Subwavelength-thick Lenses with High Numerical Apertures and Large   Efficiency Based on High Contrast Transmitarrays
  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. 
Amir Arbabi, Yu Horie, Alexander J. Ball, Mahmood Bagheri, Andrei   Faraon
physics.optics