Power from Terahertz Beams

By Belle Dume, PhysicsWeb
November 14, 2002

Terahertz beams are required in many scientific and technological applications, ranging from the imaging of biological materials to manipulating quantum states in semiconductors. However, existing terahertz sources have only been able to generate low-power beams. Now, Gwyn Williams, from the Jefferson Laboratory, and colleagues at the Brookhaven and Lawrence Berkeley National Laboratories in the US have created a beam that is several orders of magnitude more powerful than any existing source (GL Carr et al. 2002 Nature 420 153).

The terahertz region lies in the far-infrared region of the electromagnetic between about 300 GHz and 20 THz. All objects emit terahertz electromagnetic waves as "black-body radiation" but the total intensity emitted at all frequencies is less than one millionth of a watt per square centimetre.

The past decade however has seen a significant advance in the production of coherent broad-band terahertz beams. A common method of producing such a beam is to generate an electric field inside a high-resistance semiconductor. Typically, the average power of a beam generated by this method is less than 10-6 W. The beam can be used for high-resolution spectroscopy and some imaging techniques but it is of limited use, being of such low power.

Now Carr and co-workers have devised a new process in which bunches of electrons travel at nearly the speed of light, inside an accelerator at the Jefferson Laboratory in Virginia. They use a strong magnetic field to accelerate the electron bunches which emit a pulse of electromagnetic radiation lasting 500 femtoseconds. The pulse has a power peak of about 106 W and a peak frequency of 0.6 terahertz, although the detectable radiation continues up to several terahertz. When the electrons are generated at the maximum rate of 37 million each second, the average power reaches about 20 W. This is a 100,000 times higher than the power produced in previous terahertz beams.

"Of course producing and measuring the light is just a first step," Larry Carr, a member of the group, told PhysicsWeb. "Most applications depend on coherent detection, so we need to develop this." The Jefferson laboratory is already planning space for a terahertz laboratory in their Free Electron Laser facility.

Carr thinks it will be difficult to predict the most important applications for such a beam, although the team would like to use the large peak power to study advanced materials and devices, chemical reactions and biological processes. It could also be used for "full-field, real-time image capture" - in other words terahertz "movies".