Real-time analysis of nonlinear transient response of weakly confined excitons

By means of the nonlocal transient-response theory, we elucidate the characteristics of the femtosecond transient response of thin films with a thickness beyond the long wavelength approximation (LWA) regime. In this regime, the contribution of higher excitonic states with a nondipole-type spatial structure becomes dominant and the interplay between the spatial structures of excitonic wavefunction and the radiation field plays an important role, causing an anomalous enhancement of nonlinear signal at specific size-energy resonant conditions. In addition, in the femtosecond pulse excitation, the interference of the signals from the multiple excitonic states, which are excited simultaneously by the incident pulse with a wide spectral width, can generate a greater diversity of optical response than expected by the steady-state analysis. This suggests the possibility that we can control the optical function of the nano-materials by the selective excitation of the aimed excitonic states using the laser pulse. This study serves as a theoretical basis for the nonlinear transient response by ultrashort pulse excitation of excitons confined in nano-structures.