Pierre Noé

Multilevel phase shifters are key components in photonic integrated circuits. A major requirement in many applications is achieving non-volatile operation and low insertion loss simultaneously. Electrically, multiple phase levels can be encoded by controlling the heater power and employing different microheater architectures to induce varying degrees of phase-change material (PCM) amorphization, thereby modulating the device’s optical properties and the amplitude and phase of the propagating field. Here, we explore a platform based on the integration of a low-loss PCM, namely GeSe, sandwiched between microheaters and silicon waveguides. To achieve a large number of levels, we modify standard straight microheaters and propose a segmented heater design that breaks the heater’s symmetry along the propagation direction. We numerically demonstrate under pulse-width/pulse-amplitude modulation (PWM/PAM) that multilevel phase shifts can be achieved due to non-uniform heating in the GeSe PCM layer. However, the resulting phase levels for the basic configuration are highly abrupt because the constant power dissipation along the light propagation direction, associated with a uniform cross-section, does not permit a wide range of amorphization patterns. The proposed segmented heater, whose width gradually increases in steps along the light propagation direction, allows overcoming this limitation. This configuration enables the encoding of 164 well-spaced phase levels between 0 and π (>7-bit resolution), facilitated by smoother amorphization arising from the combined effects of non-uniform heating across segments and within each segment, while maintaining an insertion loss of only 0.6 dB in the fully crystalline state (worst case).

Chalcogenide ferroelectric Rashba semiconductors (FERSCs) offer a promising route to ultralow‐power spin–orbit devices, yet their integration into microelectronics has been hindered by the lack of high‐quality films grown using industry‐compatible methods. Here, we demonstrate CMOS‐compatible synthesis of rhombohedral α ‐GeTe(111) thin films on 200/300 mm Si wafers using an industrial deposition process based on van der Waals epitaxy, applied to this material class for the first time. Scanning Transmission Electron microscopy and advanced simulations of anomalous X‐ray diffraction measurements at synchrotron reveal that films are intrinsically self‐poled, exhibiting a robust upward out‐of‐plane ferroelectric polarization. The growth strategy is universal, enabling high‐quality α ‐GeTe(111) on metals and insulators without compromising structural or ferroelectric performance. Piezoresponse force microscopy confirms reversible 180° ferroelectric switching with performance comparable to molecular‐beam‐epitaxy benchmarks. This substrate‐independent growth strategy represents a decisive step toward the future large‐scale integration of FERSCs into functional spin‐orbit architectures, with the hope of bridging the gap between Rashba's fundamental physics and microelectronic implementation.

Using time-resolved optical spectroscopy, we investigate the acoustic dynamics of polycrystalline germanium telluride (GeTe) nanoscale thin films, a multifunctional material combining phase-change behavior for nonvolatile memory and reconfigurable photonics with promising thermoelectric performance. We probe the generation and propagation of longitudinal acoustic waves with submicrometer wavelengths and frequencies in the range of tens of gigahertz. Our experimental approach integrates phonon echo measurements in thicker layers (∼360 nm) with the analysis of fundamental thickness-breathing modes in thinner layers (∼80 nm), enabling separation of intrinsic and interface contributions to attenuation. This allows the direct extraction of both intrinsic acoustic attenuation and longitudinal sound velocity in polycrystalline GeTe thin films. We determine mean free paths of approximately 0.7–7 μm for 10–20 GHz acoustic waves. These findings clarify hypersonic phonon dynamics in GeTe, establish a benchmark for high-frequency acoustic dissipation in chalcogenide thin films, and support the development of devices ranging from phase-change memory to thermoelectrics.

Herein, the linear and nonlinear optical properties, together with the remarkable thermal stability and structural properties of thin films of nitrogen‐doped GeSe_1−xTe_x chalcogenide materials, are unveiled. These alloys are obtained by reactive magnetron cosputtering technique in a 300 mm wafer industrial tool, and they are promising candidates for high‐temperature nonvolatile resistive memories due to unprecedented crystallization temperatures among all known congruent phase‐change materials (PCMs). Besides, their uncommon optical properties, measured by spectroscopic ellipsometry, make them highly attractive candidates for reconfigurable and nonlinear photonic applications in the infrared range. They exhibit a reversible phase transformation between the amorphous and crystalline phases, displaying an unprecedented contrast in electronic and optical properties between these two phases. Unlike most known PCMs, they maintain high transparency in the mid‐infrared range in both phases. By tailoring nitrogen and tellurium contents in N‐doped GeSe_1−xTe_x thin films, it is shown here how it becomes feasible to customize the linear/nonlinear optical properties and thermal stability of such PCM thin films, enabling their optimization for various future optical and photonic applications.

Phase change materials for reconfigurable photonic integrated circuits: Pierre Noé, Benoît Cluzel, Stéphane Malhouitre, and Benoît Charbonnier, discuss phase change materials for reconfigurable photonic integrated circuits. Phase-change materials (PCMs) have gained increasing interest over the past decade for their potential in photonic applications. This article reviews their properties, key advantages over competing reconfigurable photonic technologies, and the challenges limiting their widespread adoption.

Phase change memories (PCMs) are at the heart of modern memory technology, offering multi‐level storage, fast read/write operations, and non‐volatility, bridging the gap between volatile DRAM and non‐volatile Flash. The reversible transition between amorphous and crystalline states of phase‐change materials such as GeTe or Ge₂Sb₂Te₅ is at the basis of PCM devices. Despite their importance, PCM devices face challenges including high power consumption during the RESET operation. Current research efforts focus on improving device architecture and exploring alternative phase‐change materials such as GeTe/Sb₂Te₃ super‐lattices (SLs), for which a reduced programming power consumption is observed compared with standard PCMs. Herein, by combining X‐ray diffraction and scanning transmission electron microscopy imaging of SL thin films with the study of the same SL in PCM devices, it is shown that it is possible to significantly decrease RESET energy of the device, without modifying the SL composition, by reducing the amount of structural defects through annealing treatment. The best device properties are obtained after transforming the SL into a defect‐free, highly out‐of‐plane oriented rhombohedral phase. These results offer a promising avenue for further improving the performance of SL‐based PCM devices through structural optimization.

The highly promising linear and nonlinear optical properties of innovative thin films of GeSe_1–xTe_x chalcogenide materials in the amorphous as‐deposited state, and after crystallization, are revealed here. These innovative alloys bridge the gap between two families of materials: chalcogenide glasses (GeSe) and phase‐change materials (GeTe). Their unique optical properties make them attractive candidates for reconfigurable and nonlinear photonic applications in the infrared. In this context, it is shown how, by varying the Te content of GeSe_1–xTe_x thin films, it is possible to tailor their linear and nonlinear optical properties to optimize them for a wide range of innovative applications.