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

Reviewers' comments: 
 
Reviewer #1 (Remarks to the Author): 
 
Review of “Giant optical nonlinearity in single silicon nanostructure: ultrasmall all-optical switch and super-
resolution” 
The authors describe in their paper the strong impact of refractive index change by absorption heating on 
the scattering intensity of nanoscale Mie-resonators. Depending on the size of the resonators a substantial 
positive or negative deviation from the usual linear behaviour is observed which was clearly observed in dark 
field microscopic measurements. Evaluating the local temperature of the Mie-resonators from Stokes-
Antistokes Raman spectra the authors could attribute this to the temperature dependent refractive index 
change of the Si. Taking also the temperature dependent absorption cross section of the Mie-resonators into 
account a simulation of the temperature increase in the Mie-resonators was carried out. The modelled 
scattering of the Mie-resonators based on this temperature affected refractive index then lead to a very good 
correspondence with the experimental results. Finally the fast switching of the nonlinearity could be 
demonstrated with time resolved pump probe measurements and a narrowing of the point spread function 
due to the linearity was also presented. 
The experimental results and theoretical simulations are convincing and present a coherent picture of the 
effect. The fast relaxation of the nonlinearity in the ns- range due to the small overall thermal capacity (fast 
cool down) is surprising and demonstrates impressively that thermal switching can be fast, when it is applied 
to nanoscale volumina. In addition the results show, that high Q-resonances (e.g. from photonic crystals 
defects) are not necessary to employ nonlinearities and that small simple Mie-resonators can already be of 
use. With the current interest in Mie-resonator based dielectric metamaterials the topic of the paper appears 
timely. The added information in the supplementary material supports the claims of the paper further and 
demonstrates that the authors investigated the presented effects thoroughly and extensively. 
Overall the paper reads well and should be accessible to a large community of readers. 
However there are still a few shortcomings which the authors must address: 
1) The critical point for the understanding that there is for some particles a negative nonlinear deviation of 
scattering intensity (100nm, 190nm) and for the 170nm a positive nonlinear deviation of scattering intensity 
is not given in the paper. In the moment it only appears in the supplementary material as caption on Fig E3: 
“Since Mie scattering spectrum red-shifts at elevated temperature, this explains…”. This crucial explanation 
has to be also well presented in the main paper! Only then the reader can understand why the nonlinearity 
acts sometimes as increasing and sometimes as decreasing. The spectral shape of the scattering spectra 
with the different resonances is crucial here. 
 
2) What is the influence of the free carriers on the refractive index. In the paper only the thermal impact on 
the refractive index is considered. However as the applied light wavelength (561nm) is well absorbed by the 
Si, a lot of free carriers are generated and should also have an impact on the refractive index or can this be 
the neglected? The authors should estimate the impact of the free carrier concentration on the refractive 
index and describe the outcome of it in the supplementary material. 
3) Fig 4c and explanation of resolution enhancement by SAX in the supplementary materials: 
From the the explanation in the supplementary materials it is not clear how the resolution enhancement is 
exactly measured and how the curve in Fig. 4c of the main paper is actually obtained. It is mentioned that 
the two beams from the AOMs are interfered and “generate a sinusoidally modulated illumination beam at 
10kHz frequency”. Does this mean that a spatially modulated light pattern is generated whose intensity is 
additionally oscillating with a frequency of 10kHz in the time domain? The authors should explain in more 
detail how the enhanced resolution (reduced PSF) is actually measured since from the existing explanations 
this appears to be a quite complex procedure. Maybe a schematic drawing of the setup, which was used for 
the SAX-measurements will help. 
 
If the authors can improve/clarify these mentioned points, I recommend the paper for publication. 
 
 
Reviewer #2 (Remarks to the Author): 
 
In the paper “Giant optical nonlinearity in single silicon nanostructure: ultrasmall all-optical switch and 
super-resolution imaging” authors show the all-optical tuning of the Si-blocks that that support the 
excitation of Mie resonances. The paper is well written and explains everything in a detailed manner. It 
shows comprehensive studies on the photothermal nonlinearity in silicon nanoblocks, that originate from the 
Mie resonance excitation. Authors provide a large amount of data including nonlinear measurements for 
different dimensions of silicon blocks, Raman measurement, ellipsometry measurements, and others. 
Authors utilize Mie resonances to enhance the absorption of Silicon and to achieve 5-orders of magnitude 
enhancement of nonlinearity. Paper looks interesting for the metamaterial community, however, I am 
concerned about novelty and if it is suitable for Nature communications, as well as believe this paper 
requires some revisions. 
 
1) As for the novelty please read recently published paper, 
Berzinš, Jonas, et al. "Laser-induced spatially-selective tailoring of high-index dielectric metasurfaces." 
Optics Express 28.2 (2020): 1539-1553. 
 
Apart from the spectral tuning of the Mie resonances due to the heating, it also provides additional data 
about the melting/reshaping and damage threshold of Silicon metasurface. Please, emphasize the 
differences in your results. 
 
2) Moreover, another paper has relevant studies that you do not cite. It provides the result of the interplay 
between free-carrier refractive index change, and thermal heating in Si-metasurface: Della Valle, Giuseppe, 
et al. "Nonlinear anisotropic dielectric metasurfaces for ultrafast nanophotonics." ACS Photonics 4.9 (2017): 
2129-2136. 
3) And another paper that can be cited: Bosch, M., et al. "Polarization states synthesizer based on a thermo-
optic dielectric metasurface." Journal of Applied Physics 126.7 (2019): 073102. 
 
Other questions: 
 
1) The authors provide the main experimental results in the numbers of NDR – nonlinear deviation. I can 
see how it can have a mathematical meaning, but from the experimental point of view, this result can lead 
readers to confusion. For example, line 98, authors claim that you have “400% enhancement or 70% 
reduction of scattering, i.e. …..”. Yes, it is defined that " i.e. deviation", but, it’s not a 400%/70% change of 
scattering. I can accept NDR after I read what does it mean; however, if I think carefully, you calculate 
deviation from some number that cannot be achieved in the experiment. Moreover, the most important 
result is shown in Fig.3 – change of scattering efficiency of the nanoblock under different excitation power. 
Here we can clearly see that schattering changes and NRD has nothing in common, – the main changes of 
scattering occur at 190 nm width of the block. But NDR fro 190 nm block has the smallest value. Therefore, 
please provide some other papers that use NDR value, since I admit, that I might not be familiar enough 
with that area, or please, consider emphasizing other experimental values. 
 
2) Moreover, when you provide a comparison with other works, you use NDR. None of these papers in refs 
[19,21] use this value to describe the modulation of the signal. There are two values that majority of 
community uses: it is the absolute value modulation of optical signal (∆I), or the relative modulation of the 
signal, ∆I/I0, where ∆I = Ipump – I0, I0 is the unperturbed reflectance/transmittance and Ipump is the 
reflectance/transmittance when high-intensity laser is used – mostly for better visualization of the changes 
(Please see papers from references [19,21], or another paper that also shows I-scan, and authors use 
absolute values: Zubyuk, Varvara V., et al. "Low-power absorption saturation in semiconductor 
metasurfaces." ACS Photonics 6.11 (2019): 2797-2806.) Therefore, please correct the comparison of the 
results with the previous works. 
 
3) Since you use CW laser, and other papers use pulsed laser, please, provide how you recalculated their 
excitation intensity, for example, in supplementary materials. 
4) I believe that the non-gaussian form of your results can come from the spectral tuning of the resonance. 
If you change the refractive index, the spectral position of the Mie resonances will tune, as I mentioned in 
the paper Berzinš, Jonas, et al. "Laser-induced spatially-selective tailoring of high-index dielectric 
metasurfaces." Optics Express28.2 (2020): 1539-1553, or any other refractive index change papers. It will 
be relevant to show numerical calculation of spectrum at least for one sample, to show how it is spectrum is 
going to be modified when you change the refractive index of silicon due to the photothermal effect. 
Therefore, it will give a better explanation of the achieved results. 
 
5) Please, provide more details regarding the nanoblock sample used: 
a) How far are the nanoblocks from each other? 
b) Is there any coupling between them that can modify the spectra of Mie resonances? 
 
6) Other small corrections: 
When you provide the number of nonlinearities, for example on line 64/66, please provide references. 
 
I believe lines 88-90 are in general incorrect. In this review you separate metasurfaces from 
nanoresonators, but if I am not mistaken, in all this works authors use metasurfaces as arrays of 
nanoresonators. 
 
Also, I recommend considering changing the title of the paper, since there is no “photothermal” anywhere. 
 
Altogether, the work is of interest to the metamaterial/silicon community and can be published after 
revision. 
 
 

Reviewer #3 (Remarks to the Author): 
 
In their manuscript, Duh et al. report on the photothermal response of Mie-resonant silicon nanoparticles. I 
think this is a technically sound study revealing an interesting property of a well-studied system: light can 
efficiently 'tune' itself in an ultra-small particle by heating and exploiting the thermo-optic response of 
silicon. The authors proceed to demonstrate several intriguing applications of this effect for switching the 
scattering efficiency and reducing the spot size of a focused laser beam down to 132 nm at a wavelength of 
592 nm. I think that the presentation quality, the rigor of the research effort, as well as the timeliness of the 
topic can secure publication in Nature Communications after the following issues are addressed: 
1. The title of the work needs to be more specific about the nature of the nonlinearity. I suggest using the 
following wording: “Photothermal nonlinearity.” Also, there is no need to emphasize the fact that it is an 
ultrasmall all-optical switch: the fact that a small silicon nanoparticle may show all-optical switching was 
discussed at length in Refs [19, 21, 26]. Finally, I am not convinced that the paper shows any explicit 
‘super-resolution imaging;’ a mere tight focal spot demonstration in Fig. 4 is not enough to claim super-
resolution imaging, especially in the title. Therefore, I recommend changing the title of the paper to “Giant 
photothermal nonlinearity in a single silicon nanoparticle.” This way is it significantly more concise, and the 
main novelty is properly emphasized. 
2. The way the silicon films were obtained is unclear. The authors used “a 150-nm-thick monocrystalline Si 
layer on the quartz substrate.” Having a monocrystalline Si film on a quartz substrate is unusual. I could not 
locate this product on the website of the supplier the authors acknowledged. Therefore, I suggest adding to 
the Methods section of the paper the procedure of how these silicon films were deposited, for the sake of 
study reproducibility. 
3. Please use caption to annotate the variables in Fig.1, such as CscaF and Cabs. 
4. Several stylistic suggestions would include getting rid of informal expressions, such as ‘aka’ (line 38), ‘etc’ 
(line 59), as well as unnecessary neologisms such as ‘meta-silicon-material’ (line 81), ‘nano-silicon 
applications’ (line 103) and others. In general, I find that the English of the paper is rough, and I strongly 
recommend invoking an English-editing service.