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Wednesday, 14 December 2011

DM: Suren: Attosecond cascades and time-delays in one-electron photo-ionization

I shall present time-resolved ab-initio study of the attosecond dynamics of electron-electron correlation during single-electron, single XUV-photon ionization of an excited two-electron atom. Specifically, we aim to time-resolve the two fundamental processes that play the key role in electronic excitations of the ion created by ionization: the shake-up and the post-ionization interaction of the outgoing electron with the core (knock-up). These two processes have different time characteristics: while the shake-up is an essentially instantaneous process, the knock-up excitation due to the interaction between the ionic and the continuum electrons depends on the movement of the latter and may be significant for longer times, leading to delays in the ionization process. We observe an interplay of the initial shake-up excitation with the excitation cascades induced by the post-ionization interaction of the outgoing electron with the core. The time-scales and the structure of these excitation cascades is directly linked to the time it takes the 'active' continuum electron, lifted from a deeper bound orbital, to traverse the orbit of the 'passive' electron residing in the outer orbit. Ideas for resolving and controlling the two-electron interaction dynamics during the ionization process will be discussed.

Tuesday, 15 November 2011

Tuesday, 8 November 2011

JC: Tobi: Plasmonic generation of ultrashort extreme-ultraviolet light pulses

I will discuss another production from Kim:
"Plasmonic generation of ultrashort extreme-ultraviolet light pulses"

The authors build on previous work "High-harmonic generation by resonant plasmon field enhancement"
and now show high harmonic emission from something like a SNOM tip.

Tuesday, 4 October 2011

DM: guest: Miron Amusia: Effect of electron exchange on atomic ionization in a strong electric field

Specific properties of Hartree-Fock equations due to exchange and non-linearity are considered. Particular attention is given to an atom in a strong electric static field. It is demonstrated that exchange between outer and inner electrons, taken into account by the so-called Fock term affects strongly the long-range behavior of the inner electron wave function. As a result, it dramatically increases its probability to be ionized.
A simple model is analyzed demonstrating that the decay probability, compared to the case of a local (Hartree) atomic potential, increases by many orders of magnitude. As a result of such increase, the ratio of inner to outer electrons ionization probability became not too small.
It is essential that the effect of exchange upon probability of inner electron ionization by strong electric field is proportional to the square of the number of outer electrons. It signals that in clusters of fullerenes the inner electron ionization by strong field, the very fact of which is manifested by e.g. high energy quanta emission, has to be essentially increased as compared to this process in gaseous atomic objects.

Tuesday, 9 August 2011

JC: Stefan: Highly radiative shock experiments driven by GEKKO XII

In this paper, recent results obtained on highly radiative shocks generated in a xenon .lled gas cell using the GEKKO XII laser facility are presented. Data show extremely high shock velocity (.150 km/s) never achieved before in gas. Preliminary analyses based on theoretical dimensionless numbers and numerical simulations suggest that these radiative shocks reach a new radiative regime where the radiative pressure plays a role in the dynamics and structure of the shock. A major effect observed is a strong anisotropic emission in the downstream gas. This unexpected feature is discussed and compared to available 2D radiation hydrodynamic simulations.

Tuesday, 12 July 2011

DM extraordinaire: Will & Henry:Talk and Poster presentation for Toulouse and Erice

Will: Poster Erice Summer school: Compression of multi-millijoule pulses using circular polarization with a differentially pumped hollow-fiber

We have compared the performance of a hollow-fiber pulse compression system using both statically filled and differentially pumped configurations. The fibre is seeded using 3mJ, carrier-envelope phase stabilized pulses with either linear or circular polarization. We have found that the most desirable performance of the system is achieved by combining the use of circularly polarized pulses with differential pumping. In this case, a significantly higher energy transmission was measured at 1bar helium pressure, compared with the transmissions for differential pumping with linear polarization, and for both polarization cases under static fill. For a differentially pumped fiber, the broadest spectra were found at 3bar for both polarizations. Linear and circular polarizations both produced spectra supporting sub-4-fs pulse durations, but a higher energy transmission was observed in the circularly polarized case. The performace of the system is improved by differential pumping and circularly polarized pulses as a result of reduced ionization and self-focusing at the fiber entrance. We envisage that our technique will be useful for increasing the energy of few-cycle pulses, for strong field experiments at higher intensities.

Henry: Talk Toulouse FASTQUAST YEP meeting: High-order Harmonic generation from plasma plumes at KHz repetition rate

I am giving my first scientific talk outside of imperial at the YEP meeting in Toulouse in just under a couple of weeks. So I would like to run through the presentation I plan to give and get your feedback. In this talk I will be presenting the some of the resent progress we have with HHG for plasma plumes at KHz repetition rate, including the presence of Quantum path signature and resonant enhancement of certain harmonics from particular tarjets.

Tuesday, 5 July 2011

JC: Felix: Attosecond control of electron-ion recollision in high harmonic generation

We show that high harmonic generation driven by an intense near-infrared (IR) laser can be temporally controlled when an attosecond pulse train (APT) is used to ionize the generation medium, thereby replacing tunnel ionization as the first step in the well-known three-step model. New harmonics are formed when the ionization occurs at a well-defined time within the optical cycle of the IR field. The use of APT-created electron wave packets affords new avenues for the study and application of harmonic generation. In the present experiment, this makes it possible to study harmonic generation at IR intensities where tunnel ionization does not give a measurable signal.

Tuesday, 21 June 2011

JC: Rashid: Attosecond emission from chromium plasma

We present the first measurement of the attosecond emission generated from underdense plasma produced on a solid target. We generate high-order harmonics of a femtosecond Ti:sapphire laser focused in a weakly ionized underdense chromium plasma. Using the .Reconstruc- tion of Attosecond Beating by Interference of Two-photon Transitions. (RABITT) technique, we show that the 11thto the 19thharmonic orders form in the time domain an attosecond pulse train with each pulse having 300 as duration, which is only 1.05 times the theoretical Fourier transform limit. Measurements reveal a very low positive group delay dispersion of 4200 as2. Beside its fundamental interest, high-order harmonic generation in plasma plumes could thus provide an intense source of attosecond pulses for applications.

Monday, 16 May 2011

DM: Tsuneto: Development of CEP-stabilized high power CPA/OPA lasers and direct generation of highly phase-matched isolated attosecond pulses

One of the most crucial issues for the present attophysics is to increase the pulse energy of isolated attosecond pulses (IAP.s). For this purpose, there are two crucial experimental challenges to demonstrate; one is to develop multi-mJ few cycle lasers with a stabilized carrier-envelope phase (CEP) and the other is to achieve the phase-matching highly between the few cycle pulses and high harmonics of them. In this talk, I will present our research activities on both challenges. First, I will present our recent results of RIKEN based on our TW-class 2-cycle laser system (5 fs, 780 nm, 5 mJ, 1 kHz). Recently, we have stabilized CEP of this system with two feedback loops and by blocking mechanical vibrations on the laser system thoroughly. Through the phase matching process, we could select the supercontinuum component of the harmonics without any use of bandpass filters such as Mo/Si mirrors. By using a heterodyne interferometry using mixed gases, we verified that we could select IAP directly. Second, I will briefly introduce our recent results at Imperial College London on constructing a CEP-stabilized high power OPA (optical parametric amplification) laser. CEP-stabilized seed pulses were generated using type II DFG of different frequency components of the supercontinuum generated with a hollow fiber compression technique, and the seed pulses were amplified by two-stage OPA.s. Final output pulses had a pulse energy of 500 μJ, pulse duration of 40fs, and tunable wavelength from 1300-1800 nm. As the first demonstration of this handmade laser, we could generate high harmonics up to 47th using a tube target filled with Xe gas.

Tuesday, 5 April 2011

JC: Stefan: Study of high Mach number laser driven blast waves in gases

A series of experiments were performed examining the evolution of blast waves produced by laser irradiation of a target immersed in gas. Blast waves were produced by illumination of wires by 1 kJ, 1 ns laser pulses from the Z-Beamlet laser at Sandia National Laboratories. The blast waves were imaged by probe laser pulses at various times to examine the trajectory, radiative precursor, and induced perturbations on the blast wave front. Well de.ned perturbations were induced on the blast wave front with arrays of wires placed in the gas and the results of the experiments are compared to the theoretical predictions for the Vishniac overstability. It is found that the experimental results are in general agreement with these theoretical predictions on thin blast wave shells and are in quantitative agreement in the simplest case.

Tuesday, 29 March 2011

DM: Damien: OPA/OPCPA in crystals and fibres

I will do a general discussion/presentation about OPA/OPCPA in non linear crystal and in fiber. I will show some experimental results related to what I did in the PETAL Petawatt project.

Tuesday, 22 March 2011

DM: Jon: Programme Grant overview & introduction

we plan to extend attosecond science to larger molecules and condensed phase systems building upon our present activity in HHG spectroscopy and attosecond pulse generation. I hope to give a short briefing on the scientific plans

Tuesday, 8 March 2011

JC: Misha: Radiation Generated by the Ultrafast Migration of a Positive Charge Following the Ionization of a Molecular System

Electronic many-body effects alone can be the driving force for an ultrafast migration of a positive charge created upon ionization of molecular systems. Here we show that this purely electronic phenomenon generates a characteristic IR radiation. The situation when the initial ionic wave packet is produced by a sudden removal of an electron is also studied. It is shown that in this case a much stronger UV emission is generated. This emission appears as an ultrafast response of the remaining electrons to the perturbation caused by the sudden ionization and as such is a universal phenomenon to be expected in every multielectron system.
What I want everyone to think about is:
- What is the role of the detached electron that they have completely neglected?
- Can this radiation be observed, and if yes, will it be coherent?
PPT slides to follow...#

Tuesday, 22 February 2011

DM: Luis: how to grow micro and nanostructures, with both, nanosecond and femtosecond laser pulses

(i) Laser irradiation of materials with laser pulses whose energy density is around the ablation limit, no matter whether they are femtosecond or nanosecond pulses, do produce microconical structures despite of the, most likely, different mechanisms involved. These structucture do have several interesting applications.
(ii) We will show how a metalic thin film of nanoparticles can be rearranged just by shining few laser pulses with a well defined intensity structure.
PPT slides to follow...

Tuesday, 8 February 2011

DM: Malte: Molecular structure dependence of recollision induced fragmentation in N2

After converting the VMI in the Red Dragon lab to an ion time-of-flight spectrometer, it is currently used to investigate those inelastic electron recollisions, which lead to fragmentation. Inelastic recollisions lead to electron-electron interactions in the outer shell of the molecule and investigating them is one of the objectives of attosecond science.
In the lab, we are trying to find out if the recollision induced fragmentation of N2 depends on the alignment of the molecule, or in other words if fragmentation is more probable if the electron hits the molecule parallel or perpendicular with respect to the bond.
I will give a detailed description of the required experimental steps to find an answer to that question and present a few intermediate results. I will try to raise a few questions about the setup and future plans to get input from the group.
Most of the experimental issues are mentioned in the following paper, written by a very familiar Irish man: Nevertheless, I will briefly discuss them during the meeting as well.
PPT slides to follow...

Tuesday, 25 January 2011