up and down. This allowed them to map the light that passed through the plastic sheets and see if any of it had been deflected to the sides. They tested the materials at frequencies ranging from 90 to 330 gigahertz, covering the critical band where these telescopes operate. They also used a different machine to test how much light the materials absorbed at even higher frequencies, up to 1400 gigahertz, to get a complete picture of their behavior.

The results painted a clear picture of which materials are safe and which might need a second look. The solid blocks of polyethylene and the polytetrafluoroethylene sheets performed exactly as hoped. They let the light pass through with almost no scattering, acting as clean, invisible windows for the telescope. However, the nylon samples told a different story. The nylon sheets arrived with tiny, parallel grooves on their surface, left over from the manufacturing process. These grooves, only about 10 micrometers deep, acted like a diffraction grating, causing the light to scatter significantly to the sides. The researchers found that this surface structure was the culprit, proving that even microscopic imperfections can disrupt the flow of light.

The most surprising finding came from the polymer foams, which are often used as lightweight, multi-layer insulation in telescopes. These foams are filled with tiny air pockets, or cells, that are roughly 400 micrometers in size. When the light hit these foams, it did not pass through cleanly. Instead, the light interacted with the large internal cells, causing it to scatter in a broad, uniform pattern that sent a significant amount of power shooting out at high angles. The researchers calculated that this scattered light could easily hit the warm parts of the telescope, potentially adding a measurable amount of noise to the detectors. This suggests that while these foams are excellent for insulation, their internal structure might be too large for the wavelengths of light these telescopes are trying to capture, causing them to act more like a foggy lens than a clear window.

The study also measured how much light these materials absorbed, which is another way they can degrade telescope performance. The nylon materials were found to be quite lossy, absorbing a significant amount of the signal, especially at higher frequencies. The polyethylene and polytetrafluoroethylene remained very transparent, absorbing very little. By combining the scattering data with the absorption measurements, the team provided a detailed map of how these materials behave across the spectrum. They found that the optical properties of the solid plastics were stable and predictable, but the foams and the grooved nylon introduced variables that could not be ignored.

This work does not declare these materials useless, but it does highlight a critical need for better characterization. The researchers emphasize that for the next generation of telescopes, which aim to map the universe with unprecedented precision, every source of noise must be accounted for. The scattered light from the foam and the grooved nylon represents a hidden source of error that could be reduced by choosing different materials or by smoothing out surface imperfections. The team plans to expand their testing to include more complex materials, such as metal mesh filters and anti-reflective coatings, to ensure that every component in the telescope chain is as transparent and clean as possible. By understanding exactly how these materials interact with light, scientists can build better instruments, ensuring that the faint whispers of the early universe are heard clearly, without the static of our own equipment.