## 2 Amplified Spontaneous Emission
### Perovskites Bulk Polycrystalline Thin Films
A similar approach has also been recently reported in MAPbI\({}_{3}\) films, observing a threshold reduction of about 1.5 times, due to the deposition of the top PMMA layer, and a strong increase of the ASE intensity (up to 13.9 times) due to the further addition of a PMMA-gold nanorod layer, and ascribed to plasmonic effects [15]. Several other experiments instead exploited the degrees of freedom of the active layer deposition from solution, thus modifying either the solutions properties or the deposition method in order to optimize the film morphology and optical properties. For example enhanced ASE properties were demonstrated in MAPbI\({}_{3}\) thin films with controlled morphology [16]. The films were realized by a two step sequential deposition process, by initially depositing a PbI layer from solution, and then dipping the film in a solution of MAI. The variation of the solvents used to dissolve PbI allowed changing the PbI film morphology and, as a consequence, the final MAPbI\({}_{3}\) one. A minimum ASE threshold of 54.1 \(\upmu\)cm\({}^{-2}\), under 5 ns pumping at 500 nm, was reached in the film with the lowest roughness and grain size, due to the combination of maximum gain and minimum waveguide losses. The role of the micromorphology and of the crystalline grain size was also investigated in green emitting MAPbBr\({}_{3}\) films by comparing the optical amplification properties of films deposited with two different methods: simple spin coating of a precursor solution and two step antisolvent deposition [17]. In the last approach the precursor solution is spin coated on the substrate and dripped with an antisolvent in order to increase the evaporation rate and to decrease the crystalline grain size, leading to the so-called \"nano-crystal pinning\" (NCP). The investigation of the film morphology allowed to observe, in the as-cast film, the presence of isolated plate-like microcrystals with a widely distributed size from several \(\upmu\)m to nearly 100 \(\upmu\)m. On the contrary the NCP film showed smooth and uniform surface due to better coverage and smaller microcrystals size (below 100 nm). Moreover the NCP film demonstrated a photoluminescence quantum yield (PLQY) 2.4 times larger and an ASE threshold about 1.7 times smaller than the as-cast film. The same approach was also used in order to optimize the ASE properties of MAPbBr\({}_{3}\) films, as a preliminary step for the realization of Distributed Feedback (DFB) lasers [18], resulting in comparable ASE threshold values of the NCP film. Another example of the role of the solution on the film morphology and on its optical properties was obtained by the development of a modified antisolvent deposition method for MAPbI\({}_{3}\)[19]. In particular the authors demonstrated that the presence of organic molecules additives to the antisolvent reduces the ASE threshold up to 4 times, down to a minimum value of 600 \(\upmu\)cm\({}^{-2}\), under 1 ns pumping at 533 nm. This result is due to the localization of the additive molecules at the grain boundaries of the perovskite film, thus passivating non radiative recombination centers at the grain boundary surface. A further approach to improve the active layer morphology was demonstrated in MAPbI\({}_{3-x}\)Cl\({}_{x}\) films deposited by double spin coating from a precursor solution [20]. In this case, the sample realized by standard spin coating did not show ASE; it was instead present in the sample deposited by double spin coating with a threshold of 50 \(\upmu\)cm\({}^{-2}\) under 880 ps at 337 nm pumping. This difference was ascribed to a higher surface coverage of the sample obtained by double spin coating, and thus better waveguiding properties. All these results evidence the possibility to realize MAPbX\({}_{3}\) films, showing promising low ASE threshold values and several possible optimization degrees of freedom. Anyway the applicative perspectives of these materials are currently limited by the known poor stability to light irradiation, heating and moisture exposure.