Ultrashort Laser Bursts To Bridge The Terahertz Gap

By creating grooves and ripples on the surface of terahertz emitting devices, scientists were able to enhance terahertz emission significantly, overcoming the ‘terahertz gap.’

AsianScientist (Sep. 29, 2015) – Researchers from Japan have found a way to overcome the problem of the terahertz (THz) gap—an electromagnetic wave spectrum that is currently unused by practical technologies. Their work, published in Optics Letters, demonstrated the increased efficiency of THz emission in a gallium arsenide (GaAs)-based device.

THz radiation lies between infrared and microwave radiation in the electromagnetic spectrum. Many materials have a unique ‘fingerprint’ in the THz band allowing their easy identification with THz scanners. Moreover, unlike X-rays and ultraviolet light, THz radiation is safe for live tissues and DNA due to its non-ionizing properties. Therefore, THz technology could be a next important breakthrough in medicine, security, chemistry, and information technology.

However, generation of THz waves is difficult since the frequency is too high for conventional radio transmitters, but too low for optical transmitters, like the majority of lasers. Furthermore, although THz radiation can penetrate fabrics, paper, plastics, wood and ceramics, it is absorbed by water, limiting the use of THz devices in the Earth’s atmosphere—laden with water vapor—to short distances

For years, the THz portion of the spectrum remained unused, giving rise to the term ‘THz gap.’ Now, research led by Professor Keshav M. Dani at Okinawa Institute of Science and Technology Graduate University (OIST) has found a method to overcome this limitation.

One of the most frequently used THz emitters is a photoconductive antenna, comprising two electric contacts and a thin film of semiconductor, often GaAs, between them. When the antenna is exposed to a short pulse from a laser, the photons excite electrons in the semiconductor and a short burst of THz radiation is produced. Thus the energy of the laser beam is transformed into a THz electromagnetic wave.

The researchers showed that microstructure of the semiconductor surface plays an important role in this process. Femtosecond-laser-ablation, in which the material is exposed to ultrashort bursts of high energy, creates micrometre-scale grooves and ripples on the surface of GaAs.

“The light gets trapped in these ripples”, says Athanasios Margiolakis, a special research student at OIST.

Since more light is absorbed by the ablated material, the efficiency of THz emission, given a sufficiently powerful laser, increases by 65 percent.

Other properties of the material change as well. For example, ablated GaAs shows only a third of the electrical current of non-ablated GaAs.

“We observe counter-intuitive phenomena,” the researchers write, “One generally expects that the material showing the higher photocurrent would give the best THz emitter.” They explain this phenomenon by shorter carrier lifetimes. That is, electrons in ablated samples return to non-agitated states much faster than in control samples.

“Femtosecond-laser ablation allows us to engineer the properties of materials and to overcome their intrinsic limitations, leading, for example, to near 100 percent photon absorption as well as broader absorption bandwidth, control of the electron concentration and lifetime,” said Dr. Julien Madéo, first author of the paper.

This technique is a fast, lower-cost alternative to existing methods of manufacturing materials for THz applications.

The article can be found at: Madéo et al. (2015) Ultrafast Properties of Femtosecond-Laser-Ablated GaAs and its Application to Terahertz Optoelectronics.

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Source: Okinawa Institute of Science and Technology.
Disclaimer: This article does not necessarily reflect the views of AsianScientist or its staff.

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