Abstract: We demonstrate the collinear generation of few-femtosecond ultraviolet and attosecond extreme ultraviolet pulses via a combination of third-harmonic and high harmonic generation in noble gases. The ultrashort coherent light bursts are produced by focusing a sub-1.5-cycle near-infrared/visible laser pulse in two subsequent quasi-static noble gas targets. This approach provides an inherently synchronized pair of UV and XUV pulses, where the UV radiation has a photon energy of ~5 eV and a pulse energy of up to 1 μJ and the XUV radiation contains up to 3.5 · 106 XUV photons per shot with a photon energy exceeding 100 eV. This source represents a novel tool for future UV pump/XUV probe experiments with unprecedented time-resolution. [Link]
E. M. Bothschafter, A. Schiffrin, V.S. Yakovlev1, A.M. Azzeer, F. Krausz1, R. Ernstorfer, and R. Kienberger

Abstract: The coherent high-order harmonic generation from the interaction of ultra-intense femtosecond laser pulses with solid density plasmas holds promise for tabletop sources of extreme ultraviolet (XUV) and soft x-ray radiation with attosecond duration and unprecedented intensities. Together with the generation of mono-energetic electron beams from gas jets and capillaries and the generation of mono-energetic ions from thin foils, this offers a unique tool box of tabletop-laser-generated radiation sources for a wide range of applications previously only accessible with large-scale accelerator and synchrotron-radiation facilities. Especially, the generation of high harmonics from laser plasmas has the potential of being applied to a wide range of experiments from plasma physics to molecular dynamics. So far the studies addressing the generation of high harmonics from laser-generated overcritical plasma surfaces have concentrated mainly on the characterization of the harmonic beams themselves not considering how, in a next step, these beams could be applied to experiments. In this paper we discuss the generation of surface harmonics with the ATLAS (800 mJ, 40 fs) laser system with the emphasis on the transport, spectral shaping refocusing of the harmonic beams, all of these being absolute prerequisites for multi-shot experiments. We also present considerations for future improvements and possible future experiments exploiting the full potential of high harmonic radiation from solid targets. [Link]
R Hörlein, Y Nomura, J Osterhoff, Zs Major, S Karsch, F Krausz, and G D Tsakiris

Abstract: The laser plasma is one of the major contenders as a high-power source for future high-volume-manufacturing extreme ultraviolet lithography systems. Such laser–plasma sources require a target system that allows high-repetition-rate operation with low debris and manageable thermal load at the required high laser power. In this paper, we review the development of the liquid-jet target laser plasmas, from droplets to filaments, with special emphasis on its applicability for high-power extreme ultraviolet generation. We focus on two target systems, the liquid-xenon-jet and the liquid-tin-jet.
Bjorn A M Hansson and Hans M Hertz

Abstract: Since 1998, the interaction of precision spectroscopy and ultrafast laser science has led to several notable accomplishments. Femtosecond laser optical frequency 'combs' (evenly spaced spectral lines) have revolutionized the measurement of optical frequencies and enabled optical atomic clocks. The same comb techniques have been used to control the waveform of ultrafast laser pulses, which permitted the generation of single attosecond pulses, and have been used in a recently demonstrated 'oscilloscope' for light waves. Here we demonstrate intra-cavity high harmonic generation in the extreme ultraviolet, which promises to lead to another joint frontier of precision spectroscopy and ultrafast science. We have generated coherent extreme ultraviolet radiation at a repetition frequency of more than 100 MHz, a 1,000-fold improvement over previous experiments. At such a repetition rate, the mode spacing of the frequency comb, which is expected to survive the high harmonic generation process, is large enough for high resolution spectroscopy. Additionally, there may be many other applications of such a quasi-continuous compact and coherent extreme ultraviolet source, including extreme ultraviolet holography, microscopy, nanolithography and X-ray atomic clocks.
Christoph Gohle, Thomas Udem, Maximilian Herrmann, Jens Rauschenberger, Ronald Holzwarth, Hans A. Schuessler, Ferenc Krausz & Theodor W. Hänsch

Abstract: In this paper the influence of the prepulse current on a capillary-discharge 46.9 nm Ne-like Ar extreme-ultraviolet laser is reported. A current pulse with a typical RC shape (decay time of ~30 μs) was used as a prepulse. Measurements indicate that when the filling pressure is low, the output can be improved by reducing the time delay between the application of the prepulse current and the onset of the main discharge current. For high pressure the reverse is true. This change is most significant for time delays between 2 and 4 μs, and beyond these time delays, the effect is less significant. This effect is attributed to the changes in the capillary channel pressure and also to the absorption of the laser emission by the plasma plume ejected during the prepulse. Thus, apart from ensuring a minimal amount of prepulse current to prevent nonuniformity effects, the timing of the application of the prepulse current is also important.
C. A. Tan and K. H. Kwek

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.
J. Seres, E. Seres, B. Landgraf, B. Ecker, B. Aurand, T. Kuehl & C. Spielmann

Abstract: Manipulation of electron dynamics calls for electromagnetic forces that can be confined to and controlled over sub-femtosecond time intervals. Tailored transients of light fields can provide these forces. We report on the generation of subcycle field transients spanning the infrared, visible, and ultraviolet frequency regimes with a 1.5-octave three-channel optical field synthesizer and their attosecond sampling. To demonstrate applicability, we field-ionized krypton atoms within a single wave crest and launched a valence-shell electron wavepacket with a well-defined initial phase. Half-cycle field excitation and attosecond probing revealed fine details of atomic-scale electron motion, such as the instantaneous rate of tunneling, the initial charge distribution of a valence-shell wavepacket, the attosecond dynamic shift (instantaneous ac Stark shift) of its energy levels, and its few-femtosecond coherent oscillations.
A. Wirth, M. Th. Hassan, I. Grguraš, J. Gagnon, A. Moulet, T. T. Luu, S. Pabst, R. Santra, Z. A. Alahmed, A. M. Azzeer, V. S. Yakovlev, V. Pervak, F. Krausz, E. Goulielmakis

Abstract: The calibration of a McPherson’s fully computer-controlled grazing-incidence spectrograph Model 248/310G (Rowland circle diameter ≈ 1 m) with a MCP/CCD detection system is described. As a DC source of the XUV radiation served the Vacuum UV Hollow Cathode Light Source (McPherson Model 629), which was delivered together with the spectrograph. We concentrated on the MCP-centre- as well as across-MCP-wavelength calibration (a more general theoretical relation than in [1] was found), MCP gain uniformity, and CCD binning linearity. No attention was paid to a resolution of the system, because it changes with the wavelengths and it is determined by properties of the detection system. In general, it is usually much worse than the resolution of the spectrograph itself.
K. Koláceka, J. Schmidta, V. Boháceka, M. Rípaa, P. Vrbaa, O. Frolovb, M. Tichýb, A. Jancárekc, M. Vrbovác, E. Skladnik-Sadowskad, M. Sadowskid, and J. Baranowskid

Abstract: Coherent soft-x-ray radiation up to photon energies of 700 eV is obtained by focusing several-mJ, 10-fs near infrared laser pulses into a He gas jet. The observed nearly constant photon yield over several hundred eVs may be attributed to nonadiabatic self-phase matching, originating from a substantial ionization within a fraction of the optical cycle of the driving laser pulse.
Enikö Seres, József Seres, Ferenc Krausz, and Christian Spielmann

Abstract: A xenon Z-pinch1,2 generating extreme ultraviolet radiation at the Mo-Si mirror wavelength of 13.5nm has been scaled to emit increased power at a higher repetition frequency. The 25mm long by 3.0mm (FWHM) diameter pinch produces 1.5W of EUV radiation (2.5% bandwidth) into an axial 0.1sr solid angle when operated at 100Hz (100J stored). The measured average pinch liner heat load at 100Hz is 37W cm-2, corresponding to an average internal wall temperature of 8(PC. Electrode and liner erosion is very slight after more than 106 pulses at 100Hz. Source cleanliness was demonstrated via a two-mirror simulation of a condenser in which throughput was unchanged during a 106 pulse run at 50Hz. Amplitude stability was 12% (3σ) and positional stability was less than 4% of source diameter (lo). 13.5nm output scaled linearly with repetition frequency to more than 150Hz (58J stored). The cost of ownership for this source is estimated to be no greater than for an excimer laser illuminator. A plan is outlined for continued development to >1kHz and usable power of 16W
MCGEOCH Malcolm W

Abstract: The high resolution camera (HRC) is one of AXAF's two focal plane instruments. It consists of two detectors: the HRC-I which is optimized for direct imaging of x-ray sources; and the HRC-S which is optimized as the spectroscopic read-out of the low energy transmission grating (LETG). Both detectors are comprised of a chevron pair of micro-channel plates (MCPs) with a crossed grid charge detector (CGCD) and a UV/ion shield (UVIS). The role of the UVIS is to minimize the detectors' sensitivity to low energy electrons, ions and UV light, while providing sufficient x-ray transmission in the 0.1 to 10 keV x-ray band. In this paper, we report on the results of the flight UVIS calibration measurements. Specifically, x-ray and UV transmission measurements obtained at the HRC X-ray Test Facility of the Smithsonian Astropysical Observatory, and x- ray transmission measurements of UVIS witness samples obtained at a synchrotron light source facility.
G. R. Meehan; Stephen S. Murray; Martin V. Zombeck; Ralph P. Kraft; K. Kobayashi; John H. Chappell; Almus T. Kenter; Marco Barbera; Alfonso Collura; Salvatore Serio

Abstract: High-order harmonic generation (HHG) from atoms and molecules offers potential application as a coherent ultrashort radiation source in the extreme ultraviolet and soft X-ray regions. In the three-step model of HHG, an electron tunnels out from the atom and may recombine with the parent ion (emitting a high-energy photon) after undergoing laser-driven motion in the continuum. Aligned molecules can be used to study quantum phenomena in HHG associated with molecular symmetries; in particular, simultaneous observations of both ion yields and harmonic signals under the same conditions serve to disentangle the contributions from the ionization and recombination processes. Here we report evidence for quantum interference of electron de Broglie waves in the recombination process of HHG from aligned CO2 molecules. The interference takes place within a single molecule and within one optical cycle. Characteristic modulation patterns of the harmonic signals measured as a function of the pump-probe delay are explained with simple formulae determined by the valence orbital of the molecules. We propose that simultaneous observations of both ion yields and harmonic signals can serve as a new route to probe the instantaneous structure of molecular systems.
Tsuneto Kanai, Shinichirou Minemoto & Hirofumi Sakai

Abstract: The X-ray Astronomy Calibration and Testing (XACT) facility of the Instituto Nazionale di Astrofisica (INAF) at Osservatorio Astronomico di Palermo has recently undergone a major upgrade with the design and construction of a 35 meter long vacuum beam-line operating in the soft X-rays (0.1-20 keV) and the addition of new hardware to meet the requirements for testing and calibration of next generation X-ray missions. We report on the present configuration of the facility and briefly survey the range of its applications
Marco Barbera, Roberto Candia, Alfonso Collura, Gaspare DiCicca, Carlo Pelliciari, Salvatore Sciortino, Salvatore Varisco

Abstract: Significant progress in high repetition rate ultrashort pulse sources based on fiber technology is presented. These systems enable operation at a high repetition rate of up to 500 kHz and high average power in the extreme ultraviolet wavelength range via high harmonic generation in a gas jet. High average power few-cycle pulses of a fiber amplifier pumped optical parametric chirped pulse amplifier are used to produce μW level average power for the strongest harmonic at 42.9 nm at a repetition rate of 96 kHz.
S. Hädrich, J. Rothhardt, M. Krebs, F. Tavella, A. Willner, J. Limpert, and A. Tünnermann

Abstract: The synthesis of isolated attosecond pulses (IAPs) in the extreme ultraviolet (XUV) spectral region has opened up the shortest time scales for time-resolved studies. It relies on the generation of high-order harmonics (HHG) from high-power few-cycle infrared (IR) laser pulses. Here we explore experimentally a new and simple route to IAP generation directly from 35fs IR pulses that undergo filamentation in argon. Spectral broadening, self-shortening of the IR pulse and HHG are realized in a single stage, reducing the cost and experimental effort for easier spreading of attosecond sources. We observe continuous XUV spectra supporting IAPs, emerging directly from the filament via a truncating pinhole to vacuum. The extremely short absorption length of the XUV radiation makes it a highly local probe for studying the elusive filamentation dynamics and in particular provides an experimental diagnostic of short-lived spikes in laser intensity. The excellent agreement with numerical simulations suggests the formation of a single-cycle pulse in the filament.
D S Steingrube, E Schulz, T Binhammer, M B Gaarde, A Couairon, U Morgner and M Kovacev

Abstract: We have spatially and spectrally resolved the high order harmonic emission from an argon gas target. Under proper phase matching conditions we were able to observe for the first time the spatial fine structure originating from the interference of the two shortest quantum paths in the harmonic beam. The structure can be explained by the intensitydependent harmonic phase of the contributions from the two paths. The spatially and spectrally resolved measurements are consistent with previous spatially integrated results. Our measurement method represents a new tool to clearly distinguish between different interference effects and to potentially observe higher order trajectories in the future with improved detection sensitivity. Here, we demonstrate additional experimental evidence that the observed interference pattern is only due to quantum-path interferences and cannot be explained by a phase modulation effect. Our experimental results are fully supported by simulations using the strong field approximation and including propagation.
F. Schapper, M. Holler, T. Auguste, A. Zaïr, M. Weger, P. Salières, L. Gallmann, and U. Keller

Abstract: High harmonic generation (HHG) and the use of coherent XUV sources has become a key tool for ultrafast dynamics experiments. This progress has been, up to date, mostly driven by the commercial availability of Ti:Sapphire amplifier systems operating with very high peak powers, but at kHz repetition rates as driving systems for HHG. However, several applications would significantly benefit from XUV sources with higher repetition rates in the MHz regime, such as experiments where rare events need to be detected or where surface charge effects should be avoided, e.g. coincidence detection or surface science experiments. Driven by the large number of applications that benefit from compact sources of XUV radiation at high repetition rate, and by progress in the availability of high-power ultrafast laser systems [1-4], numerous advances have been pursued in high-repetition rate HHG (> 100 kHz) in the last decade. Using, for example, external HHG schemes with high power fiber amplifier systems or HHG inside enhancement cavities, several μW per harmonic have already been demonstrated at MHz repetition rate, however at the expense of comparatively complex systems [5-9]. Among the laser technologies reaching the required high pulse energy at high repetition rates for HHG, SESAM modelocked thin disk lasers have shown comparable performance than amplifier systems, directly from the output of a multi-megahertz compact oscillator
Florian Emaury, Andreas Diebold, Clara J. Saraceno, and Ursula Keller

Abstract: We present a compact extreme ultraviolet (XUV) source based on high-harmonic generation (HHG) at 2.4 MHz pulse repetition rate driven from the compressed output of a mode-locked thin-disk laser (TDL) oscillator. The system generates very high peak intensities, which enable highly nonlinear frequency conversion reaching VUV/XUV energies. These sources significantly increase the signal-to-noise ratio and reduce measurement durations in many fields such as condensed matter physics. The pulse repetition rate is increased from kilohertz to megahertz with high average photon flux, while keeping the pulse energy sufficiently low to avoid space charge effects. The system uses a semiconductor saturable absorber mirror mode-locked Yb:YAG TDL delivering an average power of up to 70 W with subpicosecond pulses, which are efficiently compressed to sub-100 fs in a simple, single-stage compressor based on a Kagome-type hollow-core photonic crystal fiber. Focusing into a high-pressure xenon gas jet, we generate XUV radiation with up to >5 × 10E7 photons∕s on the 19th harmonic (23 eV). This HHG system is very compact, has low-noise performance comparable to standard ultrafast low-power laser oscillators, and provides a new tool for the study of attosecond dynamics in condensed matter physics.
Florian Emaury, Andreas Diebold, Clara J. Saraceno, and Ursula Keller

Abstract: The effect of the variation of the gas pressure on the high harmonic generation (HHG) from nitrogen molecular gas (N2) and atmospheric air with ultrashort intense laser pulses is measured. The optimum pressure in the interaction region of a gas jet for maximizing the HHG yield is determined for both gases. Enhancement of the HHG output and its extension to higher harmonic orders are observed around the optimum pressure value of 0.33 bar. Theoretical calculations based on one-dimensional models explain this effect and provide reasonable agreement with experimental results.
M.Sayraca, A.A.Kolomenskiib, H.A.Schuessler

Abstract: We demonstrate the beyond extreme ultraviolet (BEUV) emission by a laser-triggered discharge source with the laser-current delay between 0.6 µs and 3 µs. The spectra at different electron temperatures are simulated theoretically, and the spectra at different laser-current delays are measured experimentally. The theoretical and experimental results show that the lower vapor velocity at longer laser-current delay time is beneficial for increasing the steady-state time of plasma at high temperature, thereby increasing the output intensity and spectral purity of 6.76 nm. The radiation intensity of 6.76 nm (0.6% bandwidth) increases about 240% when the delay increases from 0.6 to 3 μs.
Qiang Xu, He Tian, Yongpeng Zhao, Qi Wang

Abstract: The effect of laser-current delay on extreme ultraviolet emission by laser-triggered discharge-plasma has been investigated. Typical waveforms for current, voltage, laser signals, and X-ray signals have been compared. Theoretical tin spectra were simulated among the electron temperature ranges from 10 to 50 eV to compare with the experimental results. The results show that longer laser-current delay time is propitious to increase the steady-state time of plasma at high temperatures, and it increases the intensity and spectral purity of 13.5 nm emission in 2% band. The 13.5 nm radiation intensity increases about 120% with the delay increasing from 0.7 to 5 μs, and the extreme ultraviolet (EUV) emission conversion efficiency (CE) increases from 0.5% to 1.1%
Qiang Xu, Xiaolong Deng, He Tian, Yongpeng Zhao, Qi Wang