Peer review of: Stimulated plasmon polariton scattering (2020)

Reviewers' comments: 
Reviewer #1 (Remarks to the Author): 

Taking advantage of the concept of Brillouin scattering in opto-mechanics, Wolff and Mortensen 
introduce an analog in the context of 2D polaritons. They dub it "stimulated plasmon polariton 
scattering" (SPPS) and present the principle of this mechanism, which is expected to be useful in 
measuring dispersion and lifetimes of polaritons in 2D materials in a metallic-tip-free setup, but 
also for a variety of applications. 

The paper is well written, figures are well described. The results are fully numerical (COMSOL): 
I therefore have no way to check whether they are right or wrong by myself, but they seem to be 
sound and free of macroscopic mistakes. 
I found some typos (see for example definition of Fermi energy, once called E_F and another time 
called E_f, etc) here and there. Please proofread more carefully. 

The idea is intriguing and may really work out provided that certain experimental difficulties in 
fabricating devices are surpassed. I believe that this work should be published in Nature 
Communications provided that the authors take care of the following two concerns: 

1) As a theorist, I'd like to see a small Supplementary Information file, where a brief derivation of 
the coupled nonlinear equations (1a)-(1c) is presented. I understand the authors want to postpone 
the presentation of a detailed derivation to a long technical paper, as they state in page 3, left 
column, but I find this a bit odd. 
A small Supplementary File of one page, say, will not harm their long technical paper and will be 
useful to the readers of the present Letter, who are curious about the assumptions and 
approximations that lead to Eqs. (1a)-(1c). 

2) As discussed by the authors, nonlinearities are crucial ion the physics of SPPS, and are 
parametrized via the nonlinear second-order susceptibility tensor in Eq. (2). 

In this respect, I have two remarks. 

2a) One is merely bibliographic. 
Ways of launching and detecting graphene plasmons, which do not rely on SNOM, have been 
recently discussed in the following papers, which should be cited: 

- X. Yao, M. Tokman, and A. Belyanin, Phys. Rev. Lett. 112, 055501 (2014) 
- T. J. Constant, S. M. Hornett, D. E. Chang, and E. Hendry, Nat. Phys. 12, 124 (2016) 
- H. Rostami, M.I. Katsnelson, and M. Polini, Phys. Rev. B 95, 035416 (2017) 

These rely also on a second-order nonlinear process intrinsic to graphene and parametrized via a 
nonlinear second-order susceptibility tensor. 
Since the plasmon carries a finite in-plane momentum q, graphene's inversion symmetry is 
effectively broken by q 
and this leads to a finite second-order susceptibility tensor. 

2b) The second remark is merely a curiosity. 
I was wondering whether the authors have included such intrinsic nonlinearities in their nonlinear 
second-order susceptibility tensor, or whether that would lead to "double counting" and therefore 
should not be included in the right-hand side of Eq. (2). 
This is just a personal curiosity, which I can't answer by myself, because I have not seen a 
derivation of Eqs. (1a)-(1c). 
No changes are requested in the revised manuscript in this second respect. 

In summary, I strongly believe that the present manuscript should be published immediately in 
Nature Communications, provided that the authors have taken care of 1) and 2a) above. 



Reviewer #2 (Remarks to the Author): 

In their manuscript entitled “Stimulated plasmon polariton scattering” the authors C. Wolff and N. 
A. Mortensen draw an analogy between plasmons and acoustic phonons as they propose a new 
strategy for enhancing and tailoring plasmon-polariton nonlinearities in guided-wave systems. I’m 
grateful that the authors do not sweep any of the nontrivial aspects of the system aside. However, 
the experimental feasibility of proposed interactions come with numerous challenges. That said, 
the authors are putting forward an interesting proposal, and to my knowledge, the ideas proposed 
are new. 

For these reasons I am not opposed further consideration by nature communications provided that 
they address several queries below: 

1) Following equation 3, the authors state that “ the derivation and detailed discussion of these 
equations is beyond the scope of this letter will be published separately”. It seems to me that this 
derivation and discussion are essential to this paper. I understand the desire to keep this 
publication short, on the other hand, I think it essential to include this content in a supplement. 
Otherwise, the paper seems incomplete. 

2) As for the experimental relevance of this proposal, it is unclear that slot waveguides of the type 
proposed offer a viable experimental solution. To date, slot waveguide systems suffer from 
extraordinarily high linear losses, because the same confinement mechanism also produces strong 
overlap with roughness and surface imperfections that degrade the optical performance of these 
waveguides. 

3) A nontrivial aspect of this experimental proposal that has not been addressed is heat 
dissipation. If this coupling mechanism is feasible, it is unclear whether or not lose 2D materials 
will remain stable as rather large amounts of optical power are absorbed into atomically thick 
layers. It seems that a rudimentary thermal model is necessary to address this problem 

Some minor comments and corrections follow: 

In figure 2 the authors state that the “pink and green shaded areas indicate whether the pump can 
or cannot drive independent transitions”. The statement is ambiguous please clarify. 

The authors state that "dielectric loading of the polaritonic mode is quite beneficial” on page 5. The 
intended meaning is unclear. 



Reviewer #3 (Remarks to the Author): 

I do not recommend this manuscript for publication since it contains neither experimental 
confirmation that the proposed by authors device does work, nor a careful, detailed, 
comprehensive and – most importantly – verifiable theory. The authors reports that the 
development of a detailed theory “is beyond the scope of this Letter” but then it does not make 
sense to publish this Letter.