Reflective Tunable Coded Metalens Based on Phase Change Materials

This study proposes a reflective coding metalens design based on a metal-phase change material-metal unit structure. Leveraging the Pancharatnam-Berry (PB) phase principle, we optimized a 3-bit coding unit configuration. By exploiting the phase transition properties of germanium-antimony-tellurium (GST) between its amorphous and crystalline states, dynamic modulation of the reflected electromagnetic wave phase was achieved. In the terahertz regime, two reflective coding metalenses were designed using amorphous and crystalline GST, respectively. Experimental results demonstrate that both amorphous and crystalline GST metalenses exhibit excellent focusing performance, with numerical apertures (NA) of 0.74 and 0.59 and focal lengths of 10 mu m and 15 mu m, respectively. To enhance tunability, a composite coding metalens was constructed by spatially offsetting amorphous and crystalline GST units. Dynamic focal length tuning was realized by controlling the phase state of GST. This work not only validates the potential of GST phase-change materials in a reconfigurable metalens but also provides a novel design strategy for high-performance, dynamically tunable planar optical devices. Precise control of GST phase transitions enables real-time focal length adjustment, laying the groundwork for advancements in optical imaging, terahertz communications, and photonic computing systems.