Phase-Controlled Planar Metalenses for High-Resolution Terahertz Focusing

Round 1

Reviewer 1 Report

The manuscript ‘Phase-controlled planar metalenses for high-resolution terahertz focusing’ deals with modeling and experimental realization of planar metasurfaces. The aim of the study is the high-resolution spatial focusing of the electromagnetic radiation in the THz frequency range.  Different metalenses are fabricated by conventional lithography and optically characterized, achieving an optimal focus size of about 170 µm with a 1.1 THz incident beam, when the NA of the metalens reads 0.95.

Despite the research topic is of interest within the photonics community, the novelty of the present results with respect to state-of-the-art studies is not clear at all. Additionally, recent relevant papers in the field are not cited and fair comparison between the performances achieved in the present work and the state of the art is lacking.

In the experimental section (page 5) author claim to measure the THz near-field distribution, however the measurement performed is not clear, and the eventual contributions due to background are not discussed. Indeed near-field imaging in the THz frequency range is not trivial and represents a research field on its own, that should be properly referenced in the context of the present work.

The claims of the conclusions ‘Our proposed design can facilitate the practical application of metalenes in THz imaging, communication, photonic integrated circuits, etc.’, should be properly motivated in the introduction and in the discussion.

A complete revision of the English form should be done.

Based on all these considerations I think the present version of the manuscript can’t be published in Photonics-MDPI, and an extensive revision is required.

Author Response

Response to Reviewer 1 Comments

Point 1: Despite the research topic is of interest within the photonics community, the novelty of the present results with respect to state-of-the-art studies is not clear at all. Additionally, recent relevant papers in the field are not cited and fair comparison between the performances achieved in the present work and the state of the art is lacking.

Response 1:

Thanks very much.

Actually, the novelty in our paper is that the spatial resolutions of metalens system are analyzed by angular spectrum model, and then the analysis is verified by experiment. 

The relevant papers (Refs. 23, and 30) have been cited in the revised manuscript now.  Comparing with the relevant papers, similar results are obtained in our paper. Especially, different size structures, i.e., different numerical apertures, are investigated by us to verify the analysis by angular spectrum model.

The manuscript has been revised. Please refer to page 2, lines 1; page 4, lines 26-28; and page 5, lines 25.

Point 2: The claims of the conclusions ‘Our proposed design can facilitate the practical application of metalenes in THz imaging, communication, photonic integrated circuits, etc.’, should be properly motivated in the introduction and in the discussion

Response 2:

Thanks very much.

We have repeatedly measured the near-field imaging, and the results are similar. In practice, the co-polarized transmission may affect the background, and we can remove the background signal through data processing. However, we cannot make sure that the background is only determined by the co-polarized transmission, so we keep the origin data of near-field imaging in Fig. 4.

The manuscript has been revised. Please refer to page 5, lines28.

Point 3:The claims of the conclusions ‘Our proposed design can facilitate the practical application of metalenes in THz imaging, communication, photonic integrated circuits, etc.’, should be properly motivated in the introduction and in the discussion.

Response 3:Thank you very much. We have revised the manuscript according to the comments. Please refers to page 2, lines 6-8

Point 4:A complete revision of the English form should be done.

Response 4:Thank you very much. The english form has been checked now in whole manuscript.

Author Response File: Author Response.pdf

Reviewer 2 Report

Meta lens is demonstrated to ~1 THz wavelength.  figure 1c shows the cell, however it is not clear how the cell was placed in the pattern: was in in radial, azimuthal orientation or single elements were used from the cell in each circle of the pattern. 

the lift off is not clear. it is written that Cr and Cu were deposited and then resist and lithography. why then lift off which is usually made when metals are deposited over developed resist. there is not clear statement "...copper was etched excess from the sample by reactive ion etching. The last 150
step to pattern the designed structure was a lift-off process in the acetone."

please define efficiency of light collection to the focus. using meta surfaces of Si Mie resonators 2pi phase and almost lossless transmission is already demonstrated for 1 THz (Nanophotonics 8 (7), 1263-1270 2019).  please compare dielectric and metallic metasurfaces.  what is the spectral bandwidth of the proposed meta lens and polarisation sensitivity?

Author Response

Response to Reviewer 2 Comments

Point 1: :Meta lens is demonstrated to ~1 THz wavelength. figure 1c shows the cell, however it is not clear how the cell was placed in the pattern: was in in radial, azimuthal orientation or single elements were used from the cell in each circle of the pattern.

Response 1:

Thanks very much. The super-unit-cell of the proposed structure consists of eight resonant units, which are arranged in the order of 1-3-5-7-2-4-6-8 in Fig 1 (b). The manuscript has been revised. Please refer to page 2, lines 13. and page 3, lines 7.

Point 2: the lift off is not clear. it is written that Cr and Cu were deposited and then resist and lithography. why then lift off which is usually made when metals are deposited over developed resist. there is not clear statement "...copper was etched excess from the sample by reactive ion etching. The last 150
step to pattern the designed structure was a lift-off process in the acetone."

Response 2:

Thanks very much. The lift-off process is to remove photoresist from the surface of structure. The manuscript has been revised. Please refer to page 5, lines 13.

Point 3: please define efficiency of light collection to the focus. using meta surfaces of Si Mie resonators 2pi phase and almost lossless transmission is already demonstrated for 1 THz (Nanophotonics 8 (7), 1263-1270 2019).  please compare dielectric and metallic metasurfaces.  what is the spectral bandwidth of the proposed meta lens and polarisation sensitivity?

Response 3:

Thanks very much. Efficiency of light collection to the focus has been defined and calculated in the revised manuscript.  For  NA=0.78, 0.85, 0.92, and 0.95, the efficiencies are 3.7%(simulation5.1%), 4.19%(simulation5.2%), 3.17% (simulation3.73%), 2.4%(simulation2.7%), respectively. Actually, the efficiency of metarurface by metal-resonators (Appl. Phys. Lett. 2019, 114, 71106.) is much lower than that by Si resonator (Nanophotonics 8 (7), 1263-1270 2019).  Please refer to page 5, lines 26-29 and page 6, lines 1-3.

Additionally, electric field distributions for different frequencies are illustrated in Figs. 1, which show that spectral bandwidth is approximately 0.2THz.

Electric field distributions for x-polarized and y-polarized incidences are illustrated in Figs. 2, which show that it is not sensitive to polarization of incidence.

Author Response File: Author Response.pdf

Reviewer 3 Report

I have read the manuscript by Yu et al, titled "Phase-controlled planar metalenses for high-resolution terahertz focusing", and submitted for consideration in "Photonics", and I am of the opinion that the article is unsuitable for publication.

My position is based on the lack of novelty of the proposed study. On the topic of the study, i.e. metasurface-based terahertz focusing, in the latest decade there has been plenty of publications. As a definitely non-exhausting list, see for instance the following:

1. Q. Wang et al., Adv. Opt. Mat (2015), "A Broadband Metasurface‐Based Terahertz Flat‐Lens Array", doi 10.1002/adom.201400557

2. D. Jia et al., Optics Letters (2017), "Transmissive terahertz metalens with full phase control based on a dielectric metasurface", doi 10.1364/OL.42.004494

3. C. Chang et al., Optics Letters (2017), "Demonstration of a highly efficient terahertz flat lens employing tri-layer metasurfaces", doi 10.1364/OL.42.001867

4. V. Pacheco-Pena et al., Phys. Rev. Appl. (2017), "Experimental Realization of an Epsilon-Near-Zero Graded-Index Metalens at Terahertz Frequencies", doi 10.1103/PhysRevApplied.8.034036

5. H. Chen et al., Optics Express (2018), "Sub-wavelength tight-focusing of terahertz waves by polarization-independent high-numerical-aperture dielectric metalens", doi 10.1364/OE.26.029817

More precisely, also the specific claim of the submitted manuscript, i.e. the "high-resolution focusing", has been already achieved. Indeed, if one looks at the ratio between focal spot and wavelength, that is the figure of merit to quantify the resolution, one gets:
* for Ref. 4 above, values of 0.77(x-axis) and 0.49(y-axis)
* for Ref. 5 above, a value of 0.52
* for the submitted manuscript, a value of 0.76.

Hence, the present work does not present any technological or scientifical advancement with respect to the state of the art, and does not deserve any space in a scientific journal.

Author Response

Response to Reviewer 3 Comments

Point 1: My position is based on the lack of novelty of the proposed study. On the topic of the study, i.e. metasurface-based terahertz focusing, in the latest decade there has been plenty of publications. As a definitely non-exhausting list, see for instance the following:

1. Q. Wang et al., Adv. Opt. Mat (2015), "A Broadband Metasurface‐Based Terahertz Flat‐Lens Array", doi 10.1002/adom.201400557
2. D. Jia et al., Optics Letters (2017), "Transmissive terahertz metalens with full phase control based on a dielectric metasurface", doi 10.1364/OL.42.004494
3. C. Chang et al., Optics Letters (2017), "Demonstration of a highly efficient terahertz flat lens employing tri-layer metasurfaces", doi 10.1364/OL.42.001867
4. V. Pacheco-Pena et al., Phys. Rev. Appl. (2017), "Experimental Realization of an Epsilon-Near-Zero Graded-Index Metalens at Terahertz Frequencies", doi 10.1103/PhysRevApplied.8.034036
5. H. Chen et al., Optics Express (2018), "Sub-wavelength tight-focusing of terahertz waves by polarization-independent high-numerical-aperture dielectric metalens", doi 10.1364/OE.26.029817

More precisely, also the specific claim of the submitted manuscript, i.e. the "high-resolution focusing", has been already achieved. Indeed, if one looks at the ratio between focal spot and wavelength, that is the figure of merit to quantify the resolution, one gets:
* for Ref. 4 above, values of 0.77(x-axis) and 0.49(y-axis)
* for Ref. 5 above, a value of 0.52
* for the submitted manuscript, a value of 0.76.

Response 1:

Thanks very much.

Actually, the novelty in our paper is that the spatial resolutions of metalens system are analyzed by angular spectrum model, and then the analysis is verified by experiment. 

The relevant papers (Refs. 29, and 30) have been cited in the revised manuscript now.  Comparing with the relevant papers, similar results are obtained in our paper. Especially, different size structures, i.e., different numerical apertures, are investigated by us to verify the analysis by angular spectrum model.

The manuscript has been revised. Please refer to page 2, lines 1; page 4, lines 26-28; and page 5, lines 25.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

In the revised version of the manuscript, the main critical points of the paper have been clarified. However, the introduction has not been significantly improved and I think could be better revised highlighting the main motivations of the research. 

Author Response

Answer:

Thanks very much.

The introduction has been revised. Please refer to page 1, lines37-40.

Reviewer 2 Report

The added explanations are good and makes results more clear. 

I strongly believe that it is useful to readers to see that was done before and with different techniques. so would encourage to compare you results with things discussed but omitted:

"Actually, the efficiency of metarurface by metal-resonators (Appl. Phys. Lett. 2019, 114, 71106.) is much lower than that by Si resonator (Nanophotonics 8 (7), 1263-1270 2019).  Please refer to page 5, lines 26-29 and page 6, lines 1-3."

Author Response

Thanks very much

More discussions and references (Appl. Phys. Lett. 2019, 114, 71106; Nanophotonics 8 (7), 1263-1270 2019) have been supplemented in the revised manuscript. Please refer to page 6, lines 1-2, and Refs. [33].

Author Response File: Author Response.pdf

Reviewer 3 Report

I have read the revised manuscript by Yu et al, titled "Phase-controlled planar metalenses for high-resolution terahertz focusing", and unfortunately I am still of the opinion of my previous review, that is, the article is unsuitable for publication.

In the revised version the authors have highlighted that their novel contribution is the use of the angular spectrum representation; however, that model is very basic and often automatically kept into account in any analysis of focusing metasurfaces. Also the use of the function "circ(x,y)" in the text is nothing else than a cumbersome restatement of the limits of integration of a function in polar coordinates.

Author Response

Thanks very much