Tunable terahertz generation

A DC electric field could be used to generate tunable terahertz radiation, according to researchers in the USA. The discovery opens up what has remained an essentially unnavigable part of the electromagnetic spectrum that lies between the microwave and infrared regions. Coherent artificial terahertz sources might soon be possible and lead to new analytical spectroscopic and imaging techniques. [Gumbs et al., J. Appl. Phys. 118, 054303 (2015); DOI:10.1063/1.4927101]

Terahertz radiation is known to be distinguish between living tissues of different water content or density and has already been used in airport security as the infamous "naked body" scanners. However, it has serious potential in identifying tumors and other lesions in the body as well as carrying out single-molecule imaging, detection of explosives or hidden weapons, or even as a carrier frequency for wireless telecommunications.

Godfrey Gumbs and Andrii Iurov at Hunter College of the City University of New York and the University of New Mexico, Albuquerque, working with Danhong Huang of the Air Force Research Laboratory at Kirtland Air Force Base, New Mexico, and Wei Pan of Sandia National Laboratory, also in Albuquerque, have investigated hybrid semiconductors that combine two-dimensional (2D) crystalline layers and a thick conducting material. They point out such structures could consist of a single or pair of sheets of graphene, silicene, or a 2D electron gas.

The team explains that their approach exploits instabilities in surface plasmon resonance and essentially disproves a conclusion by others that when a layer lies above a thick conductor, one can simply replace one of the frequencies in a two-layer plasma dispersion equation by the surface plasmon frequency of the underlying substrate. This is not the case and the team shows that when a direct current is applied to their hybrid semiconductor, a plasmon instability is created at a particular wavenumber, which induces terahertz emission, which can then be exploited using a surface grating to split it. Changing the density of conduction electrons in the material or the DC voltage allows the team to tune the cutoff wavenumber and so the frequency of the emission.

"Our work demonstrates a new approach for efficient energy conversation from a dc electric field to coherent, high-power and electrically tunable terahertz emission by using hybrid semiconductors," explains Iurov. "Additionally, our proposed approach based on hybrid semiconductors can be generalized to include other novel two-dimensional materials, such as hexagonal boron nitride, molybdenum disulfide and tungsten diselenide."

The team will next investigate how the critical wave vector and the required drift current might be optimized for different system designs perhaps by increasing the number of two-dimensional layers used. The researchers will also investigate the effect of temperature on plasmon instability in order to see how the device will perform in a realistic temperature regime. "We will work with experimentalists to actually produce this source of radiation under laboratory conditions," Iurov told Materials Today.

David Bradley blogs at Sciencebase Science Blog and tweets @sciencebase, he is author of the bestselling science book "Deceived Wisdom".