Optical Properties of Magnetic Monopole Excitons

Round 1

Reviewer 1 Report

In the manuscript “Optical properties of magnetic monopole excitons,” the authors propose creating a magnetic monopole via matter excitations. Specifically, they propose exciton-like complexes and study their spectrum using a simplified model where they solve the Lorentz-driven damped harmonic oscillator. Moreover, they propose coupling the magnetic monopoles to light to create polariton-like structures and analyze the eigenmodes of a coupled oscillator model.

Generally speaking, I find the idea novel and very interesting, and the calculations are rigorously done. However, I believe the proposal needs stronger support and motivation, particularly about the systems where these excitations can be actually measured and manipulated. Hence, I would recommend publication in the Condensed Matter journal as a regular article once the authors address the following points.

1. The authors should make explicit the necessary conditions for the monopole and anti-monopole emergence in condensed matter systems. This includes mentioning the platforms where the effect could be observable. Would they be a many-body or single-particle effect?

2. I suggest the authors expand the conclusions and explain the advantages of creating this kind of excitation, as the energy spectrum of both the exciton-like magnetic monopole structure and the polariton resemble the standard exciton and exciton-polariton spectra, respectively. What is new beyond creating a monopole in matter? Is there an advantage in the tunability of the magnetic parameters?

3. Exciton-polaritons emerge as a result of the strong light-matter problem. This is possible only in systems with smaller decay rates than light-matter interaction. The authors should explain the microscopic mechanism to couple the monopole excitations to light in a strong coupling regime. 

Also, I have some comments on style.

4. From Eq. 3 onwards, it is necessary to be consistent with the vector notation. 

5. Why are the authors not simplifying constants and factors in equations 12 to 19?

6. Finally, the manuscript would benefit from an in-deep style and English language revision. There are some typos like “magentic" in lines 27 and 194. 

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Reviewer 2 Report

The authors report on establishing the concept of magnetic monopole exciton-polariton. The magnetic susceptibility of a magnetic monopole under an AC magnetic field has been derived, using the damped driven oscillator model. To model the magnetic monopole in a periodic lattice, the authors have solved the Schrodinger equation of an elementary magnetic monopole in a periodic triangle potential field. Finally, a monopole polariton has been described by modeling the coupling between the magnetic monopole and a cavity mode.

The work is well organized and well presented. The analytical derivation is rigorous, systematic and clear.  However, my attention has been attracted to the last section of the manuscript, where the coupling between the exciton and a microcavity was analyzed by the authors. The coupling has been analyzed with analogy to the coupling occurring between an electric exciton and a cavity mode. The coupling is realized through a splitting in the cavity mode which is observed in the dispersion of the coupled system.

The analogy with the coupling between electronic exciton and cavity or even between vibrations and optical cavity is very interesting. However, to complete this analogy, one expects to see more extensive discussion around the last section of the manuscript. This discussion might address the energy exchange issue that normally occurs in electronic light-matter strong coupling. How is this exchange realized in the present interaction (cavity-magnetic exciton)?  Moreover, what are the key parameters that guarantee achieving a strong-coupling regime? How the authors exactly defined the strong coupling regime and how they draw the borders between strong and weak coupling regime? And importantly, how do the authors see the experiment in which one can examine the new theory? I think that this is very helpful if there is a purpose to experimentally validate the main ideas of the present work.  These are some of the questions that may raise due to this analogy. I am pretty sure that the authors can see the whole picture once they look at the interaction between matter and light from this point of view. In recent years, the strong coupling regime has gained a lot of interest, in part due to the evidence that this coupling modifies the properties of materials. For example, putting atoms or molecules inside an optical cavity might change the basic properties of these atoms and molecules. There is even a new research line, called polaritonic chemistry, that recently emerges where has been shown that light-matter interaction in the strong coupling regime might modify the chemistry. Therefore, I wish, probably like many other readers, to see more consideration of these aspects.

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

In the current version manuscript, “Optical properties of magnetic monopole excitons,” the authors have successfully addressed the reviewers’ comments and observations, improving the motivations and perspectives of the work. I believe it constitutes a relevant addition to the state-of-the-art of current research on novel quasiparticles and polariton-like structures in condensed matter systems. Hence, I recommend publication in the Condensed Matter journal as a regular article.

Reviewer 2 Report