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Abstract Advancements in nanofabrication processes have propelled nonvolatile phase change materials (PCMs) beyond storage‐class applications. They are now making headway in fields such as photonic integrated circuits (PIC), free‐space optics, and plasmonics. This shift is owed to their distinct electrical, optical, and thermal properties between their different atomic structures, which can be reversibly switched through thermal stimuli. However, the reliability of PCM‐based optical components is not yet on par with that of storage‐class devices. This is in part due to the challenges in maintaining a uniform temperature distribution across the PCM volume during phase transformation, which is essential to mitigate stress and element segregation as the device size exceeds a few micrometers. Understanding thermal transport in PCM‐based devices is thus crucial as it dictates not only the durability but also the performance and power consumption of these devices. This article reviews recent advances in the development of PCM‐based photonic devices from a thermal transport perspective and explores potential avenues to enhance device reliability. The aim is to provide insights into how PCM‐based technologies can evolve beyond storage‐class applications, maintain their functionality, and achieve longer lifetimes. 

Abstract Nonvolatile photonic integrated circuits employing phase change materials have relied either on optical switching mechanisms with precise multi-level control but poor scalability or electrical switching with seamless integration and scalability but mostly limited to a binary response. Recent works have demonstrated electrical multi-level switching; however, they relied on the stochastic nucleation process to achieve partial crystallization with low demonstrated repeatability and cyclability. Here, we re-engineer waveguide-integrated microheaters to achieve precise spatial control of the temperature profile (i.e., hotspot) and, thus, switch deterministic areas of an embedded phase change material cell. We experimentally demonstrate this concept using a variety of foundry-processed doped-silicon microheaters on a silicon-on-insulator platform to trigger multi-step amorphization and reversible switching of Sb₂Se₃and Ge₂Sb₂Se₄Te alloys. We further characterize the response of our microheaters using Transient Thermoreflectance Imaging. Our approach combines the deterministic control resulting from a spatially resolved glassy-crystalline distribution with the scalability of electro-thermal switching devices, thus paving the way to reliable multi-level switching towards robust reprogrammable phase-change photonic devices for analog processing and computing. 

Abstract Chalcogenide optical phase change materials (PCMs) have garnered significant interest for their growing applications in programmable photonics, optical analog computing, active metasurfaces, and beyond. Limited endurance or cycling lifetime is however increasingly becoming a bottleneck toward their practical deployment for these applications. To address this issue, a systematic study elucidating the cycling failure mechanisms of Ge₂Sb₂Se₄Te (GSST) is performed, a common optical PCM tailored for infrared photonic applications, in an electrothermal switching configuration commensurate with their applications in on‐chip photonic devices. Further a set of design rules building on insights into the failure mechanisms is proposed, and successfully implemented them to boost the endurance of the Ge₂Sb₂Se₄Te (GSST) device to over 67 000 cycles. 

Abstract Chalcogenide phase change materials (PCMs) have become one of the most promising material platforms for the Optics and Photonics community. The unparalleled combination of nonvolatility and large optical property modulation promises devices with low‐energy consumption and ultra‐compact form factors. At the core of all these applications lies the difficult task of precisely controlling the glassy amorphous and crystalline domains that compose the PCM microstructure and dictate the optical response. A spatially controllable glassy‐crystalline domain distribution is desired for intermediate optical response (vs. binary response between fully amorphous and crystalline states), and temporally resolved domains are sought after for repeatable reconfiguration. In this perspective, we briefly review the fundamentals of PCM phase transition in various reconfiguring approaches for optical devices. We discuss each method's underpinning mechanisms, design, advantages, and downsides. Finally, we lay out current challenges and future directions in this field. 

The properties of chalcogenide phase change materials have long attracted the scientific community due to a combination of state retention (i.e., memory) and a large contrast in electrical and optical properties between different solid phases. The last decade has witnessed a vast interest in utilizing this material family for optics and photonics, given their large refractive index modulation, nonvolatility—elusive in optics—and straightforward integration into photonic devices. Thus, designing new optical phase change materials (O-PCMs) and demonstrating high-performance applications have become fast-growing research topics. However, advances in O-PCMs have predominantly followed empirical device developments, driven by their promise in trending technological applications. Nonetheless, a growing interest in revealing their materials science intricacies is driving the much-needed effort toward a holistic understanding and codesign of O-PCMs, which is required to fill knowledge gaps, expand the materials library, and solve the most pressing device performance challenges. 

Chalcogenide phase-change materials (PCMs) offer a promising approach to programmable photonics thanks to their nonvolatile, reversible phase transitions and high refractive index contrast. However, conventional designs are limited by global phase control over entire PCM thin films between fully amorphous and fully crystalline states, which restricts device functionality and confines design flexibility and programmability. In this work, we present a novel approach that leverages pixel-level control of PCM in inverse-designed photonic devices, enabling highly reconfigurable, multi-functional operations. We integrate low-loss Sb2Se3 onto a multi-mode interferometer and achieve precise, localized phase manipulation through direct laser writing. This technique allows for flexible programming of the photonic device by adjusting the PCM phase pattern rather than relying on global phase states, thereby enhancing device adaptability. As a proof of concept, we programmed the device as a wavelength-division multiplexer and subsequently reconfigured it into a mode-division multiplexer. Our results underscore the potential of combining inverse design with pixel-wise tuning for next-generation programmable phase-change photonic systems. 

The integration of computing with memory is essential for distributed, massively parallel, and adaptive architectures such as neural networks in artificial intelligence (AI). Accelerating AI can be achieved through photonic computing, but it requires nonvolatile photonic memory capable of rapid updates during on-chip training sessions or when new information becomes available during deployment. Phase-change materials (PCMs) are promising for providing compact, nonvolatile optical weighting; however, they face limitations in terms of bit precision, programming speed, and cycling endurance. Here, we propose a novel photonic memory cell that merges nonvolatile photonic weighting using PCMs with high-speed, volatile tuning enabled by an integrated PN junction. Our experiments demonstrate that the same PN modulator, fabricated via a foundry-compatible process, can achieve dual functionality. It supports coarse programmability for setting initial optical weights and facilitates high-speed fine-tuning to adjust these weights dynamically. The result shows a 400-fold increase in volatile tuning speed and a 10,000-fold enhancement in efficiency. This multifunctional photonic memory with volatile and nonvolatile capabilities could significantly advance the performance and versatility of photonic memory cells, providing robust solutions for dynamic computing environments. 

Programmable and reconfigurable optics hold significant potential for transforming a broad spectrum of applications, spanning space explorations to biomedical imaging, gas sensing, and optical cloaking. The ability to adjust the optical properties of components like filters, lenses, and beam steering devices could result in dramatic reductions in size, weight, and power consumption in future optoelectronic devices. Among the potential candidates for reconfigurable optics, chalcogenide‐based phase change materials (PCMs) offer great promise due to their non‐volatile and analogue switching characteristics. Although PCM have found widespread use in electronic data storage, these memory devices are deeply sub‐micron‐sized. To incorporate phase change materials into free‐space optical components, it is essential to scale them up to beyond several hundreds of microns while maintaining reliable switching characteristics. This study demonstrated a non‐mechanical, non‐volatile transmissive filter based on low‐loss PCMs with a 200 × 200 µm²switching area. The device/metafilter can be consistently switched between low‐ and high‐transmission states using electrical pulses with a switching contrast ratio of 5.5 dB. The device was reversibly switched for 1250 cycles before accelerated degradation took place. The work represents an important step toward realizing free‐space reconfigurable optics based on PCMs.