The Photon Arrival and Length Monitor (PALM), a THz streak camera device developed by PSI for non-destructive hard x-ray measurements of photon pulse length and arrival time versus a pump laser, was brought to the SACLA XFEL in Japan in a cross-calibration temporal diagnostics campaign after an initial experiment where only the PALM was being used. The device was used with 9 keV pink beam and a 9.0 and 8.8 keV two-color mode, successfully measuring the temporal ifnromation of the pulses for several different FEL operating conditions. The most interesting achievement is the PALM’s ability to measure two arrival times of the two colorors as tey are shifted against each other by the FEL, opening up new possibilities in temporal accuracy for two-color experiments. SwissFEL will employ two such devices at the end stations for use by both operators and experimenters to improve the operation of the FEL and to better interpret experimental data.
P. N. Juranić et. al, Journal of Instrumentation (2014) 9.
T. Ishikawa et. al., Nature Photonics (2012) 6(8).
 P. N. Juranić et. al., Optics Express (2014) 22.
SwissFEL is aiming to produce X-ray pulses from 30 fs down to the attosecond time scale. This requires the compression of the several picosecond long electron bunches produced by a photo-injector to sub-fs level. To achieve this, 40fs accurate injection of the electron bunches into the main linear accelerator is necessary. Therefore high timing accuracy is required from the drive laser of the electron gun. Furthermore fs scan capability is foreseen for the experimental stations of the FEL. The ultra-short pulse pump-probe lasers therefore need to exhibit outstanding, below 10fs short term jitter relative to the X-rays. Timing tools for both the electron gun laser and for the experiments are developed. The former is based on electro-optical modulation of the optical reference at 1560nm by a signal produced from the gun laser at 260nm, a concept similar to beam arrival monitors in the linear accelerator, with an expected resolution below 20fs. The latter will use spectrally resolved cross-correlation technique to determine relative jitter between the optical reference and the laser used at the experiments at 800nm, with fs resolution. These systems will be complemented by electron and X-ray timing tools. In this paper we present the general concept for the laser arrival time measurement and correction, with first results obtained on a Ti:sapphire chirped pulse amplifier system. Shot to shot, short term jitter and long term timing drift measurements are presented, with discussion on the sources of the noise. Plans for the feedback stabilization and the resolution and limitation of the systems are also covered.
SwissFEL is the Free Electron Laser (FEL) facility under construction at the Paul Scherrer institute (PSI), aiming to provide users with X-ray pulses of lengths down to 2 femtoseconds at standard operation. The measurement of the length of the FEL pulses and their arrival time relative to the experimental laser is crucial for the pump-probe experiments carried out in such facilities. This work presents a new device that measures hard X-ray FEL pulses based on the THz streak camera concept. It describes the prototype setup called pulse arrival and length monitor (PALM) developed at PSI and tested in Spring-8 Angstrom Compact Free Electron Laser (SACLA) in Japan. Based on the first results obtained from the measurements, we introduce the new improved design of the second generation PALM setup that is currently under construction and will be used in SwissFEL photon diagnostics.
Intense Terahertz radiation in organic crystals is typically generated by optical rectification of short wavelength infrared femtosecond lasers between 1.3 and 1.5 μm. In this wavelength range high energy ultrashort pump sources are hardly available. Here we present results on powerful THz generation by using DAST and DSTMS pumped directly by the widely used and well-established Ti:sapphire laser technology, emitting at 0.8 μm. This approach enables straightforward THz generation by optical rectification. We present systematic studies on
nIR-to-THz conversion efficiency, damage threshold, and on the emitted THz spectrum and field strength.
We investigated intense Terahertz generation in lithium niobate pumped by a powerful Yb:CaF<sub>2</sub> laser at room temperature and 25 K. This unique amplifier system delivers transform-limited pulses of variable duration (0.38-0.65 ps) with pulse energies up to 12 mJ at a central wavelength of 1030 nm. From theoretical investigations it is expected that those laser parameters are excellently suited for efficient THz generation. In this study we present experimental results on both the conversion efficiency and the THz spectral shape for a series of transform-limited pump pulse durations and crystal temperatures and discuss the optimum pump parameters for most efficient THz generation.
Organic stilbazolium salt crystals pumped by intense, ultrashort mid-infrared laser have been investigated for efficient THz generation by optical rectification. In this paper we present our latest results in view of the generation of single-cycle and high-field THz transient in the THz gap (0.1-10 THz). The organic rectifiers like DAST, OH1 and DSTMS combine extremely large optical susceptibility with excellent velocity matching between the infrared pump and the THz radiation. Our simple collinear conversion scheme provides THz beams with excellent focusing properties and single cycle electric field larger than 1.5 MV/cm and magnetic field strength beyond 0.5 Tesla. The source can potentially cover the full THz gap at field strength which is barely provided by other THz sources. The THz pulse is carrier-envelope phase stable and the polarity of the field can be easily inverted.
We report on laser-based, high power single-cycle THz source. The THz radiation is generated by four-wave mixing in
plasma and by optical rectification in organic salt crystal pumped by powerful optical parametric amplifier. The first
approach permits the generation of electric field of hundreds of kV/cm at central frequency of 0.7 THz. The second
technique allows the synthesis of an electric field exceeding 1 MV/cm paired with an unprecedented conversion
efficiency of more than 2%, at frequency of 2 THz. The presented sources can be focused to a diffraction-limited spot
and are suitable-versatile tool for time resolved THz experiment.
We show by experiments and simulations that properly chirped laser pulses enable efficient and broadband sum
frequency generation in nonlinear crystals. We achieved high energy, picosecond deep-UV pulses with spectral width
one order of magnitude greater than the acceptance bandwidth of the nonlinear interaction. The broad spectrum supports
shaping of ps flat-top deep-UV pulses with short rise- and fall-time, which are optimal for driving high brightness
photocathode electron guns.
Experiments in laser physics often require more comprehensive information about a beam than can be extracted from temporal and spatial profile measurement alone. In particular, the wavefront has considerable effect on both irradiance and phase distribution near focus, and thus large impact on the efficiency of non-linear coherent processes such as generation of higher harmonics from femtosecond ultra-short laser pulses. Here we present Hartmann-Shack wavefront measurements of ultra-broadband laser pulses with a spectral bandwidth of >190 THz, which are produced by focusing amplified pulses from a 20 fs Ti:Sapphire oscillator-amplifier system into an Argon filled hollow fibre of 400 μm diameter. After re-compression the pulses were analyzed with the Hartmann-Shack sensor, both at a distance of 140 cm behind the fibre exit and after reflection from a concave mirror (f = 100 mm). Measurements of the overall polychromatic wavefront are faced to a couple of quasi-monochromatic ones covering the whole spectrum. Incoherent superposition of the spectral components yields excellent agreement to the measured overall wavefront, showing that the total wavefront can be sensed reliably by a single measurement. Furthermore, comparison of numerically propagated and measured wavefronts shows good agreement for different spectral components: the measured overall wavefront fits, within sensor accuracy, the numerically propagated one obtained by incoherent superposition of its quasi-monochromatic parts. Drawbacks and opportunities of the Hartmann-Shack technique in ultra-short pulse sensing are briefly discussed.