Attosecond pulse shaping using partial phase matching

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Above-threshold ionization and photoelectron spectra in atomic systems driven by strong laser fields

Above-threshold ionization (ATI) results from strong field laser-matter interaction and it is one of the fundamental processes that may be used to extract electron structural and dynamical information about the atomic or molecular target. Moreover, it can also be used to characterize the laser field itself. Here, we develop an analytical description of ATI, which extends the theoretical Strong Field Approximation (SFA), for both the direct and re-scattering transition amplitudes in atoms. From a non-local, but separable potential, the bound-free dipole and the re-scattering transition matrix elements are analytically computed. In comparison with the standard approaches to the ATI process, our analytical derivation of the re-scattering matrix elements allows us to study directly how the re-scattering process depends on the atomic target and laser pulse features -we can turn on and off contributions having different physical origins or corresponding to different physical mechanisms. We compare SFA results with the full numerical solutions of the time-dependent Schroedinger equation (TDSE) within the few-cycle pulse regime. Good agreement between our SFA and TDSE model is found for the ATI spectrum. Our model captures also the strong dependence of the photoelectron spectra on the carrier envelope phase of the laser field.Comment: 29 pages, 6 figures, submitted to PR

All-regions tunable high harmonic enhancement by a periodic static electric field

Simulations show that a static electric field periodically distributed in space can be used to control the production of coherent light by high-order harmonic generation in a wide spectral range covering extremeultraviolet and soft x-ray radiation. The radiation yield is selectively enhanced due to symmetry breaking induced by a static electric field on the interaction between the driving laser and the medium. The spectral position of the enhancement is tuned by varying the periodicity of the static electric field which matches twice the coherence length of the harmonics in the desired region. We find that the static electric field strength inducing enhancement decreases for shorter wavelengths and predict an increase of more than two orders of magnitude for harmonics in the water window spectral range with a static electric field as weak as 1:12 MV/cm.We would like to thank Dr. D. Zalvidea for helpful discussions. Support is acknowledged from the Spanish Ministry of Education and Science through its Consolider Program Science (SAUUL-CSD 2007-00013), through ‘‘Plan Nacional’’ (FIS2008-06368-C02-01/02), as well as from the Barcelona Supercomputing Center.Peer ReviewedPostprint (published version

Molecular Fine Structure from Water Window X-rays

Postprint (published version

Performance of MgO:PPLN, KTA, and KNbO3 for mid-wave infrared broadband parametric amplification at high average power

The performance of KNbO3, MgO:PPLN and KTA were experimentally compared for broadband mid-wave infrared parametric amplification at high repetition rate. The seed pulses with an energy of 6.5 μJ were amplified using 410 μJ of pump at 1064 nm to a maximum pulse energy of 28.9 μJ at 3 μm wavelength and at 160 kHz repetition rate in MgO:PPLN while supporting a transform limited duration of 73 fs. The high average powers of the interacting beams used in this study, revealed average power induced processes which limit the scaling of optical parametric amplification: in MgO:PPLN the pump peak intensity was limited to 3.8 GW/cm2 due to non-permanent beam reshaping while in KNbO3 an absorptioninduced temperature gradient in the crystal led to permanent internal distortions in the crystal structure when operated above a pump peak intensity of 14.4 GW/cm2Peer ReviewedPostprint (author's final draft

Theory of X-ray absorption spectroscopy: a microscopic Bloch equation approach for two-dimensional solid states

We develop a self-consistent Maxwell-Bloch formalism for the interaction of X-rays with two-dimensional crystalline materials by incorporating the Bloch theorem and Coulomb many-body interaction. This formalism is illustrated for graphene, by calculating the polarization-dependent XANES, formulating expressions for the radiative and Meinter-Auger recombination of core-holes, and the discussion of microscopic insights into the spectral oscillations of EXAFS beyond point scattering theory. In particular, the correct inclusion of lattice periodicity in our evaluation allows us to assign so far uninterpreted spectral features in the Fourier transformed EXAFS spectrum

Few-cycle mid-infrared pulses from BaGa2GeSe6

BaGa2GeSe6 (BGGSe) is a newly developed nonlinear material that is attractive for ultrabroad frequency mixing and ultrashort pulse generation due to its comparably low dispersion and high damage threshold. A numerical study shows the material’s capacity for octave-spanning mid-infrared pulse generation up to 18 µm. In a first experiment, we show that a long crystal length of 2.6 mm yields a pulse energy of 21 pJ at 100 MHz with a spectral bandwidth covering 5.8 to 8.5 µm. The electric field of the carrier-envelope-phase stable pulse is directly measured with electro-optical sampling and reveals a pulse duration of 91 fs, which corresponds to sub-four optical cycles, thus confirming some of the prospects of the material for ultrashort pulse generation and mid-infrared spectroscopy.European Research Council (ERC Advanced Grant “TRANSFORMER” 788218, Proof of Concept Grant “miniX” 840010); H2020 Future and Emerging Technologies (“PETACom” 829153, FET-OPEN "OPTOlogic" 899794); Laserlab-Europe (EU-H2020 654148); Ministerio de Economía y Competitividad (Plan Nacional FIS2017-89536- P, “SeveroOchoa” SEV- 2015-0522); Agència deGestió d’Ajuts Universitaris i de Recerca (2017 SGR 1639); Fundación Cellex Barcelona; Centres de Recerca de Catalunya Programme / Generalitat de Catalunya; Alexander von Humboldt-Stiftung (FriedrichWilhelmBessel Prize).Peer ReviewedPostprint (published version

Ultrashort pulse generation in the mid-IR

Recent developments in laser sources operating in the mid-IR (3–8μm) have been motivated by the numerous possibilities for both fundamental and applied research. One example is the ability to unambiguously detect pollutants and carcinogens due to the much larger oscillator strengths of their absorption features in the mid-IR spectral region compared with the visible. Broadband sources are of particular interest for spectroscopic applications since they remove the need for arduous scanning or several lasers and allow simultaneous use of multiple absorption features thus increasing the confidence level of detection. In addition, sources capable of producing ultrashort and intense mid-IR radiation are gaining relevance in attoscience and strong-field physics due to wavelength scaling of re-collision based processes. In this paper we review the state-of-the-art in sources of coherent, pulsed mid-IR radiation. First we discuss semi-conductor based sources which are compact and turnkey, but typically do not yield short pulse duration. Mid-IR laser gain material based approaches will be discussed, either for direct broadband mid-IR lasers or as narrowband pump lasers for parametric amplification in nonlinear crystals. The main part will focus on mid-IR generation and amplification based on parametric frequency conversion, enabling highest mid-IR peak power pulses. Lastly we close with an overview of nonlinear post-compression techniques, for decreasing pulse duration to the sub-2-optical-cycle regime.Peer ReviewedPostprint (author's final draft