Role of refractive index in metalens performance

Abstract

Sub-wavelength diffractive optics, commonly known as metasurfaces, have recently garnered significant attention for their ability to create ultra-thin flat lenses with high numerical aperture. Several materials with different refractive indices have been used to create metasurface lenses (metalenses). In this paper, we analyze the role of material refractive indices on the performance of these metalenses. We employ both forward and inverse design methodologies to perform our analysis. We found that, while high refractive index materials allow for extreme reduction of the focal length, for moderate focal lengths and numerical aperture (<0.6), there is no appreciable difference in focal spot-size and focusing efficiency for metalenses made of different materials with refractive indices ranging between n= 1.25 to n=3.5.

Broadband transparent and CMOS-compatible flat optics with silicon nitride metasurfaces [Invited]

“…Like SiN, this material not only exhibits a wide band gap but is a material that is widely used and is CMOS-compatible. While lossless, the lower refractive index (n ~1.5) of this material translates to lower beam deflection efficiency at high angles and reduced focusing efficiency for high NA lenses [34,35]. This reduced performance arises from the decrease in diffraction efficiency that, while minimal at low angles, becomes evident in lower index materials at moderate to high deflection angles [34,35].…”

“…While lossless, the lower refractive index (n ~1.5) of this material translates to lower beam deflection efficiency at high angles and reduced focusing efficiency for high NA lenses [34,35]. This reduced performance arises from the decrease in diffraction efficiency that, while minimal at low angles, becomes evident in lower index materials at moderate to high deflection angles [34,35]. While silicon dioxide is preferable compared to a variety of other dielectrics considering its CMOS-compatibility, its manufacturing advantages are matched by those of SiN, but it is inferior in terms of performance.…”

“…Separately, a work [54] demonstrating inverse design of arrays of dielectric spheres successfully generated single and double layer metalenses using the adjoint method and generalized multi-sphere Mie theory (GMMT). While this work used spheres with a refractive index of 1.52, the index for polymers used in state-of-the-art two-photon polymerization 3-D printers, the algorithm was recently used in a separate study [35] for indices from 1.2 to 3.5, which includes the index for SiN. GMMT is a method based on the T-matrix formalism and can calculate the electromagnetic scattering off an ensemble of spheres [55][56][57].…”

Design and optimization of ellipsoid scatterer-based metasurfaces via the inverse T-matrix method

“…The properties of these scatterers are computed with periodic boundary conditions and the metasurfaces are designed under the "local phase approximation": the scattering in any small region is taken to be the same as the scattering from a periodic surface [10]. This approximation neglects inter-scatterer coupling which is significant for metasurfaces composed of scatterers with rapidly varying geometries or with low refractive index [11]. Moreover, it is not always possible to know the phase-profile a priori, and in these cases forward design methods cannot be used.…”

Highly Efficient Bifunctional Dielectric Metasurfaces at Visible Wavelength: Beam Focusing and Anomalous Refraction in High-Order Modes

“…Metasurfaces typically consist of an array of sub-wavelength metallic or dielectric nanostructures, their geometric parameters, including size, shape, and direction can be adjusted to change the amplitude, phase, and polarization of light. Based on the obvious features, various metasurfaces with different functions have been extensively studied, including beam deflectors [7][8][9][10], metalenses [11][12][13], waveplates [14,15], and high-resolution holograms [16,17]. So far, most of metasurfaces invented by people have a single function.…”