Optical control of hard X-ray polarization by electron injection in a laser wakefield accelerator

Abstract

Laser-plasma particle accelerators could provide more compact sources of high-energy radiation than conventional accelerators. Moreover, because they deliver radiation in femtosecond pulses, they could improve the time resolution of X-ray absorption techniques. Here we show that we can measure and control the polarization of ultra-short, broad-band keV photon pulses emitted from a laser-plasma-based betatron source. The electron trajectories and hence the polarization of the emitted X-rays are experimentally controlled by the pulse-front tilt of the driving laser pulses. Particle-in-cell simulations show that an asymmetric plasma wave can be driven by a tilted pulse front and a non-symmetric intensity distribution of the focal spot. Both lead to a notable off-axis electron injection followed by collective electron-betatron oscillations. We expect that our method for an all-optical steering is not only useful for plasma-based X-ray sources but also has significance for future laser-based particle accelerators.

ARTICLE

Received 2 May 2013 | Accepted 8 Aug 2013 | Published 12 Sep 2013

Michael Schnell1, Alexander Savert1,2, Ingo Uschmann1,2, Maria Reuter1,2, Maria Nicolai1,2, Tino Kampfer2, Bjrn Landgraf1,2, Oliver Jackel1,2, Oliver Jansen3, Alexander Pukhov3, Malte Christoph Kaluza1,2 & Christian Spielmann1,2

Laser-plasma particle accelerators could provide more compact sources of high-energy radiation than conventional accelerators. Moreover, because they deliver radiation in femtosecond pulses, they could improve the time resolution of X-ray absorption techniques. Here we show that we can measure and control the polarization of ultra-short, broad-band keV photon pulses emitted from a laser-plasma-based betatron source. The electron trajectories and hence the polarization of the emitted X-rays are experimentally controlled by the pulse-front tilt of the driving laser pulses. Particle-in-cell simulations show that an asymmetric plasma wave can be driven by a tilted pulse front and a non-symmetric intensity distribution of the focal spot. Both lead to a notable off-axis electron injection followed by collective electronbetatron oscillations. We expect that our method for an all-optical steering is not only useful for plasma-based X-ray sources but also has signicance for future laser-based particle accelerators.

DOI: 10.1038/ncomms3421 OPEN

Optical control of hard X-ray polarization by electron injection in a laser wakeeld accelerator

1 Institute of Optics and Quantum Electronics, Abbe-Center of Photonics, Friedrich Schiller University, Max-Wien Platz 1, 07743 Jena, Germany. 2 Helmholtz Institute Jena, Frbelstieg 3, 07743 Jena, Germany. 3 Institute for Theoretical Physics 1, Heinrich-Heine University Dsseldorf, 40225 Dsseldorf, Germany. Correspondence and requests for materials should be addressed to M.S. (email: mailto:[email protected]

Web End [email protected] ).

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Laser-plasma-based electron acceleration is the rst step towards the generation of spatially and temporally coherent hard X-ray pulses in the femtosecond time domain. Broad

band femtosecond X-ray pulses provide new opportunities for time-resolved X-ray absorption spectroscopy, shedding light on chemical reaction dynamics1,2 or revealing ultra-fast structural changes3. The availability of a source with polarized X-rays could pave the way for studying the dynamics of magnetization via linear magnetic dichroism4 or for studying structural changes in thin lms by employing polarization-dependent X-ray absorption ne structure spectroscopy5. Polarized X-rays are also widely used in materials science and biology to signicantly enhance the sensitivity of X-ray uorescence analysis6. One possible route for generating X-rays is to induce a wiggling motion of the accelerated electrons in a conventional undulator consisting of a periodic magnetic structure like that found in a synchrotron. Using this approach, spatially coherent soft X-rays up to an energy of 200 eV have been generated7,8, holding the promise for a table-top X-ray-free electron laser9. Much more straightforward is applying the strong localized electric and magnetic elds inside the plasma and using the plasma itself as an effective wiggler structure to produce hard X-ray radiation1012. The major advantage of the latter approach is that plasmas can produce multimega-gauss elds13 and have a much shorter effective wiggler period on the order of a few hundred micrometres14, resulting in a low divergence beam of keV X-rays even for electrons in the 100-MeV range.

A laser-plasma accelerator can be realized by focusing a femtosecond laser pulse to intensities of B1019 W cm 2 into a millimetre-scale supersonic gas jet. When using helium or hydrogen as the target gas the leading edge of the pulse is already intense enough to fully ionize the gas. In the central part of the laser pulse the intensity, and hence the pondermotive potential, is strong enough to push the electrons away from the optical axis and a plasma wave is excited in its wake. This plasma wave, travelling close to the speed of light, can break after strong excitation. As a consequence of this wave breaking, electrons are injected into the plasma-wave structure. The electric eld inside the wave is strong enough (E100 GV m 1) to accelerate the electrons in the forward direction to relativistic energies in excess of 1 GeV over distances of only a few millimetres or centimetres15. Also present in the plasma wave together with an azimuthal magnetic eld is a transverse electric eld associated with the charge separation in the plasma wave, forcing the highly relativistic electrons into a transverse oscillation leading to the emission of betatron radiation in the X-ray regime12. It was shown that for electrons in an energy range of about 100 MeV the largest number of keV photons is emitted from the region in the plasma where the electrons have the maximum energy16. The radiation from the relativistically moving charges emitted into a spectral band do, centred on the frequency o, and into the solid angle dO centred on the observation direction, is a function of the electron velocity ~b (normalized to the speed of light in vacuum) and the electron trajectory ~r(t), and reads17

d2I

dodO

e2 16p3e0c

Z

1

1

1

where e is the electron charge and e0 the vacuum permittivity. Inspecting equation (1) immediately reveals that the polarization state of the output depends on the direction of the electrons oscillation in the plane perpendicular to the lasers propagation direction.

In this article, we report on a mechanism to control the off-axis injection of the electrons into the plasma wave, and hence adjust the electron trajectory. One all-optical method is to tilt the pulse front of the driving laser pulse to create a non-symmetric spatio-temporal distribution of the focal spot. For a tilted pulse front the maximum intensity will be reached at different times across the beam prole, breaking the symmetry and preferentially injecting the electrons into the plane of the lasers tilted pulse front with a spatial offset to the plasma waves axis as shown recently18. Likewise, an asymmetric laser intensity prole leads to a non-symmetric intensity distribution within the focal plane19. Since the laser intensity prole is asymmetric, the radial ponderomotive force is different in each direction, driving an asymmetric plasma wave resulting in a substantial off-axis injection of the electrons20. To fully model our experiment we analyse also the emitted X-ray radiations polarization state predicted by our three-dimensional particle-in-cell (PIC) VLPL-Code21 either for different pulse-front tilts or for an asymmetric intensity distribution in the focal spot (see Methods).

ResultsNumerical results. We rst carried out PIC simulations for a tilted pulse front and a non-symmetric intensity distribution of the focal spot. Both deviations from a perfect Gaussian beam are expected to inject electrons off-axis into the plasma wave (Fig. 1). As shown by the simulations, without any aberration of the laser beam the injected electrons oscillate about the laser axis with the betatron frequency (Fig. 1a) and emit X-ray radiation with no well-dened polarization state. With an additional pulse-front tilt, the electrons are injected off-axis, leading to a larger initial amplitude of the betatron oscillation and a signicantly higher number of X-ray photons polarized in the plane of the electrons oscillation (Fig. 1b). Similar results can be achieved by introducing an asymmetric focal spot (Fig. 1c). In summary, the simulations reveal that controlling the off-axis injection of the electrons by tilting the pulse front of the driving laser pulse is a promising method to control the polarization of the X-ray pulses.

Experimental results. Experimentally, the pulse front can be tilted by slightly misaligning one of the compressors gratings, which are used in a chirped pulse amplication (CPA) laser system22. The misaligned grating causes an angular chirp in the spectral domain corresponding to a tilted pulse front in the time domain23,24. Introducing a pulse-front tilt in this way also changes the direction of the compressed beam, which must be compensated by re-aligning the end mirror in the compressor. In our experiment, the relativistic laser pulse (see Methods) is focused into the leading edge of a pulsed gas jet to generate electron bunches that are able to produce intense X-ray beams due to betatron motion (Fig. 2a). The far-eld distribution of the accelerated electron bunches was analysed with an aiming screen suggesting a collimated electron beam with a divergence of about 4 mrad in both transverse directions. To characterize the electron and X-ray beams simultaneously, we have removed the aiming screen and analysed the electron energy distribution with a magnetic dipole spectrometer placed farther downstream in the beam path. The single-shot, far-eld distribution of the generated X-ray beam was measured with detector 3 (see Fig. 2a) and showed a Gaussian-like spatial distribution with a divergence of 205 mrad (see Supplementary Fig. S1). By averaging over 100 consecutive shots we obtained a root-mean-square (r.m.s.) uctuation of the X-ray beam pointing better than 3 mrad in the horizontal and vertical directions (Supplementary Fig. S1). Taking into account the efciency of our X-ray detector we have estimated the number of emitted photons with an energy over 3 keV to be about 5 107 X-ray photons per shot. In the

~b

_

~b

exp io t ~rt=c

h i

1

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2

dt

2

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a

Time ~ 1.2 ps Time ~ 2.1 ps Time ~ 2.7 ps Time ~ 3.4 ps

Time ~ 1.2 ps Time ~ 2.1 ps Time ~ 2.7 ps Time ~ 3.4 ps

Time ~ 2.6 ps Time ~ 3.2 ps Time ~ 4.3 ps

b

0.02

0.00

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ne / ncrit

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Figure 1 | Snapshots of the simulated electron density distribution. (a) On-axis electron injection driven by a laser pulse with no pulse-front tilt and an ideal symmetric focal spot. The white scale bar in each gure corresponds to a length of 10 mm. (b) A signicant pulse-front tilt showingan asymmetric off-axis injection of the electrons. The dashed line indicates the on-axis position. (c) An asymmetric plasma wave is also driven by a non-symmetric intensity distribution of the focal spot leading to a notable off-axis electron injection and also to collective electronbetatron oscillations. Parameters used in the simulation: ne 1 1019 cm 3, l 800 nm, laser spot-diameter 12 mm, laser pulse duration 30 fs and initially a0 2.0

(experimental case).

Gas jet

a

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Figure 2 | Setup of the single-shot X-ray radiations polarization state analyser. (a) An f/13 off-axis parabolic mirror focuses the laser pulses(730 mJ, 27 fs, 800 nm) to a 12-mm-diameter spot into a hydrogen gas jet with 2.7 mm opening diameter. The laser-accelerated electrons were analysed with an aiming screen and a magnetic dipole spectrometer. The betatron X-ray beam was measured with different CCD detectors (labelled as detector 1, 2 and 3). (b) Setup of the single-shot X-ray polarimeter. Two lithium-uoride crystals are oriented at Brewsters angle, reecting either the horizontally (crystal 1) or vertically (crystal 2) polarized X-ray component at 4.35 keV. The two orthogonal polarization components are detected simultaneously with two X-ray CCD cameras (labelled as detector 1 and 2). (c) Having an asymmetric laser focal spot without a deformable mirror generatesan asymmetric wakeeld. The white double arrow indicates a width of 12 mm.

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literature, polarization of the X-ray beam has been predicted by analysing the far-eld distribution of the electrons25 (Supplementary Fig. S2) or X-rays26,27. However, in our experiments the sub-30 fs laser pulses were slightly shorter than the excited plasma wavelength and will be further shortened by an additional longitudinal pulse compression mechanism28. According to the theory we can then exclude an interaction of the accelerated electrons with the trailing edge of the laser pulses (Supplementary Discussion). To analyse the polarization of the betatron emission we used a single-shot polarimeter (Fig. 2b). The polarizer is based on two mutually perpendicular, plain lithium-uoride (LiF) mosaic crystals. Polarization-sensitive reection is realized by fullling the Bragg condition at Brewsters angle (aB), which is close to 45 for a refractive index of nE1 for radiation in the X-ray spectral range (see Methods)29. Taking into account the lattice distance of the LiF crystals, only s-polarized

X-ray radiation at 4.35keV will be reected and detected with an X-ray-sensitive CCD camera. The extinction ratio of the polarization state measurement between the p-component and the s-component was calculated to be better than 1:33. The two crystals are sufciently far away from the X-ray source (4 m) and sufciently close to each other, ensuring a uniform illumination of both crystals.

With this arrangement we are able to simultaneously measure the amount of horizontally and vertically polarized X-ray radiation for each laser shot. Having the possibility to measure the polarization state in a single shot is important to be insensitive to shot-to-shot uctuations. This is particularly important as the output signal depends critically on the exact position of the nonlinear wave breaking, that is, the electron injection position and the inevitable intensity uctuations within the asymmetric laser focus (Fig. 2c). However, with our experimental setup we are not only able to measure the polarization state for each laser shot, but we managed to record for each shot a shadowgraphic image of the plasma wave, the electron spectrum, and the transverse electron beam prole in one dimension. These simultaneously recorded data sets allowed correlation of the polarization states to the electrons energy

spectra. Here we take advantage of the fact that our electron spectrometer disperses the electron pulse along the horizontal axis, whereas the signal is spatially resolved in the vertical direction20. This feature can be used to determine the direction of the off-axis injection of electrons into the plasma wave by tilting the pulse front of the laser pulse18. As the X-ray photons are mainly emitted from high-energy electrons16, it is sufcient to show the electron spectra only above 60 MeV (Fig. 3). Some shots show a corrugated trace of the electrons on the scintillating screen (Fig. 3ac), while others exhibit a narrow transverse distribution (Fig. 3df). Many results of the laserplasma interaction will vary from shot to shot depending on the exact longitudinal and transverse injection position of the electrons. These include the net acceleration length of the electrons, their energy, the plane of the betatron oscillation and the X-ray radiations polarization state. The main contributing effect for our setup is the self-injection of electrons due to an asymmetric intensity distribution inside the focal spot, which is likely to vary from shot to shot as well (Fig. 2c). Furthermore, we can conclude that in our acceleration regime the betatron polarization state is independent of the direction of the lasers polarization state, which was not changed during the experiment. The laterally smooth electron distribution can be explained by electrons oscillating mainly in the horizontal direction, while the corrugated structure indicates an additional collective wiggling of the electrons in the vertical direction. For the wiggled electron trace the X-ray signal is primarily vertically polarized (Fig. 3ac), whereas for the straight electron trace we detect mainly horizontally polarized X-rays (Fig. 3df). As the polarization of the emitted betatron radiation is determined by the plane of oscillation of the electrons injected into the plasma wave, the measured X-ray polarization provides us additional information about the injection of the electrons into the plasma wave.

To overcome the shot-to-shot uctuations of the polarization state of the betatron emission it is necessary to control the off-axis injection of the electrons into the plasma wave. Therefore we increased the spatio temporal asymmetry within the focal spot by

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Figure 3 | Electron spectra and simultaneously measured X-ray polarization state. Typical single-shot electron spectra on the high-energy scintillating screen exhibit (ac) either a corrugated trace (df) or are vertically well conned. For the wiggled electron traces (ac), the X-ray radiation is mainly vertically polarized (red line) and have only a minor contribution in the horizontal polarization direction (black line). For the vertically conned electron traces (df) the horizontally polarized component dominates. The photon number within only one Bragg peak corresponds to nearly 103 photons per shot. The shot-to-shot uctuations are attributed to the asymmetric intensity distribution inside the focal spot, which varies from shot to shot as well.

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a b

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Figure 4 | Controlling the X-ray polarization by tilting the pulse front. (a) Snapshot of the electron density gradient taken with a 6-fs duration probe pulse for zero horizontal angular dispersion. The laser pulse propagates from left to right and the plasma wave is vertically tilted due to a spatio-temporal asymmetry of the focal spot. We increase our a priori asymmetric intensity distribution within the focus to rule out shot-to-shot uctuation of the X-ray polarization state, which was sufcient to overcome the statistical uctuations. The white double arrow indicates a lengthof 15 mm. (b) Ratio of the vertically and horizontally polarized X-ray contribution as a function of the horizontal angular dispersion of the laser pulse. Each data point corresponds to an average over at least 100 consecutive shots, and the error bars indicate the standard error of the mean value (black squares).

Our measurements t very well to the predictions from the 3D PIC simulations (red squares). The red dashed line is to guide the eye.

introducing a vertical pulse-front tilt, which was sufcient to overcome the statistical uctuations. In Fig. 4a we present an image of the plasma wave taken with the probe beam that crosses the plasma at an early stage in the interaction. The gas jet is placed in front of the focal position of the main laser pulse propagating from the left to the right. The intensity modulation (grey scale) can be associated with the plasma wave and exhibits a very strong tilt against its propagation direction. It is important to mention that the shadowgram does not directly show the electrons density distribution. The observed spatial intensity modulation is created from the spatially homogeneous probe pulse refracting through the electrons density modulation inside the plasma. Light and dark areas in the image are formed proportional to the Laplacian of the plasmas refractive index30. From this image we can estimate a wavelength of the plasma wave on the order of 15 mm, corresponding to an electron density of 5 1018 cm 3. This is in good agreement with

the calculated density derived from interferometric characterization of the nozzle using a neutral gas31. The effect of this introduced pulse asymmetry on the polarization of the betatron emission is depicted in Fig. 4 averaged over 100 consecutive laser shots.

To reduce the effect of uctuations of the X-rays intensity between measurements of different pulse-front tilts we plotted the ratio of the spectrally and temporally integrated signal in the vertical plane to the horizontal plane. The spectral integration spans the high reectivity range of the LiF crystal in the range of4.24.5 keV. Here, for an asymmetry within the focal spot mainly in the vertical direction, a net ratio of 4:1 for vertical polarized X-rays arises. The vertically tilted pulse front, which mainly causes self-injection of electrons with a vertical offset, resulted in vertically polarized X-rays. To demonstrate the inuence of the tilt, we rotate one of the compressor gratings in the dispersive plane away from the position without angular tilt, which results in a horizontal angular dispersion. This favours injection in the plane with the tilted pulse front (horizontal plane) and this ratio starts to decline, in agreement with our theory. However, it is not possible to completely invert the intensity ratio because the electron acceleration becomes more and more inefcient due to the longer pulse duration of the laser pulse within the focal spot. Fig. 4b reveals the strong impact of the tilt on the polarization state. With increasing horizontal tilt, the electrons are injected with a higher probability off-axis in the horizontal direction, which increases the number of horizontally polarized X-rays. This measurement demonstrates the feasibility to control the X-rays polarization with an all-optical method, and is in excellent agreement with our predictions based on 3D PIC simulations.

DiscussionBy controlling the injection of the electrons in a laser wakeeld accelerator, we have demonstrated that we can tune the polarization state of the emitted X-rays. The simulation results obtained here indicate that a non-symmetric plasma wave can be driven by tilting the pulse front of the driving laser pulse or by inducing a non-symmetric intensity distribution of the focal spot. Both methods result in a notable off-axis injection of the electrons and also result in the generation of pulsed X-rays polarized either in the plane of the tilted pulse front or in the plane of the focus asymmetry, depending on the method used. Our measurements reveal the importance of controlling the injection of the electrons into the plasma wave with regard to the resulting X-ray polarization state. We also show that we can inuence the direction of an additional transverse momentum of the accelerated electrons, which will be of importance for feeding the electrons into an additional conventional accelerator or a permanent magnet-based undulator for generating X-ray radiation. To further increase the polarization purity and sustainable control of the polarization direction, it might be advantageous to work in the harmonically resonant betatron regime,32 where the electrons trajectory can be controlled via the laser polarization itself.

Methods

Laser-plasma wakeeld accelerator. The experiments were carried out at the multi-TW, Ti:Sapphire, chirped pulse amplication (CPA) laser system (JETI) in Jena, Germany, delivering 27-fs pulses with an on-target energy of 730 mJ at a central wavelength of 800 nm. The pulses were focused with an f/13 off-axis parabolic mirror into the leading edge of a pulsed supersonic hydrogen-gas jet. The jet consists of a pulsed valve and a conically shaped nozzle with a diameter of2.7 mm resulting in an adjustable peak electron density of 0.22.0 1019 cm 3,

which can be adjusted by varying the backing pressure of the gas. Without a deformable mirror we obtain an asymmetric focal spot of 12 mm diameter at full-width at half-maximum (FWHM), which leads to an intensity of 7.2

1018 W cm 2 (FWHM) corresponding to a normalized amplitude of the vector potential, a0X1.9. For most of the experiments the laser was operated in single-shot mode, but operation up to 0.5 Hz was possible, limited only by the read-out time of the X-ray detectors and the gas load in the vacuum chamber.

Electron beam diagnostic. The laser-accelerated electrons were monitored with scintillating phosphor screens. The rst screen, called the aiming screen, was placed after the laserplasma interaction and analysed the electrons single-shot far-eld distribution, their pointing and their beam divergence. When the screen was removed, the electrons could enter a magnetic dipole spectrometer (0.7 T over20 cm), which deected the electrons depending on their energy onto two different scintillating screens optimized for detecting electrons in the range 1055 and 60350 MeV, respectively. The uorescence of each screen was imaged onto a high-resolution 12-bit Basler A102f CCD camera.

X-ray beam diagnostic. The X-ray radiation in the lasers forward direction was detected with two thermoelectrically cooled, back-illuminated, deep-depletion

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X-ray CCD cameras (ANDOR DO-936NMOW-BR-DD and ANDOR DV420-BRDD). The residual laser light and the low-energy X-ray radiation (below 1 keV) were blocked by a permanent 50-mm Be-lter placed in front of the DO-936 CCD and a 30-mm Be-lter placed in front of the DV420 CCD. For the polarization measurement we used the CCD cameras in a single-photon counting mode as an energy-sensitive X-ray spectrometer with an energy resolution better than 100 eV.

Plasma imaging. For the transverse time-resolved optical probing, a fraction of the main laser pulse is further shortened by rst broadening the spectrum in a argon lled hollow core bre and then compressing it with chirped mirrors down to 5.90.4 fs. In our experiment the ultra-short pulses at a central wavelength of 750 nm have been used to image the plasmas density via shadowgraphy, relying on the dependence of the refractive index of the plasma as a function of the electron density.

X-ray polarimeter. The X-ray polarimeter consists of two LiF crystals using the strongest Bragg reection 200. For ideal mosaic crystals, the diffracted polarization components with the electric eld components parallel (p-component) and perpendicular (s-component) to the diffraction plane are dened by the incident and diffracted wave vector amount cos2(2y) and 1, respectively33. The 200 netplane distance (2d200 4.027 ) ts perfectly to the maximum of the betatron spectrum

at 4.6 keV by using the Bragg condition (l 2dhkl sin y, where l is the reected

wavelength and dhkl the lattice distance of the crystal reection with hkl being the diffraction indices) close to the Bragg angle of 45. At this Bragg angle only linearly s-polarized X-rays are diffracted. The crystals were selected for high crystalline perfection, dened by small angles between the crystal grains. In order to increase the diffraction efciency, the crystals perfection was reduced by a specially controlled grinding procedure. With this treatment the crystals provide integrated reectivities that are more than nine times higher than for a perfect crystal. The reectivity over the whole crystals area was determined to be constant within 6%. The FWHM of the experimentally determined rocking curves yield an energy resolution of 1322 eV at 4.6 keV photon energy. Both crystals were built into the polarimeter so that their dispersion planes were mutually perpendicular, allowing detection of the vertical and horizontal polarization states of the betatron radiation with an extinction ratio of 1:33 (3.5%). This assumes an alignment accuracy better than 1 for the Bragg angle and uses the measured divergence of the radiation and width of the crystals reection curves.

Numerical modelling. For predicting the polarization state of the emitted radiation we carried out numerical simulations using the three-dimensional PIC code VLPL21. We modelled a linearly polarized laser pulse with a normalized, relativistic amplitude a0 2.0, travelling through an underdense plasma of density

n 0.01ncr, with ncr being the critical density for this laser pulse. Our 3D PIC

simulations included the recoil force acting on an accelerating electron caused by the emitted radiation (radiation reaction), during which we also derived the polarization state of the emitted radiation by using the changes in the momentum of the emitting particles. We simulated different tilts of the lasers pulse front and analysed the results for the corresponding values of the electrons off-axis injection points ranging from on-axis injection (zero angular dispersion, no pulse-front tilt) to a signicant off-axis injection (angular dispersion).

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Acknowledgements

This study has been sponsored by the DFG (grant TR18 A12, B9 and GRK1203), BMBF (contracts 05K10SJ2 and 03ZIK052), TMBWK (grants B154-09030, B 715-08008) and European Regional Development Fund (EFRE). The authors acknowledge contributions of the JETI laser team (Burgard Beleites, Wolfgang Ziegler and Falk Ronneberger) for running the laser laboratory, Matthew Schwab for providing the probe beam setup and reading the manuscript, and Robert Ltzsch for fruitful discussions.

Author contributions

M.S., A.S., M.C.K., and C.S. designed the experiments and wrote the manuscript; M.S., A.S., I.U., M.R., M.N., T.K., and O. Ja carried out the experiment; B.L., O.Ja. and A.P. performed simulations and developed the theory, all authors analysed the data and contributed to the completion of the manuscript.

Additional information

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How to cite this article: Schnell, M. et al. Optical control of hard X-ray polarization by electron injection in a laser wakeeld accelerator. Nat. Commun. 4:2421 doi: 10.1038/ ncomms3421 (2013).

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