Increasing the Quality Factor (Q) of 1D Photonic Crystal Cavity with an End Loop-Mirror

Round 1

Reviewer 1 Report

The authors describes the modeling, fabrication and characterization of 1D photonic crystal (PC) cavity whose quality factor can be improved by reinjecting the waveguide transmitted light by an end-loop mirror.

The employ of a reflector at the end of the PC waveguide significantly enhance the cavity quality factor. The choice of the end-loop mirror should be motivated with respect to Bragg mirrors or a longer (more holes) PC waveguide (that can increase the Q-factor as well, as the authors noticed also in their calculations). On the other end, the author should also comment about the chosen cavity geometry with respect to double taper PC waveguides, also in terms of modal volume.

Besides the necessary extensive editing of English grammar and vocabulary, the work needs to be improved in term of article organization, result presentation and reference choice. Here below, a not-exhaustive list of concerns to be addressed in order to make the article considered for publication:

1. Page 1, line 31-32: this sentence is crucial for the motivation of the work but it is not clear and references should be added.
2. At the end of the introduction, the fabrication detail should be removed.
3. Page 3, line 101: The Q-factor monitor should better explained. Can the authors also detail why they preferred to use the Lumerical calculator instead of extracting the cavity resonance Q-factor from the transmission spectrum resonance wavelength and FWHM?
4. Can the author comment on the choice of the PC parameter? Do they refer to some previous work or they explored a wider range of hole diameters, pitches and numbers?
5. Page 4: It would be interesting for the reader to have some further detail on how measuring the back transmitted light into the waveguide neglecting the injected light?
6. Figure 4: the two graphs need an inset with the resonance peak to appreciate their with and shape. Fig 4b: I would use positive values for transmittance. Moreover the spectrum has strange features both in the stop band and at the band edges. The author should investigate and explain such behavior.
7. Figure 5: the authors reports in the caption that the figure is the electric field map. It looks like an electric field intensity (or squared electric field) instead. Moreover, it would be interesting to add the intensity distribution for the waveguide without the end-loop mirror for comparison.
8. Figure 8: the Q-factor estimation is missing. This is fundamental for the comparison of the obtained results with the calculation. It would be relevant also to include the experimentally measured transmission of a waveguide without the end-loop mirror.
9. References such as [Laser & Photonics Reviews (2020): 2000317; Appl. Lett. 94, 121106 (2009)] should be added.

Author Response

Please see the attachment. Thanks.

 

Author Response File: Author Response.pdf

Reviewer 2 Report

In this paper, the authors proposed to use an end loop-mirror to enhance the quality factor of 1D photonic crystal cavity. I found this paper is unmatured and their discussion is not convincing. Below are my comments:

1. In practical devices, the added loop mirror will add more loss to the whole system, which was not considered in the simulations.
2. The best Q-factor (33808) reported in their paper is much smaller than the Q-factor of 750000 reported in a well-known study almost 12 years ago [Appl. Phys. Lett. 94, 121106 (2009)], which the authors didn’t cite.
3. The authors didn’t mention what is the Q factor of their experimental device, which seems not impressive. They also need to fabricate and measure a device without an end loop-mirror to experimentally prove the end loop-mirror actually enhances the Q factor.
4. The device with two grating couplers used by the authors is more complex to fabricate and has a very large footprint. Moreover, as the authors also mentioned, the insertion loss times 2 in their device and Y-branch leads to an obvious drop in the transmission. All these make their device less favorable for practical applications.
5. The simulation results indicate that samples with a hole radius of 50 nm possess better Q factors, but why the authors fabricated the sample with a hole radius of 70 nm inside of 50 nm?
6. The quality of figures is not good and there are many typos and grammatical mistakes in this paper, so I suggest the authors polish this paper more.

Author Response

Please see the attachment. Thanks.

Author Response File: Author Response.pdf

Reviewer 3 Report

This is a good technical paper. It is not a breakthrough but it certainly deserves publication. The introduction provides sufficient background of the subject and quotes relevant works. Modeling was done with commercial software. Experimental verification was outsourced. It confirms calculations.

Author Response

Please see the attachment. Thanks.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

The authors partially reply to the reviewer concerns but they did not improve the English grammar and vocabulary and a careful investigation of the fundamental properties of the proposed design is still missing.

Here below, a list of the modification that should be introduced to consider this work for publication:

• Abstract, line 11: light transmission-> transmitted light
• Abstract, line 17-19: this sentence should be moved before the previous one, explaining that the calculated design has been fabricated by EBL in SOI and a Y branch has been used to extract the cavity transmission properties.
• Introduction, line 27: the subject of the sentence is missing.
• Introduction, line 32, delete “on chip optical devices’ important”
• Introduction, line 47: change “used techniques like” -> exploit
• Introduction, line 49: change “contra” -> contrast
• Introduction, line 52: change “in the past is the” -> that.
• Introduction, line 56-60: too long sentence
• Line 90-92: change “which is more useful for final design calculation for device fabrication. It”- > and it ; delete “in 2D condition”
• Line 94 (adapt this choice in the whole text): don’t use future tense for already done simulations/fabrication/characterization
• Line 110: there is no formula in the text
• Line 127: ignored-> not considered
• Line 129: if the Q factor is higher, obviously the transmission is lower in ideal cases. The authors should better explain the problem highlighting as in high quality factor cavities is also fundamental to extract cavity resonant light. This can also be done with directional couplers. In their research they decide to improve the reflectivity of one of the two mirror of the cavity.
• Line 148: delete “will monitor the reflected light from the ELM”->towards the PC cavity
• Line 154: can you discuss more about the motivation of the enhancement of the Q-factor due to the ELM?
• Figure 4: Transmission cannot be negative. Moreover, I understand that is important to show the resonant peak within the whole stop band but for the reader would be also interesting to appreciate the effect of the ELM on the resonance shape. I encourage the author to show a zoom of the peak as inset or additional panel. Line 167: In particular, did you observed a Fano shape of the resonance? Can you comment on the transmission modulation out of the stop band? what is connected to?
• It would be interesting to compare the "back transmission" with and without the ELM to check if the baseline of the spectra in those cases are comparable and which is the contribute of the multiple reflections.
• Line 190: simulation-> calculations; profile-> map (to be changed in the whole discussion)
• It is not clear why the field is always positive if you are not plotting the norm of the electric field or its absolute value.
• To appreciate the effective propagation within the ELM, a map of the electric field intensity out of cavity resonance should be added, also to evaluate eventual ELM losses.
• Figure 7: add the scale bar in the inset.
• Which numerical design does the experimental photonic chip correspond to?
• Line 234: “by comparing the FWHM” what does it mean? How the authors evaluate the experimental Q factor?
• Conclusions, line 252: change “increases”-> increase

Author Response

Dear Prof./Dr.,

Please look at the attachment for our responses.

Thank you.

Author Response File: Author Response.pdf

Reviewer 2 Report

The authors successfully answered some of my comments, but I still think there are significant experimental data and information missing in their paper. For instance, in order to verify whether ELM actually enhances the Q factor of 1D PhC experimentally, they should at least prepare one 1D PhC without ELM having the same parameters as the one with ELM for comparison. In addition, For the quality of Figures, I didn’t mean the resolution of Figures while referring to the styles, labels, captions, and etc. For example, captions in Figure 4 are a little too small and I also suggest the authors add spectra from Figure 4 (a) and (b) with the wavelength range zoom in at 1.55um for readers to see a better comparison of Q factor between them.

Author Response

Dear Prof./Dr.,

Please look at the attachment for our responses.

Thank you.

Author Response File: Author Response.pdf