Adaptive control, generation, and characterization of bright soft x-rays by quasi-phase-matching

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This programme uses recent advances made by the Oxford group in producing and controlling trains of amplified, ultrashort laser pulses, as well as a new method for producing high-energy photons in waveguides, and seeks to apply this to generating very bright sources of coherent, ultrashort, soft x-ray radiation - laser-like pulses of radiation at energies approaching x-ray photon energies, with extremely short time durations.Coherent soft x-ray radiation can be produced through a process called high harmonic generation . In this process multiple frequencies of an intense laser pulse can be generated by ionizing an atom and colliding the resulting electron into its parent ion, producing a pulse of radiation which can be as short as 100 attoseconds (1 attosecond = 1 billionth of a microsecond). Photon energies approaching x-ray energies can be generated this way. However, the efficiency of this process is very low, and gets much lower at higher energies, severely limiting many applications. The main cause for this low efficiency is the effect of dephasing . Simply put, this means that the laser pulse and the generated soft x-rays travel at different speeds, preventing the continuous growth of the soft x-ray. One way to overcome this dephasing is to employ a technique known as quasi phase-matching, whereby the harmonic generation process is switched on and off with a period equal to the coherence length , i.e. the distance over which the driving laser pulse and the generated radiation become out of phase. If this is achieved over N coherence lengths the soft x-ray signal will increase by a factor N-squared. Therefore, to maximize the power of this technique it is necessary to quasi phase-match over as many coherence lengths as possible. Switching the harmonic process on and off can be achieved in a number of ways. One method involves using a series ( or train ) of closely spaced, very short laser pulses travelling in the opposite direction to the generating laser pulse. Under EPSRC grant EP/C005449 the Oxford group has recently developed a method for producing trains of laser pulses with up to 100 pulses which can be accurately controlled. This has the potential to match up to 100 coherence lengths, which would increase the soft x-ray yield by a factor of 10,000! The adaptive nature of these pulse trains will also allow genetic algorithms to be employed to fully optimize the soft x-ray source. As part of this programme the group will seek to exploit the power of these new adaptive pulse trains to produce a soft x-ray source from harmonic generation with a brightness far exceeding anything previously recorded. Under grant EP/C005449 the Oxford group, in collaboration with a group at Queen's University Belfast (QUB), also discovered a new method for controlling the harmonic process. This relies on manipulating the way a laser pulse propagates through a narrow, hollow glass fibre. Through precise control of the laser pulse and how it couples into the fibre it is possible to excite modes in the fibre such that they create an interference pattern within the fibre which switches the harmonic generation on and off. The Oxford and QUB groups successfully used such a pattern to create the brightest source of water window x-rays from harmonic generation ever reported and, as part of this program will seek to develop this method even further. The development of such bright soft x-ray sources is crucial for a variety of scientific applications such as ultrafast measurements of chemical reactions, high-contrast biological imaging, advanced lithography, atomic and molecular spectroscopy, and ultrafast x-ray diffraction.