Publications Project 03 -
Strong-field Spectroscopy with Multicolor Pulses

Project Leader: Andrius Baltuška


Multi-millijoule few-cycle Mid-IR pulses through nonlinear self-compression in bulk

V. Shumakova, P. Malevich, S. Ališauskas, A. Voronin, A. M. Zheltikov, D. Faccio, D. Kartashov, A. Baltuška & A. Pugžlys

  The physics of strong-field applications requires driver laser pulses that are both energetic and extremely short. Whereas optical amplifiers, laser and parametric, boost the energy, their gain bandwidth restricts the attainable pulse duration, requiring additional nonlinear spectral broadening to enable few or even single cycle compression and a corresponding peak power increase. Here we demonstrate, in the mid-infrared wavelength range that is important for scaling the ponderomotive energy in strong-field interactions, a simple energy-efficient and scalable soliton-like pulse compression in a mm-long yttrium aluminium garnet crystal with no additional dispersion management. Sub-three-cycle pulses with >0.44 TW peak power are compressed and extracted before the onset of modulation instability and multiple filamentation as a result of a favourable interplay between strong anomalous dispersion and optical nonlinearity around the wavelength of 3.9 μm. As a manifestation of the increased peak power, we show the evidence of mid-infrared pulse filamentation in atmospheric air.

Published: 2016-09-13
Nature Communications 7, 12877 (2016)
DOI: 10.1038/NCOMMS12877

Press release TU Wien, 2016-09-13 [60/2016]


Coincidence spectroscopy of high-lying Rydberg states produced in strong field interaction

S. Larimian, S. Erattupuzha, R. Maurer, C. Lemell, S. Nagele, S. Yoshida, J. Burgdörfer, A. Baltuska, M. Kitzler, X. Xie

  We demonstrate the detection of high-lying Rydberg states produced in strong laser fields with coincidence spectroscopy. Electron emission after the interaction of strong laser pulses with atoms and molecules is measured together with the parent ions in coincidence measurements. These electrons originate from high-lying Rydberg states with quantum numbers from n∼20 up to n≲120 formed by frustrated field ionization. Ionization rates are retrieved from the measured ionization signal of these Rydberg states. Simulations show that both tunneling ionization by a weak dc field and photoionization by blackbody radiation contribute to delayed electron emission on the nano- to microsecond scale. Furthermore, the dependence of the Rydberg-state production on the ellipticity of the driving laser field indicates that such high-lying Rydberg states are populated through electron recapture. The present experiment provides detailed quantitative information on Rydberg production in strong-field interaction.

Published: 2016-09-01
Phys. Rev. A 94, 033401 (2016)
DOI: 10.1103/PhysRevA.94.033401
(--> also listed in Project 04)


Laser-subcycle control of sequential double-ionization dynamics of helium

M. S. Schöffler, X. Xie, P. Wustelt, M. Möller, S. Roither, D. Kartashov, A. M. Sayler, A. Baltuska, G. G. Paulus, and M. Kitzler

  We present measured momentum distributions on the double ionization of helium with intense, near-circularly-polarized few-cycle laser pulses with a known carrier-envelope offset phase (CEP). The capability of obtaining CEP-resolved momentum distributions enables us to observe signatures of the various combinations of laser-half-cycle two-electron emissions. By comparison to semiclassical trajectory simulations, we succeed in assigning the corresponding structures in the measured distributions to certain two-electron emission dynamics. Based on this possibility, we demonstrate that the sequential double-ionization dynamics can be sensitively controlled with the pulse duration and the laser peak intensity. For the shortest pulse durations and not too high intensities we find that the two electrons are dominantly emitted with a delay of roughly a laser half cycle. For a just slightly increased intensity we find evidence that at least one of the two electrons is surprisingly likely emitted in between the peaks of the laser field oscillations rather than at the field maxima. The simulations reproduce the signatures of these kinds of two-electron emissions overall relatively well.

Published: 2016-06-24
Phys. Rev. A 93, 063421 (2016)
DOI: 10.1103/PhysRevA.93.063421


Two-pulse control over double ionization pathways in CO2

S. Erattupuzha, S. Larimian, A. Baltuška, X. Xie, M. Kitzler

  We visualize and control molecular dynamics taking place on intermediately populated states during different sequential double ionization pathways of CO2 using a sequence of two delayed laser pulses which exhibit different peak intensities. Measured yields of CO22+ and of fragment pairs CO+/O+ as a function of delay between the two pulses are weakly modulated by various vibronic dynamics taking place in CO2+. By Fourier analysis of the modulations we identify the dynamics and show that they can be assigned to merely two double ionization pathways. We demonstrate that by reversing the sequence of the two pulses it becomes possible to control the pathway which is taken across CO2+ towards the final state in CO22+. A comparison between the yields of CO22+ and CO+/O+ reveals that the modulating vibronic dynamics oscillate out-of-phase with each other, thus opening up opportunities for strong-field fragmentation control on extended time scales.

Published: 2016-01-12
The Journal of Chemical Physics, 144, 024306 (2016)
DOI: 10.1063/1.4939638


Duration of an intense laser pulse can determine the breakage of multiple chemical bonds

X. Xie, E. Lötstedt, S. Roither, M. Schöffler, D. Kartashov, K. Midorikawa, A. Baltuška, K. Yamanouchi, M. Kitzler

  Control over the breakage of a certain chemical bond in a molecule by an ultrashort laser pulse has been considered for decades. With the availability of intense non-resonant laser fields it became possible to pre-determine femtosecond to picosecond molecular bond breakage dynamics by controlled distortions of the electronic molecular system on sub-femtosecond time scales using field-sensitive processes such as strong-field ionization or excitation. So far, all successful demonstrations in this area considered only fragmentation reactions, where only one bond is broken and the molecule is split into merely two moieties. Here, using ethylene (C2H4) as an example, we experimentally investigate whether complex fragmentation reactions that involve the breakage of more than one chemical bond can be influenced by parameters of an ultrashort intense laser pulse. We show that the dynamics of removing three electrons by strong-field ionization determines the ratio of fragmentation of the molecular trication into two respectively three moieties. We observe a relative increase of two-body fragmentations with the laser pulse duration by almost an order of magnitude. Supported by quantum chemical simulations we explain our experimental results by the interplay between the dynamics of electron removal and nuclear motion.

Published: 2015-08-14
Scientific Reports 5, 12877 (2015)
DOI: 10.1038/srep12877


CEP-stable tunable THz-emission originating from laser-waveform-controlled sub-cycle plasma-electron bursts

T. Balčiūnas, D. Lorenc, M. Ivanov, O. Smirnova, A.M. Zheltikov, D. Dietze, K. Unterrainer, R. Rathje, G.G. Paulus, A. Baltuška, S. Haessler

  We study THz-emission from a plasma driven by an incommensurate-frequency two-colour laser field. A semi-classical transient electron current model is derived from a fully quantum-mechanical description of the emission process in terms of sub-cycle field-ionization followed by continuum-continuum electron transitions. For the experiment, a CEP-locked laser and a near-degenerate optical parametric amplifier are used to produce two-colour pulses that consist of the fundamental and its near-half frequency. By choosing two incommensurate frequencies, the frequency of the CEP-stable THz-emission can be continuously tuned into the mid-IR range. This measured frequency dependence of the THz-emission is found to be consistent with the semi-classical transient electron current model, similar to the Brunel mechanism of harmonic generation.

Published: 2015-06-02
Optics Express 15278, Vol. 23, No. 12 (2015)
DOI: 10.1364/OE.23.015278
(--> also listed in Project 02)


Time-dependent density-functional study of the alignment-dependent ionization of acetylene and ethylene by strong laser pulses

A. Russakoff, S. Bubin, X. Xie, S. Erattupuzha, M. Kitzler, K. Varga

  The alignment-dependent ionization of acetylene and ethylene in short laser pulses is investigated in the framework of the time-dependent density-functional theory coupled with Ehrenfest dynamics. The molecular alignment is found to have a substantial effect on the total ionization. Bond stretching is shown to cause an increase of the ionization efficiency, i.e., enhanced ionization, in qualitative agreement with previous theoretical investigations. It is also demonstrated that the enhanced ionization mechanism greatly enhances the ionization from the inner valence orbitals, and the ionization of the inner orbitals is primarily due to their extended weakly bound density tails.

Published: 2015-02-19
Physical Review A 91, 023422 (2015)
DOI: 10.1103/PhysRevA.91.023422


Laser-sub-cycle two-dimensional electron-momentum mapping using orthogonal two-color fields

L. Zhang, X. Xie, S. Roither, D. Kartashov, Y. Wang, C. Wang, M. Schöffler, D. Shafir, P.B. Corkum, A. Baltuška, I. Ivanov, A. Kheifets, X. Liu, A. Staudte, M. Kitzler

  We study laser-sub-cycle control over electron trajectories concomitantly in space and time using orthogonally polarized two-color laser fields.We compare experimental photoelectron spectra of neon recorded by coincidence momentum imaging with photoelectron spectra obtained by semiclassical and numerical solutions of the time-dependent Schr¨odinger equation. We find that a resolution of a quarter optical cycle in the photoelectron trajectories can be achieved. It is shown that depending on their sub-cycle birth time the trajectories of photoelectrons are affected differently by the ion’s Coulomb field.

Published: 2014-12-01
Physical Review A 90, 061401(R) (2014)
DOI: 10.1103/PhysRevA.90.061401


High brightness table-top hard X-ray source driven by sub-100-femtosecond mid-infrared pulses

J. Weisshaupt, V. Juvé, M. Holtz, S.A. Ku, M. Woerner, T. Elsaesser, S. Ališauskas, A. Pugžlys, A. Baltuška

  Ultrafast structural dynamics in the condensed phase represents a key topic of current physics, chemistry and materials science. Femtosecond hard X-ray pulses are important structure probes that have been applied in time-resolved X-ray absorption and diffraction. Optical pump/X-ray probe schemes with compact laser-driven table-top sources have allowed for tiny changes of diffracted intensity to be measured with X-ray photon statistics, which has set the ultimate sensitivity limit. To address the strong quest for a higher X-ray flux, here we present the first hard X-ray plasma source driven by intense mid-infrared sub-100-fs pulses at 3.9 μm. The comparably long optical period allows for accelerating electrons from the Cu target to very high kinetic energies and for generating a characteristic Kα flux of 109 photons per pulse, 25 times more than with our 800 nm driver. Theoretical simulations account for the experimental results in a wide range of driving fields and predict a further enhancement of X-ray flux.

Published: 2014-11-10
Nature Photonics Letters
DOI: 10.1038/nphoton.2014.256

Press release TU Wien, 2014-11-11 [113/2014]


Exploring and Controlling Fragmentation of Polyatomic Molecules with Few-Cycle Laser Pulses

M. Kitzler, X. Xie, and A. Baltuška

  The removal of electrons from polyatomic molecules by ionization with intense, ultrashort laser pulses may trigger complex restructuring and fragmentation dynamics. Depending on the valence-shell from which the electrons are removed the molecular ion might be put into a certain dissociative or binding state by the ionization process. With control over the ionization process it might thus be possible to gain control over the subsequent restructuring and fragmentation process on a purely electronic level. Here we introduce two conceptually similar schemes that allow controlling the outcome of molecular restructuring and fragmentation processes in polyatomic molecules on sub-femtosecond time-scales. The first one involves recollision double ionization in few-cycle laser fields with a known carrier-envelope phase (CEP). We demonstrate experimentally CEP-control over various fragmentation reactions of a series of polyatomic molecules (acetylene, ethylene, 1,3-butadiene). As the recollision energy for a given intensity sensitively depends on the CEP, tuning of the CEP allows controlling the removal of inner-valence electrons and the controlled population of dissociative excited states. The second control scheme uses the strong preponderance of ionization from specific molecular orbitals to the alignment of the molecular axis with respect to the laser polarization direction for determining which valence level the electrons are removed from. We demonstrate experimental control over different two-body fragmentation and dissociation pathways from the cation and the dication of the acetylene molecule using the field-free alignment angle as a control knob. Finally, we turn from the demonstration of control schemes working at sub-femtosecond time-scales, for which the nuclear dynamics following the ionization are not essential, to the investigation of the coupled nuclear-electronic dynamics that in general take place for longer pulse durations. We explore experimentally the mechanism behind the surprisingly high charge states recently observed in the ionization of hydrocarbon molecules that have been explained by a multi-bond version of the well known enhanced-ionization (EI) mechanism taking place in parallel at many C–H bonds. Our experimental results are in agreement with the proposed multi-bond version of EI.

Published: 2014-08-10
Springer Ser. Chem. Phys. 109, 43–72 (2015)
DOI: 10.1007/978-3-319-06731-5_3


Subcycle control of electron-electron correlation in double ionization

L. Zhang, X. Xie, S. Roither, Y. Zhou, P. Lu, D. Kartashov, M. Schöffler, D. Shafir, P. B. Corkum, A. Baltuška, A. Staudte, and M. Kitzler

  Double ionization of neon with orthogonally polarized two-color (OTC) laser fields is investigated using coincidence momentum imaging. We show that the two-electron emission dynamics in nonsequential double ionization can be controlled by tuning the subcycle shape of the electric field of the OTC pulses. We demonstrate experimentally switching from correlated to anticorrelated two-electron emission, and control over the directionality of the two-electron emission. Simulations based on a semiclassical trajectory model qualitatively explain the experimental results by a subcycle dependence of the electron recollision time on the OTC field shape.

Published: 2014-05-14
Phys. Rev. Lett. 112, 193002 (2014)
DOI: 10.1103/PhysRevLett.112.193002


Interference of electron wave packets in atomic ionization by subcycle sculpted laser pulses

D.G. Arbó, S. Nagele, X.-M. Tong, X. Xie, M. Kitzler, J. Burgdörfer

  We present a theoretical analysis of the atomic photoelectron emission spectra produced by a linearly polarized sculpted laser pulse of two colors with frequencies ω and 2ω. The spectrum of the “direct” electrons with intermediate energies prominently features both intracycle and intercycle interferences. We derive a simple analytic expression for this spectral range based on a semiclassical approximation of the time-dependent distorted wave strong-field approximation generalized to strong-field ionization by a two-color pulse. We verify its applicability to approximately represent the intricate interference patterns by comparison with the exact solutions of the time-dependent Schr¨odinger equation and with the strong-field approximation.We show that the interference patterns can be tuned and its contrast enhanced by the additional “knob” available, the relative phase between the two frequency components. The present results confirm that two-color ionization allows resolving interference structures originating from trajectories launched within a time interval of less than 100 as [X. Xie et al., Phys. Rev. Lett. 108, 193004 (2012)].

Published: 2014-04-16
Physical Review A 89, 043414 (2014)
DOI: 10.1103/PhysRevA.89.043414
(--> also listed in Project 04)


Role of proton dynamics in efficient photoionization of hydrocarbon molecules

X. Xie, S. Roither, M. Schöffler, H. Xu, S. Bubin, E. Lötstedt, S. Erattuphuza, A. Iwasaki, D. Kartashov, K. Varga, G. G. Paulus, A. Baltuška, K. Yamanouchi, M. Kitzler

  We experimentally investigate the ionizationmechanism behind the formation of remarkably high charge states observed in the laser-pulse-induced fragmentation of different hydrocarbon molecules by Roither et al. [Phys. Rev. Lett. 106, 163001 (2011)], who suggested enhanced ionization occurring at multiple C-H bonds as the underlying ionization mechanism. Using multiparticle coincidence momentum imaging we measure the yield of multiply charged fragmenting ethylene and acetylene molecules at several intensities and pulse durations ranging from the few-cycle regime to 25 fs. We observe, at constant intensity, a strong increase of the proton energy with increasing laser pulse duration. It is shown that this is caused by a strong increase in the yield of highly charged parent molecular ions with pulse duration. Based on experimental evidence we explain this increase by the necessary population of precursor states in the parent ion that feature fast C-H stretch dynamics to the critical internuclear distance, where efficient ionization via enhanced ionization takes place. For increasing pulse duration these precursor ionic states are more efficiently populated, which leads in turn to a higher enhanced-ionization probability for longer pulses. Our work provides experimental evidence for the existence of a multiple-bond version of enhanced ionization in polyatomic molecules.

Published: 2014-02-26
Phys. Rev. A 89, 023429 (2014)
DOI: 10.1103/PhysRevA.89.023429


High energy and average power femtosecond laser for driving mid-infrared optical parametric amplifiers

P. Malevich, G. Andriukaitis, T. Flöry, A. J. Verhoef, A. Fernández, S. Ališauskas, A. Pugžlys, A. Baltuška, L. H. Tan, C. F. Chua, P. B. Phua

  We have developed the first (to our knowledge) femtosecond Tm-fiber-laser-pumped Ho:YAG room-temperature chirped pulse amplifier system delivering scalable multimillijoule, multikilohertz pulses with a bandwidth exceeding 12 nm and average power of 15W. The recompressed 530 fs pulses are suitable for broadband white light generation in transparent solids, which makes the developed source ideal for both pumping and seeding optical parametric amplifiers operating in the mid-IR spectral range.

Published: 2013-08-01
Opt. Lett. 38, 2746–2749 (2013)
DOI: 10.1364/OL.38.002746


Probing the influence of the Coulomb field on atomic ionization by sculpted two-color laser fields

X.H. Xie, S. Roither, S. Gräfe, D. Kartashov, E. Persson, C. Lemell, L. Zhang, M. S. Schöffler, A. Baltuška, J. Burgdörfer, M. Kitzler

  Interpretation of electron or photon spectra obtained with strong laser pulses that may carry attosecond dynamical and Ångström structural information about atoms or molecules usually relies on variants of the strongfield approximation (SFA) within which the influence of the Coulomb potential on the electron trajectory is neglected.We employ two-color sculpted laser fields to experimentally tune and probe the influence of the Coulomb field on the strong-field-driven wavepacket as observed by two-dimensional electron and ion momentum spectra. By comparison of measured spectra with predictions of the three-dimensional time-dependent Schrödinger equation as well as the quasiclassical limit of the SFA, the strong-field classical trajectory model, we are able to trace back the influence of the Coulomb field to the timing of the wavepacket release with sub-cycle precision.

Published: 2013-04-30
New Journal of Physics 15, 043050 (2013)
DOI: 10.1088/1367-2630/15/4/043050
(--> also listed in Project 04)








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