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.