Physicists at the Tata Institute of Fundamental Research (TIFR), Mumbai, have succeeded in studying electron bunches, kicked up to high speeds within a glass slab by a short duration laser pulse. They have measured the lifetime of these electron bunches within the material. And so long, this has been only guessed at. The work, published in The Physical Review Letters, lies at the forefront of high energy density science.
Only time in India
Such electron pulses, carrying mega-sized currents, are created by high intensity lasers in many labs. “We are not the first [to create such pulses]. But we are the only lab in India, and are among 10–20 labs in the world where people look at the basic physics of the transport of such mega ampere, femtosecond electron current pulses through high density (solid) media,” says Prof G. Ravindra Kumar of TIFR, Mumbai, one of the principal investigators of the experiment.
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Such mega-sized current pulses cause secondary emissions from their targets, which can be x-rays, ions and the like, which have applications in medicine and imaging technology. Hence understanding their properties and interaction with the material they travel through is important.
“They [the secondary emissions] are also of a very short duration (picosecond or less) making them very useful for these applications. On a bigger scale, such megampere electron bunches are also expected to be used in laser fusion research and how they lose energy thereby heating the fusion target is extremely important,” says Professor Kumar.
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Tabletop lasers
In the tabletop experiment done at TIFR in collaboration with P P Rajeev of Rutherford Appleton Labs, U.K., a high-intensity femtosecond laser pulse is aimed at a spot on a glass slab. Bunches of electrons from the glass slab are kicked up to high speeds close to that of light in vacuum. These continue to travel within the glass slab, but at speeds higher than that of light, because light slows down within the glass medium. Such ‘faster-than-light’ electrons emit a radiation known as Cherenkov radiation, which is what the researchers used to track them.
Using the ‘optical Kerr effect’, the researchers innovatively devise an optical ‘time gate’, which allows them to time the duration of the Cherenkov radiation. It is natural to expect that the electron bunch will stay together for a duration in the order of femtoseconds, which was the duration of the pulse that kicked them. However the electrons live approximately 2,000 times longer, for about 50 picoseconds.
“The Kerr gate time we generated was as short as 2 picosecond, allowing us to time the evolution of Cherenkov emission in the best possible manner ever. All earlier measurements had a time window [electronically generated, not optically as in this case] that was thousand times longer and could say nothing about the time evolution of the Cherenkov emission process,” says Professor Kumar.
The ability to create extreme states of matter using high power, femtosecond lasers gives a chance to create intrastellar and intraplanetary temperatures and pressures on a tabletop in the lab! You can ‘mimic’ those systems right on the earth, and that is the most exciting thing.