## 3 Optically Pumped Lasers
### Whispering Gallery Mode (WGM) Lasers
Adapted with permission from [53]. John Wiley and Sons, 2015. (**b**) CsPbBr\({}_{3}\) microrod. Adapted with permission from [54]. John Wiley and Sons, 2015. (**c**) MAPbBr\({}_{3}\) branched microrod. Adapted with permission from [55]. Royal Society of Chemistry, 2017. (**d**) CsPbBr\({}_{3}\) microcubes at different growth stages with increasing temperature. Adapted with permission from [56]. Royal Society of Chemistry, 2018. (**e**) MAPbBr\({}_{3}\) cuboids at different reaction times. Adapted with permission from [57]. Elsevier, 2019.  
wavelengths of the lasing modes and in the threshold, ascribed to the slight edge length differences between the individual platelets. The authors also developed a vapor-phase halide exchange method in order to integrate on the same substrate microlasers operating at different colors, based on the protection of part of the platelets with scotch tape followed by exposure to HBr vapors. They interestingly observed that the uncovered platelets converted from deep blue CsPbCl\({}_{3}\) to green CsPbBr\({}_{3}\) while the platelet far from the tape edge remained unaltered. Partial composition conversion took instead place in the platelets close to the tape edge, thus allowing a color tuning including blue and cyan (see Figure 9e). A remarkable improvement of the lasing reproducibility was demonstrated by the same group that succeeded in the realization [60], with the same technique, of microrings of 2D Ruddlesden-Popper (BA)\({}_{2}\)(MA)\({}_{n-1}\)Pb\({}_{n}\)Br\({}_{3n-1}\) (see Figure 9f-h) showing very similar spectra between different lasers with maximum relative threshold variations of about 10%. Finally, almost aligned CsPbBr\({}_{3}\) microwire WGM lasers were recently realized by a PDMS template-confined antisolvent crystallization method, obtaining a good control of wire length, width and position [61]. Strain induced dynamic modulation of the lasing modes was also demonstrated and ascribed to piezoelectric polarization effects on the CsPbBr\({}_{3}\) gain region and refractive index. Another strategy to realize resonant cavity with controlled and reproducible size and shape, still preserving the easy realization, is given by the perovskite growth on dielectric resonators, like microspheres [62; 63] or microcapillaries [64]. The first results with this approach were obtained by [PERSON] et al. [62] that developed a growth process allowing the conformal coating of a SiO\({}_{2}\) microsphere, with a diameter of 52 \(\upmu\)m, with a MAPbI\({}_{3}\) film with excellent thickness uniformity (see Figure 10a). The growth was obtained starting with atomic layer deposition of PbS on the microsphere, followed by its conversion to PbI\({}_{2}\) by exposure to I\({}_{2}\) vapors and concluding with the conversion to MAPbI\({}_{3}\) through treatment with MAI. The samples showed WGM lasing under 2 ns pumping at 355 nm and at a temperature of 80 K with a threshold of about 75 \(\upmu\)Jcm\({}^{-2}\). Room temperature single mode lasing was more recently demonstrated in CsPbX\({}_{3}\) microspheres with a diameter of about 1.0 \(\upmu\)m realized by standard CVD (see Figure 10b). Figure 9: **(a)** Single mode lasing spectrum from a small MAPbI\({}_{3}\) microhexagon with edge length of 2 \(\upmu\)m, shown in **(b)** (optical microscope image, size bar 5 \(\upmu\)m and SEM image in the inset, size bar 1 \(\upmu\)m). **(c)** Single mode lasing spectrum from a large MAPbI\({}_{3}\) asymmetric microhexagon, shown in **(d)** (optical microscope image, size bar 5 \(\upmu\)m). **(a–d)** Adapted with permission from [58]. John Wiley and Sons, 2016. **(e)**: Optical microscope image of CsPbCl\({}_{3-x}\)Br\({}_{x}\) above the lasing threshold providing evidence of lasing color tuning by controlled halide exchange. Adapted with permission from [59]. John Wiley and Sons, 2017. **(f–g)** Low magnification microPL and SEM images of 2D perovskites microrings. **(h)** Lasing spectrum of 9 different microrings, providing evidence of the reproducibility. **(f–h)** Adapted with permission from [60].