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Publications by
Robert Klas

All publications of HI Jena


W. Eschen, L. Loetgering, V. Schuster, R. Klas, A. Kirsche, L. Berthold, M. Steinert, T. Pertsch, H. Gross, M. Krause, J. Limpert, and J. Rothhardt
Material-specific high-resolution table-top extreme ultraviolet microscopy
Light: Science & Applications 11, 2678 (2022)

Abstract: Microscopy with extreme ultraviolet (EUV) radiation holds promise for high-resolution imaging with excellent material contrast, due to the short wavelength and numerous element-specific absorption edges available in this spectral range. At the same time, EUV radiation has significantly larger penetration depths than electrons. It thus enables a nano-scale view into complex three-dimensional structures that are important for material science, semiconductor metrology, and next-generation nano-devices. Here, we present high-resolution and material-specific microscopy at 13.5 nm wavelength. We combine a highly stable, high photon-flux, table-top EUV source with an interferometrically stabilized ptychography setup. By utilizing structured EUV illumination, we overcome the limitations of conventional EUV focusing optics and demonstrate high-resolution microscopy at a half-pitch lateral resolution of 16 nm. Moreover, we propose mixed-state orthogonal probe relaxation ptychography, enabling robust phase-contrast imaging over wide fields of view and long acquisition times. In this way, the complex transmission of an integrated circuit is precisely reconstructed, allowing for the classification of the material composition of mesoscopic semiconductor systems.


S. Skruszewicz, A. Przystawik, D. Schwickert, M. Sumfleth, M. Namboodiri, V. Hilbert, R. Klas, P. Gierschke, V. Schuster, A. Vorobiov, C. Haunhorst, D. Kip, J. Limpert, J. Rothhardt, and T. Laarmann
Table-top interferometry on extreme time and wavelength scales
Optics Express 29, 40333 (2021)

Abstract: High-resolution Fourier-transform spectroscopy using table-top sources in the extreme ultraviolet (XUV) spectral range is still in its infancy. In this contribution a significant advance is presented based on a Michelson-type all-reflective split-and-delay autocorrelator operating in a quasi amplitude splitting mode. The autocorrelator works under a grazing incidence angle in a broad spectral range (10 nm - 1 µ m) providing collinear propagation of both pulse replicas and thus a constant phase difference across the beam profile. The compact instrument allows for XUV pulse autocorrelation measurements in the time domain with a single-digit attosecond precision resulting in a resolution of E/Δ E=2000. Its performance for spectroscopic applications is demonstrated by characterizing a very sharp electronic transition at 26.6 eV in Ar gas induced by the 11th harmonic of a frequency-doubled Yb-fiber laser leading to the characteristic 3s3p⁶4p¹P¹ Fano-resonance of Ar atoms. We benchmark our time-domain interferometry results with a high-resolution XUV grating spectrometer and find an excellent agreement. The common-path interferometer opens up new opportunities for short-wavelength femtosecond and attosecond pulse metrology and dynamic studies on extreme time scales in various research fields.

W. Eschen, S. Wang, C. Liu, R. Klas, M. Steinert, S. Yulin, H. Meissner, M. Bussmann, T. Pertsch, J. Limpert, and J. Rothhardt
Towards attosecond imaging at the nanoscale using broadband holography-assisted coherent imaging in the extreme ultraviolet
Communications Physics 4, 154 (2021)

Abstract: The inherently broad bandwidth of attosecond pulses conflicts with the coherence requirements of lensless imaging. Here, broadband holography-assisted coherent imaging is demonstrated with a resolution of less than 35 nm. Nanoscale coherent imaging has emerged as an indispensable modality, allowing to surpass the resolution limit given by classical imaging optics. At the same time, attosecond science has experienced enormous progress and has revealed the ultrafast dynamics in complex materials. Combining attosecond temporal resolution of pump-probe experiments with nanometer spatial resolution would allow studying ultrafast dynamics on the smallest spatio-temporal scales but has not been demonstrated yet. To date, the large bandwidth of attosecond pulses poses a major challenge to high-resolution coherent imaging. Here, we present broadband holography-enhanced coherent imaging, which enables the combination of high-resolution coherent imaging with a large spectral bandwidth. By implementing our method at a high harmonic source, we demonstrate a spatial resolution of 34 nm in combination with a spectral bandwidth of 5.5 eV at a central photon energy of 92 eV. The method is single-shot capable and retrieves the spectrum from the measured diffraction pattern.

V. Schuster, V. Hilbert, R. Klas, C. Liu, M. Tschernajew, B. Bernhardt, J. Rothhardt, and J. Limpert
Agile spectral tuning of high order harmonics by interference of two driving pulses
Optics Express 29, 22117 (2021)

Abstract: In this work, the experimental realization of a tunable high photon flux extreme ultraviolet light source is presented. This is enabled by high harmonic generation of two temporally delayed driving pulses with a wavelength of 1030 nm, resulting in a tuning range of 0.8 eV at the 19th harmonic at 22.8 eV. The implemented approach allows for fast tuning of the spectrum, is highly flexible and is scalable towards full spectral coverage at higher photon energies.

V. Schuster, C. Liu, R. Klas, P. Dominguez, J. Rothhardt, J. Limpert, and B. Bernhardt
Ultraviolet dual comb spectroscopy: a roadmap
Optics Express 29, 21859 (2021)

Abstract: Dual Comb Spectroscopy proved its versatile capabilities in molecular fingerprinting in different spectral regions, but not yet in the ultraviolet (UV). Unlocking this spectral window would expand fingerprinting to the electronic energy structure of matter. This will access the prime triggers of photochemical reactions with unprecedented spectral resolution. In this research article, we discuss the milestones marking the way to the first UV dual comb spectrometer. We present experimental and simulated studies towards UV dual comb spectroscopy, directly applied to planned absorption measurements of formaldehyde (centered at 343 nm, 3.6 eV) and argon (80 nm, 16 eV). This will enable an unparalleled relative resolution of up to 10-9 - with a table-top UV source surpassing any synchrotron-linked spectrometer by at least two and any grating-based UV spectrometer by up to six orders of magnitude.

R. Klas, A. Kirsche, M. Gebhardt, J. Buldt, H. Stark, S. Hädrich, J. Rothhardt, and J. Limpert
Ultra-short-pulse high-average-power megahertz-repetition-rate coherent extreme-ultraviolet light source
PhotoniX 2, 4 (2021)

Abstract: High harmonic generation (HHG) enables coherent extreme-ultraviolet (XUV) radiation with ultra-short pulse duration in a table-top setup. This has already enabled a plethora of applications. Nearly all of these applications would benefit from a high photon flux to increase the signal-to-noise ratio and decrease measurement times. In addition, shortest pulses are desired to investigate fastest dynamics in fields as diverse as physics, biology, chemistry and material sciences. In this work, the up-to-date most powerful table-top XUV source with 12.9 ± 3.9 mW in a single harmonic line at 26.5 eV is demonstrated via HHG of a frequency-doubled and post-compressed fibre laser. At the same time the spectrum supports a Fourier-limited pulse duration of sub-6 fs in the XUV, which allows accessing ultrafast dynamics with an order of magnitude higher photon flux than previously demonstrated. This concept will greatly advance and facilitate applications of XUV radiation in science and technology and enable photon-hungry ultrafast studies.

M. Gebhardt, E. Amuah, R. Klas, H. Stark, J. Buldt, A. Steinkopff, and J. Limpert
Investigation of spatiotemporal output beam profile instabilities from differentially pumped capillaries
Optics Express 29, 6957 (2021)

Abstract: Differentially pumped capillaries, i.e., capillaries operated in a pressure gradient environment, are widely used for nonlinear pulse compression. In this work, we show that strong pressure gradients and high gas throughputs can cause spatiotemporal instabilities of the output beam profile. The instabilities occur with a sudden onset as the flow evolves from laminar to turbulent. Based on the experimental and numerical results, we derive guidelines to predict the onset of those instabilities and discuss possible applications in the context of nonlinear flow dynamics. (C) 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

M. Gebhardt, T. Heuermann, R. Klas, C. Liu, A. Kirsche, M. Lenski, Z. Wang, C. Gaida, J. E. Antonio-Lopez, A. Schulzgen, R. Amezcua-Correa, J. Rothhardt, and J. Limpert
Bright, high-repetition-rate water window soft X-ray source enabled by nonlinear pulse self-compression in an antiresonant hollow-core fibre
Light: Science & Applications 10, 2021 (2021)

Abstract: Bright, coherent soft X-ray radiation is essential to a variety of applications in fundamental research and life sciences. To date, a high photon flux in this spectral region can only be delivered by synchrotrons, free-electron lasers or high-order harmonic generation sources, which are driven by kHz-class repetition rate lasers with very high peak powers. Here, we establish a novel route toward powerful and easy-to-use SXR sources by presenting a compact experiment in which nonlinear pulse self-compression to the few-cycle regime is combined with phase-matched high-order harmonic generation in a single, helium-filled antiresonant hollow-core fibre. This enables the first 100 kHz-class repetition rate, table-top soft X-ray source that delivers an application-relevant flux of 2.8 x 10(6) photon s(-1) eV(-1) around 300 eV. The fibre integration of temporal pulse self-compression (leading to the formation of the necessary strong-field waveforms) and pressure-controlled phase matching will allow compact, high-repetition-rate laser technology, including commercially available systems, to drive simple and cost-effective, coherent high-flux soft X-ray sources.

R. Klas
Efficiency scaling of high harmonic generation using ultrashort fiber lasers
Doctoral thesis
Friedrich-Schiller-Universität Jena, Physikalisch-Astronomische Fakultät (2021)

Abstract: -

W. Eschen, L. Loetgering, V. Schuster, R. Klas, J. Limpert, and J. Rothhardt
Table-top high-resolution ptychographic EUV imaging

Abstract: Nanoscale imaging at 13.5 nm provides ideal opportunities for ‘at wavelength’ metrology. We present a setup and the latest results on lensless ptychographic imaging at 92 eV achieving sub 30 nm resolution.

M. Gebhardt, T. Heuermann, R. Klas, C. Liu, A. Kirsche, M. Lenski, Z. Wang, C. Gaida, J. E. Antonio-Lopez, A. Schulzgen, R. Amezcua-Correa, J. Rothhardt, and J. Limpert
100 kHz water window soft X-ray high-order harmonic generation through pulse self-compression in an antiresonant hollow-core fiber

Abstract: Coherent soft X-ray (SXR) sources that provide a high photon flux in the water window are essential tools for advanced spectroscopy (e.g. of magnetic materials [1] and organic compounds [2] ) or for lens-less bio imaging with nm-scale resolution [3] . To date, such sources are mostly large-scale facilities like synchrotrons or free electron lasers. A promising alternative are laser-driven sources, based on high harmonic generation (HHG) in noble gases. Current state-of-the-art SXR HHG uses frequency converted Ti:Sa lasers (to ~2 µm wavelength to increase the phase matching cutoff) with multi-mJ pulse energies at 1 kHz repetition rate [1] . Most recently, there has been a strong push towards increasing the repetition rate of the driving lasers, and the SXR HHG, to enable faster data acquisition, space-charge-reduced SXR photoelectron spectroscopy or coincidence detection [4] , [5] . In this contribution, we present an approach to SXR HHG that is based on nonlinear pulse self-compression and HHG in the same helium gas-filled antiresonant hollow-core fiber (ARHCF). Because of the intensity enhancement resulting from temporal pulse self-compression, the experiments can be driven by moderate-energy, multi-cycle laser pulses, which facilitates repetition rate scaling. We coupled 100 fs-, 250 µJ-pulses centered around 1.9 µm wavelength at a 98 kHz repetition rate to the ARHCF ( Fig. 1a ). When the fiber length (~1.2 m) and the gas pressure at its output (~3.8 bar) were chosen appropriately, the pulses close to its end ( Fig. 1b ) were self-compressed to <20 fs, leading to an on-axis peak intensity >4×10 14 W/cm 2 . At this point, the gas is partially ionized, and the chosen pressure ensures phase matching between the driving laser and the generated SXR light ( Fig. 1b ). It is the first time that this approach is experimentally realized, and we have generated a photon flux >10 6 Ph/s/eV at the carbon K-edge ( Fig. 1c ). To the best of our knowledge, this is the highest photon flux at 300 eV reported to date at a laser repetition rate >1 kHz.

A. Kirsche, R. Klas, M. Gebhardt, L. Eisenbach, W. Eschen, J. Buldt, H. Stark, J. Rothhardt, and J. Limpert
Continuously tunable high photon flux high harmonic source at 50-70 eV

Abstract: Table-top extreme ultraviolet (XUV) light sources based on high harmonic generation (HHG) have emerged as a user-friendly, complementary technique to large scale facilities like synchrotrons and free electron lasers [1] . The ultrashort pulse duration (femtosecond to attosecond) and coherence of these laser-driven sources are highly advantageous for many applications, e.g. coherent diffractive imaging [2] or XUV spectroscopy of atoms, molecules and ions [3] . In addition to the precise control of temporal and spatial properties of the HHG radiation, investigations of narrow band resonances and detailed spectral features in the XUV require tuning of the discrete harmonic lines. Therefore, there is a strong application-driven demand for powerful, tunable XUV combs. To date, a large variety of tunable XUV sources have been demonstrated e.g. by shifting the driving wavelength using optical parametric amplifiers [4] or soliton-plasma dynamics [5] , among others [6] .

M. Tschernajew, S. Hadrich, R. Klas, M. Gebhardt, R. Horsten, S. Werdenburg, S. Pyatchenkov, W. Coene, J. Rothhardt, T. Eidam, and J. Limpert
High repetition rate high harmonic generation with ultra-high photon flux

Abstract: High-harmonic generation (HHG) driven by ultrashort laser pulses is an established process for the generation of coherent extreme ultraviolet (XUV) to soft X-ray radiation, which has found widespread use in various applications [1] . In recent years photon-hungry applications such as coherent diffractive imaging [2] , [3] and applications based on statistical analysis [3] have required more powerful HHG sources, in particular, at high repetition rates. This need can be addressed by using high average power fiber lasers as the HHG drivers [4] . Here, we present a HHG-based XUV source, capable of providing a large photon flux across a wide range between 66 eV and 150 eV. It is driven by a commercial XUV beamline from Active Fiber Systems GmbH consisting of 100-W average power fiber-laser system, delivering up to 300 µJ at <300-fs pulse duration. For HHG this system is operated at 100 W, 600 kHz. A post-compression unit is part of the device to shorten the pulses to ~35 fs, the average power remains at 63W. The turnkey source can provide unprecedented photon fluxes of >10 11 photons/s in each harmonic between 69 eV and 75 eV (HH57-HH63). All fluxes are given at the generation point, i.e. directly after the source.

S. Hadrich, E. Shestaev, N. Walther, T. Nagy, P. Simon, A. Blumenstein, A. Klenke, R. Klas, J. Buldt, H. Stark, M. Gebhardt, S. Breitkopf, P. Jojart, I. Seres, Z. Varallyay, A. Borzsonyi, T. Eidam, and J. Limpert
High-average-power and high-pulse-energy CEP-stable few-cycle pulses: Status of the ELI-ALPS HR2 laser system

Abstract: Few-cycle laser systems providing both high repetition rates and high pulse energies are a major focus of next-generation light sources. Consequently, the research facility of the Extreme Light Infrastructure (ELI) that is devoted to the generation of isolated attoseconds pulses, (ELI-ALPS in Szeged, Hungary), has set the demand for a laser system delivering carrier-envelope phase (CEP) stable pulses with 5 mJ of pulse energy, 6 fs pulse duration at 100 kHz repetition rate, which corresponds to an average power of 500 W [1] . This laser system, that is named HR2 (the high-repetition-rate beam line), is currently under development at Active Fiber System GmbH. Achieving these ambitious laser parameters is done by merging the robust techniques of coherent combination as an average-power scaling concept and the use of stretched hollow-core fiber technology for nonlinear pulse compression [2] .

M. Gebhardt, T. Heuermann, R. Klas, A. Kirsche, C. Liu, Z. Wang, M. Lenski, C. Gaida, C. Jauregui, J. Antonio-Lopez, A. Schülzgen, R. Amezcua-Correa, J. Rothhardt, and J. Limpert
High repetition rate high-order harmonic generation up to the carbon K-edge in an antiresonant hollow-core fiber
No abstract availableLinkBibTeX
W. Eschen, V. Schuster, S. Wang, L. Loetgering, C. Liu, R. Klas, J. Limpert, and J. Rothhardt
Ultrafast nanoscale XUV table-top coherent diffractive imaging
No abstract availableLinkBibTeX


V. Hilbert, M. Tschernajew, R. Klas, J. Limpert, and J. Rothhardt
A compact, turnkey, narrow-bandwidth, tunable, and high-photon-flux extreme ultraviolet source
AIP Advances 10, 045227 (2020)

Abstract: We report on a compact high-photon-flux extreme ultraviolet (XUV) source based on high harmonic generation. A high XUV-photon flux (>10¹³ photons/s) is achieved at 21.8 eV and 26.6 eV. The narrow spectral bandwidth (ΔE/E < 10⁻³) of the generated harmonics is in the range of state-of-the-art synchrotron beamlines and enables high resolution spectroscopy experiments. The robust design based on a fiber– laser system enables turnkey-controlled and even remotely controlled operation outside specialized laser laboratories, which opens the way for a variety of applications.

R. Klas, W. Eschen, A. Kirsche, J. Rothhardt, and J. Limpert
Generation of coherent broadband high photon flux continua in the XUV with a sub-two-cycle fiber laser
Optics Express 28, 6188 (2020)

Abstract: High harmonic sources can provide ultrashort pulses of coherent radiation in the XUV and X-ray spectral region. In this paper we utilize a sub-two-cycle femtosecond fiber laser to efficiently generate a broadband continuum of high-order harmonics between 70 eV and 120 eV. The average power delivered by this source ranges from > 0.2 µW/eV at 80 eV to >0.03 µW/eV at 120 eV. At 92 eV (13.5 nm wavelength), we measured a coherent record-high average power of 0.1 µW/eV, which corresponds to 7 · 109 ph/s/eV, with a long-term stability of 0.8% rms deviation over a 20 min time period. The presented approach is average power scalable and promises up to 1011 ph/s/eV in the near future. With additional carrier-envelop phase control even isolated attosecond pulses can be expected from such sources. The combination of high flux, high photon energy and ultrashort (sub-) fs duration will enable photon-hungry time-resolved and multidimensional studies.

S. Hädrich, N. Walther, E. Shestaev, T. Nagy, P. Simon, A. Blumenstein, R. Klas, J. Buldt, H. Stark, S. Breitkopf, and . others
High Pulse Energy CEP-stable Few-cycle Pulses at High Average Power: Status of the ELI-ALPS HR2 System

Abstract: We demonstrate a two-pulse Bessel beam scheme for generating plasma waveguides guiding high intensity laser pulses over 30 cm with on-axis plasma densities as low as 5 × 10^16cm−3.

M. Tschernajew, S. Hädrich, R. Klas, M. Gebhardt, R. Horsten, S. Weerdenburg, S. Pyatchenkov, W. Coene, J. Rothhardt, T. Eidam, and . others
High Repetition Rate High Harmonic Generation with Ultra-high Photon Flux

Abstract: A 100W fiber laser system is used to drive a high repetition rate HHG beamline producing record-high photon flux of >10^11 photons/s at 69-75eV and >10^10 photons/s for harmonics between 115eV and 140eV.

S. Hädrich, E. Shestaev, N. Walther, T. Nagy, P. Simon, A. Blumenstein, A. Klenke, R. Klas, J. Buldt, and H. Stark
High-average-power and High-pulse-energy CEP-stable few-cycle Pulses: Status of the ELI-ALPS HR2 Laser System

Abstract: Multi-mJ, 10fs pulses at average powers beyond 300W are generated by compression of of an Yb-fiber-CPA in a 6m long stretched capillary. CEP stabilization of the CPA system yields ~220mrad at 1.1kW of average power.

M. Gebhardt, T. Heuermann, C. Gaida, Z. Wang, M. Lenski, R. Klas, A. Kirsche, C. Liu, S. Hädrich, C. Jauregui, and . others
High-order harmonic generation in noble gas driven by high-power ultrafast thulium-doped fiber lasers

Abstract: We present HHG results obtained with thulium-doped fiber lasers. It is the first time that a photon energy cut-off close to 400 eV has been demonstrated using this highly scalable laser technology.

R. Klas, W. Eschen, A. Kirsche, J. Rothhardt, and J. Limpert
Power scalable fiber laser driven high-harmonic source for broadband high photon flux continua

Abstract: A HHG source generating a broadband continuum from 70 eV to 120 eV with an average power of 2 µW is presented. At 92 eV (13.5 nm) 7 10^9 ph/s/eV are generated with an rms deviation of 0.8% over 20 minutes.

M. Gebhardt, T. Heuermann, C. Gaida, Z. Wang, M. Lenski, R. Klas, A. Kirsche, C. Liu, S. Hädrich, C. Jauregui, and . others
Soft x-ray high order harmonic generation from high power ultrafast thulium-doped fiber lasers

Abstract: We report on soft x-ray HHG driven by a thulium-doped fiber laser. It is the first time that a photon energy cut-off ~400 eV has been demonstrated using this highly scalable laser technology.

R. Klas, J. Buldt, H. Stark, A. Kirsche, M. Gebhardt, J. Rothhardt, and J. Limpert
Sub-20 fs high-energy pulse generation at 515 nm with 50 W of average power

Abstract: A nonlinear compression of 515 nm pulses resulting in 17.8 fs-, 50 µJ-pulses at 1 MHz, 50 W average power and near diffraction limited beam quality is presented.

T. Nagy, S. Hadrich, P. Simon, A. Blumenstein, N. Walther, R. Klas, J. Buldt, H. Stark, S. Breitkopf, P. Jojart, I. Seres, Z. Varallyay, T. Eidam, and J. Limpert
Pulse Compression to 3-Cycle Duration beyond 300 W Average Power
No abstract availableLinkBibTeX
M. Gebhardt, T. Heuermann, Z. Wang, M. Lenski, C. Gaida, R. Klas, A. Kirsche, S. Hädrich, J. Rothhardt, and J. Limpert
Soft x-ray high order harmonic generation driven by high repetition rate ultrafast thulium-doped fiber lasers

Abstract: Intense, ultrafast laser sources with an emission wavelength beyond the well-established near-IR are important tools for exploiting the wavelength scaling laws of strong-field, light-matter interactions. In particular, such laser systems enable high photon energy cut-off HHG up to, and even beyond, the water window thus enabling a plethora of subsequent experiments. Ultrafast thulium-doped fiber laser systems (providing a broad amplification bandwidth in the 2 μm wavelength region) represent a promising, average-power scalable laser concept in this regard. These lasers already deliver ~100 fs pulses with multi-GW peak power at hundreds of kHz repetition rate. In this work, we show that combining ultrafast thulium-doped fiber CPA systems with hollow-core fiber based nonlinear pulse compression is a promising approach to realize high photon energy cut-off HHG drivers. Herein, we show that thulium-doped, fiber-laser-driven HHG in argon can access the highly interesting spectral region around 90 eV. Additionally, we show the first water window high-order harmonic generation experiment driven by a high repetition rate, thulium-doped fiber laser system. In this proof of principle demonstration, a photon energy cut-off of approximately 400 eV has been achieved, together with a photon flux <105 ph/s/eV at 300 eV. These results emphasize the great potential of exploiting the HHG wavelength scaling laws with 2 μm fiber laser technology. Improvements of the HHG efficiency, the overall HHG yield and further laser performance enhancements will be the subject of our future work.


T. Nagy, S. Hädrich, P. Simon, A. Blumenstein, N. Walther, R. Klas, J. Buldt, H. Stark, S. Breitkopf, P. J\'ojárt, I. Seres, Z. Várallyay, T. Eidam, and J. Limpert
Generation of three-cycle multi-millijoule laser pulses at 318  W average power
Optica 6, 1423 (2019)

Abstract: The generation of three-cycle multi-millijoule pulses at 318 W power is reported by compressing pulses of a Yb-fiber chirped pulse amplifier in a 6 m long stretched flexible hollow fiber. This technique brings high-power lasers to the few-cycle regime.

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.


K. Bergner, M. Müller, R. Klas, J. Limpert, S. Nolte, and A. Tünnermann
Scaling ultrashort laser pulse induced glass modifications for cleaving applications
Applied Optics 57, 5941 (2018)

Abstract: Ultrashort laser pulses allow for in-volume processing of glass through non-linear absorption. This results in permanent material changes, largely independent of the processed glass, and it is of particular relevance for cleaving applications. In this paper, a laser with a wavelength of 1030 nm, pulse duration of 19 ps, repetition rate of 10 kHz, and burst regime consisting of either four or eight pulses, with an intra-burst pulse separation of 12.5 ns, is used. Subsequently, a Gaussian–Bessel focal line is generated in a fused silica substrate with the aid of an axicon configuration. We show how the structure of the modifications, including the length of material disruptions and affected zones, can be directly influenced by a reasonable choice of focus geometry, pulse energy, and burst regime. We achieve single-shot modifications with 2 μm in diameter and 7.6 mm in length, exceeding an aspect ratio of 1:3800. Furthermore, a maximum length of 10.8 mm could be achieved with a single shot.

R. Klas, A. Kirsche, M. Tschernajew, J. Rothhardt, and J. Limpert
Annular beam driven high harmonic generation for high flux coherent XUV and soft X-ray radiation
Optics Express 26, 19318 (2018)

Abstract: Separation of the high average power driving laser beam from the generated XUV to soft-X-ray radiation poses great challenges in collinear HHG setups due to the losses and the limited power handling capabilities of the typically used separating optics. This paper demonstrates the potential of driving HHG with annular beams, which allow for a straightforward and power scalable separation via a simple pinhole, resulting in a measured driving laser suppression of 5⋅10−3. The approach is characterized by an enormous flexibility as it can be applied to a broad range of input parameters and generated photon energies. Phase matching aspects are analyzed in detail and an HHG conversion efficiency that is only 27% lower than using a Gaussian beam under identical conditions is demonstrated, revealing the viability of the annular beam approach for high flux coherent short-wavelength sources and high

G. Tadesse, W. Eschen, R. Klas, V. Hilbert, D. Schelle, A. Nathanael, M. Zilk, M. Steinert, F. Schrempel, T. Pertsch, A. Tünnermann, J. Limpert, and J. Rothhardt
High resolution XUV Fourier transform holography on a table top
Scientific Reports 8, 8677 (2018)

Abstract: Today, coherent imaging techniques provide the highest resolution in the extreme ultraviolet (XUV) and X-ray regions. Fourier transform holography (FTH) is particularly unique, providing robust and straightforward image reconstruction at the same time. Here, we combine two important advances: First, our experiment is based on a table-top light source which is compact, scalable and highly accessible. Second, we demonstrate the highest resolution ever achieved with FTH at any light source (34 nm) by utilizing a high photon flux source and cutting-edge nanofabrication technology. The performance, versatility and reliability of our approach allows imaging of complex wavelength-scale structures, including wave guiding effects within these structures, and resolving embedded nanoscale features, which are invisible for electron microscopes. Our work represents an important step towards real-world applications and a broad use of XUV imaging in many areas of science and technology. Even nanoscale studies of ultra-fast dynamics are within reach.


J. Buldt, M. Müller, R. Klas, T. Eidam, J. Limpert, and A. Tünnermann
Temporal contrast enhancement of energetic laser pulses by filtered self-phase-modulation-broadened spectra
Optics Letters 42, 3761 (2017)

Abstract: We present a novel approach for temporal contrast enhancement of energetic laser pulses by filtered self-phase-modulation-broadened spectra. A measured temporal contrast enhancement by at least seven orders of magnitude in a simple setup has been achieved. This technique is applicable to a wide range of laser parameters and poses a highly efficient alternative to existing contrast-enhancement methods.

M. Müller, A. Klenke, T. Gottschall, R. Klas, C. Rothhardt, S. Demmler, J. Rothhardt, J. Limpert, and A. Tünnermann
High-average-power femtosecond laser at 258 nm
Optics Letters 42, 2826 (2017)

Abstract: We present an ultrafast fiber laser system delivering 4.6 W average power at 258 nm based on two-stage fourth-harmonic generation in beta barium borate (BBO). The beam quality is close to being diffraction limited with an M^2 value of 1.3×1.6. The pulse duration is 150 fs, which, potentially, is compressible down to 40 fs. A plain BBO and a sapphire-BBO compound are compared with respect to the achievable beam quality in the conversion process. This laser is applicable in scientific and industrial fields. Further scaling to higher average power is discussed.


R. Klas, S. Demmler, M. Tschernajew, S. Hädrich, Y. Shamir, A. Tünnermann, J. Rothhardt, and J. Limpert
Table-top milliwatt-class extreme ultraviolet high harmonic light source
Optica 3, 1167 (2016)

Abstract: Extreme ultraviolet (XUV) lasers are essential for the investigation of fundamental physics. Especially high repetition rate, high photon flux sources are of major interest for reducing acquisition times and improving signal-to-noise ratios in a plethora of applications. Here, an XUV source based on cascaded frequency conversion is presented, which, due to the drastic better single atom response for short wavelength drivers, delivers an average output power of (832±204)  μW at 21.7 eV. This is the highest average power produced by any high harmonic generation source in this spectral range, surpassing previous demonstrations by almost an order of magnitude. Furthermore, a narrowband harmonic at 26.6 eV with a relative energy bandwidth of only ΔE/E=1.8·10−3 has been generated that is of high interest for high-precision spectroscopy experiments.

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.

S. Hädrich, M. Kienel, M. Müller, A. Klenke, J. Rothhardt, R. Klas, T. Gottschall, T. Eidam, A. Drozdy, P. Jójárt, Z. Várallyay, E. Cormier, K. Osvay, A. Tünnermann, and J. Limpert
Energetic sub-2-cycle laser with 216 W average power
Optics Letters 41, 4332 (2016)

Abstract: Few-cycle lasers are essential for many research areas such as attosecond physics that promise to address fundamental questions in science and technology. Therefore, further advancements are connected to significant progress in the underlying laser technology. Here, two-stage nonlinear compression of a 660 W femtosecond fiber laser system is utilized to achieve unprecedented average power levels of energetic ultrashort or even few-cycle laser pulses. In a first compression step, 408 W, 320 μJ, 30 fs pulses are achieved, which can be further compressed to 216 W, 170 μJ, 6.3 fs pulses in a second compression stage. To the best of our knowledge, this is the highest average power few-cycle laser system presented so far. It is expected to significantly advance the fields of high harmonic generation and attosecond science.

J. Rothhardt, S. Hädrich, Y. Shamir, M. Tschnernajew, R. Klas, A. Hoffmann, G. K. Tadesse, A. Klenke, T. Gottschall, T. Eidam, J. Limpert, A. Tünnermann, R. Boll, C. Bomme, H. Dachraoui, B. Erk, M. D. Fraia, D. A. Horke, T. Kierspel, T. Mullins, A. Przystawik, E. Savelyev, J. Wiese, T. Laarmann, J. Küpper, and D. Rolles
High-repetition-rate and high-photon-flux 70 eV high-harmonic source for coincidence ion imaging of gas-phase molecules
Optics Express 24, 18133 (2016)

Abstract: Unraveling and controlling chemical dynamics requires techniques to image structural changes of molecules with femtosecond temporal and picometer spatial resolution. Ultrashort-pulse x-ray free-electron lasers have significantly advanced the field by enabling advanced pump-probe schemes. There is an increasing interest in using table-top photon sources enabled by high-harmonic generation of ultrashort-pulse lasers for such studies. We present a novel high-harmonic source driven by a 100 kHz fiber laser system, which delivers 10^11 photons/s in a single 1.3 eV bandwidth harmonic at 68.6 eV. The combination of record-high photon flux and high repetition rate paves the way for time-resolved studies of the dissociation dynamics of inner-shell ionized molecules in a coincidence detection scheme. First coincidence measurements on CH3I are shown and it is outlined how the anticipated advancement of fiber laser technology and improved sample delivery will, in the next step, allow pump-probe studies of ultrafast molecular dynamics with table-top XUV-photon sources. These table-top sources can provide significantly higher repetition rates than the currently operating free-electron lasers and they offer very high temporal resolution due to the intrinsically small timing jitter between pump and probe pulses.