place in about 1 ps, and brings the electrons in thermal equilibrium with the
lattice. The final step is cooling of the lattice, through heat diffusion to the
environment. Typically less than a nanosecond after excitation, the particle
has returned to its initial state.
The relaxation of the electron gas is described by the two-temperature
model. We can define two subsystems, each with their own temperature (Te
for the electron gas, T l for the lattice). The thermalization of the two systems
can then be described by two coupled differential equations [35, 36].