Nonlinear Absorption and Ultrafast Dynamics of Ag Nanoparticle
Round 1
Reviewer 1 Report
In the paper titled “Nonlinear absorption and ultrafast dynamics of Ag nanoparticle”, authors Jijuan Jiang, Jun Wang, Tong Wu and Yachen Gao, have studied the resonant nonlinear absorption of silver nanoparticles.
In this study, the nonlinear absorption spectra of Silver nanoparticles are studied using open aperture Z-scan techniques, which seems to be a well-established technique for this type of studies and relies on the lensing effect due to power dependence of refractive index of the material. In this regard, I have several questions, that I think need to be addressed before the paper is accepted for the publications.
- First of all, the Z-scan technique appears to me as an effective method only for homogeneous medium which posses very low scattering and the transmission enhancement through the aperture can be only assign to nonlinear phenomena like discussed above. Thereby in the case of nanoparticle composites a natural question to ask how valid is the applied method for this particular case. In particular, from SEM images in Fig1. it is difficult to tell how is the spatial dispersion of the silver particles in the composite. Are they clustered, or they are monodispersed in the matrix the authors used in the experiments? Can authors comment on this to give more details about the sample characteristics?
- Near plasma resonance, the nanoparticle clusters will scatter the light strongly so one can observe enhance transmission also due to this effect. How the authors can differentiate this linear enhancement from the nonlinear saturable absorption effect? I understand that the SA and RSA character switch is a clear behavioural transition, that is nicely captured by this measurement, however, it will be hard to quantify nonlinear absorption coefficient accurately unless the linear enhancement effects are included in the calculations.
- In general, I couldn’t read the clear message of what is the novelty of the paper. As authors mentioned themselves the subject is heavily studied with Ag nanoparticles, and there are even papers that some of the current authors have contributed earlier.
The main points highlighted by the authors are the resonant excitation and the time resolved measurements. For the resonant excitation, I agree, it is much better to excite the particles with light wavelength closed to plasmon resonance, so that the effect can benefit from stronger enhancement. It is true that many of the measurements are done far from plasma resonance, but this can be for example beneficial to discard the effect of linear enhancement as I mentioned in my comment number 2).
Also, there are other studies done for 532 nm, but the particle sizes in those experiments are larger, so the plasmon resonance is very close to excitation wavelength (see for example: Journal of Applied Physics 113, 053107 (2013); https://doi.org/10.1063/1.4790798)
Finally, in the time resolved measurements, two relaxation time scales were observed with relaxation times estimated to be: tau1 = 713 fs and tau2 = 25.2ps from double exponential fitting. The authors assigned the initial fast decay process to electron-phonon relaxation process as it matches to the estimate obtained earlier for 3nm Ag particles. However, the particle sizes in this study are an order of magnitude larger than that of the cited reference study. And it is well known that for very small particles surface scattering effects for electron relaxation processes become essential and this needs to be considered, which will be less relevant for the case of the larger particles in used in this study.
Second relaxation process is assigned to phonon-phonon scattering processes with no further clarification. I think, the relaxation processes need to be examined more carefully and in depth before assigning to certain processes.
Considering all the comments above, I would suggest the paper for publication only if the authors will make clear the novelty of their study and address the issues I have raised in my comments.
Author Response
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Author Response.pdf
Reviewer 2 Report
I have several questions to your manuscript that must be adressed before i can make a final conclusion. Comments are related to the line numbers of your manuscript:
Line 70: There is a missing information on the environment of the nanoparticles for the measurements. Were they solved in eg water for the study? Or did you use as-ablated material to fill up the cuvette? As the procudure is referenced to Ref [16] in the paper, i assume in my following report the presence of a solvent.
Line 76: Please add the Rayleigh length for the focussing parameters of your setup and set it in relation to the cuvette thickness along z-direction.
Line 77: The measurements were performed in a quartz cuvette that itself gives rise to nonlinear absorption. Please add how the impact of the cuvette (and of the solvent) on the signal was controlled.
Line 78: The authors state that different energies were used to investigate the irradiance on the nonlinear absorption. Please add how the energies were varied, i.e. was a variable filter used or was the power of the pump laser in the RGA reduced? Is it the same as written in line 84, i.e. using a polariser-halfplate combination?
Line 87: The delay stage was build using a prism. How did the authors secure to keep the pulse duration in this case?
Line 97: Different pulse energies were used for this study. The pulse energy may have an impact on the pulse duration. Did the authors measure the pulse duration (at the position of the focal point) for all individual measurements?
Line 106 and Figure 3: The authors observe a break-down of the signal at z=0 for elevated pump energies. They conclude that this effect is due to the switch from saturable absorption to reverse saturable absorption. Though i dont doubt in principle that this process may be at the origin of the observation, there is no experimental evidence given by the authors that this is the one and only explanation. Let us assume that the increase of the pulse energy may result in a local thermooptical heating of the solvant, such that bubbles appear. In this case, also a significant change of the transmission may result. Another possibility to explain the results are nonlinear effects in quartz. It is thus necessary to give experimental evidence for SRA or to state clearly, that this is an assumption, only.
Figure 4 does not reveal the decrease of the transmission at largest energy of 100 nJ. Is this value at the limit prior to the appearance of RSA?
Line 164: Please add errors to the determined time constants. I assume they will be large for the fast relaxation process.
Overall, the authors shall discuss their results more clearly in the framework of the state of the art knowledge. A discussion on the impact of particle size is missing, as well as a discussion on the impact of the pulse duration.
How does the temporal relaxation influence the z-scan measurements? I have seen in literature several articles that deal with the interplay of transient nonlinear absorption and nonlinear absorption; the result is a considerably impact on the transmission features as formulated by Sheik-Bahae
Author Response
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Reviewer 3 Report
The Photonics-1237363 manuscript deals with the non-linear properties of silver nanoparticles. The Ag nanoparticles were prepared by laser ablation and characterized by Z-scan experiments; their resonant dynamics processes were studied.
In this study, some details seem to be missing. Hence, that is for this reason that the reviewer recommends a major revision of this manuscript.
The reviewer hopes that his main comments listed below will help the authors to improve their manuscript:
1- The authors must give more details about silver nanoparticles preparation as the cited reference is not from the authors.
2- The manuscript deals with the characterization of the optical properties of silver nanoparticles. The nanoparticle sample must be fully characterized.
3- How many samples have the authors prepared and characterized? (i.e., how many replicates)
4- SEM image quality must be improved. Based on the provided image, the nanoparticle size seems to be around more 50 nm than 25 nm. (TEM? Size histogram?)
5- some references are missing within the main part of the text (e.g., l108-116).
6- An in-depth discussion and a comparison with the corresponding literature, as well as experimental details, are expected by the reader. The authors need to improve these points.
7- the abbreviations have to be defined when first met.
8- Authors should also check their manuscript for formatting and typing errors (e.g., missing space, missing special symbols, etc.).
9- In the conclusion, “in ethanol solution” is mentioned, however, this specification could not be found within the experimental part nor in the results and discussion one.
10- the references have to be checked (i.e., some elements are missing and they are not homogeneous).
Author Response
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Reviewer 4 Report
The paper by Jiang et. al. investigates resonant non-linear absorption and ultrafast relaxation dynamics of silver nanoparticles irradiated by ultrashort laser pulses. The saturable optical intensity and two-photon absorption coefficient were determined experimentally by measuring the transmission and fitting the parameters to the experimental data. The electron-phonon and phonon-phonon relaxation times have been determined using the transient differential transmission. The paper offers new insight into the physics of laser interaction with nanoparticles, the measured data are useful for the community working in this field of research. The article is solid in terms of physics, well organized and written, and I recommend it for publication with few minor suggestions and recommendations. There are numerous minor fixes to be made listed below.
The authors may address the following questions.
- page 2, line 93: the statement: “Besides, the results showed that, in short wavelength region, there is inter-band absorption.”. This is not obvious, particularly because according to line 33, “But for Ag, the wavelength of interband absorption is about 320 nm and the SPR wavelength is about 400nm”. Are you referring to the increase of absorbance between 300 and 320 nm ? Also, add “the” right before the word short.
- Figure 3: there are 2 experimental data sets, but only one fitting curve. Why ?
- The experimental data refer to electron-phonon and phonon-phonon relaxation time in clusters. What is the difference compared to bulk ? The reader will benefit by a short paragraph comparing the rates for clusters and bulk as well as explaining the dfference.
Typos and corrections:
- page 2, line 82: “By doubling 800nm…” -> “By frequency doubling 800nm…”
- page 2, line 82: “um” -> “mm” (it is quite easy: just type “m”, go to fonts and select “Symbol”). It will convert m to m.
- Figure 3 caption: ”… high energies (110nJ and 150nJ…” -> ”… high laser energies (110nJ and 150nJ…”
page 4, ine 108: “and wavelength of…” -> “and the wavelength of…”
- page 4, line 128: “Then the coefficient of positive non-linear absorption” ->“The coefficient of positive non-linear absorption”
- page 4, line 130: “The theoretical curve was presented…” -> “The theoretical curve is presented…”
Author Response
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Author Response File: Author Response.pdf
Round 2
Reviewer 1 Report
I would like to thank the authors for considering the comments. Although they have addressed some of the concerns regarding the Z-scan method and sample properties and provided more details for this and provided additional references for the relaxation processes, I still find it not fully satisfactory for the questions I have raised in my first report about the relaxation processes.
First of all, in the revised version the authors say “Maurya et al. Studied the size dependence of the ultrafast kinetics of silver nanoparticles, and found that the ultrafast kinetics of silver nanoparticles is almost unrelated to the size of the nanoparticles. [30] Although 713fs, 700fs and 530fs are not exactly equal, we consider that this is not caused by size effect, and it may be due to errors and other reasons”. However, Mauraya et. al clearly state in their conclusions that the electron-phonon relaxation time they have observed ranges in the limit 1.5-2.3 ps, which means there is almost factor of two deference in relaxation time for nanoparticles whose sizes range from 25 nm to 37 nm. In fact, the sample S3 and S7 in the study by Mauraya et. al, which are the closest in size to nanoparticles studied by current authors, show relaxation times 1.8 ± 02 ps. This is more than two times longer than the one report by current authors. Therefore, I do not see it clear to conclude about the effect of the sizes. Moreover, in my first report, I had bigger concern about the comparison authors did for the relaxation time obtained for 3nm nanoparticles which shows similar timescales what they obtain from experiments for 25 nm particles. I believe there should be significant deference in the relaxation process for 3nm particle compared to 25 nm due to finite size effects and electron wavefunction confinement effects.
Considering this, I would suggest the authors to address this question before I can make my final decision.
Author Response
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Reviewer 2 Report
The authors have carefully revised their manuscript along my considerations. I agree with all changes made and am happy to recommend this manuscript for publication.
Author Response
Thank you for your comments and suggestions。
Reviewer 3 Report
Line 104: "radius" should be used instead of "size"
Author Response
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Author Response File: Author Response.pdf
Round 3
Reviewer 1 Report
I would like to thank the authors for reconsidering my comments.
Considering the extensive changes and corrections applied compared to first version of the manuscript, I think the improved version can be considered for the publication in this journal.
