Tuesday, 10 January 2017
In this journal club I will discuss an experiment in which I participated at LCLS in October, which aimed to explore possibilities for single shot temporal characterisation of hard (~5keV) X-ray pulses (duration ~15fs) using a cross correlation with sub 10fs optical pulses in thin solids. My role was to support the development of a hollow core fibre system to deliver the short optical pulses.
In a suitable material, absorption of an X-ray pulse will transiently decreases the optical transmission as electrons are promoted to the conduction band. This process can then be probed in time using a non-colinear laser pulse to map the time dependent transmission to a spatially dependent transmission, which is then can then be recorded by a high frame rate camera. A variant of this technique is already regularly used at LCLS to measure the arrival time of the FEL pulses relative to optical pulses (i.e in laser pump, X-ray probe experiments). This experiment aimed to study the time dependent optical response of various samples to the X-ray pulses, which is complicated by subsequent electronic processes such as cascaded Auger decay. This, together with knowledge of the laser pulse temporal profile, would in principle allow the FEL pulse temporal profiles to be deconvolved from the spatially dependent transmission profiles on a single shot basis.
Using high harmonic generation (HHG) to make attosecond pulses in the soft X-ray (SXR) for time-resolved X-ray spectroscopy measurements is a topic of great interest. Recent work has demonstrated the first CEP dependent harmonic generation into the so-called "water window" region between 284 eV and 540 eV . This region is of interest because there are numerous absorption edges of interesting atoms, while water is largely transparent, allowing in principle for invivo imaging and spectroscopy. Other recent work includes improved X-ray generation up to 500 eV  and time-resolved measurements at the carbon K-edge (284 eV) .
I'll present our work on generation of CEP dependent HHG across the water window (implicitly, attosecond pulses). We have generated HHG from neon and helium up to maximum energy of 650 eV, and can move our cutoff energy (and thus, attosecond pulse) to any absorption edge in the water window. Our source also exhibits excellent spatial quality, as demonstrated by a SWORD measurement, suggesting the potential for imaging applications and even, intriguingly, X-ray non-linear optics. We also perform the first X-ray absorption measurement with an HHG source at the oxygen K-edge (540 eV).