Electron-phonon nonequilibrium during ultrashort pulsed laser heating of metals

Ultrashort pulsed lasers have repeatedly been demonstrated as an effective tool for the observation of transport properties on atomistic time and length scales. Accordingly, the number of applications of these types of lasers as diagnostic tools is rapidly increasing. To effectively use these tools, precise knowledge of the energy deposition mechanism is absolutely necessary. The accepted model for ultrashort pulsed laser heating is the “Two Temperature Model” which assumes equilibrium electron and phonon distributions that are not in equilibrium with each other. Recently the applicability of the “Two Temperature Model” has received some scrutiny for very low and very high intensity application. This model gave rise to the electron-phonon coupling factor, which, when combined with the temperature difference between the two systems, represents the rate of energy transfer for small perturbations in temperature. However, numerous applications use moderate to high intensity ultrashort pulses, which create far more than small perturbations in temperature.; In this investigation the temperature dependence of the electron-phonon coupling factor, electron heat capacity, and thermal conductivity are examined for significant changes in the electron temperature. Experimental results are presented for transient thermoreflectance data taken at moderate fluences. A significant discrepancy is apparent between the two temperature model and the experimental data taken on Au. This problem was originally thought to arise from increased electron-phonon coupling for moderate changes in the electron temperature. Investigation into the temperature dependence of the electron-phonon coupling factor did not support this hypothesis. It was discovered that the discrepancy was due to a nonlinear relationship between changes in the electron temperature and changes in reflectance.; The incident probe energy used when taking the experimental data was 1.5 eV, which is significantly less than the first interband transition energy in Au of 2.45 eV. Therefore, absorption occurs due to intraband transitions. The Drude model for a nearly free electron gas was used to model the dielectric constant as a function of temperature. The temperature dependence of intraband transitions comes from the electron collision frequency. The electron-electron collisional frequency is proportional to the square of the electron temperature, while the electron-phonon collisional frequency is linearly related to the lattice temperature. It is shown that the observed nonlinear relationship between changes in electron temperature and reflectance is the result of electron-electron collisions.