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The optical properties of phase-change materials (PCM) can be tuned to multiple levels by controlling the transition between their amorphous and crystalline phases. In multi-material PCM structures, the number of discrete reflectance levels can be increased according to the number of PCM layers. However, the effect of increasing number of layers on quenching and reversibility has not been thoroughly studied. In this work, the phase-change physics and thermal conditions required for reversible switching of single and multi-material PCM switches are discussed based on thermo-optical phase-change models and laser switching experiments. By using nanosecond laser pulses, 16 different reflectance levels in Ge2Sb2Te5 are demonstrated via amorphization. Furthermore, a multi-material switch based on Ge2Sb2Te5 and GeTe with four discrete reflectance levels is experimentally proven with a reversible multi-level response. The results and design principles presented herein will impact active photonics applications that rely on dynamic multi-level operation, such as optical computing, beam steering, and next-generation display technologies., Comment: 25 pages, 8 figures

Phase change memory devices are typically reset by melt-quenching a material to radically lower its electrical conductance. The high power and concomitantly high current density required to reset phase change materials is the major issue that limits the access times of 3D phase change memory architectures. Phase change superlattices were developed to lower the reset energy by confining the phase transition to the interface between two different phase change materials. However, the high thermal conductivity of the superlattices means that heat is poorly confined within the phase change material, and most of the thermal energy is wasted to the surrounding materials. Here, we identified Ti as a useful dopant for substantially lowering the thermal conductivity of Sb2Te3-GeTe superlattices whilst also stabilising the layered structure from unwanted disordering. We demonstrate via laser heating that lowering the thermal conductivity by doping the Sb2Te3 layers with Ti halves the switching energy compared to superlattices that only use interfacial phase change transitions and strain engineering. The thermally optimized superlattice has (0 0 l) crystallographic orientation yet a thermal conductivity of just 0.25 W/m.K in the "on" (set) state. Prototype phase change memory devices that incorporate this Ti-doped superlattice switch faster and and at a substantially lower voltage than the undoped superlattice. During switching the Ti-doped Sb2Te3 layers remain stable within the superlattice and only the Ge atoms are active and undergo interfacial phase transitions. In conclusion, we show the potential of thermally optimised Sb2Te3-GeTe superlattices for a new generation of energy-efficient electrical and optical phase change memory., Comment: 4 Figures, 7 Supplementary Figures, 27 pages including a supplement

All-dielectric metasurfaces provide unique solutions for advanced wavefront manipulation of light with complete control of amplitude and phase at sub-wavelength scales. One limitation, however, for most of these devices is the lack of any post-fabrication tunability of their response. To break this limit, a promising approach is employing phase-change-materials (PCM), which provide a fast, low energy and non-volatile means to endow metasurfaces with a switching mechanism. In this regard, great advancements have been done in the mid infrared and near infrared spectrum using different chalcogenides. In the visible spectral range, however, very few devices have demonstrated full phase manipulation, high efficiencies, and reversible switching. Here, we experimentally demonstrate a tunable all-dielectric Huygens metasurface made of antimony sulfide (Sb2S3) PCM, a low loss and high-index material in the visible spectral range with a large contrast (nearly 0.5) between its amorphous and crystalline states. We show close to 2pi phase modulation with high associated transmittance and use it to create switchable beam steering and holographic display devices. These novel chalcogenide PCM metasurfaces have the potential to emerge as a platform for next generation spatial light modulators and to impact application areas such as tunable and adaptive flat optics, LiDAR, and many more.

The switchable optical and electrical properties of phase change materials (PCMs) are finding new applications beyond data storage in reconfigurable photonic devices. However, high power heat pulses are needed to melt-quench the material from crystalline to amorphous. This is especially true in silicon photonics, where the high thermal conductivity of the waveguide material makes heating the PCM energy inefficient. Here, we improve the energy efficiency of the laser-induced phase transitions by inserting a layer of two-dimensional (2D) material, either MoS2 or WS2, between the silica or silicon and the PCM. The 2D material reduces the required laser power by at least 40% during the amorphization (RESET) process, depending on the substrate. Thermal simulations confirm that both MoS2 and WS2 2D layers act as a thermal barrier, which efficiently confines energy within the PCM layer. Remarkably, the thermal insulation effect of the 2D layer is equivalent to a ~100 nm layer of SiO2. The high thermal boundary resistance induced by the van der Waals (vdW)-bonded layers limits the thermal diffusion through the layer interfaces. Hence, 2D materials with stable vdW interfaces can be used to improve the thermal efficiency of PCM-tuned Si photonic devices. Furthermore, our waveguide simulations show that the 2D layer does not affect the propagating mode in the Si waveguide, thus this simple additional thin film produces a substantial energy efficiency improvement without degrading the optical performance of the waveguide. Our findings pave the way for energy-efficient laser-induced structural phase transitions in PCM-based reconfigurable photonic devices., Comment: 34 pages, 12 figures

Dielectric optical nanoantennas play an important role in color displays, metasurface holograms, and wavefront shaping applications. They usually exploit Mie resonances as supported on nanostructures with high refractive index, such as Si and TiO2. However, these resonances normally cannot be tuned. Although phase change materials, such as the germanium-antimony-tellurium alloys and post transition metal oxides, such as ITO, have been used to tune optical antennas in the near infrared spectrum, tunable dielectric antennae in the visible spectrum remain to be demonstrated. In this paper, we designed and experimentally demonstrated tunable dielectric nanoantenna arrays with Mie resonances in the visible spectrum, exploiting phase transitions in wide-bandgap Sb2S3 nano-resonators. In the amorphous state, Mie resonances in these Sb2S3 nanostructures give rise to a strong structural color in reflection mode. Thermal annealing induced crystallization and laser induced amorphization of the Sb2S3 resonators allow the color to be tuned reversibly. We believe these tunable Sb2S3 nanoantennae arrays will enable a wide variety of tunable nanophotonic applications, such as high-resolution color displays, holographic displays, and miniature LiDAR systems., Comment: 36 pages, 5 figures in main text, 9 figures in supporting information

Chalcogenide phase change materials (PCMs) have been extensively applied in data storage, and they are now being proposed for high resolution displays, holographic displays, reprogrammable photonics, and all-optical neural networks. These wide-ranging applications all exploit the radical property contrast between the PCMs different structural phases, extremely fast switching speed, long-term stability, and low energy consumption. Designing PCM photonic devices requires an accurate model to predict the response of the device during phase transitions. Here, we describe an approach that accurately predicts the microstructure and optical response of phase change materials during laser induced heating. The framework couples the Gillespie Cellular Automata approach for modelling phase transitions with effective medium theory and Fresnel equations. The accuracy of the approach is verified by comparing the PCM optical response and microstructure evolution with the results of nanosecond laser switching experiments. We anticipate that this approach to simulating the switching response of PCMs will become an important component for designing and simulating programmable photonics devices. The method is particularly important for predicting the multi-level optical response of PCMs, which is important for all-optical neural networks and PCM-programmable perceptrons., Comment: 26 pages, 6 figures