special issue

More documents

Recommendations

Info

The laser at 50: Visions of the futurephysicsworld.comNew kinds of lasers with differentwavelengths, ever shorter pulse lengths,ever higher powers, ever narrower spectralwidths and ever better stability have allmade possible new kinds of experimentsWilliam D PhillipsAtomic physicsWilliam D Phillips is aphysicist at the NationalInstitute of Standards andTechnology (NIST) inGaithersburg, Maryland, US.He shared the 1997 NobelPrize for Physics with Claude Cohen-Tannoudji and Steven Chu for cooling andtrapping atoms with laser lightIn the early 1970s I was a young graduate studentin Dan Kleppner’s research group atthe Massachusetts Institute of Technology,working on a thesis that involved makingprecision measurements with a high-magnetic-fieldhydrogen maser (masers were themicrowave precursors to the laser, which wasoriginally called an “optical maser”). Klep -pner and Norman Ramsey had inventeda low-field version of the hydrogen masermore than a decade earlier, and the highfieldversion was producing unprecedentedlyaccurate measurements of magnetic mo -ments in atoms – a sort of zenith for this kindof atomic physics.But then came a new development thatwould change the direction of work in Dan’slab, in my career and in atomic physics as awhole: the first continuous-wave, commercial,tunable dye lasers. The lasing mediumin these devices was an organic dye that lasedover a far wider range of wavelengths than,for example, a helium–neon laser, wherethe gain medium is an atomic gas. The introductionof these devices meant that eventhose who were not experts in laser designand construction could, by tuning a laser toan atomic-resonance transition, explore anew domain of atomic manipulation whereco herent light was the key tool.Eager to play with these new toys, I askedDan to suggest an additional thesis experimentusing lasers. He agreed, and suggestedthat I study collisions of optically excitedsodium atoms. I began to build the apparatus.Other students and postdocs in thegroup started new experiments as well. Eachissue of the research journals brought anincreasing number of laser-related papers,and each conference saw reports of newlaser experiments.The excitement of that time was palpable.New ideas and new experiments popped upeverywhere. In 1978 I was inspired by DaveWineland’s demonstration of laser coolingof ions at the National Bureau of Standards(now NIST) in Boulder, Colorado, and byan idea from Art Ashkin at Bell Laboratoriesto slow and trap a beam of sodium atoms.Later that year, when I went to the bureau’slabs in Gaithersburg, Maryland, I took mythesis apparatus with me and began to workon laser cooling and trapping of sodium.For me, the excitement I felt in the 1970sin Dan’s lab has never waned. New kindsof lasers with different wavelengths, evershorter pulse lengths, ever higher powers,ever narrower spectral widths and ever bet-54ter stability have all made possible new kindsof experiments. Laser cooling of many moretypes of atoms and ions, plus giant coldmolecules, atomic clocks ticking at opticalfrequencies, and non-classical states of lightare just some of the paths into which lasershave led atomic, molecular and optical(AMO) physics.Moreover, lasers have allowed AMO phy -sicists to realize Bose–Einstein condensation,to create optical lattices and to studyultracold Fermi gases. Each of these hasdeepened the connections between AMOand condensed-matter physics. It may bethat lasers and cold atoms will help to elucidatesome of the outstanding problems incondensed matter, such as the origins ofhigh-temperature superconductivity and thenature of fractional-quantum-Hall statesthat are useful for quantum computing.Ever since they first became available,lasers have invigorated and reinvigoratedatomic phys ics, and the adventure shows nosigns of stopping.BiophysicsSteven Block is abiophysicist at StanfordUniversity, California, USOver the past 10 years, it hasbecome possible to do ex -peri ments in biophysics thatwere previously just pipe dreams. For ex -ample, I work in a field known as single mo -lecule biophysics. In this area, the challengeis to study the molecules of life – the proteins,nucleic acids, carbohydrates and other chemicalsthat make us up – literally one moleculeat a time. This is not easy to do, because allbiomolecules are much too small to be seenusing, say, a conventional microscope. None -the less, we are finding that they can bemanipulated and measured, and the techniquesthat are in volved in doing this oftenrequire lasers.One technique that my lab has helped topioneer is known as “optical tweezers”. Theidea behind optical tweezers is that you canuse the radiation pressure supplied by aninfrared laser beam to capture and manipulatesmall materials – including individualproteins and nucleic acids – and move themaround under a microscope. To do this, wehook tiny microscopic beads up to moleculessuch as DNA. Then we can use opticaltweezers and optical traps to “hold onto”these beads and exert very tiny, controlledforces on the DNA molecules.The lasers that we use for this have somepretty extraordinary properties – they arenot like the laser in a laser pointer or in yourCD player. We need to be able to hold a laserbeam stably in space to within the diameterof a hydrogen atom, or about 1 Å, for severalseconds at a time. This is because the basepairs in a DNA molecule are only separatedby about 3.5 Å, and one of the things we areinterested in studying is how the enzymeRNA-polymerase, which “reads” the geneticcode, moves as it climbs the DNA ladder onebase pair at a time.It is amazing that we can literally watch thishappen, and it all depends on being able toshine laser light on the enzyme, scatter thatlight and measure displacements that areaccurate down to an angstrom. We are constantlylooking for lasers with higher powerin single modes and better stability prop -erties. Some of the new generation of diodelasers are now reaching the point where theycan be used for these ex peri ments, but for themost part they have not made it out of the labyet. It will be very interesting once they do.DefenceJeff Hecht is a freelancescience and technologywriter based in Auburndale,Massachusetts, US, whohas covered laser weaponssince 1980High-energy laser weapons – long the stuffof science fiction – have recently reached aturning point. But it is not the one you wouldexpect if you saw news clips of a laser-armedBoeing 747 shooting down a target missile inFebruary this year. Instead, the US military isplanning to concentrate on thwarting attacksfrom short-range targets such as rockets,mortars and artillery shells.Modern laser weaponry dates from about1980, when the main goal was to develophigh-energy lasers capable of destroyingmissiles launched hundreds or thousandsof kilometres away. Indeed, US PresidentRon ald Reagan’s “Star Wars” programmespent billions on plans for orbiting laser battlestations. But tough technology problemsPhysics World May 2010
physicsworld.comThe laser at 50: Visions of the futureand the end of the Cold War changed the re -quirements. The result was the Air borneLaser (ABL): a Boeing 747 equip ped with amegawatt chemical oxygen– iodine laser anddesigned to shoot down missiles launchedby a “rogue state”.But in May 2009, US defence secretaryRobert Gates reported that the ABL (longplagued by budget and deadline overruns)had a lethal range of less than 140 km – farshort of the planned minimum of 200 km. So,after the current round of tests (conductedat an undisclosed shorter range), efforts toreach megawatt powers will start again withlasers that use diode-pumped alkali-metalvapours. Lasers of this type currently emitjust tens of watts but may eventually offer abetter power-to-size ratio than the ABL.Until those plans get off the ground – ifthey ever do – the bold new future of laserweapons will be solid-state lasers that emit100 kW or more in a steady or repetitivelypulsed beam. It has already been demonstratedthat kilowatt-class lasers can detonateunexploded ordnance left on the battlefieldby illuminating it from a safe distance. Thehope is that lasers in the 100–400 kW rangecould also destroy rockets, mortars and shellsat distances of up to a few kilometres. Theclose range of these targets would greatlyease beam-propagation problems that hamperlaser-based missile-defence systems.More over, by detonating explosives in the airwith laser heating, rather than firing pro -jectiles at them, laser-based weapons couldreduce “collateral damage” to friendly soldiersand non-combatants.In March 2009 US defence giant NorthropGrumman reported continuous emission ofmore than 100 kW for five minutes from alaboratory diode-pumped laser. This Feb -ruary, Textron Systems reached the samegoal with its own design. These are by far thehighest continuous powers achieved in asolid-state laser. The next step will be toengineer a 100 kW laser that works on ships,trucks and planes. The US Army is movingNorthrop Grumman’s device to the HighEnergy Laser System Test Facility at theWhite Sands Missile Range, New Mexico,where it is planning to try out a mobile versioninstalled in a heavy battlefield truck. An -other defence agency, DARPA, is building alightweight 150 kW solid-state laser for usein fighter planes, while the US Navy is planningtests of similar lasers at sea.The lasers used in all these projects marka radical departure in laser-weapon design.Earlier weapon-class lasers were chemicallyfuelled, but commanders did not want themon the battlefield because handling chem -ical fuels posed major logistical issues. Theyalso wanted lasers that could be poweredby diesel generators. But other formidablechallenges remain, including damage to thelaser itself, the need to operate in a dirty battlefieldenvironment and the expected highcost of the devices.Physics World May 2010Honolulu Star BulletinBagging a few test rockets should be easy.Engineering mobile lasers that work reliablyin messy places where people are shootingat them is a much tougher problem. We willprobably see prototypes blasting targets outof the sky within a few years, but do not ex -pect battlefield deployment until the 2020sat the earliest.Free-electron lasersJohn Madey is director ofthe FEL Laboratory at theUniversity of Hawaii, US,and contributed to thedevelopment of freeelectronlasersLike all lasers, free-electron lasers (FELs)rely on the principle of stimulated emissionto amplify a beam of light as it passes througha region of space. In other words, as electronsmove from a high- to a low-energystate, they emit photons of light all at thesame wavelength and all moving in the samedirection. But unlike the transitions betweenbound electronic states in other lasers, FELsexploit another of Einstein’s key discoveries– special relativity – to provide tunable electromagneticradiation from a beam of relativisticfree electrons as they move througha spatially periodic transverse magnetic field.According to special relativity, the electronsperceive such a field as an intensetravelling wave in their rest frame, with awavelength reduced in proportion to theirkinetic energy. Photons scattered by the electronsfrom this pulse in the direction of theirmotion are reduced in wavelength once againwhen viewed from the laboratory frame. Asa result, electrons with a kinetic en ergy of50 MeV emit near-infrared radi ation whenmoving through a field with a period of 2 cm.Light of longer and shorter wavelengths canbe created by simply varying the energy ofthe electrons. FELs can readily provide laserlight with about 1% of the instantaneouspower of the electron beam – megawatts ormore – and their pulse lengths can vary fromless than a picosecond to full continuouswaveoperation. Ex cep tional phase coherenceis also attainable through the use ofsuitable interferometric resonator systems.Serious efforts to explore the possibleapplications of FELs began shortly aftercolleagues and I at Stanford University successfullydemonstrated the first opticalwavelengthFEL amplifiers and oscillatorsin 1974 and 1976, respectively. The focussince then has been on using FELs to dothings that are tricky to pull off by othermeans. Perhaps the best known applicationis to generate tunable, high peak power,coherent, femtosecond X-ray pulses at energiesabove 1 keV to carry out time-resolvedstructural and functional studies of complexindividual and interacting molecules. Thefirst such X-ray FEL is now in operation atthe SLAC National Accelerator Laboratoryin the US, with the European X-ray Free-Electron Laser due to come online at theDESY lab in Germany in 2014.Even for applications where other types oflasers might be adequate, the big advantageof FELs is that they are so flexible. FELshave therefore proved invaluable in carryingout exploratory research when the requirementsof a particular application have not yetbeen determined or when a research teamdoes not have the time or money to developa new specialized laser system needed to supportthe application. The short-pulse, highpeak-power,third-generation FELs, whichwere pioneered in the 1980s, have beenpartic ularly useful for developing new surgicaltechniques and for exploring the energylevels, band structure and mobility of electronsand holes in new electronic and opticalma terials, without having to worry aboutlonger probing laser pulses damaging thematerial.The more recently developed high-average-powerFEL systems have extended thesecapabilities to include research on possiblelaser applications for industrial-scale ma -terials processing. Of at least equal signi -ficance are the improvements in remotesensing for climate-change research madepossible by the broad tunability, high peakpower, and exceptional spatial and tem poralcoherence offered by FELs at visible andinfrared wavelengths.There are, however, a few clouds on thehorizon for FEL research. Historically, suchresearch has mainly taken place at a handfulof small and mid-sized university and governmentlabs in the US, Europe and Asia. Arecent transition to larger national labs hasbrought many scientific advances, but alsoruns the risk of making both the science andShort-pulse, high-peak-power,third-generation free-electron lasers havebeen particularly useful for developingnew surgical techniques and for exploringnew electronic and optical materialsJohn Madey55
Page 5 and 6: physicsworld.comQuantaFor the recor
Page 7 and 8: physicsworld.comFrontiersScience/AA
Page 9 and 10: physicsworld.comNews & AnalysisLHC
Page 11 and 12: physicsworld.comNews & AnalysisSpac
Page 13 and 14: physicsworld.comNews & AnalysisPoli
Page 15 and 16: physicsworld.comNews & Analysisacce
Page 17 and 18: physicsworld.comThe laser at 50Phys
Page 19 and 20: physicsworld.comThe laser at 50: A
Page 21 and 22: physicsworld.comThe laser at 50: A
Page 23 and 24: XRF Solutions• Solid State Design
Page 25 and 26: physicsworld.comThe laser at 50: Th
Page 29 and 30: physicsworld.comThe uncomfortable t
Page 31 and 32: physicsworld.comThe laser at 50: La
Page 37 and 38: Image credits (top to bottom): Amer
Page 39 and 40: physicsworld.comThe laser at 50: Bo
Page 41 and 42: physicsworld.comThe laser at 50: Bo
Page 43 and 44: ULTRAFAST LASERS FOR SCIENCE AND IN
Page 49 and 50: physicsworld.comThe laser at 50: At
Page 55: physicsworld.comThe laser at 50: Vi
Page 59 and 60: You rely on technology innovation f
Page 61 and 62: physicsworld.comCareersmonitoring s
Page 63 and 64: FEATURES OF THE MCA8000A• Success
Page 65 and 66: Oxford Instruments provides high te
Page 67 and 68: sWork in a stimulatinginternational
Page 69 and 70: Home News Blog Multimedia In depth
Page 71 and 72: The New HF2LILock-In AmplifierYour

special issue

Create successful ePaper yourself

Delete template?

Save as template?