Mid-Infrared Continuous Varifocal Metalens with Adjustable Intensity Based on Phase Change Materials

Round 1

Reviewer 1 Report

Metalenses are relevant objects of research, as they can be the basis of multifunctional devices for object visualization. At the same time, an important property is the possibility of dynamic control of the focal length of the lens. In the article under consideration, a two-layer structure based on a Ge single crystal and a new GSSe material is proposed to create dynamically controlled metalens. Each of the two layers is formed by nanopillar, that is, nanorods with a rectangular or circular cross section. The advantages of the new GSSe material are its low optical losses in the mid-infrared range, as well as the presence of a phase transition between crystalline and amorphous states. These two crystalline fractions of the GSSe material are characterized by different refractive indices, which makes it possible to adjust the focus of the metalens. An important feature is the difference in focal length for left and right circularly polarized waves, which creates additional possibilities, including for controlling the ellipticity of the transmitted wave. The proposed metalens can be used in optical communication systems, optical tomography and imaging systems.

The strong point of the article is the proof of continuous control of the focal length of the metalens during the phase transition between two states of the new GSSe material.

The weak side of the article, in our opinion, is the insufficient justification for the presence of two layers of metalens. Let's assume that metalens can consist of only one layer. This layer could be formed by nanopillars (nanorods) of a new GSSe material. If nanopillars made of the new GSSe material had a rectangular cross-section, the Pancharatnam-Berry effect would be possible with their help. Also, such nanopillars would provide a phase change during wave propagation. Do we still need Ge nanopillars? This question needs to be further substantiated.  

      Specific comments:

1. In formula (11), it is necessary to use the value m, expressed not as a percentage, but in fractions from 0 to 1.

2. For a better understanding of the definition of the right and left circularly polarized waves, on page 10, after line 374, it should be added that the wave propagates in the positive direction of the z axis, or in the negative direction this axis. 

Author Response

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Author Response File: Author Response.docx

Reviewer 2 Report

This paper entitled “Mid-infrared continuous varifocal metalens with adjustable intensity based on phase change materials” by Shao et al. presents a theoretical study of a mid-infrared varifocal metalens enabled a novel phase change material Ge₂Sb₂Se₅. The focal length tunability is realized by a double layer metasurface with Ge₂Sb₂Se₅ nanostructure controlling the propagation phase and germanium pillars provide a geometric phase manipulation. Upon the phase change of Ge₂Sb₂Se₅, the focal length could be tuned from 62.91 to 67.13μm, and from 33.84 to 36.66μm under right/left right circularly polarized incidence, and the proposed metalens also has focusing efficiency higher than 48%. The results are solid and are interesting for the community, and also this paper is well written. I recommend this paper for publication in Photonics, after providing a few additional details and minor revisions.

 

(1)   Refractive index change through manipulating the crystallization fraction of phase change materials Ge₂Sb₂Se₅ offers the continuous zooming capability of the double-layer metalens proposed in the paper, could the authors provide a detailed description on practical realization of such function in experiments to achieve this goal?

(2)   The paper reported a focusing efficiency of over 48% for the proposed metalens, but there is a lack of information about how focusing efficiency is calculated. Please add the definition of focusing efficiency in the manuscript.

(3)   What is the difference between the zoom metalens reported in Ref. [43] and the zoom metalens proposed in this manuscript in terms of phase regulation mechanism? A comparative analysis would be interesting for readers.

(4)   There is an obscure expression in the manuscript, such as " ... since the Ge unit structure rotated by an angle θ works as a half-wave plate we can create metasurfaces combining the propagation phase and geometric phase..." Please clarify whether the metasurfaces specifically referring to the Ge layer or not.

(5)   The authors should provide the full names of the GST and GSST mentioned in the introduction before they use the abbreviation.

(6)   In Figure 4, the authors are suggested to use a larger font of the legend, and the label of the horizontal axis “radius” should be “Radius”. Please carefully check other typos in the manuscript.

 

Author Response

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Reviewer 3 Report

In this submitted manuscript #Photonics-2049137, the authors present a novel continuous varifocal metalens based on the double-layer square Ge and circular GSSe platform. Besides, they achieve dynamic focusing based on the GSSe state change from crystalline to amorphous gradually and the focusing results are given and analyzed clearly. This interesting and excellent work, in my opinion, can be published after the following concerns and comments are addressed and responded properly.

1.     This is a double-layer all-dielectric metalens and bifocal function is clearly given in Fig. 1(d, e) and the focusing results in Fig. 3(a, d) under X-LP incident with different GSSe state. However, the physical mechanism of bifocal focusing is confusing since there is only one phase profile in (1) and the other is missing. Besides, which layer (upper or lower dielectric layer) control each focal length F_L and F_R individually is not clearly given at all.

2.     There is only transmission magnitude change in Fig.2 for Ge layer with different width and length parameters. How about the phase change? Besides, the other layer GSSs parameters change should also added and complemented in the manuscript.

3.     The operation wavelength should be given. What is the total thickness of this bifocal metalens, which should compared it to the state-of-art ones and state its advantages?

4.     It seems that the tunable focusing intensity is changed largely but the focal length is slightly tuned under different polarized light incidence. Could the authors further discuss how to increase the focal length variation range in the double-layer structure? There are some recent researches concerning the huge wavelength-dependent focal length variation by changing dielectric thickness or metasurface layer in low spectrum (Optics and Lasers in Engineering, 147, 106734 - December (2021), https://doi.org/10.1016/j.optlaseng.2021.106734)  and (Applied Physics Express Volume 15, No 1 (2022): 014003.) (https://iopscience.iop.org/article/10.35848/1882-0786/ac4534/meta). The two papers could be included in the manuscript and add some proper comments on that.

 

Author Response

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Author Response File: Author Response.docx

Round 2

Reviewer 2 Report

The authors have addressed all my concerns and I recommend its publication now.

Reviewer 3 Report

The authors responde to the comments very well and I suggest publish.