Limits of immersion-free recording of holographic waveguide displays

This Letter discusses the limitations of immersion-free recording schemes for holographic waveguide displays. Traditional holographic recording of waveguides requires recording angles exceeding the critical angle of the hologram-cladding interface. Achieving these angles necessitates edge-lit exposure using prisms and immersion liquids, which are challenging for roll-to-roll mass production and hinder widespread adoption. We present analytical equations describing the set of holographic gratings that can be recorded immersion-free by utilizing recently proposed methods of wavelength and recording configuration shifting. We show that by appropriately choosing the recording polarization, superficial holograms can be mitigated and large effective index modulation can be achieved. Our results demonstrate that all relevant red, green, and blue (RGB) incouplers for typical reflective holographic waveguide displays in a photopolymer can be recorded and copied immersion-free, reducing manufacturing complexity and costs. However, typical expanders cannot. These findings contribute to the industrialization of holographic waveguides for augmented reality (AR).