Wavelength-actuated varifocal and polarization-insensitive metalenses assisted by monolayer single-celled phase-change metasurfaces

Abstract Varifocal metalenses with tunable focal lengths allowing for transforming the complex and bulky tunable lens kit into a single flat lens, show great potential applications in tomography techniques, 3D displays, multi-imaging processing, among others. However, varifocal metalenses that integrate key properties, such as modest efficiency, large numerical aperture (NA) as well as polarization insensitivity into one remain elusive. Here, we present and numerically demonstrate a wavelength-actuated varifocal and polarization-insensitive metalens. This metalens is based on a novel generic design principle that relies on independently tailoring two sets of Ge 2 Sb 2 Se 4 Te 1 (GSST) anisotropic nanofins corresponding to two resonant wavelengths of λ 1 and λ 2 , respectively. Via intelligently integrating two sets of anisotropic nanofins into a monolayer single-celled phase-change metasurface in the form of ‘cross’ or ‘straight’ composite, the metalens design not only enables the crosstalks between two wavelength channels minimized, but also is endowed with polarization-insensitive and varifocal performance, while maintaining high focusing efficiency and diffraction-limited performance. Due to the noninterleaved arrangement making the meta-structure more compact, another prototype of wavelength-actuated varifocal metalens with ultrahigh-NAs (0.89 and 0.95, respectively corresponding to the target wavelengths of λ 1 and λ 2 ) are also implemented with a field of view of 4° based on this principle. We also demonstrate the versatility of our approach by successfully implementing the generation of the polarization-insensitive varifocal focusing optical vortex. Additionally, the phase transition of GSST enables the proposed meta-devices to switch between ON and OFF states for spot focusing. Our design can be easily extended to visible or THz wavebands and will enable enormous applications ranging from switchable optical scanners, dynamic optical tweezers and spanners, to miniaturized polarization-insensitive imaging devices.