Lukas Gallmann, ETH Zürich
Ultrashort laser pulses of femtosecond duration offer very high peak power, intensity and electric field strengths by concentrating moderate amounts of energy into very short bursts of light. At the same time it is precisely this property that severely limited progress of this field for a long time. The reason for this is that as peak field strengths get high enough to break chemical bonds in materials, damage of optical components and amplifier crystals occurs. Even before this regime, the beams of ultrashort laser pulses experience unwanted distortions in space and time due to the onset of optical nonlinearities.
Chirped-pulse amplification (CPA, ) invented by Donna Strickland and Gérard Mourou and recognized by this year’s Nobel Prize in Physics elegantly overcomes this technical limitation and enabled an increase in peak power and intensity by many orders of magnitude compared to prior approaches. A high-intensity laser system consists of a low-energy (~nJ) seed oscillator followed by one or more power amplification stages. The principle of CPA is to stretch the duration of the seed pulses by many orders of magnitude (i.e., from femtoseconds to nanoseconds) before they’re amplified to high energy (typically mJ to J). As a result of their increased duration, peak power and intensity remains below the threshold of nonlinearities or damage even after amplification. In a final step, the pulses are recompressed to their original femtosecond duration. The final recompression can happen in vacuum, which means that not even the optical nonlinearity or laser-induced ionization of air becomes a limitation.
Modern state-of-the-art laser systems, such as the ones presently being constructed for the European Extreme Light Infrastructure (ELI, ), can reach a peak power of 10 PW and focused intensities of 1023 W/cm2. At such high intensities, the motion of free electrons in vacuum that are accelerated by the electric field underlying the optical pulse becomes highly relativistic. The high fields enable the acceleration of protons to tens of MeV and electrons to GeV energies on micrometer to millimeter length scales. The energy density in the focused beam becomes so high than one expects to observe electron-positron pair production with the latest generation of lasers systems.
Aside from these extreme regimes, CPA enabled the design of comparably compact high-intensity laser systems that gave rise to a vast range of applications. These sources laid the technical foundation to new research fields such as attosecond science, as well as commercial applications such as in refractive eye surgery for vision correction.
This year’s Nobel Prize in Physics marks the third Nobel Prize awarded for achievements related to ultrashort laser pulses following the 1999 Nobel Prize in Chemistry (A. Zewail, for the application of ultrafast lasers for studying chemical dynamics) and the 2005 Nobel Prize in Physics (J. Hall and T. W. Hänsch, for the use of ultrafast lasers in precision frequency metrology). While the previous two prizes recognized the foundation of specific fields of research enabled by the availability of ultrashort laser pulses, the 2018 prize is different in that it was awarded for a specific technological development that gave rise to a broad range of applications from the commercial domain to fundamental science. CPA is now an indispensable ingredient of intense sources of femtosecond optical pulses and is expected to create more exciting research opportunities by yielding access to new regimes of fundamental laser-matter interaction for the foreseeable future.
 D. Strickland and G. Mourou, “Compression of amplified chirped optical pulses,” Opt. Commun. 56(3), 219 (1985)
[Published: October 2018]