Christian: JC: Generation of intense supercontinuum in condensed media

Destructive nonlinear processes have limited the useful input power to a few megawatts for supercontinuum generation in bulk material. Consequently, reliable high-power, high-pulse-energy supercontinuum in condensed media has not been realized. Here, we describe an intense femtosecond supercontinuum generated in a solid medium with pulse energy and mode quality that approach those generated in the gas phase while preserving the advantages of a condensed medium of being compact, simple to operate, and highly reproducible. This is achieved by strategically placing several thin solid plates at or near the focused waist of a high-power laser pulse. The thickness of each plate is such that the optical pulse exits the plate before undesirable effects begin to take hold of the pulse. With this approach, we have obtained pulses that have an octave-spanning spectrum that covers from 450 to 980 nm at the −20  dB intensity level while converting as much as 54% of the input pulse energy to the continuum. The highest pulse energy obtained to date is 76 μJ, nearly two orders of magnitude greater than previously reported values. The transverse mode of the pulse has a M2 of 1.25. Frequency-resolved optical grating and spectral interferometric measurements indicate that the pulse is phase coherent and could be compressed to a few femtoseconds. Furthermore, the multiple-plates approach is shown to be extremely flexible and versatile. It is applicable for a broad range of input powers and materials. The generated continuum is stable and robust. Thus, multiple-plate generated femtosecond continuum could be a promising new light source in ultrafast science and extreme nonlinear optics applications.

http://www.opticsinfobase.org/optica/abstract.cfm?uri=optica-1-6-400

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.

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.

http://www.opticsinfobase.org/ol/abstract.cfm?uri=ol-39-18-5383

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.

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).
http://www.sciencemag.org/content/338/6104/234.full.pdf

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.

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.

http://www.sciencemag.org/content/346/6207/336.abstract