Oxford Centre for High Energy Density Science (OxCHEDS)

Abstract:

Accurate characterization of laser pulses used in experiments is a crucial step to the analysis of their results. In this paper, a novel single-shot frequency-resolved optical gating (FROG) device is described, one that incorporates a dispersive element which allows it to fully characterize pulses up to 25 ps in duration with a 65 fs per pixel temporal resolution. A newly developed phase retrieval routine based on memetic algorithms is implemented and shown to circumvent the stagnation problem that often occurs with traditional FROG analysis programs when they encounter a local minimum.

Abstract:

Spatially resolved interference is observed between high-order harmonics generated in two longitudinally separated gas targets. High-contrast modulations in the intensity of each harmonic order up to the cutoff are observed on-axis in the far field of the source as the separation between the gas targets is increased. For low-order harmonics, additional off-axis modulations are observed, which are attributed to the interference between the contributions from the long quantum trajectories from each gas target. The inherent synchronization of this setup offers the prospect for high-stability metrology of quantum states with ultrafast temporal resolutions.

Abstract:

We generalize the temporally encoded spectral shifting (TESS) analysis for measuring plasma wakefields using spectral interferometry to dissimilar probe pulses of arbitrary spectral profile and to measuring nonlinear wakefields. We demonstrate that the Gaussian approximation used up until now results in a substantial miscalculation of the wakefield amplitude, by a factor of up to two. A method to accurately measure higher amplitude quasilinear and nonlinear wakefields is suggested, using an extension to the TESS procedure, and we place some limits on its accuracy in these regimes. These extensions and improvements to the analysis demonstrate its potential for rapid and accurate on-shot diagnosis of plasma wakefields, even at low plasma densities.