Revealing the simultaneous increase in transient transmission and reflectivity in InN

The ultrafast transient behavior of InN under intensive laser irradiation remains unclear. The simultaneous measurements of pump–probe transient transmission and reflectivity are reported in this study. The irradiation-induced change in the dielectric constant of InN films due to the generation of thermalized carriers gives rise to complex transient behavior, and simultaneous increase in both transient transmission and reflectivity are observed at certain wavelengths. Herein, transient transmission is interpreted as the occupation probability of thermalized electrons at the probing level originating from a hot Fermi–Dirac distribution, and our calculation results are in good agreement with the experiments. Likewise, the Drude-like response due to the collective motion of thermalized carriers causes the increase in transient reflectivity, which depends on the change of dielectric constant caused by the collective motion of thermalized carriers. The ultrafast carrier dynamics is modeled by calculating the temporal evolution of the occupation probability of thermalized electrons in the conduction band. On the basis of the two-temperature model, the electron–phonon scattering time is extrapolated to be $∼300$ fs in InN, which dominates the cooling of excited electrons.