Although radiation at terahertz frequencies has many uses, most sources cannot generate terahertz beams with great power. Magnetic manipulation of energetic electrons inside a particle accelerator offers a solution.
This page emits a broad spectrum of electromagnetic radiation, including frequencies in the region of one terahertz (1 THz = 1012 Hz). You cannot see this terahertz emission because its frequency is about 300 times smaller than the limit of human vision. Neither can you feel it: the total intensity emitted at all frequencies below 1 THz is less than a millionth of a watt per square centimetre. Not only this page, but all of the objects around you emit terahertz electromagnetic waves in all directions as 'black-body radiation'.
Terahertz beams much brighter than black-body radiation are required for scientific and technological applications, ranging from the imaging of biological and other materials1,2 to manipulating quantum states in semiconductors3. On page 153 of this issue, Carr et al.4 report the generation of a beam of radiation that contains a broad spectrum of frequencies up to about one terahertz with an average power of 20 W. Such a beam has never previously been created; Carr et al. have opened the door to new investigations and applications in a wide range of disciplines.
Researchers have at their disposal an increasing number of sources of coherent terahertz radiation — that is, terahertz radiation with a well-defined phase, such that it can be tightly focused. Lasers that can generate pulses of visible or near-infrared light (around 1014–1015 Hz) with a duration less than 10−12 s are increasingly common, and can, with small incremental costs, be used to generate terahertz radiation5.
One common method is as follows. An electric field of about 106 V cm−1 is generated in a high-resistance semiconductor by applying a d.c. voltage between a pair of electrodes bonded to its surface. An ultrafast laser pulse illuminates the semiconductor between the electrodes, creating a large density of mobile charge carriers (electrons and 'holes') through an effect that is closely related to the photoelectric effect used in solar cells. These charge carriers, sensing the large electric field, accelerate at roughly 1017 m s−2 — compare that to the gravitational acceleration felt by an object dropped near the Earth's surface, of 10 m s−2. All accelerating charges emit electromagnetic radiation. These charge carriers, reaching their maximum velocity in less than 10−12 s, emit a single electric-field pulse shorter than 10−12 s that contains a broad range of frequencies, up to a few terahertz. Typically, the average power generated by this method is less than 10−6 W. But as this power is in a stable, coherent beam with well-known temporal characteristics, it can be used for spectroscopy with high spectral resolution and excellent signal-to-noise ratio, and even for imaging3. The drawback in imaging, however, is that it is usually necessary to scan the beam spot over the object in question, which is much too slow for video-rate image acquisition.
Carr et al.4 also use accelerating electrons to generate their 20-W beam of broadband terahertz radiation. But rather than being trapped inside a semiconductor, these electrons are travelling in a vacuum at nearly the speed of light — inside an accelerator at the Jefferson Laboratory in Newport News, Virginia. The electrons are grouped in bunches that are so small they whiz past an observer in 0.5 × 10−12 s. As long as a bunch of electrons travels in a straight line, it does not accelerate or emit light. But a strong magnetic field can deflect the bunch: if its trajectory is bent along a circular arc of radius 1 m, the associated acceleration causes the bunch to emit a 500-fs pulse (1 fs = 10−15 s) of electromagnetic radiation with a peak power of roughly 106 W, a peak frequency of about 0.6 THz, and detectable radiation up to several terahertz. When electron bunches are generated at the maximum rate of 37 million each second, the average power in the beam reaches roughly 20 W. In fact, the authors even needed to reduce the power by a factor of 550 to bring the generated signal back within the dynamic range of their equipment.
The 20-W broadband beam complements other sources of terahertz radiation. These include the broadband sources based on ultrafast lasers, discussed above, that come in two types. The first emits pulses with peak powers that are similar to those reported by Carr et al.4, but the maximum repetition rate is only 103 Hz, compared with 4 × 107 Hz for the Jefferson Lab source, and the resulting average power is roughly 10−3 W (ref. 6). The other type of source emits terahertz pulses with a repetition rate of 108 Hz, but an average power of less than 10−6 W (refs 1, 3).
There are also sources that emit radiation at well-defined frequencies. These include the quantum-cascade laser7, which produces pulses with a peak power of 0.002 W at around 4 THz; microwave oscillators8, whose frequency can be multiplied by nonlinear devices (10 s of continuous power at the microwatt level and below 2 THz); and free-electron lasers that reach 103–106 W of peak power9. Each of these sources has already found its own applications and users.
As with any new technology, it is difficult to predict the most important applications — the inventors of the laser did not envisage bar-code scanners. The authors speculate that the large peak power could be used for the study of new nonlinear phenomena in advanced materials and devices, and that the large average power could allow “full-field, real-time image capture” — in effect, terahertz movies. Another possibility is that the large average and peak powers could be used to manipulate and alter materials, chemical reactions and biological processes. Perhaps, as you bask in the weak terahertz radiation emitted by this page, you will think up the 'killer' application for the new terahertz source.
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Chen, Q. & Zhang, X.-C in Ultrafast Lasers: Technology and Applications (eds Fermann, M. E., Galvanauskas, A. & Sucha, G.) 521–572 (Dekker, New York, 2001).
Cole, B. E., Williams, J. B., King, B. T., Sherwin, M. S. & Stanley, C. Nature 410, 60–63 (2001).
Carr, G. L. et al. Nature 420, 153–156 (2002).
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Colson, W. B. et al. Physics Today 55 (1), 35–41 (2002).
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