Past REU Participants & Project Pages

2006 REU Porjects
2006 REU Projects
2006 Participant: Karl A. Meyer
Project Title: Ultrashort pulsed laser heating of thin Au films

Faculty Advisor: Pamela M. Norris
The thermal behavior of nanoscale metallic and semiconductor films has become a significant factor in the performance of electronic and optical devices as the drive for miniaturization continues. As film thicknesses decrease to the nanoscale, principles of macro heat transfer, specifically Fourierís Law, no longer apply as they ignore energy transport at the atomic level. The heating of thin films by ultrashort laser pulses, often mere pico or femtoseconds in duration, is particularly useful in microelectronics due to its spatial control and high heating rate. As with small length scales, heat transfer at time scales on the order of molecular interactions invalidates Fourierís Law. Two characteristic times are particularly relevant in the laser heating of metals. The electron thermalization time, typically around 10 femtoseconds for metals, is the time for excited electrons to relax into a Fermi distribution. At this point, the hot electrons transfer energy to the lattice through electron-phonon collisions. The time for the electrons and lattice to reach thermal equilibrium is the electron-phonon thermalization time, usually around 5 picoseconds for metals. If the laser pulse is much greater than the e-p thermalization time, the heating can be described as one-step, for the electrons and lattice remain in equilibrium. However, if the laser pulse duration is longer than electron thermalization time but shorter than e-p thermalization time, the energy transfer between electrons and phonons must be considered. In order to experimentally capture temperature profiles of both the electrons and phonons, data must be collected on a picosecond time scale. The Transient Thermoreflectance Technique (TTR), which uses a pump-probe setup to monitor reflectance, can accomplish this temporal resolution. In this study, we will examine the TTR response of 20 nm Au films, set on different substrates. We will compare this data to the predictions of the Parabolic Two Step (PTS) model, fitted to reflectance data using the Drude model. Furthermore, the effect of substrate on electron-phonon coupling will be observed.
Figure C shows that for high fluence, measured vales of the electron-phonon coupling factor, G, are significantly higher than the accepted material value. A portion of this increase can be attributed to a small dependence of G on temperature. However, the primary reason is substrate interference. The substrate absorbs some of the electron systemís energy. This energy loss is misinterpreted as additional energy transfer to the lattice, resulting in a higher measured G. As displayed in Figure D, the rate of increase in measured G is higher for Si substrate, because silicon is a conductor, while glass is an insulator.
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