Mode Selection Method in Spherical Optical Cavities with Thin Metal Film
Round 1
Reviewer 1 Report
This paper describes a new method for selection of the radial modes of a spherical microresonator by the quality factor, based on the effect of absorption in a thin metal film deposited on a sphere together with dielectric layer, is proposed. The concept is quite novel and the work presented is computational results. From an experimental point of view, the thin-film coating and optical position of the thin film may pose huge challenges. The authors need to highlight the fabrication challenges in terms of the impact of the film uniformity and placement on the performance. The coating of sub10 nm films in a consistent and reproducible manner is still a technical challenge.
Further, it would help the readers if schematic drawings of the structure is presented to aid their understanding of the problem.
Author Response
Dear, reviewer
We thank you for helpful comments!
We tried to take into account all advises.
Below are our explanations to the questions.
Corrections in the manuscript are marked with yellow background color.
This paper describes a new method for selection of the radial modes of a spherical microresonator by the quality factor, based on the effect of absorption in a thin metal film deposited on a sphere together with dielectric layer, is proposed. The concept is quite novel and the work presented is computational results. From an experimental point of view, the thin-film coating and optical position of the thin film may pose huge challenges.
The authors need to highlight the fabrication challenges in terms of the impact of the film uniformity and placement on the performance. The coating of sub10 nm films in a consistent and reproducible manner is still a technical challenge.
It is unclear exactly what the reviewer's question concerns. If it mean the coating is applied with the specified characteristics, then it is really very difficult and practically unrealizable. But if the task is to apply any sufficiently absorbing thin up to 10 nm metal film, then reproduciblility can be achieved by vacuum magnetron sputtering. As our practice shows, it is possible to achieve repeatability of the result with an accuracy of up to 5% of thickness by precisely controlling the pressure, current, voltage and discharge frequency. It is possible to have very low deposition speeds of up to several nanometers per minute and rapidly rotate the sample over the magnetron. Therefore, it is possible to accurately control the thickness of the coating. This, in our opinion, may be suitable for the experimental implementation of the presented method.
In connection with remarks, the following text has been added to the Discussion section (starting with line 296):
«However, here several technological problems arise, that need to be overcame. The high reproducibility of thin metal film formation with desired optical properties and dielectric layer are among them. We suppose, that this could be done using magnetron sputtering with low deposition rate (down to nm/min) and rapid sphere rotation. Moreover, the curvature radius of microsphere should be constant along the propagation direction in order to prevent the drift of node’s position. Otherwise, it will lead to decrease of Q-factor. Another problem may be related to the quality of the dielectric coating, but this technological problem can also be solved by a suitable method of deposition.»
Further, it would help the readers if schematic drawings of the structure is presented to aid their understanding of the problem.
A schematic representation of the selection method (Fig. 1) is added in Introduction, and Figure 2 is redrawn in color as Figure 3 in corrected manuscript.
Authors.
Reviewer 2 Report
Please add some 3D figures of the resonator, and E field plots, to help visualization.
please add the formula for the Q factor calucaltion.
you can add comsol figure showing simulation domain, mesh and PMLs.
people have tried D shaped cavity to reduce the number of modes in the resonantor, please comment on the D shaped cavity for mode selection. ( or combined with thin film on the surface).
Author Response
Dear, reviewer
We thank you for helpful comments! We tried to take into account all advises. Below are our explanations to the questions.
Corrections in the manuscript are marked with yellow background color.
Please add some 3D figures of the resonator, and E field plots, to help visualization.
In our opinion, a two-dimensional drawing is more informative than a three-dimensional one. A schematic representation of the selection method (Fig. 1) is added in Introduction, and Figure 2 is redrawn in color as Figure 3 in corrected manuscript.
please add the formula for the Q factor calucaltion.
A description has been added to the Materials and Methods section (starting with line 174):
«Q-factor is the relation of energy accumulated in the cavity to the round-trip losses multiplied by angular frequency. Losses in the cavity can have different nature: radiative losses, Rayleigh scattering, material absorption and others. Most of them are negligibly small. The total Q-factor is determined as , where (i = 1, 2, 3...) state for a single loss source. The COMSOL estimates Q-factor using the relation , where ω - eigenfrequency and δ - damping in time, obtained from numerical solution [24].»
you can add comsol figure showing simulation domain, mesh and PMLs.
Due to the large number of grid elements (over 150000), it is unclear how to depict them all in the figure. However, Figure 2 is redrawn in color as Figure 3 in corrected manuscript, that should be more illustrative.
people have tried D shaped cavity to reduce the number of modes in the resonantor, please comment on the D shaped cavity for mode selection. ( or combined with thin film on the surface).
We ask the reviewer to point directly to the publication, since this method is unknown to us, and we also could not find it.
Authors.
Reviewer 3 Report
The authors presented a method of radial mode selection of spherical microresonators. The results show that the selected mode can achieve a high quality factor at 10^7, surpassing other radial modes. The authors discussed the calculation method in detail, which can inspire the development of microresonators.
However, in order to meet the standard of publication of Photonics journal, I do recommend revising the manuscript based on the comments below.
1. Could the authors discuss the definition of the quality factor in the Introduction or the Method section, as well as what impacts the quality factor. This serves as a fundamental of all discussion of this work, and can justify how this work benefits the broad research community?
2. In Figure 2, the authors only described region 1/2/3/4. Region 5 is listed in the figure, but not mentioned in the caption or main text.
3. It'd be beneficial to show a schematic of the structure overlapping with EM waves/field for better illustration.
4. Would the authors consider experimental characterization as the next step or future study? If not, what would be the challenges?
English grammar is generally okay. However, there are many long sentences in the manuscript which the authors should consider breaking them out for easier understanding.
Author Response
Dear, reviewer
We thank you for helpful comments! We tried to take into account all advises. Below are our explanations to the questions.
Corrections in the manuscript are marked with yellow background color.
The authors presented a method of radial mode selection of spherical microresonators. The results show that the selected mode can achieve a high quality factor at 10^7, surpassing other radial modes. The authors discussed the calculation method in detail, which can inspire the development of microresonators.
However, in order to meet the standard of publication of Photonics journal, I do recommend revising the manuscript based on the comments below.
- Could the authors discuss the definition of the quality factor in the Introduction or the Method section, as well as what impacts the quality factor. This serves as a fundamental of all discussion of this work, and can justify how this work benefits the broad research community?
The description has been added to the Materials and Methods section (starting with line 174):
«Q-factor is the relation of energy accumulated in the cavity to the round-trip losses multiplied by angular frequency. Losses in the cavity can have different nature: radiative losses, Rayleigh scattering, material absorption and others. Most of them are negligibly small. The total Q-factor is determined as , where (i = 1, 2, 3...) state for a single loss source. The COMSOL estimates Q-factor using the relation , where ω - eigenfrequency and δ - damping in time, obtained from numerical solution [24].»
- In Figure 2, the authors only described region 1/2/3/4. Region 5 is listed in the figure, but not mentioned in the caption or main text.
The figure number was changed to 3 and repicted in color. Descriptions have been moved to the figure. Please, see the corrected manuscript.
- It'd be beneficial to show a schematic of the structure overlapping with EM waves/field for better illustration.
A schematic representation of the selection method (Fig. 1) is added in Introduction, and Figure 2 is redrawn in color as Figure 3 in corrected manuscript.
- Would the authors consider experimental characterization as the next step or future study? If not, what would be the challenges?
The practical implementation of this method is planned to be held using vacuum evaporation to create a temperature or magnetic field sensor (since the metal film can be heated by the alternating magnetic field). In addition, this topic may be developed towards the selection of the main mode of the microsphere, but in this case it will be necessary to use structured metal film.
In connection with this remark, the followin text has been added to the Discussion section (starting with line 305):
«The proposed structure can be used as high-sensitivity sensor. For example, the metal film can absorb the external radiation, thus heating the whole microsphere and changing the propagation condition. Similar effect will arise in the external alternating magnetic field because of surface currents in the thin film.»
Authors.
