Professor and student push instrument beyond where any telescope has gone before
While there was recent controversy about the intrusiveness of airport body scanners (and the abuses of unscrupulous TSA employees) what was not widely publicized was the unique technology behind one type of scanner: terahertz imaging. Much more than just technology capable of rendering awkward semi-nude photos of a nail clippers wielding passenger, the terahertz technology can see into portions of the universe where visible light cannot travel. ASU’s Christopher Groppi is creating terahertz detectors that can look into the dark and dirty areas of space, where no telescope has gone before, to examine the making of the stars.
“A terahertz is a color of light in between radio waves and infrared. Redder than infrared and bluer than radio waves,” explains Groppi, an assistant professor in SESE.
Groppi is interested in this part of the spectrum because there are dusty gaseous sections of the universe with extremely cold dense clouds of gas, and they’re hard to see through. These clouds are so dense that molecules can form in these regions and eventually the gas collapses and form stars and planets.
“If you want to study how new stars form, you want to see the environments they form in. You want to see them right from the start, and see how the process happens. These clouds are opaque to visible and sometimes infrared light so it’s very difficult,” Groppi explains.
The complex dust particles in these clouds obstruct the view of normal telescopes as well as infrared sensors. Terahertz light is about 1,000 times redder than visible light, which allows it to penetrate dust particles. But as useful as terahertz radiation is, it is very hard to utilize and detect.
Scientists employ two main approaches to scan the skies with terahertz. Bolometers are a kind of super-sensitive thermometer that measure how light warms up the detector. But the downside to bolometers is they cannot differentiate colors so they take “black and white” images of the cloud.
Another instrument is a radio receiver; however, the difficulty with receivers is they have to work at a frequency roughly 3,000 times higher than FM radio. Only since the seventies has electronics technology been able to process the light at that speed. It is this type of receiver that Groppi has been busy building.
“We find ways to make receivers work at really high frequencies using special superconducting detectors and we have to make everything very, very small. The reason why is because you have to make parts of the receiver approximately how big the wavelength of the light is. For instance when you have an antennae on your car that antenna is about one-fourth the wavelength of FM waves at 100 megahertz; we have to do that same type of thing 3,000 times smaller,” describes Groppi.
Groppi uses a German milling machine designed to make miniscule parts of high-end Swiss watches. The machine is operated by Matt Underhill, an ASU mechanical engineering technology undergraduate student. Underhill can make parts that are as small as 25 microns or about a fourth the size of a human hair. Nearly every component of the receiver is custom built. The National Science Foundation pays mostly for the labor of Groppi and his various associates who have to hand-make almost every part of the instruments.
Terahertz telescopes are not new but what is new is Groppi’s approach to designing them. Previously, receivers could only scan the sky point by point, one pixel at a time to form a small image. Groppi’s instruments are more like a camera than a single point sensor. He has combined sixty-four detectors into an array so that he can make a picture that is sixty-four times bigger in the same amount of time.
Terahertz telescopes work best in space but can be earthbound as well. Earth’s atmosphere is made up of lots water molecules that absorb terahertz radiation so Groppi and his collaborators must place the receivers in high arid locations such as Antarctica, Atacama Desert, or Arizona’s Mount Graham.
“What we’ve tried to study with this instrument is how stars and planets form. There is a very good theory as to how a star works can predict how its life will play out. But there is no theory as to how the star begins, how it goes from gas cloud to star, so we observe the clouds to find out,” says Groppi.
The terahertz telescopes have the potential to connect characteristics of clouds to the type of stars that will form within them. The telescopes offer not only pictures but spectra of the gas so Groppi can determine what elements make up the cloud, how the cloud is moving and temperatures of areas of the cloud.
Groppi and his associates have begun pointing their telescopes at the dark patches of the sky; where there are no stars to be seen there is a cloud blocking optical light. Already they have found several young stars less than 100,000 years old, a success for Groppi and his team. The instrument has so far accomplished what it was designed to do.
What is most difficult for Groppi isn’t finding the youngest stars of time, but instead having to create a whole new machine on his own: building new eyes for man to look toward the sky, and building them without instructions or a guide.
Photo: Undergraduate Matthew Underhill uses a Kern Model 44 computer numerically controlled micromilling system. This system is used in the School of Earth and Space Exploration terahertz laboratory to fabricate radio astronomy detectors with dimensions accurate to 1 micron, or about 1/100th the diameter of a human hair. Credit: Tom Story.