# High-Endurance and Energy-Efficient All-Optical Programming of Scalable Silicon Waveguides with Integrated Phase-Change Material Patches

Rajath Sawant, Anthony Albanese, Arnaud Rogemont, Gregorio

Gonzalez-Cortes, Yoann Brûlé, Lara Karam, Jean-baptiste Jager, Stephane

Malhouitre, Benoit Charbonnier, Aurélien Coillet, et al.

► To cite this version:

Rajath Sawant, Anthony Albanese, Arnaud Rogemont, Gregorio Gonzalez-Cortes, Yoann Brûlé, et al. "High-Endurance and Energy-Efficient All-Optical Programming of Scalable Silicon Waveguides with Integrated Phase-Change Material Patches." Advanced Optical Materials, 2025, 13 (25), pp.e00775. 10.1002/adom.202500775. hal-05250600

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# High-Endurance and Energy-Efficient All-Optical Programming of Scalable Silicon Waveguides with Integrated Phase-Change Material Patches

Rajath Sawant, Anthony Albanese, Arnaud Rogemont, Gregorio Gonzalez-Cortes, Yoann Brûlé, Lara Karam, Jean-Baptiste Jager, Stephane Malhouitre, Benoit Charbonnier, Aurélien Coillet, Pierre Noé,* and Benoit Cluzel*

With the rise of optical computing, phase-change materials (PCM) are attractive candidates for implementing programmable optical functionalities in photonic circuits. Integrating them as tunable media in photonic waveguides appears to be a simple but effective way to program their optical transfer function in both amplitude and phase through near-field interactions. This is exactly what is being addressed in this work by integrating Ge<sub>2</sub>Sb<sub>2</sub>Te<sub>5</sub> (GST) patches on top of silicon waveguides. A scalable fabrication process is described, and the optical properties and programming capabilities are reported obtained from heterodyne interferometry to access the complex transmission of waveguides on the fly. It is reported that both amorphization and crystallization energy thresholds are at least an order of magnitude lower than previous reports, and programming endurance is successfully tested up to 10<sup>8</sup> cycles, exceeding all previous results obtained with GST or any other PCM for use in photonics. Finally, the morphological and chemical characterization of PCM against aging is presented, revealing a failure mechanism due to phase segregation and opening new avenues to further optimize device endurance. As such, these results will be valuable to the large community of scientists seeking to integrate phase-change materials into advanced photonic architectures where reconfigurability is required.

## 1. Introduction

Chalcogenide materials are enabling materials for advanced technologies such as neuromorphic computing, in both photonic<sup>[1–5]</sup> and electronic domains,<sup>[6–8]</sup> spinorbitronics,<sup>[9,10]</sup>

nano-photonics<sup>[11–16]</sup> or nonlinear photonics,<sup>[17]</sup> to name just a few. Within the large family of chalcogenides, alloys made of germanium (Ge), antimony (Sb), and tellurium (Te) exhibit a non-volatile and reversible structural phase change from amorphous to crystalline that is either optically, thermally, or electronically induced. Such phase changes are accompanied by large variations in both optical and electronic properties, which have been used for decades in non-volatile data storage devices, such as the RW-DVD optical disks of the last century<sup>[18–20]</sup> to the current electronic Phase-Change Memory.<sup>[21]</sup> Nowadays, optical phase change materials (O-PCMs) also appear as promising candidates in the rapidly growing field of photonic computing, which could potentially overcome the bottleneck of von Neumann computation.<sup>[11,13,22]</sup> Indeed, they enable the implementation of programmable non-volatile optical functionalities, which are essential building blocks for such low-power and ultrafast

photonic integrated circuits (PICs). O-PCM patches, when evanescently coupled to the photonic waveguides, are convenient solutions for such applications, since they provide an active control over the light transmitted through them.<sup>[23–25]</sup> Moreover, this configuration allows the phase change to be optically

R. Sawant, A. Rogemont, A. Coillet, B. Cluzel
Université de Bourgogne Europe
Laboratoire Interdisciplinaire Carnot de Bourgogne
UMR CNRS 6303, Dijon F-21078, France
E-mail: benoit.cluzel@ube.fr

The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/adom.202500775

© 2025 The Author(s). Advanced Optical Materials published by Wiley-VCH GmbH. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

DOI: 10.1002/adom.202500775

A. Albanese, Y. Brûlé, L. Karam, S. Malhouitre, B. Charbonnier, P. Noé
Université Grenoble Alpes
CEA, LETI, Grenoble F-38000, France
E-mail: pierre.noe@cea.fr

G. Gonzalez-Cortes
Laboratoire Ondes et Matière d'Aquitaine (LOMA)
UMR CNRS 5798, Talence F-33405, France

J.-B. Jager
Université Grenoble Alpes, CEA, Grenoble INP
IRIG, PHELIQS
Grenoble F-38000, France
<table><caption>Table 1. Comparison with literature.</caption><thead><tr><th>Paper ref.</th><th>Device description, switching mode</th><th>E<sub>amorph</sub></th><th>E<sub>cryst</sub></th><th>ΔT/T(%)</th><th>Δφ/L</th><th>Endurance</th></tr></thead><tbody><tr><td>[23]</td><td>GST225 on Si<sub>3</sub>N<sub>4</sub> waveguide, All-optical</td><td>430 pJ</td><td>—</td><td>21%</td><td>—</td><td>50 cycles</td></tr><tr><td>[26]</td><td>GST225 on SOI waveguide, Electrothermal</td><td>5.5 nJ</td><td>—</td><td>3 dB</td><td>—</td><td>1500 cycles</td></tr><tr><td>[26]</td><td>Sb<sub>2</sub>Se<sub>3</sub> on SOI waveguide, Electrothermal</td><td>9.25 nJ</td><td>—</td><td>7 dB</td><td>—</td><td>1100 cycles</td></tr><tr><td>[27]</td><td>GST225 on SOI waveguide, Electrical</td><td>380 nJ</td><td>6.83 μJ</td><td>10 dB</td><td>—</td><td>2800 cycles</td></tr><tr><td>[36]</td><td>Sb<sub>2</sub>S<sub>3</sub> film on SiO<sub>2</sub> substrate, All-optical</td><td>130 nJ</td><td>—</td><td>20%</td><td>—</td><td>7000 cycles</td></tr><tr><td>[28]</td><td>Ge<sub>2</sub>Sb<sub>2</sub>Se<sub>5</sub> on Si waveguide, Electrothermal</td><td>—</td><td>—</td><td>40%</td><td>—</td><td>5 × 10<sup>5</sup> cycles</td></tr><tr><td>[37]</td><td>Sb<sub>2</sub>Se<sub>3</sub> film on SiO<sub>2</sub> substrate, All-optical</td><td>> 600 pJ</td><td>> 5 nJ</td><td>21%</td><td>—</td><td>2 × 10<sup>6</sup> cycles</td></tr><tr><td>[38]</td><td>Sb<sub>2</sub>Se<sub>3</sub> film on Si substrate, All-optical</td><td>> 10 nJ</td><td>> 100 nJ</td><td>25%</td><td>—</td><td>10<sup>7</sup> cycles</td></tr><tr><td>Our work</td><td>GST225 on SOI waveguide, All-optical</td><td>100-240 pJ</td><td>7.4 nJ</td><td>60%</td><td>37°/μm</td><td>10<sup>8</sup> cycles</td></tr></tbody></table>

triggered, which has the advantage of making the device design less bulky and more energy efficient compared to other switching approaches.

Electrical and electrothermal modes of switching are highly attractive for chip-scale device integration and are independent of optical absorption of the material[26-30] but often suffer from lower endurance and higher switching energies.[31] In contrast, all-optical programming schemes, such as demonstrated here, offer a complementary approach with high energy efficiency and scalability for photonic signal processing applications.

In this work, we report on a well known PCM, Ge<sub>2</sub>Sb<sub>2</sub>Te<sub>5</sub> (hereafter referred to as GST for simplicity), integrated as an activation patch on silicon-on-insulator (SOI) photonic integrated circuit waveguides. We propose an all-optical programming setup coupled to an optical heterodyne interferometric detection that probes in situ the phase changes through the resulting modification of the waveguide complex transmittance. GST has an excellent contrast in refractive index between amorphous and crystalline phases at 1550 nm wavelength (Δn = 2.9 and Δκ = 1.3),[32] which is advantageous for achieving higher modulation in complex transmission. Compared to previous works on PCM programming, here the modulation is reported not only in amplitude but also in optical phase, which provides a complete picture of the complex refractive index change in the PCM patches. Several works have been reported along similar lines of optically switching PCM,[23,33-35] but the hereby reported performances exceed all previous results in terms of phase change endurance, contrast of optical properties between PCM states, and energy thresholds for crystallization and amorphization, as summarized in Table 1.

## 2. Fabrication and Experimental Methods

All investigated devices were fabricated on 200-mm SOI wafers using industrial nanofabrication tools ensuring both compatibility with CMOS technologies and scalability to mass production. Detailed fabrication steps are provided in the Supplementary Information S1. SOI PICs, including waveguides and grating couplers, were fabricated by deep UV lithography followed by dry etching, then encapsulated with a thick SiO<sub>2</sub> layer and further planarized by chemical mechanical polishing (CMP) to near the top surface of the waveguide. The GST thin films were deposited by magnetron sputtering from a pure GST target in an Ar atmosphere. The deposited thin films were protected from surface oxidation by in-situ deposition of a 20 nm GeN<sub>x</sub> capping layer obtained by reactive sputtering of a pure Ge target under Ar+N<sub>2</sub> atmosphere.

Finally, PCM patches were drawn on silicon waveguides by e-beam lithography, followed by dry etching and a final physical vapor deposition of a thin 20 nm SiO<sub>2</sub> layer to prevent oxidation of the GST etch walls. The scanning electron microscope (SEM) images of typical devices fabricated are shown in Figure 1. Figure 1a shows an SEM image of the device cross section obtained after Focused Ion Beam (FIB) milling performed after full layer deposition of PCM and GeN<sub>x</sub> and before PCM patch etching. The SEM view highlights the homogeneity of the layers as well as the accuracy of the CMP planarization. Figure 1b shows an SEM top image of a final device after etching of a PCM patch and SiO<sub>2</sub> capping layer deposition. The patch lengths vary from 1 µm to 3 µm, while their widths are 10 µm which are much wider than that of the waveguide itself.

A crystallization annealing under a nitrogen atmosphere at 250°C for a few minutes completes the fabrication process to ensure that all devices are in the same crystalline state prior to the programming tests described below. The I/O light intensities of the patches of different lengths are measured to determine the propagation losses (see the Supplementary Information S2). Propagation losses of 5.7 dB.µm<sup>-1</sup> were found, consistent with the high extinction coefficient of GST in its crystalline phase.

As shown schematically in Figure 1b, to switch the state of the PCM layer and to probe it, programming laser pulses and a continuous wave (CW) probe laser (hereafter referred to as the read laser) are sent into the waveguide in counter-propagating directions and are slightly wavelength-detuned to allow further spectral filtering. As shown in Figure 2b, a programming cycle consists of sending a short (400 ps) and intense amorphizing pulse (write), followed 20 µs later by a longer but less intense pulse to re-crystallize and reset the PCM state (erase). In this configuration, due to the non-zero extinction coefficient κ of the GST in both amorphous and crystalline phases, the optical pulses at 1550 nm are both sufficiently absorbed to provide the thermal energy for the amorphization and crystallization of the PCM layer.

The programming pulses are tailored from a CW laser emitting at 1552 nm using standard telecommunication fiber optic components. The programming laser scheme is shown in Figure 2a. An electro-optic modulator (EOM) driven by two arbitrary waveform generators (AWG) defines the programming
<figure><img src="image_3.png" /><figcaption>**Figure 1.** a) SEM image of SOI waveguide cross section (width: 600 nm, height: 300 nm) covered by GST PCM and GeN capping layer of height 20 nm each. b) Typical top SEM image of the final device with the GST patch and the buried silicon waveguide. A schematic of the optical experiments further conducted to program and read the PCM state is superimposed. c) Mode simulations performed in COMSOL on the waveguide-PCM system, mode indices of 2.48 + 0.01*i* for amorphous and 2.65 + 0.08*i* for crystalline state at 1550 nm wavelength were found.</figcaption></figure>

pulse sequence with a 40 dB extinction ratio. The first AWG sets asymmetric pulses with fast rise and slow fall times in the µs range to allow PCM crystallization. For amorphization, a pulse generator triggered by the second AWG delivers 400 ps pulses, short and energetic enough to quickly reach the PCM melting temperature and quench the PCM molten phase to the amorphous state thanks to fast thermal dissipation through the sili-

con waveguide. The pulse sequence is then amplified by an Erbium Doped Fiber Amplifier (EDFA), whose amplified spontaneous emission (ASE) is removed by a spectral filter before the pulse sequence is sent to the device.

The phase-state change of the patches is then detected in real time thanks to an optical heterodyne detection based on a Mach-Zehnder interferometer[39] and fed by a low power probe

<figure><img src="image_4.png" /><figcaption>**Figure 2.** a) Programming pulse generation setup, AWG -Arbitrary Waveform Generator, EOM -Electro-Optic Modulator, EDFA -Erbium Doped Fiber Amplifier. b) A sequence of amorphization (write) and crystallization (erase) pulses from the pulse generation setup. c) Heterodyne interferometer setup to detect transmission and optical phase change: AOM - Acousto-Optic Modulator, LO - Local Oscillator, Filt.- Spectral filter, D0, D1, D2 - Photodiodes.</figcaption></figure>
<figure><img src="image_5.png" /><figcaption>**Figure 3.** Transmission and optical phase change recorded upon exposure to programming pulses i.e. amorphization (write) and crystallization (erase) pulses on GST of patch length 1.5 µm.</figcaption></figure>

(P = 0.6 mW) CW laser emitting at 1550 nm. As shown in Figure 2c, the sample is placed in one of the arms of the interferometer and is sandwiched between two circulators that allow the counter-propagative combination of the write and read lasers inside the sample. A spectral filter is also added to the read laser path after the sample to suppress the residual programming pulses reflected from the sample and passing through the circulator. The other arm is the reference arm, which contains an acousto-optic modulator (AOM) driven by a local oscillator (LO) at 43 MHz that shifts the laser wavelength frequency, and a delay line that synchronizes the optical paths of the read signal and reference arms to minimize phase noise. The interference between the two arms is split with a 50:50 coupler and detected using a balanced amplified photodiode (Thorlabs PDB450C) that maximizes the contrast of the interferometric signal $x(t)$. This signal is further fed to a lock-in amplifier, where it is mixed with the LO signal to produce two separate outputs: relative changes in transmittance ($\Delta T$) and phase ($\Delta\phi$).

**3. Results and Discussion**

Typical results recorded in this configuration are shown in **Figure 3** for a 1.5 µm long GST patch. The two outputs of the lock-in amplifier are shown together with the sequence of programming pulses recorded at the photodetector $D_0$. The programming pulses cause drastic, reversible, and nonvolatile changes in both the transmitted amplitude and phase. Amorphization pulses of 400 ps pulse width and pulse energy $E_{amorph}$ of around 100 pJ, in-

crease the transmittance by about $\Delta T = 60\%$ while decreasing the optical phase delay by $\Delta\phi = 55^\circ$. Such variations are consistent with the decrease in both refractive index and extinction coefficient from crystalline to amorphous GST. It is worth noting that the transmittance change is also enhanced by the slight change in mode mismatch between the waveguide sections with and without PCM patches. In fact, the high refractive index ($n = 7.5$) of the crystalline GST slightly pulls the TE mode of the Si waveguide toward the patch, as seen on Figure 1c, while the amorphous GST. which has a refractive index closer to that of silicon ($n = 3.5$), only slightly perturbs it.

Then, crystallization pulses of width 1.2 µs and energy $E_{cryst}$ of around 7.4 nJ, reset the transmission level and the phase delay to their initial values. This demonstrates the reversibility of the state changes and validates the viability of the proposed approach for high-contrast binary data storage states. It is also noteworthy that the time to program the PCM patch to a given state is measured to be about 3 µs, which corresponds to the low-pass filter limit within the heterodyne interferometer. The switching dynamics of the device for write and erase operations are further analyzed in Supplementary Information S3 without the interferometer but with a faster photodiode. The amorphization and crystallization time scales are in the range of 3–6 ns and 0.5–1 µs, respectively.

The influence of the crystallization pulse duration $\tau_c$ and peak power $P_c$ on the changes of transmitted amplitude $\Delta T/T_c$ and phase $\Delta\phi$ are plotted as 3D bar graphs in **Figure 4a,b** respectively. This plot shows the degree of crystallization for various pulse energies $P_c\tau_c$ and allows to find the crystallization energy threshold which is found to be around 540 pJ. Similarly,
<figure><img src="image_6.png" /><figcaption>**Figure 4.** a) Transmission change and b) optical phase change studied by varying pulse width and peak power of the crystallizing pulses. Amorphizing pulse energy (legend- single pulse energy) is varied to determine amorphization threshold by recording c) the transmission change and d) the optical phase change.</figcaption></figure>

the amorphization energy was tuned by attenuating the peak power of the 400 ps pulses, the pulse duration being set by the short pulse generator itself, which cannot be altered. In addition, to test the feasibility of multi-level programming approaches, we repeated the amorphization pulse 10 times before recrys- tallization in a single programming sequence. In this case, we used a repetition rate of 50 kHz, which is slow enough to en- sure total thermal dissipation between each 400 ps pulse. The results are shown in Figure 4c (transmittance) and Figure 4d (phase). Transmittance and phase curves display similar behav- ior as previously discussed, but with additional intermediate amorphization levels, showing that we can also access more lev- els of information storage. The threshold energy above which the phase change can be observed is found to be around 40 pJ. It has to be noted that the threshold energy values are different from the actual pulse energies used for programming (see Table 1), as in this case higher pulse energies are applied to achieve maxi- mum transmission contrast with a single pulse. Upon incidence of the crystallization reset pulse, an abrupt change in complex transmission is observed, which is likely due to free carrier gener- ation in silicon waveguides. As this is a transient effect unrelated to the active material GST, it has minimal influence on the overall device performance.

A final aspect of PCM properties that is critical for practical applications is endurance, i.e., the number of cycles of write- erase operations that can be performed over it. Since injecting excess energy into the PCM can reduce cycling performance, the endurance of the GST patch is tested after optimizing the

amorphization and crystallization pulse energies. We repeated the write-erase programming sequence 1 billion times using the previously determined pulse energies of $E_{amorph}$ = 100 pJ and $E_{cryst}$ = 7.4 nJ to attain maximum transmission contrast. In practice, this is a 14-hour endurance test during which we continuously melt, quench, and recrystallize the PCM patch every 50 µs using programming laser pulses. For convenience and to avoid saturating the memory of our acquisition cards, we recorded the transmitted amplitude and phase through the PCM patches only for a few programming sequences, each $10^N$ sequences, as can be seen on **Figure 5**.

For the purpose of comparison with previous reports in the field, we summarize all characteristics of our devices in Table 1 and compare them with values reported in the literature. We stress here that the measurements were performed on several different patches and the results were reproducible. The range of switching energies (100–240 pJ) observed during these experiments is reported in the table. Only a few recent works have reported both amorphization and crystallization thresholds and device endurance. None of them has dealt with direct optical phase change per unit length, which is a key parameter for applications in non-volatile phase shifters. From this direct comparison, it is thus worth noting that the results reported here challenge or exceed previous realizations, making our approach particularly relevant for integrating non-volatile functionalities into commercial, mass-production compatible PICs.

The superior performance of our device arises from the integration of encapsulated GST patches on strip silicon waveguides,
<figure><img src="image_7.png" /><figcaption>**Figure 5.** Transmitted amplitude and phase changes through the silicon-decorated PCM patches recorded each $10^N$ sequences, with N ranging from 1 to 9, up to 1 billion sequences.</figcaption></figure>

actuated via optical pulses propagating within the waveguide. The high thermal conductivity of silicon facilitates efficient heat dissipation, enhancing device endurance. The capping layer is also expected to play a role in the efficiency of the PCM as discussed in Ref. [40]. Furthermore, the strip geometry ensures effective optical mode interaction with the GST patch, reducing programming energy, as discussed in Ref. [34]. To determine and better understand the degradation mechanism of programmable waveguides, we finally performed top-view SEM imaging of two different patches, combined with EDX (Energy Dispersive X-ray) analysis. The first patch was prepared in an amorphous state after a few tens of programming cycles (30 cycles) as shown in **Figure 6a**. The second patch shown in Figure 6b was degraded after one billion programming cycles. In Figure 6a, SEM image of the patch prepared in the amorphous state evidences various bright and dark contrast in the region located at the intersection of the patch and the underneath Si waveguide. In particular, a large darker zone, slightly off-centered toward the right side of the GST patch, is visible. The shape of this zone strongly resembles the off-centered shapes of the simulated temperature distributions shown in Figure S3 of Ref. [41] where the authors have developed a time-dependent multi-physics simulation framework to model the thermal behavior of GST patches deposited on $Si_3N_4$ rib waveguides when programmed using light pulses. Therefore, we can assume that this zone corresponds to the amorphized region of our GST patch since the programming laser pulses are applied from right of the image explaining the right off-centering of the amorphized region. Note that four small darker regions located at the corner intersections of GST patch and Si waveguide are also visible in Figure 6a. They are likely arising from high temperature and local melting of GST patch due to local field-enhancement resulting from corner effects.

The corresponding EDX line-scan profiles of Ge, Sb, and Te atoms acquired on the GST patch and along the Si waveguide direction, as indicated by the white dashed line on Figure 6a are plotted in Figure 6c. For different EDX signals, the baseline was obtained by removing the mean of each signal outside the GST patch and then normalized to a maximum of 100 for the purpose of comparison. First, we observe that the EDX signals of Ge, Sb, and Te are stronger on the left side of the patch showing a material accumulation in this region. Second, the amorphized region, corresponding to the middle-right of the patch, show a slight increase in the Sb and Te signals but is enriched in Ge. These observations suggest that phase segregation and chemical species migration occurred during programming cycles, similar to what is usually observed in electronic PCM devices.[42] However, the migration mechanism appears to be different than electro-migration in our case. Here, it seems to be temperature-related and can be explained by thermos-diffusion or the Soret effect resulting from a large temperature gradient along the patch upon programming.[43,44] When an amorphization pulse is applied to the patch through the Si waveguide, a Ge rich zone forms at the hottest spot of the patch, located on the right side of the patch. Ge being a lighter element than Sb and Te, Ge atoms move toward the hottest point of the patch. While the heavier elements Sb and Te, which have similar atomic weights, are segregated toward the cooler areas due to the thermos-migration effect.

Figure 6b,d presents an SEM image of a degraded patch after one billion of programming cycles and its corresponding EDX signal acquired in the middle of GST patch along the underneath Si waveguide direction. Figure 6b evidences cracks and a hole in the GST patch on the right side of the patch. Right side of the patch corresponding to the hottest spot of the molten phase, the hole results from a local ablation of GST and the ablated matter can be observed in the upper part of the image as a black
<figure><img src="image_8.png" /></figure>

<figure><img src="image_9.png" /></figure>

<figure><img src="image_10.png" /></figure>

<figure><img src="image_11.png" /><figcaption>**Figure 6.** SEM images of GST patches surmounting Si waveguides prepared in an amorphous state after a few tens of cycles (a) and degraded after one billion cycles (b). In (c) and (d) are plotted the results of EDX line scans acquired along the white dashed lines shown on (a) and (b) SEM images, respectively. Dotted black lines indicate patch edges.</figcaption></figure>

pyramid-like dot. In the corresponding EDX profile of Figure 6d, a gradient of Ge content along the patch is visible. Ge relative concentration is significantly higher on the right side, where the hottest region of the patch is formed upon programming, while Sb and Te are mostly located on the left side in the cooler regions, confirming a phase segregation. During crystallization over the cycling process, the volume change occurring upon crystallization or amorphization of GST leads to Ge segregation and cracks formation, and ultimately to local GST ablation which causes the final failure of the device.

## 4. Conclusion

In summary, we have reported here on the integration of one of the most popular phase change materials, GST225, into silicon photonic circuits. Fabrication was performed on 200 mm wafers using industrial-grade tools, ensuring scalability to mass production. Using heterodyne interferometry, we provided a complete picture of the device properties, including the energy thresholds for switching the PCM, the relative optical transmission and phase changes associated with structural changes, and the overall endurance of the devices. The performances reported here in terms of energy efficiency and endurance, which are two key parameters to envision any practical application, exceed all previous results in this field. Finally, we unveil the degradation mechanism

of the PCM and show that managing the temperature gradients responsible for phase segregation and ultimate device failure would open a large avenue for further improvement of device endurance. As such, this work is an important step toward future PICs dedicated to non von Neumann optical computation thanks to non-volatile functionalities. Indeed, in neuromorphic networks, a PCM patch can act both as a spiking neuron once a certain threshold is reached, and as a synapse weighting the signals to be sent to the next layer of neurons, the latter weighting being programmed according to an appropriate learning rule. The proposed integrated PCM device, in which phase changes can be accumulated and optically programmed into intermediate states, thus promises both functionalities with reduced power consumption, a particularly important aspect for the deployment of artificial intelligence hardware.

## Supporting Information

Supporting Information is available from the Wiley Online Library or from the author.

## Acknowledgements

R.S. and A.A. contributed equally to this work. The authors acknowledge funding from the ANR (Agence Nationale de la Recherche) through the
OCTANE project (grant agreement ANR-20-CE24-0019). The authors also thank the French research group "GdR CHALCO", recently created for fruitful discussions (https://gdrchalco.cnrs.fr/). This work was supported in part in the framework of the NEUROPULS project funded by the European Union's Horizon Europe research and innovation program under grant agreement no. 101070238. All optical tests were conducted on the facilities of the SMARTLIGHT platform funded by the Agence Nationale de la Recherche (EQUIPEX+ contract "ANR-21-ESRE-0040") and Région Bourgogne-Franche-Comté.

## Conflict of Interest

The authors declare no conflict of interest.

## Data Availability Statement

The data that support the findings of this study are available from the corresponding author upon reasonable request.

## Keywords

phase change materials, programmable photonics, silicon photonics

Received: March 10, 2025

Revised: June 18, 2025

Published online: July 8, 2025

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