Increasing efficiency of high-NA metasurface lenses
Diffractive optical devices based on dielectric metasurfaces have recently attracted significant attention. Small size, low weight, planar form factor, and potential for low-cost manufacturing using semiconductor fabrication techniques are some of the main features that make metasurfaces ideal candidates for implementation of low-cost miniaturized optical systems. However, to become competitive for practical applications, metasurfaces should also offer specifications (e.g. efficiency, bandwidth, and wavefront error) comparable to their refractive counterparts. We have recently demonstrated diffraction-limited metasurface lenses with high efficiency using high refractive index nano-posts. Low numerical aperture (NA) metasurface lenses have more than 90% focusing efficiency, but the efficiency of the lenses with NA>0.5 decreases with increasing NA and drops to ~40% for NA=0.9, thus resulting in a trade-off between the NA and efficiency. Here we identify the main physical origin of this trade-off as the low transmission of large diameter nano-posts for transverse-magnetic (TM) polarized light incident at large angles, and show that the low transmission is caused by the excitation of undesired high order modes in these nano-posts. To overcome this issues, we present a novel approach for evaluating different metasurface designs in implementation of high NA metasurface components. The approach is based on adiabatic approximation of aperiodic metasurfaces by periodic gratings, and considers the effect of large deflection angles. Using the proposed design approach, we experimentally demonstrate more than 75% focusing efficiency for metasurface lenses with NA=0.7, and more than 70% deflection efficiency for 50-degree beam deflectors for unpolarized light at 915 nm.
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