## 3 Optically Pumped Lasers
### Whispering Gallery Mode (WGM) Lasers
As further characterization, the authors also investigated the lasing spectra dependence on the edge length, observing a progressive reduction of the number of lasing modes and an increase of their spacing as the edge length was progressively decreased from 29 \(\upmu\)m to 9 \(\upmu\)m (see Figure 7d). Finally full color tuning was demonstrated by acting on the chemical structure in ternary and quaternary compounds, all showing lasing threshold below 10 \(\upmu\)cm\({}^{-2}\) (see Figure 7e). Very similar results were also obtained by depositing square CsPbX\({}_{3}\) (X = I, Br, Cl) nanoplatelets by CVD on Si/SiO\({}_{2}\) and sapphire [50]. An alternative deposition approach was proposed by [PERSON] et al. that demonstrated WGM lasing from MAPbBr\({}_{3}\) microstructures realized from solution [51; 52], reducing the solvent evaporation rate  
Figure 7: (**a**) Far-field optical image of two typical MAPbI\({}_{3}\) nanoplatelets under the illumination of white light (upper panel) and pump laser (bottom panel) providing evidence of clear diffraction patterns inside the WGM cavity. (**b**) Excitation density dependence of the PL spectra of a typical MAPbI\({}_{3}\) triangular nanoplatelet (optical image inset; thickness, 150 nm; edge length, 32\(\upmu\)m). Left inset: integrated output emission (P\({}_{out}\)) over the whole spectra range as a function of pumping fluence (P\({}_{in}\)) using log-log scale, providing evidence of the lasing threshold. Right inset: PL relaxation dynamics below (pink, SE) and above (dark green, lasing) the threshold. (**a,b**) adapted with permission from [48]. American Chemical Society, 2014. (**c**) Optical image of CsPbBr\({}_{3}\) nanoplatelets showing square or rectangular shapes and rich colors. (**d**) Multimode lasing spectra of four nanoplatelets with different edge length from 9 to 29 \(\upmu\)m. (**e**) Multicolor WGM microlasers based on inorganic perovskites with different composition. (**c-e**) adapted with permission from [49]. John Wiley and Sons, 2016.  
by covering the solution with a second substrate. Even if simple, this technique allowed the realization of microcrystals with a larger variety of shapes, including disks, squares and rectangles. Tridimensional MAPbBr\({}_{3}\) microcrystals were instead obtained by a single step deposition process from solution, consisting of the deposition of a precursor solution in DMF and drying in a sealer beaker containing dichloromethane as antisolvent [53]. The diffusion of dichloromethane in the solution induced the nucleation and the subsequent growth of square 3D microcrystals with very smooth outer surface and sharp edges (see Figure 8a). Single mode lasing with a threshold as low as 3.6 \(\upmu\)cm\({}^{-2}\), under 120 fs pumping at 400 nm, was demonstrated in a square microcrystal with a edge length of 2.0 \(\upmu\)m, while at higher edge lengths multimode lasing was observed. The same technique was also exploited to realize fully inorganic CsPbBr\({}_{3}\) parallelepiped microrods, with length in the 2-8 \(\upmu\)m range and height in the range 1-5 \(\upmu\)m, thus big enough to allow WGM formation (see Figure 8b) [54], and even branched microrods (see Figure 8c) [55]. A detailed description of the growth process leading to the spontaneous formation of parallelepiped microcrystal was reported by [PERSON] et al. that deposited cubic CsPbBr\({}_{3}\) microcubes by dual source CVD [56]. By investigating the shape of the microcrystals formed on substrates in different positions in the furnace tubes, and thus grown at different temperatures, the authors observed (see Figure 8d) that the microparticles formed at lower temperature have a poor crystallinity and a spherical shape, in order to minimize the internal energy. At higher temperatures a crystallinity increase was observed, resulting in a progressive shape transition toward cubes with well-defined facets and edges.