Research Group Introduces New Transmitter for Terahertz Waves

If the gallium arsenide crystal is irradiated with short laser pulses, charge carriers form. These charges are accelerated by applying a voltage that generates a terahertz wave. Credit: HZDR / Unix

Terahertz waves are becoming increasingly important in science and technology. They allow us to unravel the properties of future materials, to check the quality of automotive paints and screen envelopes. But generating these waves is still a problem. The team at the Helmholtz Center Dresden-Rossendorf (HZDR), TU Dresden and the University of Konstanz have now made significant strides. The researchers developed a germanium component that generates short terahertz pulses with the predominant property: the pulses have an extreme broadband spectrum and, therefore, give out many different terahertz frequencies at the same time. Since it was possible to manufacture the component using methods already used in the semiconductor industry, the development promises a wide range of applications in the field of research and technology, as reported by the group in the journal. Light: science and application,

Like light terahertz waves belong to the category of electromagnetic radiation. In the spectrum, they fall directly between microwaves and infrared radiation. But while microwaves and infrared have long entered our daily lives, terahertz waves are just beginning to be used. The reason is that experts were able to build only acceptable sources. terahertz waves since the early 2000s But these transmitters are still not perfect – they are relatively large and expensive, and the radiation that they emit does not always have the desired properties.

One of the established generation methods is based on a gallium arsenide crystal. If this semiconductor crystal is irradiated short laser pulsescarriers of gallium arsenide are formed. These charges are accelerated by applying a voltage that generates a terahertz wave – basically the same mechanism as in the mast of a VHF transmitter, where moving charges create radio waves.

However, this method has several drawbacks: “It can only work with relatively expensive special lasers,” explains physicist HZDR Dr. Harald Schneider. “With standard lasers of the type we use for fiber optic communications, this does not work.” Another disadvantage is that gallium arsenide crystals supply only relatively narrow-band terahertz pulses and, thus, a limited frequency range, which greatly limits the scope.

Precious metal implants

That is why Schneider and his team are betting on another material – semiconductor germanium. “With Germany, we can use less expensive lasers, known as fiber lasers,” says Schneider. "In addition, germanium crystals are very transparent and thus contribute to the emission of very broadband pulses." But so far they had a problem: if you irradiate pure germanium with a short laser pulse, it takes several microseconds before the electric charge in the semiconductor disappears. Only then can the crystal absorb the next laser pulse. Modern lasers, however, can trigger their pulses at intervals of several tens of nanoseconds – a sequence of images that is too fast for germanium.

To overcome this difficulty, experts were looking for a way to make electric charges in Germany disappear faster. And they found the answer in the famous precious metal – gold. “We used an ion accelerator to shoot gold atoms at a germanium crystal,” explains Schneider's colleague, Dr. Abhishek Singh. "Gold penetrated the crystal to a depth of 100 nanometers." Then the scientists heated the crystal for several hours at a temperature of 900 degrees Celsius. heat treatment ensure uniform distribution of gold atoms in a germanium crystal.

The success began when the team illuminated the germanium drenched with ultrashort laser pulses: instead of hanging in the crystal for several microseconds, the charge carriers again disappeared within two nanoseconds – about a thousand times faster than before. Figuratively speaking, gold works like a trap, helping to capture and neutralize charges. "Now the germanium crystal can be bombarded laser The heart rate is fast and still working, "Singh is pleased to announce.

Low-cost production possible

The new method allows you to receive pulses of the terahertz range with an extremely wide passband: instead of 7 terahertz using the installed gallium arsenide technique, it is now ten times larger – 70 terahertz. “We get a wide, continuous, gapless spectrum in one fell swoop,” says Harald Schneider enthusiastically. “This means that we have a truly universal source on hand that can be used for a wide variety of applications.” Another advantage is that germanium components can be efficiently processed using the same technology as microchips. “Unlike gallium arsenide, germanium is compatible with silicon,” says Schneider. “And since new components can work together with standard fiber optic lasers, you can make this technology compact and inexpensive enough.”

It should turn into gold germanium into an interesting option not only for scientific applications, such as a detailed analysis of innovative two-dimensional materials, such as graphene, but also for applications in medicine and environmental technology. For example, you can imagine sensors that track specific gases in the atmosphere by their terahertz spectrum. Today's terahertz sources are still too expensive for this purpose. New methods developed in Dresden-Rossendorf can help in the future significantly reduce the cost of such environmental sensors.

Record terahertz laser beam

Additional Information:
Abhishek Singh et al., Bandwidth up to 70 THz from an implanted Ge photoconductive antenna excited by a femtosecond Er: fiber laser, Light: science and application (2020). DOI: 10.1038 / s41377-020-0265-4

The research team presents a new transmitter for terahertz waves (2020, March 16)
restored March 16, 2020

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