University of Minnesota
School of Physics & Astronomy


EBEX and the search for B-Modes

Karl Young
Karl Young
Shaul Hanany

Karl Young is a graduate student in Shaul Hanany’s observational cosmology group, working on EBEX. EBEX is a planned balloon-borne telescope designed to observe polarization patterns in the Cosmic Microwave Background (CMB). The CMB is a remnant from the Big Bang. Physicists are searching the CMB for evidence of “B Modes”, a theorized polarization pattern imprinted in the CMB by the gravitational waves produced by the expansion of the universe during the Big Bang. Gravitational waves were theorized by Einstein and are a wave caused by the movement of a very large body in space.

Physicists have yet to find direct evidence of gravitational waves. Since B-modes were produced during the Big Bang, finding them would show that the Universe underwent very rapid expansion in the first fraction of a second after the Big Bang, confirming a theory called Inflation that predicts much of the universe we see today.

EBEX’s approach to looking for B-modes is to fly in the upper atmosphere of the Antarctic sky. This allows the instrument to get above the water vapor which can play havoc with polarimetry (the measurement of polarized light). EBEX last flew in 2012, but Young didn’t join the experiment until 2013. “My job is to help plan the next detector,” he says. Young has been working on developing optics for the 2nd generation detector which is planned to fly in 2017.
Young is redesigning the lenses using crystalline silicon instead of the high density polyethelyne used in the previous experiment. With silicon he can make thinner and flatter lenses, can increase how much light gets to detectors, increase sensitivity, decrease polarization measurement errors, and decrease emission from lenses which can actually effect the signal. “The CMB is so faint it can be affected by emission from the telescope’s lenes.” Young and others have increased the field of view of the telescope and increased the sensitivity as well by making the lenses bigger, but leaving the telescope's structure the same.

These new optics for EBEX have a higher index of refraction, which cause more reflection. The change of material meant going from 5% reflection to 30% reflected away. “If you have three lenses like we do, it would mean almost no light gets to detector. So we have to create new anti-reflection coating.” The group’s approach is to make a “meta-material” AR coating. This involves machining small grooves into the surface of the lens. The grooves are significantly smaller than the wavelength of the CMB. Grooves are machined in one direction, the lens is rotated 90 degrees and machined again. This grid pattern means that surface is evenly coated with little pyramids all the same size and shape. The pyramids are smaller than the wavelength, so incident light doesn’t 'see' the pyramids. Instead it sees an effective index of refraction that’s dependent on the average amount of silicon relative to air at a given point on the pyramid.. At the top of the pyramid the light sees an average material that is almost entirely air. Nearer the base of the pyramid there is more silicon relative to air so the average material begins to look more and more like silicon. This means the index changes gradually from air to silicon and the specific shape of the pyramid controls this change. By adjusting the geometry of the pyramids Young can plan and control the reflection from each surface. “We predict we can get below 2% reflection across the broad band we are searching.”

The next step is to prove it can be made and to test them. They have made a prototype structure on Rexolite (a millimeter wave plastic) on silicon and alumina (aluminum oxide ceramic) and sapphire (the kind used in optics, a synthetic sapphire) using a laser to etch the pattern on small 2 inch samples. Young says that these coatings aren’t just EBEX specific, and have the potential to be used in all CMB experiments. As part of this project Young got to go to Japan to work with Tomotake Matsumura (former EBEX grad student) to measure transmission of the Rexolite, and set up his experiment. “The transition measurements we took in January 2014, matched with the predictions. We’re hopeful.” Since Young’s trip, Matsumura has measured the more recent work on alumina and sapphire and that seems to be matching predictions as well. “Now we’re in the middle of trying to scale it up from 5 to 30 cm.” Young did the machining work on silicon at the Nanotechnology lab at the University of Minnesota.