Novel Approaches for Designing Broadband Achromatic and Polarization-Insensitive Metalenses

With powerful wavefront control of phase, amplitude and polarization of light based on sub-wavelength structures, metasurface has been demonstrated great potential applications in the field of compact planar optical device. However, there are still some key challenges, especially the correction of achromatic aberration intrinsically originating from the specific resonance and limited working bandwidth of each nanostructure, to be solved before the technology can be applied in practice. At present, the integrated resonance phase compensation method and the dispersion-tailored method have become the mainstream design schemes to obtain broadband achromatic metalenses. However, these two strategies often require computationally intensive full-system simulations and suffer from polarization sensitivity, which undoubtedly increases the complexity of the design and significantly limits the application scenarios. Here, inspired by the Rayleigh criterion for spot resolution, we present a novel design principle for broadband achromatic and polarization-insensitive metalenses using two sets of anisotropic nanofins based on phase change materials of Ge2Sb2Se4Tel. By limiting the rotation angles of all nanofins to either 0 or 90 degrees, a metalens with a suitable numerical aperture constructed by this fashion allows for achromatic and polarization-insensitive performance across the wavelength range of $\lambda=4\sim 5\text{um}$, while maintaining high focusing efficiency and diffraction-limited performance. We also demonstrate the versatility of our approach by successfully implementing the generation of a wavelength-actuated varifocal and polarization-insensitive metalens. Our design with significant advancements in achromatic and varifocal metalenses can be easily extended to visible or THz wavebands and will enable enormous applications in miniaturized imaging devices such as cell phones, wearable displays, and virtual reality (VR)/augmented reality (AR).