Terahertz Radiation Detector Facilitates Greater Understanding of the Cosmos

This content is excerpted from Sensor Technology Alert and Newsletter, a sensor intelligence service published by the Technical Insights unit of Frost & Sullivan.

Sensor Technology AlertA minuscule yet ultrasensitive sensor, developed by Merlijn Hajenius for Delft University of Technology's Kavli Institute of Nanoscience, in close cooperation with the SRON Netherlands Institute for Space Research, could help further our understanding of the cosmos and outer space. Hajenius received a PhD from Delft University of Technology in January 2007 based on this research subject.

The detector, termed a "hot electron bolometer," is based on the phenomenon of an increase in electrical resistance when an item is heated. The use of a superconductor renders the detector extremely sensitive and allows it to be used for radiation that, purportedly, heretofore, could not be so well detected.

The detector is capable of terahertz (THz) frequencies, which astronomers and atmospheric scientists are very interested in. The detector's core is comprised of a small piece of superconducting niobium nitride. Clean superconducting contacts that are kept at a constant temperature of -268 degrees C (five degrees above absolute zero) are attached to both ends of the superconducting niobium nitride.

A miniscule gold antenna catches the terahertz radiation and sends it via the contacts to the small piece of niobium nitride, which serves as an extremely sensitive thermometer. Hajenius noted that very accurate measurements of terahertz radiation can be made by reading the thermometer; and that, at Delft, the researchers have achieved world record sensitivities using this detector in the following frequency ranges: 1.6 THz (receiver noise temperature = 750 K); 2.5 THz (950 K); and 2.8 THz (1050 K). The results have convinced astronomers to use the detectors for the new observatory in Antarctica (HEAT), and a new space mission (ESPRIT) has also been proposed.

Hajenius told Sensor Technology that the hot electron bolometer detector is based on a small niobium nitride superconducting bridge. The superconductor is brought to an operating temperature close to the critical temperature (that is, close to the point where it switches from superconducting to normal conduction). The terahertz radiation heats the electrons to above the critical temperature point, which causes a strong resistive response. The resistive response can be monitored by a bias current through the device.

Hajenius explained that the detector is able to achieve very high sensitivity because the researchers "developed a new method to contact the extremely thin NBN films to the antenna using special contact structures." They found that clean contacts with an extra intermediate superconducting layer double the sensitivity of the detector.

The maiden flight of the detector is planned for next year. It will not take place in a satellite used for studying cosmic clouds, but in a balloon that will study the earth's atmosphere. The TELIS instrument, which SRON is working on, will be equipped with a Delft University of Technology detector and will measure the molecules in the atmosphere above Brazil that influence the formation of the hole in the ozone layer.

Hajenius noted that the detector's terahertz spectrum measurements will be used to identify molecules, and the line profile will contain information about temperature, pressure, and relative velocities.

Hajenius explained that SIS heterodyne receivers are typically used to measure cosmic radiation up to 1.5 THz; and hot electron bolometers (HEBs) are the "most practical option" for measurements beyond 1.5 THz. He mentioned that, in principle, Schottky diodes can be used for measurements beyond 1.5 THz, but they are much less sensitive and require much more local oscillator (LO) power than is currently available from tunable LO sources.

The measurement of cosmic radiation, Hajenius noted, provides detailed information about the chemical composition of a wide range of astronomical objects. If the detection system's spatial resolution is high enough (for example, a large dish or interferometer setup), it could be used to probe the atmospheres of planets outside our solar system to search for signs of life.