Terahertz waves have numerous advantages ranging from medical applications in imaging tissues to airport security systems. However, until now there was not a single unified source of terahertz emission which could provide usable terahertz radiation over a wide frequency range.
Physicists from the Fritz Haber Institute, Berlin and Johannes Gutenberg-University Mainz (JGU) along with national and international partners have realized a new concept for the production of this electromagnetic radiation using spintronic emitters. These emitters are in the form of thin multi-layered metal films and make use of the spin property of the electron rather than the conventional semiconductor emitters, which use only the electron charge. “Using this property, we were able to show that it is possible to produce broadband emitters fully covering the 1‑to‑30‑THz range which are also cost-efficient in terms of their industrial applications,” said Professor Matthias Kläui of the JGU Institute of Physics. In recent years, this quantum property, i.e., the spin of the electron, has led to a new branch of spin-based electronics or spintronics. The new findings have recently been published in the scientific journal Nature Photonics.
Terahertz radiation is a part of the electromagnetic spectrum between microwave frequencies and infrared light, in the frequency range of 0.3 to 30 THz. Many materials absorb THz radiation in a characteristic manner, while textiles and plastics are largely transparent. Unlike X-rays, terahertz rays are harmless to biological structures. As a consequence, terahertz waves can be used for bioimaging – such as in body scanners at airports, for quality control of food, and for material identification.
One obstacle which prevents the wide-scale usage of these terahertz rays is the fact that current technologies require expensive and large apparatus to generate broadband THz. The spintronics-based emitters fabricated by researchers at the Fritz Haber Institute in Berlin and at Johannes Gutenberg University Mainz are scalable and can be used as table top emitters. Current semiconductor-based emitters can cover only a limited range of frequencies. However, using these novel spintronics emitters it is possible to cover the complete range of terahertz frequencies from 1 to 30 THz without gap. It is more energy-efficient, cheaper to manufacture, and easier to use than conventional sources.
Thin metal film at the heart of the emitter
“The new THz emitter resembles a photodiode or a solar cell: on illuminating the material with an ultrashort laser pulse, an ultrafast spin-current is generated. This spin current is then converted to a charge current via the Inverse Spin-Hall effect. Consequently, a transmitter antenna radiates an equivalent electromagnetic pulse with frequencies in the terahertz range,” explained Samridh Jaiswal, co-author of the study and a member in Professor Mathias Kläui’s group at Mainz University. In contrast to a solar cell, the metal films of the spintronics variant are only 5.8 nanometers thick, which ensures the impulse to be extremely short while at the same time preventing attenuation of the terahertz wave inside the emitter. The metal and layer thicknesses used were systematically optimized such that a relatively weak laser radiation is sufficient to produce the entire terahertz spectrum from 1 to 30 THz. “These results pave the way to novel sources of terahertz emitters,” said Jaiswal, who is a fellow in the Marie Curie Initial Training Network WALL “Controlling domain wall dynamics for functional devices,” of which Mainz University is a partner. Samridh Jaiswal was in charge of the materials development.
To optimize the emitter performance, the scientists had to screen a large number of materials with varying material composition and geometry. The high-throughput Rotaris sputter deposition system installed at the Institute of Physics at Mainz University by Singulus Technologies was a crucial prerequisite to fabricate a large number of samples in a short time. The optimization procedure was further supported by calculations of theorists at Forschungszentrum Jülich. The topic of converting between spin and charge currents due to spin-orbit effects is part of the recently established Collaborative Research Center CRC/Transregio 173 “Spin+X: Spin in its collective environment,” funded by the German Research Foundation (DFG).