David Kroon, Van-Thai Pham, Jörgen Larsson, Amélie Jarnac, Henrik Enquist, Andrius Jurgilaitis
The studied sample is an indium antimonide (InSb) coated with 60 nanometres of gold. This type of structure is called photo-acoustic transducer which is a device that can convert the energy in light to a sound wave. The sample is illuminated with light for a very short time (50 fs). The light is absorbed in the gold film and that energy is converted to heat within a few picoseconds. The rapid expansion due to heat creates sound waves both at the gold-vacuum interface and at the gold-InSb interface.
By using very short bursts of x-rays you can measure how the sound wave changes the local density of the InSb sample. The time-dependant intensity of the diffracted X-rays gives information about the shape of the acoustic waves which in turn sheds light on how the wave was generated. The particular design allows for modulating the intensity of X-rays with light. We have demonstrated an on-switch which allows reflection of X-rays for only 20 ps.
Although these types of experiments are very much basic science – the scientific field itself is no more than fifteen years old – one could foresee the knowledge obtained being used for example in making materials for data storage. Our daily lives become more and more dependent on storing ever growing amounts of data. The ability to write and read such data at high speeds is an important factor in this development.
In this particular experiment, the photo-acoustic transducer generated a series of acoustic strain pulses which pairwise made the studied acoustic phonons interfere destructively. Since the experiment probe the acoustic phonon, the X-ray reflectivity shows a peak structure with a 20 picosecond FWHM.
A lot of work still remains before FemtoMAX is a fully up-and-running beamline ready for user experiments. The most important improvement is that the new in-vacuum undulators from Hitachi needs to be installed. These are anticipated to give more than a factor 100 higher flux at this photon energy (5 keV) compared to the present undulator. This work is planned for the time around the start-up of the 1.5 GeV storage ring in the fall. The performance will also be enhanced by increasing the repetition rate of the electron pulses in the Linear accelerator from 2 Hz to 10 Hz and eventually 100 Hz. In order to demonstrate that even faster dynamics can be studied, the pulse duration and jitter needs to be measured and optimized. This work is joint with the linac team. In order to accommodate higher repetition rates the data acquisition and storage speed needs to be increased. In the spring, work will be done with pulse-duration measurements and tests aimed at demonstrating flexibility in laser wavelength for the excitation.