Photoinduced High-Density Phase of Exciton-Polariton Masses and their Coherent Propagation at Stacking Fault Interface in BiI 3

Photoinduced high-density phase of exciton-polariton masses excited at a two-dimensional stacking fault interface in BiI 3 has been studied by observing the degenerate four-wave-mixing (DFWM) signals. It was found that the increase in the dephasing rate due to exciton-exciton collisions is suppressed above a threshold excitation density. Above such density, a coherently propagating component of the exciton masses was detected on the space-resolved DFWM signals obtained by applying the spatially isolated two-spot excitation with a picosecond laser. In this scheme the off-diagonal element of the density matrix of the excitons at the one exciting laser spot propagates to the other, bringing about nonlinear polarization to yield the DFWM light. Time evolution of the space-resolved DFWM signals reveals evidently the macroscopic expansion of the exciton-polariton masses in a photoinduced condensed phase. Energy splitting of the DFWM spectra obtained with a narrow-band laser shows a repulsive interaction between the excitons.