Directional and Polarized Lasing Action on Pb‐free FASnI₃ Integrated in Flexible Optical Waveguides

Abstract In this work, high‐quality FASnI₃ (FA, formamidinium) lead‐free perovskite thin films are successfully incorporated in a flexible polyethylene terephthalate (PET) substrate to demonstrate amplified spontaneous emission (ASE) and lasing. The waveguide (WG) consists of polymethylmethacrylate(PMMA)/FASnI₃ bilayer deposited on a PET substrate and is properly designed to allow single‐mode propagation at the photoluminescence wavelength. This geometry optimizes the excitation of the emitting FASnI₃, enhances the light−matter interaction in the semiconductor thin film, provides a preferable direction for the emitted light and allows its direct outcoupling for on‐chip or fiber‐optic applications. As far as the authors know, ASE and random lasing are obtained for the first time in a flexible‐based WG integrating a highly efficient lead‐free perovskite. The high quality of the deposited films and the optimized design of the structure result in an extremely low ASE/lasing threshold in the range of 1 µJ cm⁻², which is only ten times higher than that measured in the same PMMA/FASnI₃ structure deposited on a rigid substrate (Si/SiO₂). More interestingly, these WGs exhibit a strong polarization anisotropy for the outcoupled ASE/lasing light with a preferable transverse electric polarization. This work is the base for the future development of ecofriendly, flexible, and efficient photonic devices.