The use of light as a tool in science has profoundly enhanced our understanding of the universe, on scales ranging from the distribution of galaxies, to the finest structure of the microscopic world. On the smallest scales, laser based sources offer the ultimate precision in terms of spatial and temporal localization. Focusability, and hence spatial localization, is a characteristic feature of nearly all laser-based sources, and since the development of the pulsed laser, the time durations that we can observe have been refined from microseconds to attoseconds. Combined, these features have allowed scientists to track the progress of chemical reactions and even the motion of individual electron wave-packets in matter. However, the precise selection of specific photon energies, while maintaining sub-femtosecond pulse durations, for example to excite or probe specific electronic or vibrational transitions, is a demanding requirement. While laser-based sources offer precise frequency control, their selection is limited to natural lasing transitions and are mostly narrowband. This can be overcome by making use of nonlinear optical effects to convert laser frequencies, or through the use of synchrotron or other accelerator based sources—although these lack high temporal resolution and come at a significant cost in terms or resources and complexity. Additionally, current attosecond sources are limited to the XUV spectral region, by virtue of their generation process.
In this work I proposed to use a new regime of high-energy optical soliton dynamics to revolutionize our access to the shortest, sub-femtosecond, high-energy optical pulses in the near-infrared down to the vacuum-ultraviolet range: a region of huge importance to spectroscopy, but poorly served by current sources.
Solitons, a central concept in nonlinear physics, are particle-like nonlinear wave-packets or pulses that maintain their shape upon propagation and interaction. They appear in many areas of physics, including plasmas, magnetic circuits and the atmosphere. Optical solitons form through the balance of linear effects such as dispersion or diffraction, and the intensity dependent refractive index. A wide range of fascinating and useful soliton dynamics have been discovered in solid-core fibres at up to kilowatt peak powers and nanojoule energies, including: pulse self-compression, the emission of radiation analogous to the Čerenkov radiation, inter-soliton collisions, and soliton self-frequency red-shifting due to interactions with molecular oscillations (the Raman effect). By orchestrating soliton dynamics in both time and frequency an extreme transient polarization of the electrons and molecules of the nonlinear medium can be created and a supercontinuum can be formed. Supercontinua can contain the spectral content, for example, of sunlight, spanning from the ultraviolet to infrared, but possess other properties usually associated with lasers, such as directionality and extreme brightness.
The objectives of my proposal are:
1. To study a new regime of ultra high intensity and energy temporal optical soliton dynamics in gas and plasma media in large-bore hollow capillaries. I aim to achieve millijoule energy-scale, soliton dynamics, and thus combine high-field laser science with the physics of solitons.
2. To use these soliton dynamics to create a coherent, ultrafast, table-top and tunable light sources deep into the vacuum ultra-violet (100 nm to 200 nm, 6 eV to 12 eV). I plan to exceed the energy and pulse characteristics of any other known sources in the VUV region.
3. To use these soliton dynamics to produce millijoule scale sub-femtosecond pulses in the visible-infrared spectral region—the shortest isolated optical pulses ever generated there.
4. To combine points 2 and 3 above, in one experiment, to perform sub-femtosecond VUV pump-probe spectroscopy experiments.
Nuclear motion and charge dynamics occur on femtosecond and attosecond timescales respectively. Therefore, to probe the evolution of molecular structure and its interplay with electron dynamics, in real-time, requires pump-probe spectroscopy experiments operating on these time-scales. A very wide variety of atoms, molecules and solids have electronic resonances in the VUV spectral region. To observe the excitation and evolution of electronic wave-packets in these systems, similar to what has been achieved in the XUV region using high-harmonic based attosecond sources, requires a sub-femtosecond VUV source. Indeed, it has been suggested that one might even be able to control the motion of the electronic wave-packets. However, there is a distinct lack of sources in the vacuum ultraviolet (VUV) range (see state of the art below). The tunable, sub-femtosecond, VUV source I propose will address this need. More generally, many of the techniques that provide access to the fundamental properties of matter, such as attosecond transient absorption spectroscopy, photoemission spectroscopy, or photoionization mass-spectroscopy, would directly benefit from a sub-femtosecond VUV source.
