## 3 Optically Pumped Lasers
### Distributed Feedback (DFB) Lasers
The combination of NIL and ion beam milling (IBM), together with a proper choice of the imprint resist in order to allow its deposition on top of the perovskite layers, was used to realize an array  
Figure 11: **(a)** SEM images of the DFB grating imprinted into a MAPbBr3 layer with a periodicity of 300 nm (inset: high resolution cross-section, scale bar is 100 nm). Adapted with permission from [66]. John Wiley and Sons, 2018. **(b)** PL, ASE, and DFB lasing spectra of MAPbI3. **(c)** Emission intensities at the peak wavelengths as a function of the pump pulse intensity of the same MAPbI3 sample in a position without and with underlying grating, leading to ASE (blue) and lasing (red) at thresholds of 190 \(\upmu\)Jcm\({}^{-2}\) (\(\pm\)38 \(\upmu\)Jcm\({}^{-2}\)) and 120 \(\upmu\)Jcm\({}^{-2}\) (\(\pm\)24 \(\upmu\)Jcm\({}^{-2}\)), respectively. **(b,c)** adapted with permission from [67]. AIP publishing, 2016. **(d,e)** Photograph of the output beam of a MAPbBr3 DFB laser projected on a screen, **(d)** shows the TM mode from a 300 nm grating, while **(e)** shows the TE mode from a 290 nm grating. **(d,e)** adapted with permission from [18]. Springer Nature Publishing, 2017. **(f)** SEM image at a 52\({}^{\circ}\) inclination with respect to the normal of a MAPbI3 film on top of a 2D grating with a portion of the perovskite removed by focused ion beam to evidence the underlying pattern. Adapted with permission from [68]. OSA Publishing, 2016. **(g)** Cross-sectional SEM of a MAPbBr3 inverse opal (scale bar 2 \(\upmu\)m). Adapted with permission from [69]. American Chemical Society, 2017. **(h)** Top-view SEM image of a MAPbBr3 microwire on silicon grating with a periodicity of 5 \(\upmu\)m (scale bar 10 \(\upmu\)m). Adapted with permission from [70]. American Chemical Society, 2016.  
of micro-DFB lasers, exploiting MAPbI\({}_{3}\) as active material [72] with a lasing threshold of about 225 \(\upmu\)cm\({}^{-2}\) under 5 ns pumping at 532 nm. Similar threshold, but under 300 ps pumping at 532 nm, were recently obtained in MAPbI\({}_{3}\) DFB lasers realized by solution deposition on imprinted plastic substrates, by using as precursor lead acetate (Pb(CH\({}_{3}\)COO)\({}_{2}\)), instead that PbI\({}_{2}\)[73]. DFB lasing in the green spectral region with a 1D pattern was reported for the first time by [PERSON] et al. [66] with a MAPbBr\({}_{3}\) active film, directly imprinted by thermal nanoimprinting (see Figure 11a). The authors demonstrated an evident improvement of the film cristallinity and morphology due to the application of high pressure (100 bar) and temperature (100 \({}^{\circ}\)C), even when flat stamps were used, that allowed to obtain single mode lasing at about 555 nm with a remarkably low threshold of 3.4 \(\upmu\)cm\({}^{-2}\), and linewidth of 0.14 nm, under 300 ps pumping at 532 nm. The search of high operational stability of the lasers also stimulated the development of fully inorganic perovskites with optical gain. Single mode emission at 654 nm was reported in CsPbBrI\({}_{2}\) films [74], spin coated on nanoimprinted polymer substrates, but no lasing was claimed by the authors, due to a particularly high linewidth of about 5 nm. Clear single mode lasing in the green was instead very recently demonstrated from CsPbBr\({}_{3}\) thin films by [PERSON] et al. [24], who optimized the film morphology, crystallinity and photoluminescence quantum yield (PLQY) by thermal imprinting. The DFB lasing properties were investigated in samples realized both by direct imprinting of the active layer and by deposition on an imprinted substrate, achieving in both cases a low lasing threshold of 10.0 \(\upmu\)Jcm\({}^{-2}\) and 7.2 \(\upmu\)Jcm\({}^{-2}\), respectively, under 300 ps pumping at 355 nm. A further degree of freedom for the optimization of the DFB lasing properties is the dimensionality of the nanopatterning used to introduce the interference necessary for the lasing feedback.