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 1.