Allan S. Johnson

My phd forms is taking place in the Imperial College Laser Consortium. We aim to enable measurements and control of matter at the attosecond timescale, the timescale of electron motion and the fastest events in normal matter. The basic enabling technology of attosecond physics is attosecond laser pulses in the extreme ultraviolet (XUV) and x-ray wavelength ranges, created by a process called high harmonic generation. The first attosecond pulses were demonstrated over a decade ago, but using this exciting new technology to examine nature at its most basic levels is an as yet unrealized dream.

Recently, with new advances in XUV optical components and control over the spatial properties of harmonic beams, allowing for efficient focusing, the major remaining challenge to attosecond physics is the generation of  individual attosecond pulses. This requires generating few-cycle laser pulses to drive high harmonic generation process. Once a stable source of high energy, high brightness individual attosecond pulses becomes available, new avenues for studying real time (attosecond) electron dynamics, the main focus of my phd.

The Laser Consortium has recently installed a new hollow core fibre compression system which allows for the generation of ultrashort pulses with a central wavelength of 1.8µm.

This sort of fibre compression system provides the necessary high power, broadband source for the generation of isolated attosecond pulses. In order to utilize this source to its greatest extent, however, a great deal of optimization is still necessary, requiring a host of online beam diagnostic and control systems. Thus far I have built a single shot frequency resolved optical gate (FROG) for shot to shot diagnostics of the temporal properties of the pulse, and an imaging spectrometer in the infrared (IR). I will also begin building a 4f pulse shaper to allow for arbitrary waveform synthesis, which together with the online diagnostics will allow for dynamic optimization of the harmonic generating source. We are simultaneously developing a high pressure gas target for efficiently generating water window harmonics


This project is the work of a team comprised of Dr. Dane Austin, Paloma Matia-Hernando, Dr. Tobias Witting, Dr. Thomas Siegel, Prof. John Tisch, and Prof. Jon Marangos.