Peer review of: Giant photothermal nonlinearity in a single silicon nanostructure (2020)

 
RR2
REVIEWER COMMENTS
Reviewer #1 (Remarks to the Author):
Reviewer #2 (Remarks to the Author): Dear authors,
  The authors considered the remarks in the reviews and improved th manuscript accordingly. I therefore
 recommend to publish the paper now in its new form.
   Thank you for your response. Even though the paper has a lot of exciting results, I believe they should be
 modified to be understood by the audience. Unfortunately, there are quite a lot of questions that arise when
 I read the paper. I will focus on two significant issues.
 1) One of the main problems is NDR. As you mentioned, it is something that no one uses. “We agree that no
 other literature use NDR, but we have a good reason for it.” And that fact creates large misunderstanding of
 the whole results of the paper and I believe that it isn’t something that can be stated as the main result of
 the paper in the abstract and conclusions.
 The reason for that is that apples should be compared to apples. And the use of NDR confused not only
 readers but authors too.
 Despite all that is said, NDR is definitely not something that you can compare to pump/probe experiments.
 For example, paper “Shcherbakov, Maxim R., et al. "Ultrafast all-optical switching with magnetic resonances
 in nonlinear dielectric nanostructures." Nano letters 15.10 (2015): 6985-6990.” provide not only pump-
 probe measurements with 1% change but also I-scan measurements for silicon metasurface. Here authors
 also mention thermal effects, similar to what you show in Fig. 2, I include the figure from the paper below,
 please see additional file.
 Therefore, for that figure, we can also estimate NRD, and it will be at least 10% for top and bottom cases.
 Please note that the sign here is opposite because they measured here transmission. Therefore, the fact that
 NDR is something that no-one uses confused not only readers but also authors. Thus, the statement from
 the response letter “NDR is, in fact, the same as ∆S/S, by defining S as linearly extrapolated value, and ∆S
 is a deviation from S. To further emphasize this similarity” is incorrect, same as a claim in the paper “ 74
 However, the required intensity is ~104 mW/μm2, and the corresponding nonlinear deviation is only less
 than 1%”. In my opinion, all shown results should be compared not to the pump-probe measurements but
 only to I-scan measurements.
 2) The following question is related the comparison of the shown results with the previous works and power
 used.
 As I mentioned before, for me, it’s questionable If static results can be compared to pump-probe results.
 They can be if we compare values of the absolute change of reflectance/transmittance as it is done in all
 pump/probe works. No one compares Delta I/I between papers because it can be enormous when you divide
 by small value (dip in scattering, exactly what you have for some cases), and it has no meaning if we want
 to think about applications. Delta I/I is mostly used to visualize the changes more clearly. I strongly
 recommend to the authors to get rid of the NDR value, your results are excellent without it.
 Also, I agree that for nonlinear optic processes for pulsed lasers, we need to compare peak power with
 average power for the CW case. However, for thermal processes that have nanosecond-microsecond lifetime
 time to achieve the same amount of heating, we need to have the same average power. Please correct me if
 I am wrong.
 3) Same as another reviewer, I am completely confused about the results from pump-probe measurements

 in Fig.4. There are absolutely no details in supplementary, and actually, the reader should be referred to the
 Methods section. To claim that there is a heating process with ns switching time, all equations that you used
 for the model need to be shown, parameters of the simulations, etc. Or you can refer the reader to another
 paper, but at least you should write how theoretical data was achieved. Otherwise, none of these results can
 be reproduced. The main question is, how can you separate heating from carrier relaxation processes that
 also can be quite long (Augier recombination, for example). You mention that it is possible to distinguish the
 FC effect from thermal, but for the reader, it is impossible.
 Moreover, the statement that claims that you are first to show photothermal relaxation with ns switching
 times is wrong.
 320 “Here, we show for the first time, theoretically and experimentally, that photothermal relaxation time of
 an isolated silicon nanostructure reaches nanoseconds, i.e. GHz operation potential, with very large
 modulation depth, which has not been previously reported. ”
 Please see the paper “Della Valle G, Hopkins B, Ganzer L, Stoll T, Rahmani M, Longhi S, Kivshar YS, De
 Angelis C, Neshev DN, Cerullo G. Nonlinear anisotropic dielectric metasurfaces for ultrafast nanophotonics.
 ACS Photonics. 2017 Sep 20;4(9):2129-36.”
 In the abstract, authors mention that they have nanosecond lattice dynamics: “We reveal a set of operating
 windows where ultrafast all-optical modulation of transmission
 is achieved with full return to zero in 20 ps. This is made
 possible because of the distinct dispersive features exhibited by
 the competing nonlinear processes in transmission and despite the slow (nanosecond) internal lattice
 dynamics. ”
 4) Small details:
 In Methods: thank you for providing a lot of details; however, section “Temperature estimation during laser
 irradiation” is about temperature, used rate equations that need to be written; otherwise it’s all just
 describing.
 Reviewer #3 (Remarks to the Author):
 I have carefully studued the responses of the authors and re-read their revised manuscript. I think that the
 authors did a great job addressing all the issues raised by the reviewers, and the manuscript can be
 accepted for publication in its present form.