| Phonon transport is essential in both understanding and characterizing materials, particularly in nanoscale systems. In this thesis, I use coherent ultrafast extreme ultraviolet (EUV) beams from high-order harmonic generation, to study the dynamics of photoacoustics and energy-carrying phonons at the nanoscale.;I first generate and detect short-wavelength photoacoustic waves by impulsively heating suboptical phononic crystals with an infrared laser. By monitoring the diffraction dynamics of EUV beams I observe the shortest-wavelength surface acoustic waves to date at 35 nm, corresponding to an interface layer sensitivity of sub-10 nm. I also achieve coherent control of SAW generation and preferentially enhance higher-order SAWs which allows us to reduce the generated SAW wavelength by a factor of two for a defined nanostructure period. I apply this photoacoustic technique to thin film metrology metrology: by generating nanoscale longitudinal and surface acoustic waves simultaneously, I am able to characterize the mechanical properties of ultrathin film samples.;Secondly, I study thermal transport dynamics in nano-to-bulk systems where phonons are heat carriers. I first observed quasi-ballistic thermal transport in 1D nano-to-bulk systems, and detect a stronger ballistic effect in 2D nanostructured materials. Temperature- and polarizationdependent experiments are also reported in this thesis. Furthermore, I was able to make a first attempt in dynamic thermal imaging using coherent diffraction of EUV beams. |
