## 3 Optically Pumped Lasers
### Whispering Gallery Mode (WGM) Lasers
A short growth time leads to the formation of cubic microcrystals, while long times result in appreciable edges length differences due to growth rate dependence on the facets orientation, thus resulting in parallelepiped microcrystals. Single mode lasing was again observed in small microcubes both under 80 femtoseconds pumping at 470 nm, with a threshold of 16.9 \(\upmu\)cm\({}^{-2}\) and a remarkably low linewidth of 0.064 nm, and under 1.1 nanosecond pumping at 355 nm, with a higher threshold of 214 \(\upmu\)cm\({}^{-2}\). Another fascinating demonstration of the perovskite capability to spontaneously form complex structures was recently obtained by [PERSON] et al. that realized [57] MAPbBr\({}_{3}\) microcubes by a solvatothermal process at a temperature of 120 \({}^{\circ}\)C, observing the initial formation of cubic microcrystals with an edge length increasing to about 10 \(\upmu\)m with the reaction time, followed by the formation of a step on the (001) surfaces and, as the reaction time further increased, of a multistep structure inside the microcubes (see Figure 8e). All these approaches show that perovskites have the capability to form bidimensional or tridimensional microcrystals with well-defined polygonal or polyedron shape if the growth process is adequately controlled, providing easy microcavity formation processes that allow to test the lasing properties of a given active material. However these spontaneously formed microlasers are not suitable for real devices applications as the lasing properties in terms of threshold and lasing wavelength are different between individual microcrystals and basically impossible to control and predict _a priori_. For this reason several groups worked to realize WGM microresonators with reproducible spectra and thresholds. A first possibility in this direction is the micropatterning of the substrate, in order to guide the spontaneous formation of identical microcavities. A nice demonstration of this approach was reported by [PERSON] et al. [58] that realized hexagonal micropatelets with an edge length of about 15 \(\upmu\)m of MAPbI\({}_{3}\) by physical vapor deposition on a Si substrate covered by a single layer boron nitride (BN), prepatterned in hexagons by photolitography. The BN hexagons act as nucleation centers for the perovskites, allowing the growth of MAPbI\({}_{3}\) crystals replicating the BN geometry. Multimode lasing was observed, under 50 fs pumping at 400 nm, with a threshold down to 9 \(\upmu\)cm\({}^{-2}\) and single mode lasing was demonstrated for edge length shortened to about 2 \(\upmu\)m. The authors also demonstrated the possibility to obtain single mode lasing in larger microcrystals by acting on the crystal geometrical symmetry rather than on its size, obtaining a single mode in asymmetric hexagonal crystal with an edge lengths between 5 and 10 \(\upmu\)m and with a threshold about 3 times smaller than the 2 \(\upmu\)m regular hexagon (see Figure 9a-d). Another fascinating approach to obtain arrays of microcrystals with similar shape and size is the template confined growth from solution. In this case, a precursor solution is deposited on a substrate and covered by a PDMS mold with patterned voids. The PDMS is then pressed on the substrate, leaving the solution only in the mold voids. The slow solvent evaporation in a geometrically confined region leads to perovskite crystallization and to the formation of microcrystals of the same shape and size. For example [PERSON] et al. [59] realized an array of CsPbCl\({}_{3}\) square microplates, showing a rather uniform edge length of 2.5 \(\pm\) 0.3 \(\upmu\)m, and multimode lasing around 427 nm. The comparison of the lasing spectra of nine different platelets anyway showed differences in the number and in the  
Figure 8: SEM images of (**a**) parallelepiped microcrystals of bromide perovskite MAPbBr\({}_{3}\) at low magnification and high magnification (top inset). The circled regions indicate the thickness of square MDs. The right bottom inset shows a cartoon of a square MD.