RESEARCH NEWS
Guiding and filtering crystallization of phase-change materials to program nanophotonic devices
Integrating designer nanostructures on a chip opens up unprecedented functionalities for microelectronics and photonics, for various applications ranging from lightspeed data communications and calculations to beam shaping and focusing with optical metasurfaces. Passive photonic integrated circuits now provide the full control of nearly all aspects of light at the nanoscale, and can now readily be included in complex microelectronic chips. Research on large-scale systems for optical datacoms, metalenses, and neuromorphic computing is now close to leaping industrialization but requires tunable building blocks. In that context, photonic integrated devices are progressively evolving beyond passive components into fully programmable systems, notably driven by the progress in chalcogenide phasechange materials( PCMs) for non-volatile reconfigurable nanophotonics. Recently, Sb 2 S 3 emerged as an alternative PCMs with experimentally demonstrated large refractive index changes upon crystallization and ultra-low extinction coefficients from the visible range to infrared. However, the stochastic nature of their crystal grain formation and subsequent uncontrolled growth results in strong spatial and temporal crystalline inhomogeneities, leading to inefficient and lossy devices.
In this work, researchers propose the concept of spatially-controlled planar guided crystallization, a novel method for programming the crystal growth of optically homogeneous low-loss Sb 2 S 3 PCM, leveraging the seeded directional and progressive crystallization within confined channels. By strategically designing crystallization reservoirs and channels, the guided crystallization enables precise control over the temporal and spatial evolution of crystals on the chip, as well as their overall quality. This enables controlling where and when the PCM will crystallize on a chip and, under certain conditions, filtering the planar growth to produce quasi-monocrystals! This guided crystallization method is experimentally shown to circumvent the current limitations of conventional PCMbased nanophotonics in two different devices: a programmable optical phase shifter and a reconfigurable metasurface. By patterning an Sb 2 S 3 patch onto one arm of a Mach-Zehnder interferometer, ten distinct phase shifts were successfully programmed, leveraging the refractive index change of the PCM during crystallization, all while maintaining negligible optical absorption. Going beyond conventional phase-change modulation, the concept of in-plane guided growth of‘ filtered’ optically homogeneous Sb 2 S 3 crystals was shown, analogous to a planar and spatially-controlled version of conventional single crystal production processes such as grain selection in directional solidification or the Czochralski growth. The ability to precisely control the phase transition of Sb 2 S 3 between amorphous and optically homogeneous states was leveraged in reconfigurable metasurfaces, enabling a 100 nm spectral modulation of a bound state in the continuum( BIC) mode, while preserving the metasurface functionalities, including optical coherence across a large area of the device. Precisely controlling the phase transformation of PCMs to ensure optically uniform crystalline properties across devices is a cornerstone for the industrial development of non-volatile reconfigurable photonic integrated circuits. These results establish a new platform for high precision and efficient programmable nanophotonic devices.
RÉFÉRENCE F. Bentata, A. Taute, C. Laprais, R. Orobtchouk, E. Kempf, A. Gassenq, Y. Pipon, M. Calvo, V. Martinez, S. Monfray, G. Saint-Girons, N. Baboux, H. S. Nguyen, X. Letartre, L. Berguiga, P. Genevet and S. Cueff. Spatially-Controlled Planar Guided Crystallization of Low-Loss Phase Change Materials for Programmable Photonics Adv. Mater.( 2025): e06609. https:// doi. org / 10.1002 / adma. 202506609
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