Efficient high NA flat micro-lenses realized using high contrast transmitarrays

We present design, fabrication, and characterization results of high numerical aperture (NA) micro-lenses based on a high contrast transmitarray platform. The high contrast transmitarray is created by periodic arrangement of amorphous silicon posts with different diameters on a fused silica substrate. We report near infrared high NA micro-lenses with spot sizes as small as 0.57λ and focusing efficiencies in excess of 80%. We demonstrate a trade-off relation between NA and efficiency of high contrast array flat micro-lenses, and attribute it to the spatial discretization of their phase profiles.