High-harmonic generation (HHG) traditionally combines ~100 near-infrared laser photons to generate bright, phase-matched, extreme ultraviolet beams when the emission from many atoms adds constructively. Here, we show that by guiding a mid-infrared femtosecond laser in a high-pressure gas, ultrahigh harmonics can be generated, up to orders greater than 5000, that emerge as a bright supercontinuum that spans the entire electromagnetic spectrum from the ultraviolet to more than 1.6 kilo-electron volts, allowing, in principle, the generation of pulses as short as 2.5 attoseconds. The multiatmosphere gas pressures required for bright, phase-matched emission also support laser beam self-confinement, further enhancing the x-ray yield. Finally, the x-ray beam exhibits high spatial coherence, even though at high gas density the recolliding electrons responsible for HHG encounter other atoms during the emission process.

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Photosynthesis begins with light harvesting, where specialized pigmentprotein complexes transform sunlight into electronic excitations delivered to reaction centres to initiate charge separation. There is evidence that quantum coherence between electronic excited states plays a role in energy transfer. In this review, we discuss how quantum coherence manifests in photosynthetic light harvesting and its implications. We begin by examining the concept of an exciton, an excited electronic state delocalized over several spatially separated molecules, which is the most widely available signature of quantum coherence in light harvesting. We then discuss recent results concerning the possibility that quantum coherence between electronically excited states of donors and acceptors may give rise to a quantum coherent evolution of excitations, modifying the traditional incoherent picture of energy transfer. Key to this (partially) coherent energy transfer appears to be the structure of the environment, in particular the participation of non-equilibrium vibrational modes. We discuss the open questions and controversies regarding quantum coherent energy transfer and how these can be addressed using new experimental techniques.

We demonstrate fully phase-matched high harmonic emission spanning the water window spectral region important for nano- and bioimaging and a breadth of materials and molecular dynamics studies. We also generate the broadest bright coherent bandwidth (≈300  eV) to date from any light source, small or large, that is consistent with a single subfemtosecond burst. The harmonic photon flux at 0.5 keV is 10³ higher than demonstrated previously. This work extends bright, spatially coherent, attosecond pulses into the soft x-ray region for the first time.

Using a simple model of strong-field ionization of atoms that generalizes the well-known 3-step model from 1D to 3D, we show that the experimental photoelectron angular distributions resulting from laser ionization of xenon and argon display prominent structures that correspond to electrons that pass by their parent ion more than once before strongly scattering. The shape of these structures can be associated with the specific number of times the electron is driven past its parent ion in the laser field before scattering. Furthermore, a careful analysis of the cutoff energy of the structures allows us to experimentally measure the distance between the electron and ion at the moment of tunnel ionization. This work provides new physical insight into how atoms ionize in strong laser fields and has implications for further efforts to extract atomic and molecular dynamics from strong-field physics. DOI: 10.1103/PhysRevLett.109.073004 PACS numbers: 32.80.Fb, 32.80.Rm, 34.80.Qb When an atom or molecule is illuminated with a moderately intense femtosecond laser field ($ 10 14 W=cm 2 ), an electron wave packet will tunnel ionize and accelerate in the field before being turned around by the field and returning to the parent ion. The returning electron can either recombine with the parent ion, releasing its kinetic energy as a high-energy photon [1-3], or can elastically scatter from the potential of the ion. The photons and electrons generated by these strong-field processes have the potential to probe the dynamic structure of molecules and materials on the subnanometer length scale and femtosecond-to-attosecond time scale. Several recent papers have suggested that structures seen in angledependent photoelectron spectra may be useful for determining time-resolved molecular structures [4], characterizing attosecond electron wave packets [5], and studying the dynamics of electron wave packet propagation [6]. However, despite extensive analyses [7][8][9][10][11], many features observed in angle-resolved photoelectron spectra still lack a simple physical explanation.The recent development of midinfrared (mid-IR) femtosecond lasers [12] and angle-resolved detection schemes [13] has enabled new advances in visualizing strong-field physics. Electrons that are ionized in a mid-IR laser field reach higher velocities because of the larger ponderomotive energy, given by U P / I 2 , where I is the intensity and is the wavelength. The possibility of harnessing the high-energy electrons that are first ionized and then driven back to a molecule by a strong laser field has inspired several theoretical and experimental efforts to use strongfield ionization to probe molecular structure [4,[14][15][16]. Recently, Huismans and co-workers [17] used 7 m mid-IR lasers, in combination with angle-resolved detection, to observe angular interference structures in the photoelectron spectra. They presented a theoretical model that explains these structures based on the difference in the phase between two different paths that electrons can take to...