T-rays could probe materials at unprecedented level of detail

Powerful pulses of terahertz radiation (T-rays) could be used to probe the fine internal structure of materials at an unprecedented level of detail.

A team of researchers from France and the UK have built a T-ray laser that is around 10,000 times more powerful than any similar device and can emit in separate pulses rather than a continuous stream.

Terahertz radiation — which refers to the far-infrared and microwave portion of the electromagnetic spectrum — is an area of increasing interest to scientists and engineers.

The technology was first developed by astronomers for detecting cosmic T-rays, but in the past decade or so there has been considerable progress in producing and detecting T-rays from terrestrial sources.

Active systems that emit T-rays then detect and image the resulting absorption and reflection patterns are now commercially available, while passive systems that can detect endogenous T-rays from various objects are also under development.

The method used by the current team is based on quantum cascade lasers that use a semiconductor as an active medium. They devised a way of ‘mode locking’ the laser by modulating its bias current with a radio-frequency synthesiser. The result was a powerful train of laser pulses that could be detected as a spectral signature.

‘This opens up opportunities for imaging that are not possible with passive systems, since one can investigate how the pulse shape is changed or delayed after propagating through a material,’ said Leeds University’s Prof Edmund Linfield, who worked on the project.

Linfield told The Engineer that the new method could offer spectroscopy at an unprecedented level of detail, probing structures at the nanoscale.

‘If we know that an object has a specific absorption line at a given frequency, say a rotational line of a gas molecule, we can tune our source so that all the power is focused at or close to this absorption line.’

The team has essentially validated its method and will now begin testing with materials in the lab, although Linfield stresses that the field of T-rays is still an emerging one.

‘There is not the “right” or “best” way of generating and detecting terahertz radiation as yet — each system has its advantages and disadvantages, depending on the targeted application. 

‘It is going to be exciting watching the field develop over the next decade and seeing whether a single terahertz technology will become the market leader.’

Researchers from Denis Didercot University in Paris and the French National Centre for Scientific Research (CNRS) also worked on the project, which was supported by the EPSRC, as well as several European grants.