  In this paper we present two design generations of metasurface-based planar
microlenses implemented on Front-Side Illumination SPAD pixels. This kind of
microlens is an alternative to conventional reflow microlens. It offers more
degrees of freedom in term of design, especially the capability to design
off-axis microlens to gather light around the SPAD photodiode. The two
generations of microlenses have been fabricated on STMicroelectronics SPAD and
characterized. We validated the sensitivity improvement offered by extended
metasurface-based microlens. We also confirmed the impact of lithography
capability on metasurface performances, highlighting the
