  Time-dependent femtosecond pump-probe spectroscopic ellipsometry studies on
zincblende gallium-nitride (zb-GaN) are performed and analyzed between
2.9-3.7eV. An ultra-fast change of the absorption onset (3.23eV for zb-GaN) is
observed by investigating the imaginary part of the dielectric function. The
266nm (4.66eV) pump pulses induce a large free-carrier concentration up to
$4\times 10^{20}$cm$^{-3}$, influencing the transition energy between
conduction and valence bands due to many-body effects, like band filling and
band gap renormalization, up to $\approx$500meV. Additionally, the absorption
of the pump-beam creates a free-carrier profile within the 605nm zb-GaN layer.
This leads to varying optical properties from sample surface to substrate,
which are taken into account by grading analysis for an accurate description of
the experimental data. A temporal resolution of 100fs allows in-depth
investigations of occurring ultra-fast relaxation and recombination processes.
We provide a quantitative description of the free-carrier concentration and
absorption onset at the sample surface as a function of relaxation,
recombination, and diffusion yielding a characteristic relaxation time of
0.19ps and a recombination time of 26.1ps.
