Edge-lit volume holograms recorded by free space exposure: diffraction by 2nd Harmonics in Bayfol HX film

Miniaturization of optical components, specifically the reduction in thickness created by using planar optical devices, makes light manipulation by diffraction more and more attractive. Optical gratings based on volume Holographic Optical Elements (vHOEs) have the advantage over surface gratings as they reconstruct only a single diffraction order and hence provide high diffraction efficiencies, selectivity and remain fully transparent in the off-Bragg condition. Guiding light inside an optically transparent medium by total internal reflection (TIR) is common and useful in thin planar optical devices. vHOEs offer unique ways to create selective in- and out-coupling of TIR light. As such vHOEs typically have to be recorded in an edge-lit configuration as the necessary high diffraction angles could not be generated by two free-space beams outside the medium. To record such an edge-lit vHOE, bulky recording blocks or liquid baths are used in complex and hard to align recording setups. We present in this paper our findings to use instant-developing photopolymer film (Bayfol® HX) to generate 2nd harmonics in the index profile of phase gratings while using free-space recording setups. Those 2nd harmonic components enable the vHOE to diffract at such large angles that they replay in an edge-lit configuration. We will discuss in this paper selected materials and beneficial recording parameters to tune the diffraction efficiency towards 2nd harmonic replay. By this - in reported specific cases - the cumbersome and complex edge-lit recording can be substituted by easy-to-use free-space setups. This process significantly simplifies master recordings for vHOEs with edge-lit functionalities which later can be used in contact copy schemes for mass replication.