Reexamination of electron-phonon coupling constant in continuum model by comparison with Boltzmann transport theory

Abstract

The electron-phonon (e-ph) coupling process in ultrafast dynamics involves non-equilibrium effects including non-thermalized electrons and temporal non-equilibrium between different phonon branches. During such process, the two-temperature model (TTM) is employed to extract the relevant e-ph coupling constant though experiments themselves have demonstrated the invalidity of TTM. In this work, through fitting the results of Boltzmann transport theory by that of TTM, we quantitatively investigate the influence of the non-equilibrium effects on extraction of the e-ph coupling constant in TTM. The extracted e-ph coupling constant will be indeed underestimated yet not significantly by about 7% under the influence of non-equilibrium between different phonon branches for Au, Ag. Additionally, when the excitation pulse width is comparable to the e-ph relaxation time of electrons, non-thermalized electrons exists after excitation and thus the extracted e-ph coupling constant in TTM shows over 20% deviation. Therefore, the excitation pulse width is required to be larger than 300 fs and 500 fs for Au, Ag, respectively at room temperature if the error is controlled within 10%. This work will provide insightful indication for the measurement of the e-ph coupling constant in femtosecond pump-probe experiments.