Laser action and photoexcitations dynamics in PbI_2 films

We have grown high quality PbI2 films by thermal evaporation. We obtained in these films two amplified spontaneous emission (ASE) lines around ~2.23 eV at relatively low pump excitation threshold using 10 ns pulsed laser, along with a broad photoluminescence (PL) band that results from trapped electron and hole (e-h) pairs in shallow traps. We discuss the two ASE lines as due to the anisotropic PbI2 crystal structure. In addition, we also studied the ultrafast transient response of photoexcitations in the PbI2 films using a broadband pump-probe correlation spectroscopy in the broad spectral range of 0.3-2.4 eV with 150 fs time resolution. The transient photomodulation spectrum is dominated by two photoinduced absorption bands and a stimulated emission band that agrees with the obtained ASE band. We ascribed the low threshold laser action in PbI2 films as due to the e-h confinement in ‘quantum wells’ like structures formed between the main 2H-PbI2 crystalline and other polytypes with lower optical gap.