Chalcogenide phase change materials (PCMs) are truly remarkable compounds whose unique switchable optical and electronic properties have fueled an explosion of emerging applications in electronics and photonics. Key to any application is the ability of PCMs to reliably switch between crystalline and amorphous states over a large number of cycles. While this issue has been extensively studied in the case of electronic memories, current PCM-based photonic devices show limited endurance. This review discusses the various parameters that impact crystallization and re-amorphization of several PCMs, their failure mechanisms, and formulate design rules for enhancing cycling durability of these compounds.
Liquid metals based on gallium have attracted considerable attention for soft and bioelectronics, thanks to their excellent combination of stretchability and conductivity. Nevertheless, owing to their large surface tension, these materials are notoriously difficult to pattern and shape into thin continuous films, or nanoscale 2D architectures, hindering practical use in systems with reduced dimensions. Herein, thanks to fine control in both substrate surface state and oxidation dynamics, a process for producing stretchable gallium-based conducting films with percolation down to 90 nm thickness is presented. By further combining this process with lithography, it is also demonstrated that the approach enables, for the first time, stable stretchable gallium-based optical metasurfaces with tunable resonance in the infrared. It is shown that oxygen partial pressure during evaporation determines the initial film percolation via an interplay between oxidation and dewetting. With this approach, conducting films with relative resistance change as low as 3% over 50% strain, with an excellent stability over 15k cycles are also demonstrated. Tunable soft optical metasurfaces with sub-micrometer feature sizes are also realized, paving the way toward a novel paradigm in soft electronics and photonics.
Dielectric metasurfaces have shown prominent applications in nonlinear optics due to strong field enhancement and low dissipation losses at the nanoscale. Chalcogenide glasses are one of the promising materials for the observation of nonlinear effects thanks to their high intrinsic nonlinearities. Here, we demonstrate, experimentally and theoretically, that significant second harmonic generation (SHG) can be obtained within amorphous Selenium (Se)-based chalcogenide metasurfaces by exploiting the coupling between lattice and particle resonances. We further show that the high-quality factor resonance at the origin of the SHG can be tuned over a wide wavelength range using a simple and versatile fabrication approach. The measured second harmonic intensity is orders of magnitude higher than that from a dewetted Se film consisting of random Se nanoparticles. The achieved conversion efficiency in the resonance region is of the order of 10−6 which is comparable with direct bandgap materials and at least two orders of magnitude higher than that of conventional plasmonics- and Si-based structures. Fabricated via a simple and scalable technique, these all-dielectric architectures are ideal candidates for the design of flat nonlinear optical components on flexible substrates.
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