A new device could open new avenues for the generation of high-frequency radiation with applications in science, radar, communications, security and medical imaging.
Ever since the discovery of X-rays in 1895, their ability to reveal things hidden to the human eye has created endless opportunities. But X-rays by far aren’t the only option to see the world with different eyes. Researchers hope to make better use of a different form of light, called terahertz radiation, which has broad applications in science, radar, security, medicine and communications.
With wavelengths ranging from one-tenth of a millimeter to a few millimeters, terahertz light is several hundred times less energetic than visible light and occupies a middle ground between infrared radiation and microwaves. It could be used in radar systems to detect small objects, such as space debris. Or it could be used in navigation systems, security scanners and devices that search for explosives and drugs. In medicine, it has been used as an alternative to X-rays in some areas, such as 3-D imaging of teeth. Terahertz radiation could also be used in communications, where it would enable high data transfer rates.
The problem, however, is that there are only few options for terahertz sources, which are often not very practical. Some require large superconducting magnets or use giant particle accelerators, others produce radiation that is not powerful enough.
The SLAC team opted for another idea for the production of high-power radiation with shorter wavelengths: They used an electron beam that interacts with a specifically shaped cavity – a hollow metal structure – in a microwave vacuum tube. As the electron beam passes through the cavity, it excites electromagnetic radiation, or light, of a particular wavelength. Since the wavelength of the excited radiation scales with the cavity dimension, the output radiation can be shifted to shorter wavelengths by making smaller cavities. However, making these cavities very small has several unwanted side effects, including a drop-in output power and issues with heating and manufacturing.
“Our design eliminates these issues because it uses a cavity that is very large compared to the wavelength it produces and because it has a wide opening at one end,” says Filippos Toufexis, a graduate student at the Department of Energy’s SLAC National Accelerator Laboratory and Stanford University. “It also doesn’t use magnets and is relatively small.”
In fact, the new device has two different cavities. The first cavity uses microwaves to deflect an electron beam coming from an “electron gun,” forcing it onto a corkscrew-like path into the second cavity – the one with the wide opening. As a result, the deflected beam appears to be rotating along that opening.
The electron beam excites a rotating electromagnetic field in the second cavity, which generates the output radiation. Since beam and field move synchronously along the rim, the field can continuously draw energy from the beam. The traveling field is known as a “whispering gallery” mode because it has an analogy in acoustics. The effect occurs, for instance, in St. Paul’s Cathedral in London, where whispers – soundwaves – travel around a gallery beneath the cathedral’s dome and can be clearly heard anywhere on the gallery. Due to its particular shape, the second cavity produces radiation with a wavelength five-times shorter than the microwave radiation that goes into the first cavity, shifting the output wavelength toward the terahertz region.
In experiments at SLAC’s Klystron Test Lab, the device, for which the researchers obtained a provisional patent, produced stable radiation with a wavelength of about 5 millimeters and an output power of 50 watts. Calculations show that the source even has the potential to produce radiation that is 1,000 times more powerful. However, more development work is needed to push the technology into the terahertz region with wavelengths of about a millimeter, says SLAC accelerator scientist Sami Tantawi, who had the idea for the new design, which borrows elements from an established method for the generation of radiation with longer wavelengths.