Highly efficient broadband terahertz radiation from metamaterials

Jan. 17, 2014 — Scientists at the U.S. Department of Energy's Ames Laboratory have demonstrated broadband terahertz (THz) wave generation using metamaterials. The discovery may help develop noninvasive imaging and sensing, and make possible THz-speed information communication, processing and storage. The results appeared in the Jan. 8 issue of Nature Communications.

Terahertz electromagnetic waves occupy a middle ground between electronics waves, like microwave and radio waves, and photonics waves, such as infrared and UV waves. Potentially, THz waves may accelerate telecom technologies and break new ground in understanding the fundamental properties of photonics. Challenges related to efficiently generating and detecting THz waves has primarily limited their use.

Traditional methods seek to either compress oscillating waves from the electronic range or stretch waves from the optical range. But when compressing waves, the THz frequency becomes too high to be generated and detected by conventional electronic devices. So, this approach normally requires either a large-scale electron accelerator facility or highly electrically-biased photoconductive antennas that produce only a narrow range of waves.

To stretch optical waves, most techniques include mixing two laser frequencies inside an inorganic or organic crystal. However, the natural properties of these crystals result in low efficiency.

So, to address these challenges, the Ames Laboratory team looked outside natural materials for a possible solution. They used human-made materials called metamaterials, which exhibit optical and magnetic properties not found in nature.

Institute of Technology in Germany, created a metamaterial made up of a special type of meta-atom called split-ring resonators. Split-ring resonators, because of their u-shaped design, display a strong magnetic response to any desired frequency waves in the THz to infrared spectrum.

Ames Laboratory physicist Jigang Wang, who specializes in ultra-fast laser spectroscopy, designed the femto-second laser experiment to demonstrate THz emission from the metamaterial of a single nanometer thickness.

"The combination of ultra-short laser pulses with the unique and unusual properties of the metamaterial generates efficient and broadband THz waves from emitters of significantly reduced thickness," says Wang, who is also an associate professor of Physics and Astronomy at Iowa State University.

The team demonstrated their technique using the wavelength used by telecommunications (1.5 microns), but Wang says that the THz generation can be tailored simply by tuning the size of the meta-atoms in the metamaterial.

"In principle, we can expand this technique to cover the entire THz range," said Soukoulis, who is also a Distinguished Professor of physics and astronomy at Iowa State University.

What's more, the team's metamaterial THz emitter measured only 40 nanometers and performed as well as traditional emitters that are thousands of times thicker.

"Our approach provides a potential solution to bridge the 'THz technology gap' by solving the four key challenges in the THz emitter technology: efficiency; broadband spectrum; compact size; and tunability," said Wang.

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Chemical synthesis: A simple technique for highly functionalized compounds

Sep. 25, 2013 — Researchers at Kanazawa University have demonstrated a technique that allows direct functionalization of alkenes without the need for metallic reagents, photolysis or extreme reaction conditions.

The addition of functional groups to certain unsaturated hydrocarbons, known as alkenes, is a crucial stage in the synthesis of various compounds, including many plastics.

For these functionalization reactions to occur a carbon-hydrogen (C-H) bond must be activated, which is traditionally achieved using transition metal catalysts. However use of these catalysts has both economical and environmental drawbacks. Now researchers at Kanazawa University have demonstrated a technique that allows direct functionalization of alkenes without the need for metallic reagents, photolysis or extreme reaction conditions.

Tsuyoshi Taniguchi and colleagues at Kanazawa University developed work where they had reported a reaction of alkenes using tert-butyl nitrite and molecular oxygen. They monitored the reaction products -- ?-lactol and nitrate ester -- using different solvents, and found that a high polarity aprotic (hydrogen-free) solvent gave the best yield, with ?-lactol as the major product.

They then experimented with different alkenes and observed how the products differed for branched and linear alkenes. Further reduction reactions demonstrated how the new synthesis technique could yield a range of useful derivatives, producing highly functionalized compounds from simple alkenes in only one or two steps.

The researchers were also able to propose a possible reaction mechanism. While the exact pathway remains uncertain, they suggest that the key step is the cleavage of an oxygen-oxygen bond to form a highly reactive alkoxy radical -- a molecular component comprising an oxygen with single bonds either side to hydrocarbon chains.

The work demonstrates how substantial yields of highly functionalized compounds can be achieved from simple organic molecules in simple conditions with no metal catalyst. The authors conclude, "We believe that such 'simple and advanced reactions' are promising in the development of useful synthetic methods involving direct C-H functionalization."

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