A THz Spectrometer Using Band Pass Filters
Round 1
Reviewer 1 Report
The terahertz spectrometer developed by the authors is of great interest for the scientific community that is involved in terahertz technology. A new compact, los cost THz spectrometer is presented based on global source and pyroelectric detector. The system cover the range from 1.2 to 10.5 THz using a band of filters made with metasurfaces. The authors need to take into account the following comments to improve the paper.
1/ The authors needs to give more datas on the spot size of the terahertz beam, the exact distance between the filters and the detector, the acronym HDPE is not specified and explained.
2/ How about the resolution of the system? when the results obtained from the FTIR is compared withe MFA ones it's clearly seen that the resolution of the MFA is low.
3/ Discrepancy of the results are seen in all figures. For figure3 for example, there is an absorption of BaCO3 that is observed around 9 THz with FTIR and around 10 THz with MFA. Also the transmission is almost flat and 0 in the range 5 to 7 THz for FTIR but with MFA it's different. Comments and discussion of this discrepancy is needed.
4/ Similar behaviour is observed in figure 4. Only one absorption is observed around 4 THz with MFA but with FTIR, there is two absorptions at 3 THz and 7 THz.
Author Response
We want to thank the referee because his/her comments were useful to improve our paper.
1/ The authors needs to give more datas on the spot size of the terahertz beam, the exact distance between the filters and the detector, the acronym HDPE is not specified and explained.
The spot has a radius of 5.7 mm, now this datum has been added to the text on page 3 line 89, the measure was easily made using the step motor to slowly move the sample holder over a large slot. The distance between filters and the detector is about 3 mm, being not exact, it was not added to the text. On page 3 line 85 of the new version we explained the acronym HDPE.
2/ How about the resolution of the system? when the results obtained from the FTIR is compared withe MFA ones it's clearly seen that the resolution of the MFA is low.
We added the frequency resolution of the two apparatus in the section of Experimental validation page 6 line 165-173. The frequency resolution of MFA is very low compared with that one of FTIR.
3/ Discrepancy of the results are seen in all figures. For figure3 for example, there is an absorption of BaCO3 that is observed around 9 THz with FTIR and around 10 THz with MFA. Also the transmission is almost flat and 0 in the range 5 to 7 THz for FTIR but with MFA it's different. Comments and discussion of this discrepancy is needed.
We changed the text to show why the discrepancies in figure 3 are not significant. The absorption of BaCO3, that is observed around 9 THz on sample C with FTIR, is due to interference (as many other features on the same sample), the curve of sample D with FTIR is flat in the same regions of spectrum. The absorption around 10 THz with MFA disappears when the measurement is made in pure Nitrogen, that is a less noisy environment. The transmission is almost flat in the range 5 to 7 THz for FTIR, it cannot be flat with MFA because its filters frequency resolution is poor.
4/ Similar behaviour is observed in figure 4. Only one absorption is observed around 4 THz with MFA but with FTIR, there is two absorptions at 3 THz and 7 THz.
We changed the text to show why the discrepancies in figure 4 are not significant. We were speaking of peaks, but we agree, it is better to look at absorption: the absorption at 7 THz has a frequency span too small to be seen with MFA. In the text now we put the attention on absorption.
Reviewer 2 Report
The paper by the authors presents the development of a new THz spectrometer, called Metasurface Filters Apparatus (MFA). The instrument is cheaper and simpler compared to the commercial options presently available (Fourier Transform Infrared Spectrometer, or FTIR) and it can be used for those applications where a coarser frequency resolution is acceptable.
I have found this paper useful and well written. The quality of the results, figures and selection of references is appropriate. My main comments concern the experimental validation of the MFA with respect to the FTIR. Here I suggest the authors to discuss in more detail the comparison of the results, following my questions below. I expect this manuscript to be suitable for publication in Instruments after the authors have addressed my comments.
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Detailed comments
1) Page 6, line 164: "similar results". Please explain what you qualify as being "similar". Is it enough to identify the same minimum in the trasmittance curve? Should the relative change in the trasmittance curve between two different samples be the same when it is measured with the FTIR and MFA?
2) Page 6, figure 3 to 5: I would use "Trasmittance" rather than just "T" as the label of the y axis.
3) Page 6, line 165: "then the measure". Did you mean "then the measurement"?
4) Page 6, figure 4: why is the difference of the trasmittance curves between the E and F samples more pronounced with the MFA compared with the FTIR measurement? Is this significant?
5) Page 7, figure 5: the curve measured with the MFA has a local maximum between 2 and 4 THz compared with that measured with the FTIR. Is this difference significant?
6) Page 6 and 7, figures 4 to 5: sometimes the curve measured with the MFA reaches zero while that measured with the FTIR does not, or viceversa. Is this significant?
7) Page 8, lines 186 to 205: the description of figure 6 is a bit schematic, unlike that of figures 3 to 5. I suggest to improve the structure of this paragraph accordingly.
Author Response
We want to thank the referee because his/her comments were useful to improve our paper.
We made some minor language corrections.
1) Page 6, line 164: "similar results". Please explain what you qualify as being "similar". Is it enough to identify the same minimum in the trasmittance curve? Should the relative change in the trasmittance curve between two different samples be the same when it is measured with the FTIR and MFA?
We removed “similar” and we better described the differences. Same minima in transmittance curve are good identifiers of features. The spot sizes of FTIR and MFA are different, if the samples are uniform the relative change is the same, but the samples are usually not uniform, so the results may be different.
2) Page 6, figure 3 to 5: I would use "Trasmittance" rather than just "T" as the label of the y axis.
We changed T into Transmittance in all figures
3) Page 6, line 165: "then the measure". Did you mean "then the measurement"?
We changed “measure” in “measurement”.
4) Page 6, figure 4: why is the difference of the trasmittance curves between the E and F samples more pronounced with the MFA compared with the FTIR measurement? Is this significant?
We added:
The difference in amplitude between the samples E and F is more pronounced in MFA measurements compared with those of FTIR probably because MFA spot size is larger and it mediates on both whole samples.
5) Page 7, figure 5: the curve measured with the MFA has a local maximum between 2 and 4 THz compared with that measured with the FTIR. Is this difference significant?
It is not significant, we added:
We think that the difference between FTIR and MFA at low frequency are due to the noise in MFA for small signals.
6) Page 6 and 7, figures 4 to 5: sometimes the curve measured with the MFA reaches zero while that measured with the FTIR does not, or viceversa. Is this significant?
It is not exact: FTIR absorption reaches zero, but on the same region MFA never reaches zero, it never happens the opposite: the reason is in the large bandwidth of filters.
7) Page 8, lines 186 to 205: the description of figure 6 is a bit schematic, unlike that of figures 3 to 5. I suggest to improve the structure of this paragraph accordingly.
We added:
All measurements give the same result, three regions of absorption: lower than 2 THz, around 4.3 THz and for frequency higher than 6 THz. The measurement with FTIR (see Figure S 9 of supplementary material) shows the same results.
Round 2
Reviewer 1 Report
The author made the change requested by the author. The paper can published
