# Towards reconfigurable photonics in Ge-based platforms for nonlinear mid-infrared broadband sources

Marko Perestjuk, Adam Bieganski, Remi Armand, Lamine Ferhat, Miguel Campos, Ujjal Chettri, Alberto Della Torre, Capucine Laprais, Vincent Reboud, Nicolas Bresson, et al.

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## ► To cite this version:

Marko Perestjuk, Adam Bieganski, Remi Armand, Lamine Ferhat, Miguel Campos, et al. Towards reconfigurable photonics in Ge-based platforms for nonlinear mid-infrared broadband sources. Integrated Optics: Devices, Materials, and Technologies XXIX, SPIE, Jan 2025, San Francisco, CA, United States. pp.38, 10.1117/12.3050570. hal-05393076

HAL Id: hal-05393076

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Submitted on 19 Jan 2026

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# Towards reconfigurable photonics in Ge-based platforms for nonlinear mid-infrared broadband sources (Invited Paper)

Marko Perestjuk<sup>a,b</sup>, Adam Bieganski<sup>a,b</sup>, Rémi Armand<sup>a</sup>, Lamine Ferhat<sup>a</sup>, Miguel Campos<sup>a</sup>, Ujjal Chettri<sup>a</sup>, Alberto della Torre<sup>a</sup>, Capucine Laprais<sup>a</sup>, Vincent Reboud<sup>c</sup>, Nicolas Bresson<sup>c</sup>, Pierre Brianceau<sup>c</sup>, Jean-Michel Hartmann<sup>c</sup>, Guanghui Ren<sup>b</sup>, Andreas Boes<sup>d</sup>, Thach Nguyen<sup>b</sup>, Arnan Mitchell<sup>b</sup>, Sébastien Cueff<sup>a</sup>, Christelle Monat<sup>a</sup>, Christian Grillet*<sup>a</sup>

<sup>a</sup>Institut des Nanotechnologies de Lyon, CNRS (France), Ecole Centrale de Lyon (France);

<sup>b</sup>RMIT Univ. (Australia);

<sup>c</sup>Univ. Grenoble Alpes (France), CEA-LETI (France);

<sup>d</sup>The Institute for Photonics and Advanced Sensing, The Univ. of Adelaide (Australia)

*christian.grillet@ec-lyon.fr

## ABSTRACT

Mid-infrared (MIR 3-15 µm) photonics is a burgeoning scientific and technological field with wide potential application domains including pollution detection, environmental monitoring, security, safety, astrophysics and many more...One of the key aspect is that many important molecules have fundamental ro-vibrational absorption lines in the MIR. An appealing approach, to create power efficient, sensitive, precise, compact molecular sensing devices is to develop an on-chip MIR platform integrating a bright and broadband light source, such as an integrated supercontinuum or a Kerr micro-comb. Ge-based platform (e.g. silicon germanium) has emerged as an attractive platform for nonlinear MIR photonics. We report on our latest progress on reconfigurable Ge-based photonics for bright and broadband MIR sources. We demonstrate that Phase change materials, like Sb<sub>2</sub>S<sub>3</sub>, are suitable for reconfigurable supercontinuum generation. We also report Ge-based ring resonators with quality factors reaching up to one million which is highly promising for power efficient MIR micro-comb generation.

**Keywords:** Mid-infrared, integrated photonics, supercontinuum, phase change materials, resonators

## 1. INTRODUCTION

The MIR wavelength range has shown a large potential for sensing applications, as many molecules have strong fundamental absorption lines in this range [1]. However, the high cost and bulky MIR technology has prevented a real breakthrough so far. The integration of MIR components on-chip can provide a crucial step towards large scale applications. One critical challenge is to integrate a broadband light source. Two approaches, where nonlinear material properties are used are either supercontinuum generation (SCG) [2] or micro-combs [3].

Very promising integrated photonic platforms for SCG are germanium (Ge) and silicon germanium (SiGe) due to their excellent nonlinear properties and CMOS compatibility, where broad SCG has been demonstrated recently [4-6]. One disadvantage of on-chip SCG is currently the lack of tunability and reconfigurability of the output spectrum. One method of circumventing it is to use the birefringence of waveguides for dispersion tuning. Another option of post-fabrication tuning is the deposition of thin films on the waveguides used for SCG, however this process is irreversible [7]. Phase change materials (PCMs), which can exist in two switchable and stable states, amorphous and crystalline, with different optical properties in each of them, have recently experienced an enormous amount of interest for applications in integrated photonics [8]. Their properties are extremely useful for reconfigurable and programmable photonics [9], which can be a solution to the reconfigurability issue of SCG. Many different PCMs are being investigated in integrated photonics, including VO<sub>2</sub>, Ge<sub>2</sub>Sb<sub>2</sub>Te<sub>5</sub> (GST), Ge<sub>2</sub>Sb<sub>2</sub>Se<sub>4</sub>Te<sub>1</sub> (GSST), Ge<sub>2</sub>Sb<sub>2</sub>Te<sub>3</sub>S<sub>2</sub> (GSTS), Sb<sub>2</sub>S<sub>3</sub> and Sb<sub>2</sub>Se<sub>3</sub> [9-13]. However, this is rarely done in the MIR. For most of the emerging PCMs, the MIR properties are completely unknown, for others at least the material properties were characterized, though often with not consistent results [11], and almost none were characterized integrated on-chip, and if so only at the short-wavelength tail of the MIR [13].
The other very promising approach for such broadband sources for integrated photonics are microcombs. There have been demonstrations of integrated microcombs in the MIR [14-16], but the current wavelength record is at ~4.3µm cutoff [16]. Hence, so far the largest portion of the MIR is inaccessible by microcombs. The challenges to realize such devices is to find material platforms with broad transparency win-dows in the MIR and be able to fabricate cavities with high quality (Q) factor. SiGe and Ge are also very promising for these applications and very recently we demonstrated a SiGe ring resonator with a loaded Q of 236,000 [17] and a Ge ring with a loaded Q of 154,000 [18] at 4.18µm wavelength. This starts to lower threshold powers for nonlinear effects and comb generation to levels reachable by modern equipment. One interesting nonlinear effect is optical bistability [19, 20]. It happens when the power buildup in a cavity is large enough to change the refractive index of the cavity material significantly. This leads to a feedback loop and a "tilt" of the resonance, resulting in a two state system. For one given wavelength there are two resonance states, so the cavity acts as a bistable switch. Besides being a precursor for the use of other nonlinearities, bistability itself has many applications, for example all optical switching [19], and, specifically interesting in the MIR, sensing applications [21].

Here we present our recent work on these different aspects. We demonstrate the integration of thick (> 800 nm) Sb₂S₃ layers on the SiGe-on-Si platform and SCG in those hybrid waveguides. We also demonstrate the ability of dispersion tuning through waveguide birefringence on this platform. Regarding the ring resonators, we first present fabrication improvements which greatly improve the coupling to the cavity. Second, we demonstrate optical bistability.

## 2. DISPERSION TUNING BY SB₂S₃ – A LOW LOSS PHASE CHANGE MATERIAL

We previously demonstrated that integration of 100 nm Sb₂S₃ layer onto SiGe-on-Si photonic platform introduces negligible additional propagation loss in the mid-IR range [22]. However, due to the relative small thickness of the PCM layer in comparison with the waveguide dimensions, interaction between guided mode and Sb₂S₃ layer is small and tunability possibilities are limited. Here, we use the same platform and deposit 812 nm thick Sb₂S₃ layer. This thickness provides more significant shift of the mode effective refractive index (Δn ≈ 0.01 at 4.0 µm) and dispersion profile (ΔD ≈ 10 at 4.0 µm, see Fig. 1a). We study the impact of its integration on linear propagation loss in the wavelength range of 3.3–4.5 µm, as well as its suitability for supercontinuum generation when pumped at 3.9 µm.

<figure><img src="image_3.png" /><figcaption>Figure 1. a) Waveguide dispersion profiles for air- (red), aSb₂S₃- (blue) and cSb₂S₃- (green) cladding calculated with Lumerical and b) measured linear propagation loss.</figcaption></figure>

Loss measurement are performed with a pulsed OPA laser source (MIROPA-fs, Hotlight Systems) with a repetition rate of 63 MHz and ~200 fs pulse duration. We calculate propagation loss using the cut-back method on three spiral waveguides with different lengths (4.2, 6.2, and 8.2 cm) under relatively low average power (1 mW). Results are presented in Fig. 1b. We notice that waveguides exhibit low propagation loss below 0.5 dB/cm with moderate increase after amorphous Sb₂S₃ (aSb₂S₃) layer deposition following the procedure described in [22]. The absolute loss level is below 1 dB/cm for wavelengths beyond 3.4 µm which is fully acceptable for nonlinear phenomena.
In the next step, we use the aSb₂S₃-clad sample to generate supercontinuum from a pump at 3.9 µm for TE polarization and varying average input power up to 60 mW. Experimental spectra are presented in Fig 2. For the highest input power we observe spectral broadening from 3.0 µm up to beyond 5.0 µm. These results indicate that amorphous phase of Sb₂S₃ is well suitable for nonlinear application.

<figure><img src="image_4.png" /><figcaption>Figure 2. Spectral broadening measured for aSb₂S₃-clad waveguide for different average input power.</figcaption></figure>

We crystallize the cladding (cSb₂S₃) by heating the sample up to the temperature of 270°C under ambient pressure for the time of 1 hour and we repeat the measurement. The microscope images, shown in Fig. 3, reveal randomly oriented crystal grains present on both the top of the waveguides and the area surrounding it.

<figure><img src="image_5.png" /><figcaption>Figure 3. Optical microscope images of cSb₂S₃-cladded chip. Multiple crystal grains with different orientations are visible.</figcaption></figure>

After crystallization, we observed a complete attenuation of the guided signal, suggesting a substantial increase in propagation loss. This significant increase is likely attributed to light scattering at the crystal grain boundaries, as has been previously demonstrated for Sb-based PCMs [23]. These findings suggest that further optimization of both the integration with our platform and crystallization processes is necessary to enable the realization of nonlinear light sources based on the crystalline phase of Sb₂S₃.
### 3. DISPERSION TUNING BY WAVEGUIDE'S BIREFRINGENCE

Another convenient way to control SC properties is exploiting birefringent waveguides that operate in different dispersion regimes for different pump pulse polarizations. Extreme polarization-dependent SC generation has been already demonstrated in chalcogenide [24] and SiN [25] waveguides, but their practical use for large scale spectroscopy devices would be limited by difficult mass production for the former, and limited spectral coverage in the mid-IR for the latter. Here, we demonstrate mid-IR supercontinuum generation in a birefringent SiGe-on-Si waveguide, and we show that, by changing the pump polarization, we can simultaneously control the spectral and the coherence properties of the generated spectra. For this, we designed a 2.2 cm long, 3.5 µm wide, and 2.7 µm thick air-clad SiGe-on-Si waveguide to achieve all-normal dispersion in TM polarization and a low anomalous region between ~3.8 and ~4.6 µm in TE polarization. Waveguide geometry and corresponding dispersion profiles for TE and TM polarization are presented in Fig 3.

<figure><img src="image_6.png" /><figcaption>Figure 4. a) Waveguide geometry and b) dispersion profiles for TE (red) and TM (blue) polarizations.</figcaption></figure>

Fig. 4a shows the measured SC spectra at the waveguide output for 100 mW average pump power (at 3.9 µm) in TE and TM polarization, respectively, using the same experimental setup as described in Paragraph 2. In TE polarization the -20 dB bandwidth of generated SC extends over more 2.0 µm, but exhibits low spectral flatness. In TM polarization, the spectrum covers a narrower band (-20 dB bandwidth below 2.0 µm), but the spectral flatness is improved. The dips at around 4.3 µm are due to the absorption from CO₂ in the atmosphere. The spectra are faithfully reproduced by simulations (Fig. 4b). Our results show the potential of birefringent waveguides for flexible and small footprint spectroscopy devices.

<figure><img src="image_7.png" /><figcaption>Figure 5. a) Experimental and b) simulated spectra for TE (red) and TM (blue) polarizations. Simulations were performed by solving the nonlinear Schrödinger equation, as described in [4].</figcaption></figure>

### 4. HIGH-Q SILICON GERMANIUM RING RESONATORS IN THE MID-INFRARED

The general design and fabrication of the ring resonator is described in [17]. It has a radius of 250 µm with 3.25 µm wide and 3.3 µm high waveguides of SiGe on a Si substrate. The dispersion is designed to be slightly anomalous to be suitable for Kerr comb generation. As was shown in [17], it was a difficulty to achieve critical coupling for SiGe waveguides with tight mode confinement as this required small gaps. As can be seen in Fig. 5a, this can be attributed to difficulties to control the etching of the very narrow gap between the ring and the bus waveguide (here 250 nm). We improved the fabrication and Fig. 5b shows the optimized gap that is fully etched.
<figure><img src="image_8.png" /><figcaption>Figure 6. Gap between ring and bus waveguide before (a) and after (b) optimization.</figcaption></figure>

To characterize the fabricated rings and show that we have better control of coupling, and hence can load more power in the cavity, we used the setup shown in Fig. 6a. We use a tunable (idler ~2.4 - 4.5 µm) continuous wave optical parametric oscillator (OPO) from Excelitas, 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. We can either measure the transmission by a photodetector or the in-cavity power by a MIR camera (FLIR). Imaging the ring with the camera (Fig. 6b) shows that we can achieve high loading power. Fig. 6c shows an example of a Q-factor measurement with this method, with a retrieved Q of 386,000. The record value of all Q-factors obtained across different rings and resonances is around one million. In Fig. 6d we show the resonance by the recorded in-cavity power for increasing input power (as measured before the coupling lens). For increasing power the typical bistability shape appears with a “tilted” resonance. Optical bistability can result from various different sources of nonlinearity, of which the most important ones for SiGe are the thermal nonlinear optical effect, where the high intensities result in a temperature change and hence through the thermo-optic coefficient in a refractive index change, and the Kerr nonlinearity, where the $\chi_3$ of SiGe leads to a direct dependence of the refractive index on optical intensity. In the future we envision to analyze the contribution of them to the observed bistability, which can be done by fast modulation of the input as the thermal and Kerr nonlinearities have different time scales [20].

<figure><img src="image_9.png" /><figcaption>Figure 7. (a) Measurement setup. (b) Image of MIR camera showing loaded ring. (c) Q-factor measurement for a SiGe ring with optimized etching and with gap of 250 nm (wavelength 4.05 µm, TE polarization, radius 250 µm, waveguide width 3.75 µm). (d) Ring resonances (in-cavity power recorded by MIR camera) showing bistability behavior for increasing power.</figcaption></figure>
## 5. CONCLUSION

In conclusion, our results confirm good suitability of group IV photonic platforms based on SiGe and Ge for reconfigurable MIR broadband sources. We have shown that the integration of thick (>800 nm) amorphous layers of low-loss PCMs on SiGe waveguides is compatible with SCG, which paves the way for emerging on-chip reconfigurable sources in the MIR. We also demonstrated that reconfigurability can be achieved by utilizing waveguide birefringence. Furthermore, our work on high-Q resonators in the MIR sets the directions towards MIR microcombs and the demonstration of optical bistability is another step towards that goal. Combined, these aspects lead the way towards fully integrated and efficient sensing systems.

## ACKNOWLEDGEMENTS

We acknowledge the support of the Horizon 2020 research and innovation program under the Marie Skłodowska-Curie Actions (ECLAUSion, Grant No. 801512; REDI, Grant No. 01034328), the Agence Nationale de la Recherche (ANR) (Grant No. MIRSiCOMB, ANR-17-CE24-0028; MIRthFUL, ANR-21-CE24-0005; MetaOnDemand, ANR-20-CE24-0013), 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 the French National Research Agency (PEPR OFCOC, ANR-22-PEEL-0005). We acknowledge the support from the EquipEx French Government program "IMPACT" (ANR-10-EQPX-33) and the French National Nanofabrication Network Renatech.

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