Generation and detection of quasi-monochromatic sound waves
Quasi-monochromatic strain waves can be detected by Ultrafast X-Ray Diffraction (UXRD) as sidebands of the substrate reflection. Depending on the sensitivity of the experiment, several higher-order peaks can be detected. The propagating waves are either generated by optical excitation of a transducer with multiple pump pulses or by the excitation of a superlattice with a single pulse. In particular, this method allows for the generation of coherent phonons with different frequencies. UXRD is a direct probe of the phonon lifetime in different wavelength and temperature regimes. It also allows for the detection of nonlinear transport phenomena, e.g. frequency conversion of the propagating phonons.