  We report subwavelength-thick, polarization insensitive micro-lenses
operating at telecom wavelength with focal spots as small as 0.57 wavelengths
and measured focusing efficiency up to 82%. The lens design is based on high
contrast transmitarrays that enable control of optical phase fronts with
subwavelength spatial resolution. A rigorous method for ultra-thin lens design,
and the trade-off between high efficiency and small spot size (or large
numerical aperture) are discussed. The transmitarrays, composed of silicon
nano-posts on glass, could be fabricated by high-throughput photo or
nanoimprint lithography, thus enabling widespread adoption.
