Research team presents novel transmitter for terahertz waves

If a gallium-arsenide crystal is irradiated with short laser pulses, charge carriers are formed. These charges are accelerated by applying a voltage which enforces the generation of a terahertz wave. Credit: HZDR/Juniks

by Helmholtz Association of German Research Centres

https://phys.org/news/2020-03-team-transmitter-terahertz.html

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, as the team reports in the journal Light: Science & Applications.

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 Dr. 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.

Precious metal implants

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, Dr. 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.

Inexpensive manufacture possible

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, developed in Dresden-Rossendorf, could help to make environmental sensors like this much cheaper in the future.

Abstract-An Integrated Germanium-Based THz Impulse Radiator with an Optical Waveguide Coupled Photoconductive Switch in Silicon

Peiyu Chen,  Mostafa Hosseini, Aydin Babakhani

https://www.mdpi.com/2072-666X/10/6/367/htm

This paper presents an integrated germanium (Ge)-based THz impulse radiator with an optical waveguide coupled photoconductive switch in a low-cost silicon-on-insulator (SOI) process. This process provides a Ge thin film, which is used as photoconductive material. To generate short THz impulses, N++ implant is added to the Ge thin film to reduce its photo-carrier lifetime to sub-picosecond for faster transient response. A bow-tie antenna is designed and connected to the photoconductive switch for radiation. To improve radiation efficiency, a silicon lens is attached to the substrate-side of the chip. This design features an optical-waveguide-enabled “horizontal” coupling mechanism between the optical excitation signal and the photoconductive switch. The THz emitter prototype works with 1550 nm femtosecond lasers. The radiated THz impulses achieve a full-width at half maximum (FWHM) of 1.14 ps and a bandwidth of 1.5 THz. The average radiated power is 0.337 μ$$W. Compared with conventional THz photoconductive antennas (PCAs), this design exhibits several advantages: First, it uses silicon-based technology, which reduces the fabrication cost; second, the excitation wavelength is 1550 nm, at which various low-cost laser sources operate; and third, in this design, the monolithic excitation mechanism between the excitation laser and the photoconductive switch enables on-chip programmable control of excitation signals for THz beam-steering.

Abstract-Modulated Photoluminescence of Germanium via Intense Terahertz Pulse Electric Fields

Mary A. B. Narreto, Chenxi Huang, David N. Purschke,  Frank A. Hegmann,

https://www.osapublishing.org/abstract.cfm?uri=LS-2018-JW3A.33

We show a nonlinear effect of intense terahertz pulse electric fields (>200 kV/cm) on the photoluminescence of bulk germanium. Terahertz-pulse-induced direct-gap photoluminescence quenching is observed.

© 2018 The Author(s)

Abstract-Exciton ionization by THz pulses in germanium

M Stein, C Lammers, J T Steiner, P-H Richter, S W Koch, M Koch,  M Kira,

http://iopscience.iop.org/article/10.1088/1361-6455/aabac7/meta

After optical pumping, strong single cycle THz pulses are used to probe and manipulate excitons in bulk germanium. For strong THz fields a significant broadening and bleaching of the 1s–2pTHz absorption peak is observed. The experimental results are analyzed using a microscopic many-body theory attributing the observations to a shortening of the excitonic state lifetime and eventual exciton ionization. Simultaneously, the ac THz Stark effect leads to a shift in the 1s–2ptransition energy.  

© 2018 IOP Publishing Ltd

Graphene and Germanium: A Happy Marriage With Exceptional Conductivity

                                                                                 Image: University of Wisconsin-Madison

By Alexander Hellemans

http://spectrum.ieee.org/nanoclast/semiconductors/materials/graphene-and-germanium-a-happy-marriage-with-exceptional-conductivity

Graphene became the subject of much research because its electrical, mechanical, and optical properties make it an excellent material for electronics. The conductivity of freestanding graphene is comparable to that of copper. However, using graphene in electronic components requires a substrate to support it, and researchers were faced with a problem: graphene's electrical properties degrade when bonded to most substrates. For example, bonded to silicon dioxide, a material widely used in electronics because of its good insulating properties, graphene's conductivity decreases by two to three orders of magnitude.

Now a team of researchers has shown that graphene, when deposited on a germanium substrate covered with a thin germanium oxide layer, acquires excellent electrical properties, and its conductivity even improves compared to pure graphene. The team, from the University of Wisconsin-Madison and University of Notre Dame, reported their findings in ACS Nano earlier this month.

One reason the researchers decided to try germanium as a substrate was its low cost. "People have tried, besides silicon dioxide, boron nitride and cadmium telluride, which are expensive," says Francesca Cavallo, who was formerly at the University of Wisconsin-Madison and is now a professor of electrical and computer engineering at the University of New Mexico. A literature search of germanium showed that this material, a cheap substrate, could be a good candidate. "We found that germanium has a very high density of surface states which could provide charge to the graphene, so we tried that," she says.

For the experiment, the team transferred a graphene layer from a plastic support film directly onto the surface of the germanium substrate. Next the researchers passed a current through contacts at both ends of the graphene strip and measured the voltage between them [pictured above]. When a current of 5 µA passed through the strip, practically no voltage was measurable over the contacts, indicating that the conductivity was very high. For comparison, the researchers repeated the same experiment with graphene placed on a silicon dioxide substrate. This time they measured a voltage of about 60 mV, which suggested a decrease in conductivity.

To measure the electron density in the graphene layer, the researchers applied a magnetic field to the strip and measured the voltage across its width. At very low temperatures they could measure a voltage caused by a force acting on the electrons, known as the Lorentz force, resulting in the build-up of charge on one side of the strip (a phenomenon known as the Hall effect). At higher temperatures the build-up of electrons on one side of the strip became counterbalanced by the build-up of holes in the underlying germanium oxide layer, lowering the Hall voltage.

"There are so few terahertz sources available that the terahertz region is called the 'terahertz gap' "

Trying to understand better how the graphene-germanium interface modifies the properties of graphene is the next step for Cavallo and her colleagues. She says they'll modify the interface, removing the oxide and performing more chemical analysis to find out "exactly what is going on."

For Cavallo, the discovery of the happy marriage between graphene and germanium could be an important step for a particular system she—and many others—are interested in because of its promising application: a terahertz emitter. "There are so few terahertz sources available that the terahertz region is called the 'terahertz gap', " she says.

One possibility would be using the new graphene-germanium material in a device called a mechanical wiggler, designed to produce terahertz radiation. The wiggler is a serpentine-shaped conductor that works like a miniature and mechanical version of a free-electron laser. But instead of forcing the electrons to follow an alternating magnetic field, it forces them to follow the bends of the serpentine conductor. As a result, the electrons give off what is known as synchrotron radiation at each curve.

One of Cavallo's colleagues and a co-author of the ACS Nano paper, Max Lagally, a physicist at the University of Wisconsin-Madison, explains that the challenge is that the serpentine cannot consist of metals because they are polycrystalline materials and the electrons wouldn't remain focused, scattering on the crystal boundaries. Enter graphene, which acts like a single crystal and could be potential candidate to make the mechanical wiggler a reality, he says. Lagally and another scientist, Robert Blick, have a patent for the device.

The researchers are hopeful that graphene wigglers on germanium will perform well. "You need a high density of carriers and a high mobility," Cavallo says, "so the combination of the two materials seems to be ideal for that."