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.

http://journals.aps.org/pra/abstract/10.1103/PhysRevA.91.013405

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

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 .

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.

http://www.opticsinfobase.org/ol/abstract.cfm?uri=ol-40-8-1675

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.

http://pubs.acs.org/doi/abs/10.1021/jz502493j

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.

Allan: JC: Generation of bright isolated attosecond soft X-ray pulses driven by multicycle midinfrared lasers

Generation of bright isolated attosecond soft X-ray pulses driven by multicycle midinfrared lasers
http://www.pnas.org/content/111/23/E2361