Phase-change-driven dielectric-plasmonic transitions in chalcogenide metasurfaces

Abstract

Chalcogenides—alloys based on group-16 ‘chalcogen’ elements (sulfur, selenium, and tellurium) covalently bound to ‘network formers’ such as arsenic, germanium, antimony, and gallium—have a variety of technologically useful properties, including infrared transparency, high optical nonlinearity, photorefractivity and readily induced, reversible, non-volatile structural phase switching. Such phase-change materials are of enormous interest in the fields of plasmonics and nanophotonics. However, in such applications, the fact that some chalcogenides accrue plasmonic properties in the transition from an amorphous to a crystalline state, i.e., the real part of their relative permittivity becomes negative, has gone somewhat unnoticed. Indeed, one of the most commercially important chalcogenide compounds, germanium antimony telluride (Ge2:Sb2:Te5 or GST), which is widely used in rewritable optical and electronic data storage technologies, presents this behavior at wavelengths in the near-ultraviolet to visible spectral range. In this work, we show that the phase transition-induced emergence of plasmonic properties in the crystalline state can markedly change the optical properties of sub-wavelength-thickness, nanostructured GST films, allowing for the realization of non-volatile, reconfigurable (e.g., color-tunable) chalcogenide metasurfaces operating at visible frequencies and creating opportunities for developments in non-volatile optical memory, solid state displays and all-optical switching devices. Inorganic compounds that transform on demand from glassy (dielectric) to crystalline (plasmonic) states can be employed to manipulate the color of reflected and transmitted light. The ability of germanium antimony telluride (GST) to change structural phase in response to brief laser heating pulses is the foundation of rewritable optical data storage technologies. Researchers from the University of Southampton, UK and Nanyang Technological University, Singapore, now report on a novel application of GST’s phase-change character to switching color. They etched a series of nanoscale grating patterns into a thin GST film, creating structures that control levels of reflection, transmission and absorption for different colors of light, in a way that changes when GST is switched between its amorphous and crystalline states. This type of laser-controlled color switching could benefit applications including solid-state display technologies. Phase change materials are of enormous interest in the field of plasmonics and nanophotonics. For such applications, it has gone largely unnoticed that some chalcogenides accrue plasmonic properties in the transition from an amorphous to a crystalline state. In this work, we show that the phase transition-induced emergence of plasmonic properties in the crystalline state can markedly change the optical properties of subwavelength-thickness nanostructured GeSbTe films, providing for the realization of non-volatile, reconfigurable (e.g. color-tunable) chalcogenide metasurfaces operating at visible frequencies, and thus creating opportunities for developments in non-volatile optical memory, solid state displays and all-optical switching devices.