Time-to-space mapping in a gas medium for the temporal characterization of vacuum-ultraviolet pulses

Abstract

The authors introduce a method for cross correlating vacuum-ultraviolet with near-infrared femtosecond light pulses in a perpendicular geometry. Photoelectrons generated in an atomic gas by laser-assisted photoionization are used to create a two-dimensional image of the cross-correlation volume, thereby mapping time onto a space coordinate. Thus, information about pulse duration and relative timing between the pulses can be obtained without the need to scan an optical delay line. First tests using vacuum-ultraviolet pulses from the free-electron laser at the Deutsches Elektronen Synchrotron set an upper limit for their temporal jitter with respect to external optical laser pulses. © 2007 American Institute of Physics. ͓DOI: 10.1063/1.2714999͔For measurement of the temporal properties of optical pulses in the visible, near-ultraviolet, and near-infrared ͑NIR͒ range the nonlinear response of an optical medium-usually a crystal-on two optical fields of different or the same color is often used to realize cross-or autocorrelation schemes. 1 The recent advent of sources of femtosecond-and even attosecond 2 -pulses of ionizing radiation, based on laser plasmas, 3 high harmonic generation, 4 and free-electron lasing calls for a transfer of the correlation principle to the vacuum-ultraviolet ͑vuv͒ and the x-ray range. Since, however, crystals are no longer transparent in the vuv and do not exhibit sufficient nonlinearity in the x-ray range, alternative approaches for pulse characterization are necessary. Freeelectron lasers ͑FELs͒ are the most powerful femtosecond laser sources at short wavelengths, currently operating in the vuv range ͓e.g., free-electron laser Hamburg ͑FLASH͔͒ 5 and down to x-rays in the future. 6,7 So far, the utilized mode of operation for short-wavelength FELs relies on the process of self-amplified spontaneous emission ͑SASE͒. 8 High photon energies combined with greatly enhanced pulse intensities as compared to other sources make FELs a promising source for time-resolved visible/vuv pump-probe studies. FELs, however, are based on linear accelerators and undulators, which together measure from a few hundred meters up to a few kilometers in length. This makes these instruments susceptible to path length variations on a micrometer scale. Moreover, the statistical nature of SASE comes with a frequently varying spectral and temporal profile for each individual shot. 9 As a result, the arrival time of the FEL pulses at the experiment fluctuates in the order of a few hundred femtoseconds. This jitter inhibits an exact synchronization to an external laser source needed for reliable pump-probe studies. Therefore, an adequate characterization method is needed to determine the pulse duration together with the actual delay with respect to an external laser on a shot-to-shot basis. State-of-the-art x-ray streak cameras have reached subpicosecond temporal resolution, 10 but the photocathode completely dissipates the beam. In addition, their readout rate is limited to a few hertz, while FLASH is operated at a...

A femtosecond X-ray/optical cross-correlator

“…So far, approaches like laser induced side band generation and streaking in vacuum ultraviolet photoemission from noble gases have been put forward to measure temporal overlap 32,27 . This can only be done using dedicated electron spectrometers under high vacuum conditions 27 and is limited by space charge effects.…”

“…This can only be done using dedicated electron spectrometers under high vacuum conditions 27 and is limited by space charge effects. Second harmonic and sum frequency generation, routinely used to correlate optical pulses from different lasers 4 , suffer -when applied to X -rays -from low cross sections.…”

LCLS Ultrafast Science Instruments:Conceptual Design Report

“…However, a hard X-ray/optical laser cross-correlation technique with sufficient temporal resolution and signal-to-noise ratio appropriate for single shot operation has yet to be realized. Technical advancements are being made in the XUV spectral regime at the FLASH facility but it is unclear if these techniques can be extended into the hard X-ray regime [17], [18]. A robust timing diagnostic was developed at the Sub-Picosecond Pulse Facility (SPPS) facility that measured the relative timing of a femtosecond optical laser with respect to the SLAC electron bunches using electro-optic sampling [19].…”

X-ray–optical cross-correlator for gas-phase experiments at the Linac Coherent Light Source free-electron laser

“…X-ray-optical cross-correlator for gas-phase experiments at the Linac Coherent Light Source free-electron laser S. Schorb, 1 T. Gorkhover, 2 J. P. Cryan, 1 J. M. Glownia, 1 M. R. Bionta, 1 R. N. Coffee, 1 B. Erk, 3,4 R. Boll, 3,4 C. Schmidt, 3,4 D. Rolles, 3,5 A. Rudenko, 3,4 A. Rouzee, 6 X-ray-optical pump-probe experiments at the Linac Coherent Light Source (LCLS) have so far been limited to a time resolution of 280 fs fwhm due to timing jitter between the accelerator-based free-electron laser (FEL) and optical lasers. We have implemented a single-shot cross-correlator for femtosecond x-ray and infrared pulses.…”

“…4 First experiments have utilized sidebands from two-photon above threshold ionization of atomic targets to characterize the temporal jitter between the FEL and optical pulses. 5,6 For the hard x-ray regime strong field ionization of atomic targets opening resonant absorption channels has been proposed as x-ray-optical cross correlation technique. 7 Alternative approaches to twocolor ionization of atomic targets are to measure ultrafast x-ray induced changes in surface reflectivity 8 or transient changes in the optical transmission of a membrane via spectral encoding.…”