## 4 Applicative Perspectives  
### Pumping Regime  
A first important aspect to consider in order to evaluate the possibilities to build a real laser from a given active material is the pumping temporal regime necessary to have optical gain and lasing. By looking at the 73 results reported in Tables 1 and 2 relative to room temperature data and with specified pump pulse time length (among the 80 in total) it can be observed than only 20 (about 27%) were obtained by using pump pulses not shorter than 1 nanosecond, while about 50% of the experiments were performed with ultrafast pumping (between 50 fs and 300 fs).  
To quantitatively investigate the role of the pump pulse temporal length on the ASE and lasing properties we can compare the experimental results obtained in the literature in the actually few experiments in which the same active material is investigated by using both ultrafast (fs) and quasi-CW (ns) excitation, thus allowing a direct threshold comparison between the two regimes (see Table 1).  
Concerning ASE experiments [PERSON] et al. [15] demonstrated a threshold increase of about 1.7 times and 2 times when passing from 120 fs- to 7 ns-pumping in MAPbI\({}_{3}\) and in MAPbI\({}_{3}\)-PMMA films, respectively. Larger differences were instead observed in FAPbI\({}_{3}\) thin films [21], showing an ASE increase of 6.5 times under nanosecond pumping, and in state of the art CsPbBr\({}_{3}\) thin films deposited by one step coevaporation [23]) providing evidence of an ASE threshold under nanosecond pumping about 20 times higher than under femtosecond pumping. Finally, [PERSON] et al. [5] observed a threshold of 450 \(\upmu\)cm\({}^{-2}\) in CsPbBr\({}_{3}\) NCs films pumped by 10 ns pulses, thus 85 times higher than the one under 100 fs pumping (5.3 \(\upmu\)cm\({}^{-2}\)).  
Concerning lasers to date only three examples of pumping in both regimes are present, providing evidence of qualitatively similar higher thresholds under nanosecond pumping than under femtosecond pumping, but with higher quantitative variation with respect to the results on ASE. In particular, a threshold increase of 15 times was demonstrated in MAPbI\({}_{3}\) VCSEL [78] passing from 340 ps to 5 ns pump pulses, while a higher variation (27.5 times) was obtained in MAPbI\({}_{3}\) 2-D DFB VCSEL [68] passing from 200 fs to 910 ps. Much higher differences were observed in CsPbBr\({}_{3}\) NCs films used in a VCSEL laser, with a threshold increase of 250 times between excitation with 50 fs- and 5 ns-lasers [79].  
By looking at these data and considering that femtosecond pumping typically also results in higher damage threshold and higher photostability, it could be ingenuously concluded that femtosecond pumping is better than nanosecond pumping. Anyway on an applicative perspective this conclusion is wrong as the generation of ultrafast and highly energetic pulses requires big and expensive amplified pump lasers, not suitable for the excitation of any realistic compact and low cost laser device. On the contrary nanosecond pulses can be obtained by compact and low cost laser diodes and, eventually, also by inorganic LEDs that could be exploited for the realization of compact optically pumped perovskite lasers, as already demonstrated for optically pumped organic lasers [84].  
Thus the investigation of the ASE and lasing threshold and of their intensity stability under nanosecond optical excitation is more suitable to reproduce the possible working conditions of optically pumped perovskite lasers exploiting cheap and compact pumping sources.