Stardust in terahertz

Researchers at the South Pole launch test balloons. As soon as the atmosphere is stable enough, they will launch an enormous balloon carrying a telescope with detectors built in Delft.

Ten years ago, Dr Jian-Rong Gao, project leader in the department of Quantum NanoScience at the Faculty of Applied Sciences and the TU Delft Space Institute made up his mind to develop a better technology for detecting terahertz radiation: electromagnetic radiation that falls in between infrared radiation and microwave radiation. Terahertz radiation provides insight into the composition of dust clouds in our Milky Way galaxy; the ‘incubators’ of new solar systems.

Since then, Gao has worked steadily and continually on the development of his terahertz detectors and, in collaboration with the Netherlands Institute for Space Research (SRON), managed to get them selected by NASA for the Stratospheric Terahertz Observatory (STO2)mission. Shortly before the launch of his life’s work (mid-December) he declares, slightly nervously: “We are hoping for God’s help in making the launch a success.” Hence the test balloons.

Primitive
Since the beginning of November, SRON researchers Darren Hayton and Wouter Laauwen have been working at the McMurdo base station in Antarctica, building the balloon gondola and testing all the components. This was the continuation of work that had begun last August in Palestine, Texas, where the whole gondola was lifted and tested. After the satisfactory completion of the ‘hang test’, the colossus was dismantled and shipped to the South Pole in crates. In mid-November, Laauwen wrote in his blog: “The balloon gondola is in our hangar. It’s well on the way to completion. Colleagues from APL (Applied Physics Laboratory, Johns Hopkins University, Ed.) and the CSBF (Columbia Scientific Balloon Facility, Ed.) are continuing the work. We’re going to build our instrument into a cryostat, a sort of large thermos flask filled with liquid helium that cools the whole instrument to around 270 degrees below zero, in other words 4 kelvin, 4 degrees above absolute zero.
Although it’s all a bit more primitive, in the lab it’s easy to forget where you are; there aren’t any windows. As soon as you open the door, you remember: I really am in Antarctica.”

Four days later he announced: “The good news is that we’ve just got the whole signal chain from the 4.7 THz receiver to work (see ‘Delft detector’, Ed.). We’re receiving a local signal, we can see the skies and the detector is sending out a required signal. There is still some optimising to be done, but all the steps are working. Success! The bad news is that the toilet doesn’t work.”

Gao_Sterrenstof_2302_SITE

Dr Jian-Rong Gao: “We are the eyes of the mission.”

Star formation
The first stars were formed around 13.5 billion years ago, from light elements such as hydrogen, helium and  lithium. Nuclear fusion in the stars gave rise to heavier elements such as carbon, nitrogen and oxygen – elements that are now abundant in the dust clouds of ‘our’ Milky Way. It’s strange to think that the building blocks of our planet and life on it were once forged in the interior of stars, and were scattered as stardust billions of years later following an enormous explosion.

New stars and planets are still forming from that interstellar dust in an eternal dance of energy and matter. That is the general picture, but we still know very little about the proportions in which the elements carbon, oxygen and nitrogen occur, about how fast the gases cool, how many stars are formed, how fast that process is and how heavy the stars become.

Professor of Submillimetre Astronomy at the University of Groningen, Prof. Floris van der Tak, hopes that measurements made during the STO2 mission will lead to a greater understanding of this. Admittedly, the balloon mission will have a limited duration – an estimated two weeks, the length of time the helium will last. That is why only a limited area of the Milky Way, 10 degrees wide by 2 degrees high, can be scanned. Within that area,the various detectors will map the distribution lines of nitrogen (N at 1.4 THz), carbon (C at 1.9 THz) and oxygen (O at 4.7 THz). Van der Tak, who works at SRON, is particularly interested in the oxygen line, because oxygen glows at 300 Kelvin (room temperature), quite a lot warmer than the environment of 100 Kelvin. In other words: the oxygen line shows the first temperature rise of a forming star. Van der Tak: “If you think of a gas cloud as a womb in which stars develop, the oxygen line is the ultrasound scan that tells you what’s going on inside.” Depending on the measurements, the plan is to scan larger areas in a higher resolution in later missions. In that sense, the STO2 mission itself is a sort of test balloon.

Gao’s dream
“Terahertz radiation from space was first observed twenty years ago, with semiconductor detectors in a plane at an altitude of ten kilometres. There was too much noise in the measurements to do anything with them, but they showed that there was radiation in the far infrared range”, explains Dr Jian-Rong Gao (born Shanghai, 1959), describing the beginnings of submillimetre astronomy. Ten years ago, the detector specialist working for SRON and TU Delft decided to develop the best detector for terahertz radiation (see ‘Delft detector’). The detectors he developed have been used in NASA and ESA missions, and have revealed to us a hitherto obscure area of the spectrum.

His efforts were rewarded when mission leader Prof. Christopher Walker of the University of Arizona chose the Delft detectors for the STO2 balloon mission. Walker had decided that Gao’s team at the department of Quantum NanoScience built the best detectors. Eventually, the Delft team were even the saviours of the STO2 mission, because when it became apparent last summer that the Jet Propulsion Lab could not deliver the promised infrared detectors (for 1.4 and 1.9 THz), Gao provided some that he had made for the European Herschel mission (2009-2013). As a result, all the detectors used on STO2 are from Delft. Gao says: “We are the eyes of the mission”.

Gao would have liked to have been the first one to measure the oxygen lines with the latest technology. Unfortunately, German researchers did that earlier this year, from a plane at an altitude of 14 kilometres. They were the first, but if everything goes according to plan, the measurements at 40 kilometres will not only be better (the atmosphere absorbs terahertz radiation from space), but also much more detailed.

A plan for the follow-up mission, GUSSTO (Galactic/Xtragalactic Ultra long duration balloon Spectroscopic Stratospheric THz Observatory), is already with NASA. GUSSTO will have 3 x 16 pixels on board (for the three different wavelengths of oxygen, carbon and nitrogen) and will be active for more than 100 days. Gao’s dream that came true has apparently led to a taste for more.

Read the latest news in Laauwen’s blog: sron.nl/sto2/antartica

How it works
A parabolic reflector concentrates the incoming radiation onto a one-way mirror. which also receives the radiation from a terahertz laser that serves as a local oscillator. This specially developed quantum cascade laser (QCL) that generates the 4.7 THz radiation is kept at a working temperature of 50 Kelvin (minus 223 degrees Celsius) by a compact Stirling cooler. The laser itself is tiny: only one millimetre long and 20-40 micrometres wide.

The mixing of a signal with a reference radiation from a local oscillator is called heterodyne detection. The advantages of the method are that it converts the frequency of the signal from terahertz to gigahertz, and that good amplifiers, filters and meters are available for that microwave radiation.

A silicon lens concentrates the mixed radiation (signal and reference) onto a small antenna that is connected to a superconducting detector known as a hot-electron bolometer or HEB mixer. Put simply, these detectors consist of a superconducting niobium-nitride bridge between two gold contacts. The radiation absorbed heats the bridge (2 by 0.2 micrometres), which causes the bridge to lose some of its superconducting properties. The resulting increased electrical resistance can be measured.

A star camera fixes the position of the telescope in relation to the firmament. Once a reading of one pixel has been measured (on three wavelengths simultaneously), its position shifts very slightly to the next pixel. Working thus, the telescope takes around 15 days to complete a pixel-by-pixel scan of a previously-determined observation framework 2 degrees high and 10 degrees wide.

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