Ge2Sb2Se4Te1-based multifunctional metalenses for polarization-independent, switchable and dual-mode focusing in the mid-infrared region

Abstract

Reconfigurable metasurfaces, whose optical responses can be tailored post-fabrication, hold great promise to enable multifunctional and versatile optical components. However, the singularity of adjustment, usually manifested as switching functions or wavelength-tunable features, hinders their further development in practice. Particularly, active polarization-insensitive metasurfaces usually appear to be forced by using the isotropic nanostructure as their constituent elements, rendering them to lose a degree of freedom in the design space and plaguing them in many practical scenarios. In this paper, we report a polarization-insensitive, switchable and dual-mode multifunctional metalens. By using anisotropic Ge2Sb2Se4Te1 nanofins as building blocks, we can accurately impart the phases to each atom in order to realize the full 2π coverage by restricting their rotation angle to either 0 or 90 degrees. This guarantees that the metalens can elegantly focus any incident polarized light at the operation wavelength of λ0 = 4200 nm. Meanwhile, the proposed metalens can also focus the oblique incident light into one spot within the incident angle range of [−2.4°, +2.4°], providing a field of view (FOV) of 4.8°. Moreover, the designed scheme supports a stable focusing efficiency (above 60%), a near-constant focal length (the focal length shifts less than 6%) and diffraction-limited focal spots over λ = 4000–4500 nm. The point to emphasize here is that when the amorphous Ge2Sb2Se4Te1 (aGSST) is converted into the crystalline state (cGSST), the designed metalens can be switched off in terms of focusing features at λ0 = 4200 nm, while the bright focal spot reappears on the focal plane at the wavelength of λ = 4700 nm unexpectedly. The cGSST-based metalens works well both in the reflection and transmission modes simultaneously at the wavelength of λ = 4000 nm owing to phase matching conditions. The generality of our polarization-insensitive, switchable and dual-mode design allows it to be implemented in a plethora of metasurface devices with promising applications, such as: optical imaging, optical encryption, augmented reality and sensing systems.