Femtosecond-class radiative decay of multinode-type excitons in nano-to-bulk crossover regime

Transient grating (TG) spectrum and its delay-time dependence have been investigated for CuCl ultrathin films in a crossover regime of nano-to-bulk scale. The CuCl films whose crystalline quality is extremely improved by electron-beam irradiation at the start of deposition show anomalous mode structures in the TG spectrum. We succeeded in demonstrating that the photon energy at each peak in the TG spectrum is in good agreement with the theoretically predicted size-dependent eigenenergy including the radiative shift. Ultrafast radiative decay of excitons in a few hundred femtoseconds is observed for several eigenmodes, in which the radiative widths are extremely large due to a long-range coherent coupling between the weakly confined exciton and the radiation field.