Dielectric metasurfaces with broadband achromatic response

Abstract

Controlling and manipulating the wavefront of light is pivotal in modern optics, especially the ability to do so on the nano- and microscale. Metasurfaces present a unique platform for realising flat lenses - or metalenses - with thicknesses on the order of the wavelength, which is expected to pave the way for miniaturising cameras, displays, and wearable optics among other things. However, suppressing the chromatic aberrations over large operational bandwidths of such metalenses remains a challenge and has largely been overlooked in the mid-infrared spectrum. Here, a new design method enabling polarisation-insensitive simultaneous control of the phase and group delay of a light beam - based on libraries of transmission-mode dielectric meta-elements - is developed. The proposed methodology, which can be used as a general design rule for all spectra, even provides a design scheme for metalenses operating with ultrashort femtosecond pulses. The validity, of the methodology, is extensively evaluated and verified through analytical theory, mode analysis, dipole summations, and full-wave simulations. For this purpose metalenses, operating in a $2.5\:\mu\mathrm{m}$ wide bandwidth in the mid-infrared range ($6-8.5\:\mu\mathrm{m}$), are analysed and shown to extinguish chromatic aberrations, while maintaining high focusing efficiencies above 45\% and diffraction-limited focusing. This new design method represents a major advance toward practical implementations of metalenses.