Very high contrast volume holographic gratings recorded in photopolymerizable nanocomposite materials

Abstract

Volume holographic phase gratings possessing the saturated refractive index modulation amplitudes as large as 4.5×10−2 were recorded at a wavelength of 532 nm in a photopolymerizable nanoparticle-polymer composite (NPC) film dispersed with ultrahigh refractive index hyperbranched-polymer (HBP) organic nanoparticles. This prominent result was achieved by a combination of the HBP nanoparticles with triazine and aromatic ring units and an electron donor/acceptor photo-initiator system doped in an acrylate monomer blend with low viscosity. As a result, efficient mutual diffusion of HBP nanoparticles and monomer having their very large refractive index difference took place. Obtained results suggest a potentiality of our newly developed HBP-dispersed NPC gratings as efficient volume holographic optical elements for various photonic applications including wearable headsets for augmented and mixed reality.

Cyclic allylic sulfide based photopolymer for holographic recording showing high refractive index modulation

“…The effect of the thermal treatment becomes more relevant at higher monomer concentration and a remarkably high refractive index modulation of 0.0346 is achieved at 19.5 wt% (sample 8). Such value is among the largest reported in the literature for photopolymeric systems 24,25,31,63 . Increasing even more the concentration up to 23.6 wt% (sample 9) does not induce a ∆n increase as high as expected.…”

“…Such value is among the largest reported in the literature for photopolymeric systems. 24,25,31,63 Increasing even more the concentration up to 23.6 wt% (sample 9) does not induce a Δn increase as high as expected. In that case, a saturation of polymer concentration in the high index regions could cause end chains to remain in dark areas, mitigating the index modulation.…”

“…[21][22][23] Exploiting the mismatch of the refractive index and using hyperbranched-polymer (HBP) organic nanoparticles, Tomita et al have been able to reach Δn of the order of 0.03 and 0.045 depending on the composition and writing wavelength. 24,25 Considering the standard photopolymers, only a certain fraction of the monomer present in the formulation usually diffuses and polymerizes with the desired spatial distribution, thus contributing to Δn. 26,27 This limitation in the Δn may be mitigated by increasing the concentration of the active monomer, but a solubility limit exists and it depends on the monomer/matrix combination.…”

“…A parallel approach is at the basis of photopolymerizable nanoparticle‐polymer composite (NPC), where there is a mutual diffusion of the reactive monomer from the dark to the bright areas and the inert nanoparticles from the bright to the dark areas 21–23 . Exploiting the mismatch of the refractive index and using hyperbranched‐polymer (HBP) organic nanoparticles, Tomita et al have been able to reach Δn of the order of 0.03 and 0.045 depending on the composition and writing wavelength 24,25 . Considering the standard photopolymers, only a certain fraction of the monomer present in the formulation usually diffuses and polymerizes with the desired spatial distribution, thus contributing to ∆n 26,27 .…”

Advances on holographic polymer nanocomposites

“…(Holographic composites) [32] 、纳米粒子-聚合物复合材料体全息光栅(Nanoparticle-polymer composite volume holographic gratings) [33,34] 、 可 光 聚 合 的 纳 米 复 合 材 料 (Photopolymerizable nanocomposite materials) [35] 、纳米粒子-光 聚 合 物 复 合 材 料 (Nanoparticle-photopolymer composites) [36] 高分子材料的高性能化和多功能化提供了新思路和新机遇。在 HPNC 中，液晶不仅提供 高的折射率，还起到降低复合体系黏度、促进有序相分离的作用。此外，通过合理的分 子设计，还能够通过引入新型液晶分子赋予复合材料新的功能(如聚集诱导发光) [6] 。类 似地，HPNC 中的纳米粒子也不一定具有高折射率，还可以是折射率较低、但具有特定 功能的纳米粒子(如上转换纳米粒子) [19,29,37] 导相分离原理成型，光引发阻聚剂在构筑有序相分离结构过程中起着关键作用 [19,20,29] 。 这是因为在激光全息记录过程中， 复合体系的光聚合反应速率通常较快， 凝胶化时间短， 导致相分离 过程受阻，有序相分离结构难以形成 [4,38] 。有趣的是， 光引发阻聚剂 (photoinitibitor)作为一类新型光引发体系，在可见光的辐照下能够同时产生具有光引发 聚合(photoinitiation)和光阻聚(photoinhibition)两种功能不同的自由基(图 3) [5,39,40] 。 两种自 由基的竞争与协同效应，显著扩大了相干亮区(bright region，见图 1，也称为 constructive region)与相干暗区(dark region，也称为 destructive region)之间的凝胶化时间差异，促进 了有序相分离 [5,30,38] 。 对照实验结果显示， 当光引发体系仅产生引发聚合功能时(如双(2,6-二氟-3-吡咯苯基)二茂钛(Irgacure 784)与过氧化苯甲酰(BPO)构成的光引发体系)， 复合体 系的聚合反应速率显著加快，但相分离结构从有序变为无序，说明光引发阻聚剂对全息 记录过程中的有序相分离结构构筑具有难以替代的作用 [5,30] 。 A c c e p t e d https://engine.scichina.com/doi/10.1360/TB-2021-1116 图 3 光引发阻聚剂对调控复合材料相分离结构的关键作用。 (a) KCD/NPG 蓝光光引发阻 聚剂和 RB/NPG 绿光光引发阻聚剂的作用原理示意图 [5,41] ; (b) KCD 含量分别为 0.2wt% 和 0.6wt%时，复合材料光栅结构的扫描电子显微镜(SEM)照片 [5] ; (c) RB 含量分别为 0.1 wt%和 1.0wt%时，复合材料光栅结构的 SEM 照片 [41] . SEM 表征前用正己烷除去了液晶 RB/NPG green light photoinitibitor [5,41] .…”