Numerical study of mid-infrared tunable metalenses based on Ge2Sb2Te5 phase-change material

Abstract

The mid-infrared (mid-IR) metalens is an advanced but also fundamental element for a variety of dynamic optical applications, e.g. night visualization and spectroscopic sensing. However, the lens quality and tunability is challenging at this strategically important band. Here, we numerically propose a reflective metasurface platform based on Ge2Sb2Te5 (GST) phase-change material to resolve this problem. Our two paradigms, i.e. bifocal and bifunctional metalenses, prove that a diversified range of functions can be modulated in the mid-IR without structural reconfiguration. By converting GST between amorphous and crystalline states, the focus of the bifocal metalens shifts 35.8 μm (>6λ), with maximum focusing efficiency of up to 8%. The continuous modulation of the intensity profile at two focuses is realized by GST phase transition. We have also demonstrated that a one-dimensional focal metalens with focusing efficiency of 28% can be switched into a mid-IR light deflector by converting the GST phase states. The metasurface uses mature optical materials and can be fabricated by standard microfabrication technologies, which makes the tunable mid-IR platform promising for integrated optical systems.