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From the lab: Liquids as a source for elusive, high-power terahertz radiation

Written by Amitabh Sinha
| Pune |

Published: January 1, 2018 12:44 am

terahertz, electromagnetic waves, gamma rays, X-rays, radio waves, high-power terahertz Liquids, science news The TIFR results, published in Nature Communications on October 30, are one of the best attempts so far to break the terahertz gap.

For most part of history, light has been the only natural radiation that human beings have been aware of. It was only in the 19th and 20th centuries that it became known, through a series of discoveries, that there was nothing very special about light, that a number of electromagnetic waves exist in nature and that “light”, as we normally know it, is part of them. What humans see is in fact a tiny part of this very broad electromagnetic spectrum that comprises waves whose wavelengths range from a few trillionths of a metre at one end to thousands of metres at the other.

Depending on their wavelength, these radiations exhibit signature properties that could be very useful in certain situations. For example, X-rays, because of the fact that they could penetrate human skin but not the bones, came to be used extensively in medical imaging. In the last nearly 100 years, scientists have mastered the exploitation of nearly every bit of the electromagnetic spectrum for different kinds of purposes — from gamma rays and X-rays, to microwaves and radio waves.

A very small part of this spectrum, tucked between the microwave and optical infra-red region, and corresponding to frequencies in the range of 0.1 to about 3 terahertz (1 THz = 1 trillion Hz), however, has remained largely unutilised till now. Radiations in the terahertz frequency range are known to have a number of properties that can be put to good use. For example, it too can penetrate human skin and could be used for body imaging, but unlike X-rays, these are not very harmful to human tissues, making them ideal for use in the medical field and security scanners like those at the airports. Terahertz radiation is also great for communication. Besides, it can be used in a number of research activities.

The problem is that scientists have so far not been able to develop a reliable source of terahertz radiations that can generate these waves as and when required, like they can with X-rays or other radiations. The terahertz region in the electromagnetic spectrum, in fact, is often referred to as the terahertz gap.

About three decades ago, scientists looked at a new way of producing terahertz radiation using semiconductors such as silicon and gallium arsenide. A visible laser pulse of a few ten femtoseconds (1 femtosecond is a thousandth of a trillionth of a second) was used to excite this semiconductor layer, producing a pulsed current. This current was seen to be emitting terahertz radiation. However, the energy of these radiations was very low, of the order of a few pico-joule (a trillionth of a joule) or less. There was very little that could be done with them. They needed to have more power to be put to any meaningful use.

A decade later, attempts were made to produce terahertz radiation through excitation of air. Again, a femtosecond laser but at a much higher power, was used to ionise air molecules to produce a plasma which, under certain conditions, could be made to emit terahertz radiation. This time, this radiation carried a few microjoules of energy, making a big improvement over the fluxes produced by semiconductors.

All this while, scientists had not tried using liquids to produce terahertz radiation, probably because it was very well known that liquids exhibit very strong absorption in the terahertz region. So, even if the terahertz radiation was produced, there was every likelihood that it would get reabsorbed.

Prof G Ravindra Kumar, Dr Indranuj Dey and colleagues at the Tata Institute of Fundamental Research in Mumbai had been carrying out a very different set of experiments in liquids using high-powered lasers. It was during the course of these experiments that they decided to check out whether terahertz radiation could indeed be produced using liquids. They exposed the liquid — different liquids were used, even water — to very high intensity femtosecond lasers. Indeed, they observed huge amounts of terahertz radiation coming out. What was more, these radiations had a very broad spectrum of frequency ranges, from a few terahertz to about 100 terahertz. Such a broad spectrum of terahertz radiation had not been produced from any of the earlier attempts. The energy of these radiations was in the range of 60 to 70 microjoules, depending on the liquid used, which again was a big improvement on previous experiments.

The TIFR team checked and rechecked the results, blanking out all other radiations to confirm that what was being emitted was indeed terahertz. A theoretical simulation was used to explain what could be happening. Exposed to the femtosecond laser light, the liquid was producing secondary radiation, which was then mixing non-linearly with the input laser light radiation to produce terahertz radiation.

The TIFR results, published in Nature Communications on October 30, are one of the best attempts so far to break the terahertz gap. It opens up new possibilities to exploit the terahertz radiation for a variety of applications.

The research

A way to generate waves in the terahertz range out of liquids, which were not explored as a source earlier under the impression that they would reabsorb the radiation.

Naresh Patwari & team, Department of Chemistry, IIT Bombay

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