High-Efficiency Broadband Grating Couplers for Silicon Hybrid Plasmonic Waveguides
, Weijie Liu 2, Yuheng Zhang 1, Shaojie Yin 3, Daoxin Dai 1,3, Shiming Gao 1,4 and Xiaowei Guan 1,3,*Round 1
Reviewer 1 Report
The authors demonstrate numerically highly efficient grating coupler for 3 different HPWs, with apodized grating design and two-stage-tapers, the manuscript is overall good, and instructive, but deeper insights into these theoretical design are missing, for example, what could be the possible maximum coupling efficiency? what is the trade-off between the most important parameters, i.e., efficiency, bandwidth, size... I recommend if possible, the authors may make a table, list all parameters along with pros and cons to show it more clearer to the readers. Please see my comments below.
(1) Why the etching depth in the uniform and apodized grating is set to be 167 nm?
(2) I would suggest the authors to elaborate where the power goes, for a better comparison between the uniform and apodized grating. for example, in the uniform grating, at 1550 nm, power to the SMF is 73%, then where is the other 27%? How much goes to absorption loss from the HPW, and how much is due to the mode mismatch between the scattering field and the fiber single mode, etc. By doing this, it would be very clear about what the theoretical maximum coupling efficiency could be.
(3) In the simulation models in figure 1, should there also be a silica superstrate as a cover layer? Also, did the authors include a index-matched UV polymer between the fiber and the on-chip silicon waveguide, it is of course always exist in the real packaging. Of course, the silica cover layer and the UV polymer would be also influencing the CE. Also, think more about possible experimental configurations for Fig.4 and 6 cases.
(4) Why the thickness of the Si, SiO2 and silver layers are set to be 320 nm, 95 nm and 100 nm for the silicon HPWs with a bottom metal case, and then 250 nm, 60 nm and 94 nm for the silicon HPWs with a metal cap case?
Author Response
Please see the attachment.
Author Response File: 
Author Response.docx
Reviewer 2 Report
The paper under consideration provides the numerical study of the grating couplers for the hybrid plasmonic waveguide structures. The idea is quite new and well executed. The author has discussed several configurations of the HPWG such as metal on bottom and metal on top. Moreover, etchless structure has also been presented. I am quite satisfied with the performance of the paper. However, I have few queries, which need to be addressed before the final acceptance of the paper:
1) In figure 5, the shade of gray color should be changed. It is difficult to distinguish them from one another. Better to use different colors.
2) In the Introduction section, the applications of HPWGs should be discussed in detail with proper referencing.
3) The author has mentioned, that the cladding layer has been used. Why the cladding layer is not visible in the images? Please draw it and mention the thickness of the cladding layer.
4) The paper contains English grammar errors which needs to be rectified by consulting it with a native English speaker.
5) Is it TM0 mode or TM00 mode? Please confirm.
6) I am not sure, if in figure 8 the fabrication tolerance of bottom metal design of HPWG should be discussed or the meta top HPWG. I suggest to also add the fabrication tolerance of top metal HPWG design as the author has suggested the advantages of having metal top design.
Author Response
Please see the attachment.
Author Response File: Author Response.docx
Round 2
Reviewer 2 Report
I am willing to accept the paper in its current form.
