Active Optical Metasurfaces Based on Low Loss Phase Change Materials

<br/>Metasurfaces are promising optical components that enable light engineering by controling the phase, amplitude or/and polarization achieving large panel of applications such as optical filtering, focusing, holographic projection, spatial light modulation, lidar and neuromorphic computing. Inherent passive functionality limits full integration into smart devices and multi-functionality on demand. Active metasurfaces are a potential solution for a wide range of applications. Several use cases have been detected based on the level of design complexity. First, the correction of manufacturing aberrations in post-fabrication for mass production and highly sensitive applications. Second, increasing the functionality of fixed features into binary or continuously adjustable features is of great interest. Finally, an on-demand addressable meta-atom would allow arbitrary light control.<br/>Among the different tuning mechanisms, the exploitation of phase change materials (PCMs), such as GeSbTe, GeTe, seems particularly relevant as they possess remarkable optical properties that can be strongly modulated by reversibly changing the crystallinity of the material. This also-called standard PCMs, widely used in non-volatile memories, exhibit high optical losses in the visible and near infrared range, which limits the efficiency of optical devices. A new class of PCMs called low-loss PCMs, such as Sb2Se3, Sb2S3, GeSbSeTe, is more suitable for optical metasurfaces given the low optical losses and phase change properties[1].<br/><br/>In this work, we focus on the design, fabrication and characterization of metasurfaces based on low-loss PCMs, especially Sb2S3. First, an active metasurface based on the Pancharatnam Berry principle is optimized to passively encode the deflection angle information in a given polarization state, and then, depending on the crystallinity state, we switch from one polarization state to the other, realizing a binary beam deflection. Next, an active filter will be presented, allowing both independent passive control of an extremely high Q factor using design-controlled perturbation and active spectral control using the active properties of PCMs. After showing examples of potential designs of the active spectral and spatial light control, we finally present the nanofabrication and characterization challenges that have been addressed.<br/><br/><b>Reference:</b><br/>[1] Elena Mikheeva, Christina Kyrou, <b>Fouad Bentata</b>, Samira Khadir, Sébastien Cueff, Patrice Genevet “Space and Time Modulations of Light with Metasurfaces: Recent Progress and Future Prospects<i>”, </i>ACS Photonics, 2022<br/><br/><b>Acknoledgements:</b><br/><br/>F.B., S.C., L.B., X.L., S.M. and P.G. acknowledge support by the French National Research Agency (ANR) under the projects MetaOnDemand (ANR-20-CE24-0013), National Association of Research and Technology ANRT, and STMicroelectronics.