Ultrafast nonlinear acoustics in crystals and nanostructures

Abstract

This research aims at experimentally monitoring nonlinear generation and propagation of picosecond coherent acoustic strain wave packets in solids. The experiments are performed by ultrafast pump-probe reflectometry and interferometry. At first, nonlinear strain generation in thin nickel and chromium films is characterized. At high pump fluences, the elevated electron and lattice temperatures in the absorption layer significantly modify thermal and mechanical material parameters, thereby increasing the strain amplitude superlinearly. Electron diffusion is suppressed by electron-electron collisions. The results are accurately described by a two-temperature model for fluences up to 80% of the damage threshold, above which nonthermal processes come into play. At room temperature, the high-amplitude strain waves generated in a thin chromium film and launched into the sapphire substrate, transform into an acoustic shock wave within tens of micrometer due to large atomic displacements and the nonparabolic interatomic potential. When lowering the temperature, thermal attenuation gradually decreases and lattice dispersion comes into play. At 20 K, propagation is undamped, thus leading to the formation of acoustic solitons. A maximum number of seven solitons is measured. By performing measurements on different sample thicknesses, the superlinear soliton velocity can be determined. Since the soliton velocity is intimately linked to its spatial width, a soliton width as short as two nanometer can be derived. The measured soliton velocities and volumes are in excellent agreement with numerical simulations of propagation, as well with the exact predictions by the Korteweg – De Vries equation. Following the demonstration of acoustic solitons, an experiment was devised in which these solitons are used to modulate nanostructures on ultrafast timescales. Modulation of exciton states inside a semiconductor within times shorter than the exciton lifetime leads to chirping, i.e. modification (broadening) of the reflection spectrum. These results provide an outlook for the use of nanometer-sized acoustic solitons as a means of nano-ultrasonic material characterization and modulation. Using time-resolved Brillouin scattering, the strain generation in (semi-)transparent materials was investigated. This way, strain generation in GaN/InGaN quantum wells in the saturation regime was characterized. By monitoring the spectral intensity at the Brillouin frequency in yttrium vanadate as a function of pump fluence and temperature, the dynamics of electronic and structural phase transitions are studied. For initial temperatures below 80 K, the structural phase transition appears to occur fast in the crystalline c-direction, and slow in the a-direction. Finally, an explorative study on the ultrafast carrier dynamics in ZnO nanowires is presented. The lasing behaviour of these nanowires is quantitatively analyzed in the electron-hole plasma regime.