GaAs crystal advances terahertz “telephone”
Researchers in Germany have transmitted data at terahertz frequencies for the first time.
Martin Koch and colleagues at the Technical University of Braunschweig encoded the electrical output from an audio CD player onto a terahertz beam and sent it over a distance of almost 1 m. The beam was then decoded by an antenna, which forwarded the signal to a loudspeaker.
Communication systems using terahertz radiation (1012-1014 Hz) could offer much higher bandwidth than today’s gigahertz systems. However, researchers have only recently started to develop ways of generating and detecting terahertz radiation.
In Koch’s experiment, the terahertz radiation was generated by shining femtosecond pulses of infrared light onto a GaAs crystal.
The pulses were focused onto a semiconductor modulator comprising a two-dimensional sheet of electrons sandwiched between layers of GaAs and AlGaAs, generating radiation with frequencies between 100 GHz and 3 THz.
The reflectivity of the electron sheet varies with the electrical output from the CD player, which is encoded in the terahertz signal. The terahertz signal is transmitted to an antenna and converted back into an electrical signal.
Koch emphasizes that the experiment is merely the first step towards the development of wireless terahertz communications systems. These could be used to achieve line-of-site transmission over distances of several meters.
However, UK-based physicist Michael Pepper, who co-founded the terahertz-imaging firm TeraView, is cautious about the implications of the work.
He points out that the terahertz pulses used in the experiment are weak and are limited by their low repetition rate of 75 MHz. "It is therefore impossible to modulate the signal any faster than that, which means that you are not really communicating at terahertz frequencies at all," he said. “What you need for genuine terahertz communication is to generate the terahertz pulses at a higher frequency.”
Author
Hamish Johnston is the editor of Wireless Europe magazine.
