Huge retardation of grating formation in holographic polymer-dispersed liquid crystals

Phototriggered kinetics is investigated in the dark for holographic polymer-dispersed liquid crystal gratings. Subsequently to holographic recording, the diffraction efficiency first slightly increases, then decreases, and afterwards starts to grow again. The remarkable growth continues for several days until the diffraction efficiency stabilizes at values several times larger than the initial ones. The initial increase is attributed to a dark polymerization reaction as confirmed by electron paramagnetic resonance spectroscopy. The slow process is associated with the kinetics of a phase-separation mechanism. The non-monotonic behaviour of the diffraction efficiency is explained by superposition of refractive-index and extinction modulations.