In this paper, the physical mechanisms responsible for optical gain in ${\mathrm{CH}}_3{\mathrm{NH}}_3{\mathrm{Pb}\mathrm{I}}_3$ (MAPI) polycrystalline thin films are investigated experimentally and theoretically. Waveguide structures composed by a MAPI film embedded in between PMMA and silica layers are used as an efficient geometry to confine emitted light in MAPI films and minimize the energy threshold for amplified spontaneous emission (ASE). We show that photogenerated exciton density at the ASE threshold is as low as $(2.4-12)\times {10}^{16}{\mathrm{cm}}^{-3}$, which is below the Mott transition density reported for this material and the threshold transparency condition deduced with the free-carrier model. Such a low threshold indicates that the formation of excitons plays an important role in the generation of optical gain in MAPI films. The rate-equation model including gain is incorporated into a beam-propagation algorithm to describe waveguided spontaneous emission and ASE in MAPI films, while using the optical parameters experimentally determined in this work. This model is a useful tool to design active photonic devices based on MAPI and other metal-halide semiconductors.

• Received 24 November 2019
• Accepted 13 May 2020

DOI:https://doi.org/10.1103/PhysRevApplied.13.064071