Compact solid-state lasers can now produce pulses with intensities rivalling those from the large laser systems housed in central facilities. Physicists are using them to investigate novel phenomena with exciting potential applications. High-intensity lasers could be a source of coherent X-rays for biological imaging, accelerate subatomic particles, and even trigger nuclear fusion

Lasers, regarded only a few decades ago as exotic scientific tools, are now commonplace - used, for example, in supermarket barcode scanners and in CD players. Today, research into new types of laser and laser phenomena is still a hugely active area in physics laboratories all over the world, with breakthroughs regularly reported. One of the most exciting ongoing developments is that of moderately-sized lasers capable of producing light pulses each with a power of more than a terawatt, or 1012 watts. The newest lasers are portable and can be installed on a laboratory bench. What is more, they are relatively inexpensive, costing not much more than £250,000. Not surprisingly, the potential of these instruments is enormous.

A Ti:sapphire crystal
pumped by green laser
beams is used to produce
very intense pulses of light

Until recently, the push to brighter beams meant building ever larger, composite lasers that could deliver very high energy (kilojoules). These giant systems are therefore based at national facilities. Indeed, lasers such as Vulcan at the CLRC's Rutherford Appleton Laboratory are still among the most powerful in the world. Work is under way to develop a petawatt (1015/W) laser at the Lawrence Livermore National Laboratory in the US.

There is now, however, another route to achieving high intensities - thanks to a combination of new solid-state lasers, which emit very short pulses, and a clever way of amplifying them. The shorter the pulse, the higher is its power and therefore intensity. Crystals of lasing materials, Ti:sapphire or the newer CrLiSAF, can spit out bolts of light as short as 10 femtoseconds. However, amplifying a short pulse is a problem because the high brilliance results in damage to the amplifier. The answer is ingenious but simple in concept: employ a pair of diffraction gratings to stretch the pulse so that its energy is spread out in time - thereby reducing the power to safe levels - then amplify the pulse, and finally re-compress it back to its original duration. This technique is called chirped pulse amplification (CPA).

High harmonic generation
Lasers with CPA are opening up many exciting possibilities, some of them completely unexpected. When focused with a lens or mirror, the beam can have intensities as high as 1019 Wcm-2, which implies an electric field considerably greater than that in an atom. When such a laser beam interacts with a gas, say helium, the motion of the atomic electrons becomes almost completely dominated by the oscillating light field. Each electron is periodically driven away from the parent ion and then back towards it. As the electron approaches the nucleus, its electric field distorts its harmonic motion in the light field, creating higher-order harmonic frequencies in addition to those of the laser. Since all the electrons in the gas are oscillating in unison, a coherent beam of radiation is produced with frequencies that are multiples of the fundamental laser frequency. These harmonics go up to 150 times the original frequency - well into the soft X-ray range.

Most work has been carried out on laser interactions with inert gases such as xenon and, as yet, the energy conversion efficiency is not very high. Nevertheless just a few months ago, American researchers showed that by exploiting some novel optics they could increase the efficiency one-hundredfold. Another approach is to aim for shorter laser pulses.

Such a coherent X-ray source has many advantages. The system is portable and emits a tightly collimated beam of very short pulses generated at high repetition rates - and because the input laser is tunable, so is the output X-ray beam. Laser physicists already envisage many applications such as the holographic imaging of molecular structure in living cells in the desirable 'water window' region of the spectrum (where water is transparent to X-rays but carbon is not), and time-resolved chemistry experiments.

The interaction of high-intensity laser beams with other forms of matter, as well as gases, has also revealed some remarkable and useful phenomena. Reactions with solids are energetic enough to ionise atoms and generate very hot plasmas. These are sources of very bright, incoherent X-rays of energies up to 1 MeV.

Clusters' last stand
However, even more extraordinary is current work carried out by John Tisch and colleagues at Imperial College, London on the interaction with clusters of rare gas atoms. Such atomic conglomerates, which may contain from four up to a million atoms, are held together by van der Waals forces. Physicists became interested in clusters because they represent a bridge between a gas and a solid, in other words, they have the properties of both single atoms and bulk matter.

What the researchers discovered was a great surprise. When the bright laser light is shone on clusters of 200 or more atoms, the reaction is even more energetic than in the case of the solid. Virtually all the laser energy is absorbed by the clusters causing them to 'melt' into tiny balls of extremely hot plasma. The interaction is enhanced partly as a result of the heat being trapped in the clusters by the surrounding vacuum (unlike in the solid where it can conduct away) and partly because a resonance condition is reached which increases the electric field inside the mini-plasmas. This leads to an elevated ionisation rate and increased collisions between ions. The hot plasma of very highly charged ions (up to 40+) then blows apart in all directions. The ions then slowly recombine, emitting X-rays at a reasonably efficient level (some of the X-ray emission is believed to be fast). Again, X-rays produced thus would be useful for a number of applications such as high-resolution radiography in medicine, X-ray lithography, and as a diagnostic tool for plasmas (increasingly used in industrial manufacturing processes).

A spectrum of high
harmonics showing the
excellent beam quality

There are more speculative possibilities for the cluster configuration. The intense laser radiation, combined with the efficient coupling into a gas of selected atomic clusters, could be used to produce exotic plasmas of the kind that are of interest to astrophysicists, or even to trigger nuclear fusion between clusters of deuterium and/or tritium atoms. Researchers in the UK are thinking of testing out this idea with experiments on deuterium clusters. They are also investigating whether high harmonic radiation can be produced as efficiently from clusters as from ordinary inert gases. UK, French and American researchers are very much at the forefront of such research in high intensity lasers. We can look forward to many more rewarding developments over the next few years.

Plasmas induced by high-intensity lasers could also be used to accelerate subatomic particles such as electrons, thus providing a compact device for particle physics experiments. Although a practical accelerator is still a long way from realisation, active progress is being made. Researchers are working on a number of schemes that take advantage of the powerful electric fields generated in such plasmas. The principle is to convert the oscillating electric field of a picosecond laser pulse into an oscillating electric field in the plasma - a plasma wave. As the laser pulse moves through the plasma, the free electrons oscillate, causing periodic variations in the charge density resulting from separation of electrons and ions in the plasma. The travelling plasma wave can be extremely large, generating fields up to 1000 times stronger than in a typical linear particle accelerator. Electrons can then 'surf' on the wave, accelerated by the strong electric field. One new scheme is called the self-modulated wake-field approach, in which the start of the laser pulse generates a plasma. This interacts with the rest of the pulse in a way that causes periodic variations in plasma density along the length of the pulse. This scheme is currently being tested with Vulcan.

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Thanks go to Dr John Tisch, Professor Peter Knight and the Laser Consortium at Imperial College, London for their considerable help with this Vision paper.

Copyright © Institute of Physics and IOP Publishing Ltd. 1999 - 2000