Fano resonance lineshapes in a waveguide-microring structure enabled by an air-hole

Hanlin Fang; Jianbang Zheng; Jianlin Zhao; Juntao Li; Liang Fang; Linpeng Gu; Qiang Zhao; Xuetao Gan

arxiv: 1907.02186 · v1 · pith:YSXPYR36new · submitted 2019-07-04 · ⚛️ physics.optics

Fano resonance lineshapes in a waveguide-microring structure enabled by an air-hole

Linpeng Gu , Liang Fang , Hanlin Fang , Juntao Li , Jianbang Zheng , Jianlin Zhao , Qiang Zhao , Xuetao Gan This is my paper

Pith reviewed 2026-05-25 09:37 UTC · model grok-4.3

classification ⚛️ physics.optics

keywords Fano resonancemicroring resonatorair-holewaveguide couplingphase shifttransmission spectrumLorentzian lineshape

0 comments

The pith

Inserting an air-hole next to a microring resonator turns its transmission spectrum from Lorentzian to Fano lineshapes at all resonances.

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The paper demonstrates that placing a single air-hole in the bus waveguide side-coupled to a microring resonator induces a phase shift between the resonator's discrete resonant modes and the waveguide's continuum propagating mode. This phase shift changes the interference from symmetric Lorentzian to asymmetric Fano lineshapes across every resonant mode. Fabricated devices achieve slope rates over 400 dB/nm and extinction ratios over 20 dB. The approach requires only this one modification, remains compact, tolerates fabrication variations, and functions over a broad wavelength range.

Core claim

By inserting an air-hole in the waveguide side-coupling with a microring resonator, the transmission spectrum presents Fano lineshapes at all of the resonant modes due to an air-hole-induced phase-shift between the discrete resonant modes and the continuum propagating mode, which modifies their interference lineshapes from symmetric Lorentzian to asymmetric Fano.

What carries the argument

Air-hole-induced phase-shift between the discrete resonant modes of the microring and the continuum mode of the bus waveguide.

If this is right

Varying air-hole location and diameter produces different Fano asymmetry parameters.
The structure provides simple design, compact footprint, large fabrication tolerance, and broadband operation.
Performance of on-chip microring devices for sensors, switches, and filters can be expanded and improved.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

The same phase-shift approach could be tested in other resonator-waveguide geometries to introduce controlled asymmetry without separate phase elements.
Direct phase measurements on the air-hole section would allow quantitative prediction of the Fano parameter from hole size and position.
Multiple air-holes of different sizes placed along the waveguide might enable engineered lineshapes with multiple asymmetry features in a single device.

Load-bearing premise

The asymmetry arises specifically from the phase shift caused by the air-hole rather than other fabrication effects or measurement artifacts.

What would settle it

Fabricating identical structures without the air-hole and finding persistent Fano lineshapes, or measuring the phase directly and finding no correlation with the extracted asymmetry parameter, would disprove the claimed cause.

Figures

Figures reproduced from arXiv: 1907.02186 by Hanlin Fang, Jianbang Zheng, Jianlin Zhao, Juntao Li, Liang Fang, Linpeng Gu, Qiang Zhao, Xuetao Gan.

**Figure 2.** Figure 2: FIG. 2. Fano parameter [PITH_FULL_IMAGE:figures/full_fig_p003_2.png] view at source ↗

**Figure 3.** Figure 3: FIG. 3. (a) Optical microscope image and (b) the zoomed SEM [PITH_FULL_IMAGE:figures/full_fig_p004_3.png] view at source ↗

**Figure 5.** Figure 5: FIG. 5. (a) Measured (blue dots) and fitted (red solid, by Eq. 4) [PITH_FULL_IMAGE:figures/full_fig_p004_5.png] view at source ↗

read the original abstract

We propose and demonstrate, by simply inserting an air-hole in the waveguide side-coupling with a microring resonator (MRR), the transmission spectrum presents Fano lineshapes at all of the resonant modes. Measured from the fabricated devices, Fano lineshapes with slope rates over 400 dB/nm and extinction ratios over 20 dB are obtained. We ascribe it to the air-hole-induced phase-shift between the discrete resonant modes of the MRR and the continuum propagating mode of the bus-waveguide, which modifies their interference lineshapes from symmetric Lorentzian to asymmetric Fano. From devices with varied locations and diameters of the air-hole, different Fano asymmetric parameters are extracted, verifying the air-hole-induced phase-shifts. This air-hole-assisted waveguide-MRR structure for achieving Fano resonance lineshapes has advantages of simple design, compact footprint, large tolerance of fabrication errors, as well as broadband operation range. It has great potentials to expand and improve performances of on-chip MRR-based devices, including sensors, switchings and filters.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Desk Editor's Note private letter to a colleague

A single air hole in the bus waveguide turns standard MRR Lorentzian dips into Fano lineshapes at every mode, backed by fabricated devices and parameter sweeps, though the phase-shift mechanism is shown only indirectly.

read the letter

The core result is straightforward: placing one air hole in the waveguide next to a microring resonator produces asymmetric Fano transmission at all resonant modes instead of the usual symmetric Lorentzian. Fabricated devices reach slopes above 400 dB/nm and extinction ratios above 20 dB, and the effect holds across multiple resonances. That is useful for anyone who wants steeper responses for sensing or filtering without redesigning the whole coupler or adding extra elements. The authors also fabricated variants with different hole positions and diameters, extracted different Fano q parameters, and showed the asymmetry tracks with those changes. That correlation is the main evidence they offer for an air-hole-induced phase shift between the ring resonance and the waveguide continuum mode. The experimental spectra look clean and the fabrication tolerance claim is plausible given the simple geometry. The main limitation is that the causal story stays at the level of correlation. No quantitative model or simulation is described that predicts the observed q values from hole size and location, so other effects such as modified coupling strength or added scattering loss could contribute to the asymmetry. A reader can accept the practical demonstration while still wanting a tighter link between geometry and lineshape. This work is aimed at device engineers in integrated photonics who already use MRRs and would like a low-complexity route to Fano responses. The measured data are concrete enough to justify sending the paper to referees, even if the modeling section needs strengthening in revision.

Referee Report

2 major / 2 minor

Summary. The manuscript proposes and experimentally demonstrates that inserting a single air-hole into the bus waveguide side-coupled to a microring resonator produces Fano resonance lineshapes at all resonant modes. The authors attribute the transition from Lorentzian to asymmetric Fano profiles to an air-hole-induced phase shift between the discrete MRR resonances and the waveguide continuum. Fabricated devices exhibit measured slopes exceeding 400 dB/nm and extinction ratios above 20 dB; devices with varied hole locations and diameters yield different extracted Fano asymmetry parameters q, which the authors interpret as verification of the phase-shift mechanism. The structure is presented as simple, compact, fabrication-tolerant, and broadband, with potential applications in on-chip sensors, switches, and filters.

Significance. If the experimental observations are reproducible and the mechanism is placed on firmer quantitative footing, the result supplies a minimal-modification route to Fano lineshapes in standard silicon-photonic MRR platforms. The reported slope rates and extinction ratios are competitive for sensing and switching applications, and the broadband character across multiple resonances is a practical advantage. The experimental demonstration itself is a clear strength; the absence of a predictive model relating hole geometry to phase is the principal limitation on the depth of the mechanistic claim.

major comments (2)

[Abstract / discussion] Abstract and discussion sections: the central claim that the air-hole produces Fano lineshapes via a controllable phase shift between discrete and continuum modes rests on correlation (different q values for varied hole positions/diameters) rather than a quantitative mapping. No analytic model, coupled-mode calculation, or FDTD simulation is presented that predicts the phase offset from hole diameter or offset; alternative explanations (modified coupling coefficient, wavelength-dependent scattering loss, or local index perturbation) therefore cannot be excluded on the basis of the data shown. This attribution is load-bearing for the title and the proposed mechanism.
[Results / device variation] Results section (device variation): while intentional changes in hole location and diameter are used to extract different q parameters, the manuscript does not report a direct measurement or calculation of the phase difference itself (e.g., via interferometric test structures or extracted phase response). Without this, the causal link remains indirect and the claim that the air-hole “enables” the Fano shape via phase shift is not fully substantiated.

minor comments (2)

[Abstract / conclusions] The statement of “large tolerance of fabrication errors” is asserted but not supported by explicit tolerance data (e.g., yield versus hole-size variation or misalignment); if this is inferred only from the presented devices, the claim should be qualified or backed by additional measurements.
[Figures] Figure captions and text should explicitly label which spectra correspond to which hole diameters and offsets so that the correlation between geometry and q can be traced without ambiguity.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for the constructive and detailed review. The comments correctly identify that our mechanistic interpretation relies primarily on experimental correlation rather than a quantitative predictive model. We address each major comment below and indicate where revisions will be made to improve clarity and substantiation while preserving the experimental demonstration.

read point-by-point responses

Referee: [Abstract / discussion] Abstract and discussion sections: the central claim that the air-hole produces Fano lineshapes via a controllable phase shift between discrete and continuum modes rests on correlation (different q values for varied hole positions/diameters) rather than a quantitative mapping. No analytic model, coupled-mode calculation, or FDTD simulation is presented that predicts the phase offset from hole diameter or offset; alternative explanations (modified coupling coefficient, wavelength-dependent scattering loss, or local index perturbation) therefore cannot be excluded on the basis of the data shown. This attribution is load-bearing for the title and the proposed mechanism.

Authors: We acknowledge that the attribution to a phase shift is supported by correlation across devices rather than a first-principles calculation of the phase from hole geometry. The systematic dependence of the extracted asymmetry parameter q on both hole diameter and offset, together with the preservation of Fano lineshapes at every resonance, is difficult to reconcile with alternatives such as a simple change in coupling strength or wavelength-dependent loss, which would not produce the observed broadband behavior. Nevertheless, we agree that a quantitative model would place the claim on firmer footing. In the revised manuscript we will add a short coupled-mode-theory estimate of the air-hole-induced phase and a brief discussion of why the alternative mechanisms are less consistent with the full data set. revision: yes
Referee: [Results / device variation] Results section (device variation): while intentional changes in hole location and diameter are used to extract different q parameters, the manuscript does not report a direct measurement or calculation of the phase difference itself (e.g., via interferometric test structures or extracted phase response). Without this, the causal link remains indirect and the claim that the air-hole “enables” the Fano shape via phase shift is not fully substantiated.

Authors: The referee is correct that we present no direct interferometric measurement of the phase. The causal link is therefore indirect, resting on the well-established relation between the Fano parameter q and the relative phase between the discrete and continuum paths. We will revise the results and discussion sections to state this limitation explicitly and to clarify that the observed trends in q constitute supporting but not conclusive evidence for the phase-shift picture. No additional experimental structures will be added, as that would require a separate fabrication run outside the scope of the present work. revision: partial

Circularity Check

0 steps flagged

No circularity; experimental demonstration with parameter variation is self-contained

full rationale

The paper reports fabrication of waveguide-MRR devices with inserted air-holes at varied locations and diameters, followed by spectral measurements and extraction of Fano q parameters showing asymmetry. The attribution to phase shift is presented as an ascription verified by the observed correlation across devices, but no equations, models, or derivations are provided that reduce the outcome to a fitted input renamed as prediction or to a self-referential definition. No self-citations or uniqueness theorems are invoked as load-bearing steps. The central result remains an empirical observation independent of any circular construction.

Axiom & Free-Parameter Ledger

0 free parameters · 1 axioms · 0 invented entities

The claim rests primarily on experimental observation rather than a first-principles derivation; standard waveguide and resonator interference principles are invoked without new free parameters or invented entities.

axioms (1)

standard math Electromagnetic interference between a discrete resonator mode and a continuum waveguide mode produces transmission lineshapes that can be modified by relative phase.
Invoked to explain the transition from Lorentzian to Fano upon insertion of the air-hole.

pith-pipeline@v0.9.0 · 5739 in / 1185 out tokens · 27140 ms · 2026-05-25T09:37:12.346681+00:00 · methodology

discussion (0)

Reference graph

Works this paper leans on

23 extracted references · 23 canonical work pages

[1]

Del’Haye, A

P. Del’Haye, A. Schliesser, O. Arcizet, T. Wilken, R. Holzwarth, and T. J. Kippenberg, Optical frequency comb gen- eration from a monolithic microresonator, Nature 450, 1214 (2007)

work page 2007
[2]

Guo, C.-l

X. Guo, C.-l. Zou, C. Schuck, H. Jung, R. Cheng, and H. X. Tang, Parametric down-conversion photon-pair source on a nanophotonic chip, Light. Sci. & Appl. 6, e16249 (2017)

work page 2017
[3]

Q. Xu, B. Schmidt, S. Pradhan, and M. Lipson, Micrometre- scale silicon electro-optic modulator, nature. 435, 325 (2005)

work page 2005
[4]

T. J. Kippenberg, R. Holzwarth, and S. A. Diddams, Microres- onatorbased optical frequency combs, science. 332, 555559 (2011). 6

work page 2011
[5]

G. Wang, A. Shen, C. Zhao, L. Yang, T. Dai, Y . Wang, Y . Li, X. Jiang, and J. Yang, Fano-resonance-based ultra-high- resolution ratio-metric wavelength monitor on silicon, Opt. let- ters 41, 544547 (2016)

work page 2016
[6]

G. Zhao, T. Zhao, H. Xiao, Z. Liu, G. Liu, J. Yang, Z. Ren, J. Bai, and Y . Tian, Tunable fano resonances based on microring resonator with feedback coupled waveguide, Opt. express 24, 2018720195 (2016)

work page 2016
[7]

C. Qiu, P. Yu, T. Hu, F. Wang, X. Jiang, and J. Yang, Asymmet- ric fano resonance in eye-like microring system, Appl. Phys. Lett. 101, 021110 (2012)

work page 2012
[8]

T. Hu, P. Yu, C. Qiu, H. Qiu, F. Wang, M. Yang, X. Jiang, H. Yu, and J. Yang, Tunable fano resonances based on two-beam interference in microring resonator, Appl. physics letters 102, 011112 (2013)

work page 2013
[9]

L. Y . Mario, S. Darmawan, and M. K. Chin, Asymmetric fano resonance and bistability for high extinction ratio, large modulation depth, and low power switching, Opt. express 14, 1277012781 (2006)

work page 2006
[10]

Z. Tu, D. Gao, M. Zhang, and D. Zhang, High-sensitivity com- plex refractive index sensing based on fano resonance in the subwavelength grating waveguide micro-ring resonator, Opt. express 25, 2091120922 (2017)

work page 2017
[11]

Chao and L

C.-Y . Chao and L. J. Guo, Biochemical sensors based on polymer microrings with sharp asymmetrical resonance, Appl. Phys. Lett. 83, 15271529 (2003)

work page 2003
[12]

A. E. Miroshnichenko, S. Flach, and Y . S. Kivshar, Fano res- onances in nanoscale structures, Rev. Mod. Phys. 82, 2257 (2010)

work page 2010
[13]

Absil, J

P. Absil, J. Hryniewicz, B. Little, R. Wilson, L. Joneckis, and P.- T. Ho, Compact microring notch ﬁlters, IEEE Photonics Tech- nol. Lett. 12, 398400 (2000)

work page 2000
[14]

Zhou and A

L. Zhou and A. W. Poon, Fano resonance-based electrically reconﬁgurable add-drop ﬁlters in silicon microring resonator- coupled machzehnder interferometers, Opt. letters 32, 781783 (2007)

work page 2007
[15]

Darmawan, Y

S. Darmawan, Y . Landobasa, and M. Chin, Phase engineering for ring enhanced mach-zehnder interferometers, Opt. express 13, 45804588 (2005)

work page 2005
[16]

Zhang, W

W. Zhang, W. Li, and J. Yao, Optically tunable fano reso- nance in a grating-based fabryperot cavity-coupled microring resonator on a silicon chip, Opt. letters 41, 24742477 (2016)

work page 2016
[17]

Liang, L

W. Liang, L. Yang, J. K. S. Poon, Y . Huang, K. J. Vahala, and A. Yariv, Transmission characteristics of a fabry-perot etalon- microtoroid resonator coupled system, Opt. letters 31, 510512 (2006)

work page 2006
[18]

A. E. Miroshnichenko, S. F. Mingaleev, S. Flach, and Y . S. Kivshar, Nonlinear fano resonance and bistable wave transmis- sion, Phys. Rev. E 71, 036626 (2005)

work page 2005
[19]

M. F. Limonov, M. V . Rybin, A. N. Poddubny, and Y . S. Kivshar, Fano resonances in photonics, Nat. Photonics 11, 543 (2017)

work page 2017
[20]

Avrutsky, R

I. Avrutsky, R. Gibson, J. Sears, G. Khitrova, H. Gibbs, and J. Hendrickson, Linear systems approach to describing and clas- sifying fano resonances, Phys. Rev. B 87, 125118 (2013)

work page 2013
[21]

J. W. Yoon and R. Magnusson, Fano resonance formula for lossy two-port systems, Opt. express 21, 1775117759 (2013)

work page 2013
[22]

L. Gu, H. Fang, J. Li, L. Fang, S. J. Chua, J. Zhao, and X. Gan, A compact structure for realizing lorentzian, fano, and elec- tromagnetically induced transparency resonance lineshapes in a microring resonator, Nanophotonics 8, 841848 (2019)

work page 2019
[23]

Y . Ding, H. Ou, and C. Peucheret, Ultrahigh-efﬁciency apodized grating coupler using fully etched photonic crystals, Opt. letters 38, 27322734 (2013)

work page 2013

[1] [1]

Del’Haye, A

P. Del’Haye, A. Schliesser, O. Arcizet, T. Wilken, R. Holzwarth, and T. J. Kippenberg, Optical frequency comb gen- eration from a monolithic microresonator, Nature 450, 1214 (2007)

work page 2007

[2] [2]

Guo, C.-l

X. Guo, C.-l. Zou, C. Schuck, H. Jung, R. Cheng, and H. X. Tang, Parametric down-conversion photon-pair source on a nanophotonic chip, Light. Sci. & Appl. 6, e16249 (2017)

work page 2017

[3] [3]

Q. Xu, B. Schmidt, S. Pradhan, and M. Lipson, Micrometre- scale silicon electro-optic modulator, nature. 435, 325 (2005)

work page 2005

[4] [4]

T. J. Kippenberg, R. Holzwarth, and S. A. Diddams, Microres- onatorbased optical frequency combs, science. 332, 555559 (2011). 6

work page 2011

[5] [5]

G. Wang, A. Shen, C. Zhao, L. Yang, T. Dai, Y . Wang, Y . Li, X. Jiang, and J. Yang, Fano-resonance-based ultra-high- resolution ratio-metric wavelength monitor on silicon, Opt. let- ters 41, 544547 (2016)

work page 2016

[6] [6]

G. Zhao, T. Zhao, H. Xiao, Z. Liu, G. Liu, J. Yang, Z. Ren, J. Bai, and Y . Tian, Tunable fano resonances based on microring resonator with feedback coupled waveguide, Opt. express 24, 2018720195 (2016)

work page 2016

[7] [7]

C. Qiu, P. Yu, T. Hu, F. Wang, X. Jiang, and J. Yang, Asymmet- ric fano resonance in eye-like microring system, Appl. Phys. Lett. 101, 021110 (2012)

work page 2012

[8] [8]

T. Hu, P. Yu, C. Qiu, H. Qiu, F. Wang, M. Yang, X. Jiang, H. Yu, and J. Yang, Tunable fano resonances based on two-beam interference in microring resonator, Appl. physics letters 102, 011112 (2013)

work page 2013

[9] [9]

L. Y . Mario, S. Darmawan, and M. K. Chin, Asymmetric fano resonance and bistability for high extinction ratio, large modulation depth, and low power switching, Opt. express 14, 1277012781 (2006)

work page 2006

[10] [10]

Z. Tu, D. Gao, M. Zhang, and D. Zhang, High-sensitivity com- plex refractive index sensing based on fano resonance in the subwavelength grating waveguide micro-ring resonator, Opt. express 25, 2091120922 (2017)

work page 2017

[11] [11]

Chao and L

C.-Y . Chao and L. J. Guo, Biochemical sensors based on polymer microrings with sharp asymmetrical resonance, Appl. Phys. Lett. 83, 15271529 (2003)

work page 2003

[12] [12]

A. E. Miroshnichenko, S. Flach, and Y . S. Kivshar, Fano res- onances in nanoscale structures, Rev. Mod. Phys. 82, 2257 (2010)

work page 2010

[13] [13]

Absil, J

P. Absil, J. Hryniewicz, B. Little, R. Wilson, L. Joneckis, and P.- T. Ho, Compact microring notch ﬁlters, IEEE Photonics Tech- nol. Lett. 12, 398400 (2000)

work page 2000

[14] [14]

Zhou and A

L. Zhou and A. W. Poon, Fano resonance-based electrically reconﬁgurable add-drop ﬁlters in silicon microring resonator- coupled machzehnder interferometers, Opt. letters 32, 781783 (2007)

work page 2007

[15] [15]

Darmawan, Y

S. Darmawan, Y . Landobasa, and M. Chin, Phase engineering for ring enhanced mach-zehnder interferometers, Opt. express 13, 45804588 (2005)

work page 2005

[16] [16]

Zhang, W

W. Zhang, W. Li, and J. Yao, Optically tunable fano reso- nance in a grating-based fabryperot cavity-coupled microring resonator on a silicon chip, Opt. letters 41, 24742477 (2016)

work page 2016

[17] [17]

Liang, L

W. Liang, L. Yang, J. K. S. Poon, Y . Huang, K. J. Vahala, and A. Yariv, Transmission characteristics of a fabry-perot etalon- microtoroid resonator coupled system, Opt. letters 31, 510512 (2006)

work page 2006

[18] [18]

A. E. Miroshnichenko, S. F. Mingaleev, S. Flach, and Y . S. Kivshar, Nonlinear fano resonance and bistable wave transmis- sion, Phys. Rev. E 71, 036626 (2005)

work page 2005

[19] [19]

M. F. Limonov, M. V . Rybin, A. N. Poddubny, and Y . S. Kivshar, Fano resonances in photonics, Nat. Photonics 11, 543 (2017)

work page 2017

[20] [20]

Avrutsky, R

I. Avrutsky, R. Gibson, J. Sears, G. Khitrova, H. Gibbs, and J. Hendrickson, Linear systems approach to describing and clas- sifying fano resonances, Phys. Rev. B 87, 125118 (2013)

work page 2013

[21] [21]

J. W. Yoon and R. Magnusson, Fano resonance formula for lossy two-port systems, Opt. express 21, 1775117759 (2013)

work page 2013

[22] [22]

L. Gu, H. Fang, J. Li, L. Fang, S. J. Chua, J. Zhao, and X. Gan, A compact structure for realizing lorentzian, fano, and elec- tromagnetically induced transparency resonance lineshapes in a microring resonator, Nanophotonics 8, 841848 (2019)

work page 2019

[23] [23]

Y . Ding, H. Ou, and C. Peucheret, Ultrahigh-efﬁciency apodized grating coupler using fully etched photonic crystals, Opt. letters 38, 27322734 (2013)

work page 2013