Laser ablation boosts terahertz emission

September 17, 2015, Okinawa Institute of Science and Technology
Athanasios Margiolakis and Bala Murali Krishna Mariserla, OIST Posdoctoral researcher, perform an experiment in Femtosecond laser laboratory.

From almost instantaneous wireless transfer of huge amounts of data and easy detection of explosives, weapons, or harmful gases, to safe 3-D medical imaging and new advances in spectroscopy —technologies based on terahertz (THz) radiation, the electro-magnetic band with wavelengths from 0.1 to 1 mm, can transform science fiction into reality. However, scientists and engineers still do not have cheap and efficient solutions for mass production of THz-based devices.  

For years, the THz portion of the spectrum remained unused, giving rise to the term "terahertz gap". Research, recently published in Optics Letters by the Femtosecond Spectroscopy Unit at the Okinawa Institute of Science and Technology Graduate University (OIST), suggests one possible solution for this problem: a method to increase efficiency of THz emission gallium arsenide (GaAs)-based devices.

THz radiation lies between infrared and microwave radiation in the electro-magnetic spectrum. It is absorbed by water —which limits the use of THz devices in the Earth's atmosphere, laden with water vapour, to short distances—but it can penetrate fabrics, paper, cardboard, plastics, wood, and ceramics. 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-ionising properties. THz technology could be a next important breakthrough in medicine, security, chemistry, and information technology. 

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. Therefore, researchers have to come up with new innovative devices. 

Laser ablation boosts terahertz emission
Femtosecond-laser-ablation creates micrometre-scale grooves and ripples on the surface of GaAs.

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 electro-magnetic wave.

OIST researchers showed that micro-structure 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%. 

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. 

Dr Julien Madéo, one of the OIST team members, says that "femtosecond-laser ablation allows us to engineer the properties of materials and to overcome their intrinsic limitations, leading, for example, to near 100% photon absorption as well as broader absorption bandwidth, control of the electron concentration and lifetime". This technique is a fast, lower-cost alternative to existing methods of manufacturing materials for THz applications.

Explore further: Broadband THz wave generation with metamaterials demonstrated

More information: "Ultrafast properties of femtosecond-laser-ablated GaAs and its application to terahertz optoelectronics." Optics Letters Vol. 40, Issue 14, pp. 3388-3391 (2015) DOI: 10.1364/OL.40.003388

Related Stories

Photonics: strong vibrations

May 10, 2012

A new approach to generating terahertz radiation will lead to new imaging and sensing applications. The low energy of the radiation means that it can pass through materials that are otherwise opaque, opening up uses in imaging ...

Laser guided codes advance single pixel terahertz imaging

June 25, 2013

The universe is awash in terahertz (THz) waves, as harmless as they are abundant. But unlike other regions of the electromagnetic spectrum, THz has proven to be extremely difficult to manipulate in order to capture novel ...

Terahertz imaging goes the distance

April 26, 2007

Terahertz (THz) radiation, or far-infrared light, is potentially very useful for security applications, as it can penetrate clothing and other materials to provide images of concealed weapons, drugs, or other objects. However, ...

Recommended for you

Coffee-based colloids for direct solar absorption

March 22, 2019

Solar energy is one of the most promising resources to help reduce fossil fuel consumption and mitigate greenhouse gas emissions to power a sustainable future. Devices presently in use to convert solar energy into thermal ...

Physicists reveal why matter dominates universe

March 21, 2019

Physicists in the College of Arts and Sciences at Syracuse University have confirmed that matter and antimatter decay differently for elementary particles containing charmed quarks.

ATLAS experiment observes light scattering off light

March 20, 2019

Light-by-light scattering is a very rare phenomenon in which two photons interact, producing another pair of photons. This process was among the earliest predictions of quantum electrodynamics (QED), the quantum theory of ...

How heavy elements come about in the universe

March 19, 2019

Heavy elements are produced during stellar explosion or on the surfaces of neutron stars through the capture of hydrogen nuclei (protons). This occurs at extremely high temperatures, but at relatively low energies. An international ...

0 comments

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.