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Plasma switch as a temporal overlap tool for pump-probe experiments at FEL facilities
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P UBLISHED BY IOP P UBLISHING FOR S ISSA M EDIALAB R ECEIVED: June 22, 2012 ACCEPTED: July 10, 2012 P UBLISHED: August 10, 2012
Plasma switch as a temporal overlap tool for pump-probe experiments at FEL facilities
a Deutsches
Elektronen-Synchrotron DESY, Notkestr. 85, 22607 Hamburg, Germany b Clarendon Laboratory, University of Oxford, Parks Road, Oxford OX1 3PU, U.K. c LCLS, SLAC National Accelerator Laboratory, 2575 Sand Hill Road, Menlo Park, CA 94025-7015, U.S.A. d Lawrence Livermore National Laboratory, University of California, P.O. Box 808, Livermore, CA 94551, U.S.A. e IOQ, Friedrich-Schiller-Universit¨ at, Max-Wien Platz 1, 07743 Jena, Germany f European XFEL GmbH, Albert-Einstein-Ring 19, 22761 Hamburg, Germany g Institut f¨ ur Physik, Universit¨at Rostock, 18051 Rostock, Germany h GSI Helmholtzzentrum f¨ ur Schwerionenforschung GmbH, Planckstr. 1, 64291 Darmstadt, Germany i Laboratoire pour l’Utilisation des Lasers Intenses, Ecole ´ Polytechnique, Route de Saclay, 91128, Palaiseau Cedex, France j Helmholtz-Institut Jena, Fr¨obelstieg 3, 07743 Jena, Germany k EMMI, Planckstr. 1, 64291 Darmstadt, Germany l AWE Aldernaston, Reading RG7 4PR, U.K.
E-mail:
[email protected] 1 Corresponding
author.
c 2012 IOP Publishing Ltd and Sissa Medialab srl
doi:10.1088/1748-0221/7/08/P08007
2012 JINST 7 P08007
d ¨ M. Harmand,a,1 C.D. Murphy,b C.R.D. Brown,b,l M. Cammarata,c T. Doppner, a D. Fritz,c E. Forster, e, j E. Galtier,i J. Gaudin, f S.H. Glenzer,d S. Gode, g ¨ ¨ S. Dusterer, ¨ G. Gregori,b V. Hilbert,e D. Hochhaus,h T. Laarmann,a H.J. Lee,c H. Lemke,c K.-H. Meiwes-Broer,g A. Moinard,i P. Neumayer,h,k A. Przystawik,a H. Redlin,a M. Schulz,a S. Skruszewicz,g F. Tavella,a T. Tschentscher, f T. White,b U. Zastraue and S. Toleikisa
K EYWORDS : Timing detectors; Instrumentation for FEL; Beam-line instrumentation (beam position and profile monitors; beam-intensity monitors; bunch length monitors)
2012 JINST 7 P08007
A BSTRACT: We have developed an easy-to-use and reliable timing tool to determine the arrival time of an optical laser and a free electron laser (FEL) pulses within the jitter limitation. This timing tool can be used from XUV to X-rays and exploits high FELs intensities. It uses a shadowgraph technique where we optically (at 800 nm) image a plasma created by an intense XUV or X-ray FEL pulse on a transparent sample (glass slide) directly placed at the pump - probe sample position. It is based on the physical principle that the optical properties of the material are drastically changed when its free electron density reaches the critical density. At this point the excited glass sample becomes opaque to the optical laser pulse. The ultra-short and intense XUV or X-ray FEL pulse ensures that a critical electron density can be reached via photoionization and subsequent collisional ionization within the XUV or X-ray FEL pulse duration or even faster. This technique allows to determine the relative arrival time between the optical laser and the FEL pulses in only few single shots with an accuracy mainly limited by the optical laser pulse duration and the jitter between the FEL and the optical laser. Considering the major interest in pump-probe experiments at FEL facilities in general, such a femtosecond resolution timing tool is of utmost importance.
Contents Introduction
1
2
Experiment details
2
3
Results 3.1 Timing tool in the XUV range at FLASH 3.2 Timing tool in the X-ray range at LCLS 3.3 A jitter limited method
3 3 3 5
4
Conclusion
5
1
Introduction
As short wavelength free electron laser facilities develop and are applied to a growing range of scientific studies, it becomes increasingly important to elaborate reliable and easy to use diagnostic tools. Characterization of the timing between two independent photon sources is a particularly challenging aspect in the femtosecond time regime for pump-probe experiments. For such experiments, knowledge of when temporal overlap occurs (“time zero”) and the uncertainty in the arrival time between the two pulses (the jitter) directly at the interaction point of the experiment is of paramount importance. Several timing methods have already been studied and developed: e.g., reflectivity changes of FEL excited materials [1–5], generation of sidebands in photoelectron TOF spectra [6], transient molecular Nitrogen alignment of molecules [7], or phase transitions in solids [8, 9]. We report here a competitive in-situ method to determine the temporal overlap between optical laser and FEL pulses within the jitter limitation (∼ 100fs), with only few single shots and for a broad range a FEL photon energy. The method presented here is a shadowgraph technique where the transmission change of the optical laser pulse is imaged through a transparent sample, in our case a simple glass slide, which is irradiated by an intense FEL pulse. The main idea is to transform rapidly the material from a highly transparent state to an opaque one. This is achieved by quickly changing the free electron density in the sample. This process is commonly used for optical gates [10] and plasma mirrors in laser physics [11]. In contrast to those applications, the change of the free electron density is accomplished with XUV or X-ray photons from an FEL. Photoionization and subsequently collisional ionization increase the free electron density within femtosecond time scale [12, 13]. In the case of high FEL intensity, one creates a plasma on the surface that exceeds the critical electron density Nc , where the plasma frequency equals the optical laser frequency. For an optical laser at a wavelength of λOL = 800 nm, the critical free electron density is Nc = 1.7 · 1021 cm−3 . In order to reach the critical electron density on the irradiated surface, the intensity of the Xray pump pulse has to be sufficiently high to create a significant amount of free carriers. In the case
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1
Glass Slide
Lens
CCD
OL @ 800nm (probe) FEL (pump) Plasma shadow
Plasma (opaque @ 800nm for N > Nc)
of optical lasers, the ultrafast ionization mechanism is mainly based on multiphoton and tunnelling ionization and ensure the feasibility of 10 fs gating of optical gates [10]. In the XUV to X-ray range, the ionization process is mainly due to single photon photoionization followed by the emission of less energetic Auger electron in light elements. Subsequently, collisional ionization also occurs via electron impact of the primary photoelectrons and secondary electrons are produced in this dense environment. Due to the high brilliance of FEL pulses [14], this effect is very efficient, especially in the XUV range. For example at FLASH [15], typical XUV photon intensities around 1013 − 1014 W/cm2 are sufficient to get to the critical density within the FEL pulse duration (typically ∼ 10 fs to ∼ 200 fs). However, there is an important difference between the XUV and the X-ray regime concerning the absorption: the attenuation length. For glass (SiO2 ), the attenuation length at photon energy of 200 eV is only 0.12 µm, while at 8 keV it is about 130 µm. From this fact one could expect that it might be more difficult to reach the critical density at higher photon energies, because the photons are then absorbed in a larger volume. But at higher energies (8 keV) also the number of electron-hole pairs which are created is much higher than in the XUV range which allows to get to the critical density within ∼ 100 fs or less [16].
2
Experiment details
During previous pump-probe experiments at FLASH [17], we have developed an experimental procedure to overlap spatially and temporally the FEL and the optical laser pulses at the point of interest for pump-probe experiment. A first temporal overlap with a ∼ 50 ps time resolution is performed with an ultrafast photodiode placed at the sample position to adjust the electronic delay set on a masterclock of the optical laser beam on the FEL arrival time [18]. In a second step, the precise arrival time of the optical laser is adjusted with a delay line and by using the described shadowgraph based technique in the XUV regime to reach a synchronization limited by the jitter between the optical laser and the FEL pulses. In our case, the FEL intensity was around 1014 − 1015 W/cm2 , ensuring a plasma effect on the surface [17]. A sketch of the experimental setup is shown in figure 1. We position a movable glass slide in the FEL focal plane, which is backlit by a low intensity optical laser which does not damage the glass substrate. A single FEL pulse generates a plasma on the glass surface by ultrafast ionization. If the plasma is present when the 800 nm laser pulse reaches the glass, the laser is reflected at the critical density position of the plasma and less
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Figure 1. Experimental setup of the plasma timing tool.
T=
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T=
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light is transmitted through the area where the free electron density is above the critical density. This “shadow” is then observed in transmission by imaging the glass surface onto a CCD camera using a single lens. A synchronized camera is acquiring the magnified images on a single shot basis. If the laser pulse arrives at the same time or later than the intense FEL pulse, either the shadow from the critical surface, or the permanent damage at very long delays, will be visible in transmission. If the laser pulse arrives before the intense FEL pulse, no changes will be observed on the transmission during the shot, while a permanent damage is observed afterwards showing the interaction of the intense FEL pulse with the sample occurs after the optical laser arrival time. By scanning iteratively the delay from one situation to another while observing the transmitted signal during a single shot, one can easily find the temporal overlap between the two fs photon sources. In this way, a delay scan of few single pulses has allowed us to synchronize the optical laser and the FEL to few hundreds of fs.
3 3.1
Results Timing tool in the XUV range at FLASH
An example of this shadowgraph technique used at FLASH [15] is shown in figure 2. The FEL beam with a wavelength of λ = 13.5 nm, an averaged energy of 90 µJ and a pulse duration of ∼ 100 fs is focused to a 30 µm focal spot and then illuminated by an optical laser pulse at 800 nm with a pulse duration of ∼ 60 fs [18]. The imaging system was composed of a single lens ( f = 50 mm) reaching a magnification of 10. This simple system allows the determination of temporal overlap within the given jitter limitation [3] and also to finalize the spatial overlap. An upper value for the jitter can be estimated by recording enough statistics for a few delay steps around the time overlap position. At FLASH, this diagnostic has been used for various experimental conditions (energy varying from 15 µJ to 200 µJ, pulse durations from 30 fs to 130 fs). 3.2
Timing tool in the X-ray range at LCLS
Such a diagnostic was also tested at LCLS [19] during a commissioning shift at the XPP station with FEL pulses of 8 keV photon energy and 80 fs pulse duration. The FEL beam was focused down to 10 µm focal spot and the following results were obtained with a full FEL beam (0.4 mJ on target).
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Figure 2. Single shot background subtracted images obtained with a single lens imaging system at FLASH. The two images correspond to two different delay steps adjusted with an optical delay line (negative delay: optical laser before the FEL), indicating that the temporal overlap position is between -0.5 ps (left) and +0.5 ps (right). The shadow is due to a plasma created by the intense FEL radiation focused on the glass slide surface.
Op#cal
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Damage
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a=er
the
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1.617 ps
Object = 2 x 2 µm
Object = 5 x 5 µm
Object = 10 x 8 µm
X‐ray‐Tracing
Experiment
Damage
Object = 20 x 15 µm
Object = 1 x 1 µm
Figure 4. The first row is showing images in transmission of the optical laser through the FEL plasma at different delays. The second row is showing ray tracing Zemax calculations for an increasing object size. The first image called “Damage” is corresponding to an acquisition a long time after the FEL shot and has been used to optimize initial parameters of the Zemax calculation.
Figure 3 shows some images taken at LCLS with an imaging system composed of a single lens only and a CCD camera. The sample is a commercial microscope glass slide (Borosilicate glass) of 1.2 mm thickness. The observed diffraction pattern is due to a slight defocused configuration of the imaging system. The four pictures are corresponding to the following situations (from left to right): a) an average of four reference pictures (optical laser without FEL); b) an image taken during a single shot at a fixed time delay (the optical laser pulse arrives 2.2 ps after the FEL pulse); c) an average of five pictures taken without the FEL, few ms after the shot; and d) a differential interference contrast (DIC) microscopy image of the damage. The optical laser and the FEL are arriving at a normal incidence to the target. As we have seen at FLASH, by taking a series of images for different delays, we can find the time overlap of the two beams with only a few shots and within the jitter limitation. Figure 4 shows a delay scan between the optical laser and the FEL beam. The positive delays corresponds to the optical laser arriving after the FEL. As described before, we can see the shadow of the high electron density volume evolving in time. We compared those images with a Zemax
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Figure 3. Images with a maximum FEL fluence, before, during and after the FEL shot. The delay between the optical laser and the FEL is fixed at 2.2 ps. The last picture shows the damage spot taken with a microscope.
3.3
A jitter limited method
The accuracy of this method is mainly limited by the jitter between the optical laser and the FEL arrival time. This has been measured up to 120fs RMS at LCLS [7] and up to 250 fs RMS at FLASH [21]. After a recent upgrade at FLASH, the jitter has been significantly improved to yield less than 100 fs. The temporal jitter will result in fluctuations at time zero if sometimes one observes a shadow on the image and sometimes not. In this way we are able to evaluate the maximum value of the jitter as soon as the shadow appear and disappear for a fixed delay. A statistical approach could also allow us to extract the average value of the jitter by accumulating measurements for each delay step around the temporal overlap between the optical laser and the FEL pulse. The pulse duration of the optical laser (∼ 40 fs at LCLS) is a second limitation of our temporal resolution assuming that the plasma critical density is reached even faster [16]. This could be overcome by using few fs laser pulses. By tilting the sample, it would be possible to turn this simple diagnostic into a complete hard X-rays timing tool as described in the references [2–5]. In this way, we would convert the arrival time information into a spatial effect by introducing a delay between one side of the FEL beam spot and the other. We would then project the time information spatially and measure directly the exact jitter shot to shot between the FEL and the optical laser. Such timing tool would then allow ultrafast pump - probe experiments for a broad range of FEL wavelength. The physical phenomena based on ultrafast photoionization is expected to be so fast for FEL [16] that it could potentially support few 10’s of femtoseconds resolution.
4
Conclusion
We have developed a timing tool that has already been used successfully at FLASH and LCLS for several experiments. In these cases we were mainly limited by the jitter between the FEL and the optical laser of ∼200 fs. In this paper we were able to demonstrate that one can adopt this timing tool to the X-ray regime. This diagnostic, based on a really simple set up (single lens imaging system) combined with a fast physical process affords a synchronization up to the jitter limitation with only a few single shots. This study underlines the versatility of such reliable in-situ diagnostic adapted for pump-probe experiments on FEL facilities in the XUV to X-ray regime. This technique could easily allow a measurement of the average jitter. A major development of such diagnostic would consist in measuring the shot-to-shot jitter online.
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ray tracing calculation [20] including an 2-D object (a disc) placed on the glass slide. The Zemax parameters were optimized by reproducing the image of the damage impact (images at the extreme left) that has been precisely measured with a microscope afterwards (see figure 3). For the comparison with the scan delay images, we only changed the object size to reproduce the diffraction pattern evolution. We can understand the image evolution by the increase of the electron density volume as function of time. The FEL focus has been measured with a knife-edge to ∼10 x 10 µm while the evolution of the diffraction pattern would suggest that at the beginning the opaque area is smaller. This would suggest that an intensity effect occurs and that only an intense central part of the FEL gaussian beam has to be taken into account. The time-dependance is related to the rise of the excited volume.
Acknowledgments We acknowledge all our FLASH, LCLS and Peak Brightness collaborators. We also thank B. Ziaja and N. Medvedev (CFEL, Center for Free Electron Science science, DESY) for stimulating discussions. The authors are greatly indebted to the machine operators, run coordinators, scientific and technical teams of the FLASH and LCLS facilities for enabling an outstanding performance. E. F¨orster, U. Zastrau and V. Hilbert are grateful to the German Federal Ministry for Education and Research (BMBF) via project FSP 301-FLASH. U. Zastrau acknowledges the VolkswagenStiftung via a Peter-Paul-Ewald Fellowship.
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References
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[21] P. Radcliffe et al., Single-shot characterization of independent femtosecond extreme ultraviolet free electron and infrared laser pulses, Appl. Phys. Lett. 90 (2007) 131108.
