THz receiver installed on Ast/Ro telescope provides an order of magnitude increase in sensitivity over previous technology for studying the interstellar composition of the Milky Way
A Sifir-50 terahertz (THz) laser system developed by Coherent is enabling a novel THz receiver that will be used in conjunction with the South Pole-based Ast/Ro telescope for studying the interstellar composition of the Milky Way.
Through their observations, scientists hope to form a more complete image of our galaxy's interstellar medium, and thus enhance their understanding of the lifecycle of stars.
Named Trend, the terahertz receiver with niobium nitride hot electron bolometer (HEB) device is the result of a US National Science Foundation grant.
The three-year-long project is a joint effort between the University of Massachusetts at Amherst and Lowell, the University of Arizona, the Smithsonian Astrophysical Observatory, Harvard University, and Chalmers University of Technology in Sweden.
The installation of the receiver at the South Pole occurred mid-December 2002.
Most interstellar observations will take place in the austral summer season of 2002/2003.
"With the THz Trend device, we're getting into a new frequency range for astronomy," said Sigfrid Yngvesson, professor at the University of Massachusetts, Amherst, and principal investigator for the project.
"People have worked in this frequency range before, but they used old Schottky barrier receiver technology".
"What we have now is a receiver that has more than ten times the sensitivity compared to the old technology".
"This will enable us to map the NII and CO components of the Milky Way, something that was not accessible with last-generation tools." By studying the THz spectral lines specific to NII and CO, scientists hope to form a more complete image of the interstellar medium.
Said Yngvesson, "It is out of this medium stars are formed".
"Stars go through generations - they're formed from the gas in the interstellar medium and then they live as stars for a while, and then they explode and throw out whatever they've processed into the interstellar medium again".
"So to really understand the whole cycle of the birth and death of stars we'd like to be able to observe as many components of the interstellar medium as possible." Studying THz frequencies from a ground-based telescope, however, poses some unique challenges.
Since THz radiation is typically absorbed by water vapour in the Earth's atmosphere, the telescope itself must be located in a very dry climate.
Ast/Ro, a 1.7-meter-diameter telescope operated by Smithsonian Astrophysical Observatory, is located at the South Pole, the most desirable location on Earth for observing THz radiation due to its unusually low wind speeds, absence of rain, and extremely cold and dry atmosphere.
Another challenge was the necessity for a more sensitive THz receiver.
While THz work done in the 1980s and early 90s centered on Schottky barrier diodes, Yngvesson's team focused its efforts on HEB technology.
The HEB device at the heart of the Trend receiver is a niobium nitride superconducting film created using UV lithography that measures just 4x1 microns".
"The HEB technology is in some ways very simple, because it's just one piece of film that does the whole receiving detection function," said Yngvesson.
"Around the film we build a very complicated system that operates on the heterodyne principal".
"And since we're recording a spectral line that is specific in frequency for NII or CO, it means that we're only interested in radiation over a small terahertz frequency band".
"In order to do that we needed to have a very stable local oscillator that's nearby in frequency - within one to two gigahertz - and that's what Coherent's Sifir laser provides." Although there are electronic solid-state sources that produce accurate frequencies in lower ranges, the Coherent Sifir-50 is currently the only available source for producing THz radiation from a package rugged enough for deployment to the South Pole.
According to Eric Mueller, Coherent's manager of CO2 engineering and specialty products, "There are just a few outfits in the world that make terahertz lasers at all".
"Coherent is the only one that makes a unit that's reliable enough to be easily transported and installed at the South Pole, while also easy enough to be used by someone other than a laser engineer." A technology originally developed for the US military, the Sifir-50 system consists of an optically pumped THz laser and a controller module containing all of the electronics necessary to support the laser head.
Sifir-50 systems produce over 50mW of power on lines ranging from 0.3 to 7.0THz.
This system has also been the THz source of choice for a wide range of demanding applications, ranging from a 5-year atmospheric measurement mission on Nasa's Aura satellite, to characterizing advanced next-generation devices and materials for telecommunications.
While it is likely that THz astronomy will eventually be pursued from platforms in space or from balloons in the upper atmosphere, with the Sifir-50 now installed at the Ast/Ro site, the next step in the project will likely be the development of future extensions to THz focal plane arrays for ground-based observations.
"Although we have the best receiver in this frequency range, we still can't see past our galaxy - we need a larger telescope to do that," added Yngvesson.
"And there will be a larger telescope at the South Pole soon".
"We're just three or four years away from its installation."
