Terahertz radiation has a wavelength typically just under a millimeter – a technically difficult range. Electromagnetic waves with longer wavelengths can be generated with ordinary electronic components (such as transistors) and antennas. Smaller wavelengths can be achieved with ordinary light sources, such as lasers or LEDs. However, the terahertz range between the two remains a technical challenge. Still, radiation in this range can be very useful. It is needed in many fields, from materials testing or airport security screening to radio astronomy, and perhaps also in future telecommunications systems.
TU Wien (Vienna) has now succeeded in producing an extremely simple and compact terahertz radiation source: a resonant tunneling double diode oscillator. Its radiation power greatly surpasses similar devices. The new technology has just been published in the scientific journal Letters of applied physics.
Chip size instead of lab bench size
“Today, there are different ways to generate terahertz waves,” explains Professor Michael Feiginov (Institute of Electrodynamics, Microwaves and Circuit Engineering, TU Wien). One can use quantum cascade lasers, for example. With these it is possible to reach high intensities, but they must be cooled to very low temperatures. Additionally, large, complicated photonic systems can be used, with multiple optical lasers whose radiation is mixed to produce longer wavelengths. This makes it possible to produce the desired wavelengths very flexibly. “However, our goal was to develop a simple and extremely compact terahertz source”, emphasizes Michael Feiginov. “If we want any technology to be incorporated into everyday devices in the future, terahertz sources need to be small and operate at normal room temperature.”
To do this, the team no longer used optical or quantum cascade lasers, but simple oscillators. “Oscillators are something quite common in electrical engineering,” says Petr Ouředník (TU Vienna), the first author of the current publication. If certain electronic components, such as coils and capacitors, are coupled together, energy flows between them, generating electromagnetic radiation. “But the problem is usually losses, which you can imagine as electrical resistance,” explains Petr Ouředník. “This normally ensures that the oscillations in these resonant circuits stop after a very short time.”
Quantum trick for negative resistance
However, this can be changed with quantum physics tricks: “We use resonant tunneling diodes, where current flows between two barriers as a result of tunneling,” explains Petr Ouředník. “The quantum well between the barriers is particularly narrow in our structures, so that only very specific and very few electronic states can exist there.” By applying a voltage, these electronic states and their energies can be changed.
Normally, current flow increases as electrical voltage increases – electrical resistance indicates by how much. In resonant tunneling diodes, however, the opposite effect is possible: if the voltage increases, it may happen that the electronic states in the quantum well no longer correspond to the electronic states in the other parts of the structure. This means that electrons can no longer flow from one area to another and the current flow decreases instead of increasing. This means: the electrical resistance becomes negative. “A negative resistance in the oscillating circuit, however, means that the oscillating circuit does not lose energy, it instead gains energy. The electromagnetic oscillations continue on their own and the external direct current is converted into radiation terahertz,” says Michael Feiginov.
From cell phones to radio astronomy
The special feature of this technology is not only the considerably high intensity of the terahertz radiation, but above all its small size: the entire structure is considerably smaller than one millimeter. It would therefore be potentially suitable for integration into compact devices such as smartphones.
“There are so many application ideas that we can’t even say today which one is the most realistic”, says Michael Feiginov. “The terahertz range is used in radio astronomy, it can be used to see through optically opaque objects, for example during airport security checks or even in materials testing. Another exciting application are chemical sensors : different molecules can be recognized by the fact that they absorb specific frequencies in the terahertz range All these technologies will benefit from simple and compact terahertz sources, and this is what we wanted to make an important contribution to.