Novel Terahertz Source: Potential for Scanners and Quality Control
Terahertz waves can be used for numerous applications, such as body scanners for instance, but until now generating them has been difficult and fraught with limitations. Scientists from Jülich, together with international partners, have implemented a novel concept for the production of this electromagnetic radiation. An emitter in the form of a thin metal layer is able to generate the full terahertz spectrum by the clever exploitation of the spin properties of electrons. On the basis of this principle, efficient sources can be built which for the first time are able to emit radiation seamlessly over a wide bandwidth from 1 - 30 THz. Moreover, the new emitter is more compact in design and can be produced more cost-effectively than before.
Terahertz waves occur between microwaves and infrared light on the electromagnetic spectrum, in the frequency range from approximately 1 to 30 THz. This type of radiation is extremely useful, as it is able to penetrate many types of materials, including textiles and plastics, and is absorbed in a characteristic way by other substances. Unlike X-rays, terahertz radiation is harmless to humans. It is therefore used for example in body scanners at airports or in the quality control of foodstuffs.
An obstacle to its use in a wider field of application is however that the equipment capable of producing gap-free radiation over the entire terahertz spectrum is expensive and unwieldy. Scientists at Forschungszentrum Jülich along with partners from Germany, the United States, Sweden and France have now implemented a terahertz emitter that is scalable and suitable as a desktop device. "Our prototype is capable of producing the entire terahertz spectrum from 1 - 30 THz and is also more energy efficient, simple to use and cheaper to build than existing sources", enthused Prof. Yuriy Mokrousov, head of the Helmholtz Young Investigators Group for Topological Nanoelectronics at Forschungszentrum Jülich. "We are expecting the uptake of this technology to be rapid and wide-ranging."
The novel source uses a femtosecond laser, which produces 80 million ultrashort light pulses per second. Conventional devices require significantly more powerful lasers, which are much more expensive, complex and cumbersome, and consume more energy.
The new type of emitter resembles a photodiode or solar cells; lighting up the material with an ultrashort laser flash produces an electric pulse which then directly emits an electromagnetic signal like a transmission antenna. Unlike solar cells, the novel emitter is made up of a thin metal film, just 5.8 nanometres thick, so the electrical impulse is extremely short and the terahertz radiation in the emitter material is barely weakened (see image for further details). After the scientists had systematically optimized the metal and layer thicknesses, relatively weak laser radiation was sufficient to produce the entire terahertz spectrum from 1 to 30 THz.
Mokrousov's colleague, Dr. Frank Freimuth, explained another important element of the new terahertz source: "The emitter is able to work so well because as well as using the electrons' charge, we are also taking advantage of their spin." Spin is a magnetic property of electrons and accounts for the fact that current behaves differently in magnetic metals than in non-magnetic ones. This effect is cleverly put to good use in the new source to control the transport of electrons so that the terahertz wave can be emitted extremely efficiently. With the aid of a computer code developed in Jülich, the two physicists were able to choose materials suitable for this research and then to interpret the experimental results.
Source: Forschungszentrum Jülich