NEWS FEATURE NATURE|Vol 446|1 March 2007 EXTREME LIGHT Physicists are planning lasers powerful enough to rip apart the fabric of space and time. Ed Gerstner is impressed. 16 NATURE|Vol 446|1 March 2007 NEWS FEATURE lmost 100 million times more pow- something altogether more exotic, unpredict- High-power lasers achieve their awesome erful than its earliest ancestor, the able and nonlinear. intensities by squeezing moderate amounts of Large Hadron Collider (LHC) is Pump in enough energy, and the paired vir- energy into very short-lived bursts, thus driv- C. DARKIN Athe latest triumph in a century of tual particles become real, separated at birth by ing up the power — which is energy divided astounding physics. Officially inaugurated the extraordinarily strong fields involved. This by time. So although the power of NIF’s lasers later this year, the €3.7-billion (US$4.9-bil- energy level, currently thought to require fields sounds incredible, they actually use relatively lion) accelerator at CERN, Europe’s particle- of a little more than 8×1018 volts per metre, is small amounts of energy. A single pulse con- physics laboratory, will next year be fully up to known as the Schwinger limit, and it is the tains just 2,000 kilojoules — roughly half a speed in its search for exotic phenomena, such point at which the vacuum sea begins to boil. kilowatt hour. It’s just that all that energy is as the Higgs boson, that can only be discovered “When I give talks to general audiences, delivered in a few billionths of a second. at the extreme energies it makes available. Yet that’s the thing that they really seem to get even before it is turned on, the particle-physics drawn in by,” says Tom Katsouleas, an extreme- Power trip community is already looking beyond it, with laser enthusiast at the University of Southern The ELI takes the same principle further. By plans for an even more powerful machine, the California in Los Angeles. The problem is that generating pulses a million times shorter than International Linear Collider (ILC), which will the Schwinger limit is a long way away. For those of NIF — five femtoseconds — the ELI cost some $6.7 billion. fields of 8×1018 V m−1 you need a laser with an should fairly quickly be able to generate peak At about the time that the LHC hopes to intensity of more than 1030 W cm−2 — a thou- powers of more than a petawatt (1015 watts) be homing in on the Higgs, construction of sand trillion times more intense than NIF. from just a few joules of energy. This radically a much less feted multibillion-dollar physics Given NIF’s multibillion-dollar pricetag, that reduced need for energy makes things much research facility, the National Ignition Facility seems an overly ambitious target. easier than they are at NIF. By shortening the (NIF) at Lawrence Livermore National Labo- But a team led by Gérard Mourou, director pulse lengths by a factor of a hundred more, ratory in California, will be reaching comple- of the Laboratory of Applied Optics near Paris, down to tens of attoseconds (10−18 seconds), tion. At a cost of about US$4 billion, NIF is believes it can meet this target for a relatively the ELI’s proponents hope to reach peak inten- an assembly of 192 lasers that, for a billionth moderate price. And the researchers predict all sities of more than 100 petawatts. of a second at a time, can pump out energy at manner of wonders on the way to their even- The ELI’s extraordinarily short pulses will be more than 50 times the rate that it is generated tual goal. Throughout their made possible by a technique in all of Earth’s power stations put together. history, lasers have excited “I am always called chirped-pulse amplifi- Its aim is to ignite a fusion reaction that turns physicists by opening up new cation (CPA), which Mourou a tiny pellet of hydrogen at the lasers’ focus possibilities with light. In the astonished at what developed at the University into helium. 1960s, the fact that early lasers experimentalists can of Rochester, New York, in Science at NIF will bring astrophysics into were powerful enough to actually do if they the mid-1980s. CPA works by the laboratory by aping stars in microcosm, change the refractive index of decomposing the light in a laser and it could conceivably provide the basis for the medium through which put their minds to it.” pulse using a diffraction grat- future energy generation. But the main ration- they travelled opened up fresh — William Unruh ing known as a stretcher, which ale for NIF, and the reason it has been able to vistas in ‘nonlinear’ optics. acts like a prism. Having been command the budget that it has, is to help the Today the frontier buzzword stretched, the pulse’s compo- United States assure the operability and safety is ‘relativistic optics’ — systems in which the nents, now spread out in space and time, are of its nuclear arsenal. Similar motivations lie fields associated with the laser light can accel- fed individually through an optical amplifier, behind the French Laser MegaJoule (LMJ) erate every electron in the medium the light is before a similar grating designed for the oppo- facility, which is expected to achieve ignition passing through close to the speed of light. site effect — a ‘compressor’ — re unites them a few years after NIF. into a pulse far shorter and more intense than The lasers of the LMJ and NIF don’t come Need for speed the original. close to an accelerator such as the LHC in Mourou’s proposal — the Extreme Light Infra- These stretchers and compressors are cur- terms of generating excitement among physi- structure (ELI) — would up the ante further, rently used on almost all of the world’s most cists. At least, not yet. But laser beams even moving into ‘ultrarelativistic systems’ in which powerful lasers except those, such as NIF and more intense than NIF’s — and far cheaper to not only electrons but also the ions from which the LMJ, that need pulses that are relatively generate — might in the next decades begin to they have been stripped move close to the long (of the order of nanoseconds). Osaka Uni- take over from particle accelerators in explor- speed of light. Mourou notes that the nonlin- versity and the Central Laser Facility at Ruther- ing the outermost frontiers of the physical ear effects of lasers revealed in the 1960s far ford Appleton Laboratory in Didcot, UK, both world. The world may never see conventional exceeded expectations. “Only the tip of the have CPA lasers that can generate a petawatt, particle accelerators much more powerful iceberg was predicted,” he says. He is similarly and the University of Rochester is also build- than the LHC and ILC. But lasers a million optimistic about what the new energies avail- ing one, as are other institutions. Mourou and times more intense than NIF are already to be able at the ELI could deliver. others feel that the technique has, as yet, no found in the presentations of physicists look- On 15 February, the French government obvious limitations; pulses can go on getting ing for funding. announced that it had bought into the vision shorter and shorter. enough to pay for a new laser beamline at the In 2006, the ELI was one of 35 projects Let there be light Laboratory of Applied Optics to show that the short-listed for consideration under the Euro- The guiding inspiration for extreme lasers lies ELI could work. If it goes ahead, the full facility pean Roadmap for Research Infrastructures, in the way light interacts with the vacuum. would provide unprecedented opportunities a programme that will provide money to help Quantum field theory sees the vacuum as a for scientists to pursue fundamental, curiosity- develop proposals for international projects. strange sea of possibilities, where pairs of ‘vir- driven science, says Mourou. Perhaps more Also on the shortlist is another, costlier tual’ particles and antiparticles ceaselessly pop importantly, it would be relatively cheap, cost- project that plans to use a CPA-enabled laser. in and out of existence. When light is bright ing €138 million to build and €6 million per A consortium led by the Central Laser Facil- enough, the electromagnetic fields it is com- year to run — considerably less than the £380- ity wants to build HiPER — the High Power posed of begin to interact with that sea in unu- million (US$746-million) Diamond synchro- Laser Energy Research facility — as a civilian sual ways. The vacuum no longer behaves as tron X-ray source recently opened in Britain, equivalent to NIF and the LMJ, but pursuing a simple, predictable medium — it becomes for example. a subtly different path to fusion. Whereas NIF 17 NEWS FEATURE NATURE|Vol 446|1 March 2007 and the LMJ use megajoule beams to hitch a ride on the wake left behind by crush their targets into fusion, the an intense pulse of laser light blasting €855-million HiPER would com- through a plasma. press the target comparatively gently “If you look at the progress that has and then ignite it with a much shorter gone on in laser wakefield accelera- high-power pulse. tors,” says Katsouleas, “and extrapolate One advantage of this approach is that to the kinds of powers that peo- that the laser could be fired far more ple are talking about for the ELI, then frequently than NIF.