Couple of articles on going for smaller pixels

"Optical confinement methods for continued scaling of CMOS image sensor pixels"
opg.optica.org/oe/fulltext.cfm?uri=oe-16-25-20457&id=175035

"Microlens performance limits in sub-2μm pixel CMOS image sensors"
opg.optica.org/oe/fulltext.cfm?uri=oe-18-6-5861&id=196475

There was some later research on the topic, by other authors, following his steps.