# Hybrid Ge/Sb 2 S 3 /SiGe waveguide for tunable Mid-IR supercontinuum generation

Adam Biegański, Rémi Armand, Marko Perestjuk, Lamine Ferhat, Vincent Reboud, Jean-Michel Hartmann, Thach Nguyen, Arnan Mitchell, Christelle Monat, Sébastien Cueff, et al.

► To cite this version:

Adam Biegański, Rémi Armand, Marko Perestjuk, Lamine Ferhat, Vincent Reboud, et al. Hybrid Ge/Sb 2 S 3 /SiGe waveguide for tunable Mid-IR supercontinuum generation. Integrated Photonics Research, Silicon and Nanophotonics, Jul 2025, Marseille, France. pp.IM4A.1, 10.1364/IPRSN.2025.IM4A.1. hal-05325877

HAL Id: hal-05325877

https://hal.science/hal-05325877v1

Submitted on 22 Oct 2025

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# Hybrid Ge/Sb<sub>2</sub>S<sub>3</sub>/SiGe Waveguide for Tunable Mid-IR Supercontinuum Generation

Adam Biegański<sup>1,2,*</sup>, Rémi Armand<sup>1</sup>, Marko Perestjuk<sup>1,2</sup>, Lamine Ferhat<sup>1</sup>, Vincent Reboud<sup>3</sup>, Jean-Michel Hartmann<sup>3</sup>, Thach Nguyen<sup>2</sup>, Arnan Mitchell<sup>2</sup>, Christelle Monat<sup>1</sup>, Sébastien Cueff<sup>1</sup> and Christian Grillet<sup>1</sup>

**Abstract:** We demonstrate supercontinuum generation in the mid-IR using a hybrid Ge/Sb<sub>2</sub>S<sub>3</sub>/SiGe-on-Si waveguide. By changing the Sb<sub>2</sub>S<sub>3</sub> state, we modify the waveguide's dispersion regime and therefore the properties of generated spectra. © 2025 The Authors

**1. Introduction**

Spectroscopy in the mid-IR wavelength range is attracting significant attention due to its potential for gas sensing devices. These devices would greatly benefit from a bright and broadband light source such as a supercontinuum (SC) generated in integrated waveguides[1]. The temporal and spectral characteristics of the generated SC, such as bandwidth, flatness and coherence, are determined by the waveguide dispersion profile[2]. The ability to adjust the dispersion on demand could enable the development of a reconfigurable light source.

An attractive and innovative method of post-fabrication control in photonic devices is the use of phase-change materials (PCMs). These materials exhibit two different structural states: amorphous and crystalline with different optical properties, and can be reversibly switched. Integration of emerging low-loss PCMs such as Ge<sub>2</sub>Sb<sub>2</sub>Te<sub>3</sub>S<sub>2</sub> (GSTS) or Sb<sub>2</sub>S<sub>3</sub> on waveguides operating in the mid-IR regime was recently demonstrated[3-4]. The waveguide dispersion profile can be modified by depositing a dielectric cladding[5]. However, direct implementation of PCMs in this scenario is not feasible. Significant dispersion shift would require high light confinement in the PCM, inevitably leading to high propagation loss[6]. Moreover, large dimensions of waveguides operating in the mid-IR regime (compared to their near-IR counterparts) will involve a micrometer-scale PCM thickness, effectively making them difficult to re-amorphize and hence irreversible.

Here, we present a hybrid Ge/Sb<sub>2</sub>S<sub>3</sub>/SiGe-on-Si waveguide. Using high refractive index (n = 3.77 at 4.0 µm) germanium (Ge) as waveguide top cladding allows us to boost the tunability while keeping low light fraction confined in the PCM. We fabricate the waveguide using CMOS-compatible techniques and demonstrate its potential by generating SC pumped at 3.9 µm with very distinct spectral signatures in amorphous and crystalline Sb<sub>2</sub>S<sub>3</sub> states respectively.

**2. Waveguide design and mode dispersion profiles**

The waveguide structure is illustrated in Figure 1a. A 50 nm thick Sb<sub>2</sub>S<sub>3</sub> layer, sandwiched between two 6 nm thick MgO layers, and a 300 nm thick Ge capping layer were deposited atop a SiGe-on-Si waveguide using electron beam evaporation. The role of the MgO is to improve adhesion on the interfaces between Sb<sub>2</sub>S<sub>3</sub>, SiGe and Ge layers. SiGe-on-Si platform fabrication details can be found in Ref. [1].

<figure><img src="image_2.png" /><figcaption>Fig 1. (a) Waveguide's cross-section and (b) dispersion profile of the fundamental TE mode in amorphous (red) and crystalline (indigo) Sb<sub>2</sub>S<sub>3</sub> phases simulated with Lumerical.</figcaption></figure>

Dispersion profiles of the waveguide's fundamental TE mode in both Sb<sub>2</sub>S<sub>3</sub> states are illustrated in Figure 1b. In the amorphous phase it exhibits low anomalous dispersion with zero dispersion wavelengths (ZDWs) around 3.54 and 4.40 µm. After the PCM's crystallization, the ZDWs shift to 3.64 and 4.48 µm respectively.

<footnote n="1"><sup>1</sup>Univ Lyon, CNRS, ECL, INSA Lyon, UCBL, CPE, INL UMR5270, 69134 Ecully, France</footnote>

<footnote n="2"><sup>2</sup>Integrated Photonics and Application Centre (InPAC), School of Engineering, RMIT University, Melbourne, VIC 3001, Australia</footnote>

<footnote n="3"><sup>3</sup>Université Grenoble Alpes, CEA-Leti, 38054 Grenoble, France</footnote>

<footnote>*adam.bieganski@ec-lyon.fr</footnote>
### 3. Supercontinuum generation in the mid-IR

We pump the waveguide with a pulsed OPA laser source (MIROPA-fs, Hotlight Systems) operating at a wavelength of 3.9 µm with a repetition rate of 63 MHz and ~200 fs pulse duration. We measure the SC spectra generated with the average input power of 20, 40 and 60 mW using an FTIR. Results, plotted together with a pump spectrum, are shown in Figure 2a. We next crystallize the Sb₂S₃ layer by heating the sample up to 270°C under ambient pressure for 1 hour and we repeat the measurement.

<figure><img src="image_3.png" /><figcaption>Fig 2. Spectral broadening measured for Sb₂S₃ in (a) amorphous and (b) crystalline state for increasing average input power.</figcaption></figure>

We notice a significant change in spectral shape and wavelength coverage before and after Sb₂S₃ crystallization. In the amorphous state, the spectrum exhibits a 20 dB bandwidth from 3.55 to 4.92 µm with the presence of a dispersive wave centered at ~3.35 µm. In the crystalline state, the bandwidth expands from 3.41 to 5.05 µm. Its profile is more irregular and rippled compared to the smooth shape before crystallization. These changes in spectral response confirm shifting of the waveguide dispersion characteristics.

### 4. Conclusion

We designed and fabricated a hybrid Ge/Sb₂S₃/SiGe waveguide with a significant change of the dispersion characteristics upon the Sb₂S₃ phase transition. We experimentally showed SC generation in the mid-IR spectral region with distinct properties before and after sample crystallization. To our best knowledge, this is the first demonstration of tuning SC features using a phase-change material in any wavelength regime. Given the small thickness (50 nm) of Sb₂S₃, the material can be re-amorphized using an integrated microheater. Consequently, the structure has the potential to act as a broadband reconfigurable light source. Its tunability range can be further improved by increasing the thickness of the Ge layer and therefore enhancing light interaction with the PCM.

### Acknowledgements

We acknowledge the support of the International Associated Laboratory in Photonics between France and Australia (LIA ALPhFA), the Agence Nationale de la Recherche (MIRthFUL, ANR-21-CE24-0005 and PEPR OFCOC, ANR-22-PEEL-0005), the Horizon 2020 research and innovation programme under the MSCA (REDI, grant agreement No 101034328 and ECLAUSion, Grant No. 801512) and the Horizon Europe research and innovation programme under the MSCA (MIRCOCOMBSYS, GA n°101119968).

### References

[1] M. Sinobad et al., "Mid-infrared octave spanning supercontinuum generation to 8.5 µm in silicon-germanium waveguides," *Optica*, vol. 5, no. 4, p. 360 (2018).

[2] C.-S. Brès et al., "Supercontinuum in integrated photonics: generation, applications, challenges, and perspectives," *Nanophotonics*, vol. 12, no. 7, pp. 1199-1244 (2023).

[3] A. Biegański et al., "Sb₂S₃ as a low-loss phase-change material for mid-IR photonics," *Opt. Mater. Express*, vol. 14, no. 4, p. 862 (2024).

[4] Y. Miyatake et al., "Proposal of Low-Loss Non-Volatile Mid-Infrared Optical Phase Shifter Based on Ge₂Sb₂Te₃S₂," *IEEE Trans. Electron Devices*, vol. 70, no. 4, pp. 2106-2112 (2023).

[5] M. Sinobad et al., "Dispersion trimming for mid-infrared supercontinuum generation in a hybrid chalcogenide/silicon-germanium waveguide," *J. Opt. Soc. Am. B*, vol. 36, no. 2, p. A98 (2019).

[6] S. Blundell et al., "Ultracompact Programmable Silicon Photonics Using Layers of Low-Loss Phase-Change Material Sb₂ Se₃ of Increasing Thickness," *ACS Photonics*, p. acsphotonics.4c01789 (2025).