2, Data Storage Institute, A*STAR (Agency for Science, Technology and Research), Singapore, , Singapore
Nanostructures made of dielectric materials can have analogous properties to plasmonic structures for manipulation of light, with the advantage of having lower dissipative losses . Wide bandgap phase change chalcogenides may be tailored to have a large refractive index with a low absorption in the visible and near infrared spectrum, and thus they are a promising platform for tunable metasurfaces in the visible and near infrared spectrum. For one of the most commonly used chalcogenides, Ge2Sb2Te5 (GST), the refractive index at 840nm is rather large, approximately 4.5 for the amorphous state and 5.5 for the crystalline state. However, the extinction coefficient of GST at 840nm is approximately 1.5 and 3.6 for the amorphous and crystalline states respectively, which renders it very lossy and, therefore, impractical for realistic applications. In comparison, the properly designed wide bandgap phase change chalcogenide, which is used herein, has a refractive index of approximately 3.0 and 3.5 for the amorphous and crystalline states at 840nm, with an extinction coefficient near 0 at 840nm for both states, thus meeting the high index and low loss requirements for high efficiency devices.
We designed nanoantenna arrays metasurfaces based on a wide bandgap chalcogenide for the visible and NIR spectrum. The device operates in transmission mode and allows manipulation of the phase of the transmitted wave, and is tunable through structural phase transitions in the chalcogenide material. The proposed device exploits both the refractive index change in chalcogenide and the concept of Huygens’ metasurface  to exhibit a very high transmission (>80%) with full angular 2π phase control. Based on the structural phase change property of the chalcogenide, we designed a gradient metasurface using two different mechanisms: geometrical tuning and partial crystallization. Both designs allow dynamic control of the transmitted light at a wavelength of 840nm. The transmitted beam deflection could be tuned by adjusting the individual crystallization levels of the wide bandgap phase change material, with overall efficiencies exceeding 40% with respect to the incident power.
In conclusion, both simulations and experimental results will be presented that demonstrate nanoantenna array metasurfaces, which are based on a wide bandgap phase change materials, can achieve tunable control of light beams in the visible and NIR spectrum. These results suggest that phase change materials have a further application beyond data storage in high-speed spatial light modulator and phase arrays, with potential applications in dynamic holography.
Li Lu acknowledges his scholarship from Singapore Ministry of Education.
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