Focus on Phase Change Materials for Photonics

The growing demand for multifunctional nanophotonic devices has led to the exploration, and utilization, of a plethora of exotic electro-optical materials. Recently, chalcogenide glass based phase change materials (PCMs) have shown utility as a tuning material for a range of nanophotonic devices. Owing to their low loss, ultrafast switching speeds and wide waveband operation, PCMs are integrated in an increasing number of next-generation tunable components, including integrated photonic switches, metasurface optics and tunable spectral filters. Nonetheless, modelling of PCM-based devices is challenging—both in terms of accurate representation of experimentally derived material properties in different phase states, and standardization of results across the research community. Further, as each device requires optimization of specific performance metrics dependent on their respective application, any inaccuracies will lead to erroneous outcomes. In this work, we introduce PCM-net (http://nekocloud.com/pnet/): an online database of the complex refractive indices of a variety of chalcogenide glass PCMs (such as GeSbTe), as an accessible and indexed repository for data sharing across the PCM community. Refractive indices (n) and extinction coefficients (k) between amorphous and crystalline states are directly extracted from experimentally derived data in numerous academic research articles, and collated into the material resource database. Due to the inaccuracies associated with our data collection methods, this data is supplemented with additional computationally generated data, obtained through WVASE^® —a commercial ellipsometry analysis software package. To demonstrate the utility of PCM-net, we provide a NASA application-driven device optimization example using the optical properties of PCMs collected with our database. We anticipate the database providing great use to the PCM community and coordinated research efforts enabled by PCM-net will promote the shared repository for the selection of appropriate PCMs for tunable nanophotonic device design for a range of applications.

Chalcogenide phase-change materials (PCMs) show a significant contrast in optical reflectivity and electrical resistivity upon crystallization from the amorphous phase and are leading candidates for non-volatile photonic and electronic applications. In addition to the flagship Ge₂Sb₂Te₅ phase-change alloy, doped Sb₂Te alloys, in particular AgInSbTe used in rewritable optical discs, have been widely investigated for decades, and nevertheless the theoretical insights on the optical properties of this important family of PCMs are scarce. Here, we carry out thorough ab initio simulations to gain an atomistic understanding of the optical properties of Sb₂Te and AgInSbTe. We show that the large optical contrast between the amorphous and crystalline phase stems from the change in bond type in the parent compound Sb₂Te. Ag and In impurities serve mostly the purpose of stabilization of the amorphous phase, and have marginal impact on the large variation in the dielectric function upon the phase transitions.

Chalcogenide phase change semiconductors have played a crucial role in the evolution of photonic technologies. From their decades-long utilization at the core of optical disks to their emergence as a highly promising reconfigurable component for a variety of nanophotonic modulation, switching and sensing platforms, the field of optics has continuously recognized their potential and sought to engineer their properties through a variety of material, device and fabrication level schemes. Most recently, the integration of phase change semiconductors within various photonic metamaterials, metadevices and metasystems has ignited research interest worldwide. This has facilitated the development of a wealth of highly promising application-driven nanophotonic device platforms that address growing societal demands requiring higher data storage capacity, faster and more efficient telecommunication, as well as adaptive sensing and imaging with reduced size, weight and power requirements. Here, we present a comprehensive review on the evolution of reconfigurable phase change chalcogenide metamaterials that focuses not just on a device level perspective but also examines the underlying material and fabrication considerations that are critical to obtaining optimal performance in these groundbreaking devices.