Facebook just published a new research paper at SIGGRAPH which shows a proof of concept for a thin VR optic system that essentially uses a holographic lens recording of folded optics.
Link to paper: https://research.fb.com/publications/holographic-optics-for-thin-and-lightweight-virtual-reality/
Link to an article with a simplified explanation of how it works: https://www.roadtovr.com/facebook-folded-holographic-optics-thinnest-vr-display-glasses/