Femtosecond synchrotron pulses were generated directly from an electron storage ring. An ultrashort laser pulse was used to modulate the energy of electrons within a 100-femtosecond slice of the stored 30-picosecond electron bunch. The energy-modulated electrons were spatially separated from the long bunch and used to generate approximately 300-femtosecond synchrotron pulses at a bend-magnet beamline, with a spectral range from infrared to x-ray wavelengths. The same technique can be used to generate approximately 100-femtosecond x-ray pulses of substantially higher flux and brightness with an undulator. Such synchrotron-based femtosecond x-ray sources offer the possibility of applying x-ray techniques on an ultrafast time scale to investigate structural dynamics in condensed matter.

Pulses of x-rays 300 femtoseconds in duration at a wavelength of 0.4 angstroms (30,000 electron volts) have been generated by 90° Thomson scattering between infrared terawatt laser pulses and highly relativistic electrons from an accelerator. In the right-angle scattering geometry, the duration of the x-ray burst is determined by the transit time of the laser pulse across the ∼90-micrometer waist of the focused electron beam. The x-rays are highly directed (∼0.6° divergence) and can be tuned in energy. This source of femtosecond x-rays will make it possible to combine x-ray techniques with ultrafast time resolution to investigate structural dynamics in condensed matter.

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Light-matter interactions are ubiquitous, and underpin a wide range of basic research fields and applied technologies. Although optical interactions have been intensively studied, their microscopic details are often poorly understood and have so far not been directly measurable. X-ray and optical wave mixing was proposed nearly half a century ago as an atomic-scale probe of optical interactions but has not yet been observed owing to a lack of sufficiently intense X-ray sources. Here we use an X-ray laser to demonstrate X-ray and optical sum-frequency generation. The underlying nonlinearity is a reciprocal-space probe of the optically induced charges and associated microscopic fields that arise in an illuminated material. To within the experimental errors, the measured efficiency is consistent with first-principles calculations of microscopic optical polarization in diamond. The ability to probe optical interactions on the atomic scale offers new opportunities in both basic and applied areas of science.Light-matter interactions have advanced our understanding of atoms, molecules and materials, and are also central to a number of areas of applied science. Although optical interactions have received a great deal of study, the microscopic details of how light manipulates matter are poorly understood in many circumstances. A material's optical response is complex, being determined by coupled many-body interactions that vary on the scale of atoms rather than on the scale of a long-wavelength applied field. Data are needed to combat this complexity, and so far it has not been possible to probe the microscopic details of light-matter interactions. X-ray and optical wave mixing, specifically sum-frequency generation (SFG), was proposed nearly half a century ago as an atomic-scale probe of light-matter interactions 1,2 . The process is, in essence, optically modulated X-ray diffraction: X-rays inelastically scatter from optically induced charge oscillations and probe optically polarized charge in direct analogy to how standard X-ray diffraction probes ground-state charge. Furthermore, the optically induced microscopic field is determined because it is closely related to the induced charge [3][4][5][6] . So far it has not been possible to measure these two quantities directly. X-ray and optical wave mixing has frequently been discussed 1,2,4,[7][8][9][10][11][12] , but it has not yet been demonstrated owing to a lack of sufficiently intense X-ray sources. More generally, although there have been theoretical studies of nonlinear X-ray scattering [13][14][15][16][17][18] , experimental observations have largely been confined to the spontaneous processes of X-ray parametric down-conversion [19][20][21][22][23] and resonant inelastic X-ray scattering 24,25 . X-ray free-electron lasers offer unprecedented brightness and new scientific opportunities 26 . Here we use an X-ray laser to demonstrate X-ray/optical SFG through the nonlinear interaction of the two fields in single-crystal diamond. Optically modulated X-ray diffract...

We report on the first demonstration of femtosecond x-ray absorption spectroscopy, made uniquely possible by the use of broadly tunable bending-magnet radiation from "laser-sliced" electron bunches within a synchrotron storage ring. We measure the femtosecond electronic rearrangements that occur during the photoinduced insulator-metal phase transition in VO2. Symmetry- and element-specific x-ray absorption from V2p and O1s core levels (near 500 eV) separately measures the filling dynamics of differently hybridized V3d-O2p electronic bands near the Fermi level.

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We present a setup which allows the measurement of time-resolved x-ray absorption spectra with picosecond temporal resolution on liquid samples at the Advanced Light Source at Lawrence Berkeley National Laboratories. The temporal resolution is limited by the pulse width of the synchrotron source. We characterize the different sources of noise that limit the experiment and present a single-pulse detection scheme.

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