Dimensional changes in slanted diffraction gratings recorded in photopolymers

Abstract

Photopolymers are appealing materials for many optical applications. For most of them, shrinkage plays an important role in the final properties of the display, especially in holographic data storage applications. In this paper, we demonstrate that to quantify correctly the shrinkage, it is mandatory to measure the angle of propagation for both diffracted orders ± 1, so that an accurate value of the grating vector can be calculated. Experimental evidence from three different photopolymers supports this affirmation. Firstly, polyvinyl alcohol acrylamide based photopolymer, which has been studied by many research groups; secondly, one environmentally compatible photopolymer developed by our group; and thirdly, a photopolymer with dispersed liquid crystal molecules. We studied the deviation from the sinusoidal profile analyzing the higher diffracted orders.

Holographic waveguides in photopolymers

“…SiO 2 nanoparticles (the average size of 13 nm), dissolved in methyl isobutyl ketone (MIBK) at a concentration of 30 wt. % were used for their dispersion in the thiol-ene monomer blend [20][21][22]. The refractive index difference between SiO 2 nanoparticles and the formed thiol-ene polymer is close to 0.1, which helps to obtain large values of the refractive index modulation.…”

“…For the fabrication of diffraction gratings to be used in holographic waveguides, the material must be able to adequately record highly slanted gratings. This kind of holographic optical elements, HOEs, are highly affected by the shrinkage process, which deforms the grating structure, and also the Bragg condition, resulting in a reduction in the refractive index modulation [20].…”

“…In this paper, we investigate the responses for different spatial frequency and slanted gratings of three different photopolymers for recording such waveguides: polyvinylacrylamide based photopolymer [20], PVA/AA, with a functionality slightly higher than one, a nanoparticle-(thiol-ene) polymer composite dispersed with SiO 2 nanoparticles [21][22][23] and average functionality higher than two, NPC, and a photopolymer with dispersed liquid crystal molecules [24], with average functionality higher than five, PDLC. This investigation is initially carried out recording symmetric sinusoidal gratings.…”

Multiplexed holograms recorded in a low toxicity Biophotopol photopolymer

“…4 shows the angular displacement (Δθ m ) of DE m versus time. This displacement is related to shrinkage 8 as we assessed and quantified in previous studies 7 . Firstly, Δθ m is close to zero for the unslanted grating stored at the zero position (θ= 0º).…”

“…From the Fig 1 it can be seen that the slanted gratings recorded have a shift from their original register angle. This displacement is a consequence of the shrinkage that appears due to a film thickness variation 2,7 . In some gratings, it is also possible to see the secondary lobes that appear in the slanted gratings ( i.e.…”