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Vulcan laser zaps its way to a world record

(Credit: STFC)

Scientists using the Vulcan laser at the Central Laser Facility (CLF) have succeeded in setting a world record for the highest energy in a single pulse of terahertz radiation achieved in a laboratory. Terahertz radiation, also known as T-rays, is used for everything from airport security to medical scans and, while the practical applications for such powerful T-ray pulses are unclear, the type of scanning ability it promises could give scientists a new way to explore the very extremes of physics.

Until recently, T-rays, which sit between infrared and microwaves, have been the least explored and used part of the electromagnetic spectrum. Although scientists have known of its existence for most the last century, this part of the spectrum, known as the ‘terahertz gap’, long eluded analysis and detection. The development of technology in the 1980s that could both detect and provide a source of terahertz radiation gave scientists a whole new way of examining the world.

Like X-rays, T-rays let us see through many materials and, although they are far less penetrating than X-rays, they do come without their potentially-damaging side-effects. X-rays are what is known as ionising radiation, which means they carry so much energy they can knock electrons away from atoms as they penetrate an object (such as a human body). Overexposure to ionising radiation can damage cells and DNA and lead to cancers developing, which is why a patient’s exposure to X-rays is carefully monitored and restricted.

T-rays, on the other hand, are non-ionising and so do not damage cells and DNA. Unlike X-rays though, T-rays cannot travel far through materials that contain water, so they can’t be used to look deep inside the human body. They can penetrate several millimetres of tissue with a low water content, such as skin and fatty tissues, which makes them ideal for diagnosing skin cancers without exposing the patient to ionising radiation.

Their inability to pass through the human body however, does mean that they have found a role in airport security. They can pass through clothing with ease, but are stopped by the body and by metallic objects such as concealed guns – the X-ray specs of fiction, were actually made real through the harnessing of T-rays (so we should maybe call them T-ray specs from now on!).

T-rays can also be used to identify the chemical composition of an object, which makes them ideal for inspecting goods that are sealed in otherwise opaque containers. Despite many useful applications, the adoption of T-rays has been slow because of the limited output power of existing sources, which is what makes Vulcan’s record-breaking power output so significant.

The record T-ray pulse set at CLF is the result of a collaboration between UK and Chinese scientists, which was supported by the Newton Fund – a UK government funding scheme set up to promote and establish just such an international collaboration. The collaborations made possible by the Newton Fund are bringing together the sort of new ideas and outlooks that make landmark science like this possible.

The team, led by principle investigator Prof. David Neely, achieved the record-breaking pulse containing millijoules of energy by shooting a high-intensity pico-second (one millionth of a millionth of a second) laser pulse onto a millimeter-sized target made of metal foil. As the laser pulse is fired onto the target, it accelerates 10 million million million (10 quintillion) energetic electrons from the foil, which, in turn, emit the powerful pulse of T-ray radiation.

A millijoule in itself may not sound like a great deal of energy, but considering a T-ray photon’s energy is very low (about one thousandth of the energy of a visible light photon), it means 10 million million million photons were emitted in a pulse of just one millionth of a millionth of second (a very, very large number of photons in a very, very short time), which represents a significant step-up in power from existing T-ray sources.

The experiment is part of a long-term series of planned experiments and, having achieved this level of T-ray power, the team will proceed with a follow-on experiment where they will attempt to use T-rays to alter the quantum structure of a crystal – potentially stepping into an unknown area of scientific discovery and allow scientists to peer into the very extremes of physics.

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Last updated: 15 April 2019


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