|
The demand for holographic optical elements (HOEs) is continuously increasing due to their potential applications in a wide range of quickly advancing technologies. The main challenge for HOE development is in the design and preparation of holographic materials, which satisfy environmental requirements during device exploitation.
Photosensitive glasses are potential candidates for the development of HOEs with improved environmental stability, mechanical stability and hardness. Recently, a novel photopolymerisable hybrid sol-gel (PHSG) for holographic recording of volume gratings with high performance has been reported. Due to its valuable combination of properties, PHSG has potential for mass-production of highly efficient and durable optical devices with long-term stable performance at different ambient conditions. Nevertheless, the mechanism of grating formation is not fully characterised and understood. To achieve control over the performance of the material and further improve its properties, systematic study is required.
In this research, the mechanism of grating formation in the PHSG is investigated by analysing two main effects contributing to the refractive index modulation. The first effect is density change due to both polymerization of monomers under holographic patterning and polycondensation caused by natural aging of the sol-gel material. The second effect is diffusion of mobile species such as dye molecules induced by concentration gradients and/or distribution of small nanoparticles caused by spatially varying heat dissipation resulting from light absorption during holographic patterning. Experimental results confirn that the mechanism of grating formation in the PHSG can be defined as the creation of spatially-varying refractive index within the bulk of the material mainly due to changes in density induced by photopolymerisation and polycondensation. Diffusion processes are observed to have low impact on the grating formation. The proposed model facilitates controlled and predictable diffraction efficiency of gratings recorded on the PHSG, thereby furthering the prospect of the development of HOEs with customisable specification.
|
