At the Jefferson Laboratory (JLab) in Virginia, US, a multilaboratory team using beams of relativistic electrons has generated broadband terahertz radiation at nearly 20 W average power, several orders of magnitude higher than any other source. The terahertz band – at the far-infrared interface between electronics and photonics – has drawn increasing attention in the past decade, despite the lack of high-average-power sources. The team reported its demonstration of high power in the 14 November edition of Nature.
The terahertz work is a spin-off from the superconducting radiofrequency (SRF) electron accelerator central to JLab’s mission of probing the quark structure of nuclei. In a news commentary accompanying the Nature report, Mark Sherwin of the Center for Terahertz Science and Technology at the University of California, Santa Barbara, wrote that the high-power demonstration has “opened the door to new investigations and applications in a wide range of disciplines”.
Terahertz imaging could reveal interesting features of the many materials with distinct absorptive and dispersive properties in this spectral range, which corresponds revealingly with biomolecular vibrations. The demonstration source would allow full-field, real-time imaging of the distribution of specific proteins or water in tissue, or buried metal layers in semiconductors. High-peak and average-power terahertz sources are also critical for driving new nonlinear phenomena, and for pump-probe studies of dynamical properties of materials.
Non-ionizing terahertz radiation can pass through clothing, paper, cardboard, wood, masonry, plastic and ceramics. It can penetrate fog and clouds. Since the light cannot penetrate metal or water, it cannot be used to inspect seagoing cargo containers or diagnose conditions deep inside the human body. However, eventual applications could include better detection of concealed weapons, hidden explosives and land mines; improved medical imaging and more productive study of cell dynamics and genes; real-time “fingerprinting” of chemical and biological terrorist materials in envelopes, packages or air; better characterization of semiconductors; and widening the frequency bands available for wireless communication.
Whichever applications may ultimately materialize, many will require high-average-power broadband terahertz light. Free-electron lasers (FELs) and fast diodes can produce useful quantities of narrow-band light. Thermal sources and tabletop laser-driven sources can produce broadband terahertz at low average power. The JLab experimenters produced high-average-power broadband emission from subpicosecond electron bunches in the JLab FEL’s unique SRF “driver” accelerator – a small, energy-recovering, high-current cousin of the 6 GeV CEBAF, the SRF accelerator that serves JLab’s nuclear and particle physics users.
Unlike most linear accelerators (linacs), the JLab FEL’s driver linac operates at a very high repetition rate – up to 75 MHz – using SRF cavities and recovering the energy of the spent electron bunches, so that the average current is orders of magnitude higher than in conventional linacs. This energy-recovery linac (ERL) runs with beam current up to 5 mA, compared with only 200 mA in CEBAF. The linac typifies the widening transdisciplinary applicability of smaller accelerators. In 1999, it provided the first substantial proof of the ERL principle, which is now being incorporated in or envisioned for machines worldwide.
JLab’s Gwyn Williams conceived and led the high-power terahertz demonstration experiment, which took place during late 2001 and involved researchers from JLab, Brookhaven National Laboratory and Lawrence Berkeley National Laboratory. They generated the light as synchrotron radiation from very short electron bunches (500 fs) that were accelerated to the relativistic energy of 40 MeV and then transversely accelerated by a magnetic field. Because the electron bunch dimensions are small – in particular, the bunch length is less than the wavelength of observation – the experimenters obtained multiparticle coherent enhancement.
Their demonstration of high-power terahertz radiation (also called T-rays, T-light or T-lux) adds a new dimension to Science magazine’s 16 August report, in an article called “Revealing the Invisible”, that “much research is being directed toward the development of T-ray sources and detectors.” Tochigi Nikon Corporation and Teraview (a Cambridge, UK, start-up associated with Toshiba) have begun commercializing low-power terahertz systems. A few hospitals are testing comparatively dim terahertz light for detecting skin cancer. Daniel M Mittleman of Rice University says that for low-power terahertz light, “perhaps the most promising applications lie in the area of quality control of packaged goods.” He illustrates by showing how the light can check the raisin count in boxes of raisin bran. Dr Xi-Cheng Zhang, a terahertz expert at Rensselaer Polytechnic Institute, predicts that “the future ‘killer application’…will be in biomedicine.”
These developments, statements and predictions were made when terahertz average power was still measured in milliwatts, not the tens of watts now demonstrated, or the still higher power that is expected. Nevertheless, the terahertz region still constitutes a gap in the science and technology of light – a region of the electromagnetic spectrum remaining to be better understood, and much better exploited. With commissioning of the 10 kW JLab FEL upgrade under way, Williams and his colleagues are planning an even higher-power terahertz beamline for further attempts to contribute toward those ends.