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Publikationen von
Dr. Christian Rödel

Alle Publikationen des HI Jena

2022

S. Keppler, N. Elkina, G. A. Becker, J. Hein, M. Hornung, M. Mäusezahl, C. Rodel, I. Tamer, M. Zepf, and M. C. Kaluza
Intensity scaling limitations of laser-driven proton acceleration in the TNSA-regime
Physical Review Research 4, 013065 (2022)

Abstract: We report on experimental results on laser-driven proton acceleration using high-intensity laser pulses. We present power law scalings of the maximum proton energy with laser pulse energy and show that the scaling exponent 4 strongly depends on the scale length of the preplasma, which is affected by the temporal intensity contrast. At lower laser intensities, a shortening of the scale length leads to a transition from a square root toward a linear scaling. Above a certain threshold, however, a significant deviation from this scaling is observed. Two-dimensional particle-in-cell simulations show that, in this case, the electric field accelerating the ions is generated earlier and has a higher amplitude. However, since the acceleration process starts earlier as well, the fastest protons outrun the region of highest field strength, ultimately rendering the acceleration less effective. Our investigations thus point to a principle limitation of the proton energy in the target normal sheath acceleration regime, which would explain why a significant increase of the maximum proton energy above the limit of 100 MeV has not yet been achieved.

2021

M. Zimmer, S. Scheuren, T. Ebert, G. Schaumann, B. Schmitz, J. Hornung, V. Bagnoud, C. Rödel, and M. Roth
Analysis of laser-proton acceleration experiments for development of empirical scaling laws
Physical Review E 104, 045210 (2021)
Kein Abstract vorhandenLinkBibTeX
H. Abramowicz, U. Acosta, M. Altarelli, R. Aßmann, Z. Bai, T. Behnke, Y. Benhammou, T. Blackburn, S. Boogert, O. Borysov, M. Borysova, R. Brinkmann, M. Bruschi, F. Burkart, K. Büßer, N. Cavanagh, O. Davidi, W. Decking, U. Dosselli, N. Elkina, A. Fedotov, M. Firlej, T. Fiutowski, K. Fleck, M. Gostkin, C. Grojean, J. Hallford, H. Harsh, A. Hartin, B. Heinemann, T. Heinzl, L. Helary, M. Hoffmann, S. Huang, X. Huang, M. Idzik, A. Ilderton, R. Jacobs, B. Kämpfer, B. King, H. Lahno, A. Levanon, A. Levy, I. Levy, J. List, W. Lohmann, T. Ma, A. J. Macleod, V. Malka, F. Meloni, A. Mironov, M. Morandin, J. Moron, E. Negodin, G. Perez, I. Pomerantz, R. Pöschl, R. Prasad, F. Quéré, A. Ringwald, C. Rödel, S. Rykovanov, F. Salgado, A. Santra, G. Sarri, A. Sävert, A. Sbrizzi, S. Schmitt, U. Schramm, S. Schuwalow, D. Seipt, L. Shaimerdenova, M. Shchedrolosiev, M. Skakunov, Y. Soreq, M. Streeter, K. Swientek, N. Hod, S. Tang, T. Teter, D. Thoden, A. I. Titov, O. Tolbanov, G. Torgrimsson, A. Tyazhev, M. Wing, M. Zanetti, A. Zarubin, K. Zeil, M. Zepf, and A. Zhemchukov
Conceptual design report for the LUXE experiment
European Physical Journal Special Topics 230, 2445 (2021)
Kein Abstract vorhandenLinkBibTeX
S. Skruszewicz, S. Fuchs, J. J. Abel, J. Nathanael, J. Reinhard, C. Rödel, F. Wiesner, M. Wuensche, P. Wachulak, A. Bartnik, K. Janulewicz, H. Fiedorowicz, and G. G. Paulus
Coherence tomography with broad bandwidth extreme ultraviolet and soft X-ray radiation
Applied Physics B 127, 55 (2021)

Abstract: We present an overview of recent results on optical coherence tomography with the use of extreme ultraviolet and soft X-ray radiation (XCT). XCT is a cross-sectional imaging method that has emerged as a derivative of optical coherence tomography (OCT). In contrast to OCT, which typically uses near-infrared light, XCT utilizes broad bandwidth extreme ultraviolet (XUV) and soft X-ray (SXR) radiation (Fuchs et al in Sci Rep 6:20658, 2016). As in OCT, XCT\textquotesingle s axial resolution only scales with the coherence length of the light source. Thus, an axial resolution down to the nanometer range can be achieved. This is an improvement of up to three orders of magnitude in comparison to OCT. XCT measures the reflected spectrum in a common-path interferometric setup to retrieve the axial structure of nanometer-sized samples. The technique has been demonstrated with broad bandwidth XUV/SXR radiation from synchrotron facilities and recently with compact laboratory-based laser-driven sources. Axial resolutions down to 2.2 nm have been achieved experimentally. XCT has potential applications in three-dimensional imaging of silicon-based semiconductors, lithography masks, and layered structures like XUV mirrors and solar cells.

F. Wiesner, M. Wünsche, J. Reinhard, J. Abel, J. Nathanael, S. Skruszewicz, C. Rödel, S. Yulin, A. Gawlik, G. Schmidl, U. Huebner, J. Plentz, G. Paulus, and S. Fuchs
Material-specific imaging of nanolayers using extreme ultraviolet coherence tomography
Optica 8, 230 (2021)

Abstract: Scientific and technological progress depend substantially on the ability to image on the nanoscale. In order to investigate complex, functional, nanoscopic structures like, e.g., semiconductor devices, multilayer optics, or stacks of 2D materials, the imaging techniques not only have to provide images but should also provide quantitative information. We report the material-specific characterization of nanoscopic buried structures with extreme ultraviolet coherence tomography. The method is demonstrated at a laser-driven broadband extreme ultraviolet radiation source, based on high-harmonic generation. We show that, besides nanoscopic axial resolution, the spectral reflectivity of all layers in a sample can be obtained using algorithmic phase reconstruction. This provides localized, spectroscopic, material-specific information of the sample. The method can be applied in, e.g., semiconductor production, lithographic mask inspection, or quality control of multilayer fabrication. Moreover, it paves the way for the investigation of ultrafast nanoscopic effects at functional buried interfaces. Published by The Optical Society under the terms of the Creative Commons Attribution 4.0 License.

2020

S. Fuchs, F. Wiesner, M. Wünsche, J. Nathanael, J. Abel, J. Reinhard, C. Rodel, and G. Paulus
Quantitative nanoscale coherence tomography with extreme ultraviolet light
(2020)

Abstract: We present nanoscale coherence tomography (XCT) in the extreme ultraviolet range driven by a high-harmonic generation (HHG) light source. Using a novel phase retrieval algorithm, XCT enables non-destructive, quantitative, cross-sectional imaging, of, e.g., semiconductor devices.

2019

J. Nathanael, M. Wünsche, S. Fuchs, T. Weber, J. Abel, J. Reinhard, F. Wiesner, U. Hübner, S. Skruszewicz, G. Paulus, and C. Rödel
Laboratory setup for extreme ultraviolet coherence tomography driven by a high-harmonic source
Review of Scientific Instruments 90, 113702 (2019)

Abstract: We present a laboratory beamline dedicated to nanoscale subsurface imaging using extreme ultraviolet coherence tomography (XCT). In this setup, broad-bandwidth extreme ultraviolet (XUV) radiation is generated by a laser-driven high-harmonic source. The beamline is able to handle a spectral range of 30-130 eV and a beam divergence of 10 mrad (full width at half maximum). The XUV radiation is focused on the sample under investigation, and the broadband reflectivity is measured using an XUV spectrometer. For the given spectral window, the XCT beamline is particularly suited to investigate silicon-based nanostructured samples. Cross-sectional imaging of layered nanometer-scale samples can be routinely performed using the laboratory-scale XCT beamline. A depth resolution of 16 nm has been achieved using the spectral range of 36-98 eV which represents a 33% increase in resolution due to the broader spectral range compared to previous work.

Y. S. You, J. Lu, E. F. Cunningham, C. Rödel, and S. Ghimire
Crystal orientation-dependent polarization state of high-order harmonics
Optics Letters 44, 530 (2019)

Abstract: We analyze the crystal orientation-dependent polarization state of extreme ultraviolet high-order harmonics from bulk magnesium oxide crystals subjected to intense linearly polarized laser fields. We find that only along high-symmetry directions do high-order harmonics follow the polarization direction of the laser field. In general, there are strong deviations that depend on harmonic order, strength of the laser field, and crystal orientation. We use a real-space electron trajectory picture to understand the origin of polarization deviations. These results have implications in all-optical probing of electronic band structure in momentum space and valence charge distributions in real space, and in producing attosecond pulses with time-dependent polarization in compact setups.

M. Wünsche, S. Fuchs, T. Weber, J. Nathanael, J. Abel, J. Reinhard, F. Wiesner, U. Hübner, S. Skruszewicz, G. Paulus, and C. Rödel
A high resolution extreme ultraviolet spectrometer system optimized for harmonic spectroscopy and XUV beam analysis
Review of Scientific Instruments 90, 023108 (2019)

Abstract: We present a modular extreme ultraviolet (XUV) spectrometer system optimized for a broad spectral range of 12-41 nm (30-99 eV) with a high spectral resolution of lambda/Delta lambda greater than or similar to 784 +/- 89. The spectrometer system has several operation modes for (1) XUV beam inspection, (2) angular spectral analysis, and (3) imaging spectroscopy. These options allow for a versatile use in high harmonic spectroscopy and XUV beam analysis. The high performance of the spectrometer is demonstrated using a novel cross-sectional imaging method called XUV coherence tomography.

F. Wiesner, S. Fuchs, M. Wünsche, J. Nathanael, J. Abel, J. Reinhard, S. Skruszewicz, C. Rödel, A. Gawlik, G. Schmidl, and . others
Label-free quantitative material sensitive tomography with extreme ultraviolet light
(2019)

Abstract: We report on quantitative material-sensitive cross-sectional imaging with nanoscale axial resolution. First experimental results show that in addition to the structural information element-specific identification of buried layers is possible.

S. Fuchs, M. Wünsche, J. Nathanael, J. Abel, J. Reinhard, F. Wiesner, S. Skruszewicz, C. Rödel, and G. Paulus
XUV coherence tomography with nanoscale resolution using one-dimensional phase retrieval
(2019)

Abstract: We present XUV Coherence Tomography (XCT) driven by a high-harmonic generation (HHG) light source. Using a novel one-dimensional phase retrieval algorithm, XCT enables non-destructive, artifact-free, nanoscale, cross-sectional imaging, of, e.g., semiconductor devices.

2018

L. Obst-Huebl, T. Ziegler, F.-E. Brack, J. Branco, M. Bussmann, T. E. Cowan, C. B. Curry, F. Fiuza, M. Garten, M. Gauthier, S. Göde, S. H. Glenzer, A. Huebl, A. Irman, J. B. Kim, T. Kluge, S. D. Kraft, F. Kroll, J. Metzkes-Ng, R. Pausch, I. Prencipe, M. Rehwald, C. Rödel, H.-P. Schlenvoigt, U. Schramm, and K. Zeil
All-optical structuring of laser-driven proton beam profiles
Nature Communications 9, 5292 (2018)

Abstract: Extreme field gradients intrinsic to relativistic laser-interactions with thin solid targets enable compact MeV proton accelerators with unique bunch characteristics. Yet, direct control of the proton beam profile is usually not possible. Here we present a readily applicable all-optical approach to imprint detailed spatial information from the driving laser pulse onto the proton bunch. In a series of experiments, counter-intuitively, the spatial profile of the energetic proton bunch was found to exhibit identical structures as the fraction of the laser pulse passing around a target of limited size. Such information transfer between the laser pulse and the naturally delayed proton bunch is attributed to the formation of quasi-static electric fields in the beam path by ionization of residual gas. Essentially acting as a programmable memory, these fields provide access to a higher level of proton beam manipulation.

M. Cerchez, M. Swantusch, M. Toncian, X. M. Zhu, R. Prasad, T. Toncian, Ch. Rödel, O. Jäckel, G. G. Paulus, A. A. Andreev, and O. Willi
Enhanced energy absorption of high intensity laser pulses by targets of modulated surface
Applied Physics Letters 112, 221103 (2018)

Abstract: Investigations of energy transfer of high intensity (I=5x10^19 W/cm2), ultrashort (<30 fs) Ti:Sa laser pulses to solid targets with a randomly rough surface have been performed. We investigated the influence of the target surface morphology on the efficiency of energy transfer of p-polarized laser pulses characterized by a very high contrast. Targets with a roughness r larger than 20% of the laser wavelength proved to absorb a remarkably large fraction of energy reaching up to 70%, almost independent of the incidence angle. Numerical simulations of various interaction conditionsare in agreement with the experimental data and confirm the effect of the target morphology and its surface parameters on the enhanced energy absorbed fraction.

L. Obst, J. Metzkes-Ng, S. Bock, G. E. Cochran, T. E. Cowan, T. Oksenhendler, P. L. Poole, I. Prencipe, M. Rehwald, C. Rödel, H.-P. Schlenvoigt, U. Schramm, D. W. Schumacher, T. Ziegler, and K. Zeil
On-shot characterization of single plasma mirror temporal contrast improvement
Plasma Physics and Controlled Fusion 60, 054007 (2018)

Abstract: We report on the setup and commissioning of a compact recollimating single plasma mirror (PM) for temporal contrast enhancement at the Draco 150 TW laser during laser-proton acceleration experiments. The temporal contrast with and without PM is characterized single-shot by means of self-referenced spectral interferometry with extended time excursion at unprecedented dynamic and temporal range. This allows for the first single-shot measurement of the PM trigger point, which is interesting for the quantitative investigation of the complex pre-plasma formation process at the surface of the target used for proton acceleration. As a demonstration of high contrast laser plasma interaction we present proton acceleration results with ultra-thin liquid crystal targets of ~ 1 μm down to 10 nm thickness. Focus scans of different target thicknesses show that highest proton energies are reached for the thinnest targets at best focus. This indicates that the contrast enhancement is effective such that the acceleration process is not limited by target pre-expansion induced by laser light preceding the main laser pulse.

U. Zastrau, C. Rödel, M. Nakatsutsumi, T. Feigl, K. Appel, B. Chen, T. Döppner, T. Fennel, T. Fiedler, L. B. Fletcher, E. Förster, E. Gamboa, D. O. Gericke, S. Göde, C. Grote-Fortmann, V. Hilbert, L. Kazak, T. Laarmann, H. J. Lee, P. Mabey, F. Martinez, K.-H. Meiwes-Broer, H. Pauer, M. Perske, A. Przystawik, S. Roling, S. Skruszewicz, M. Shihab, J. Tiggesbäumker, S. Toleikis, M. Wünsche, H. Zacharias, S. H. Glenzer, and G. Gregori
A sensitive EUV Schwarzschild microscope for plasma studies with sub-micrometer resolution
Review of Scientific Instruments 89, 023703 (2018)

Abstract: We present an extreme ultraviolet (EUV) microscope using a Schwarzschild objective which is optimized for single-shot sub-micrometer imaging of laser-plasma targets. The microscope has been designed and constructed for imaging the scattering from an EUV-heated solid-density hydrogen jet. Imaging of a cryogenic hydrogen target was demonstrated using single pulses of the free-electron laser in Hamburg (FLASH) free-electron laser at a wavelength of 13.5 nm. In a single exposure, we observe a hydrogen jet with ice fragments with a spatial resolution in the sub-micrometer range. In situ EUV imaging is expected to enable novel experimental capabilities for warm dense matter studies of micrometer-sized samples in laser-plasma experiments.

2017

S. Fuchs, M. Wünsche, J. Nathanael, J. J. Abel, C. Rödel, J. Biedermann, J. Reinhard, U. Hübner, and G. G. Paulus
Optical coherence tomography with nanoscale axial resolution using a laser-driven high-harmonic source
Optica 4, 903 (2017)

Abstract: Extreme ultraviolet microscopy is technologically demanding and thus largely confined to synchrotron radiation facilities. However, specific benefits like high resolution and exceptional material contrast provide strong motivation for the development of table-top alternatives. We report on the first demonstration of coherence tomography, i.e., noninvasive cross-sectional imaging, with high harmonics. A depth resolution of 24 nm and very good material contrast are achieved. Excessively demanding optics for extreme ultraviolet radiation are avoided and artifacts due to the elementary geometry are suppressed with a novel three-step one-dimensional phase-retrieval algorithm. The images are recorded in reflection geometry, facilitating the analysis of, e.g., operating semiconductor samples.

M. Wünsche, S. Fuchs, S. Aull, J. Nathanael, M. Möller, C. Rödel, and G. G. Paulus
Quasi-supercontinuum source in the extreme ultraviolet using multiple frequency combs from high-harmonic generation
Optics Express 25, 6936 (2017)

Abstract: A quasi-supercontinuum source in the extreme ultraviolet (XUV) is demonstrated using a table-top femtosecond laser and a tunable optical parametric amplifier (OPA) as a driver for high-harmonic generation (HHG). The harmonic radiation, which is usually a comb of odd multiples of the fundamental frequency, is generated by near-infrared (NIR) laser pulses from the OPA. A quasi-continuous XUV spectrum in the range of 30 to 100 eV is realized by averaging over multiple harmonic comb spectra with slightly different fundamental frequencies and thus different spectral spacing between the individual harmonics. The driving laser wavelength is swept automatically during an averaging time period. With a total photon flux of 4×10^9 photons/s in the range of 30 eV to 100 eV and 1×10^7photons/s in the range of 100 eV to 200 eV, the resulting quasi-supercontinuum XUV source is suited for applications such as XUV coherence tomography (XCT) or near-edge absorption fine structure spectroscopy (NEXAFS).

2016

S. Kuschel, D. Hollatz, T. Heinemann, O. Karger, M. B. Schwab, D. Ullmann, A. Knetsch, A. Seidel, C. Rödel, M. Yeung, M. Leier, A. Blinne, H. Ding, T. Kurz, D. J. Corvan, A. Sävert, S. Karsch, M. C. Kaluza, B. Hidding, and M. Zepf
Demonstration of passive plasma lensing of a laser wakefield accelerated electron bunch
Physical Review Accelerators and Beams 19, 071301 (2016)

Abstract: We report on the first demonstration of passive all-optical plasma lensing using a two-stage setup. An intense femtosecond laser accelerates electrons in a laser wakefield accelerator (LWFA) to 100 MeV over millimeter length scales. By adding a second gas target behind the initial LWFA stage we introduce a robust and independently tunable plasma lens. We observe a density dependent reduction of the LWFA electron beam divergence from an initial value of 2.3 mrad, down to 1.4 mrad (rms), when the plasma lens is in operation. Such a plasma lens provides a simple and compact approach for divergence reduction well matched to the mm-scale length of the LWFA accelerator. The focusing forces are provided solely by the plasma and driven by the bunch itself only, making this a highly useful and conceptually new approach to electron beam focusing. Possible applications of this lens are not limited to laser plasma accelerators. Since no active driver is needed the passive plasma lens is also suited for high repetition rate focusing of electron bunches. Its understanding is also required for modeling the evolution of the driving particle bunch in particle driven wake field acceleration.

S. Fuchs, C. Rödel, A. Blinne, U. Zastrau, M. Wünsche, V. Hilbert, L. Glaser, J. Viefhaus, E. Frumker, P. Corkum, E. Förster, and G. G. Paulus
Nanometer resolution optical coherence tomography using broad bandwidth XUV and soft x-ray radiation
Scientific Reports 6, 20658 (2016)

Abstract: Optical coherence tomography (OCT) is a non-invasive technique for cross-sectional imaging. It is particularly advantageous for applications where conventional microscopy is not able to image deeper layers of samples in a reasonable time, e.g. in fast moving, deeper lying structures. However, at infrared and optical wavelengths, which are commonly used, the axial resolution of OCT is limited to about 1 μm, even if the bandwidth of the light covers a wide spectral range. Here, we present extreme ultraviolet coherence tomography (XCT) and thus introduce a new technique for non-invasive cross-sectional imaging of nanometer structures. XCT exploits the nanometerscale coherence lengths corresponding to the spectral transmission windows of, e.g., silicon samples. The axial resolution of coherence tomography is thus improved from micrometers to a few nanometers. Tomographic imaging with an axial resolution better than 18 nm is demonstrated for layer-type nanostructures buried in a silicon substrate. Using wavelengths in the water transmission window, nanometer-scale layers of platinum are retrieved with a resolution better than 8 nm. XCT as a nondestructive method for sub-surface tomographic imaging holds promise for several applications in semiconductor metrology and imaging in the water window.

2015

M. Yeung, J. Bierbach, E. Eckner, S. Rykovanov, S. Kuschel, A. Sävert, M. Förster, C. Rödel, G. Paulus, S. Cousens, M. Coughlan, B. Dromey, and M. Zepf
Noncollinear Polarization Gating of Attosecond Pulse Trains in the Relativistic Regime
Physical Review Letters 115, 193903 (2015)

Abstract: High order harmonics generated at relativistic intensities have long been recognized as a route to the most powerful extreme ultraviolet pulses. Reliably generating isolated attosecond pulses requires gating to only a single dominant optical cycle, but techniques developed for lower power lasers have not been readily transferable. We present a novel method to temporally gate attosecond pulse trains by combining noncollinear and polarization gating. This scheme uses a split beam configuration which allows pulse gating to be implemented at the high beam fluence typical of multi-TW to PW class laser systems. Scalings for the gate width demonstrate that isolated attosecond pulses are possible even for modest pulse durations achievable for existing and planned future ultrashort high-power laser systems. Experimental results demonstrating the spectral effects of temporal gating on harmonic spectra generated by a relativistic laser plasma interaction are shown.

T. Hahn, J. Bierbach, C. Rödel, D. Hemmers, M. Yeung, B. Dromey, S. Fuchs, A. Galestian, S. Kuschel, M. Zepf, G. Paulus, and G. Pretzler
Broadband XUV polarimetry of high harmonics from plasma surfaces using multiple Fresnel reflections
Applied Physics B 118, 241 (2015)

Abstract: High-harmonic generation (HHG) by nonlinear interaction of intense laser pulses with gases or plasma surfaces is the most prominent way of creating highly coherent extreme ultraviolet (EUV/XUV) pulses. In the last years, several scientific applications have been found which require the measurement of the polarization of the harmonic radiation. We present a broadband XUV polarimeter based on multiple Fresnel reflections providing an extinction rate of 5–25 for 17–45 nm which is particularly suited for surface harmonics. The device has first been tested at a gas harmonic source providing linearly polarized XUV radiation. In a further experiment using HHG from plasma surfaces, the XUV polarimeter allowed a polarization measurement of high harmonic radiation from plasma surfaces for the first time which reveals a linear polarization state as predicted for our generation parameters. The generation and control of intense polarized XUV pulses - together with the availability of broadband polarizers in the XUV - open the way for a series of new experiments. For instance, dichroism in the XUV, elliptically polarized harmonics from aligned molecules, or the selection rules of relativistic surface harmonics can be studied with the broadband XUV polarimeter.

B. Aurand, S. Kuschel, C. Rödel, O. Jäckel, J. Polz, B. Elkin, H. Zhao, A. Karmakar, P. Gibbon, M. Kaluza, and T. Kühl
Reduction of X-ray generation in high-intensity laser ion acceleration
Applied Physics B 118, 247 (2015)

Abstract: In this paper, we report on measurements of bremsstrahlung in laser ion acceleration experiments from ultra-thin, polymer-based target foils. The influence of laser polarization on the generated γ radiation, the maximum achievable proton energy and the total proton number is investigated. A clear benefit in terms of γ radiation reduction by the use of circular polarized light can be observed. At the same time, the total number of accelerated protons was increased.

2014

L. Senje, M. Yeung, B. Aurand, S. Kuschel, C. Rödel, F. Wagner, K. Li, B. Dromey, V. Bagnoud, P. Neumayer, M. Roth, C.-G. Wahlström, M. Zepf, T. Kuehl, and D. Jung
Diagnostics for studies of novel laser ion acceleration mechanisms
Review of Scientific Instruments 85, 113302 (2014)

Abstract: Diagnostic for investigating and distinguishing different laser ion acceleration mechanisms has been developed and successfully tested. An ion separation wide angle spectrometer can simultaneously investigate three important aspects of the laser plasma interaction: (1) acquire angularly resolved energy spectra for two ion species, (2) obtain ion energy spectra for multiple species, separated according to their charge to mass ratio, along selected axes, and (3) collect laser radiation reflected from and transmitted through the target and propagating in the same direction as the ion beam. Thus, the presented diagnostic constitutes a highly adaptable tool for accurately studying novel acceleration mechanisms in terms of their angular energy distribution, conversion efficiency, and plasma density evolution.

V. Hilbert, C. Rödel, G. Brenner, T. Döppner, S. Düsterer, S. Dziarzhytski, L. Fletcher, E. Förster, S. H. Glenzer, M. Harmand, N. J. Hartley, L. Kazak, D. Komar, T. Laarmann, H. J. Lee, T. Ma, M. Nakatsutsumi, A. Przystawik, H. Redlin, S. Skruszewicz, P. Sperling, J. Tiggesbäumker, S. Toleikis, and U. Zastrau
Spatio-temporal coherence of free-electron laser radiation in the extreme ultraviolet determined by a Michelson interferometer
Applied Physics Letters 105, 101102 (2014)

Abstract: A key feature of extreme ultraviolet (XUV) radiation from free-electron lasers (FELs) is its spatial and temporal coherence. We measured the spatio-temporal coherence properties of monochromatized FEL pulses at 13.5 nm using a Michelson interferometer. A temporal coherence time of (59±8) fs has been determined, which is in good agreement with the spectral bandwidth given by the monochromator. Moreover, the spatial coherence in vertical direction amounts to about 15% of the beam diameter and about 12% in horizontal direction. The feasibility of measuring spatio-temporal coherence properties of XUV FEL radiation using interferometric techniques advances machine operation and experimental studies significantly.

B. Aurand, S. Kuschel, O. Jäckel, C. Rödel, H. Zhao, S. Herzer, A. Paz, J. Bierbach, J. Polz, B. Elkin, A. Karmakar, P. Gibbon, M. C. Kaluza, and T. Kühl
Enhanced radiation pressure-assisted acceleration by temporally tuned counter-propagating pulses
Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment 740, 83 (2014)

Abstract: Within the last decade, laser-ion acceleration has become a field of broad interest. The possibility to generate short proton- or heavy ion bunches with an energy of a few tens of MeV by table-top laser systems could open new opportunities for medical or technical applications. Nevertheless, today's laser-acceleration schemes lead mainly to a temperature-like energy distribution of the accelerated ions, a big disadvantage compared to mono-energetic beams from conventional accelerators. Recent results [1] of laser-ion acceleration using radiation-pressure appear promising to overcome this drawback. In this paper, we demonstrate the influence of a second counter-propagating laser pulse interacting with a nm-thick target, creating a well defined pre-plasma.

B. Aurand, B. Elkin, L.-O. Heim, B. Lommel, B. Kindler, M. Tomut, C. Rödel, S. Kuschel, O. Jäckel, and T. Kühl
Ultra-thin polymer foils for laser-ion acceleration
Journal of Radioanalytical and Nuclear Chemistry 299, 965 (2014)

Abstract: We report on the development of new materials for laser-ion acceleration applicable for the advanced mechanism of radiation-pressure-acceleration. These targets are ultra-thin with thicknesses of just a few nm. For several years, diamond-like carbon foils in this thickness range can be produced. An alternative material containing more than one ion-species has the potential to further improve the acceleration mechanism. The fabrication and characterization of self-supporting polymer-based targets will be described in this paper. Furthermore, we show the significant influence on a radiation-pressure induced acceleration process by experimental data.

C. Rödel
Synthese von extrem ultravioletter Strahlung an relativistischen Plasmaoberflächen
Dissertation
Friedrich-Schiller-Universität Jena, Physikalisch-Astronomische Fakultät (2014)

Abstract: In this thesis, high harmonic radiation is studied which is generated by the relativistic interaction of intense laser pulses with dense plasma surfaces. Laser plasma simulations are performed by the author and by colleagues from the University of Dusseldorf for interpreting the experimental results. At first glance, these simulations predict such a high generation efficiency of harmonics from relativistically oscillating mirrors (ROM) that they have been considered as the next generation attosecond light source for the last 15 years. The objective of this thesis is the spectral characterization of the harmonics' efficiency and the ROM process utilizing calibrated XUV diagnostics. The first step, that has been pursued in the thesis work, is the generation of ROM harmonics at the terawatt laser systems JETI and ARCTURUS operated by the University of Jena and the University of Düsseldorf, respectively.
According to the wide-spread belief, the efficient generation of ROM harmonics requires extremely short plasma density gradients which calls for high intensity laser pulses with excellent temporal contrast. For this reason, a plasma mirror system has been installed at both laser systems to improve the pulse contrast by two or three orders of magnitude depending of the target material. In experiments using contrast-enhanced laser pulses, a stable emission of ROM harmonics was observed. However, the highest yield has been measured for the intermediate pulse contrast which results in a plasma scale length L^ROM_P=lambda/5. Surprisingly, the overall efficiency of ROM harmonics decreases for shorter scale lengths < lambda/10 or high contrast, respectively. A strong signal of ROM harmonics could even be measured - indeed unstable - without any contrast improvement. Laser plasma simulations confirm the experimental observation of an optimum plasma scale length L^ROM_P=lambda/5. Two effects have been identified which lead to the reduction of the ROM harmonics' yield for short plasma scale lengths: First, the laser field at the plasma surface is reduced for very short plasma scale lengths. Second, the oscillating electron plasma at the plasma surface is held back by strong electrostatic fields due to the immobile ion background for short plasma density gradients. As a conclusion, the use of an intermediate plasma density gradient for generating ROM harmonics with highest efficiency has to be considered as a paradigm shift in this research field since previous work has called for the highest possible pulse contrast or the shortest plasma scale length, respectively, in order to generate ROM harmonics at all.
Using the optimized plasma scale length, a significant modulation and broadening of the ROM harmonic lines has been observed which is unfavorable for most of the potential applications of ROM harmonics. Laser plasma simulations reproduce the fine structure of the harmonic lines. They further reveal an unequally spaced attosecond pulse train and a positive chirp of the harmonics which is associated with the line broadening. This positive chirp is characteristic for ROM harmonics generated at expanded plasma density profiles and is explained by a temporal denting of the plasma surface due to radiation pressure. It is shown by simulations and experiments that the harmonics' linewidth can be minimized when the harmonics' chirp is compensated by chirped driving laser pulses.
For optimized preplasma conditions, the efficiency of the ROM harmonics was measured to be 10^-4 at 40nm and 10^-6 at 20nm per harmonic order and falls short of expectations nurtured by 1D PIC-simulations and plasma theory. Having a pulse energy in the order of a µJ per harmonic order, ROM harmonics are indeed suited, e. g., for seeding XUV free-electron lasers or coherent diffraction imaging. However, the efficiency of ROM harmonics of 10^-4 at 40nm is comparable to that of high harmonic generation in gaseous media which is state-of-the-art and technologically much less demanding. Considering the present results of the ROM harmonics' efficiency, the high expectations of a highly-efficient, next-generation attosecond source have not been met yet. The reason for the rather low efficiency of ROM harmonics has been investigated by means of 2D simulations. These simulations reveal surface plasma waves which can be generated in addition to the ROM oscillation and lead to a reduced harmonic emisson in the direction of reflection. Surface plasma waves could thus be responsible for the low efficiency of ROM harmonics measured in the experiments. The simulations suggest that shorter pulses with few-cycle pulse duration should be used in the future for a more efficient generation since surface plasma waves can not be built up at these time scales.
A prerequisite for most of the potential applications of ROM harmonics is the generation with a high repetition rate. Using fast-rotating targets and frequency-doubled laser pulses surface harmonics have been generated with the 10-Hz repetition rate of the JETI laser system. Due to the frequency-doubling process the pulse contrast is enhanced by several orders of magnitude such that extremely short plasma density gradients are obtained. Surprisingly, an effect was discovered which was not predicted by theory so far: The high harmonic spectra show a significant enhancement of particular harmonic orders located at twice the maximum plasma frequency 2 omega_P or 2 omega_P +- 2 omega_L. By using targets of different density we were able to tune the enhancement in a certain frequency range in the XUV. Moreover, the efficiency of the amplified harmonics is even higher than the one which is measured for the optimized plasma scale length. Laser plasma simulations confirm then experimental results and reveal the origin of the enhancement: The plasma surface oscillates relativistically with the laser frequency omega_L and the plasma frequency omega_P. The enhanced harmonics are due to a ROM-like oscillation at omega_P. A simple model based on the ROM model can explain the enhanced harmonics as a frequency-mixing process which utilizes the relativistic nonlinearity induced by retardation. This relativistic frequency synthesis at plasma surfaces can be regarded as a new regime of nonlinear optics in the XUV which employs plasma frequencies of dense surface plasmas in the order of several PHz.
At the end of the thesis, two selected applications of ROM harmonics are discussed: The generation of intense attosecond pulses by the ROM process would enable XUV-XUV pump-probe experiments providing attosecond time resolution. However, such experiments would require the determination of the attosecond time structure of ROM harmonics first. An apparatus has been constructed which allows the measurement of an attosecond pulse train by using a nonlinear autocorrelation technique. The second potential application of ROM harmonics is a non-invasive cross-sectional imaging technique which has been developed during the thesis work. This method provides a depth resolution of a few nanometers and employs broad-bandwidth XUV or soft x-ray radiation. ROM harmonics with a nearly continuous spectrum could be a suitable radiation source for this application of technical and industrial relevance.

2013

B. Aurand, B. Elkin, L.-O. Heim, B. Lommel, B. Kindler, M. Tomut, C. Rödel, S. Kuschel, O. Jäckel, J. Barz, and T. Kühl
Preparation and characterization of nanometer-thin freestanding polymer foils for laser-ion acceleration
Journal of Polymer Science Part B: Polymer Physics 51, 1355 (2013)

Abstract: We report on the production and characterization of polymer-based ultra-thin (sub 10 nm) foils suited for experiments on laser-ion acceleration in the regime of radiation pressure acceleration. Beside the remarkable mechanical stability compared with commonly used diamond-like-carbon foils, a very homogeneous layer thickness and a small surface roughness have been achieved. We describe the technical issues of the production process as well as detailed studies of the mechanical stability and surface roughness tests. The capability of producing uniform targets of large area is essential for advanced laser-ion acceleration projects which are dealing with high repetition rate and extended measurement series, but might also be useful for other applications which require ultra-thin and freestanding substrates of high quality.

V. Hilbert, A. Blinne, S. Fuchs, T. Feigl, T. Kämpfer, C. Rödel, I. Uschmann, M. Wünsche, G. Paulus, E. Förster, and U. Zastrau
An extreme ultraviolet Michelson interferometer for experiments at free-electron lasers
Review of Scientific Instruments 84, 095111 (2013)

Abstract: We present a Michelson interferometer for 13.5 nm soft x-ray radiation. It is characterized in a proof-of-principle experiment using synchrotron radiation, where the temporal coherence is measured to be 13 fs. The curvature of the thin-film beam splitter membrane is derived from the observed fringe pattern. The applicability of this Michelson interferometer at intense free-electron lasers is investigated, particularly with respect to radiation damage. This study highlights the potential role of such Michelson interferometers in solid density plasma investigations using, for instance, extreme soft x-ray free-electron lasers. A setup using the Michelson interferometer for pseudo-Nomarski-interferometry is proposed.

B. Aurand, S. Kuschel, O. Jäckel, C. Rödel, H. Y. Zhao, S. Herzer, A. E. Paz, J. Bierbach, J. Polz, B. Elkin, G. G. Paulus, A. Karmakar, P. Gibbon, T. Kühl, and M. C. Kaluza
Radiation pressure-assisted acceleration of ions using multi-component foils in high-intensity laser-matter interactions
New Journal of Physics 15, 033031 (2013)

Abstract: Experimental results on the acceleration of protons and carbon ions from ultra-thin polymer foils at intensities of up to 6 × 10^(19) W cm^(−2) are presented revealing quasi-monoenergetic spectral characteristics for different ion species at the same time. For carbon ions and protons, a linear correlation between the cutoff energy and the peak energy is observed when the laser intensity is increased. Particle-in-cell simulations supporting the experimental results imply an ion acceleration mechanism driven by the radiation pressure as predicted for multi-component foils at these intensities.

M. Yeung, B. Dromey, C. Rödel, J. Bierbach, M. Wünsche, G. Paulus, T. Hahn, D. Hemmers, C. Stelzmann, G. Pretzler, and M. Zepf
Near-monochromatic high-harmonic radiation from relativistic laser-plasma interactions with blazed grating surfaces
New Journal of Physics 15, 025042 (2013)

Abstract: Intense, femtosecond laser interactions with blazed grating targets are studied through experiment and particle-in-cell (PIC) simulations. The high harmonic spectrum produced by the laser is angularly dispersed by the grating leading to near-monochromatic spectra emitted at different angles, each dominated by a single harmonic and its integer-multiples. The spectrum emitted in the direction of the third-harmonic diffraction order is measured to contain distinct peaks at the 9th and 12th harmonics which agree well with two-dimensional PIC simulations using the same grating geometry. This confirms that surface smoothing effects do not dominate the far-field distributions for surface features with sizes on the order of the grating grooves whilst also showing this to be a viable method of producing near-monochromatic, short-pulsed extreme-ultraviolet radiation.

S. Fuchs, C. Rödel, M. Krebs, S. Hädrich, J. Bierbach, A. E. Paz, S. Kuschel, M. Wünsche, V. Hilbert, U. Zastrau, E. Förster, J. Limpert, and G. G. Paulus
Sensitivity calibration of an imaging extreme ultraviolet spectrometer-detector system for determining the efficiency of broadband extreme ultraviolet sources
Review of Scientific Instruments 84, 023101 (2013)

Abstract: We report on the absolute sensitivity calibration of an extreme ultraviolet (XUV) spectrometer system that is frequently employed to study emission from short-pulse laser experiments. The XUV spectrometer, consisting of a toroidal mirror and a transmission grating, was characterized at a synchrotron source in respect of the ratio of the detected to the incident photon flux at photon energies ranging from 15.5 eV to 99 eV. The absolute calibration allows the determination of the XUV photon number emitted by laser-based XUV sources, e.g., high-harmonic generation from plasma surfaces or in gaseous media. We have demonstrated high-harmonic generation in gases and plasma surfaces providing 2.3 μW and μJ per harmonic using the respective generation mechanisms.

2012

A. Paz, S. Kuschel, C. Rödel, M. Schnell, O. Jäckel, M. C. Kaluza, and G. G. Paulus
Thomson backscattering from laser-generated, relativistically moving high-density electron layers
New Journal of Physics 14, 093018 (2012)

Abstract: We show experimentally that extreme ultraviolet radiation is produced when a laser pulse is Thomson backscattered from sheets of relativistic electrons that are formed at the rear surface of a foil irradiated on its front side with a high-intensity laser. An all-optical setup is realized using the Jena titanium:sapphire ten-terawatt laser system with an enhanced amplified spontaneous emission background of 10^{−12}. The main pulse is split into two: one of them accelerates electrons from thin aluminium foil targets to energies of the order of some MeV and the other, counterpropagating probe pulse Thomson-backscatters off these electrons when they exit the target rear side. The process produced photons within a wide spectral range of some tens of eV as a result of the broad electron energy distribution. The highest scattering intensity is observed when the probe pulse arrives at the target rear surface 100 fs after irradiation of the target front side by the pump pulse, corresponding to the maximum flux of hot electrons at the interaction region. These results can provide time-resolved information about the evolution of the rear-surface electron sheath and hence about the dynamics of the electric fields responsible for the acceleration of ions from the rear surface of thin, laser-irradiated foils.

C. Rödel, D. an der Brügge, J. Bierbach, M. Yeung, T. Hahn, B. Dromey, S. Herzer, S. Fuchs, A. Pour, E. Eckner, M. Behmke, M. Cerchez, O. Jäckel, D. Hemmers, T. Toncian, M. C. Kaluza, A. Belyanin, G. Pretzler, O. Willi, A. Pukhov, M. Zepf, and G. G. Paulus
Harmonic Generation from Relativistic Plasma Surfaces in Ultrasteep Plasma Density Gradients
Physical Review Letters 109, 125002 (2012)

Abstract: Harmonic generation in the limit of ultrasteep density gradients is studied experimentally. Observations reveal that, while the efficient generation of high order harmonics from relativistic surfaces requires steep plasma density scale lengths (L_p/λ < 1), the absolute efficiency of the harmonics declines for the steepest plasma density scale length L_p → 0, thus demonstrating that near-steplike density gradients can be achieved for interactions using high-contrast high-intensity laser pulses. Absolute photon yields are obtained using a calibrated detection system. The efficiency of harmonics reflected from the laser driven plasma surface via the relativistic oscillating mirror was estimated to be in the range of 10^{-4} – 10^{-6} of the laser pulse energy for photon energies ranging from 20 – 40 eV, with the best results being obtained for an intermediate density scale length.

J. Bierbach, C. Rödel, M. Yeung, B. Dromey, T. Hahn, A. Pour, S. Fuchs, A. E. Paz, S. Herzer, S. Kuschel, O. Jäckel, M. C. Kaluza, G. Pretzler, M. Zepf, and G. G. Paulus
Generation of 10 µW relativistic surface high-harmonic radiation at a repetition rate of 10 Hz
New Journal of Physics 14, 065005 (2012)

Abstract: Experimental results on relativistic surface HHG at a repetition rate of 10 Hz are presented. Average powers in the 10 μW range are generated in the spectral range of 51 to 26 nm (24 - 48 eV). The surface harmonic radiation is produced by focusing the second-harmonic of a high-power laser onto a rotating glass surface to moderately relativistic intensities of 3 × 10^{19} W cm^{−2}. The harmonic emission exhibits a divergence of 26 mrad. Together with absolute photon numbers recorded by a calibrated spectrometer, this allows for the determination of the extreme ultraviolet (XUV) yield. The pulse energies of individual harmonics are reaching up to the μJ level, equivalent to an efficiency of 10^{−5}. The capability of producing stable and intense high-harmonic radiation from relativistic surface plasmas may facilitate experiments on nonlinear ionization or the seeding of free-electron lasers.

Ph. A. Korneev, S. V. Popruzhenko, S. P. Goreslavski, T.-M. Yan, D. Bauer, W. Becker, M. Kübel, M. F. Kling, C. Rödel, M. Wünsche, and G. G. Paulus
Interference Carpets in Above-Threshold Ionization: From the Coulomb-Free to the Coulomb-Dominated Regime
Physical Review Letters 108, 223601 (2012)

Abstract: The velocity map recorded in above-threshold ionization of xenon at 800 nm exhibits a distinct carpetlike pattern of maxima and minima for emission in the direction approximately perpendicular to the laser polarization. The pattern is well reproduced by a numerical solution of the time-dependent Schrödinger equation. In terms of the simple-man model and the strong-field approximation, it is explained by the constructive and destructive interference of the contribution of the long and the short orbit. Strictly perpendicular emission is caused by ionization at the two peaks of the laser field per cycle, which results in a 2ℏω separation of the above-threshold ionization rings.

S. Fuchs, A. Blinne, C. Rödel, U. Zastrau, V. Hilbert, M. Wünsche, J. Bierbach, E. Frumker, E. Förster, and G. G. Paulus
Optical coherence tomography using broad-bandwidth XUV and soft X-ray radiation
Applied Physics B 106, 789 (2012)

Abstract: We present a novel approach to extend optical coherence tomography (OCT) to the extreme ultraviolet (XUV) and soft X-ray (SXR) spectral range. With a simple setup based on Fourier-domain OCT and adapted for the application of XUV and SXR broadband radiation, cross-sectional images of semiconductors and organic samples becomes feasible with current synchrotron or laser-plasma sources. For this purpose, broadband XUV radiation is focused onto the sample surface, and the reflected spectrum is recorded by an XUV spectrometer. The proposed method has the particular advantage that the axial spatial resolution only depends on the spectral bandwidth. As a consequence, the theoretical resolution limit of XUV coherence tomography (XCT) is in the order of nanometers, e.g., 3 nm for wavelengths in the water window (280 - 530 eV). We proved the concept of XCT by calculating the reflectivity of one-dimensional silicon and boron carbide samples containing buried layers and found the expected properties with respect to resolution and penetration depth confirmed.

B. Aurand, C. Rödel, H. Zhao, S. Kuschel, M. Wünsche, O. Jäckel, M. Heyer, F. Wunderlich, M. C. Kaluza, G. G. Paulus, and T. Kühl
Note: A large aperture four-mirror reflective wave-plate for high-intensity short-pulse laser experiments
Review of Scientific Instruments 83, 036104 (2012)

Abstract: We report on a four-mirror reflective wave-plate system based on a phase-shifting mirror (PSM) for a continuous variation of elliptical polarization without changing the beam position and direction. The system presented and characterized here can replace a conventional retardation plate providing all advantages of a PSM, such as high damage-threshold, large scalability, and low dispersion. This makes reflective wave-plates an ideal tool for ultra-high power laser applications.

2011

B. Aurand, S. Kuschel, C. Rödel, M. Heyer, F. Wunderlich, O. Jäckel, M. C. Kaluza, G. G. Paulus, and T. Kühl
Creating circularly polarized light with a phase-shifting mirror
Optics Express 19, 17151 (2011)

Abstract: We report on the performance of a system employing a multi-layer coated mirror creating circularly polarized light in a fully reflective setup. With one specially designed mirror we are able to create laser pulses with an ellipticity of more than ε = 98% over the entire spectral bandwidth from initially linearly polarized Titanium:Sapphire femtosecond laser pulses. We tested the homogeneity of the polarization with beam sizes of the order of approximately 10 cm. The damage threshold was determined to be nearly 400 times higher than for a transmissive quartz-wave plate which suggests applications in high intensity laser experiments. Another advantage of the reflective scheme is the absence of nonlinear effects changing the spectrum or the pulse-form and the scalability of coating fabrication to large aperture mirrors.

C. Rödel, M. Heyer, M. Behmke, M. Kübel, O. Jäckel, W. Ziegler, D. Ehrt, M. C. Kaluza, and G. G. Paulus
High repetition rate plasma mirror for temporal contrast enhancement of terawatt femtosecond laser pulses by three orders of magnitude
Applied Physics B 103, 295 (2011)

Abstract: We present a plasma mirror configuration that improves the temporal pulse contrast of femtosecond terawatt laser pulses by a factor of thousand using a single antireflection coated glass target. The device provides ultra-high contrast for experiments with a maximum repetition rate of 10 Hz. A third-order cross-correlator has been used to measure the temporal pulse contrast for several different plasma mirror targets. It is shown that the ASE can be suppressed to a level of 10^(−11.) A comparison between a triggered and an untriggered plasma mirror reveals differences in the intensity distribution of the focused beam. The triggered plasma mirror produces a slightly larger focus due to the expansion of the triggered plasma mirror at -3 ps before the main pulse. We propose a cost-effective AR-coated and a blank glass target to reduce the costs of the consumable target material. High-harmonic radiation on solid surfaces has been generated with different plasma mirror targets to demonstrate the high laser contrast.

M. Behmke, D. an der Bruegge, C. Rödel, M. Cerchez, D. Hemmers, M. Heyer, O. Jäckel, M. Kübel, G. G. Paulus, G. Pretzler, A. Pukhov, M. Toncian, T. Toncian, and O. Willi
Controlling the Spacing of Attosecond Pulse Trains from Relativistic Surface Plasmas
Physical Review Letters 106, 185002 (2011)

Abstract: When a laser pulse hits a solid surface with relativistic intensities, XUV attosecond pulses are generated in the reflected light. We present an experimental and theoretical study of the temporal properties of attosecond pulse trains in this regime. The recorded harmonic spectra show distinct fine structures which can be explained by a varying temporal pulse spacing that can be controlled by the laser contrast. The pulse spacing is directly related to the cycle-averaged motion of the reflecting surface. Thus the harmonic spectrum contains information on the relativistic plasma dynamics.