Nano-detectors for terahertz radiation

A Dutch researcher’s PhD project has resulted in a novel electromagnetic sensor that should have application in a number of areas including astronomy, atmospheric science and health monitoring.

Merlijn Hajenius from the Kavli Institute of Nanoscience in Delft, together with colleagues at the SRON Netherlands Institute for Space Research in Utrecht, has developed a hot electron bolometer (HEB) that works at terahertz frequencies. Bolometers measure the intensity of radiant energy via the variation in a material’s electrical resistance with temperature.

Hajenius’ detector is highly sensitive, and ideal for detecting radiation that has so far proved difficult to measure. Terahertz radiation falls between the microwave and far-infrared bands of the electromagnetic spectrum, and its frequencies correspond to energy level transitions in biological systems, space plasmas and atmospheric chemistry that are of great interest to scientists.

Hot electron bolometer (taken from Hajenius et al., J. Appl. Phys. 100, 074507, 2006)

A tiny gold antenna on the HEB captures terahertz radiation and feeds the resulting electrical signal to a temperature-sensitive, superconducting film of niobiumnitride deposited on a silicon substrate, and maintained at a temperature of five degrees above absolute zero.

Until now, detectors for performing high resolution spectroscopy have been lacking, says Hajenius: ‘Other options exist, but do not have the spectral resolution required for heterodyne spectroscopy that can provide information on relative velocities, local temperature and pressure.’

The impressive results achieved by Hajenius have convinced astronomers to use the detectors for a new ground-based observatory in Antarctica (HEAT), and a new European space mission (ESPRIT) has been proposed. But proof of concept for Hajenius’ detector will come in a balloon experiment (TELIS) looking at ozone chemistry in the atmosphere above Brazil.

Hajenius’ work represents the state of the art in hot electron bolometry technology, according to SRON-based HEB expert Pourya Khosropanah: ‘It is basically the only suitable mixer technology today for heterodyne spectroscopy above 1.2 THz that can be used for radio astronomy among other applications.’

Figure: Sketch of the HEB detector structure, showing the niobiumnitride bridge, contact pads and part of the gold terahertz antenna. Taken from: Hajenius et al., J. Appl. Phys. 100, 074507, 2006 (subscription required).

Article first published in Nanomaterials News.