## 3 Optically Pumped Lasers
### Random Lasers
The correlation between the wrinkles formation and the random lasing was demonstrated by measuring the PL spectra, at high pumping density, progressively removing the strain and observing that only spontaneous emission was present when the PDMS-GO substrate was fully strained (and thus planar), while random lasing peaks appeared when the residual strain decreased below 60%. The possibility to control the wrinkle size and geometry by engineering the  
Figure 5: (**a**) SEM image of the CH\({}_{3}\)NH\({}_{3}\)PbI\({}_{3}\) film showing evidence of the presence of cuboids with a typical size of about 200 nm. The inset shows the sample cross section. (**b**) Excitation density dependence of the PL spectra of the film, showing evidence of the progressive appearance of random lasing peaks as the excitation density increases. Inset: excitation density dependence of the PL intensity, showing the random lasing threshold. (**c**) Pictorial view of the formation of a closed-loop laser cavity via random scattering provided by the polycrystalline grain boundary. Adapted with permission from [40]. Royal Chemical Society, 2016.  
strain conditions of the substrate opens the way to the realization of random lasers with optimized emission properties. While this result is still not present in perovskites random lasers it was recently demonstrated that the control of the wrinkles height, shape and size allows to realize organic active layers showing random lasing, ASE or even no light amplification [45]. A further elegant experiment on the perovskite random lasing engineering was recently performed by [PERSON] et al. [46] that deposited MAPbBr\({}_{3}\) NCs on a network of graphene-sheathed SiC nanowalls (EGNWs) (see Figure 6a). The sample PL spectra clearly showed coherent random lasing peaks with a very low threshold of 10 nJcm\({}^{-2}\) (see Figure 6c), under 55 ps pumping at 374 nm, ascribed to the extremely high photon trapping within the graphene network. As further step the authors improved the scattering properties of the EGNWs by coating them with 40 nm of Ag nanoparticles and then with 100 nm of SiO\({}_{2}\). This coating process left basically unaltered the EGNWs morphology (see Figure 6b) but strongly improved the scattering cross section, due to the plasmonic field enhancement induced by the Ag nanoparticles. The random lasing properties of the EGNW-Ag-NCs samples were similar to the EGNW-NCs one, due to the interplay between improved scattering and non radiative NCs emission quenching, both due to the Ag plasmons. On the contrary a remarkable improvement of the emission properties showed both by a PL intensity increase and by a random lasing threshold decrease of one order of magnitude was observed in the EGNW-Ag-SiO\({}_{2}\)-NCs samples due to suppression of the PL quenching at the Ag/PNCs interface induced by the incorporation of the SiO\({}_{2}\) spacer layer. Despite the attempts to control their emission properties the applicative perspectives of random lasers are to date still limited by the lack of reproducibility of mode wavelength and threshold from sample to sample, and by the poor directionality of the emission, which makes unrealistic any claim of their use in applications requiring the emission features of standard lasers (monochromaticity, directionality and spatial and temporal coherence). Anyway the multimode emission and the low spatial coherence propose random lasers as bright and cheap sources for applications in which coherence can produce artifacts, like the speckle patterns formation in illumination. A nice evidence of the perovskite random lasers potentialities as speckle free lighting sources was very recently obtained in MAPbBr\({}_{3}\) thin films deposited by spin coating and showing a morphology dominated by the aggregation of irregular perovskite islands with an in-plane size in the range 20-80 \(\upmu\)m [47]. Coherent random lasing with a threshold of about 91 \(\upmu\)Jcm\({}^{-2}\) was observed, under 100 fs pumping at 400 nm, ascribing the feedback to scattering at the islands edges.