An Over-Coupled Phase-Change Metasurface for Efficient Reflection Phase Modulation

An over-coupled, thermally tunable metasurface reflect-array that employs the phase change material Ge₂Sb₂Te₅ (GST) is presented. The metasurface is constructed from gap plasmon cavities in which GST is incorporated as the active switching medium. Upon annealing at 200 °C for 10 min, the GST layer undergoes a transition from the amorphous state to the crystalline state, and this leads to a unity-order increase of the refractive index. This is accompanied by a spectral shift of 1.6 µm in the resonance wavelength of the plasmonic cavities, larger than their resonance linewidth of 1.2 µm. It is shown that the low material absorption of the GST layer enables operation of the metasurface in the desirable over-coupled regime. The numerical analysis indicates that this facilitates large changes in the reflection phase (up to 270°) and amplitude. The work opens the possibility of creating efficient reconfigurable metasurfaces for various applications, including holographic displays and image sensors.