Setting up of a heterodyne pump-probe bench : application for the imaging with picosecond acoustic waves
Acoustic waves in the Gigahertz or Terahertz frequency range allow the mechanical characterization of submicronic structures. The generation and the detection of these waves can be performed with the use of femtosecond lasers combined with pump-probe setups. This report describes the setting-up of an opto-acoustic imaging experiment with a submicronic spatial resolution. The association of asynchronous optical sampling with the use of low repetition rate femtosecond lasers considerably increases acquisition rates and offers a high spectral resolution, respectively. The first part of this report presents the two laser cavities with slightly different repetition rates in order to perform asynchronous optical sampling. The scheme and the performances of the synchronization stage are described. In the second part, the implementation of this dual-oscillator in a pump-probe experiment is detailed and the ability to detect sub-THz acoustic waves with a 50 MHz-spectral resolution is demonstrated. Finally, in the last chapter, the strong potential of this experiment to perform ultrafast imaging is illustrated through two examples : the measurement of the dispersion of GHz surface acoustic waves due to the presence of a thin film and the imaging and the sizing of submicronic elastic heterogeneities.