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Lasers & Sources
Subattosecond electron pulses from multiterawatt laser beams
A new free-space laser electron acceleration scheme could help scientists reach subattosecond timescales.
28 November 2006, SPIE Newsroom. DOI: 10.1117/2.1200611.0489
Particle accelerators are common in industry, medicine, and the fundamental sciences. They are used in a broad range of applications, including controlled semiconductor doping, sterilization, radiotherapy for cancer treatment, and experimental particle physics. But present accelerator technology has reached its limits, and scientists and engineers are now working on developing new, compact accelerating devices. In this context, laser acceleration schemes have great potential. Here we describe one that could produce relativistic electron pulses as short as a fraction of an attosecond (1as=10-18s). Such ultrashort electron pulses would offer the possibility of probing atomic and subatomic phenomena on an unprecedented timescale.
Laser acceleration of electrons
Three main approaches are used for laser particle acceleration. The first is referred to as ‘slow-light’ acceleration. In this scheme, the presence of a specially designed structure (waveguides, diffraction gratings, and so on) ensures sustained interaction between light waves and particles. A second approach is plasma wake field acceleration, a technique that takes advantage of the strong space-charge waves produced by intense laser beams in plasma channels. The third approach is free-space acceleration, in which particles are accelerated by focused laser beams in vacuo. Although each type of acceleration has its own specialized range of applications, free-space schemes can potentially produce electron pulses of much shorter duration than that typically obtained with the other techniques. For the scheme we propose, the zeptosecond timescale (1zs=10-21s) could be reached with present-day laser technology.1
We showed previously that subcycle electron acceleration can be achieved in free space using transverse magnetic (TM) laser beams.2 More recently, we demonstrated that the use of a few-cycle laser beam and a compact initial electron cloud forces the particles to interact with a single half-cycle of the laser field and form a pulse of attosecond duration.3 Effectively, our simulations predict that pulse duration decreases below 1as when the absolute phase of the laser pulse is optimized.
Subattosecond relativistic electron pulses
The main problem with free-space acceleration schemes is to synchronize the displacement of laser beams to that of accelerated particles. According to the Lawson-Woodward theorem, synchronization is impossible at low intensity. However, at relativistic intensities, a substantial amount of energy can be transferred to free electrons due to nonlinear effects. In this regime, beam geometry is crucial. Our work showed that a special laser beam, called the TM01 beam,4 provides conditions favorable to the production of monoenergetic collimated ultrashort electron pulses.
In the ultrarelativistic regime, the electric field of a laser beam accelerates electrons to relativistic velocities on a subcycle scale. In the case of Gaussian (TEM00) laser beams, electrons are expelled from the beam waist into the transverse plane when the v × B term of the Lorentz force bends the trajectory forward. For TM01 beams, electrons are accelerated along the propagation axis by the longitudinal electric field component, while transverse field components (electric and magnetic) help maintain the particles close to the beam propagation axis.3 This allows for subcycle acceleration during several picoseconds: when the particles escape the beam waist, they remain confined within a single half-field cycle.2
The interaction of an electron cloud with a powerful and ultrafast TM01 laser beam has been simulated using the time-dependent 3D Maxwell-Lorentz equations.3 We assumed a low electron density, so as to neglect the electrostatic repulsion occurring between the electrons. When the electron cloud had a diameter much smaller than the laser wavelength and was initially at rest at the waist of a 100TW TM01 laser beam, our simulations revealed the formation of an ultrashort electron pulse. Depending on the value of the absolute phase of the driving laser pulse, this pulse had a peak energy between 10 and 50MeV with a 0.27–10% spread and a 0.3–2.8mrad divergence. With phase optimization, pulse duration was as short as 0.5as and remained, overall, below 90as.1
Laser particle acceleration schemes may revolutionize accelerator-based research and applications. We outlined the key concepts of a free-space scheme that takes advantage of the longitudinal component of the electric field of ultraintense transverse magnetic laser beams. This approach allows for the production of attosecond and subattosecond electron pulses with a peak energy in the multimillion electronvolt range. These ultrashort relativistic electron pulses offer the possibility to probe physical processes with an unmatched temporal resolution. Preliminary results indicate that billion electronvolt energy ranges and sub-100zs timescales could eventually be reached with multipetawatt (1PW=1015W) laser power. Although the proposed method can be implemented with presently available laser technology, it still requires experimental validation.
This work has been supported by Le fonds québécois de la recherche sur la nature et les technologies (Québec), the Natural Sciences and Engineering Research Council (Canada), and the Canadian Institute for Photonic Innovations.
Laser-Matter Interaction Theory Group, University of Ottawa
Ottawa, Ontario, Canada
Charles Varin received a PhD in physics from Université Laval, Québec, Canada, in 2006. Specialized in high-field physics and ultrafast phenomena, he is currently pursuing postdoctoral studies with Thomas Brabec at the University of Ottawa.
Québec, Qc, Canada
Michel Piché received his PhD from Université Laval in 1980, and is currently professor of physics at that institution. His research includes theoretical and experimental aspects of laser technology, nonlinear optics, short pulse generation, Bessel beams, and laser-matter interaction.