Picosecond Ultrasonics for Nanoscale Subsurface Structural Characterization

Picosecond ultrasonics (PU), also known as ultrafast photoacoustics, is a pump-probe experimental technique that utilizes laser-generated acoustic waves in the gigahertz to terahertz frequency range. In this method, an ultrashort laser pulse (pump) irradiates an opaque material, causing localized heating and rapid thermal expansion, which generates an acoustic wave. This wave propagates through the sample, reflects from internal interfaces, and returns to the surface, where it is detected by a time-delayed probe pulse. By systematically varying the temporal delay between the pump and probe pulses, one can investigate the generation, propagation, and reflection dynamics of the acoustic waves with sub-picosecond temporal resolution. The picosecond timescale of acoustic generation enables several significant opportunities. Firstly, it generates ultrahigh-frequency acoustic modes, corresponding to acoustic wavelengths on the order of 100 nm. This short wavelength promises successful application of PU in high-resolution metrology. In particular, it has already been implemented as a tool for characterizing thin films and evaluating interfacial adhesion. Meanwhile, the detection and imaging of buried structures are in the active research and development phase. Secondly, PU provides an opportunity for exploring fundamental processes in solids governed by electron dynamics, such as electron transport and electron-phonon coupling, which is of high interest in fundamental physics studies. In this thesis, we focus on exploiting PU for the detection and imaging of buried nanostructures. Despite the demonstrated potential for high-resolution imaging, several challenges remain. These include improving the signal-to-noise ratio in photoacoustic measurements, enabling the detection of complex (non-binary) and three-dimensional nanostructures, and overcoming the spatial resolution limits imposed by optical diffraction. Although light-induced acoustic waves can possess much shorter wavelengths than light itself, current implementations of PU remain constrained by optical probing methods. In this work, we address these challenges and propose approaches to advance the capabilities of PU for nanoscale detection and imaging.