Independent tuning of bright and dark meta-atoms with phase change material on EIT metasurfaces

The realization of tunable metasurfaces is of fundamental importance in boosting the electromagnetic field control ability. Especially, it is significant to put forward new modulation methods for further understanding the underlying modulation mechanism and expanding the application ranges. In this paper, tunable electromagnetically induced transparency (EIT) metasurfaces based on phase change material Ge2Sb2Te5 (GST) are proposed and experimentally demonstrated. Different from previous modulation methods of directly introducing GST film below the metasurfaces, here the two-step lithography method is introduced to combine independent GST strips with the bright and dark meta-atoms in the EIT structures respectively, achieving the independent modulation of the EIT-like spectra. In addition, by applying temporal coupled-mode theory (TCMT), the EIT-like spectra with different GST crystallization levels were analysed and the corresponding characteristic parameters were extracted simultaneously. The fitting results reveal that GST strips can modulate the resonances of the bright and dark meta-atoms independently by shifting the resonant frequency and increasing the decay rate, which give rise to the different modulation features of the EIT-like spectra accordingly. This method improves the freedom of active modulation and provides a new way for tunable slow light device.