Spatially Resolved Spectral Imaging by A THz-FEL
, Masaki Fujimoto 1,†, Keigo Kawase 1,‡, Ryukou Kato 1,§, Hidenori Fujiwara 2, Atsushi Higashiya 3, Salvatore Macis 4,5, Luca Tomarchio 4, Stefano Lupi 4,5, Augusto Marcelli 5,6 and Shigemasa Suga 1Round 1
Reviewer 1 Report
In this revision, the authors have addressed all my issues and hesitations. The quality and significance of paper have been improved strongly. Therefore, I would like to recommend this paper for publication in Condensed Matter.
Author Response
We thank the referee for having addresed comments and criticisms for upgrading this paper.
Reviewer 2 Report
The paper describes THz transmission imaging of several meterials and objects. Images are qualitative; optical parameters are not derived.
The authors appear to deliberately overlook and disregard the dominant area of THz measurements and science, which is time-domain spectroscopy, in order to make their approach look more attractive. A more realistic and balanced view must be presented.
Line 125: TDS is not challenging. There are plenty of relatively inexpensive commercial instruments that are compact and robust, and measurements are quick and straightforward.
Lines 130-135: This is highly tendentious, misleading and inaccurate. There are hundreds of publications reporting THz spectral properties of a large number and variety of materials, measured using TDS. This should be mentioned and relevant references included, e.g. doi 10.1002/lpor.201000011; doi 10.1063/1.5047659. TDS also has the crucial advantage of coherent measurements, which is critically important for spectroscopic studies. In contrast, measurements using a FEL source are incoherent. The frequency resolution of TDS is commonly ~5 GHz and can be as low as 1 GHz, which is also better than FEL.
Lines 136-153: It is true that thermal sources and sources used in FTIR have low brilliance. In contrast, THz pulses generated by mode-locked lasers (used in TDS) have peak brilliance comparable or exceeding FEL. As above, this paragraph is tendentious and misleading.
Author Response
Thank you for your comments. The characteristic feature of this paper, which has been specified in the former revised text is spectral-imaging at any wavelength in the range of 3-6 THz. We never wanted to under-evaluate the role of TDS, which is a great and established method in the 0.1-3 THz region that can be easily extended to 5 THz. However, as far as only spectroscopic measurements are concerned, standard light sources and detectors may cover from the far-infrared to the visible / UV regions without difficulty. This is because these sources do not require high intensity or brightness.
In imaging, regardless of whether the detector is single-element type or multi-element type, the light source must have enough intensity and brightness. In other words, the method using FEL sources, as demonstrated in this paper is currently the most promising approach for a simple and intuitive spectral-imaging in the range of 3-6 THz within a practical measuring time. Although spectroscopic measurements in the range of 3-6 THz by TDS exist, spectral-imaging in this range by TDS is absent because of the low intensity and brightness in monochromatic condition.
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The paper describes THz transmission imaging of several meterials and objects. Images are qualitative; optical parameters are not derived.
>>>>>reply>>>>>>>>>>
In this paper, the infrared spectrum (in both transmission and reflection configuration) by FEL illumination is collected with a straightforward sample set (in-situ). Since this infrared spectroscopic method uses a diffraction grating, the optical constants n and k can not be directly obtained. But the transmittance or reflectance can be obtained as shown in Figs. 5 (a) and 6 (a). A He-Ne laser, which goes through the same optics of the FEL layout, is used as a guide of the FEL focal spot.
----------comment----------------
The authors appear to deliberately overlook and disregard the dominant area of THz measurements and science, which is time-domain spectroscopy, in order to make their approach look more attractive. A more realistic and balanced view must be presented.
Line 125: TDS is not challenging. There are plenty of relatively inexpensive commercial instruments that are compact and robust, and measurements are quick and straightforward.
>>>>>>>reply>>>>>>>>
Some expressions are deleted to avoid misleading (line 125-127). As will be described next, the main purpose of this contribution is to present and discuss spectral-imaging and not to show just spectroscopy or imaging. We also recognize that the experimental environment in which spectral imaging can be performed in the range of 3 to 6 THz is limited.
-----------comment---------------
Lines 130-135: This is highly tendentious, misleading and inaccurate. There are hundreds of publications reporting THz spectral properties of a large number and variety of materials, measured using TDS. This should be mentioned and relevant references included, e.g. doi 10.1002/lpor.201000011; doi 10.1063/1.5047659. TDS also has the crucial advantage of coherent measurements, which is critically important for spectroscopic studies. In contrast, measurements using a FEL source are incoherent. The frequency resolution of TDS is commonly ~5 GHz and can be as low as 1 GHz, which is also better than FEL.
>>>>>>>>reply>>>>>>>
In Chapter 3 we describe "Spectral-imaging using THz-FEL" and we claimed "THz imaging using various light sources and detectors ...". We do not intend to show a general spectroscopic contribution, but want to show how to combine spectroscopy and imaging in the THz range. Imaging which does not necessarily require spectroscopy has already been studied [ref19-24] and for which commercially available devices are used. Still, experiments are limited within 1-3 THz (not GHz), and there are only a few cases of imaging taken at several wavelengths. In other words, as far as we know, this is the first paper showing examples of spectral-imaging in the 3-6 THz region. In addition, FEL (Free Electron Laser) is a coherent laser light source.
------------comment---------------
Lines 136-153: It is true that thermal sources and sources used in FTIR have low brilliance. In contrast, THz pulses generated by mode-locked lasers (used in TDS) have peak brilliance comparable or exceeding FEL. As above, this paragraph is tendentious and misleading.
>>>>>>>>reply>>>>>>>
With the same total photon number, a short-pulse light source is superior in peak power or maximum of electric field in the time domain. The "brilliance" is defined as the photon number per unit time normalized to the source size and divergences and per a defined bandwidth (tipically 0.1%). Actually, it has different meanings depending upon whether the light source is monochromatic or not. The light intensity at the single wavelength required for spectral-imaging, i.e., the monochromatic photon number, is rather disadvantageous for short-pulse light sources having a wavelength distribution output. This is also true for FELs, and in the Appendix Figure A2 shows the relationship between the total photon number (area of spectrum) and the monochromatic photon number (intensity at certain wavelength) which changes vs pulse length. As shown in Figure A2, in terms of the area of the spectrum (total photon number), the wideband condition is nearly three times as large as that of the monochromatic condition, but the peak intensity (monochromatic photon number) at around rambda=70 um is about 1.5 times stronger under the monochromatic condition. And, this (quasi-) monochromatic FEL used in our experiments is powerful enough to induce irreversible changes on different materials [ref1-4].
We thank the referee for having addresed comments and criticisms for upgrading this paper. We have revised the paper and now hope that the modified text clearly point out the relevance of FEL spectral-imaging in the THz region, showing the possibility of challenging researchers in different scientific areas in both the linear and non-linear regimes.
Reviewer 3 Report
The authors have implemented and address all of my previous concerns. I do not have any further questions to the authors. Thanks
Author Response
We thank the referee for having addresed comments and criticisms for upgrading this paper.
This manuscript is a resubmission of an earlier submission. The following is a list of the peer review reports and author responses from that submission.
Round 1
Reviewer 1 Report
In this submission by Akinori Irizawa and co-authors, entitled “Spatially resolved spectral-imaging by a THz-FEL”, the authors describe experimental setup for THz imaging exploiting the radiation of their free-electron laser. In my opinion, this work is not well-organized and worthy. It does not present any interesting results, neither in the area of THz imaging technologies, not in the area of condensed matter.
I have the following list of issues and hesitations to be addressed to authors:
ISSUE 1. You describe some novel modality of THz imaging and state that high power of your source is beneficial. At the same time, I did not found any advanced performance of your setup in your manuscript.
Indeed, it could be interesting and novel technique, if you would demonstrate some spatially-resolved study of non-linear medium response at THz frequencies. However, you present conventional experiment, the results of which reveal only differences in a linear response of medium over its aperture. One could easily obtain the same images without using such a high-power source, with similar or even much higher frame rate.
In order to make this paper more important and worthy, I suggest the authors to select better object of imaging, and to study both linear and non-linear character of its response at THz frequencies. This would make sense for the condensed matter community, as well as would be interesting to the Condensed Mater journal readership.
Furthermore, if we would compare the presented imaging modality with all other existing ones from the viewpoint of the imaging information, spatial resolution, frame rate and performance (for example see the following latest review and research articles, just as an example)
– Advances in Optics and Photonics 10(4), 843 (2018), https://doi.org/10.1364/AOP.10.000843;
– Journal of Optics 22(1), 013001 (2020), https://doi.org/10.1088/2040-8986/ab4dc3;
– Progress in Quantum Electronics 62, 1 (2018), https://doi.org/10.1016/j.pquantelec.2018.10.001;
– Applied Physics Letters 113(11), 111102 (2018), https://doi.org/10.1063/1.5045480;
– APL Photonics 2, 056106 (2017); https://doi.org/10.1063/1.4983114;
– IEEE Transactions on Terahertz Science and Technology 8(4), 403 (2018), https://doi.org/10.1109/TTHZ.2018.2844104)
we would not find its advantages. There is a number of methods, which yield the same imaging information (spatially-resolved linear response of matter), but with much higher resolution and frame rate, even in case of imaging of larger square areas than that presented in this paper.
Please, in your revision, consider other modalities of THz imaging and objectively compare them to the developed method, in order to uncover its weaknesses and benefits. Due to small repetition rate of your laser, the frame rate of your system is even smaller than that of, for example, the THz solid immersion microscopy based on backward-wave oscillator, as a THz wave source, and a Goley cell, as a THz wave detector
– Applied Physics Letters 113(11), 111102 (2018), https://doi.org/10.1063/1.5045480.
Thereby, in my opinion, the only way to demonstrate advanced character of your method is to study and analyze spatially-resolved non-linear response of some medium, which is inhomogeneous at the scale posed by the THz wavelengths.
ISSUE 2. My second issue is about the structure and contents of your paper, which is much more dedicated to the description of your THz-FEL, than to the discussion of the developed THz imaging setup. Please, add a scheme of your imaging system, and provide in-depth description of its main optical and opto-electronic parts. Maybe it would be better to extend experimental part of submission and to provide additional THz imaging of other representative samples. It would be interesting to study THz imaging of the same object, but at different powers of the THz-FEL using an attenuator (this could help you to reveal some non-linear response of media).
ISSUE 3. The author did not discuss any modern modalities of THz imaging, as well as they did not compare their imaging modality with the existing ones. Please, extend the introduction or discussions. I was surprised that the authors did not even mention the results of THz imaging based on the Novosibirsk FEL system. For example, see Rerfs.:
– Physics Procedia 84, 13 (2016), https://doi.org/10.1016/j.phpro.2016.11.004;
– IEEE Transactions on Terahertz Science and Technology 5(5), 836 (2015), https://doi.org/10.1109/TTHZ.2015.2460465;
– IEEE Transactions on Terahertz Science and Technology 5(5), 798 (2015), https://doi.org/10.1109/TTHZ.2015.2453121.
Please, discuss the results of this research group, as compared to your data.
ISSUE 4. The authors demonstrated 107-μm-resolution of their THz imaging system. In turn, they use only 100-μm-scanning step in their imaging experiments. Such a pair of the resolution and the scanning step does not satisfy the Nyquist–Shannon sampling theorem, which might lead to artifacts in THz images. Please, make additional imaging experiments with correct scanning step, which should be, at least, twice smaller than the imaging resolution.
Finally, considering all this major issues, I recommend a very major revision or rejection otherwise.
Author Response
Manuscript-ID: condensedmatter-725332
Comments and Suggestions for Authors by reviewer1
------------------
In this submission by Akinori Irizawa and co-authors, entitled “Spatially resolved spectral-imaging by a THz-FEL”, the authors describe experimental setup for THz imaging exploiting the radiation of their free-electron laser. In my opinion, this work is not well-organized and worthy. It does not present any interesting results, neither in the area of THz imaging technologies, not in the area of condensed matter.
>>>>>>>>>>
We apologize to the reviewer for the negative impression of this contribution. We made significant revisions and add more original materials, so we hope this revised text deserves publication.
The main content of this contribution is the establishment of the imaging technology for materials of various shapes. This technique is based on a monochromatic wavelength in the wavelength range 50-100 μm (3-6 THz), and is easy and fast. The results show the original purpose of this spectral-imaging, and point out the advantages compared to previous THz imaging researches. Most of the previous examples, except for a few cases, have been indeed performed at the most intense wavelength of light sources, and not to the wavelength matching specific absorption features, i.e., fingerprints of the materials. Moreover, these few cases were only collected in the range 100-300 μm (1-3 THz) [see references 24 and 25].
In the THz region, researches involving FELs are still focused on advanced technologies and methods, and applications in materials science are not yet abundant. At present, the availability of a user-oriented facility where researchers that do not have expensive advanced equipments or that are not yet “experts” in the use of THz radiation can perform advanced researches with original ideas is very important.
Also, for this reason, we are confident in materials science researches on inorganic and organic materials and in biomedical and medical researches that the simple and fast method using THz radiation generated from FELs described in this paper can be used to achieve advanced results.
-----------------
I have the following list of issues and hesitations to be addressed to authors:
>>>>>>>>>
We are grateful to the reviewer that pointed several examples of recent imaging researches in the THz region and evaluated our research from various perspectives. All suggestions have been added in the references. In addition, in order to make the manuscript more meaningful to readers in condensed matter physics, the referee recommended to discuss examples of linear and nonlinear responses, which has a great relevance in the future research plan of this facility. Thanks to your suggestion, and we added one example of a nonlinear response on imaging.
About some specific comments/criticisms, below some detailed answers/comments.
--------------------
ISSUE 1. You describe some novel modality of THz imaging and state that high power of your source is beneficial. At the same time, I did not found any advanced performance of your setup in your manuscript.
>>>>>>>>>>>>>>
The absolute photon counts, i.e., the intensity of light source is essential for true spectral-imaging with a monochromatic wavelength. No studies have been published in the range 3-6 THz. Even in the 1-3 THz domain are a small number.
----------------------
Indeed, it could be interesting and novel technique, if you would demonstrate some spatially-resolved study of non-linear medium response at THz frequencies. However, you present conventional experiment, the results of which reveal only differences in a linear response of medium over its aperture. One could easily obtain the same images without using such a high-power source, with similar or even much higher frame rate.
In order to make this paper more important and worthy, I suggest the authors to select better object of imaging, and to study both linear and non-linear character of its response at THz frequencies. This would make sense for the condensed matter community, as well as would be interesting to the Condensed Mater journal readership.
>>>>>>>>>>>>>>>>>>
The suggestion regarding nonlinear imaging was very helpful. Thanks. Further research work is needed, but one example has been included in the manuscript.
We are planning new experiments on other samples, and we hope to get new results soon. Although not described in this manuscript, we have also been studying other methods, e.g., signal detection in micro-pulse units with a high-speed detector, and acquisition of 2D images using a THz imager with a phase recovery method for speckles.
---------------
Furthermore, if we would compare the presented imaging modality with all other existing ones from the viewpoint of the imaging information, spatial resolution, frame rate and performance (for example see the following latest review and research articles, just as an example)
r1-1– Advances in Optics and Photonics 10(4), 843 (2018), https://doi.org/10.1364/AOP.10.000843;
r1-2– Journal of Optics 22(1), 013001 (2020), https://doi.org/10.1088/2040-8986/ab4dc3;
r1-3– Progress in Quantum Electronics 62, 1 (2018), https://doi.org/10.1016/j.pquantelec.2018.10.001;
r1-4– Applied Physics Letters 113(11), 111102 (2018), https://doi.org/10.1063/1.5045480;
r1-5– APL Photonics 2, 056106 (2017); https://doi.org/10.1063/1.4983114;
r1-6– IEEE Transactions on Terahertz Science and Technology 8(4), 403 (2018), https://doi.org/10.1109/TTHZ.2018.2844104)
we would not find its advantages. There is a number of methods, which yield the same imaging information (spatially-resolved linear response of matter), but with much higher resolution and frame rate, even in case of imaging of larger square areas than that presented in this paper.
>>>>>>>>>>>>>>>
As mentioned above, it is important in materials science that collecting images at a specific wavelength that is a fingerprint of the material under analysis can be performed easily and fast. No results clearly support this approach. These manuscripts are, however, important for imaging and all have been added to the bibliography.
-----------------------
Please, in your revision, consider other modalities of THz imaging and objectively compare them to the developed method, in order to uncover its weaknesses and benefits. Due to small repetition rate of your laser, the frame rate of your system is even smaller than that of, for example, the THz solid immersion microscopy based on backward-wave oscillator, as a THz wave source, and a Goley cell, as a THz wave detector
r1-4– Applied Physics Letters 113(11), 111102 (2018), https://doi.org/10.1063/1.5045480.
>>>>>>>>>>>>>>
This experiment is also performed at 0.6 THz. In other words, in the range 3-6 THz there are almost no suitable light sources for spectral-imaging other than FEL.
---------------------------
Thereby, in my opinion, the only way to demonstrate advanced character of your method is to study and analyze spatially-resolved non-linear response of some medium, which is inhomogeneous at the scale posed by the THz wavelengths.
>>>>>>>>>>>>>
We agree on that in any case. We are working to future developments.
-----------------------
ISSUE 2. My second issue is about the structure and contents of your paper, which is much more dedicated to the description of your THz-FEL, than to the discussion of the developed THz imaging setup. Please, add a scheme of your imaging system, and provide in-depth description of its main optical and opto-electronic parts. Maybe it would be better to extend experimental part of submission and to provide additional THz imaging of other representative samples. It would be interesting to study THz imaging of the same object, but at different powers of the THz-FEL using an attenuator (this could help you to reveal some non-linear response of media).
>>>>>>>>>>>>>
As requested, we added a figure and clarified our simple optical configuration.
----------------------
ISSUE 3. The author did not discuss any modern modalities of THz imaging, as well as they did not compare their imaging modality with the existing ones. Please, extend the introduction or discussions. I was surprised that the authors did not even mention the results of THz imaging based on the Novosibirsk FEL system. For example, see Rerfs.:
r1-7– Physics Procedia 84, 13 (2016), https://doi.org/10.1016/j.phpro.2016.11.004;
r1-8– IEEE Transactions on Terahertz Science and Technology 5(5), 836 (2015), https://doi.org/10.1109/TTHZ.2015.2460465;
r1-9– IEEE Transactions on Terahertz Science and Technology 5(5), 798 (2015), https://doi.org/10.1109/TTHZ.2015.2453121.
Please, discuss the results of this research group, as compared to your data.
>>>>>>>>>>>>>>>
When selecting THz/FIR as a light source for imaging, this radiation is not so powerful compared to shorter wavelengths in term of spatial resolution and even for time resolved measurement or material permeability. Therefore, the first goal of imaging in the THz region is to perform imaging at a specific wavelength corresponding to specific fingerprinting absorption of the investigated materials, i.e., THz spectral-imaging. This contribution is the first case in which we show spectral-imaging in the range 3-6 THz. We are convinced that examples given in our manuscript are competitive: the range 50-100 um or 3-6 THz is unique, and only FEL sources may satisfy the condition of high intensity coherent emission. Some similar FEL facilities in the THz region are operational such as NovoFEL, CLIO, FELBE, FELIX, and KAERI, but none reported "spectral-imaging" in this fingerprinting region.
--------------------------
ISSUE 4. The authors demonstrated 107-μm-resolution of their THz imaging system. In turn, they use only 100-μm-scanning step in their imaging experiments. Such a pair of the resolution and the scanning step does not satisfy the Nyquist–Shannon sampling theorem, which might lead to artifacts in THz images. Please, make additional imaging experiments with correct scanning step, which should be, at least, twice smaller than the imaging resolution.
>>>>>>>>>>>>>>>>
Actually the Nyquist–Shannon sampling theorem is important when reproducing a real-space image from a diffraction image or a speckle image by Fourier transform. In this contribution, real-space images are acquired by a simple raster scanning of samples at a focal point. In this case, the scanning size and the spatial resolution of the image can be modified by changing the degree of focusing and the step scan width. However, referee's point is very important for advanced researches. We are already trying to reproduce a real-space image from a speckle image by the phase recovery method. In this case, it is necessary to take the correct sampling interval as the referee pointed out. Thank for having pointed out this fundamental issue.
---------------------------
Finally, considering all this major issues, I recommend a very major revision or rejection otherwise.
>>>>>>>>>>>>>>>
According to the criticism, we strongly revised the original text and add further results from spectral-imaging experiments. We are convinced that the present content is now unique and useful for readers interested to materials science researches.
Thanks once more to the referee for his/her suggestions. They were fundamental to improve significantly the manuscript.
Reviewer 2 Report
The paper is mostly a trivial description of the history and capabilities of the ISIR-FEL. There is one spectral transmission measurement on CuO and Cu2O over a limited frequency range and giving no optical properties (no absorption coefficient or refractive index). Notably, accurate THz optical properties of these materials can be easily and accurately measured over a broader frequency range using a time domain spectrometer - a table-top, inexpensive instrument - without resorting to a massive FEL facility.
The abstract is highly misleading. It states "... to investigate the non-linear response of different materials and to characterize materials and surfaces after irradiation." - there is none of that in the paper. Also "... we present results of micro-spectroscopy experiments and two-dimensional spectral-imaging." - there is none of that either. (There is indeed imaging, but not spectral imaging.)
How were the spectra in Fig 2 measured? If a grating spectrometer was used, as mentioned later, how was it designed and what were its specifications (e.g. frequency resolution)? It is known that designing a high-performance spectrometer at THz frequencies is problematic.
What detector was used in all measurements, and what were its specifications?
In Fig 3b, a knife-edge measurement produces integrated intensity, which for a a Gaussian beam is an error function. The beam profile is then obtained by differentiation. Because the measured power is integrated over both axes, such measurement cannot reveal features or localised distortions. As a result, even distorted beam profiles tend to appear close to Gaussian. Fig 3a clearly shows astigmatism and some distortions.
Author Response
Manuscript-ID: condensedmatter-725332
Comments and Suggestions for Authors by reviewer2
----------------------------
The paper is mostly a trivial description of the history and capabilities of the ISIR-FEL. There is one spectral transmission measurement on CuO and Cu2O over a limited frequency range and giving no optical properties (no absorption coefficient or refractive index). Notably, accurate THz optical properties of these materials can be easily and accurately measured over a broader frequency range using a time domain spectrometer - a table-top, inexpensive instrument - without resorting to a massive FEL facility.
>>>>>>>>>>>>>>>>
We apologize to the reviewer for the negative impression of this contribution. We made significant revisions and add more original materials, so we hope this revised text deserves publication.
The main content of this contribution is the establishment of the imaging technology for materials of various shapes. This technique is based on a monochromatic wavelength in the wavelength range 50-100 μm (3-6 THz), and is easy and fast. The results show the original purpose of this spectral-imaging, and point out the advantages compared to previous THz imaging researches. Most of the previous examples, except for a few cases, have been indeed performed at the most intense wavelength of light sources, and not to the wavelength matching specific absorption features, i.e., fingerprints of the materials. Moreover, these few cases were only collected in the range 100-300 μm (1-3 THz) [see references 24 and 25].
In the THz region, researches involving FELs are still focused on advanced technologies and methods, and applications in materials science are not yet abundant. At present, the availability of a user-oriented facility where researchers that do not have expensive advanced equipments or that are not yet “experts” in the use of THz radiation can perform advanced researches with original ideas is very important.
Also, for this reason, we are confident in materials science researches on inorganic and organic materials and in biomedical and medical researches that the simple and fast method using THz radiation generated from FELs described in this paper can be used to achieve advanced results.
-----------------
The abstract is highly misleading. It states "... to investigate the non-linear response of different materials and to characterize materials and surfaces after irradiation." - there is none of that in the paper. Also "... we present results of micro-spectroscopy experiments and two-dimensional spectral-imaging." - there is none of that either. (There is indeed imaging, but not spectral imaging.)
>>>>>>>>>>>>>>>
Thanks to reviewer’s comments, and we fix the sentences in the text. However, as for spectral-imaging, it can be described as shown in Figure 3 at the URL:
http://zeiss-campus.magnet.fsu.edu/articles/spectralimaging/introduction.html
In our study, it is also possible to collect 2D images at continuous wavelengths, but the top priority for spectral-imaging can be observing the changes in 2D images when characteristic fingerprint wavelengths are selected. In that sense, we consider our method as a spectral-imaging.
---------------------
How were the spectra in Fig 2 measured? If a grating spectrometer was used, as mentioned later, how was it designed and what were its specifications (e.g. frequency resolution)? It is known that designing a high-performance spectrometer at THz frequencies is problematic.
＞＞＞＞＞＞＞＞＞＞＞
The spectrometer used here is a conventional cross Czerny-Turner type with diffraction grating, but the size is quite large, ~1 m square. The equipment including diffraction grating is custom made. Since the corresponding wavelength is longer, no better accuracy of microns is required for the diffraction grating. The wavelength resolution is ~1 μm or less achieved both from simulation and from the resolution of the absorption lines, e.g., CuO and Cu2O spectra. The optical layout has been added as a figure.
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What detector was used in all measurements, and what were its specifications?
＞＞＞＞＞＞＞＞＞＞＞＞
We added this information in the text. Since FEL has a high intensity, we use an available energy detector for near-infrared lasers calibrated in the THz range.
----------------------
In Fig 3b, a knife-edge measurement produces integrated intensity, which for a Gaussian beam is an error function. The beam profile is then obtained by differentiation. Because the measured power is integrated over both axes, such measurement cannot reveal features or localised distortions. As a result, even distorted beam profiles tend to appear close to Gaussian. Fig 3a clearly shows astigmatism and some distortions.
＞＞＞＞＞＞＞＞＞＞＞＞
In the revised text, we added a more detailed analysis. As pointed out by the reviewer, the image presents some astigmatism. This is because the optical system is not an optimized solution. We are aware of this, and are planning the upgrade. However, the optical system is made by 10 mirrors going from the FEL generation point to the experimental hall about 10 m beyond the radiation shielding wall. The optimization is then not an easy process and the results is also affected by the alignment errors of each optical element. However, this does not significantly affect the spatial distribution as far as seen in the THz viewer image, and the focal depth is about 7 mm.
Thanks to the reviewer for his/her report that was extremely helpful to improve our contribution.
Reviewer 3 Report
In this paper, the authors investigated the non-linear response of different materials and to characterize materials and surfaces after irradiationby using ISIR-FEL using ISIR-FEL. The manuscript could be considered for publication after the following concerns are appropriately addressed with requested changes in the manuscript.
a) The problem statement and motivation behind this research and contribution of this research is very weak. The authors must rewrite abstract and Introduction to address the current status of this imaging technology comparing with similar technology (with reference), what's the gap/issues/problem statement (with reference) in this and other technology that warranteed to conduct this research. And overall must clearly mention in the abstract and conclusion what's the contribution of this research comparing similar work to this technology? Add a discussion section mentioning the limitation of this research and future works (with reference).
b) Please provide experimental setup schematic mimicking the actual setup with ALL the apparatus mentioned so the readers can easily understand the setup or re-conduct the experiment if interested.
c) Provide full form of ISIR, THz... etc. Please check all the acronym if they were elaborated.
Author Response
Manuscript-ID: condensedmatter-725332
Comments and Suggestions for Authors by reviewer3
--------------------------------
In this paper, the authors investigated the non-linear response of different materials and to characterize materials and surfaces after irradiation by using ISIR-FEL using ISIR-FEL. The manuscript could be considered for publication after the following concerns are appropriately addressed with requested changes in the manuscript.
- a) The problem statement and motivation behind this research and contribution of this research is very weak. The authors must rewrite abstract and Introduction to address the current status of this imaging technology comparing with similar technology (with reference), what's the gap/issues/problem statement (with reference) in this and other technology that warranteed to conduct this research. And overall must clearly mention in the abstract and conclusion what's the contribution of this research comparing similar work to this technology? Add a discussion section mentioning the limitation of this research and future works (with reference).
>>>>>>>>>>>>>>>>>>
We apologize to the reviewer for the negative impression of this contribution. We made significant revisions, and add more original materials and references, so we hope this revised text deserves publication.
The main content of this contribution is the establishment of the imaging technology for materials of various shapes. This technique is based on a monochromatic wavelength in the wavelength range 50-100 μm (3-6 THz), and is easy and fast. The results show the original purpose of this spectral-imaging, and point out the advantages compared to previous THz imaging researches. Most of the previous examples, except for a few cases, have been indeed performed at the most intense wavelength of light sources, and not to the wavelength matching specific absorption features, i.e., fingerprints of the materials. Moreover, these few cases were only collected in the range 100-300 μm (1-3 THz) [see references 24 and 25].
---------------------------
- b) Please provide experimental setup schematic mimicking the actual setup with ALL the apparatus mentioned so the readers can easily understand the setup or re-conduct the experiment if interested.
>>>>>>>>>>>>>>>
As reviewer pointed out, the main purpose of this article is that FEL users can reproduce the experiments described here, so we added also the layout of the optical setup. No special equipment has been used, and it is an easy optical layout to arrange. At present, we consider that the availability of a user-oriented facility where researchers lacking of own equipment or that are not “experts” in the use of THz radiation can perform their advanced researches is very important. Also, for this reason, we are convinced that the experiments using THz radiation generated from FELs by a relatively "simple and fast" method as described in this contribution is worth to be performed for material scientists on inorganic and organic materials and in biomedical and medical researches.
-----------------------------
- c) Provide full form of ISIR, THz... etc. Please check all the acronym if they were elaborated.
>>>>>>>>>>>>>>>>>
As reviewer pointed out, we added all full names and abbreviations.
Thanks to the reviewer for his/her report that was extremely helpful to improve our contribution.
