Coupling of Integrated Waveguide and Optomechanic Cavity for Microwave Phonon Excitation in Si Nanobeams

Round 1

Reviewer 1 Report

A strong interaction of light with the mechanical degrees of freedom of a dielectric object is one of the key features in optomechanics and used to develop resonators with large cooperativity. PhoXonic crystal cavity belongs to this class and it is an evolution of zipper optomechanical cavity (from Oskar Painter group).  PhoXonic crystals realise a full 1D phononic bandgap by adding a series of stubs to the array of circular holes and can be fabricated by planar silicon-chip technology. 

In this work, Authors present the optical design of a PhoXonic resonator coupled to a tapered Bragg mirror waveguide. They show a configuration that increases the optical Q factor when transversal size of the OM cavity has been parabolically tapered. This configuration is compared to the reference (no-tapered) configuration known in the literature. They optimise two geometrical parameters (in plane distances) with the aim to increase the OM coupling without leaking e.m. energy from the cavity volume. 

The topic described in the paper is within Journal scope, and interesting in the field of integrated photonics and optomechanics. The article is well-written and the methodology for the analysis (full-wave FEM analysis) is rigorous and detailed. Numerical issues that could  affect the accuracy of the analysis (mesh refinements, PML boundary conditions, excitation waveport) are fully discussed. To my view, the Article is ready to be published after minor changes. 

I would suggest to enrich the introduction with a small discussion on OM PhoXonic cavities in the context of communication networks. For instance, in reference [1] it is shown how the mechanical interaction between two OM PhoXonic cavity that can be exploited for syncronization purpose to build-up  future OMC networks.   

In case of a tapered OMC, please find a general explanation on how stub size and their distribution across the beam's length can affect mechanical performances (mechanical Q-factor). An example on how 1D tapered nano-string endowed with stubs changes the mechanical properties can be found in [2]. 

Finally, to better follow the discussion in section 2 and 4,  can Authors include the 3D image of the phononic modes in Fig.4 and Fig.7 ?

There are also some misspelling/typos, please check:  

Line 215 - band gap of the pariodic crystal

Line 233 -  was accounted for in order 

[1] Synchronization of Optomechanical Nanobeams by Mechanical Interaction

M. F. Colombano et al. Phys. Rev. Lett. 123, 017402 – Published 1 July 2019

[2] Elastic strain engineering for ultralow mechanical dissipation

A. H. Ghadimi et al.  Science  18 May 2018, Vol. 360, Issue 6390, pp. 764-768

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Reviewer 2 Report

In this article, the authors propose an optomechanical device consisting on integrated optical waveguides together with a suspended nanobeam where a resonant cavity has been patterned. The study is relevant in the optomechanics field, assisting experimentalist to implement improved devices. Based on finite element method simultations, they study the impact of geometrical parameters on the optical coupling between the wavewide and the suspended nanobeam, the optical quality factor of the modes suported by the suspended nanobeam, and the optomechanical coupling existing in between the optical and the mechanical modes. The work is novel, properly written and presented. I recommend it to be published, subject to minor corrections detailed bellow:

- In the abstact, authors highlight the importance of these devices on the sensing field. However, they do not mention it during the introduction. I miss it.  I propose to include a sentence about the importance of OM in the sensing field, and particularly, biological sensing in the introduction. It could be include in the sentence of the 1st parragraph "current research trends on OM structures include topics..." or just after it. Please also include some reference on this issue.

-Figure 1: It is not clear what does it means 0, I think it should be N, but not sure. How many stubs and holes has the complete structure?. In addition, number of the holes conforming the integrated waveguide are not reported, neither their sizes and pitches. The same for the waveguide thickness and width.

-Table 1 reports the values of the geometrical parameteres of the cavity. These parameters are not defined in any figure until Fig. 12.a. I suggest to include a description of this parameters in figure 1. The same problem occurs for table 2, and it could be solved modifying figure 5.

-The nomenclature of the parameters representing the dimentions should be more clear. W use to be used for representing the width of an structure, in this article it represents a longitudinal dimention. I propose to change the letter representing the longitudinal shift all along the article. L0 or X0 would be more clear.

-Figure 3: The inset of the figure should show the meaning of solid, dashed and points lines.

-Figure 10 summarizes the optical features of optimized uniform and optimized tapered cavities. Some of the parameters used, such as h, are not shown.

-Along the conclusions, a comparison between the structure propose by the researchers with the state of the art ones is missed. 

Author Response

Please see the attachment.

Author Response File: Author Response.pdf