Imaging the structure and motion of molecular-scale systems is of fundamental importance in science. Imaging such small spatial features requires the use of light with wavelengths of the order of a few nanometers, while resolving fast dynamics in these systems requires pulses of light with durations of attoseconds (1as = 1 thousandth of a billionth of a nanosecond). Producing bright light sources which meet these requirements represents a significant challenge but the potential rewards are huge, with applications spanning biomedical science to advanced engineering and nanotechnology.
One source of short wavelength radiation is high-harmonic generation (HHG). This is a nonlinear process which can occur when an intense laser pulse is focussed into a gas, causing the atoms to emit bursts of soft x-rays. HHG is a very attractive source for many applications since the resulting soft x-ray beams are coherent, or laser-like, and can be generated using compact laser systems. The HHG pulses are also extremely short - on the order of attoseconds - and correspond to the natural timescales of electronic motion.
Recent experiments have shown that HHG can be used to drive coherent diffractive imaging (CDI) experiments, previously only possible at large-scale facilities. In CDI an object is illuminated by a coherent beam of x-rays and the diffracted light recorded. Algorithms are then used to determine the shape of the object from the diffracted pattern. CDI performed at large-scale facilities has proven to be an extraordinarily powerful technique, enabling the structures of cells and nanocrystals to be determined. Laboratory-scale CDI driven by HHG is therefore an attractive route towards compact, ultrafast, nanoscale imaging, and would be a revolutionary scientific tool.
However, a key step in realizing the full potential of HHG is to control the spatial structure of these beams. CDI is very sensitive to the coherence and shape of the illuminating beam while the nonlinearity of the HHG process can result in beams which exhibit complex structures. Controlling these beams is non-trivial since conventional optics, such as lenses, absorb strongly at these wavelengths. In this programme we will achieve precise control of HHG beams by implementing two innovations. The first uses a new approach for manipulating the shape of intense laser pulses which was developed by our project partners at Oxford University. This technique combines two programmable optical elements, one reflective and one refractive, in order to allow arbitrary shaping of an ultrafast laser beam. The sensitively of the HHG process to the driving laser means that by precisely controlling the laser beam we will be able to modify the properties of the generated harmonics. The second innovation is that we will be able to measure directly, for the first time, the changes in the structure of the HHG field as the driving beam is varied. To do this we will implement a technique which we developed recently which uses a pair of pinholes in the HHG beam to create an interference pattern. By analysing the interference pattern as the pinholes are moved we can determine the intensity, curvature, and coherence of the HHG beam with high accuracy.
This control system will allow us to dramatically improve the power of HHG-driven CDI. For example, we will be able to greatly increase the brightness of the HHG source, improve its coherence, and control its wavefront curvature. This will result in much sharper images, higher resolutions, and faster image acquisitions. It will also enable us to image complex targets, such as viruses and nanoparticles.
This research programme will bring together powerful new techniques in pulse-shaping and x-ray metrology to dramatically extend the scope of compact x-ray imaging. The development of such a system is crucial for a variety of areas such as high-contrast biological imaging, advanced lithography, materials engineering, and medicine.
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