Feb 18 2021
The electromagnetic echo of the Big Bang, called the cosmic microwave background (CMB), is a kind of radiation that has been moving through time and space ever since the birth of the very first atoms around 380,000 years after the evolution of the universe.
Left: One of the lenses developed by McMahon’s team is installed in a camera assembly. Top right: This shows a close-up view of the stepped pyramid metamaterial structure responsible for the lens’ antireflective properties. Bottom right: Members of the McMahon lab stand by recently fabricated silicon lenses. Image Credit: Jeff McMahon.
Plotting very small changes in the CMB informs researchers about the evolution of the universe and what it is composed of. Generally, to capture the cold, ancient light from the CMB, scientists employ dedicated telescopes that are integrated with ultrasensitive cameras for identifying millimeter-wavelength signals. These advanced cameras will feature up to 100,000 superconducting detectors
Jeff McMahon, a scientist at Fermilab and an Associate Professor from the University of Chicago, and his research team have designed a new kind of metamaterials-based antireflection coating for the silicon lenses used in these cameras.
“There are at least half a dozen projects that would not be possible without these,” stated McMahon.
Metamaterials are essentially engineered materials, whose properties do not occur naturally. The magic lies in the microstructure—that is, minute and repeating features that are smaller than the wavelength of the light they are developed to interact with. Such features block, bend, or otherwise exploit light in unusual ways.
Antireflection coatings generally function by reflecting light from both sides of the coating so that the reflected light particles interfere and cancel one another, and thus eliminate reflection. In the case of the metamaterials designed by McMahon, the “coating” has a million micrometer-accurate cuts on both sides of every silicon lens.
On closer inspection, the features appear like a pattern of stepped pyramids—three layers of square pillars closely arranged on top of one another. The thickness and spacing of the pillars are further adjusted to produce the highest destructive interference between the reflected light.
Light just goes sailing right through with a tenth of a percent chance of reflecting.
Jeff McMahon, Scientist, Fermilab, Office of Science, U.S. Department of Energy
The single-crystal silicon lenses are ultrapure so that the light traveling via the lens will not be scattered or absorbed by impurities. They are also transparent to microwaves. Silicon has the required light-bending characteristics to direct the light from the telescope onto a huge array of sensors, and the reflection is handled by the structure of the metamaterial.
Since every lens is created from just one pure silicon crystal, it can tolerate cryogenic temperatures (the detectors have to work at 0.1 K) without the risk of peeling or cracking, similar to lenses that have anti-reflective coatings made from a different kind of material.
According to McMahon, these lenses are perhaps the most superior technology available for CMB instruments.
“It’s not exactly that you couldn’t do the experiment otherwise,” added McMahon, but for the durability and performance required by both existing and advanced CMB surveys, these lenses are indeed sophisticated—and McMahon’s research team are the only individuals in the world to develop such lenses.
Along with his research team, McMahon started to develop the technology around a decade ago while they were working on a new kind of detector array and then realized that a better and less reflective lens was needed to go with it.
But learning how to make this kind of lens proved to be very difficult, added McMahon. While techniques were available for making tiny, precise cuts in flat wafers of silicon, none had ever used them on a lens before. The very first lens made by the team, for the Atacama Cosmology Telescope, known as ACT, took 12 weeks to develop. This was because of the large number of cuts that had to be made.
Now, with better automation and machines available at Fermilab, the same process takes only four days for each lens, and McMahon believes that they will be able to simplify this process even further.
McMahon’s research team, who was working at the University of Michigan until January 2020, created around 20 lenses, such as PIPER, ACTPol, Advanced ACTPol, TolTEC, and CLASS, for current CMB experiments.
At present, the team is creating lenses for the Simons Observatory, which will begin to collect data from 2022. From there, they will start making more lenses for Cosmic Microwave Background Stage 4 (CMB-S4)—a state-of-art project, in which Fermilab is also a member.
CMB-S4 had planned to collect data in 2027 using a total of 21 telescopes at observatories in the South Pole and Chile for the most comprehensive CMB survey yet.
The second we finish a lens, it’s doing science, and that’s what makes it fun for me. All the metamaterial stuff is cool, but at the end of the day I just want to figure out how the universe began and what’s in it.
Jeff McMahon, Scientist, Fermilab, Office of Science, U.S. Department of Energy
For McMahon, the CMB-S4 project is more like opening a treasure chest full of jewels and gold. While McMahon and other scientists contributing to the CMB-S4 project are not exactly aware of what they would find in the data, they do know that it will prove to be very useful.
But even if they fail to identify primordial gravitational waves—one of the key goals of the CMB-S4 project—the experiment will still provide a better understanding of cosmic mysteries such as neutrino masses, dark matter, and dark energy.
According to McMahon, what his research team has accomplished with their new lens technology is a testament to the outsize effect small efforts can have on big science.
The endeavor is to begin to understand the beginning of the universe. And the way we’re doing it is by figuring out how to machine little features in silicon.
Jeff McMahon, Scientist, Fermilab, Office of Science, U.S. Department of Energy
The study was funded by the Department of Energy Office of Science.