New Transmitter for Terahertz Waves

Terahertz waves are becoming ever more important in science and technology. They enable us to unravel the properties of future materials, test the quality of automotive paint and screen envelopes. But gene­rating these waves is still a challenge. A team at Helmholtz-Zentrum Dresden-Rossendorf (HZDR), TU Dresden and the University of Konstanz has now made significant progress. The researchers have developed a germanium component that generates short terahertz pulses with an advan­tageous property: the pulses have an extreme broadband spectrum and thus deliver many different terahertz frequencies at the same time. As it has been possible to manu­facture the component employing methods already used in the semi­conductor industry, the development promises a broad range of appli­cations in research and tech­nology.

A germanium component generates short terahertz pulses that have an advantageous property: the pulses have an extremely broad bandwidth and thus deliver many different terahertz frequencies at the same time. (Source: Juniks, HZDR)

Just like light, terahertz waves are categorized as electro­magnetic radiation. In the spectrum, they fall right between microwaves and infrared radiation. But while microwaves and infrared radia­tion have long since entered our everyday lives, terahertz waves are only just beginning to be used. The reason is that experts have only been able to construct reasonably acceptable sources for terahertz waves since the beginning of the 2000s. But these trans­mitters are still not perfect – they are relatively large and expensive, and the radiation 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 with short laser pulses, gallium arsenide charge carriers are formed. These charges are acce­lerated by applying voltage which enforces the generation of a terahertz wave – basically the same mechanism as in a VHF transmitter mast where moving charges produce radio waves. However, this method has a number of drawbacks: “It can only be operated with relatively expensive special lasers,” explains HZDR physicist Harald Schneider. “With standard lasers of the type we use for fiber-optic communi­cations, it doesn’t work.” Another shortcoming is that gallium-arsenide crystals only deliver relatively narrowband terahertz pulses and thus a restricted frequency range – which signi­ficantly limits the appli­cation area.

That is why Schneider and his team are placing their bets on another material – the semiconductor germanium. “With germanium we can use less expensive lasers known as fiber lasers,” says Schneider. “Besides, germanium crystals are very transparent and thus faci­litate the emission of very broadband pulses.” But, so far, they have had a problem: If you irradiate pure germanium with a short laser pulse, it takes several micro­seconds before the electrical charge in the semi­conductor disappears. Only then can the crystal absorb the next laser pulse. Today’s lasers, however, can fire off their pulses at intervals of a few dozen nano­seconds – a sequence of shots far too fast for germanium.

In order to overcome this diffi­culty, experts searched for a way of making the electrical charges in the germanium vanish more quickly. And they found the answer in a prominent precious metal – gold. “We used an ion accelerator to shoot gold atoms into a germanium crystal,” explains Schneider’s colleague, Abhishek Singh. “The gold pene­trated the crystal to a depth of 100 nanometers.” The scientists then heated the crystal for several hours at 900 degrees Celsius. The heat treatment ensured the gold atoms were evenly distributed in the germanium crystal.

Success kicked in when the team illuminated the peppered germanium with ultrashort laser pulses: instead of hanging around in the crystal for several micro­seconds, the electrical charge carriers disap­peared again in under two nano­seconds – about thousand times faster than before. Figura­tively speaking, the gold works like a trap, helping to catch and neutralize the charges. “Now the germanium crystal can be bom­barded with laser pulses at a high repetition rate and still function,” Singh is pleased to report.

The new method faci­litates terahertz pulses with an extremely broad bandwidth: instead of 7 terahertz using the established gallium-arsenide technique, it is now ten times greater – 70 terahertz. “We get a broad, continuous, gapless spectrum in one fell swoop”, Harald Schneider enthuses. “This means we have a really versatile source at hand that can be used for the most diverse applications.” Another benefit is that, effec­tively, germanium components can be processed with the same tech­nology that is used for microchips. “Unlike gallium arsenide, germanium is silicon compatible,” Schneider notes. “And as the new components can be operated together with standard fiber-optic lasers, you could make the tech­nology fairly compact and inex­pensive.”

This should turn gold-doped germanium into an interes­ting option not just for scientific applications, such as the detailed analysis of innovative two-dimensional materials such as graphene, but also for appli­cations in medicine and environ­mental technology. One could imagine sensors, for instance, that trace certain gases in the atmo­sphere by means of their terahertz spectrum. Today’s terahertz sources are still too expensive for the purpose. The new methods could help to make environ­mental sensors like this much cheaper in the future. (Source: HZDR)

Reference: A. Singh et al.: Up to 70 THz bandwidth from an implanted Ge photoconductive antenna excited by a femtosecond Er:fibre laser, Light Sci Appl 9, 30 (2020); DOI: 10.1038/s41377-020-0265-4

Link: Institute of Ion Beam Physics and Materials Research, Helmholtz-Zentrum Dresden-Rossendorf, Dresden, Germany

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