## 3 Optically Pumped Lasers
### Random Lasers
These evidence of morphological defects inducing random lasing stimulated further research aiming to control the lasing properties of the samples by acting on the film structure and morphology. Figure 4: (**a**) Emission spectra collected below (black), near (blue), and above lasing threshold (red). The spectra were offset vertically for clarity; horizontal line-segments at the left of each spectrum indicate their corresponding zero-levels. (**b**) Integrated emission intensity as a function of excitation pulse fluence, showing evidence of the presence of a slope variation due to the random lasing threshold. (**c**,**d**) Micro-PL images showing the spatial distribution of emission at the two highest pump intensities in (**a**). Bright spots observed in (**d**) above threshold correspond to high scattering locations where laser light is efficiently out-coupled from the perovskite network, as also evident by the comparison with the SEM image (**e**). Adapted with the permission from [39]. AIP Publishing, 2014. A possible approach in this direction is based on the variation of the sample preparation conditions, in order to modify its morphology. For example, [PERSON] et al. compared the emission properties of MAPbI\({}_{3}\) films obtained by a two step spin coating process, using four different concentrations of the PbI\({}_{2}\) DMF solution (namely 10%, 20%, 30% and 40% in weight) [41]. The films morphology showed an inhomogeneous substrate coverage, with a rugged surface and randomly distributed grains when the 10% PbI\({}_{2}\) solution was used and a progressive morphology improvement for higher concentrations. In particular the sample from the 10% PbI\({}_{2}\) solution was characterized by an air-void occupation area of about 50%, progressively decreasing down to about 2% for the film deposited from the 40% PbI\({}_{2}\) solution. The emission spectra of the films were investigated at a temperature of 77 K, showing a clear ASE band as the excitation density increased, with a threshold depending on the film, and an evident modulation due to random lasing in the sample deposited from the 30% PbI\({}_{2}\) solution. The ASE and lasing differences between the samples were ascribed to the variation of the morphology, changing the balance between optical gain and grain boundaries scattering. More recently morphological differences were observed by depositing MAPbI\({}_{3}\) films by spin coating of a PbI\({}_{2}\) DMF solution, followed by dipping for various times in a solution of MAI in IPA [42]. The dipping time also affected the film emission properties, showing only spontaneous emission, ASE or random lasing depending on the preparation conditions. Enhanced random lasing was also claimed in CsPbBr\({}_{3}\) thin films thanks to the addition of ZnO nanoparticles to the perovskite precursor solution [43]. In this experiment, no lasing like peaks were present in the PL spectra at high excitation density, providing evidence of the lack of coherent scattering in the films. The smaller ASE threshold in the ZnO containing film was ascribed to smaller crystalline grains of the film and lower density of air voids on the film surface, related to the scattering efficiency of the film. A second possibility to introduce, and control, scattering in the samples is the deposition of the active film on a structured substrate. A first demonstration of this approach was obtained by depositing MAPbBr\({}_{3}\) nanocrystals on a substrate corrugated by strain release wrinkles [44]. The sample was realized by unidirectionally stretching a polydimethylsiloxane (PDMS) film up to 100% of its original length and by depositing on it a graphene oxide (GO) layer and a NCs layer. The difference in elastic constant between PDMS, GO and NCs leads to an almost unidirectional corrugation of the surface when the strain is released.