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Publications by
Prof. Dr. Christian Spielmann

All publications of HI Jena


Ş. Zähter, O. N. Rosmej, P. Beloiu, A. Bogdanov, A. Golubev, M. Gyrdymov, J. Jacoby, A. Kantsyrev, R. Loetzsch, M. Nicolai, V. Panyushkin, A. Skobliakov, P. M. Tavana, I. Uschmann, N. Zahn, and C. Spielmann
Monitoring of the heavy-ion beam distribution using poly- and monochromatic x-ray fluorescence imaging
Review of Scientific Instruments 93, 113301 (2022)

Abstract: In this work, the first proof of the principal of an in situ diagnostics of the heavy-ion beam intensity distribution in irradiation of solid targets is proposed. In this scheme, x-ray fluorescence that occurs in the interaction of heavy-ions with target atoms is used for imaging purposes. The x-ray conversion to optical radiation and a transport-system was developed, and its first test was performed in experiments at the Universal Linear Accelerator in Darmstadt, Germany. The Au-beam intensity distribution on thin foils and Cu-mesh targets was imaged using multiple x-ray pinholes (polychromatic imaging) and 2D monochromatic imaging of Cu Kα radiation by using a toroidally bent silicon crystal. The presented results are of importance for application in experiments on the investigation of the equation of states of high energy density matter
using high intensity GeV/u heavy-ion beams of ≥10^10 particles/100 ns.

E. Eftekhari-Zadeh, A. Bensalama, G. Roshani, A. Salama, C. Spielmann, and A. Iliyasu
Enhanced Gamma-Ray Attenuation-Based Detection System Using an Artificial Neural Network
Photonics 9, 382 (2022)

Abstract: Scale deposition is the accumulation of various materials in the walls of transmission lines and unwanted parts in the oil and gas production system. It is a leading moot point in all transmission lines, tanks, and petroleum equipment. Scale deposition leads to drastic detrimental problems, reduced permeability, pressure and production losses, and direct financial losses due to the failure of some equipment. The accumulation of oil and gas leads to clogged pores and obstruction of fluid flow. Considering the passage of a two-phase flow, our study determines the thickness of the scale, and the flow regime is detected with the help of two Multilayer Perceptron (MLP) networks. First, the diagnostic system consisting of a dual-energy source, a steel pipe, and a NaI detector was implemented, using the Monte Carlo N Particle Code (MCNP). Subsequently, the received signals were processed, and properties were extracted using the wavelet transform technique. These features were considered as inputs of an Artificial Neural Network (ANN) model used to determine the type of flow regimes and predict the scale thickness. By accurately classifying the flow regimes and determining the scale inside the pipe, our proposed method provides a platform that could enhance many areas of the oil industry.

E. Eftekhari-Zadeh, M. S. Blümcke, Z. Samsonova, R. Loetzsch, I. Uschmann, M. Zapf, C. Ronning, O. N. Rosmej, D. Kartashov, and C. Spielmann
Laser energy absorption and x-ray generation in nanowire arrays irradiated by relativistically intense ultra-high contrast femtosecond laser pulses
Physics of Plasmas 29, 013301 (2022)

Abstract: We report here on the results of comparative experimental measurements of laser energy absorption in a bulk and different morphology nanowire arrays interacting with relativistically intense, ultra-high temporal contrast femtosecond laser pulses. We compare polished, flat bulk samples with vertically and randomly oriented nanowires made of ZnO semiconductor material. The optical absorption of the 45° incident laser pulses of ∼40 fs duration with a central wavelength of 400 nm at intensities above 1019Wcm2 was determined using an integrating Ulbricht sphere. We demonstrate an almost twofold enhancement of absorption in both nanowire morphologies with an average of (79.6±1.9)% in comparison to the flat bulk sample of (45.8±1.9)%. The observed substantially enhanced absorption in nanowire arrays is also confirmed by high-resolution x-ray emission spectroscopy. The spectral analysis of the K-shell x-ray emission lines revealed that the He-like resonance line emission from highly ionized Zn (Zn28+) is only present in the case of nanowire arrays, whereas, for the flat bulk samples, only neutral and low charge states were observed. Our numerical simulations, based on radiative-collisional kinetic code FLYCHK, well reproduce the measured He-like emission spectrum and suggest that high charge state observed in nanowire arrays is due to substantially higher plasma temperature. Our results, which were measured for the first time with femtosecond laser pulses, can be used to benchmark theoretical models and numerical codes for the relativistic interaction of ultrashort laser pulses with nanowires.


Z. Sun, F. Tuitje, and C. Spielmann
A review of high‐resolution microscopic ghost imaging with a low‐dose pseudothermal light
Journal of Microscopy 284, 3 (2021)
No abstract availableLinkBibTeX
R. Hollinger, E. Haddad, M. Zapf, V. Shumakova, P. Herrmann, R. Roeder, I. Uschmann, U. Reisloehner, A. Pugzlys, A. Baltuska, F. Legare, M. Zuerch, C. Ronning, C. Spielmann, and D. Kartashov
Role of free-carrier interaction in strong-field excitations in semiconductors
Physical Review B 104, 035203 (2021)

Abstract: The interaction of laser pulses with condensed matter forms the basis of light-wave-driven electronics potentially enabling tera- and petahertz switching rate applications. Carrier control using near- and midinfrared pulses is appealing for integration into existing platforms. Toward this end, a fundamental understanding of the complexity of phenomena concerning sub-band-gap driven semiconductors such as high harmonic generation, carrier excitation due to multiphoton absorption, and interband tunneling as well as carrier-carrier interactions due to strong acceleration in infrared transients is important. Here, stimulated emission from polycrystalline ZnO thin films for pump wavelengths between 1.2 mu m (1 eV) and 10 mu m (0.12 eV) is observed. Contrary to the expected higher intensity threshold for longer wavelengths, the lowest threshold pump intensity for stimulated emission is obtained for the longest pump wavelength corroborating the importance of collisional excitation upon intraband electron acceleration.

T. Helk, E. Berger, S. Jamnuch, L. Hoffmann, A. Kabacinski, J. Gautier, F. Tissandier, J.-P. Goddet, H.-T. Chang, J. Oh, C. Das Pemmaraju, T. Pascal, S. Sebban, C. Spielmann, and M. Zuerch
Table-top extreme ultraviolet second harmonic generation
Science Advances 7, 2265 (2021)

Abstract: The lack of available table-top extreme ultraviolet (XUV) sources with high enough fluxes and coherence properties has limited the availability of nonlinear XUV and x-ray spectroscopies to free-electron lasers ( FELs). Here, we demonstrate second harmonic generation (SHG) on a table-top XUV source by observing SHG near the TiM2,3 edge with a high-harmonic seeded soft x-ray laser. Furthermore, this experiment represents the first SHG experiment in the XUV. First-principles electronic structure calculations suggest the surface specificity and separate the observed signal into its resonant and nonresonant contributions. The realization of XUV-SHG on a table-top source opens up more accessible opportunities for the study of element-specific dynamics in multicomponent systems where surface, interfacial, and bulk-phase asymmetries play a driving role.

Z. Sun, F. Tuitje, and C. Spielmann
Improving the Contrast of Pseudothermal Ghost Images Based on the Measured Signal Distribution of Speckle Fields
Applied Sciences 2021, 2621 (2021)

Abstract: In this study, we examine the quality of microscale ghost images as a function of the measured histographic signal distribution of the speckle fields from a nonuniform pseudothermal light source. This research shows that the distribution of the detected signal level on each pixel of the camera plays a significant role in improving the contrast-to-noise ratio (CNR) of pseudothermal ghost imaging. To our knowledge, the scaling of CNR with different pixel intensity distributions of the speckle fields is observed for the first time in the field of pseudothermal microscale ghost imaging. The experimental observations are in very good agreement with numerical analysis. Based on these findings, we can predict the settings for light sources that will maximize the CNR of microscale ghost images.

R. Hollinger, P. Herrmann, V. Korolev, M. Zapf, V. Shumakova, R. Roeder, I. Uschmann, A. Pugzlys, A. Baltuska, M. Zurch, C. Ronning, C. Spielmann, and D. Kartashov
Polarization Dependent Excitation and High Harmonic Generation from Intense Mid-IR Laser Pulses in ZnO
Nanomaterials 11, 4 (2021)

Abstract: The generation of high order harmonics from femtosecond mid-IR laser pulses in ZnO has shown great potential to reveal new insight into the ultrafast electron dynamics on a few femtosecond timescale. In this work we report on the experimental investigation of photoluminescence and high-order harmonic generation (HHG) in a ZnO single crystal and polycrystalline thin film irradiated with intense femtosecond mid-IR laser pulses. The ellipticity dependence of the HHG process is experimentally studied up to the 17th harmonic order for various driving laser wavelengths in the spectral range 3-4 mu m. Interband Zener tunneling is found to exhibit a significant excitation efficiency drop for circularly polarized strong-field pump pulses. For higher harmonics with energies larger than the bandgap, the measured ellipticity dependence can be quantitatively described by numerical simulations based on the density matrix equations. The ellipticity dependence of the below and above ZnO band gap harmonics as a function of the laser wavelength provides an efficient method for distinguishing the dominant HHG mechanism for different harmonic orders.

R. Hollinger, E. Haddad, M. Zapf, V. Shumakova, P. Herrmann, R. Roder, I. Uschmann, U. Reislohner, A. Pugzlys, A. Baltuska, F. Legare, M. Zurch, C. Ronning, C. Spielmann, and D. Kartashov
Contribution of free carriers to light absorption upon intense light-semiconductor interaction

Abstract: In this work we investigated the role of free carriers in the interaction of a wide-band gap semiconductor with strong light fields at long wavelengths. Motivated by the beneficial scaling law of the pondermotive potential (U p ~ Iλ 2 ), the interaction of intense long wavelength laser pulses with condensed matter has attracted huge attention over the last decade [Kruchinin] . After excitation of quasi free electrons in the conduction band (CB) via multiphoton absorption or tunnelling the strong pondermotive force leads to highly energetic free electrons. Bound electrons can be collisionally excited if the energy of the free electrons exceed the band gap energy. Here, we use the onset of near ultraviolet (NUV) stimulated emission in ZnO thin films ( Fig. 1a ) to study off-resonance light absorption and the role of free carriers thereby.

P. Herrmann, R. Hollinger, V. Korolev, M. Zapf, V. Shumakova, R. Roder, I. Uschmann, A. Pugzlys, A. Baltuska, M. Zurch, C. Ronning, C. Spielmann, and D. Kartashov
Ellipticity dependent excitation and high harmonic generation from intense mid-IR laser pulses in ZnO

Abstract: In this contribution we experimental examine the ellipticity dependence of strong off resonance absorption and high harmonic generation (HHG) in the wide band gap semiconductor ZnO upon irradiation with intense mid-IR laser pulses. HHG in semiconductors originates from nonlinear intraband currents and interband transitions of the electrons driven by the strong mid-IR fields [Ghimire]. Here, we show that the intra- and interband contribution to HHG in a ZnO thin film are affected differently by the driving laser ellipticity.

T. Helk, E. Berger, L. Hoffmann, A. Kabacinski, J. Gautier, F. Tissandier, J. P. Goddet, S. Sebban, C. Spielmann, and M. Zurch
Extreme Ultraviolet Second Harmonic Generation using a seeded soft X-ray laser

Abstract: Non-linear interactions between light and matter are crucial for widespread applications in physical sciences, life science and engineering. Second harmonic generation (SHG) and sum-frequency generation spectroscopy in the near infrared and optical range have enabled intriguing insights into surface properties and how they influence for instance chemical reactions [1] . Expansion of these methods by developing non-linear X-ray spectroscopies has recently added the capability of studying surfaces [2] , symmetry-breaking [3] and buried interfaces [4] with elemental specificity. However, widespread application is currently limited by access to free-electron laser facilities. Here we report the first generation of second harmonic emission in the extreme ultraviolet (XUV-SHG) at the titanium M-edge. The experiments were carried out with a high-harmonic seeded SXRL [5] bringing nonlinear XUV spectroscopy with atomic specificity to the table-top. The SXRL pulses with an energy of (14 ± 2) nJ, a pulse duration of (1.73 ± 0.13) ps, wavelength of 32.8 nm and a Gaussian-like beam profile is focused down with a gold ellipsoidal mirror down to an elliptical spot with a size of roughly 20 µ m x 40 µ m. The estimated intensity on target is about of (1.0 ± 0.1)•10 10 W/cm 2 . In the focus we exceeded the damage threshold fluence of 2 mJ/cm 2 and observed single-shot damage of 50 nm Ti foils. At these intensities we also generate second harmonic light at 75.6 eV. The fundamental and SHG beams are refocused with a toroidal mirror, spectrally separated by a grating and imaged on a cooled CCD camera.

R. Hollinger, N. Harshitha, V. Korolev, Z. Gan, A. George, V. Shumakova, M. Zürch, T. Vogl, A. P. zlys, A. B. ska, F. Eilenberger, C. Spielmann, A. Turchanin, and D. Kartashov
Ellipticity controlled high-order harmonic generation in 2D materials

Abstract: High harmonic generation (HHG) in a single-atomic-layer non-centrosymmetric semiconductor is investigated experimentally for different driving laser field polarizations. The ellipticity enhanced even-order HHG for certain crystal orientations reveals linked laser and valley polarizations.


R. Hollinger, D. Gupta, M. Zapf, M. Karst, R. Röder, I. Uschmann, U. Reislöhner, D. Kartashov, C. Ronning, and C. Spielmann
Polarization dependent multiphoton absorption in ZnO thin films
Journal of Physics D: Applied Physics 53, 055102 (2020)

Abstract: We present a simple non-destructive approach for studying the polarization dependence of nonlinear absorption processes in semiconductors. The method is based on measuring the yield of the near UV photoluminescence as a function of polarization and intensity of femtosecond laser pulses. In particular, we investigated the polarization dependence of three photon laser absorption in intrinsic and Al-doped ZnO thin films. Both specimen show stronger emission for linearly polarized excitation compared to circular polarization. The ratios for the three-photon absorption coefficients are about 1.8 and independent of the doping. It is shown that Al-doped films have lower threshold for stimulated emission in comparison to the intrinsic films.

N. Geib, R. Hollinger, E. Haddad, P. Herrmann, F. Légaré, T. Pertsch, C. Spielmann, M. Zürch, and F. Eilenberger
Discrete dispersion scan setup for measuring few-cycle laser pulses in the mid-infrared
Optics Letters 45, 5295 (2020)

Abstract: In this work, we demonstrate a discrete dispersion scan scheme using a low number of flat windows to vary the dispersion of laser pulses in discrete steps. Monte Carlo simulations indicate that the pulse duration can be retrieved accurately with less than 10 dispersion steps, which we verify experimentally by measuring few-cycle pulses and material dispersion curves at 3 and 10 µm wavelength. This minimal measuring scheme using only five optical components without the need for linear positioners and interferometric alignment can be readily implemented in many wavelength ranges and situations.

F. Tuitje, P. Martinez Gil, T. Helk, J. Gautier, F. Tissandier, J. -P. Goddet, A. Guggenmos, U. Kleineberg, S. Sebban, E. Oliva, C. Spielmann, and M. Zuerch
Nonlinear ionization dynamics of hot dense plasma observed in a laser-plasma amplifier
Light: Science & Applications 9, 187 (2020)

Abstract: Understanding the behaviour of matter under conditions of extreme temperature, pressure, density and electromagnetic fields has profound effects on our understanding of cosmologic objects and the formation of the universe. Lacking direct access to such objects, our interpretation of observed data mainly relies on theoretical models. However, such models, which need to encompass nuclear physics, atomic physics and plasma physics over a huge dynamic range in the dimensions of energy and time, can only provide reliable information if we can benchmark them to experiments under well-defined laboratory conditions. Due to the plethora of effects occurring in this kind of highly excited matter, characterizing isolated dynamics or obtaining direct insight remains challenging. High-density plasmas are turbulent and opaque for radiation below the plasma frequency and allow only near-surface insight into ionization processes with visible wavelengths. Here, the output of a high-harmonic seeded laser-plasma amplifier using eight-fold ionized krypton as the gain medium operating at a 32.8 nm wavelength is ptychographically imaged. A complex-valued wavefront is observed in the extreme ultraviolet (XUV) beam with high resolution. Ab initio spatio-temporal Maxwell-Bloch simulations show excellent agreement with the experimental observations, revealing overionization of krypton in the plasma channel due to nonlinear laser-plasma interactions, successfully validating this four-dimensional multiscale model. This constitutes the first experimental observation of the laser ion abundance reshaping a laser-plasma amplifier. The presented approach shows the possibility of directly modelling light-plasma interactions in extreme conditions, such as those present during the early times of the universe, with direct experimental verification.

F. Tuitje, W. Eschen, G. Tadesse, J. Limpert, J. Rothhardt, and C. Spielmann
Reliability of ptychography on periodic structures
OSA Continuum 3, 1691 (2020)
No abstract availableLinkBibTeX
R. Hollinger, D. Hoff, P. Wustelt, S. Skruszewicz, Y. Zhang, H. Kang, D. Würzler, T. Jungnickel, M. Dumergue, A. Nayak, R. Flender, L. Haizer, M. Kurucz, B. Kiss, S. Kühn, E. Cormier, C. Spielmann, G. G. Paulus, P. Tzallas, and M. Kübel
Carrier-envelope-phase measurement of few-cycle mid-infrared laser pulses using high harmonic generation in ZnO
Optics Express 28, 7314 (2020)

Abstract: High-harmonic generation (HHG) in crystals offers a simple, affordable and easily accessible route to carrier-envelope phase (CEP) measurements, which scales favorably towards longer wavelengths. We present measurements of HHG in ZnO using few-cycle pulses at 3.1 µm. Thanks to the broad bandwidth of the driving laser pulses, spectral overlap between adjacent harmonic orders is achieved. The resulting spectral interference pattern provides access to the relative harmonic phase, and hence, the CEP.

F. Tuitje, M. Zürch, T. Helk, J. Gautier, F. Tissandier, J.-P. Goddet, E. Oliva, A. Guggenmos, U. Kleineberg, H. Stiel, S. Sebban, and C. Spielmann
Ptychography and single-shot nanoscale imaging with plasma-based laser sources
Springer Proceedings in Physics 241, 155 (2020)

Abstract: We report the direct wavefront characterization of an intense ultrafast high-harmonic seeded soft X-ray laser at 32.8 nm wavelength and monitor the exit of the laser plasma amplifier depending on the arrival time of the seed pulses with respect to pump pulses. For the wavefront measurement in phase and intensity, we used high-resolution ptychography. After propagating the wavefront back to the source, we are able to observe the rear end of the plasma amplifier. We compare the characteristics of the seeded soft X-ray Laser to an unseeded one and find an increasing beam stability and lateral coherence important for lensless imaging techniques.


Z. Sun, F. Tuitje, and C. Spielmann
Toward high contrast and high-resolution microscopic ghost imaging
Optics Express 27, 33652 (2019)

Abstract: In this study, the influence of speckle size on contrast-to-noise ratio (CNR) and resolution is examined based on the object dimensions in the macroscopic and microscopic regimes. This research shows that for microscopic samples the conventional scaling laws are no longer effective and the CNR does not counter-propagate in the same manner as the resolution. To our knowledge, a deviation in CNR scaling on speckle size is observed for the first time in the field of microscopic ghost imaging. This result was verified using two different sample shapes. In addition, numerical analysis revealed that the noise of the photodiode is a limiting factor for the CNR. Based on these findings, the conditions for identifying the parameter set that maximizes the CNR and provides high resolution images was defined, which achieving high-quality microscopic ghost images.

R. Hollinger, D. Gupta, M. Zapf, R. Röder, D. Kartashov, C. Ronning, and C. Spielmann
Single nanowire defined emission properties of ZnO nanowire arrays
Journal of Physics D: Applied Physics 52, 295101 (2019)

Abstract: We report on stimulated emission from vertically aligned, vapor transport grown, ZnO nanowire arrays, and pumped by three-photon absorption in intense near-infrared femtosecond laser pulses. In respect to single nanowires, arrays have the advantage of a higher light absorption and emission rate. The intensity and bandwidth of the emitted ultraviolet radiation as a function of the pump intensity is compared for nanowire arrays with different wire lengths, diameters, and spacing. The measured lasing thresholds for all arrays can be well described by the geometry of individual nanowire lasers, showing that coupling effects between the individual emitters in the arrays are negligible, even for the smallest 100 nm diameter wires with an average distance of 200 nm.

R. Hollinger, P. Malevich, V. Shumakova, S. Ališauskas, M. Zapf, R. Röder, A. Pugžlys, A. Baltuška, C. Ronning, C. Spielmann, and D. Kartashov
Strong Light-Field Driven Nanolasers
Nano Letters 19, 3563 (2019)

Abstract: Einstein established the quantum theory of radiation and paved the way for modern laser physics including single-photon absorption by charge carriers and finally pumping an active gain medium into population inversion. This can be easily understood in the particle picture of light. Using intense, ultrashort pulse lasers, multiphoton pumping of an active medium has been realized. In this nonlinear interaction regime, excitation and population inversion depend not only on the photon energy but also on the intensity of the incident pumping light, which can be still described solely by the particle picture of light. We demonstrate here that lowering significantly the pump photon energy further still enables population inversion and lasing in semiconductor nanowires. The extremely high electric field of the pump bends the bands and enables tunneling of electrons from the valence to the conduction band. In this regime, the light acts by the classical Coulomb force and population inversion is entirely due to the wave nature of electrons, thus the excitation becomes independent of the frequency but solely depends on the incident intensity of the pumping light.

Z. Samsonova, S. Höfer, V. Kaymak, S. Ališauskas, V. Shumakova, A. Pugžlys, A. Baltuška, T. Siefke, S. Kroker, A. Pukhov, O. Rosmej, I. Uschmann, C. Spielmann, and D. Kartashov
Relativistic Interaction of Long-Wavelength Ultrashort Laser Pulses with Nanowires
Physical Review X 9, 021029 (2019)

Abstract: We report on experimental results in a new regime of relativistic light-matter interaction employing midinfrared (3.9-mu m wavelength) high-intensity femtosecond laser pulses. In the laser-generated plasma, electrons reach relativistic energies already for rather low intensities due to the fortunate lambda(2) scaling of the kinetic energy with the laser wavelength. The lower intensity efficiently suppresses optical field ionization and creation of the preplasma at the rising edge of the laser pulse, enabling an enhanced efficient vacuum heating of the plasma. The lower critical plasma density for long-wavelength radiation can be surmounted by using nanowires instead of flat targets. Numerical simulations, which are in a good agreement with experimental results, suggest that approximate to 80% of the incident laser energy has been absorbed resulting in a long-living, key-temperature, high-charge-state plasma with a density more than 3 orders of magnitude above the critical value. Our results pave the way to laser-driven experiments on laboratory astrophysics and nuclear physics at a high repetition rate.

F. Tuitje, T. Helk, M. Zürch, and C. Spielmann
Extreme ultraviolet lensless imaging of biological specimen
Proceedings of SPIE 10890, 80 (2019)

Abstract: Imaging of biological specimen is one of the most important tools to investigate structures and functionalities in organic components. Improving the resolution of images into the nanometer range call for short wavelengths light sources and large aperture optics. Subsequently, the use of extreme ultraviolet light in the range of 2 nm to 5 nm provides high contrast and high resolution imaging, if it is combined with lensless imaging techniques. We describe important parameters for high resolution lensless imaging of biological samples and specify the required light source properties. To overcome radiation based damage of biological specimen, we discuss the concept of ghost imaging and describe a possible setup towards biological imaging in the extreme ultraviolet range.

F. Tuitje, T. Helk, M. Zürch, J. Gautier, F. Tissandier, J.-P. Goddet, E. Oliva, A. Guggenmos, U. Kleineberg, S. Sebban, and C. Spielmann
Following the plasma dynamics in a seeded soft x-ray laser with lensless imaging
Proceedings of SPIE 10903, 9 (2019)

Abstract: We evaluated the capabilities of an intense ultrafast high-harmonic seeded soft X-ray laser at 32.8 nm wavelength regarding single-shot lensless imaging and ptychography. Additionally the wave front at the exit of the laser plasma amplifier is monitored in amplitude and phase using high resolution ptychography and backpropagation techniques.Characterizing the laser plasma amplifier performance depending on the arrival time of the seed pulse with respect to pump pulses provides insight into the light plasma interaction in the soft X-ray range.

G. Tadesse, W. Eschen, R. Klas, M. Tschernajew, T. Frederik, M. Steinert, M. Zilk, V. Schuster, M. Zürch, T. Pertsch, C. Spielmann, J. Limpert, and J. Rothhardt
Wavelength-scale ptychographic coherent diffractive imaging using a high-order harmonic source
Scientific Reports 9, 1735 (2019)

Abstract: Ptychography enables coherent diffractive imaging (CDI) of extended samples by raster scanning across the illuminating XUV/X-ray beam, thereby generalizing the unique advantages of CDI techniques. Table- top realizations of this method are urgently needed for many applications in sciences and industry. Previously, it was only possible to image features much larger than the illuminating wavelength with table-top ptychography although knife-edge tests suggested sub-wavelength resolution. However, most real-world imaging applications require resolving of the smallest and closely-spaced features of a sample in an extended field of view. In this work, resolving features as small as 2.5 \lambda (45 nm) using a table-top ptychography setup is demonstrated by employing a high-order harmonic XUV source with record-high photon flux. For the first time, a Rayleigh-type criterion is used as a direct and unambiguous resolution metric for high-resolution table-top setup. This reliably qualifies this imaging system for real-world applications e.g. in biological sciences, material sciences, imaging integrated circuits and semiconductor mask inspection.

T. Helk, M. Zürch, and C. Spielmann
Perspective: Towards single shot time-resolved microscopy using short wavelength table-top light sources
Structural Dynamics 6, 010902 (2019)

Abstract: Time-resolved imaging allows revealing the interaction mechanisms in the microcosm of both inorganic and biological objects. While X-ray microscopy has proven its advantages for resolving objects beyond what can be achieved using optical microscopes, dynamic studies using full-field imaging at the nanometer scale are still in their infancy. In this perspective, we present the current state of the art techniques for full-field imaging in the extreme-ultraviolet- and soft X-ray-regime which are suitable for single exposure applications as they are paramount for studying dynamics in nanoscale systems. We evaluate the performance of currently available table-top sources, with special emphasis on applications, photon flux, and coherence. Examples for applications of single shot imaging in physics, biology, and industrial applications are discussed.

R. Hollinger, V. Shumakova, A. Pugzlys, A. Baltuska, S. Khujanov, C. Spielmann, and D. Kartashov
High-order harmonic generation traces ultrafast coherent phonon dynamics in ZnO
No abstract availableLinkBibTeX
M. Zuerch, F. Tuitjea, T. Helk, J. Gautier, F. Tissandier, J. -P. Goddet, E. Oliva, A. Guggenmos, U. Kleineberg, H. Stiel, S. Sebban, and C. Spielmann
Lab-scale soft X-ray ptychography: advanced nanoscale imaging and beam diagnostics
No abstract availableLinkBibTeX
M. Zuerch, F. Tuitje, T. Helk, J. Gautier, F. Tissandier, J.-P. Goddet, A. Guggenmos, U. Kleineberg, S. Sebban, and C. Spielmann
Single shot XUV nanoimaging using an intense femtosecond soft X-ray laser
No abstract availableLinkBibTeX


N. Jayakumar, R. Sollapur, A. Hoffmann, T. Grigorova, A. Hartung, A. Schwuchow, J. Bierlich, J. Kobelke, M. A. Schmidt, and C. Spielmann
Polarization evolution in single-ring antiresonant hollow-core fibers
Applied Optics 57, 8529 (2018)

Abstract: Understanding polarization in waveguides is of fundamental importance for any photonic device and is particularly relevant within the scope of fiber optics. Here, we investigate the dependence of the geometry-induced polarization behavior of single-ring antiresonant hollow-core fibers on various parameters from the experimental perspective, showing that structural deviations from an ideal polygonal shape impose birefringence and polarization-dependent loss, confirmed by a toy model. The minimal output ellipticity was found at the wavelength of lowest loss near the center of the transmission band, whereas birefringence substantially increases toward the resonances. The analysis that qualitatively also applies to other kinds of hollow-core fibers showed that maximizing the amount of linearly polarized light at the fiber output demands both operating at the wavelength of lowest loss, as well as carefully choosing the relative orientation of input polarization. This should correspond to the situation in which the difference of the core extent along the two corresponding orthogonal polarization directions is minimal. Due to their practical relevance, we expect our findings to be very important in fields such as nonlinear photonics or metrology.

Z. Samsonova, S. Höfer, R. Hollinger, T. Kämpfer, I. Uschmann, R. Röder, L. Trefflich, O. Rosmej, E. Förster, C. Ronning, D. Kartashov, and C. Spielmann
Hard X-ray generation from ZnO nanowire targets in a non-relativistic regime of laser-solid interactions
Applied Sciences 8, 1728 (2018)

Abstract: We present a detailed investigation of X-ray emission from both flat and nanowire zinc oxide targets irradiated by 60 fs 5E16 W/cm^2 intensity laser pulses at a 0.8 µm wavelength. It is shown that the fluence of the emitted hard X-ray radiation in the spectral range 150–800 keV is enhanced by at least one order of magnitude for nanowire targets compared to the emission from a flat surface, whereas the characteristic Kα line emission (8.64 keV) is insensitive to the target morphology. Furthermore, we provide evidence for a dramatic increase of the fast electron flux from the front side of the nanostructured targets. We suggest that targets with nanowire morphology may advance the development of compact ultrafast X-ray sources with an enhanced flux of hard X-ray emission that could find wide applications in high energy density (HED) physics.

T. Grigorova, R. Sollapur, A. Hoffmann, A. Hartung, A. Schwuchow, J. Bierlich, J. Kobelke, M. A. Schmidt, and C. Spielmann
Measurement of the Dispersion of an Antiresonant Hollow Core Fiber
IEEE Photonics Journal 10, 1 (2018)

Abstract: Due to unique properties, antiresonant hollow core fibers have found widespread use in various fields of science and application. Particular regarding applications that involve ultrashort pulses, precise knowledge of group velocity dispersion is essential to understand the underlying physics and to optimize device performance. Here we report on the successful measurement of the spectral distribution of the group velocity dispersion of the fundamental mode of an antiresonant hollow core fiber in close proximity to and away from a strong strand resonance. The results show the variations of the hundreds of fs^2/cm near the resonance region, whereas the dispersion is identical to that of a perfect cylindrical waveguide away from the resonance in accordance with a literature. An additional zero dispersion wavelength that is not present in the case of a capillary was experimentally verified. The possibility to tune dispersion via strand resonances opens up a novel pathway towards engineering pulse dispersion, with applications in fields such as nonlinear science and pulse propagation management.

O. N. Rosmej, Z. Samsonova, S. Höfer, D. Kartashov, C. Arda, D. Khaghani, A. Schoenlein, S. Zähter, A. Hoffmann, R. Loetzsch, A. Saevert, I. Uschmann, M. E. Povarnitsyn, N. E. Andreev, L. P. Pugachev, M. C. Kaluza, and C. Spielmann
Generation of keV hot near-solid density plasma states at high contrast laser-matter interaction
Physics of Plasmas 25, 083103 (2018)

Abstract: We present experimental evidence of ultra-high energy density plasma states with the keV bulk electron temperatures and near-solid electron densities generated during the interaction of high contrast, relativistically intense laser pulses with planar metallic foils. Experiments were carried out with the Ti:Sapphire laser system where a picosecond pre-pulse was strongly reduced by the conversion of the fundamental laser frequency into 2ω. A complex diagnostics setup was used for evaluation of the electron energy distribution in a wide energy range. The bulk electron temperature and density have been measured using x-ray spectroscopy tools; the temperature of supra-thermal electrons traversing the target was determined from measured bremsstrahlung spectra; run-away electrons were detected using magnet spectrometers. Analysis of the bremsstrahlung spectra and results on measurements of the run-away electrons showed a suppression of the hot electron production in the case of the high laser contrast. Characteristic x-ray radiation has been used for evaluation of the bulk electron temperature and density. The measured Ti line radiation was simulated both in steady-state and transient approaches using the code FLYCHK that accounts for the atomic multi-level population kinetics. The best agreement between the measured and the synthetic spectrum of Ti was achieved at 1.8 keV electron temperature and 2 10^23 cm^{−3} electron density. By application of Ti-foils covered with nm-thin Fe-layers, we have demonstrated that the thickness of the created keV hot dense plasma does not exceed 150 nm. Results of the pilot hydro-dynamic simulations that are based on a wide-range two-temperature Equation of States, wide-range description of all transport and optical properties, ionization, electron, and radiative heating, plasma expansion, and Maxwell equations (with a wide-range permittivity) for description of the laser absorption are in excellent agreement with experimental results. According to these simulations, the generation of keV-hot bulk electrons is caused by the collisional mechanism of the laser pulse absorption in plasmas with a near solid step-like electron density profile. The laser energy, first deposited into the nm-thin skin-layer, is then transported into 150 nm depth by the electron heat conductivity. This scenario is opposite to the volumetric character of the energy deposition produced by supra-thermal electrons.

M. Zurch, A. Guggenmos, R. Jung, J. Rothhardt, C. Spath, J. Tümmler, S. Demmler, S. Haedrich, J. Limpert, A. Tünnermann, U. Kleineberg, H. Stiel, and C. Spielmann
Coherent Diffraction Imaging with Tabletop XUV Sources

Abstract: Coherent diffraction imaging (CDI) at wavelengths in the extreme ultraviolet range has become an important tool for nanoscale investigations. Employing laser-driven high harmonic sources allows for lab-scale applications such as cancer cell classification and phase-resolved surface studies in reflection geometry. The excellent beam properties support a spatial resolution below the wavelength, i.e., close to the Abbe limit. Unfortunately, the usually low photon flux of HHG sources limits their applicability. Recent advances in ultrafast fiber laser development cumulated in sources delivering average powers approaching the milliwatt level in the extreme ultraviolet. In comparison, a tabletop soft X-ray laser driven by moderate pump energies was recently employed for CDI featuring excellent temporal coherence and extraordinary high flux allowing for single-shot imaging.


R. Sollapur, D. Kartashov, M. Zürch, A. Hoffmann, T. Grigorova, G. Sauer, A. Hartung, A. Schwuchow, J. Bierlich, J. Kobelke, M. Chemnitz, M. A. Schmidt, and C. Spielmann
Resonance-enhanced multi-octave supercontinuum generation in antiresonant hollow-core fibers
Light: Science & Applications 6, e17124 (2017)

Abstract: Ultrafast supercontinuum generation in gas-filled waveguides is an enabling technology for many intriguing applications ranging from attosecond metrology towards biophotonics, with the amount of spectral broadening crucially depending on the pulse dispersion of the propagating mode. In this study, we show that structural resonances in a gas-filled antiresonant hollow core optical fiber provide an additional degree of freedom in dispersion engineering, which enables the generation of more than three octaves of broadband light that ranges from deep UV wavelengths to near infrared. Our observation relies on the introduction of a geometric-induced resonance in the spectral vicinity of the ultrafast pump laser, outperforming gas dispersion and yielding a unique dispersion profile independent of core size, which is highly relevant for scaling input powers. Using a krypton-filled fiber, we observe spectral broadening from 200 nm to 1.7 μm at an output energy of ∼ 23 μJ within a single optical mode across the entire spectral bandwidth. Simulations show that the frequency generation results from an accelerated fission process of soliton-like waveforms in a non-adiabatic dispersion regime associated with the emission of multiple phase-matched Cherenkov radiations on both sides of the resonance. This effect, along with the dispersion tuning and scaling capabilities of the fiber geometry, enables coherent ultra-broadband and high-energy sources, which range from the UV to the mid‐infrared spectral range.

R. Hollinger, Z. Samsonova, D. Gupta, C. Spielmann, R. Röder, L. Trefflich, C. Ronning, and D. Kartashov
Enhanced absorption and cavity effects of three-photon pumped ZnO nanowires
Applied Physics Letters 111, 213106 (2017)

Abstract: Semiconductor nanowire (NW) lasers attract a lot of attention as potential elements of nanophotonic circuits and lab-on-a chip devices. Here, we report on the experimental investigation of stimulated near ultraviolet (NUV) emission, pumped by three-photon absorption from near infrared femtosecond laser pulses, from ZnO NW arrays of different morphologies and compare it to the bulk. The spectrally and temporally resolved measurements of the NUV emission show both strong enhancements in the absorption and emission properties of the nanowire arrays compared to bulk samples. Thus, we determine a many times higher three-photon absorption in the nanostructure morphology compared to the bulk material. Furthermore, the threshold pumping intensity for stimulated emission in a vertically oriented nanowire array is twice lower and the emission onset time is shorter than in randomly oriented arrays, revealing strong influence of the macroscopic nanowire arrangement.

M. Zürch, R. Jung, C. Späth, J. Tümmler, A. Guggenmos, D. Attwood, U. Kleineberg, H. Stiel, and C. Spielmann
Transverse Coherence Limited Coherent Diffraction Imaging using a Molybdenum Soft X-ray Laser Pumped at Moderate Pump Energies
Scientific Reports 7, 5314 (2017)

Abstract: Coherent diffraction imaging (CDI) in the extreme ultraviolet has become an important tool for nanoscale investigations. Laser-driven high harmonic generation (HHG) sources allow for lab scale applications such as cancer cell classification and phase-resolved surface studies. HHG sources exhibit excellent coherence but limited photon flux due poor conversion efficiency. In contrast, table-top soft X-ray lasers (SXRL) feature excellent temporal coherence and extraordinary high flux at limited transverse coherence. Here, the performance of a SXRL pumped at moderate pump energies is evaluated for CDI and compared to a HHG source. For CDI, a lower bound for the required mutual coherence factor of |μ12| ≥ 0.75 is found by comparing a reconstruction with fixed support to a conventional characterization using double slits. A comparison of the captured diffraction signals suggests that SXRLs have the potential for imaging micron scale objects with sub-20 nm resolution in orders of magnitude shorter integration time compared to a conventional HHG source. Here, the low transverse coherence diameter limits the resolution to approximately 180 nm. The extraordinary high photon flux per laser shot, scalability towards higher repetition rate and capability of seeding with a high harmonic source opens a route for higher performance nanoscale imaging systems based on SXRLs.

Z. Samsonova, S. Höfer, A. Hoffmann, B. Landgraf, M. Zürch, I. Uschmann, D. Khaghani, O. Rosmej, P. Neumayer, R. Röder, L. Trefflich, C. Ronning, E. Förster, C. Spielmann, and D. Kartashov
X-ray emission generated by laser-produced plasmas from dielectric nanostructured targets
AIP Conference Proceedings 1811, 180001 (2017)

Abstract: We present an experimental study of X-ray generation from nanostructured ZnO targets. Samples of different morphology ranging from nanowires to polished surfaces are irradiated by relativistically intense femtosecond laser pulses. X-ray emission of plasma is generated by 45 fs 130 mJ laser pulses at 400 nm with picosecond temporal contrast better than 1E−9 interacting with an array of ZnO nanowires. The measured spectra indicate the existence of highly ionized states of Zn (up to He-like Zn). The obtained flux of ∼1E10 photons per laser shot at the neutral Zn Kα energies around 8.65 keV and at the Zn Heα energies around 9 keV is almost 3 times higher for nanostructured targets compared to the reference polished sample and implies 1E−4 conversion efficiency from the laser energy to the total energy of the emitted X-ray photons.

A. Hoffmann, C. Egelkamp, D. Winters, T. Kühl, and C. Spielmann
Online Monitoring of Laser-Generated XUV Radiation Spectra by Surface Reflectivity Measurements with Particle Detectors
Applied Sciences 7, 70 (2017)

Abstract: In this contribution, we present a wavelength-sensitive method for the detection of extreme ultraviolet (XUV) photon energies between 30 eV and 120 eV. The method is based on 45° reflectivity from either a cesium iodide-coated or an uncoated metal surface, which directs the XUV beam onto an electron or ion detector and its signal is used to monitor the XUV beam. The benefits of our approach are a spectrally sensitive diagnosis of the XUV radiation at the interaction place of time-resolved XUV experiments and the detection of infrared leak light though metal filters in high-harmonic generation (HHG) experiments. Both features were tested using spectrally shaped XUV pulses from HHG in a capillary, and we have achieved excellent agreement with XUV spectrometer measurements and reflectivity calculations. Our obtained results are of interest for time-resolved XUV experiments presenting an additional diagnostic directly in the interaction region and for small footprint XUV beamline diagnostics.


G. K. Tadesse, R. Klas, S. Demmler, S. Hädrich, I. Wahyutama, M. Steinert, C. Spielmann, M. Zürch, T. Pertsch, A. Tünnermann, J. Limpert, and J. Rothhardt
High speed and high resolution table-top nanoscale imaging
Optics Letters 41, 5170 (2016)

Abstract: We present a table-top coherent diffractive imaging (CDI) experiment based on high-order harmonics generated at 18 nm by a high average power femtosecond fiber laser system. The high photon flux, narrow spectral bandwidth, and high degree of spatial coherence allow for ultrahigh subwavelength resolution imaging at a high numerical aperture. Our experiments demonstrate a half-pitch resolution of 15 nm, close to the actual Abbe limit of 12 nm, which is the highest resolution achieved from any table-top extreme ultraviolet (XUV) or x-ray microscope. In addition, sub-30 nm resolution was achieved with only 3 s of integration time, bringing live diffractive imaging and three-dimensional tomography on the nanoscale one step closer to reality. The current resolution is solely limited by the wavelength and the detector size. Thus, table-top nanoscopes with only a few-nanometer resolutions are in reach and will find applications in many areas of science and technology.

B. Landgraf, B. Aurand, G. Lehmann, T. Gangolf, M. Schnell, T. Kühl, and C. Spielmann
Broadband stimulated Raman backscattering
New Journal of Physics 18, 073048 (2016)

Abstract: Broadband amplification employing stimulated Raman backscattering is demonstrated. Using seed pulses with a bandwidth of about 200 nm, we study the amplification in a wide spectral range in a single laser shot. With chirped pump pulses and a Ne gas jet, we observed under optimized conditions, amplification in range of about 80 nm, which is sufficient to support the amplification of sub-20 fs pulses. This broad amplification range is also in excellent agreement with PIC simulations. The conversion efficiency is at certain wavelengths as high as 1.2% and was measured to be better than 6 × 10^−3 on average.

C. Serrat, D. Roca, J. M. Budesca, J. Seres, E. Seres, B. Aurand, A. Hoffmann, S. Namba, T. Kuehl, and C. Spielmann
Avalanche of stimulated forward scattering in high harmonic generation
Optics Express 24, 8028 (2016)

Abstract: Optical amplifiers in all ranges of the electromagnetic spectrum exhibit an essential characteristic, namely the input signal during the propagation in the amplifier medium is multiplied by the avalanche effect of the stimulated emission to produce exponential growth. We perform a theoretical study motivated and supported by experimental data on a He gas amplifier driven by intense 30-fs-long laser pulses and seeded with attosecond pulse trains generated in a separated Ne gas jet. We demonstrate that the strong-field theory in the frame of high harmonic generation fully supports the appearance of the avalanche effect in the amplification of extreme ultraviolet attosecond pulse trains. We theoretically separate and identify different physical processes taking part in the interaction and we demonstrate that X-ray parametric amplification dominates over others. In particular, we identify strong-field mediated intrapulse X-ray parametric processes as decisive for amplification at the single-atom level. We confirm that the amplification takes place at photon energies where the amplifier is seeded and when the seed pulses are perfectly synchronized with the driving strong field in the amplifier. Furthermore, propagation effects, phase matching and seed synchronization can be exploited to tune the amplified spectral range within the seed bandwidth.


A. Hoffmann, M. Zürch, and C. Spielmann
Extremely Nonlinear Optics Using Shaped Pulses Spectrally Broadened in an Argon-or Sulfur Hexafluoride-Filled Hollow-Core Fiber
Applied Sciences 5, 1310 (2015)

Abstract: In this contribution we present a comparison of the performance of spectrally broadened ultrashort pulses using a hollow-core fiber either filled with argon or sulfur hexafluoride (SF6) for demanding pulse-shaping experiments. The benefits of both gases for pulse-shaping are studied in the highly nonlinear process of high-harmonic generation. In this setup, temporally shaping the driving laser pulse leads to spectrally shaping of the output extreme ultraviolet (XUV) spectrum, where total yield and spectral selectivity in the XUV are the targets of the optimization approach. The effect of using sulfur hexafluoride for pulse-shaping the XUV yield can be doubled compared to pulse compression and pulse-shaping using argon and the spectral range for selective optimization of a single harmonic can be extended. The obtained results are of interest for extending the range of ultrafast science applications drawing on tailored XUV fields.

C. Kern, M. Zürch, and C. Spielmann
Limitations of Extreme Nonlinear Ultrafast Nanophotonics
Nanophotonics 4, 303 (2015)

Abstract: High-harmonic generation (HHG) has been established as an indispensable tool in optical spectroscopy. This effect arises for instance upon illumination of a noble gas with sub-picosecond laser pulses at focussed intensities significantly greater than 1012W/cm2. HHG provides a coherent light source in the extreme ultraviolet (XUV) spectral region, which is of importance in inner shell photo ionization of many atoms and molecules. Additionally, it intrinsically features light fields with unique temporal properties. Even in its simplest realization, XUV bursts of sub-femtosecond pulse lengths are released. More sophisticated schemes open the path to attosecond physics by offering single pulses of less than 100 attoseconds duration.

Resonant optical antennas are important tools for coupling and enhancing electromagnetic fields on scales below their free-space wavelength. In a special application, placing field-enhancing plasmonic nano antennas at the interaction site of an HHG experiment has been claimed to boost local laser field strengths, from insufficient initial intensities to sufficient values. This was achieved with the use of arrays of bow-tie-shaped antennas of ∼ 100nm in length. However, the feasibility of this concept depends on the vulnerability of these nano-antennas to the still intense driving laser light.We show, by looking at a set of exemplary metallic structures, that the threshold fluence Fth of laser-induced damage (LID) is a greatly limiting factor for the proposed and tested schemes along these lines.We present our findings in the context of work done by other groups, giving an assessment of the feasibility and effectiveness of the proposed scheme.

B. Landgraf, A. Hoffmann, D. Kartashov, F. Gärtner, Z. Samsonova, P. Polynkin, J. Jacoby, T. Kühl, and C. Spielmann
Generation of multi-millijoule red-shifted pulses for seeding stimulated Raman backscattering amplifiers
Optics Express 23, 7400 (2015)

Abstract: The efficient generation of redshifted pulses from chirped femtosecond joule level Bessel beam pulses in gases is studied. The redshift spans from a few 100/cm to several 1000/cm corresponding to a shift of 50–500 nm for Nd:glass laser systems. The generated pulses have an almost perfect Gaussian beam profile insensitive of the pump beam profile, and are much shorter than the pump pulses. The highest measured energy is as high as 30 mJ, which is significantly higher than possible with solid state nonlinear frequency shifters.

M. Zürch, and C. Spielmann
Extreme ultraviolet digital in-line holography using a tabletop source
Applied Optics 54, 5992 (2015)

Abstract: Digital in-line holography (DIH) offers fast, lensless, and aberration-free imaging with diffraction-limited resolution and inherently combines phase- and amplitude-contrast imaging, as well as three-dimensional imaging. Extending this technique to shorter wavelengths allows increasing the achievable spatial and phase-contrast resolution, as well as accessing material parameters not accessible in the optical domain. In this paper, we report on DIH experiments conducted with a coherent tabletop ultrafast high harmonic source operated at 38 nm wavelength. Applying a twin-image-free reconstruction scheme optimized for highly absorbing samples, we were able to demonstrate the phase-contrast imaging of silicon nitride sheets of 15 nm thickness and the use of the strong absorption of extreme ultraviolet in matter for amplitude-contrast imaging of thin films with spatial resolution below 1 μm. High-resolution morphology determination in combination with phase-contrast imaging is of special importance in thin-film characterization and applications arising thereof.

M. Schnell, A. Sävert, I. Uschmann, O. Jansen, M. C. Kaluza, and C. Spielmann
Characterization and application of hard x-ray betatron radiation generated by relativistic electrons from a laser-wakefield accelerator
Journal of Plasma Physics 81, 1 (2015)

Abstract: The necessity for compact table-top x-ray sources with higher brightness, shorter wavelength and shorter pulse duration has led to the development of complementary sources based on laser-plasma accelerators, in contrast to conventional accelerators. Relativistic interaction of short-pulse lasers with underdense plasmas results in acceleration of electrons and in consequence in the emission of spatially coherent radiation, which is known in the literature as betatron radiation. In this article, we report on our recent results in the rapidly developing field of secondary x-ray radiation generated by high-energy electron pulses. The betatron radiation is characterized with a novel setup allowing to measure the energy, the spatial energy distribution in the far-field of the beam and the source size in a single laser shot. Furthermore, the polarization state is measured for each laser shot. In this way, the emitted betatron x-rays can be used as a non-invasive diagnostic tool to retrieve very subtle information of the electron dynamics within the plasma wave. Parallel to the experimental work, 3D particle-in-cell simulations were performed, proved to be in good agreement with the experimental results.

A. Sävert, S. P. D. Mangles, M. Schnell, E. Siminos, J. M. Cole, M. Leier, M. Reuter, M. B. Schwab, M. Möller, K. Poder, O. Jäckel, G. G. Paulus, C. Spielmann, S. Skupin, Z. Najmudin, and M. C. Kaluza
Direct Observation of the Injection Dynamics of a Laser Wakefield Accelerator Using Few-Femtosecond Shadowgraphy
Physical Review Letters 115, 055002 (2015)

Abstract: We present few-femtosecond shadowgraphic snapshots taken during the nonlinear evolution of the plasma wave in a laser wakefield accelerator with transverse synchronized few-cycle probe pulses. These snapshots can be directly associated with the electron density distribution within the plasma wave and give quantitative information about its size and shape. Our results show that self-injection of electrons into the first plasma-wave period is induced by a lengthening of the first plasma period. Three-dimensional particle-in-cell simulations support our observations.

J. Seres, E. Seres, B. Landgraf, B. Aurand, T. Kuehl, and C. Spielmann
Quantum Path Interference and Multiple Electron Scattering in Soft X-Ray High-Order Harmonic Generation
Photonics 2, 104 (2015)

Abstract: High-order harmonic generation is an important mechanism to generate coherent radiation in the few–100-eV spectral range with ultrashort laser pulses. Moreover, a closer inspection of the measured spectra provides unique information about the underlying physics and allows deriving guidelines for improvements. The long-range modulation of the spectral envelope is linked to phase matching, and we will show how to improve it with a double-pulse excitation scheme. Additionally, the spectrum contains only every fourth harmonic, which can be well explained by the quantum interference of multiple scattered electrons, and two dominant electron trajectories were selected by X-ray parametric interaction.


M. Gräfe, A. Hoffmann, and C. Spielmann
Ultrafast fluorescence spectroscopy for axial resolution of flurorophore distributions
Applied Physics B 117, 833 (2014)

Abstract: A new method for determining the fluorophore distribution along the propagation axis of an ultrashort optical pulse is presented. The axial resolution is obtained by temporal gating of the backward emitted fluorescence via optical parametric amplification, and we demonstrated a resolution in the order of a few 100 μm. With this approach, sampling of the fluorophore concentration of thin layers without using optics with a large numerical aperture will be possible, such as investigating the human retina via time-resolved fluorescence measurements. Additionally, we verified the gain is orders of magnitude higher for coherent seeding, making optical parametric gating very interesting for discriminating between coherently and incoherently scattered light for other multimodal imaging applications.

C. Spielmann
Electrons take the fast track through silicon
Science 346, 1293 (2014)

Abstract: The photoelectric effect, the emission of electrons from a metal surface after absorbing light, was explained by Einstein's model, where light particles (photons) must have a minimum energy (frequency) to ionize atoms. The number of excited atoms is proportional to the intensity (the number of photons delivered). However, when the light is supplied by very intense, very fast pulses from lasers, the number of ionized atoms will depend on the electric field strength - the amplitude of the light seen as an electromagnetic wave. This change occurs because ionization occurs via quantum tunneling through the relevant energy barrier during a short time window near the maxima of the electric field. Isolated attosecond pulses recently enabled studies of the dynamics of tunneling ionization of atoms in gases. On page 1348 of this issue, Schultze et al. experimentally show that atoms in a solid are also excited via the tunneling process.

M. Zürch, J. Rothhardt, S. Hädrich, S. Demmler, M. Krebs, J. Limpert, A. Tünnermann, A. Guggenmos, U. Kleineberg, and C. Spielmann
Real-time and Sub-wavelength Ultrafast Coherent Diffraction Imaging in the Extreme Ultraviolet
Scientific Reports 4, 7356 (2014)

Abstract: Coherent Diffraction Imaging is a technique to study matter with nanometer-scale spatial resolution based on coherent illumination of the sample with hard X-ray, soft X-ray or extreme ultraviolet light delivered from synchrotrons or more recently X-ray Free-Electron Lasers. This robust technique simultaneously allows quantitative amplitude and phase contrast imaging. Laser-driven high harmonic generation XUV-sources allow table-top realizations. However, the low conversion efficiency of lab-based sources imposes either a large scale laser system or long exposure times, preventing many applications. Here we present a lensless imaging experiment combining a high numerical aperture (NA = 0.8) setup with a high average power fibre laser driven high harmonic source. The high flux and narrow-band harmonic line at 33.2 nm enables either sub-wavelength spatial resolution close to the Abbe limit (Δr = 0.8λ) for long exposure time, or sub-70 nm imaging in less than one second. The unprecedented high spatial resolution, compactness of the setup together with the real-time capability paves the way for a plethora of applications in fundamental and life sciences.

M. Zürch, A. Hoffmann, M. Gräfe, B. Landgraf, M. Riediger, and Ch. Spielmann
Characterization of a broadband interferometric autocorrelator for visible light with ultrashort blue laser pulses
Optics Communications 321, 28 (2014)

Abstract: We present a compact interferometric autocorrelator that allows the characterization of ultrashort laser pulses in the visible light domain (370–740nm). The presented device uses a GaN photodiode with corresponding two-photon absorption. Different GaN and AlGaN photodiodes were characterized for this purpose. Despite AlGaN diodes have a better matched bandgap for this application, we have found that only the GaN diodes show sufficient nonlinear behavior. Using the autocorrelator we were able to characterize ultrashort frequency doubled Ti:Sapphire laser pulses with a pulse duration down to 18fs in the second harmonic having just a few hundred nanojoules of pulse energy. The broadband behavior and extension towards the UV along with the need for only low energetic pulses are the novelties of this device.

A. Hoffmann, M. Zürch, M. Gräfe, and C. Spielmann
Spectral broadening and compression of sub-millijoule laser pulses in hollow-core fibers filled with sulfur hexafluoride
Optics Express 22, 12038 (2014)

Abstract: Spectral broadening in gas-filled hollow-core fibers is discussed for sulfur hexafluoride, a molecular gas with Raman activity. Experimental results for compressed pulses are presented for input pulses longer than the Raman period and shorter than the dephasing time at a central wavelength of 800 nm and 400 nm, respectively. For both wavelengths we compress the pulses by a factor of three and maintain a good pulse quality. The obtained results are of interest for compressing pulses generated with Yb doped lasers.

M. Zürch, S. Foertsch, M. Matzas, K. Pachmann, R. Kuth, and C. Spielmann
Apparatus and fast method for cancer cell classification based on high harmonic coherent diffraction imaging in reflection geometry
Proceedings of SPIE 9033, 1 (2014)

Abstract: In cancer treatment it is highly desirable to identify and /or classify individual cancer cells in real time. Nowadays, the standard method is PCR which is costly and time-consuming. Here we present a different approach to rapidly classify cell types: we measure the pattern of coherently diffracted extreme ultraviolet radiation (XUV radiation at 38nm wavelength), allowing to distinguish different single breast cancer cell types. The output of our laser driven XUV light source is focused onto a single unstained and unlabeled cancer cell, and the resulting diffraction pattern is measured in reflection geometry. As we will further show, the outer shape of the object can be retrieved from the diffraction pattern with sub-micron resolution. For classification it is often not necessary to retrieve the image, it is only necessary to compare the diffraction patterns which can be regarded as a spatial fingerprint of the specimen. For a proof-of-principle experiment MCF7 and SKBR3 breast cancer cells were pipetted on gold-coated silica slides. From illuminating each single cell and measuring a diffraction pattern we could distinguish between them. Owing to the short bursts of coherent soft x-ray light, one could also image temporal changes of the specimen, i.e. studying changes upon drug application once the desired specimen is found by the classification method. Using a more powerful laser, even classifying circulating tumor cells (CTC) at a high throughput seems possible. This lab-sized equipment will allow fast classification of any kind of cells, bacteria or even viruses in the near future.

J. Seres, E. Seres, B. Landgraf, B. Ecker, B. Aurand, A. Hoffmann, G. Winkler, S. Namba, T. Kuehl, and C. Spielmann
Parametric amplification of attosecond pulse trains at 11nm
Scientific Reports 4, 4254 (2014)

Abstract: We report the first experimental demonstration of the parametric amplification of attosecond pulse trains at around 11 nm. The helium amplifier is driven by intense laser pulses and seeded by high-order harmonics pulses generated in a neon gas jet. Our measurements suggest that amplification takes place only if the seed pulse-trains are perfectly synchronized in time with the driving laser field in the amplifier. Varying the delay, we estimate the durations of the individual extreme ultraviolet pulses within the train to be on the order of 0.2 fs. Our results demonstrate that strong-field parametric amplification can be a suitable tool to amplify weak attosecond pulses from non-destructive pump-probe experiments and it is an important step towards designing amplifiers for realization of energetic XUV pulses with sub-femtosecond duration using compact lasers fitting in university laboratories.

J. Seres, E. Seres, B. Landgraf, B. Ecker, B. Aurand, T. Kuehl, and C. Spielmann
High-harmonic generation and parametric amplification in the soft X-rays from extended electron trajectories
Scientific Reports 4, 4234 (2014)

Abstract: We report, for the first time, the generation of high-order harmonics in a spectral range between 200 eV and 1 keV with an unusual spectral property: only every 4th (4i + 1, i∈ℵ) harmonic line appears, whereas the usual high-harmonic spectra consist of every odd (2i + 1) harmonic. We attribute this unique property to the quantum path interference of two extended electron trajectories that experience multiple re-scattering. In the well-established theory, electrons emitted via tunnel ionisation are accelerated by a laser field, return to the ion and recombine. The acceleration typically lasts for less than one optical cycle, and the electrons radiate in the extreme ultraviolet range at recombination. In contrast, for extended trajectories, electrons are accelerated over two or more optical cycles. Here, we demonstrate that two sets of trajectories dominate and provide substantial contributions to the generated soft X-ray radiation because they fulfil the resonance condition for X-ray parametric amplification.


M. Schnell, A. Sävert, I. Uschmann, M. Reuter, M. Nicolai, T. Kämpfer, B. Landgraf, O. Jäckel, O. Jansen, A. Pukhov, M. C. Kaluza, and C. Spielmann
Optical control of hard X-ray polarization by electron injection in a laser wakefield accelerator
Nature Communications 4, 2421 (2013)

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.

M. Zürch, C. Kern, and C. Spielmann
XUV coherent diffraction imaging in reflection geometry with low numerical aperture
Optics Express 21, 21131 (2013)

Abstract: We present an experimental realization of coherent diffraction imaging in reflection geometry illuminating the sample with a laser driven high harmonic generation (HHG) based XUV source. After recording the diffraction pattern in reflection geometry, the data must be corrected before the image can be reconstructed with a hybrid-input-output (HIO) algorithm. In this paper we present a detailed investigation of sources of spoiling the reconstructed image due to the nonlinear momentum transfer, errors in estimating the angle of incidence on the sample, and distortions by placing the image off center in the computation grid. Finally we provide guidelines for the necessary parameters to realize a satisfactory reconstruction within a spatial resolution in the range of one micron for an imaging scheme with a numerical aperture NA < 0.03.

D. F. A. Winters, V. Bagnoud, B. Ecker, U. Eisenbarth, S. Götte, T. Kühl, P. Neumayer, C. Spielmann, Th. Stöhlker, and B. Zielbauer
A beamline for x-ray laser spectroscopy at the experimental storage ring at GSI
Physica Scripta 2013, 014089 (2013)

Abstract: By combining an x-ray laser (XRL) with a heavy-ion storage ring, precision laser spectroscopy of the fine-structure splitting in heavy Li-like ions will be possible. An initial study has been performed to determine the feasibility of a first experiment at the experimental storage ring at GSI in Darmstadt, which also has great potential for the experiments planned for FAIR. We plan to perform a unique, direct and precise measurement of a fine-structure transition in a heavy Li-like ion. Such a measurement will test state-of-the-art atomic structure calculations in strong fields. This endeavour will require that the existing infrastructure is complemented by a dedicated beamline for the XRL. In this paper, we will discuss the details of this project and outline a proof-of-principle experiment.


M. Zürch, C. Kern, P. Hansinger, A. Dreischuh, and C. Spielmann
Strong-field physics with singular light beams
Nature Physics 8, 743 (2012)

Abstract: Light beams carrying a point singularity with a screw-type phase distribution are associated with an optical vortex. The corresponding momentum flow leads to an orbital angular momentum of the photons. The study of optical vortices has led to applications such as particle micro-manipulation, imaging, interferometry, quantum information and high-resolution microscopy and lithography. Recent analyses showed that transitions forbidden by selection rules seem to be allowed when using optical vortex beams. To exploit these intriguing new applications, it is often necessary to shorten the wavelength by nonlinear frequency conversion. However, during the conversion the optical vortices tend to break up. Here we show that optical vortices can be generated in the extreme ultraviolet (XUV) region using high-harmonic generation. The singularity impressed on the fundamental beam survives the highly nonlinear process. Vortices in the XUV region have the same phase distribution as the driving field, which is in contradiction to previous findings, where multiplication of the momentum by the harmonic order is expected. This approach opens the way for several applications based on vortex beams in the XUV region.

J. Seres, E. Seres, and C. Spielmann
Classical model of strong-field parametric amplification of soft x rays
Physical Review A 86, 013822 (2012)

Abstract: We present a detailed theoretical description of laser driven x-ray parametric amplification, which has been experimentally demonstrated by Seres et al. [Nature Phys. 6 455 (2010)] together with a supporting basic model. The process is based on the parametric interaction of an x-ray photon with a laser accelerated electron in a Coulomb field. With the extended model we are able to estimate the gain cross section also for a finite energy distribution of the interacting electrons as well as to consider dephasing between the electrons and the x-ray field. The improved model is capable of describing the recent experimental findings much more accurately.

E. Seres, J. Seres, and C. Spielmann
Extreme ultraviolet light source based on intracavity high harmonic generation in a mode locked Ti:sapphire oscillator with 9.4 MHz repetition rate
Optics Express 20, 6185 (2012)

Abstract: We report on the realization of an intracavity high harmonic source with a cutoff above 30 eV. The EUV source is based on a high power, hard-aperture, Kerr-lens mode-locked Ti:sapphire oscillator with a repetition rate of 9.4 MHz. The laser is operated in the net negative dispersion regime resulting in intracavity pulses as short as 17 fs with 1 µJ pulse energy. In a second intracavity focus, intensity more than 10^(14) W/cm^2 has been achieved, which is sufficient for high harmonic generation in a Xenon gas jet.

S. Eyring, C. Kern, M. Zürch, and C. Spielmann
Improving high-order harmonic yield using wavefront-controlled ultrashort laser pulses
Optics Express 20, 5601 (2012)

Abstract: In this work we show that it is possible to increase the high-order harmonic yield when using wavefront-shaped laser beams. The investigation of the beam profile near the interaction region shows that the optimized beam is asymmetric and has a larger diameter. Thus, the optimized beam leads to a higher yield even if the peak intensity is lower compared to an unoptimized beam. This indicates that the wavefront of the fundamental laser beam and, accordingly, the focal profile play an important role in the efficient generation of high-order harmonic radiation.

M. Schnell, A. Sävert, B. Landgraf, M. Reuter, M. Nicolai, O. Jäckel, C. Peth, T. Thiele, O. Jansen, A. Pukhov, O. Willi, M. C. Kaluza, and C. Spielmann
Deducing the Electron-Beam Diameter in a Laser-Plasma Accelerator Using X-Ray Betatron Radiation
Physical Review Letters 108, 075001 (2012)

Abstract: We investigate the properties of a laser-plasma electron accelerator as a bright source of keV x-ray radiation. During the interaction, the electrons undergo betatron oscillations and from the carefully measured x-ray spectrum the oscillation amplitude of the electrons can be deduced which decreases with increasing electron energies. From the oscillation amplitude and the independently measured x-ray source size of (1.8 ± 0.3)  μm we are able to estimate the electron bunch diameter to be (1.6 ± 0.3)  μm.


B. Aurand, J. Seres, V. Bagnoud, B. Ecker, D. C. Hochhaus, P. Neumayer, E. Seres, C. Spielmann, B. Zielbauer, D. Zimmer, and T. Kühl
Laser driven X-ray parametric amplification in neutral gases-a new brilliant light source in the XUV
Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment 653, 130 (2011)

Abstract: In this paper we present the experimental setup and results showing a new type of strong-field parametric amplification of high-order harmonic radiation. With a simple semi-classical model, we can identify the most important experimental parameters, the spectral range and the small signal gain in gases. Using a single stage amplifier, a small signal gain of 8000 has been obtained in argon for the spectral range of 40 - 50 eV, using 350 fs, 7 mJ pulses at 1.05 μm. An outlook for an experiment employing a double stage gas system will be given.

B. Landgraf, M. Schnell, A. Sävert, M. C. Kaluza, and C. Spielmann
High resolution 3D gas-jet characterization
Review of Scientific Instruments 82, 083106 (2011)

Abstract: We present a tomographic characterization of gas jets employed for high-intensity laser-plasma interaction experiments where the shape can be non-symmetrically. With a Mach-Zehnder interferometer we measured the phase shift for different directions through the neutral density distribution of the gas jet. From the recorded interferograms it is possible to retrieve 3-dimensional neutral density distributions by tomographic reconstruction based on the filtered back projections. We report on criteria for the smallest number of recorded interferograms as well as a comparison with the widely used phase retrieval based on an Abel inversion. As an example for the performance of our approach, we present the characterization of nozzles with rectangular openings or gas jets with shock waves. With our setup we obtained a spatial resolution of less than 60 μm for an Argon density as low as 2 × 10^{17} cm^{−3}.

C. Kern, M. Zürch, J. Petschulat, T. Pertsch, B. Kley, T. Käsebier, U. Hübner, and C. Spielmann
Comparison of femtosecond laser-induced damage on unstructured vs. nano-structured Au-targets
Applied Physics A 104, 15 (2011)

Abstract: The combination of high-field physics with nano-plasmonics has proven to be feasible in producing high harmonics of intense laser radiation from noble gases, assisted by the field-enhancement effect in the proximity of metallic nano-antennas. However, the intensity region where harmonics can be generated without irreversible damage to these delicate structures is rather narrow. We explore the damage threshold of gold targets that exhibit regular structures on a nanoscopic scale, either explicitly resonant to the used laser frequency, or off-resonance. These are compared to values for bulk material in order to gain insight into the role of plasmonic resonances in the response of solid targets on intense laser radiation. We find that the presence of such a resonance lowers the threshold fluence (J/cm^2) where global structural damage sets in by about an order of magnitude. Statistical deviations either in local pulse energy of the damage inducing laser radiation or in the exact resonance behaviour of singular structures prove to be limited. These results should serve as a guideline for future experiments working near the damage threshold of more sophisticated antenna designs.


J. Seres, E. Seres, B. Ecker, D. Hochhaus, D. Zimmer, V. Bagnoud, B. Aurand, B. Zielbauer, C. Spielmann, and T. Kühl
Reply to 'The super-quadratic growth of high-harmonic signal as a function of pressure'
Nature Physics 6, 928 (2010)

Abstract: The high-harmonic generation (HHG) yield depends on several important experimental parameters and can be described with models including single atom response and propagation effects. In our recent paper we extended this description by adding a stimulated emission process and named it self-seeded X-ray parametric amplification (XPA).

D. C. Hochhaus, J. Seres, B. Aurand, B. Ecker, B. Zielbauer, D. Zimmer, C. Spielmann, and T. Kühl
Tuning the high-order harmonic lines of a Nd:Glass laser for soft X-ray laser seeding
Applied Physics B 100, 711 (2010)

Abstract: We report here more than 50% coverage of the XUV spectral range between 18 nm and 35 nm by tuning the high-order harmonics generated by a fixed frequency Nd:Glass laser system. The tuning range achieved is suitable to seed Ni-like Y, Zr and Mo soft X-ray lasers.

J. Seres, E. Seres, D. Hochhaus, B. Ecker, D. Zimmer, V. Bagnoud, T. Kühl, and C. Spielmann
Laser-driven amplification of soft X-rays by parametric stimulated emission in neutral gases
Nature Physics 6, 455–461 (2010)

Abstract: We present a new method for parametric amplification of soft-X-ray radiation. The laser-driven amplifier is based on parametric stimulated emission and is seeded with high-order-harmonic radiation generated in the same medium. The exponential increase of the soft-X-ray yield with increasing atomic density is experimentally demonstrated for two different sets of laser parameters. A small-signal gain up to 8 × 10^{3} is obtained in both experiments at about 40 eV in argon using 350-fs-long laser pulses and with 6-fs-long ones at about 260 eV in helium, respectively. This new scheme reduces the pumping threshold for lasing with a comparable conversion efficiency into the millijoule level, which is about two orders of magnitude smaller compared with the conventional plasma X-ray lasers. With a simple model, we can estimate the necessary experimental conditions for identifying the spectral range and the magnitude of the maximum gain, which are in reasonable agreement with our measurements.

T. Kühl, B. Aurand, V. Bagnoud, B. Ecker, U. Eisenbarth, O. Guilbaud, J. Fils, S. Goette, J. Habib, D. Hochhaus, D. Javorkova, P. Neumayer, S. Kazamias, M. Pittman, D. Ros, J. Seres, C. Spielmann, B. Zielbauer, and D. Zimmer
Progress in the applicability of plasma X-ray lasers
Hyperfine Interactions 196, 233 (2010)

Abstract: Proposed as satellite-based weapons during the 1980s, X-ray lasing was for a long time only achieved with enormous amounts of pump energy in either nuclear explosions or at kilojoule-class laser installations. During the last few years a tremendous development was achieved, most visible in the realisation of the FEL lasers at DESY and SLAC. As important for a wider applicability is the enormous reduction in pump energy for laser pumped plasma X-ray lasers, which now brings such devices into the range of applications for diagnostics and spectroscopy even in smaller laboratories. Main developments were the transient excitation scheme and the optimized pumping concepts. This paper concentrates on developments at the GSI Helmholtzcenter at Darmstadt aiming towards reliable X-ray laser sources in the range from 50 to several 100 eV. The main driving forces for the laser development at GSI are the possible application for the spectroscopy of Li-like ions in the storage ring ESR and the future storage ring NESR at FAIR, and the interest in novel plasma diagnostics.