special issue

News & Analysisphysicsworld.comBreaking the vacuumEurope is planning to build the world’s most powerful laser that willliterally rip empty space apart. Michael Banks lifts the lid on theExtreme Light InfrastructureThis year is one of celebration for Gér -ard Mourou – and not just because2010 marks the 50th anniversary of theinvention of the laser. It is also 25 yearssince the 65-year-old French physicistpublished details of one of his mostcoveted contributions to laser science.Going by the rather ungainly name ofchirped-pulse amplification (CPA),the technique has enabled physiciststo create lasers that are orders of magnitudesmore powerful than wereachievable without it (see box).CPA now lies at the heart of mosthigh-powered laser facilities in theworld. It was used in the now-decommissionedNova PW system at theLawrence Livermore National La bor -atory in the US, which generatedrecord-breaking 1.3 PW (1.3 × 10 15 )pulses, and in the 1 PW Vulcan laserat the UK’s Rutherford Appleton La -boratory in the UK, which is in themidst of being upgraded to go beyondthe 10 PW level.But now Mourou is designing alaser facility that will be so powerfulthat it can rip apart empty space itself.Mourou’s parting shot to the lasercommunity, the Extreme Light Infra -structure (ELI) will create very shortpulses of light barely 1 femtosecond(10 –15 s) long with energies of severalkilojoules corresponding to petawattsof power. While other lasers such asVulcan can provide a high-poweredpulse every 20 minutes, ELI will beable to deliver one every few minutes.Four for the futureThe Extreme LightInfrastructure willconsist of fourfacilities, includingthis one in theCzech Republic thatwill use short pulsesof light to testacceleratingelectrons with lasers.Although ELI will be used for nuc -lear physics, attosecond physics andstudies of laser-based particle acceleration,perhaps its most exciting possibilityis to test the properties of thevacuum, or empty space, itself. “Thisis not just a laser that is about breakingthe next re cord,” says Mou rou,who is ELI’s project coordinator anddirector of the Institut de la Lu mièreExtrême at the Ecole Na tion ale Su -périeure de Tech niques Avancées inFrance. “There is a fundamental reasonbe hind building it.”Mourou first proposed ELI fiveyears ago and he has been the drivingforce behind the project ever since. In2006 it was chosen as one of 35 projectson a “wish list” of scientific facilitiesdrawn up by the European StrategyForum on Research Infra structuresthat researchers in Europe want to seeHamiltons Architectsbuilt within the next decade.The new laser facility quickly garneredsupport with laser scientists inEurope, including Wolfgang Sander,director of the Max Born Institute fornonlinear optics and short-pulsespectroscopy in Berlin and the president-electof the German PhysicalSociety. “ELI offers a factor of 100more in achievable power than anywhereelse in the world,” he says. “Alot of new physics could be done withit – it is revolutionary.”A competition to build ELI wasbegun in 2007. Five countries – theCzech Republic, France, Hungary,Romania and the UK – initially bid tohost the project. But after the UK andFrance pulled out of the running, inOctober 2009 the ELI steering committeedecided to not build one singlefacility, but four – one in Romania onnuclear physics, another in Hungaryon attosecond physics, a third onlaser-based particle-beam productionin the Czech Republic and a fourth inultrahigh-powered lasers. The latter’slocation is still up for grabs.The 7250m needed to build each ofthe first three of these facilities will bemet by the host nation and constructionis due to start at the end of theyear. Once up and running in 2015, anumber of European member statesbe longing to the European ResearchInfrastructure Consortium are expectedto pay for labs’ operational costs.Surfing electronsThe Czech facility, which will be builtin Prague, will seek to generate forthe first time pulses with a few peta -watts in power at a frequency of about100 Hz. These femtosecond laser pul -ses will be fired into a gas to create anelectron–proton plasma that could beused to make a very compact particleShining light in the femtosecond regimeAll four sites belonging to the Extreme LightInfrastructure project have one aspect in common:a way of generating very short pulses of light atvery high energies. At their heart, the four facilitieswill use the chirped-pulse amplification (CPA)technique invented 25 years ago by GérardMourou, now director of the Institut de la LumièreExtrême at the Ecole Nationale Supérieure deTechniques Avancées in France.To generate the high-energy beams, a standardoff-the-shelf table-top laser source will be used togenerate pulses that are a femtosecond in length.These pulses, however, only have a small amountof energy – about a nanojoule. To get a highpoweredpetawatt beam, the energy needs to beincreased by a factor of about 10 12 . However, asthe energy of a short-pulse laser beam is12amplified, the refractive index of the medium it ispassing through starts to change; and once thepower of the beam goes beyond a few gigawatts, itstarts to produce nonlinear effects in the medium.This can lead to so-called self-focusing, where theintensity of the beam increases rapidly damagingthe optics in the process.To keep the intensity of laser pulses below thethreshold of nonlinear effects, laser systems hadto be very large and expensive, and the peak powerof laser pulses was still limited to a few terawattsfor very large multibeam facilities. In 1985Mourou, then at Rochester University, US, andhis colleague Donna Strickland, developedCPA to get around the nonlinear effects (OpticsCommunications 56 219). It works by taking theshort pulse and passing it through a pair ofgratings that stretch the pulse in time by a factor ofa 100 000. The gratings are arranged so that thelow- frequency component of the laser pulse travelsa shorter path than the high-frequency componentdoes, so the high-frequency component lagsbehind the low-frequency component and thepulse spreads out in time.As the pulse is longer, its power is lower and itsenergy can then easily be increased by passing thepulse through a amplifier such as a titanium–sapphire crystal. The amplified pulse is thenpassed through a second pair of gratings thatreverse the dispersion – forcing the high-frequencycomponent of the laser pulse to travel a shorterpath and the low-frequency component to travel alonger path, so the pulse then “recombines” into ashort femtosecond pulse.Physics World May 2010