## 2 Amplified Spontaneous Emission
### Perovskites Bulk Polycrystalline Thin Films
This issue is relevant for all the possible device application of perovskites, including solar cells, thus a large research is running in order to improve the active materials and the devices stability. In the particular case of laser applications it is important to observe that, in order to reach the population inversion, the optical pumping regime of the active layers is typically very strong. Moreover, in order to realize low cost devices, either optically or electrically pumped, it is necessary to use long excitation pulses (at least in the nanosecond scale) or, hopefully, continuous wave excitation thus requiring stable materials against heating. A powerful strategy to improve the active layer stability is the chemical engineering of the active material, aiming to exclude less stable components and, in particular, to replace MA with more stable cations. For example it was demonstrated that formamidinium (CH(NH\({}_{2}\))\({}_{2}\), FA)-based perovskites and, in particular, in FAPbI\({}_{3}\), show low threshold ASE and almost constant ASE intensity during continuous under 5 ns pumping at 532 nm pumping up to at least 2 \(\times\) 10\({}^{5}\) pulses (while, under the same pumping conditions, the MAPbI\({}_{3}\) ASE intensity drops to only 9% of the initial value) [21]. Another promising class of materials with improved stability are fully inorganic compounds in which the organic cation is substituted by an inorganic specie (typically Cs). The first results on ASE in fully inorganic lead halide perovskites were obtained by [PERSON] et al. [22] demonstrating ASE in CsPbBr\({}_{3}\) films at cryogenic temperatures (up to 150 K) with a minimum threshold of 26 \(\upmu\)Jcm\({}^{-2}\) at 10 K, under 3 ns pumping at 337 nm. The investigation of the temperature dependence of the PL spectra allowed to demonstrate that the ASE properties are strongly affected by the presence of defects and by their energy distribution, causing the presence of thermally activated exciton trapping or detrapping. Room temperature ASE was later demonstrated in CsPbBr\({}_{3}\) films deposited by one step coevaporation of CsBr and PbBr\({}_{2}\), also showing unchanged ASE threshold after 10 days of storage in air and almost constant ASE intensity during exposure to 2 \(\times\) 10\({}^{5}\) 5.5 nanosecond pulses at 353 nm [23]. More recently, [PERSON] et al. [24] succeeded in obtaining room temperature ASE in CsPbBr\({}_{3}\) films deposited from solution thanks to high temperature and pressure induced recrystallization of pristine films not showing ASE. This approach strongly modified both the morphology and the optical properties of the active films, reducing the roughness from 23.8 nm to only 0.5 nm and increasing the PLQY from 43% to 53%, thus allowing to observe room temperature ASE with a threshold of 12.5 \(\upmu\)Jcm\({}^{-2}\), under 300 ps pumping at 355 nm. A very recent breakthrough toward future application of lead halide perovskites to real lasers was reported by [PERSON] et al. [25] that demonstrated, for the first time, ASE under continuous wave optical excitation at 532 nm in the temperature range 80-120 K, with a minimum threshold of 400 Wcm\({}^{-2}\) in a phase stable triple cation non stoichiometric Cs\({}_{0.1}\)(MA\({}_{0.17}\)FA\({}_{0.83}\))\({}_{0.9}\)Pb\({}_{0.84}\)(I\({}_{0.84}\)Br\({}_{0.16}\))\({}_{2.68}\). At higher temperatures the active material degradation prevented to reach the ASE threshold. While apparently still far from the requirements of CW operations at room temperature, or at least in the temperature range accessible by Peltier cooling (T > 220 K), this result is really impressive if one considers that it was obtained only five years later than the first ASE demonstration under femtosecond pumping.