John T. Costello

Femtosecond x-ray pulse length characterization at the Linac Coherent Light Source free-electron laser

Two-color, single-shot time-of-flight electron spectroscopy of atomic neon was employed at the Linac Coherent Light Source (LCLS) to measure laser-assisted Auger decay in the x-ray regime. This x-ray-optical cross-correlation technique provides a straightforward, non-invasive and on-line means of determining the duration of femtosecond (>40?fs) x-ray pulses. In combination with a theoretical model of the process based on the soft-photon approximation, we were able to obtain the LCLS pulse duration and to extract a mean value of the temporal jitter between the optical pulses from a synchronized Ti-sapphire laser and x-ray pulses from the LCLS. We find that the experimentally determined values are systematically smaller than the length of the electron bunches. Nominal electron pulse durations of 175 and 75?fs, as provided by the LCLS control system, yield x-ray pulse shapes of 120?20?fs full-width at half-maximum (FWHM) and an upper limit of 40?20?fs FWHM, respectively. Simulations of the free-electron laser agree well with the experimental results.

Femtosecond all-optical synchronization of an X-ray free-electron laser

Many advanced applications of X-ray free-electron lasers require pulse durations and time resolutions of only a few femtoseconds. To generate these pulses and to apply them in time-resolved experiments, synchronization techniques that can simultaneously lock all independent components, including all accelerator modules and all external optical lasers, to better than the delivered free-electron laser pulse duration, are needed. Here we achieve all-optical synchronization at the soft X-ray free-electron laser FLASH and demonstrate facility-wide timing to better than 30 fs r.m.s. for 90 fs X-ray photon pulses. Crucially, our analysis indicates that the performance of this optical synchronization is limited primarily by the free-electron laser pulse duration, and should naturally scale to the sub-10 femtosecond level with shorter X-ray pulses.

Non-linear processes in the interaction of atoms and molecules with intense EUV and X-ray fields from SASE free electron lasers (FELs)

The advent of free electron laser (FEL) facilities capable of delivering high intensity pulses in the extreme-UV to X-ray spectral range has opened up a wide vista of opportunities to study and control light matter interactions in hitherto unexplored parameter regimes. In particular, current short wavelength FELs can uniquely drive non-linear processes mediated by inner shell electrons and in fields where the photon energy can be as high as 10 keV and so the corresponding optical period reaches below one attosecond. Combined with ultrafast optical lasers, or simply employing wavefront division, pump probe experiments can be performed with femtosecond time resolution. As single photon ionization of atoms and molecules is by now very well understood, they provide the ideal targets for early experiments by which not only FELs can be characterised and benchmarked but can also be the natural departure point in the hunt for non-linear behaviour of atomistic systems bathed in laser fields of ultrahigh photon energy. In this topical review we illustrate with specific examples the gamut of apposite experiments in atomic, molecular physics currently underway at the SCSS Test Accelerator (Japan), FLASH (Hamburg) and LCLS (Stanford).

Optimization of the Emission Characteristics of Laser-Produced Steel Plasmas in the Vacuum Ultraviolet: Significant Improvements in Carbon Detection Limits

A detailed optimization study for laser-produced steel plasmas using time-integrated, spatially resolved emission spectroscopy in the vacuum ultraviolet (VUV) (40–160 nm) is presented. The influences of the laser focusing lens type, laser power density, laser wavelength, laser pulse energy, ambient atmospheres, and pressure, as well as spatial distribution of emitting species, on the emission characteristics of the steel plasmas are investigated. The aim of the work is to improve the detection power of the technique for the quantitative determination of carbon in solid steel alloys. In most of the work, Q-switched Nd:YAG (1064 nm, 820 mJ max. energy) laser pulses were used to create the steel plasmas. For the laser harmonics investigations, a second Q-switched Nd:YAG laser system that generated radiation at the second, third, and fourth harmonics as well as at the fundamental was employed. Air, argon, and helium were used as the surrounding atmospheres, and the pressure was varied from 0.005 mbar to 5.0 mbar depending on the gas composition. A 1 m normal incidence vacuum spectrometer, equipped with a 1200 grooves/mm concave reflective grating, was used to disperse the VUV radiation. The radiation was detected by a back-illuminated, anti-reflection coated, charge-coupled device (CCD) array detector. In general, the emission characteristics of the VUV spectral lines studied are similar to those previously investigated in the UV-visible spectral range. An unprecedented limit of detection for carbon in steels of 1.2 ± 0.2 μg/g was measured in this work.

Single-shot characterization of independent femtosecond extreme ultraviolet free electron and infrared laser pulses

Two-color above threshold ionization of helium and xenon has been used to analyze the synchronization between individual pulses of the femtosecond extreme ultraviolet (XUV) free electron laser in Hamburg and an independent intense 120fs mode-locked Ti:sapphire laser. Characteristic sidebands appear in the photoelectron spectra when the two pulses overlap spatially and temporally. The cross-correlation curve points to a 250fs rms jitter between the two sources at the experiment. A more precise determination of the temporal fluctuation between the XUV and infrared pulses is obtained through the analysis of the single-shot sideband intensities.

Time-resolved pump-probe experiments beyond the jitter limitations at FLASH

Using a noninvasive, electro-optically based electron bunch arrival time measurement at FLASH (free electron laser in Hamburg) the temporal resolution of two-color pump-probe experiments has been significantly improved. The system determines the relative arrival time of the extended ultraviolet pulse of FLASH and an amplified Ti:sapphire femtosecond-laser pulse at the interaction region better than 90 fs rms. In a benchmarking pump-probe experiment using two-color above threshold ionization of noble gases, an enhancement in the timing resolution by a factor of 4 compared to the uncorrected data is obtained.

Spectroscopic characterization of vacuum ultraviolet free electron laser pulses

Because of the stochastic nature of self-amplified spontaneous emission (SASE), it is crucial to measure for single pulses the spectral characteristics of ultrashort pulses from the vacuum ultraviolet free electron laser (FLASH) at DESY, Germany. To meet this particular challenge, we have employed both photon and photoelectron spectroscopy. Each FEL pulse is composed of an intense and spectrally complex fundamental, centered at a photon energy of about 38.5 eV, with a bandwidth of 0.5% accompanied by higher harmonics, each carrying an intensity of typically 0.3 to 0.6% of that of the fundamental. The correlation between the harmonics and the fundamental is in remarkable agreement with a simple statistical model of SASE FEL radiation.

X-UV Absorption Spectroscopy with Laser-Produced Plasmas: A Review

Exteme-ultraviolet (X-UV) absorption spectroscopy has gained impetus in recent years, spurred on by a growing interest in the physics of core excited atomic, molecular and solid species. Progress in this field has been helped in no small measure by developments in X-UV light sources. One such source is the laser-produced plasma which, by suitable choice of target material, can be used to produce a clean, line free continuum virtually throughout the X-UV and V-UV spectral regions from 40-2000 ?. We briefly address here the issues surrounding the origin and characteristics of laser-plasma continuum light sources. In addition we review progress to date on the application of these sources to the study of the X-UV photoabsorption spectra of atoms, ions and molecules.

Study of a colliding laser-produced plasma by analysis of time- and space-resolved image spectra

The interaction of two counterpropagating laser-produced plasmas was studied using simultaneous imaging and spectroscopic techniques. Spectrally filtered time-gated intensified charge coupled device imaging was used to obtain information about the spatial dynamics and temporal evolution of the collision process, while time-resolved imaging spectroscopy was used to determine the spatial and temporal distributions of electron temperature and density within the interaction region. We examine specifically the interaction of plasmas whose parameters match those typically used in pulsed laser deposition of thin films. These low temperature plasmas are highly collisional leading to the creation of a pronounced stagnation layer in the interaction region.

Time-integrated laser-induced plasma spectroscopy in the vacuum ultraviolet for the quantitative elemental characterization of steel alloys

This paper demonstrates that time-integrated space-resolved laser-induced plasma spectroscopy (TISR-LIPS) is a useful technique in the vacuum ultraviolet (VUV) for the quantitative determination of the carbon content in steels. The standard reference samples used were carbon-iron alloys containing a relatively wide concentration range of carbon (0.041-1.32%). In the experiments the output of a Q-switched Nd:YAG (1064?nm) laser, with approximately a 1?J maximum output pulse energy and approximately a 12?ns temporal pulse width, was focused onto the surface of each sample (under vacuum) in order to produce the emitting plasma. A fore-slit mounted in the target chamber allowed spatially-resolved spectral measurements in the axial direction of the plasma and provided emission lines that were almost free of the background continuum. A 1?m normal incidence vacuum spectrometer, equipped with a 1200 grooves mm-1 concave grating and a micro-channel plate/photodiode array detector combination, was used as the detection system. A particularly interesting feature of this work is the demonstration that VUV spectroscopy allows ionic lines to be used and linear calibration curves were obtained for the five carbon spectral lines (from C+ and C2+) under investigation. The limits of detection for all lines were determined; the lowest detection limit (87?10?ppm) was obtained from the C2+ 97.70?nm line, which compares favourably with the only available value in the literature of 100?ppm.