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 Ge 2 Sb 2 Se 4 Te 1 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 λ 0 = 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 Ge 2 Sb 2 Se 4 Te 1 (aGSST) is converted into the crystalline state (cGSST), the designed metalens can be switched off in terms of focusing features at λ 0 = 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.

Topics & Concepts