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Wednesday, 25 November 2015

Dane: JC/DM: benzene HHG spectroscopy / HHG in aromatic systems

I will discuss progress in understanding our benzene HHG spectroscopy results, as well as some recent results from Ravi Bhardwaj's group on the similar topic of HHG in aromatic systems:
http://journals.aps.org/pra/abstract/10.1103/PhysRevA.92.041801

Tuesday, 17 November 2015

Emilio: JC: Chirality in HHG

Non-collinear circularly polarized high harmonic generation (NCP-HHG).

Non-collinear generation of angularly isolated circularly polarized high harmonics

Probing molecular chirality on a sub-femtosecond timescale
http://www.nature.com/nphys/journal/v11/n8/abs/nphys3369.html

A table-top ultrashort light source in the extreme ultraviolet for circular dichroism experiments

Tuesday, 10 November 2015

Tuesday, 3 November 2015

JC: Bernd: THz induced field emission from nanostructures


In the first part I will summarise how single-cycle THz pulses can be efficiently generated, and I will present corresponding detection schemes. Due to the low photon energies in the meV range, THz radiation is typically understood to be nonionising. However, several groups have recently demonstrated that field emission from nanostructures can be induced by intense THz fields (see e.g. Herink et al., NJP 16, 123005 (2014); Iwaszczuk et al., Optica 2, 116 (2015).). I will present some of these results and discuss the possibility to generate ultrashort electron bunches with very high kinetic energies.

Wednesday, 28 October 2015

Alvaro: DM: Summary of the experimental setup and current results from experiment LF62 at LCLS (also serves as Late Stage Review)

As a main outcome from our last experiment at LCLS, a paper on Auger spectroscopy in Glycine has been accepted for publication. Apart from the results discussed in the paper, different experimental aspects as the experimental setup and the calibration of the different devices will be presented.


Tuesday, 14 July 2015

Margarita Khokhlova: Analytical theory of resonant high-order harmonic generation

abstract as PDF file here: PDF file

Analytical theory of resonant high­order harmonic generation
M. A. Khokhlova1,2, V. V. Strelkov1
1. Theoretical Department, General Physics Institute of Russian Academy of Sciences,119991, 38 Vaviliva str., Moscow, Russia
2. Department of Physics, M. V. Lomonosov Moscow State University, 119991, Leninskie gory str., Moscow, Russia
Properties of resonant high­order harmonics generated in intense laser field are actively studied both 
experimentally [1­3] and theoretically [4­7]. Very efficient generation of the harmonic resonant with the 
transition from the bound to the autoionizing state was demonstrated in the experiments using plasma plumes 
[1,2] and Xe jet [3].
We suggested analytical quantum­mechanical theory describing the effect of quasi­stationary state on HHG 
[7]. We start with the time­dependent Schrodinger equation for an atom or ion in an external laser field. The 
wave function is presented as a sum of the ground state, unperturbed continuum and the quasi­stationary state. 
To solve the Schrödinger equation we derived the perturbation method in which the solution obtained in the 
absence of the quasi­stationary state by Lewenstein et al. in Ref. [5] is taken as an unperturbed solution. 
Assuming that (i) the ionization rate is low, (ii) the quasi­stationary state population is low, (iii) the quasi­
stationary state is not affected with the laser field, and (iv) the quasi­stationary state width * is much less than 
the quasi­stationary state energy, we find the following equation for the spectral complex amplitude of the 
microscopic response at the frequency Z
 (atomic units are used):
 P ( Z ) P nr ( Z ) F (Z )
a
 * / 2 o
F (Z )
 « 1 Q »
¬
 'Z i* / 2 1⁄4
where is the 
 P
 nr
 (Z
 )
 spectrum of the non­resonant contribution, * is the resonance width, Q is a complex 
parameter 
 defined by the properties of the generating atom or ion, but not 
depending on the laser field. So the resonant harmonic line is presented as a product of the F (Z ) Fano­like 
[8] factor and the harmonic line which would be emitted in the absence of the AIS. 
Our theory allows calculating not only the resonant harmonic intensity, but also its phase. We show that there 
is a rapid variation of the phase in the vicinity of the resonance. Our calculations reasonably agree with recent 
harmonic phase measurements [9]. 
The other direction of our research is the study of the phase properties of the cut­off harmonics. We study the 
harmonic phase dependence on the laser intensity both analytically and numerically. Moreover, we investigate 
the dephasing between adjacent harmonics to study the emission time of the attosecond pulses. Thus, the 
optimum conditions for the shortest attosecond pulse generation using the cut­off harmonics are suggested.
References
>1@ R. A. Ganeev, "High­order harmonic generation in a laser plasma: a review of recent achievements", J. Phys. B: At. Mol. Opt. Phys. 40, 
R213 (2007).
[2] R. A. Ganeev, V. V. Strelkov, C. Hutchison, A. Zaïr, D. Kilbane, M. A. Khokhlova, and J. P. Marangos, "Experimental and theoretical 
studies of two­color­pump resonance­induced enhancement of odd and even harmonics from a tin plasma", Phys. Rev. A 85, 023832 (2012).
[3] A. D. Shiner, B. E. Schmidt, C. Trallero­Herrero, H. J. Wörner, S. Patchkovskii, P. B. Corkum, J­C. Kieffer, F. Légaré & D. M. 
Villeneuve, "Probing collective multi­electron dynamics in xenon with high­harmonic spectroscopy", Nature Physics 7, 464 (2011).
[4] V. Strelkov, "Role of Autoionizing State in Resonant High­Order Harmonic Generation and Attosecond Pulse Production", Phys. Rev. 
Lett. 104, 123901 (2010).
[5] M. Lewenstein, Ph. Balcou, M. Yu. Ivanov, Anne L’Huillier, and P. B. Corkum, "Theory of high­harmonic generation by low­frequency 
laser fields", Phys. Rev. A 49, 2117­2132 (1994).
[6] Jan Rothhardt, Steffen Hädrich, Stefan Demmler, Manuel Krebs, Stephan Fritzsche, Jens Limpert, and Andreas Tünnermann, "Enhancing 
the Macroscopic Yield of Narrow­Band High­Order Harmonic Generation by Fano Resonances", Phys. Rev. Lett. 112, 233002 (2014).
[7] V. V. Strelkov, M. A. Khokhlova, and N. Yu Shubin, "High­order harmonic generation and Fano resonances", Phys. Rev. A 89, 053833 
(2014).
[8] U. Fano, "Effects of Configuration Interaction on Intensities and Phase Shifts", Phys. Rev. 124, 1866­1878 (1961).
[9] S Haessler, V Strelkov, L B Elouga Bom, M Khokhlova, O Gobert, J­F Hergott, F Lepetit, M Perdrix, T Ozaki and P Salières, "Phase 
distortions of attosecond pulses produced by resonance­enhanced high harmonic generation", New J. Phys. 15, 013051 (2013).
[10] J. Seres, E. Seres, D. Hochhaus, B. Ecker, D. Zimmer, V. Bagnoud, T. Kuehl and C. Spielmann, "Laser­driven amplification of soft X­
rays by parametric stimulated emission in neutral gases", Nature Physics 6, 455–461 (2010)

Tuesday, 9 June 2015

JC: Daniel: Disclosing intrinsic molecular dynamics on the 1-fs scale through extreme-ultraviolet pump-probe measurements

Through frequency up-conversion of polarization-shaped, femtosecond laser pulses nonlinearly interacting with xenon atoms, energetic, broadband, coherent, XUV continuum radiation is generated. By exploiting the thus-formed short-duration XUV pulses, all the optically allowed excited states of H2are coherently populated. Nuclear and electronic 1-fs-scale dynamics are subsequently investigated by means of XUV-pump–XUV-probe measurements, which are compared to the results of ab initiocalculations. The revealed dynamics reflects the intrinsic molecular behavior, as the XUV probe pulse hardly distorts the molecular potential.



Figure 2

Tuesday, 26 May 2015

Peter: JC: Effect of multiple conduction bands on high-harmonic emission from dielectrics

We find that, for sufficiently strong mid-IR fields, transitions between different conduction bands play an important role in the generation of high-order harmonics in a dielectric. The transitions make a significant contribution to the harmonic signal, and they can create a single effective band for the motion of an electron wave packet. We show how high harmonic spectra produced during the interaction of ultrashort laser pulses with periodic solids provide a spectroscopic tool for understanding the effective band structure that controls electron dynamics in these media.
Figure 3
http://journals.aps.org/pra/abstract/10.1103/PhysRevA.91.013405

Tuesday, 12 May 2015

No JC, but OSA/SPIE student chapter seminar

No Jc today, but please go to Blackett 630 for 15:00 for this seminar organised by the local OSA/SPIE student chapter!

Microcirculation imaging with light and sound

Martin J. Leahy, Haroon Zafar, Sean O’Gorman, Aedán Breathnach,  and Hrebesh M. Subhash

Tissue Optics & Microcirculation Imaging Facility, National University of Ireland, Galway, Ireland and National Biophotonics & Imaging Platform, Ireland.

Structural and functional imaging of the microcirculation is necessary to understand many diseases such as diabetes mellitus, heart disease, peripheral vascular disease and arteriosclerosis. We have developed several methods for assessment of both the structural and dynamic properties of the capillaries in the upper dermis and the vessels which supply them.

We have developed 2D devices based on polarisation spectroscopy which output colour-coded maps which are sensitive to the concentration of red blood cells in the skin tissue. The hardware design consists of two orthogonally placed polarisation filters over the light source and sensor of a standard digital camera and utilises the video mode to provide 15/30 frames per second at a resolution of ~ 200 mm. We have also developed correlation mapping optical coherence tomography (cmOCT) to render the 3D microcirculation. To obtain microvascular maps without motion artefact, we used the full-field technique and applied the cmOCT algorithm to the data. We are developing miniature low cost OCT. and nanosensitive OCT [3&4]. Since the depth is limited to one or two millimetres with OCT and much less with full-field OCT, we have recently begun to work with photoacoustic tomography where we find a backscatter type probe, similar to existing clinical ultrasound, has advantages of ease of use, speed and familiarity for radiographers.


References:
1. Microcirculation Imaging. (Wiley-VCH), Leahy, M.J. editor, 2012.
2. Enfield, J., Jonathan, E. and Leahy, M.J., 2011. In vivo imaging of the microcirculation of the volar forearm using correlation mapping optical coherence tomography (OCT). Biomedical Optics Express 2 (5) 1184-1193.
3. Dsouza, R., Subhash, H.M., Neuhaus, K., Hogan, J., Wilson, C., and Leahy, M.J. 2014, Dermascope guided multiple reference optical coherence tomography, Biomedical Optics Express, 5(9) 2870-2882. http://dx.doi.org/10.1364/BOE.5.002870
4. Alexandrov, S., Subhash, H.M., Zam, A. and Leahy M.J. 2014, Nano-sensitive optical coherence tomography, Nanoscale, 6, 3545-3549, DOI: 10.1039/C3NR06132A

Tuesday, 28 April 2015

Daniel: DM: Experimental methods for time-resolved measurements of molecular electron dynamics

In the ‘attosecond lab’ we can now generate synchronised attosecond pulses at 20eV and 90eV, which we hope to use to make time resolved measurements of ultrafast electronic processes in atoms and molecules, such as charge migration. In conjunction, a new electron-ion particle detector has been installed (the ‘VMI-TOF’) in order to be able to get as much information as possible from the probe step in a ‘pump-probe’ measurement. I will try to give a brief overview of the theory of charge migration (from a non-theorist stand point!) and the challenges of making attosecond pump - attosecond probe measurements. I will then discuss the experimental implementation of spLEAD (single photon Laser Enabled Auger Decay) , a scheme to measure electron dynamics in the glycine molecule, and the operation of the VMI-TOF, with a view to informing future experiments .

Tuesday, 21 April 2015

Christian: JC: Efficient generation of below-threshold harmonics for high-fidelity multi- photon physics in the VUV spectral range

We demonstrate the generation of microjoule level, sub-20-fs, Ti:Sa fifth-harmonic pulses utilizing a loose-focusing geometry in a long Ar gas cell. The VUV pulses centered at 161.8 nm reach pulse energies of 1.1 μJ per pulse, while the corresponding pulse duration is measured with a second-order, fringe-resolved autocorrelation scheme to be 18±1  fs . Nonresonant, two-photon ionization of Kr and three-photon ionization of Ne verify the fifth-harmonic pulse high-intensity content and indicate the feasibility of multi-photon VUV pump-VUV probe studies of ultrafast atomic and molecular dynamics.




Tuesday, 17 March 2015

Felicity: JC: Attosecond Hole Migration in Benzene Molecule Surviving Nuclear Motion

Hole migration is a fascinating process driven by electron correlation, in which purely electronic dynamics occur on a very short time scale in complex ionized molecules, prior to the onset of nuclear motion. However, it is expected that due to coupling to the nuclear dynamics, these oscillations will be rapidly damped and smeared out, which makes experimental observation of the hole migration process rather difficult. In this Letter, we demonstrate that the instantaneous ionization of benzene molecules initiates an ultrafast hole migration characterized by a periodic breathing of the hole density between the carbon ring and surrounding hydrogen atoms on a subfemtosecond time scale. We show that these oscillations survive the dephasing introduced by the nuclear motion for a long enough time to allow their observation. We argue that this offers an ideal benchmark for studying the influence of hole migration on molecular reactivity.



Abstract Image

Tuesday, 24 February 2015

DM: Laser consortium LCLS subteam: Overview of our latest LCLS experiment: beamtime progress and results

Most of the members of the laser consortium already had a good idea of our aim for the LCLS experiment. However, experiments don't always go as expected, specially when they are done at facilities with a short pre-allocated beamtime, and the final results can be quite different from the original aim. In this journal club, we will show the progress of our latest experiment at LCLS shift by shift, and how the original plan had to be modified on the fly to to adapt to the different circumstances and issues we were presented with. The talk will include an overview of practical aspects, as how we calibrated the different devices within the beamtime, and will give a very quick overview of the potentially most relevant results, still open for discussion and interpretation.

Tuesday, 20 January 2015

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.