Tiny transmitter sets frequency record: Revolutionary terahertz transmitter developed

Although terahertz (THz) electromagnetic radiation, which has wavelengths ranging from 0.1 mm and 1 mm, penetrates common materials, such as plastics, paper, fabrics, or ceramics, allows, e.g., nondestructively testing workpieces, analyzing processes occurring in engine combustion chambers while engines are running, inspecting packages and letters for hazardous biological substances without need for opening them, it has yet to establish a reputation for itself in scientific and engineering fields. One of the hindrances involved was that, until now, transmitters and receivers operating at THz‑frequencies were bulky and very expensive.

However, that situation might soon be reversed, since a team of physicists and engineers led by Dr. Michael Feiginov at the TU‑Darmstadt's Institute for Microwave Technology and Photonics has developed a resonance tunnel diode (RTD) for generating terahertz electromag­netic radiation that takes up less than a square millimeter and may be produced using more or less conventional semiconductor-device fabrication technologies. Furthermore, their innovative transmitter has set a new frequency record, 1.111 THz, for microelectronic devices.

The highest frequency ever generated by an active semiconductor device

Feiginov, a physicist, noted, that, "That is the highest frequency ever generated by an active semiconductor device." He was also able to theoretically prove that a minuscule transmitter, like that developed by his group, should be capable of generating much higher frequencies extending up to 3 THz. As Feiginov, who intends to continue pursuing development work on the transmitter over the coming years until generation of such higher frequencies has been achieved, went on to say, "That was formerly regarded as impossible by those involved in terahertz research." Achieving such higher frequencies would allow attaining better spatial resolutions, i.e., recognizing finer details, employing terahertz electromagnetic radiation in materials testing and analysis than would be possible at lower frequencies.

That the RTD his group has developed operates at room temperature makes it even more attractive for use in engineering applications. He further commented that, "It might, for example, be utilized in spectroscopic analyses of molecules that have transitions falling within the THz‑range." According to Feiginov, that would mean that substances that have thus far escaped spectroscopic analysis in the THz‑range could be investigated employing that widely practiced, scientific method, which would be of great benefit in various fields, among them medicine, where it might, e.g., allow distinguishing diseased body tissues from healthy body tissues in vivo. Since active semiconductor devices, such as the THz‑transmitter developed by the TU‑Darmstadt group, represent the heart of modern informatics and telecommunications technologies, as well as all sorts of electronic equipment, Feiginov presumes that the device developed by his group will prove useful in many other application areas that cannot readily be foreseen at this stage. As he put it, "Extracting higher frequencies from the device would lead to new applications, or application areas, in the fields of computers, mobile telephones, and other types of electronic equipment."

In the course of miniaturizing their new device, the group of TU‑Darmstadt researchers spent the past few years taking microelectronics close to the limits of the technically feasible. The heart of their RTD is a dual-barrier structure, within which a quantum well (QW) is embedded. A QW is a very thin layer of indium-gallium arsenide semiconductor sandwiched between a pair of ultrathin barrier layers of aluminum-arsenide semiconductor. Every one of those layers is just one nanometer to a few nanometers thin. This dual-barrier structure, plus a quantum-mechanical effect, provides that electromagnetic waves generated within a terahertz oscillator will be repeatedly amplified, rather than attenuated, which means that the oscillator will emit continuous-wave electromagnetic radiation at terahertz frequencies. The group of TU‑Darmstadt researchers collaborated with ACST GmbH, a local fabricator of microelectronic circuit components, in producing their diode.