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Tuesday, 16 December 2014

Konstantin: DM: Probing controlled dynamics

In this week's Journal Club I will discuss a proposed scheme to use the interference between short and long trajectories in HHG, so called Quantum Path Interference (QPI), to probe sub-femtosecond hole dynamics. The TDSE for a model H2 molecule in an intense IR field and a weaker UV field (resonant with the ionic transition between HOMO and HOMO-2) is solved numerically (idea and calculations by Suren Sukiasyan et al.). The population transfer driven by the resonant UV field changes the recombination probabilities of the short and long trajectories, leading to a change in QPI contrast. I will then discuss how we plan to implement this and similar experiments in the lab.

Tuesday, 9 December 2014

Allan: JC: High-flux table-top soft x-ray source driven by sub-2-cycle, CEP stable, 1.85-μm 1-kHz pulses for carbon K-edge spectroscopy

We report on the first table-top high-flux source of coherent soft x-ray radiation up to 400 eV, operating at 1 kHz. This source covers the carbon K-edge with a beam brilliance of (4.3±1.2)×1015  photons/s/mm2/strad/10%bandwidth and a photon flux of (1.85±0.12)×107  photons/s/1% bandwidth. We use this source to demonstrate table-top x-ray near-edge fine-structure spectroscopy at the carbon K-edge of a polyimide foil and retrieve the specific absorption features corresponding to the binding orbitals of the carbon atoms in the foil.

Tuesday, 25 November 2014

Morgane: DM: Effect of nuclear distribution on electron dynamics.

Electron dynamics following ionisation is often simulated at a single nuclear geometry, the equilibrium geometry of the neutral species. However, even in the vibrational ground state, there is a distribution of nuclear geometries. We have sampled the initial distribution with hundreds geometries and simulated the electron dynamics at each of them. In toluene cation, we observe a fast decoherence of the electronic superposition: the average oscillation in the electronic density is damped.

Tuesday, 11 November 2014

Renjie: DM: Ab initio calculation of Penning ionization widths for millikelvin atom-molecule collisions

Penning ionization is a fundamental collision process between two neutral species. One of the atoms or molecules is electronically excited so that the whole system is above the ionization threshold, resulting in an auto-ionization process. The ionization width of the process is calculated using a combination of algebraic diagrammatic construction, Stieltjes imaging, Davidson and Lanczos diagonalization algorithms. This project is motivated by an observation of Penning ionization at sub-kelvin temperatures (see the link below).

Tuesday, 4 November 2014

Daniel: DM: Third harmonic enhancement of VUV generation

I will report on simulations that suggest sub-femtosecond VUV pulse generation can be enhanced by mixing the driving IR field with its third harmonic. Previous experimental work has shown that below threshold harmonics can boost HHG from longer pulses.

Tuesday, 28 October 2014

Tobi: JC: Ultrafast electron dynamics in phenylalanine initiated by attosecond pulses

In the past decade, attosecond technology has opened up the investigation of ultrafast electronic processes in atoms, simple molecules, and solids. Here, we report the application of isolated attosecond pulses to prompt ionization of the amino acid phenylalanine and the subsequent detection of ultrafast dynamics on a sub–4.5-femtosecond temporal scale, which is shorter than the vibrational response of the molecule. The ability to initiate and observe such electronic dynamics in polyatomic molecules represents a crucial step forward in attosecond science, which is progressively moving toward the investigation of more and more complex systems.

Tuesday, 21 October 2014

Alvaro: X-ray pulse characterization at LCLS using XTCAV

X-ray free electron lasers rely on an accelerated electron bunch passing through an undulator to create and amplify x-ray pulses. This generation/amplification process leaves a signature on the electron bunch than is dependent on the temporal profile of the generated x-rays. By streaking the electron bunch after the generation and looking at the electron spectrum as a function of time it is possible to characterize the generated x-ray pulses on a shot to shot basis.

I am not going to present the paper itself, but if you want to include the link it could be useful to the curious attendee:

Tuesday, 14 October 2014

Will: JC: Attosecond Photoscopy of Plasmonic Excitations

We propose an experimental arrangement to image, with attosecond resolution, transient surface plasmonic excitations. The required modifications to state-of-the-art setups used for attosecond streaking experiments from solid surfaces only involve available technology. Buildup and lifetimes of surface plasmon polaritons can be extracted and local modulations of the exciting optical pulse can be diagnosed in situ.
  • Figure
  • Figure
  • Figure
  • Published 12 September 2014
  • Received 20 January 2014

© 2014 American Physical Society

Tuesday, 15 July 2014

Jon: LCLS briefing + Zara: JC: Quasi-phase-matching-induced enhancement of high-order harmonics during two-colour pump of multi-jet plasmas

Jon: LCLS: probing hole dynamics in glycine 

Zara: Quasi-phase-matching-induced enhancement of high-order harmonics during two-colour pump of multi-jet plasmas

The first demonstration of quasi-phase-matching during high-order harmonic generation (HHG) of laser radiation in multi-jet plasmas produced on the silver and chromium targets under the conditions of two-colour pump (804 and 402 nm) is presented. We analyse the enhancement of the groups of odd and even harmonics along the plateau range and compare the harmonic yields using a different number of plasma jets. The comparison of extended line plasma and multi-jet plasma shows the advanced properties of the latter formation for the efficient HHG using the two-colour pump. The 25× and 13× enhancement factors of quasi-phase-matched harmonics were obtained in the cases of the two-colour and single-colour pumps of multi-jet plasmas.

Tuesday, 8 July 2014

Davide: JC: Coherent phase-matched VUV generation by field-controlled bound states

The generation of high-order harmonics1 and attosecond pulses2 at ultrahigh repetition rates (>1 MHz) promises to revolutionize ultrafast spectroscopy. Such vacuum ultraviolet (VUV) and soft X-ray sources could potentially be driven directly by plasmonic enhancement of laser pulses from a femtosecond oscillator3, 4, but recent experiments suggest that the VUV signal is actually dominated by incoherent atomic line emission5, 6. Here, we demonstrate a new regime of phase-matched below-threshold harmonic generation, for which the generation and phase matching is enabled only near resonance structures of the atomic target. The coherent VUV line emission exhibits low divergence and quadratic growth with increasing target density up to nearly 1,000 torr mm and can be controlled by the sub-cycle field of a few-cycle driving laser with an intensity of only ~1 × 1013 W cm−2, which is achievable directly from few-cycle femtosecond oscillators with nanojoule energy7.

Tuesday, 3 June 2014

Nick: JC: Frequency domain optical parametric amplification

Today’s ultrafast lasers operate at the physical limits of optical materials to reach extreme performances. Amplification of single-cycle laser pulses with their corresponding octave-spanning spectra still remains a formidable challenge since the universal dilemma of gain narrowing sets limits for both real level pumped amplifiers as well as parametric amplifiers. We demonstrate that employing parametric amplification in the frequency domain rather than in time domain opens up new design opportunities for ultrafast laser science, with the potential to generate single-cycle multi-terawatt pulses. Fundamental restrictions arising from phase mismatch and damage threshold of nonlinear laser crystals are not only circumvented but also exploited to produce a synergy between increased seed spectrum and increased pump energy. This concept was successfully demonstrated by generating carrier envelope phase stable, 1.43?mJ two-cycle pulses at 1.8?µm wavelength.

Tuesday, 27 May 2014

Malte: DM: Phase characterisation of plasmons

The field of plasmonics promises a brave new world of microprocessing, sensing and low-power nonlinear optics. Existing approaches to measuring plasmon fields using streaking are very involved, so an all-optical method to catch a glimpse of the surface dynamics is highly desirable. I carried out spectral interferometry on the reflected residual pump beam used to excite surface plasmon polaritons on a diffraction grating in order to characterise the phase of the propagating plasmon. I furthermore simulated the experimental situation using FDTD methods to connect the far-field results to the surface fields. Initial results show qualitative agreement between experiment and simulation, with the latter most likely critically limited by computational resources. As such, the progress so far demonstrates that the technique can indeed yield information about the plasmon dynamics in the far field with comparatively little experimental effort.

Tuesday, 20 May 2014

Will: JC: In situ measurement of nonlinear carrier-envelope phase changes in hollow fiber compression

We demonstrate a simple and robust single-shot interferometric technique that allows the in situ measurement of intensity-dependent phase changes experienced by ultrashort laser pulses upon nonlinear propagation. The technique is applied to the characterization of carrier-envelope phase noise in hollow fiber compressors both in the pressure gradient and in the static cell configuration. Measurements performed simultaneously with conventional f-to-2f interferometers before and after compression indicate that the noise emerging in the waveguide adds up arithmetically to the phase noise of the amplifier, thus being strongly correlated to the phase noise of the pulses coupled into the compressor.

Tuesday, 13 May 2014

Zsolt: JC: How to focus an attosecond pulse

Attosecond experiments involving focusing of attosecond light pulses can suffer from a spread of the attosecond radiation both in space and time due to optical aberrations. We present a detailed numerical study of the distortions induced in the most common focusing geometries that make use of parabolic, spherical, toroidal and ellipsoidal mirrors. We deduce the consequences on the pulse duration and possible issues that could arise in applications of attosecond pulses. This should serve as a guideline for setting up attosecond focusing optics.

Tuesday, 6 May 2014

Zolt: JC: Laser dressed absorption studies

Two papers will serve the bases:

Lucchini et al. Role of electron wavepacket interference in the optical response of helium atoms, NJP, 15, (2013)
Gallmann et al. Resolving intra-atomic electron dynamics with attosecond transient absorption spectroscopy, Molecular Physics, 111, (2013)

Tuesday, 1 April 2014

Thoma: JC: High-order-harmonic generation from dense water microdroplets

We report on high-order-harmonic generation from micrometer-sized liquid water droplets. In pump-probe experiments, the influence of the time delay onto the emission of harmonic radiation is systematically studied. Phase-matching aspects are observed by controlling the focal position and the intensity of the probe pulse. The spatiotemporal dynamics of the droplet during interaction with intense laser pulses are studied by controlling the intensity of the pump pulse. We find transient phase-matching conditions and the expansion dynamics of the droplet to be of major influence on the harmonic yield.

Tuesday, 4 March 2014

Allan: JC: Frequency-resolved optical gating capable of carrier-envelope phase determination

Recent progress of the coherent light synthesis technology has brought the generation of single-cycle pulses within our reach. To exploit the full potential of such a single-cycle pulse in any applications, it is highly important to obtain the full information of its electric field. Here we propose a novel pulse characterization scheme, which enables us to determine not only the intensity and phase profiles of ultrashort pulses but also their absolute carrier-envelope phase values. The method is based on a combination of frequency-resolved optical gating and electro-optic sampling, which can be extended to a self-referencing scheme to determine the electric field evolution of few-cycle ultrashort pulses. We have experimentally demonstrated the technique to characterize sub-single-cycle infrared pulses, and numerically studied the capability of the scheme to incorporate a self-referencing technique and to extend the wavelength range to visible region.

Tuesday, 25 February 2014

Lukas: JC: Femtosecond M2,3-Edge Spectroscopy of Transition-Metal Oxides: Photoinduced Oxidation State Change in a-Fe2O3

Femtosecond M2,3-Edge Spectroscopy of Transition-Metal Oxides: Photoinduced Oxidation State Change in a-Fe2O3

Tuesday, 18 February 2014

Marina: DM: Laser mass spectroscopy

Mass spectrometry is the major physical method for structure determination of organic and biological molecules. The principle of this technique is generation, fragmentation and mass analysis of gaseous charged species using a variety of mass analysers (e.g. TOF, ion trap) to provide key information about the ion structure. The tremendous growth in application of mass spectrometry for detection, quantification and characterisation of molecules, especially of those of clinical interest (such as proteins), has stimulated exploration of new ionisation/dissociation techniques to enable reliable and comprehensive mass spectral analysis. Photodissociation at various light wavelengths, implemented on both ion trapping and TOF mass spectrometers, was shown to be one of the most powerful ion activation technologies. In particular, the recently introduced femtosecond laser-induced ionisation/dissociation (fs-LID) holds high promise for protein structural analysis delivering both the nonergodic ion dissociation and the indifference to the peptide molecular ion charge state. In this technique, coherent evolution of coupled many-electron and nuclear wavepackets is taking place causing an ultrafast macromolecule electron loss via the tunneling ionisation mechanism, on a timescale similar to or faster than that of bond vibrations, resulting in multiple series of peptide product ions in the mass spectra. I will discuss the laser MS technology, ion trap mass spectral instrumentation and will tell you about our plans to develop the first-of-its-kind two-dimensional femtosecond laser mass spectrometry for molecular structure determination on the basis of the covariance mapping of laser induced trapped fragments.

Tuesday, 14 January 2014