Quantitative analysis of picosecond duration thermoelastic transients in polycrystalline metal thin films.

摘要

In the past decade, the interaction of nonequilibrium electrons with ultrafast laser pulses has seen significant study. These electrons are intimately involved in the absorption of optical energy, and the subsequent transformation of energy from the laser pulse to thermal energy in the absorbing material. For metals, the result is a local increase in temperature, which is accompanied by thermal expansion. While the acoustic pulses generated from rapid thermal expansion are routinely used to inspect the elastic properties and the thickness of opaque films, the thermal behavior of this phenomenon has yet to be described in a suitable framework. The difficulty in describing the observed picosecond duration thermal behavior of metals is that the time and length scales involved violate many of the tenants used to build both a thermodynamical and a statistical mechanical description of the phenomena.; The current work derives a thermoelastic model to describe the thermal and acoustic phenomena with classical thermodynamics. This theory is then analyzed from a statistical mechanical perspective in order to identify its weaknesses. A corrected nonlinear two-temperature thermoelastic model is then constructed. These theoretical results are used with a model that simulates reflectively sensitive measurements facilitating the direct comparison of theory with experiment. An electropolished aluminum single crystal is used to examine the validity of the presented model and the shortcomings of the conventional theories. The thermal behavior is experimental explored through a variety of metallic samples. The influence of film thickness, microstructure and annealing are examined on aluminum and tungsten samples.; A new experimental technique that shows promise as an analytical tool for the large area industrial implementation of thin film inspection through picosecond duration thermal and acoustic transients is presented. This technique encodes the picosecond duration material transients onto a chirped, super-continuum pulse, which is detected with a spectrometer. The high temporal resolution and fast acquisition time of this new technique outperforms the optical-correlation detection scheme that is in widespread use today.