Peer review of: Ultrashort laser pulse doubling by metal-halide perovskite multiple quantum wells (2020)

Reviewers' comments: 
Reviewer #1 (Remarks to the Author): 

In the manuscript of "ultrafast coherent light pulse doubling by metal-halide perovskite multiple 
quantum wells", Guo et al have studied the double optical pulse in perovskite multiple quantum wells 
(MQW). With the excitation at 400 nm, the authors have observed two pulse with 40 ps pulse duration 
and 90 ps interval. The authors have also carefully discussed the underlying mechanism for the 
formation of double pulses. A nearly Auger free process has also been revealed. These results are 
supposed to be very important for the low threshold perovskite microlasers. Overall, this research is 
very interesting and can be considered for publish after addressing the foloowing comments. 
1. In case of conventional multiple pulse lasers, the pulse interval can be precisely controlled. How the 
authors control their pulse interval? This should at least be discussed. 
2. The authors have discussed the mechanism for two kinds of two-dimensional perovskites. One is n 
< 5 and the other has n >5. This is relatively rough for a model since quantum wells with n = 1 should 
definitely be different from n = 4. The emission with n = 5 is also different from 3D perovksites. The 
authors should stated clearly in the main text that the dominant layer number. This will help 
understanding the physics. 
3. In page-7, the authors have compared the pumping with 650 nm laser. For this experiment, the 
threhsold information should also be added in the main text. This can be an additional information ofr 
the energy transfer. 
4. Figure 2b and c are too crowd. The pumping density is missing. 
5. The same problem happens in Figure 4 a and c. Pumping density should be added. For this figure, 
the authors should consider a different plot since it is almost impossible to distinguish the data. 
6. The reference number is too few. More references should be provided. 
7. Please plot the data in Figure 1d in logscale to identify the gain of StEs. 



Reviewer #2 (Remarks to the Author): 

In this manuscript, the authors reported the observation of clearly resolved dual pulsed stimulated 
emission in perovskite multiple quantum wells. The mechanism of double-pulsed stimulated emission 
has been investigated in detail by using transient spectroscopy measurements and theoretical 
modelling. Furthermore, the Auger recombination in the perovskite multiple quantum wells in this 
work behaves obviously different from the normal gain medium. These observations of the ultrashort 
laser pulse doubling by using optical gain medium are novel and very interesting. The conclusions are 
convincing, and the manuscript is well-written. I believe this paper will attract extensive attentions in 
the field of metal-halide perovskites and ultrafast photonics. Therefore, I recommend the publication 
of this manuscript in Nature Communications. 
I have few minor suggestions listed below and hope the authors would consider before publication: 
1. There is very little discussion about previous works of the light-induced lattice dynamics on the 
stimulated emission in perovskites. The coherent acoustic phonon was observed in 2D perovskites 
[Nat. Commun. 9, 2019 (2018)]. How do the authors reconcile their observations with this previous 
report? 
2. Can the authors rule out other possible reasons for the double pulsed stimulated emission? For 
example, the multiple reflection of the stimulated emission pulse within the film. 
3. Do the authors observe any sample degradation under these conditions, and could this influence 
the overall carrier dynamics? 
4. Can the authors provide more explanation for the shoulder on the right of the second pulse in Fig. 
2b at~ 170ps at high pump fluence. Is it from the third stimulated emission pulse, why it is much 
weaker than second pulse? 


Reviewer #3 (Remarks to the Author): 

The authors of this manuscript present a study on the amplified spontaneous emission (ASE) in 2D 
perovskites. The particular structure of the material allows for a charge carrier reservoir, such that, 
after an initial ASE peak, a second peak is observed at longer time delay, due to funneling of carriers 
from the reservoir into the emitting state on a picosecond time scale. 

Notwithstanding the technical merits of the manuscript, I do have the following two major concerns: 

1. The authors are overselling their study. 
-They study concerns the observation of ASE with two time constants. This is not equal to coherent 
light pulse doubling, at no point the coherence of the light pulses is demonstrated. 
-They also claim to produce ultrashort pulses, yet the pulsed produced are more than two orders of 
magnitude longer than the 150 fs excitation pulses. 

2. They also state that their results now enable a miniaturization of devices capable of light pulse 
doubling. However, on-chip manipulation of light pulses has already been demonstrated in silicon 
integrated photonics. Typically passive control of the light pulses is achieved, for instance, by 
microring resonators or photonic crystals acting as an optical delay line (e.g. doi: 
10.1109/JPHOT.2010.2044989). Combined with waveguide splitters, one could be able to achieve a 
pulse train with controllable delay between pulses, in essence what the researchers aim to show here 
but with a higher degree of control. 

In conclusion, considering the elements above, I find the manuscript more suitable for a more 
specialized journal, as it doesn’t meet the stringent criteria of novelty and potential impact that merits 
publication in Nature Communications.

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