J. Frisch

Mega-electron-volt ultrafast electron diffraction at SLAC National Accelerator Laboratory

Ultrafast electron probes are powerful tools, complementary to x-ray free-electron lasers, used to study structural dynamics in material, chemical, and biological sciences. High brightness, relativistic electron beams with femtosecond pulse duration can resolve details of the dynamic processes on atomic time and length scales. SLAC National Accelerator Laboratory recently launched the Ultrafast Electron Diffraction (UED) and microscopy Initiative aiming at developing the next generation ultrafast electron scattering instruments. As the first stage of the Initiative, a mega-electron-volt (MeV) UED system has been constructed and commissioned to serve ultrafast science experiments and instrumentation development. The system operates at 120-Hz repetition rate with outstanding performance. In this paper, we report on the SLAC MeV UED system and its performance, including the reciprocal space resolution, temporal resolution, and machine stability.

Few-femtosecond time-resolved measurements of X-ray free-electron lasers

X-ray free-electron lasers, with pulse durations ranging from a few to several hundred femtoseconds, are uniquely suited for studying atomic, molecular, chemical and biological systems. Characterizing the temporal profiles of these femtosecond X-ray pulses that vary from shot to shot is not only challenging but also important for data interpretation. Here we report the time-resolved measurements of X-ray free-electron lasers by using an X-band radiofrequency transverse deflector at the Linac Coherent Light Source. We demonstrate this method to be a simple, non-invasive technique with a large dynamic range for single-shot electron and X-ray temporal characterization. A resolution of less than 1 fs root mean square has been achieved for soft X-ray pulses. The lasing evolution along the undulator has been studied with the electron trapping being observed as the X-ray peak power approaches 100 GW.

Direct observation and imaging of a spin-wave soliton with p−like symmetry

Spin waves, the collective excitations of spins, can emerge as nonlinear solitons at the nanoscale when excited by an electrical current from a nanocontact. These solitons are expected to have essentially cylindrical symmetry (that is, s-like), but no direct experimental observation exists to confirm this picture. Using a high-sensitivity time-resolved magnetic X-ray microscopy with 50u2009ps temporal resolution and 35u2009nm spatial resolution, we are able to create a real-space spin-wave movie and observe the emergence of a localized soliton with a nodal line, that is, with p-like symmetry. Micromagnetic simulations explain the measurements and reveal that the symmetry of the soliton can be controlled by magnetic fields. Our results broaden the understanding of spin-wave dynamics at the nanoscale, with implications for the design of magnetic nanodevices.

Microwave soft x-ray microscopy for nanoscale magnetization dynamics in the 5–10 GHz frequency range

We present a scanning transmission x-ray microscopy setup combined with a novel microwave synchronization scheme for studying high frequency magnetization dynamics at synchrotron light sources. The sensitivity necessary to detect small changes in the magnetization on short time scales and nanometer spatial dimensions is achieved by combining the excitation mechanism with single photon counting electronics that is locked to the synchrotron operation frequency. Our instrument is capable of creating direct images of dynamical phenomena in the 5-10 GHz range, with high spatial resolution. When used together with circularly polarized x-rays, the above capabilities can be combined to study magnetic phenomena at microwave frequencies, such as ferromagnetic resonance (FMR) and spin waves. We demonstrate the capabilities of our technique by presenting phase resolved images of a ∼6 GHz nanoscale spin wave generated by a spin torque oscillator, as well as the uniform ferromagnetic precession with ∼0.1° amplitude at ∼9 GHz in a micrometer-sized cobalt strip.

Remote two-color optical-to-optical synchronization between two passively mode-locked lasers.

Using balanced detection in both the radio frequency (RF) and the optical domain, we remotely synchronize the repetition rate of a Ti:sapphire oscillator to an Er-doped fiber oscillator through a 360 m length-stabilized dispersion compensated fiber link. The drift between these two optical oscillators is 3.3 fs root mean square (rms) over 24 hours. The 68 MHz Er-doped fiber oscillator is locked to a 476 MHz local RF reference clock, and serves as a master clock to distribute 10 fs-level timing signals through stabilized fiber links. This steady remote two-color optical-to-optical synchronization is an important step toward an integrated femtosecond fiber timing distribution system for free-electron lasers (FELs); it does not require x-ray pulses, and it makes sub-10-fs optical/x-ray pump-probe experiments feasible.

Ultrafast X-ray-pump, laser-probe spectroscopy at LCLS

We report the first pump-probe spectra using 1 keV pulses from LCLS to excite N2 in delayed coincidence with 800 nm laser pulses. The delay between pump and probe was controlled to within 50 fsec.

Optimizing ultrashort laser pulse compression by two photon absorption

Demonstrated is an approach for relative optimization of ultrashort pulses using two-photon generated photocurrent in a GaAsP photodiode. Two-photon absorption is a nonlinear process, allowing for highly sensitive tuning of ultrashort laser systems.

Time resolved magnetic imaging at 10Ghz and beyond

Summary form only given. Understanding magnetic properties at ultrafast timescales is crucial for the development of new generations of magnetic devices. Such devices will employ the spin torque or spin Hall effect, whose manifestation at the nanoscale is not yet sufficiently understood, which is why studies addressing these effects are of great fundamental significance as well. The samples of interest are often thin film magnetic multilayers with thicknesses in the range of a atomic layers. This fact alone presents a sensitivity challenge in STXM microscopy, which is more suited toward studying thicker samples. In addition the relevant time scale is of the order of 10 ps, which is well below the typical x-ray pulse length of 50-100 ps. Altogether this means that pushing the time resolution of a synchrotron x-ray microscopy experiment is synonymous with improving the signal to noise ratio on the detector and providing stable, low jitter excitation to not further dilute the already small magnetic signals. To achieve the required stability and sensitivity the SSRL STXM is equipped with a single photon counting electronics that effectively allows us to use a double lock-in detection at 476MHz (the x-ray pulse frequency) and 1.28MHz (the synchrotron revelation frequency). The pulsed or continuous sample excitation source is synchronized with the synchrotron source with a few picosecond drift over 24 hours (see figure). This setup currently allows us to achieve a signal to noise ratio of better than 10000, enabling us to detect miniscule variations of the x-ray absorption cross section. In this talk I will describe the time resolved STXM setup developed at SSRL and present firsts results that have been obtained using the instrument in collaboration with an outstanding group of external users. The instrument operates in ultra high vacuum (~10-8torr) and allows us to apply electrical pulses to our samples that can be placed in out of plane magnetic fields up to 0.8 Tesla or in plane magnetic fields up to 0.3 Tesla. We have used the instrument to successfully image spin waves excited in spin-torque and spin Hall oscillators with nano contacts of the size of ~100nm. We also succeeded in imaging different excitation modes of magnetic samples in ferromagnetic resonance at 9.6GHz excitation frequency, where the opening angle of the precession cone is of the order of 10mrad. The facility that is dedicated to ultrafast studies of materials under electric and magnetic fields is open to general users who are interested in this field.

Femtosecond fiber timing distribution system for the Linac Coherent Light Source

We present the design and progress towards implementation of a femtosecond fiber timing distribution system for the Linac Coherent Light Source at SLAC enabling machine diagnostic at the 10 fs level.

Compton scattering techniques for the measurement of the transverse beam size of particle beams at future linear colliders

21st Beam Dynamics Workshop on Laser-Beam Interactions, State University of New York, Stony Brook, NY, USA, 11 Jun 2001 - 15 Jun 2001; 6 pp. (2001).