  The electron-phonon coupling in ultrafast heating systems is studied within
the framework of Boltzmann transport equation (BTE) with coupled electron and
phonon transport. A discrete unified gas kinetic scheme is developed to solve
the BTE, in which the electron/phonon advection, scattering and electron-phonon
interactions are coupled together within one time step by solving the BTE again
at the cell interface. Numerical results show that the present scheme can
correctly predict the electron-phonon coupling constant, and is in excellent
agreement with typical two-temperature model (TTM) and experimental results in
existing literatures and our performed time-domain thermoreflectance technique.
It can also capture the ballistic or thermal wave effects when the
characteristic length/time is comparable to or smaller than the mean free
path/relaxation time where the TTM fails. Finally, the electron-phonon coupling
in transient thermal grating geometry and Au/Pt bilayer metals with interfacial
thermal resistance is simulated and discussed. For the former, heat flow from
phonon to electron is predicted in both the ballistic and diffusive regimes.
For the latter, the reflected signal increases in the early tens of picoseconds
and then decreases with time after the heat source is removed.
