Attosecond Light and Science at the Time-scale of the Electron - Coherent XRays from Tabletop Ultrafast Lasers
Margaret M. Murnane and Henry C. Kapteyn
JILA, University of Colorado at Boulder, Boulder, CO 80309-0440
Ph. (303) 210-0396; FAX: (303) 492-5235; E-mail: murnane@jila.colorado.edu
Abstract
Ever since the invention of the laser 50 years ago and its application in nonlinear optics,
scientists have been striving to extend coherent laser beams into the x-ray region of the spectrum.
Very recently however, the prospects for tabletop coherent sources at very short wavelengths,
even in the hard x-ray region of the spectrum at wavelengths < 1nm, have brightened
considerably. This advance is possible by taking nonlinear optics techniques to an extreme—
physics that is the direct result of a new ability to manipulate electrons on the fastest, attosecond,
time-scales of our natural world.
Why are these new sources of coherent x-ray beams so exciting? Because making a laser
work in the hard x-ray region of the spectrum is very difficult. The basic physics of the laser
process dictates that the energy required to implement a laser scales roughly as 1 4 i.e a laser

at 10x shorter wavelength requires 10,000x the input power. Thus, the first soft x-ray lasers
implemented in the 1980’s used the building-size NOVA fusion laser at Lawrence Livermore as
a power source (at the time the largest laser in the world). Coherent light can be generated at

short wavelengths using synchrotron sources, or by using the new free-electron
lasers such as the
Linac Coherent Light Source (LCLS) that cost ≈ $0.5B. Again these are very large-scale and
expensive light sources, of which only a few will ever be built. This is unfortunate because a
large part of the impact of the laser has been as a practical device to use in applications such as
CD players and fiber optic communications. However, given the energy requirements, smallscale coherent x-ray sources would seem to be more a topic of science fiction than reality.
Or are they? Surprisingly, during the past few years, an alternative approach for
generating coherent x-rays has made rapid progress. For the first time, there is a practical way to
implement efficient tabletop coherent x-ray sources. The scheme can be thought of as a coherent
version of the x-ray tube. In an x-ray tube, an electron is boiled off a filament, then is accelerated
in an electric field before hitting a solid target, where the kinetic energy of the electron is
converted into incoherent x-rays. These incoherent x-rays are much like the incoherent light from
a light bulb or flashlight. To generate coherent laser-like beams of x-rays, we use an extreme
version of nonlinear optics (called high harmonic generation) to coherently combine many laser
photons together, thus generating coherent x-ray beams from a visible laser. Instead of boiling an
electron from a filament, we pluck part of the quantum wavefunction of an electron from an
atom using the electric field of a very intense laser pulse. The electron is then accelerated by this
laser field, first moving away from the atom and then returning to recombine with the atom and
liberating its kinetic energy as an x-ray photon. Since the laser field controls the motion of the
electron, the x-rays emitted can retain the coherence properties of the driving laser.
My talk will cover a new breakthrough discussed in detail in the following paper, that
describes how laser-like beams of x-rays at 0.5 keV photon energies can be generated efficiently
from a tabletop femtosecond laser. (This new work has just been submitted for publication). By
using a mid-infrared laser to drive the high harmonic generation process, the x-rays liberated by
many atoms can add together constructively, thus generating a bright laser-like beam. This work
solves a grand challenge in laser science, because it overcomes a roadblock that prevented this
upconversion process from working efficiently.
Several applications have already been demonstrated in the soft-x-ray region of the
spectrum at wavelengths > 10 nm, including making a movie of how the electron cloud in a
chemical bond changes shape as a molecule breaks apart, following how fast a magnetic material
can flip orientation, understanding how fast heat flows in a nanocircuit, or building a microscope
without lenses.
This advance also leads to an obvious question—just how far can we go? Is it possible to
generate bright beams of coherent hard x-rays, which could revolutionize crystallography,
biological, materials and medical imaging? The answer is yes in theory - efficient conversion to
very short wavelengths < 1 nm (corresponding to very high > 10keV photon energies) is
possible, guaranteeing more exciting advances in extreme nonlinear optics.
In another piece of news, Henry Kapteyn (my husband) and I were fortunate to learn last
week that we will share the American Physical Society 2010 Arthur L Schawlow Prize in Laser
Science, “For pioneering work in the area of ultra-fast laser science, including development of
ultra-fast optical and coherent soft x-ray sources.” The Schawlow Prize was endowed in 1991 by
the NEC Corporation "To recognize outstanding contributions to basic research which uses
lasers to advance our knowledge of the fundamental physical properties of materials and their
interaction with light." http://www.aps.org/programs/honors/prizes/schawlow.cfm
Please contact me at the above address should you need any of the following references.
The paper that follows discusses the exciting breakthrough that we have made in detail.
H. Kapteyn et al., “Attosecond Nonlinear Optics: Coherent x-rays from lasers”, Laser Focus World, p 89 (May
2007).
2. T. Popmintchev et al., “Phase matched upconversion of coherent ultrafast laser light into the soft and hard x-ray
regions of the spectrum”, Proceedings of the National Academies of the US 106, 10516 (2009).
3. Margaret M. Murnane and John Miao, “Ultrafast X-Ray Photography”, Nature 460, 1088 (2009).
4. Henry C. Kapteyn, Oren Cohen, Ivan Christov, Margaret M. Murnane, “Harnessing Attosecond Science in the
Quest for Coherent X-Rays,” Science 317, 775 (2007).
5. Henry C. Kapteyn, Margaret M. Murnane and Ivan P. Christov, “Coherent X-Rays from Lasers: Applied
Attosecond Science”, invited article, Physics Today, page 39 (March 2005).
6. C. La-O-Vorakiat et al., “Ultrafast Soft X-Ray Magneto-Optics at the M-edge Using a Tabletop High-Harmonic
Source”, Physical Review Letters 103, 257402 (2009).
7. M. Siemens et al. “Measurement of quasi-ballistic heat transport across nanoscale interfaces using ultrafast
coherent soft x-ray beams”, Nature Materials 9, 26 (2010).
8. K. Raines et al., “Three-dimensional structure determination from a single view,” Nature 463, 214 (2010).
9. M.C. Chen et al., “Bright, Coherent, Ultrafast Soft X-Ray Harmonics Spanning the Water Window from a
Tabletop Source,” submitted (2010).
10. W. Li et al., “Time-resolved Probing of Dynamics in Polyatomic Molecules using High Harmonic Generation”,
Science 322, 1207 (2008) (featured on cover); “Direct Observation of the Transition from Molecules to Atoms”,
submitted (2010).
11. http://jila.colorado.edu/kmgroup
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