Review of refractive index modulation achievable in photosensitive polymer materials in view of their application with diffractive optical elements for LEDs

LEDs have long been hailed as a sustainable light source. Indeed, its adoption in recent decades has greatly improved the efficiency of artificial light sources. Nonetheless, the sustainability claims are challenged by mounting evidence pointing to the detrimental impact of light pollution due to blue-rich white LEDs on wildlife, human health, and astronomy. A potential solution to this is the use of holographically recorded diffractive optical elements (DOEs) to redirect LED light to regions where illumination is required. However, current limitations on the use of DOEs are high costs, low angular working range (Δθ), and relatively narrow spectral working range (Δλ). This restricts their efficient use to narrow wavelength range light sources at highly specific ranges of incident angles. In order for DOEs to be operational with broad Δθ and Δλ, a low thickness and suitably high refractive index modulation (Δn) are required, but it must still operate in the Kogelnik regime and not lose energy to higher diffraction orders. The aim of this review is to identify candidate materials for the design of a novel low-cost DOEs with higher Δn.