# One Million Q-Factor SiGe-on-Si Ring Resonator in the Mid-Infrared

Rémi Armand, Marko Perestjuk, 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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Rémi Armand, Marko Perestjuk, Miguel Gerardo Sandoval Campos, Ujjal Chettri, Lamine Ferhat, et al.. One Million Q-Factor SiGe-on-Si Ring Resonator in the Mid-Infrared. 2025 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC), IEEE, Jun 2025, Munich, Germany. pp.1-1, 10.1109/CLEO/Europe-EQEC65582.2025.11109834. hal-05363712

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Submitted on 13 Nov 2025

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Distributed under a Creative Commons Attribution 4.0 International License
One Million Q-Factor SiGe-on-Si Ring Resonator in the Mid-Infrared

Rémi Armand¹,², Marko Perestjuk¹,³, 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, Universite 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

The mid-infrared (MIR) wavelength range (3–15 µm) is attractive for applications in several fields, particularly in sensing due to the strong fundamental absorption lines exhibited by numerous molecules [1]. MIR integrated photonics is gaining interest in the quest for compact, efficient and cost-effective sensors. CMOS-compatible Group-IV platforms based on silicon and germanium can provide the required performances to achieve this quest [2]. One crucial integrated photonic structure on these platforms is a compact resonator with high light confinement, characterised by its quality factor (Q-factor). Integrated SiN-on-SiO₂ [3] and SOI [4] resonators reaching one million Q-factor have been demonstrated. However, these platforms are limited to the lower end of the MIR spectrum (< 3.8 µm) due to absorption by the SiO₂. Demonstrations on platforms that can operate at longer MIR wavelengths, such as graded-index SiGe-on-Si [5], SiGe-on-Si [6], Ge-on-Y₂O₃ [7] have so far struggled to reach Q-factors above 250 000. In this work, we demonstrate a significant improvement in the performance of integrated SiGe-on-Si ring resonators, reaching a record Q-factor of 10⁶ at λ = 4.42 µm.

Our platform consists of an air-clad 3.3 µm thick silicon-germanium (SiGe) alloy (40% germanium) on a silicon substrate. The fabrication is carried on a 200 nm CMOS pilot line at CEA-Leti using deep ultraviolet photolithography and reactive ion etching. The cavities are ring resonators with waveguide widths from 3 to 6 µm and gaps between the bus waveguide and the ring from 250 nm to 1 µm. We perform the Q-factor measurement using the setup presented in Fig. 1a, consistent of a tunable CW-OPO (Excelitas iFLEX-Agile) and a MIR camera in a top-view configuration. When the OPO wavelength is resonant in the ring, the intensity of scattered light increases, as shown in the MIR camera images in Fig. 1b. For analysis, we assume that the detected scattered light is proportional to the intra-cavity power as it results from the waveguide sidewall roughness. By scanning the laser wavelength 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 10⁶ at λ = 4.42 µm (see Fig. 1c). Additional measurements have been performed over a wider wavelength range (3.5 to 4.6 µm) and on different ring resonators (waveguide widths of 4 and 5 µm). These measurements exhibit resonance splitting and thermal bistability.

<figure><img src="image_2.png" /><figcaption>**Fig. 1** (a) Experimental setup for Q-factor measurements in the MIR. (b) MIR camera images showing the light scattered by a ring resonator when excited off (left) and on resonance (right). (c) Scattered light from a Si<sub>0.6</sub>Ge<sub>0.4</sub>-on-Si ring resonator as a function of wavelength. The ring waveguides dimensions are 5 µm × 3.3 µm.</figcaption></figure>

References

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

[2] V. Reboud et al. "Photonic ge-based platforms for mid-infrared applications." Photoniques 125, 50-57 (2024).

[3] K. Luke et al. "Broadband mid-infrared frequency comb generation in a Si3N4 microresonator." Optics letters 40.21, 4823-4826 (2015).

[4] S. A. Miller et al., "Low-loss silicon platform for broadband mid-infrared photonics," Optica 4.7, 707-712 (2017).

[5] N. Koompaï et al., "Long-wave infrared integrated resonators in the 7.5-9 µm wavelength range," Applied Physics Letters 123.3 (2023).

[6] R. Armand et al., "Mid-infrared integrated silicon-germanium ring resonator with high q-factor," APL Photonics 8.7 (2023).

[7] J. Lim et al., "Experimental demonstration of high-q mrr based on a germanium-on-insulator platform with an yttria insulator in the mid-ir range," Photonics Research 11.11 (2023).