Breakthrough in super-terahertz detection technology for astronomy

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An international team of researchers led by SRON and TU Delft have successfully demonstrated a superconducting heterodyne receiver with an unprecedented high sensitivity at 4,7 THz in frequency or 63 micrometers in wavelength. The so called super-terahertz heterodyne receiver - about 85 times more sensitive than its predecessor, operating near the quantum noise limit - is able to operate at the exact frequency that is needed to enable the detection of the astrophysically important neutral atomic oxygen (OI) line. This is very good news for astronomers who need to study the oxygen lines from different astronomic sources to trace star formation and galactic evolution. It is also an important milestone in the development of the technology for the candidate balloon mission GUSSTO (NASA). The research results appeared this month in Applied Physics Letters.

This photo shows the key contributors; Jenna Kloosterman (right), Darren Hayton (middle) and Ren Yuan. The photo was taken at the laboratory of the TU Delft, where the measurements started

A heterodyne receiver can convert a (high-frequency) signal from space into a lower frequency without losing any information. Similar to FM radio, this mixing of frequencies makes the reception clearer and the signal from space can be amplified better. The heterodyne receiver can offer line detection with a nearly quantum noise limited sensitivity and an unparalleled high spectral resolution. The new superconducting heterodyne receiver is based on a novel solid-state terahertz quantum cascade laser to generate the local signal that is to be mixed with the signal from space (local oscillator, operating at 4,7 THz). A superconducting hot electron bolometer is used as a mixing detector..

A terahertz quantum cascade laser (QCL) is actually a tiny semiconductor chip that is based on a repeated stack of semiconductor multiple quantum well heterostructures, where laser emission is achieved through the use of intersubband transitions. Although a QCL has been demonstrated as a local oscillator in the lab before, it is the first time that the QCL is applied at 4,7 THz with an exact targeting frequency. The new QCL is based on a so-called third-order distributed feedback grating design. The grating has a double purpose in realizing both controllable single-frequency emission and a good output beam. The laser used was developed by a research group at the MIT in USA.   

The detector used is a hot electron bolometer that consists in essence of a superconducting Niobium-Nitride nanobridge and an on-chip planar antenna. Such a detector was developed by SRON in a close collaboration with the Kavli Institute of Nanoscience at the TU Delft.  

Interstellar medium
The observations of key astronomic cooling lines allow astronomers to trace star formation and galactic evolution. The fine-structure line of electrically neutral atomic oxygen (OI) at 4.7 THz is the dominant cooling line of warm, dense, and neutral atomic gas. In strongly UV irradiated photodissociation regions (PDRs), the OI line flux is generally larger than that of the carbon CII line, making it an ideal diagnostic for probing the physical conditions in regions of massive star formation and galactic centers. The spectrally resolved OI line is necessary to untangle the complexities of the interstellar medium.

The NASA balloon mission GUSSTO

The success of the newly developed QCL-bolometer receiver technology, not only demonstrated by the sensitivity, but also supported by a beautiful methanol line spectrum, is an important milestone in demonstrating the 4.7 THz technology for GUSSTO, which stands for Galactic/Xgalactic Ultra long duration balloon Spectroscopic Stratospheric THz Observatory. GUSSTO was selected by NASA as one of five Explorer Mission of Opportunity proposals. Its Phase A-Concept Study has been completed. Now NASA should announce the Phase–B decision in February this year. The success also demonstrates the technology for follow-up instruments of the molecule hunter HIFI aboard the Herschel Space Telescope (ESA).

Scientific paper
The key results were obtained at SRON by a group of young scientists: Darren Hayton (SRON), Jenna Kloosterman (University of Arizona), Ren Yuan (TU Delft and Purple Mountain Observatory), and Wilt Kao (MIT).  This joint effort was coordinated by Jian-Rong Gao(SRON/TU Delft) The research results were published in a scientific paper that appeared inApplied Physics Letters, vol. 102, 011123(2013), authored by J. L. Kloosterman, D. J. Hayton, Y. Ren, T. Y. Kao, J. N. Hovenier, J. R. Gao, T. M. Klapwijk, Q. Hu, C. K. Walker, and J. L. Reno.