SCIENTIFIC LASERS FEATURE
Attosecond x-ray science could soon be within the grasp of not only the wider scientific community but industry as well
However, the shortest pulses available
were (and still are) from titanium-doped sapphire lasers emitting in the near-infrared (NIR), producing at-best few-femtosecond pulses. Attosecond pulses required shorter wavelength, higher energy laser light. The solution came when researchers began experimenting with high-order harmonic generation (HHG). HHG is a process where CPA produces an intense infrared laser pulse that is focused on a target, causing strong nonlinear interactions that lead to a tiny fraction of the laser power being converted to very high harmonics of the optical frequency of the pulse. With the later advent of carrier envelope phase stabilisation providing the ability to control the carrier envelope phase of an amplified laser pulse, in 2001, researchers generated a train of pulses with a temporal duration of 250 attoseconds – and, in the same year, the first single attosecond pulse with duration of 650 attoseconds. Attosecond science was born.
pivotal in allowing researchers to reach the attosecond scale. Introduced by Donna Strickland and Gérard Mourou (who received the 2018 Nobel Prize in Physics for their work), CPA stretches, amplifies and then compresses again ultrashort laser pulses to produce enormously high optical intensities. “What that means is that now you have the ability to use a laser field to kind of control or move around the electron in the way that you would like,” explains Chini.
Recent advances Since then, huge progress has been made. In isolating single attosecond pulses, several gating techniques have been developed to confine the HHG process to a single event. These developments led to shorter and shorter attosecond flashes, with the record tumbling from an 80 attoseconds pulse in 2008 to a 67 attosecond pulse generated by Chini and colleagues in 2012 and more recently 53 attoseconds in 2017. Later the same year, ETH Zurich researchers succeeded in shortening the pulse duration to only 43 attoseconds, representing the shortest controlled event that has ever been created by humans. In addition, researchers have explored laser
technology and other laser source spectral regions, such as the mid-infrared. “The mid-infrared is very good for attosecond science because the oscillation cycle is longer so you have more control over what the electron does,” explains Chini. “And that allows attosecond pulses to be generated
‘People in attosecond science will remain those pushing the frontiers of laser technology’
not just in the extreme ultraviolet, but now into the X-ray regime.” Only recently have fully coherent, soft X-ray attosecond pulses through HHG driven by MIR femtosecond laser sources become available. Other researchers have advanced
attosecond spectroscopy to investigate various physical processes of interest, mainly employing pump-probe spectroscopy first developed by Ahmed Zewail (1999 Nobel Prize in Chemistry) using longer femtosecond pulses. But using attosecond pulses for both the pump and probe has proven challenging, so most practical approaches have used only one attosecond pulse, pump or probe, with a femtosecond pulse employed for the other step. Just this year, an international team (Max Born Institute, Germany, University College London, UK, and ELI-ALPS in Hungary) managed to use an attosecond- pump attosecond-probe method to study nonlinear multiphoton ionisation of atoms. Yet more have focused on extending the applicability of attosecond techniques to different targets. “When attosecond science came out, it was all about gases, but now 50 per cent if not more of the research is done in solids,” explains Giulio Vampa at the National Research Council of Canada & University of Ottawa. “My first major contribution in 2011 was to discover that the physics that underlies high harmonic generation in gases is mirrored in solids, which meant we could pour all the technology developed for gases into solids to investigate what happens.” In a completely different direction, a
new way of generating attosecond pulses has emerged: X-ray free electron lasers (X-FELs). X-FELs accelerate electrons to very high energy, close to the speed of g
Most x-ray free electron laser facilities span several kilometres. The European XFEL X-ray laser is a 3.4-km-long facility which runs essentially underground near the city of Hamburg. Tau Systems hopes to achieve the same electron energy over a distance of about 10 centimetres
www.electrooptics.com | @electrooptics November 2022 Electro Optics 21
European XFEL / Luſtaufnahmen 2015/2017: FHH, Landesbetrieb Geoinf. und Vermessung
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