Simulation of Photonic-Crystal Surface-Emitting Lasers with Air-Hole and Air-Pillar Structures

Round 1

Reviewer 1 Report

Feedback for authors:
In the paper by Zi-Xian Yang et al. entitled "Simulation of Photonic-Crystal Surface-Emitting Lasers with Air-Hole and Air-Pillar Structures", the authors theoretically compared the two types of PCSEL by using 3D CWT which is powerful method to consider the behavior of the lasing mode in the PCSEL, and showed the impressive result that their Air-Pillar structure which is intensively studied in the author's university for years shows the superior mode stability. As the authors pointed out, the PCSEL have attracted much attention recently due the promissing applications such as LiDAR or material processing. For the purpose, the mode stability even in large area is severely required which results in high power and high beam quality. In this point of view, this paper provide timely and useful insight to the reader. In particular, their air-pillar structure is simple in fabrication without regrowth process which is required in the air-hole sturcture so that it might draw the interest of the reader in the field not only the nanophotonics but also wide fields such as semiconductor lasers. 
Meanwhile, as discussed in Noda's group (S. Iwahashi, et al., "Air-hole design in a vertical direction for high-power two-dimensional photonic-crystal surface emitting lasers", J. Opt. Soc. Am. B, 27, 1204-1207 (2010).), height of the Air-pillar must severely affects the  lasing behavior including slope efficiency due to veritcal interference in the PC, although it is not mentioned. In addition, the double hole PC structure in this paper gives strange impression because the hole spacing and sizes are completely different from that in ref. [5]. As discussed in ref. [5], the hole spacing should be carefully designed around a/4 as well as the size and depth of holes to obtained the benefit of the double hole PC. In this paper, the hole spacing is a/3 and the double hole PC shows inferior mode stability to RIT PC as shown in fig. 7 and  9 which gives the strange impression.
Some modifications should be required to improve the quality of this work.

My concerns are as follows: 
(1) As aforementioned, the authours should clarify the dependecy of height of theri Air-pillar structure.
(2) As aforementioned, the authours should comment on the reason why the hole spacing a/3 and sizes are different from that in ref. [5].
(3) In bottom of the pages 6, the authours mentioned "The air-pillar PC-SEL can achieve stable single-mode resonance over emissions area about twice larger than air-hole PC-SEL.", and showed the results in fig. 7 without any discussion. Is it considered as following? In-plane loss of the Air-piller PC is grater than that of Air-hole PC due to the lower in-plane coupling coefficients caused by the lower confinement factor in PC. As the result, the modal power loss of each mode become larger and the differnce of modal loss between lowest and next lower modes become larger. Perhaps, the vertical interference in PC might contribute the result. The authours should comment on it. 

Minor comments
(4) The wavelength is not shown in this paper. Is it 960 nm same as ref. [18]?
(5) In line 9 at introduction in page 1, "dell" should be "cell".
(6) In section 3.1 in page 3, the notation of vertical profile in the text is large letter while that in eq. (2) is small letter. The notation should be unified.

Author Response

Attached please find our response.

Author Response File: Author Response.pdf

Reviewer 2 Report

The authors report on simulated results of photonic-crystal surface-emitting lasers (PC-SELs) using a 3D coupled-wave theory. Characteristics of air-hole and air-pillar PC-SELs having a triangular or circular PC are discussed. The paper is well organized and conclusions are supported by the simulated results. Although the paper may be published as is, it would be better to comment on a thermal conductance of the air-pillar structure compared with the air-hole structure.

Author Response

Thanks for your encouraging comments. We add the following paragraph in the fourth section of discussions.

Regarding to electrical and thermal conductance, we believe that air-pillar PC-SEL is inferior to air-hole PC-SEL. The air pillars reduce the semiconductor/ITO contact area where current to be injected as well as heat to be dissipated; moreover, the electrical and thermal conductivity of ITO is poorer than that of GaAs where air holes are embedded. To improve its electrical and thermal properties, we propose to fabricate substrate-emitting version of air-pillar PC-SEL. The window contact is replaced by p-electrode which facilitates not only uniform current spreading but also die bonding on thermally conductive submount.

Reviewer 3 Report

photonics-1222207 

authors:Zi-Xian Yang , Chia-Yu Kuo , Gray Lin

Title: Simulation of Photonic-Crystal Surface-Emitting Lasers with Air-Hole and Air-Pillar Structures 

Round 1

The authors study photonic-crystal surface-emitting lasers (PC-SELs) theoretically, simplified as the air-hole and air-pillar structures, respectively. Square-latticed air-hole and air-pillar PC-SELs are simulated by three-dimensional coupled-wave theory model and design guideline is illustrated with PC basis of right isosceles triangular and double circular shapes. 

1) I believe the manuscript is extremely short on references, I suggest that
the authors broden their research background a little with respect to the
following papers.

Phys. Rev. Lett. 53, 2169 (1984)
Phys. Rev. Lett. 58, 2486 (1987)
Phys. Rev. Lett. 61, 1214 (1988)
Physical Review B 61 (4), R2389 (1998)
Nature Mater. 11, 997–999 (2012)
Chem. Mater. 2001, 13, 12, 4486–4499 (2001)
Nature 430 (7000), 654-657 (2004)
Science 281 (5378), 802-804 (1998)
Nature 430 (7000), 654-657 (2004)
J Phys Chem B 109(20):9980-8 (2005)
Appl. Sci. 9 (12), 2477 (2019)
Appl. Sci. 10 (5), 1836 (2020)
ACS Photonics 3 (9), 1575-1580 (2016)
Tunable laser applications, 197-226 (2008)

2) As I understand, this paper is meant to be a theory paper. As such I am
missing the comparison to state of the art methods. Apart from the wave
equation extremely little is shown what can be peer-reviewed. The wave
equation however is textbook knowledge.
Even though the given results can be interesting, I definitely miss the link
to present day research.

3) Usually the normalized frequency is a unit of measurement of frequency
equivalent to cycles/sample. I am not sure whether I understand the authors
units.

4) The authors refer to ordered Photonic Crystals where they find for certain
filling fraction the ovelap with quantum wells. Is this really surprising when
thinking in bandstructes ? If it is, how is this explained ?

5) They compare certain cavity sizes to "infinite cavities", can the authors
provide a scheme of how this is meant to be understood ?

6) What is the size of the square lattice? array of Number x Number ? How large is it?

I believe the manuscript should be profoundly revised and the english must be checked. Following revision one can think of publications.

Author Response

Attached please find our response.

Author Response File: Author Response.pdf

Round 2

Reviewer 3 Report

photonics-1222207 

Title: Simulation of Photonic-Crystal Surface-Emitting Lasers with Air-Hole and Air-Pillar Structures 

Authors: Zi-Xian Yang , Chia-Yu Kuo , Gray Lin

The authors have added some more information to the text and they responded in parts on my questions, however the manuscript has not been revised sufficiently:

1) The authors claim to search only for "band-edge lasing". They certainly miss, that lasers have thresholds, they feature gain, gain narrowing and gain saturation. These crucial effects are not included in their work.

However, these crucial effects of lasers are included e.g. here:

Appl. Sci. 9 (12), 2477 (2019);  Appl. Sci. 10 (5), 1836 (2020)

This is why I believe the authors should cite mentioned references and broaden their research background.

2) Normalized units are used here Appl. Sci. 9 (12), 2477 (2019); Is this what the authors mean ?

3) The authors mention "The square lattice constant is 295 nm. For the cavity size of 300 \mu m, the PC area is in square of 300  x 300 \mu m^2  and the PC array numbers are over 1000 x 1000."

The authors claim to "calculate" sample sizes of one million cylinders, thus this computation is not self-consistent, which is a profound disadvantage when talking about gain, gain saturation, especially ad the band edges, since those band edges change with excitation.

The cylinders are huge compared to optical wavelengths, thus the results will heavily fluctuate.

In summary I ask for profound revision of this manuscript.

Author Response

Attached please find our response.

Author Response File: Author Response.pdf