Determination of threshold exposure and intensity for recording holograms in thick green-sensitive acrylamide-based photopolymer

For optical data storage applications, it is essential to determine the lowest intensity (also known as threshold intensity) below or at which no data page or grating can be recorded in the photosensitive material, as this in turn determines the data capacity of the material. Here, experiments were carried out to determine the threshold intensity below which the formation of a simple hologram-a holographic diffraction grating in a green-sensitized acrylamide-based photopolymer-is not possible. Two main parameters of the recording layers-dye concentration and thickness-were varied to study the influence of the density of the generated free radicals on the holographic properties of these layers. It was observed that a minimum concentration per unit volume of free radicals is required for efficient cross-linking of the created polymer chains and for recording a hologram. The threshold intensity below which no hologram can be recorded in the Erythrosin B sensitized layers with absorbance less than 0.16 was 50 μW=cm². The real-time diffraction efficiency was analyzed in the early stage of recording. It was determined that the minimum intensity required to obtain diffraction efficiency of 1% was 90 μW=cm², and the minimum required exposure was 8 mJ=cm². It was also determined that there is an optimum dye concentration of 1:5 × 10^-7 mol=L for effective recording above which no increase in the sensitivity of the layers is observed.