Attosecond Pulse Generation and Intense Ultrafast Laser Technology

The unique ability of X-rays for elemental and chemically-specific imaging of thick samples at the nano-angstrom scalehave inspired the development of X-ray free-electron laser sources, as well as ultrafast high harmonic (HHG) X-rays from tabletop-scale femtosecond lasers. The unprecedented attosecond pulse duration and full spatial coherence of the HHG light make it possible to capture the motions of electrons, atoms, and molecules at their natural space-time scales, to observeelement-specific dynamics at the M-shell absorption edges of magnetic materials, to understand heat flow in nanostructures, and to do 3D imaging with record spatial resolutions of 20 nmon a tabletop.

However, to date, most techniques employ HHG light have been limited to the EUV region of the spectrum <150 eV. Since matter is more transparent at higher photon energies, extending HHG to photon energies in the keV region would open up a hostof important applications in capturing dynamics at the magnetic, catalytic, and superconduction L-edges with attosecond temporal resolution, and in imaging with sub-nanometer scale spatial resolution in thick samples. The grand challenge for extending bright HHG into the keV region is the development of phase matching techniques that enable efficient nonlinear upconversion. Improved understanding of the microscopicquantum physics and macroscopic nonlinear optics of HHG, as well as the development of novel ultrafast mid-IR lasers havelead to rapid progress in the past few years, essentially solving the phase matching problem of HHG in the X-ray region. Our recent experiments have demonstrated that full phase matchingof HHG scales very strongly with wavelength of the driving laser [1], making it possible to obtain bright coherent emission that can support single-cycle X-ray pulses as short as 2.5-10 attoseconds in the soft X-ray region around 0.3 keV using 1.3 µm laser [1], 0.5 keV using 2 µm driver [2], and 1.6 keV, orharmonic order >5000th, using 3.9 μm OPCPA system [3,4].

Extrapolating this approach further by using longer-wavelength mid-IR lasers, bright ultrafast emission can extend even into the hard X-ray region with potential for sub-attosecond, zeptosecond, X-ray pulse durations, promising to realize the coherent, ultrafast version of the Roentgen X-ray tube.

[1] T. Popmintchev et al., Opt. Lett. 33, 2128 (2008); PNAS 106, 10516 (2009); Nature Photonics 4, 822-832 (2010).

[2] M.C. Chen et al., Phys. Rev. Lett. 105, 173901 (2010).

[3] T. Popmintchev et al., Science 336, 1287 (2012).

[4] Andriukaitis G et al., Opt. Let. 36 2755 (2011).