# One Million Quality Factor CMOS-based Integrated Ring Resonators in the Mid-Infrared

Marko Perestjuk, Rémi Armand, Miguel Gerardo Sandoval Campos, Ujjal Chettri, Lamine Ferhat, Jean-Michel Hartmann, Vincent Mathieu, Guanghui Ren, Andreas Boes, Arnan Mitchell, et al.

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Marko Perestjuk, Rémi Armand, Miguel Gerardo Sandoval Campos, Ujjal Chettri, Lamine Ferhat, et al. One Million Quality Factor CMOS-based Integrated Ring Resonators in the Mid-Infrared. Integrated Photonics Research, Silicon and Nanophotonics, Jul 2025, Marseille, France. pp.IW4A.2, 10.1364/IPRSN.2025.IW4A.2. [https://hal.science/hal-05393112v1](https://hal.science/hal-05393112v1)

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# One Million Quality Factor CMOS-based Integrated Ring Resonators in the Mid-Infrared

Marko Perestjuk¹,³, Rémi Armand¹,², Miguel Gerardo Sandoval Campos¹, Ujjal Chettri¹, Lamine Ferhat¹, Jean-Michel Hartmann², Vincent Mathieu², Guanghui Ren³, Andreas Boes⁴, Arnan Mitchell³, Nicolas Bresson², Vincent Reboud², Christelle Monat¹, and Christian Grillet¹

1. Institut des Nanotechnologies de Lyon INL, UMR5270, CNRS, Ecole Centrale de Lyon, INSA Lyon, Université Claude Bernard Lyon 1, CPE Lyon, F-69134 Ecully, France

2. CEA-Leti, Université Grenoble Alpes, F-38054 Grenoble, France

3. School of Engineering, RMIT University, VIC 3001 Melbourne, Australia

4. Institute for Photonics and Advanced Sensing, The University of Adelaide, SA 5005 Adelaide, Australia
Christian.grillet@ec-lyon.fr

**Abstract:** We report Silicon Germanium (SiGe) Ring Resonators with quality factors reaching up to one million in the Mid-Infrared (MIR) wavelength range between 3.5 - 4.6 µm. Optical bistability is observed. © 2025 The Author(s)

## 1. Introduction

The mid-infrared (MIR) wavelength range (~3 - 13 µm) offers enormous potential for sensing and detection applications, as many molecules exhibit strong absorption lines [1]. However, the high cost and bulky components still prevent a real breakthrough for MIR technologies. On-chip integration of MIR components would be a decisive step towards widespread applications [1]. Broadband, bright and coherent sources like Microcombs are an essential ingredient in this context [2]. The challenge in realizing such a device is to find material platforms with large transparency windows in the MIR, good nonlinear properties and to be able to fabricate cavities with a high-quality factor (Q). Silicon germanium (SiGe) [3] is one such promising material due to its transparency window of up to 15 µm and MIR supercontinuum generation has already been demonstrated on Ge-based waveguides [4-7]. Demonstrations of ring resonators on platforms, such as SiGe-on-Si [8,9], Ge-on-Y₂O₃ [10] have reached Q-factors in the 100 000 to 250 000 range. In this work, we demonstrate a significant improvement in the performance of integrated SiGe-on-Si ring resonators, reaching a record 10⁶ Q-factor at λ = 4.42 µm. which is a record for group-IV photonics. In addition to an optimized fabrication process, we developed a simple characterization method that allows a precise Q- factor measurement without the need for any complex instrumentation on the detector side as well as the ability to probe a wide range of resonances with less constraints on the coupling regime. Thanks to this, we perform an analysis of the statistical distribution of Q-factors in a relatively broad part (3.5 - 4.6 µm) of the MIR band. With a Q factor of one million, this puts SiGe/Si ahead of other integrated MIR photonic platforms in the quest for high-Q.

## 2. Design and fabrication

Our platform consists of an air-clad 3.3 µm thick silicon-germanium (SiGe with a 40 % germanium) alloy core waveguide on a silicon substrate, with a ring of 250 µm radius (Fig. 1a and b). The fabrication is carried on a 200 nm CMOS pilot line at CEA-Leti using deep ultraviolet photolithography and deep reactive ion etching. The cavities are ring resonators with waveguide widths from 3 µm to 6 µm and gaps from 250 nm (Fig. 1c) to 1 µm (Fig. 1c).

<figure><img src="image_2.png" /><figcaption>Fig. 1 (a) Sketch of the ring resonator. (b) Optical microscope image of the SiGe on Si ring resonator (c) SEM image of the gap (250nm) between ring and bus waveguide</figcaption></figure>
### 3. MIR characterization and results

We perform the Q-factor measurement using the setup presented in Fig. 2a. The source was a tunable continuous wave optical parametric oscillator (OPO) from Excelitas (iFLEX-Agile cw-OPO, idler ~2.4 - 4.5 µm, ~10 MHz linewidth), which is pumped by an amplified near infrared seed laser. Modulating the seed laser gives fine tuning of the OPO output wavelength and hence allows us to scan through the ring resonance continuously. A function generator fed into the piezo controller of the seed laser enables a continuous tuning and modulation up to kHz speed over a 13 GHz range (~700 pm at λ = 4 µm). We can then either measure the transmission by a photo detector or image the in-cavity power, i.e image the light scattered off the cavity in the vertical direction using a MIR camera in top-view (FLIR, A6750 MWIR) with a MIR objective lens (Fig 2.b). The detected scattered light is proportional to the intra-cavity power and hence allows us to directly access the resonance Q factor. In Fig. 1c, by scanning the wavelength of the source on a 250 µm radius ring resonator with a waveguide width of 5 µm, we extract the full width at half maximum (FWHM) of the resonance, yielding the loaded Q-factor of $10^6$ at λ = 4.42 µm [8]. A systematic study of the resonance Q factors was performed on a large number of resonances over a wide wavelength range (3.5 - 4.6 µm). we investigate the behavior of resonators under strong power loading and observe optical bistability.

<figure><img src="image_3.png" /></figure>

In conclusion, we have demonstrated a record Q-factor for integrated photonics in the MIR of one million in a SiGe/Si platform. Application fields include sensing and nonlinear integrated photonics, in particular Kerr comb generation.

**Research funding:** We acknowledge the support of the Horizon 2020 research and innovation program under the Marie Skłodowska-Curie Actions (Grant No. ECLAUSion, 801512, MICROCOMBSYS, GA n°101119968), the Agence Nationale de la Recherche (ANR) (Grant No. MIRthFUL, ANR-21-CE24-0005), the Australian Research Council Centre of Excellence in Optical Microcombs for Breakthrough Science (CE230100006) and the International Associated Laboratory in Photonics between France and Australia (LIA ALPhFA). This work benefits from a France 2030 government grant managed by ANR (PEPR OFCOC, ANR-22-PEEL-0005).

### 4. References

[1] D. Popa and F. Udrea, "Towards integrated mid-infrared gas sensors," Sensors, vol. 19, no. 9, pp. 1-15, 2019.

[2] A. Pasquazi, et al., Physics Reports 729, 1-81 (2018).

[3] R. Soref, "Mid-infrared photonics in silicon and germanium," Nat. Photonics 4.8, 495-497 (2010).

[4] M. Sinobad et al., "Mid-infrared octave spanning supercontinuum generation to 8.5 µm in silicon-germanium waveguides," Optica, vol. 5, no. 4, 2018.

[5] M. Montesinos-Ballester et al., "On-Chip Mid-Infrared Supercontinuum Generation from 3 to 13 µm Wavelength," ACS Photonics, vol. 7, no. 12, pp. 3423-3429, 2020.

[6] A. Della Torre et al., "Mid-infrared supercontinuum generation in a low-loss germanium-on-silicon waveguide," APL Photonics, vol. 6, no. 1, 2021.

[7] C. S. Bres, A. Della Torre, D. Grassani, V. Brasch, C. Grillet, and C. Monat, "Supercontinuum in integrated photonics: generation, applications, challenges, and perspectives," Nanophotonics, vol. 12, no. 7, pp. 1199-1244, 2023.

[8] R. Armand et al., "Mid-infrared integrated silicon-germanium ring resonator with high Q-factor," APL Photonics, vol. 8, no. 7, 2023.

[9] N. Koompai et al., Applied Physics Letters, vol. 123, no. 3, 2023.

[10] J. Lim, J. Shim, I. Kim, and S. Kim, "Photonics Research, vol. 11, no. 11, 2023.

[11] M. Peresjtuk et al., "One Million Quality Factor Integrated Ring Resonators in the Mid-Infrared," accepted in Nanophotonics https://doi.org/10.1515/nanoph-2024-0761