New terahertz laser could help study molecules, bodies and stars.
by Philip Ball
A new laser could boost medical imaging, chemical screening, telecommunications and even astronomy. It produces coherent radiation at terahertz frequencies - a long-overlooked region of the electromagnetic spectrum, between infrared and microwaves, that looks set for an explosion of scientific interest.
Medical researchers hope to use terahertz radiation to get more detailed pictures of soft tissues than ultrasound can provide - to track wound healing or cancer growth, for example. And physicists plan to use it to probe the Earth's atmosphere and beyond.
"The problem is getting convenient sources," says Alan Migdall, who investigates the movement of biological molecules such as DNA and proteins using terahertz radiation at the National Institute of Standards and Technology in Gaithersburg, Maryland.
Migdall currently works with a cumbersome, power-hungry argon infrared laser. A compact, low-power solid-state terahertz laser would make his life much easier.
And that's exactly what Rźdeger Kšhler of the Scuola Normale Superiore in Pisa, Italy, and co-workers in Turin and Cambridge, UK, have come up with. Their prototype is based on layers, millionths of a millimetre thick, of the same semiconductors that are used in conventional visible and infrared lasers: gallium arsenide (GaAs) and aluminium gallium arsenide (AlGaAs).
In conventional semiconductor lasers, light is emitted when electrons shed their energy falling from a high-energy state to a lower-energy one. The difference between these energy states in semiconductors means that the light is typically visible or infrared, with wavelengths up to ten times longer than terahertz radiation.
To make a terahertz laser, Kšhler's team had to create electron energy states that are much closer together - it's like the electrons drop off a doorstep, rather than a cliff. They did this by using GaAs layers of slightly different thickness separated by barriers of AlGaAs. Electrons can move about in the GaAs layers, but the AlGaAs barriers stop them falling between the layers.
But the barriers are a little leaky and the electron energy states in the GaAs layers differ only very slightly. So if electrons move from one layer to another, they lose just enough energy to produce terahertz radiation.
There were already ways of generating laser radiation at terahertz frequencies, involving mixing the beams of two infrared lasers. In fact, Russian researchers reported a semiconductor-based terahertz laser in 1984, but it was too impractical to catch on.
* Kohler, R. et al. Terahertz semiconductor-heterostructure laser. Nature, 417, 156 - 159, (2002).
© Nature News Service / Macmillan Magazines Ltd 2002