Direct observation of ultrafast free carrier dynamics in InSe

The optical properties of semiconductors are specified by the dynamics of free carriers and bound excitons, both originating from dynamics of electron-hole pair excitations. However, the contribution of free carrier to the optical properties is masked by the excitons-related features. We overcome this issue by assessing the free carrier dynamics in bulk InSe with TR-ARPES measurements.
Snapshots of conduction band pumped by 1.55 eV photons at different time delays is explored. The peak excitation is reached at t = 0.2 ps. The parabolic dispersion around the conduction band edges can be extracted from the peak intensity difference at t = 0.2 ps, where the full parabolic dispersions of the band extrema emerge due to the increased population of carriers. This then relaxes at later time delays. During interband excitons, a non-thermal distribution of excited electron-hole pairs is generated at the earliest time delays, relaxing into a quasi-equilibrium state with a temperature higher than that of the lattice.
Our measurements provide a direct observation of population inversion occurs immediately after interband excitation. By directly measuring the band occupancy at all energies and times, we have made a quantitative evaluation of the carrier dynamics.