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physicsworld.comThe laser at 50: Attosecond lasers and beyondGrasping timescalesEleftherios Goulielmakisone optical cycle( ~ 2.5 femtoseconds)Thorsten Naeser, Max Planck Institute of Quantum OpticsAs laser performance improves, the quantities that define it sound evermore impressive, with each successive prefix more exotic than the last. Forpower, the numbers become increasingly large – gigawatt (10 9 W),terawatt (10 12 W) and petawatt (10 15 W). Pulses, however, are getting evershorter – picosecond (10 –12 s), femtosecond (10 –15 s) and attosecond(10 –18 s). It is these short pulses from ultrafast lasers that are now lettingus explore processes on these timescales in real time.But these quantities can remain abstract, as it is difficult to relate themto anything in our daily lives. One second is a quantity with palpablemeaning, as it is roughly the time between beats of the human heart and isan interval that we can easily perceive. A picosecond, in contrast, is thecharacteristic time taken for molecules to move back and forth. We areactually able to sense these motions without any fancy instruments, as theyare responsible for the temperature of the air and everyday objects.Getting shorter, the femtosecond regime is the characteristic timescaleon which chemical reactions take place, or the time required for bondsbetween atoms and molecules to be broken and formed. Observing this inreal time has been called the “holy grail” of chemistry, letting us studyeverything from the storage and release of energy in batteries to thefundamental process of photosynthesis.And finally to the timescale that is the most recent barrier broken bypulsed lasers: the attosecond. On the attosecond timescale, even themaking and breaking of chemical bonds appear to occur slowly, and wouldbe akin to watching a slow-motion nature video of shoots unfurling theirleaves. In this regime, we are able to resolve charge dynamics – themovement of electrons between the energy levels of an atom, or ofelectrons or holes (electron counterparts) through an insulating interfacesuch as a p–n junction in a transistor.Physics World May 2010the SLAC National Accelerator Laboratory in the USand the European XFEL currently under constructionin Germany.As attosecond laser systems improve, they will let usaccess ever more complex systems and dynamics. Inmost experiments, a system can be prepared, its dy -namics triggered and subsequently observed every fewmicroseconds on a fresh sample to build up measurementstatistics. Therefore, ideally, attosecond laserswould pulse much faster at repetition rates higher thanthose that are currently possible. How ever, increasingthe repetition rate of current attosecond laser systemsis impossible owing to the heating that this causes inconventional laser gain media.In the future, it will be possible to use nonlinear opticsto move beyond the limits of standard laser gain mediain attosecond lasers. In a process called parametricamplification, the nonlinear response of certain transparentcrystals can be used to couple energy from onefrequency of light to another. For femtosecond pulses,parametric amplification can be used to transfer lightfrom a single frequency into a broad band of other frequencies,as is the case in Ti:sapphire. How ever, in contrastto traditional laser systems, during parametricamplification light is not strongly absorbed, so heatingis no longer a problem, allowing higher pulse repetitionrates and average powers. The average power outputof optical parametric amplifier (OPA) systems could beincreased to kilowatt levels. Further more, it is possibleto achieve even higher gain bandwidths in parametricamplifiers that allow direct amplification of quasi-fewcycleoptical light pulses, which is not possible in today’schirped-pulse amplification systems.OPAs can also be used to create HHG drive pulseswith different carrier wavelengths. The maximum photonenergy that can result from the HHG processdepends on the amount of kinetic energy that the ionizedelectron can accumulate while being accelerated inthe carrier laser field. The longer the electron travelsin the electric field, the more kinetic energy it has timeto amass, and the greater the emitted photon energy.This interaction time can be increased by using longerwavelengthdrive pulses. By moving to longer wavelength,ultrashort drive pulses, the photon energy ofisolated attosecond XUV pulses can be increased, thusallowing the efficient generation of pulses deeper inthe XUV or even in the “soft” (or low energy) X-rayregime. This is critical, as carbon absorbs XUV radi -ation at 284 eV making it visible, while water remainstransparent, allowing researchers to probe deep insideorganic materials with attosecond resolution on atomiclength scales.Looking further ahead, the development of a techniquecalled “quasi-phase-matching” may lead to atto -second light sources producing hard X-ray photonenergies and orders of magnitude more photons perpulse, which would allow these sources to compete withlarge-scale facilities. It has been predicted that thebandwidth of emission will also increase substantially,allowing for the generation of sub-attosecond, that iszeptosecond (10 –21 s), pulses, at which point it wouldbe safe to say that we have gone beyond ultrafast. ■51