## 3 Optically Pumped Lasers
### Vertical Cavity Surface Emitting Lasers (VCSELs)  
VCSELs are basically made by an active film cladded between two parallel highly reflecting mirrors. This kind of cavity allows to obtain laser beam with lower divergence, lower threshold and better quality with respect to DFBs and, for thin enough active layers, single mode emission. However as the lasing wavelength is dependent on the active layer thickness and the lasing feedback is given by multiple reflection in direction perpendicular to the mirrors, a good control of the active layer morphology and optical quality is fundamental to realize VCSELs. Moreover, the propagation losses in the cavity strongly depend on the mirrors reflectivity and absorption, thus requiring the use of two highly reflecting dielectric mirrors (Distributed Bragg Reflectors, DBR) for the losses minimization. The first perovskite VCSEL was demonstrated in 2014 by realizing a hybrid dielectric-metallic cavity made by a bottom DBR, a 200 nm-thick spin coated MAPbI3\({}_{-x}\)Cl\({}_{x}\) active layer, a 1 \(\upmu\)m-PMMA layer acting as spacer and a thermally evaporated 200 nm-thick top gold layer, acting as second mirror [6]. The microcavity emission spectra showed three modes at low excitation density and an almost single mode emission at high excitation densities due to the amplification of the mode at 775 nm (1.60 eV) (see Figure 12a). A lasing threshold of 0.2 \(\upmu\)J per pulse was reported (the absence of indications on the pump spot size prevents to determine the corresponding energy density), under 400 ps pumping at 532 nm. Figure 12: (**a**) Emission spectra of the MAPbI3\({}_{-x}\)Cl\({}_{x}\) and of the VCSEL below and above the lasing threshold. Inset: scheme of the cavity structure. Reproduced with permission from [6]. American Chemical Society, 2014. (**b**) Pictorial view of the CsPbBr3 VCSEL realization by thermal imprinting and of the VCSEL pumping and lasing. (**c**) Excitation density dependence of the emission intensity of the same sample, providing evidence of the lasing threshold. (**b**) and (**c**) adapted with permission from [24]. John Wiley and Sons, 2019. This first result was followed by further VCSELs demonstrators only after three years, with the first report of perovskite VCSELs exploiting two DBR mirrors and MAPbI\({}_{3}\) as active material [78]. The active layer was deposited by spin coating of a precursor solution on the bottom DBR covered by a PPMA layer, followed by toluene dripping. The careful optimization of the precursor solvent selection, the molar ratio of precursors, the toluene dripping timing protocol, the thermal annealing temperature and duration, and the treatment of substrate surface to make it hydrophilic, showed the extreme importance of good optical quality of the VCSELs active layer. The full cavity was mechanically assembled by bonding the top DBR above the perovskite film with wafer edges sealed with optical epoxy, also depositing a 10 nm-thin PMMA layer on the perovskite film before applying the top DBR bonding process. The laser showed single mode emission at 778.4 nm, with a threshold of 7.6 \(\upmu\)cm\({}^{-2}\), with TEM\({}_{00}\) lasing and low divergence (<3\({}^{\circ}\)) under 340 ps pulses pumping at 532 nm, and with a threshold of about 114 \(\upmu\)cm\({}^{-2}\) under 5 ns pulses pumping at the same pumping wavelength. Fully dielectric cavities assembled by gluing the top mirror on the active layer were also reported in devices exploiting as active layer a CsPbBr\({}_{3}\) nanocrystals thin film [79], showing single mode lasing with a threshold as low as 0.39 \(\upmu\)cm\({}^{-2}\) under 50 fs pulses pumping at 400 nm and of 98 \(\upmu\)cm\({}^{-2}\) under 5 ns pulses pumping at 355 nm. The importance of the quality of the mirrors in determining the VCSEL lasing properties is clearly demonstrated by the results obtained in DBR-metal cavities with a MAPbI\({}_{3}\) active layer [80]. Even if the active layer is expected to be nominally comparable with the one in Ref.