**3. Results and discussion**

#### **3.1 The influence of ionic liquids on photopolymerizable holographic materials**

Ionic liquids have significant influence on the kinetics of various polymerization reactions [26-27]. The nature of ionic liquids has strongly influenced the polymerization rate and conversion of oligomer. For instance, Chesnokov et al. reported that the addition of imidazonium salts suppress the polymerization of PEGDMA, however, tetraalkylphosphonium salts improve the photopolymerization [58]. Photopolymerizable holographic materials for practical use need to have high sensitivity, high diffraction efficiency and high resolution. The sensitivity of photopolymerizable holographic material and the photopolymerization rate have different meanings. The sensitivity here is defined as the needed exposure time (or energy) to reach the highest diffraction efficiency. The overall rate of polymerization (Rp) is considered as the rate of disappearance of monomer with respect to time, -d[M]/dt. In our research, we investigated the influence of ionic liquids on the sensitivity, the diffraction efficiency and the resolution of the photopolymerizable holographic materials.

Poly(ethylenylglycol)dimethacrylate (PEGDMA) (average Mn~ 330) was used as monomer, and Irgacure 184 (Irg184) was used as photoinitiator. In some samples, polyvinyl acetate (PVAC) or Epoxy L20/Hardener EPH161 were used as polymer binders. The structures of ionic liquids used are shown in Figure 4. 1,3-dialkylimidazolium, pyridium, and phosphonium with various counter anions were used as additives.

Due to the low solubility of PEGDMA in some ionic liquids, such as BMIMCl, BMIMBr, BMIMSO3Me, BMIMSO4Me, BMIMSO3Ph, and BMIMHSO4, BPMCl, BPMPF6, Bu4NPF6, etc, PEGDMA cannot be solved fully in these ionic liquids to form homogenous composites. Table 1 collects the test composites used to record holographic gratings. Figure 5 shows the representative diffraction pattern by probing the hologram with 633 nm He-Ne laser beam. The diffraction efficiencies of the gratings are showed in Table 1 (the angle between the two beams was 2o). Although PEGDMA with IRG184 as initiator gave rise to low diffraction efficiency (Table 1, Sample 1), most of the tested samples gave rise to good diffraction efficiencies (η) except the composites with BMIMNCN2, BMIMSCN or BMIMFeCl4 as additives (Table 1, Sample 2-20), and the formed gratings have better resolution than the sample 1 without ionic liquids (Figure 6a, b). In the presence of PVAC (polyvinyl acetate) (Table 1, Sample 21-32), the diffraction efficiency was further increased except for C32H68PCl, C32H68PPF6 and C32H68PBF4, which formed inhomogeneous composites with PEGDMA/PVAC. 34 % of the theoretical maximum diffraction efficiencies for thin hologram [1,2] were obtained. Using Epoxy L20/Hardener EPH161 as polymer binder, satisfying diffraction efficiencies were obtained as well (Sample 35-48). Interestingly, pholymerizable ionic liquids can also be used as additive in photopolymerizable holographic materials. The application of polymerizable ionic liquids may lead to form a more stable hologram. For instance, 1-butyl-3-vinylimidazolium tetrafluoroborate (BVIMBF4) and 1-allyl-3-butylimidazolium tetrafluoroborate (ABIMBF4) were used as additive of photopolymerizable holographic materials. 17% and 19% diffraction efficiency was obtained, respectively (Table 1, Sample 19, 20). Nevertheless, in the presence of PVAC, the theoretical maximum diffraction efficiencies were obtained as well (Table 1, Sample 33, 34). To test the polymerizability of ionic liquids can carry out polymerization under this exposure condition, BVIMBF4 (2.0 g) was mixed with Irg184 (0.05 g) to form a composite. The hologram was formed successfully, but only gave rise to 2% diffraction efficiency.

$$\left[ \begin{array}{c} \begin{array}{c} \text{(}\text{)} \\ \text{(}\text{)} \end{array} \right]\_{\mathbf{M}\_{\text{(}\text{M}\text{e})}} \end{array} \qquad \qquad \left| \begin{array}{c} \text{(}\text{)} \\ \text{(}\text{)} \\ \text{(}\text{)} \end{array} \right]\_{\mathbf{M}\_{\text{(}\text{)}}} $$

6 Holograms – Recording Materials and Applications

Oriel® Flood Exposure Source (Model 92540-1000, Newport Co.) was used as the UV source. An optic setup was shown in Figure 3. A textured mask with random 2-15 micrometer apertures was used as mask, and polyester (PET) film was used as substrate. 75 µm thickness PET film was used as spacer. The films of photopolymerizable holographic materials were exposed vertically or 60o to collimated UV light through the mask, followed by exposing from another side to complete the polymerization process. The direct transmittance versus the tilt angle was measured with Haze meter (BYK Gardner) and the

**3.1 The influence of ionic liquids on photopolymerizable holographic materials** 

Ionic liquids have significant influence on the kinetics of various polymerization reactions [26-27]. The nature of ionic liquids has strongly influenced the polymerization rate and conversion of oligomer. For instance, Chesnokov et al. reported that the addition of imidazonium salts suppress the polymerization of PEGDMA, however, tetraalkylphosphonium salts improve the photopolymerization [58]. Photopolymerizable holographic materials for practical use need to have high sensitivity, high diffraction efficiency and high resolution. The sensitivity of photopolymerizable holographic material and the photopolymerization rate have different meanings. The sensitivity here is defined as the needed exposure time (or energy) to reach the highest diffraction efficiency. The overall rate of polymerization (Rp) is considered as the rate of disappearance of monomer with respect to time, -d[M]/dt. In our research, we investigated the influence of ionic liquids on the sensitivity, the diffraction efficiency and the resolution of the photopolymerizable

Poly(ethylenylglycol)dimethacrylate (PEGDMA) (average Mn~ 330) was used as monomer, and Irgacure 184 (Irg184) was used as photoinitiator. In some samples, polyvinyl acetate (PVAC) or Epoxy L20/Hardener EPH161 were used as polymer binders. The structures of ionic liquids used are shown in Figure 4. 1,3-dialkylimidazolium, pyridium, and

Due to the low solubility of PEGDMA in some ionic liquids, such as BMIMCl, BMIMBr, BMIMSO3Me, BMIMSO4Me, BMIMSO3Ph, and BMIMHSO4, BPMCl, BPMPF6, Bu4NPF6, etc, PEGDMA cannot be solved fully in these ionic liquids to form homogenous composites. Table 1 collects the test composites used to record holographic gratings. Figure 5 shows the representative diffraction pattern by probing the hologram with 633 nm He-Ne laser beam. The diffraction efficiencies of the gratings are showed in Table 1 (the angle between the two beams was 2o). Although PEGDMA with IRG184 as initiator gave rise to low diffraction

phosphonium with various counter anions were used as additives.

**2.2 The fabrication of optic diffuser** 

diffuser was rotated from -60o to +60o around the vertical axis.

Fig. 3. The optic set-up for fabricating optic diffuser.

**3. Results and discussion** 

holographic materials.

RMIMX R = Bu, or Oct X = Cl, BF4, PF6, NTf2, or OTf etc

RPYX R = Bu, or Oct X = Cl, BF4, Pf6, or NTf2 etc

BVIMBF4

Bu4PX X = Cl, PF6, or BF4 etc C32H68PX

X = Cl, PF6, or BF4 etc

Ionic Liquids in Photopolymerizable Holographic Materials 9

With PVAC sample[c]


BMIMBF4 2 16 22 34 35 27

BMIMPF6 3 4 23 34 36 13

OMIMBF4 4 13 24 31 37 24

OMIMPF6 5 9 25 34 38 22

BMIMNTf2 6 16 26 34 39 24

BMIMOTf 7 15 27 34 40 26

BMIMNCN2 8 7 - - - -

BMIMSCN 9 4 - - - -

BMIMFeCl4 10 1 - - - -

BMIMSO4Oct 11 9 28 25 41 21

BPMBF4 12 22 29 34 42 20

OPMBF4 13 15 30 34 43 26

OPMNTf2 14 25 31 34 44 21

Bu4PBF4 15 27 32 34 45 29

C32H68PCl 16 11 - - 46 4

C32H68PPF6 17 28 - - 47 26

C32H68PBF4 18 15 - - 48 20

BVIMBF4 19 17 33 34 - -

ABIMBF4 20 19 34 34 - -

Epoxy L20/Hardener EPH161 (0.5 g, w/w = 4: 1), Irg184 (0.1 g), Ionic liquid (1.0 g).

Table 1. Properties of the holographic films.[a]

[a] All of the films were exposed for 8 seconds (E ~ 0.26 J·cm-2) with 351 nm Ar laser (Power ~ 32 mW·cm-2). The diffraction efficiencies were measured with He-Ne laser (633 nm). The thicknesses were about 10 µm, and the grating space is about 10.0 µm. [b] PEGDMA (4.0 g), Irg184 (0.2 g), Ionic liquid (1.0 g); [c] PEGDMA/PVAC (4.0 g, w/w = 10: 1), Irg184 (0.08 g), Ionic liquid (1.0 g); [d] PEGDMA (3.4 g),

DE (η, %)

With EPOLH sample[d]

DE (η, %)

Additive

No matrix Sample[b]

DE (η, %)

Fig. 5. The diffraction pattern obtained by 633 nm He-Ne laser beam.

Fig. 6. Comparison of the optic microscopy images of the gratings. (a) Sample 1 (PEGDMA, 4.0g; Irg184, 0.2 g), η = 1%, Λ = 10.0 µm, θ = 1.0o. (b) Sample 2 (PEGDMA, 4.0g; BMIMBF4, 1.0g; Irg184, 0.2 g), η = 16%, Λ = 10.0 µm, θ = 1.0o. (c) Sample 22 (PEGDMA/PVAC, 4.0 g, w/w = 10: 1; BMIMBF4, 1.0 g; Irg184, 0.08 g), η = 34%, Λ = 10.0 µm, θ = 1.0o. (d) Sample 22, η = 14%, Λ = 5.6 µm, θ = 1.8o. (e) Sample 22, η = 11%, Λ = 4.7 µm, θ = 2.1o. (f) Sample 22, η = 8%, Λ = 4.0 µm, θ = 2.5o. (g) Sample 22, η = 3%, Λ = 2.9 µm, θ = 3.5o. (h) Sample 22, η = 1%, Λ = 2.2 µm, θ = 4.6o.


Fig. 5. The diffraction pattern obtained by 633 nm He-Ne laser beam.

Fig. 6. Comparison of the optic microscopy images of the gratings. (a) Sample 1 (PEGDMA, 4.0g; Irg184, 0.2 g), η = 1%, Λ = 10.0 µm, θ = 1.0o. (b) Sample 2 (PEGDMA, 4.0g; BMIMBF4, 1.0g; Irg184, 0.2 g), η = 16%, Λ = 10.0 µm, θ = 1.0o. (c) Sample 22 (PEGDMA/PVAC, 4.0 g, w/w = 10: 1; BMIMBF4, 1.0 g; Irg184, 0.08 g), η = 34%, Λ = 10.0 µm, θ = 1.0o. (d) Sample 22, η = 14%, Λ = 5.6 µm, θ = 1.8o. (e) Sample 22, η = 11%, Λ = 4.7 µm, θ = 2.1o. (f) Sample 22, η = 8%, Λ = 4.0 µm, θ = 2.5o. (g) Sample 22, η = 3%, Λ = 2.9 µm, θ = 3.5o. (h) Sample 22, η = 1%, Λ

= 2.2 µm, θ = 4.6o.


[a] All of the films were exposed for 8 seconds (E ~ 0.26 J·cm-2) with 351 nm Ar laser (Power ~ 32 mW·cm-2). The diffraction efficiencies were measured with He-Ne laser (633 nm). The thicknesses were about 10 µm, and the grating space is about 10.0 µm. [b] PEGDMA (4.0 g), Irg184 (0.2 g), Ionic liquid (1.0 g); [c] PEGDMA/PVAC (4.0 g, w/w = 10: 1), Irg184 (0.08 g), Ionic liquid (1.0 g); [d] PEGDMA (3.4 g), Epoxy L20/Hardener EPH161 (0.5 g, w/w = 4: 1), Irg184 (0.1 g), Ionic liquid (1.0 g).

Table 1. Properties of the holographic films.[a]

Ionic Liquids in Photopolymerizable Holographic Materials 11

than the polymerization rate at the beginning. At the end triggered by the monomer concentration, the diffusion rate is less than the polymerization rate. This has an impact on the diffraction efficiency. In the presence of some ionic liquids, such as BMIMBF4, BMIMPF6, OMIMPF6, BMIMNTf2, BPMBF4, etc, the diffraction efficiencies increased continually during the hologram formation (Figure 8b-e), which indicates that the diffusion rate is higher than the polymerization rate during the polymerization process. For the formation mechanism of the hologram it can be proposed that, a monomer is polymerized in the exposure region by light activation while writing the information into the material. Since the concentration of

Fig. 8. The diffraction efficiency (%) vs time (second). The power of the laser beam is approximately 32 mW·cm-2. The different exposure time may be seen from the different mark of the skeleton. (a) Sample 1; (b) Sample 2 (BMIMBF4); (c) Sample 12 (BPMBF4); (d)

Sample 15 (Bu4PBF4); (e) Sample 20 (ABIMBF4); (f) Sample 8 (BMIMNCN2).

Other different spatial frequency gratings were also fabricated to find that the ionic liquidphotopolymerizable holographic materials perform better at larger grating spacings. For instance, the diffraction efficiency of the grating based on composite 4 is 34% for 10.0 µm, 14% for 5.6 µm, 11% for 4.7 µm, 8% for 4.0 µm, and 3% for 2.9 µm, respectively. Figure 7 shows the spatial frequency response of the material. The spatial frequency increasing leads to the decreasing of the diffraction efficiency. These results are similar to the hologram material based on a photopolymerizable nematic acrylate [20]. Figure 6c-f shows the optic images of the gratings.

Fig. 7. The spatial frequence (mm-1) vs the diffraction efficiency (%) of composite 4. The spatial frequencies increasing led to the decreasing of the diffraction efficiencies.

Figure 8 shows the representative curves of the diffraction efficiencies with reference to time. In the presence of BMIMNCN2, BMIMSCN or BMIMFeCl4, an unstable hologram was formed (Figure 8f). In contrast, in the presence of BMIMBF4, OMIMBF4, BMIMPF6, OMIMPF6, BMIMNTf2, BMIMOTf, BMIMSO4Oct, BPMBF4, OPMBF4, OPMNTf2, Bu4PBF4, C32H68PCl, C32H68PF6, C32H68BF4, BVIMBF4 or ABIMBF4, the materials were more sensitive only needing about 5-6 seconds exposure to reach the maximum stable value and had higher diffraction efficiencies (Figure 8 b-e) compared to the composite in the absence of ionic liquid needing about 8 seconds exposure (Figure 8 a). The diffraction efficiency continued to increase to a stable value after stopping the exposure (Figure 8 b-e). According to the diffusion theory for formation of the hologram, the concentration of the monomer (c) is related to the diffraction efficiency (I/I0): -dc/dt = kIc, whereas k depends on the extinction of the quantum yield [59]. For the composites without ionic liquid or with BMIMNCN2, BMIMSCN, or BMIMFeCl4 as additive, the diffraction efficiencies first increase, then drop sharply (Figure 8a, f), which indicates that the diffusion rate is bigger

Other different spatial frequency gratings were also fabricated to find that the ionic liquidphotopolymerizable holographic materials perform better at larger grating spacings. For instance, the diffraction efficiency of the grating based on composite 4 is 34% for 10.0 µm, 14% for 5.6 µm, 11% for 4.7 µm, 8% for 4.0 µm, and 3% for 2.9 µm, respectively. Figure 7 shows the spatial frequency response of the material. The spatial frequency increasing leads to the decreasing of the diffraction efficiency. These results are similar to the hologram material based on a photopolymerizable nematic acrylate [20]. Figure 6c-f shows the optic

0 5 10 15 20 25 30 35

Diffraction Efficiency (%)

Figure 8 shows the representative curves of the diffraction efficiencies with reference to time. In the presence of BMIMNCN2, BMIMSCN or BMIMFeCl4, an unstable hologram was formed (Figure 8f). In contrast, in the presence of BMIMBF4, OMIMBF4, BMIMPF6, OMIMPF6, BMIMNTf2, BMIMOTf, BMIMSO4Oct, BPMBF4, OPMBF4, OPMNTf2, Bu4PBF4, C32H68PCl, C32H68PF6, C32H68BF4, BVIMBF4 or ABIMBF4, the materials were more sensitive only needing about 5-6 seconds exposure to reach the maximum stable value and had higher diffraction efficiencies (Figure 8 b-e) compared to the composite in the absence of ionic liquid needing about 8 seconds exposure (Figure 8 a). The diffraction efficiency continued to increase to a stable value after stopping the exposure (Figure 8 b-e). According to the diffusion theory for formation of the hologram, the concentration of the monomer (c) is related to the diffraction efficiency (I/I0): -dc/dt = kIc, whereas k depends on the extinction of the quantum yield [59]. For the composites without ionic liquid or with BMIMNCN2, BMIMSCN, or BMIMFeCl4 as additive, the diffraction efficiencies first increase, then drop sharply (Figure 8a, f), which indicates that the diffusion rate is bigger

Fig. 7. The spatial frequence (mm-1) vs the diffraction efficiency (%) of composite 4. The spatial frequencies increasing led to the decreasing of the diffraction efficiencies.

images of the gratings.

100

150

200

Spatial Frequency (mm-1)

250

300

350

than the polymerization rate at the beginning. At the end triggered by the monomer concentration, the diffusion rate is less than the polymerization rate. This has an impact on the diffraction efficiency. In the presence of some ionic liquids, such as BMIMBF4, BMIMPF6, OMIMPF6, BMIMNTf2, BPMBF4, etc, the diffraction efficiencies increased continually during the hologram formation (Figure 8b-e), which indicates that the diffusion rate is higher than the polymerization rate during the polymerization process. For the formation mechanism of the hologram it can be proposed that, a monomer is polymerized in the exposure region by light activation while writing the information into the material. Since the concentration of

Fig. 8. The diffraction efficiency (%) vs time (second). The power of the laser beam is approximately 32 mW·cm-2. The different exposure time may be seen from the different mark of the skeleton. (a) Sample 1; (b) Sample 2 (BMIMBF4); (c) Sample 12 (BPMBF4); (d) Sample 15 (Bu4PBF4); (e) Sample 20 (ABIMBF4); (f) Sample 8 (BMIMNCN2).

Ionic Liquids in Photopolymerizable Holographic Materials 13

source or mask. Comparing to other diffusers, the holographic diffusers have unique properties, such as controllable diffusion angle, directional property, volume refractive index variation and high transmittance. Hologram materials such as silver halide sensitized gelatine [72], dichromated gelatine [73], photopolymer [74-76] and azobenzene polymer [77] have been used to fabricate the diffusers. The properties of source diffuser or mask and

As we discuss in the 3.1 section, ionic liquids can be used as additives to increase the sensitivity, the diffraction efficiency and the resolution of photopolymerizable holographic materials. Interestingly, there is strong dark diffusion of the monomers during the polymerization process. In this section, we described the applications of ionic liquidsphotopolymerizable holographic materials in fabricating optic diffusers via lithographic writing process. A textured mask with random 2 – 15 µm apertures was used as the mask. The films of the materials were exposed to collimated UV light through the mask. Figure 10 illustrates the lithographic writing process. The film of ionic liquids-photopolymerizable holographic materials exposes to the UV light. During the exposure to the UV light, the monomers in the bright region were polymerized. Due to the reduction of the monomer concentration in the bright region, the monomers in the dark region diffuse to the bright

Symmetric and asymmetric diffusers with directional diffusion properties were both fabricating based on the ionic liquids-photopolymerizable holographic materials. For instance, using BMIMBF4 as additive (sample 2, Table 1), the optic diffusers were obtained successfully with directional diffusion properties. The transmittance values varied from 7- 57% within the measured angle (Figure 11a). In comparison, the composites without ionic liquids only afforded a transmittance film. Figure 11b, c show the photos of the diffusion patterns using 633 nm wavelength laser incident to the diffuser (a) a commercial particletype diffuser and (b) the diffuser fabricated with sample 2. Comparing to the particle-type

diffuser, our diffuser can scatter the light more uniformly and effectively.

holographic medium have important effects on the diffuser.

region to form gradient structure with volume refractive index variation.

Fig. 10. Illustration of the lithographic writing process.

the monomer is reduced, monomers in the dark and unexposed regions of the material diffuse to the exposed region. Due to the diffusion-controlled polymerization in the presence of some ionic liquids, the diffraction efficiencies increase continually during the hologram formation. This gives rise to higher diffraction efficiencies and bigger refractive index modulations compared to the other composites, whose diffraction efficiencies first increase, then drop sharply because of the polymerization rate controlled polymerization. On the other hand, the characters of ionic liquid have important effect on the properties of photopolymerizable holographic materials. For example, although both [BMIM][BF4] and [BMIM][PF6] can improve the sensitivity of the materials, only the former gave rise to high diffraction efficiency. This may be due to the different solubility property, viscosity or polarity of ionic liquids [27].

Additionally, we have also looked after the morphology of the gratings with scanning electron microscopy. In absence of polymer binder, sample 2 gave rise to a homogeneous grating, where we can not find any obvious phase separation (Figure 9a). But in the presence of epoxy resin, an obvious phase separation occurs which forms droplets of approx. 1.5 µm on the grating (Figure 9b). Possibly these are due to the different solubilization of polymers in ionic liquids [60,61]. Phase separation is often seen in the holographic polymer dispersed liquid crystal (H-PDLC) [62,63].

Fig. 9. SEM micrographs of the gratings. (a) sample 2 (PEGDMA, 4.0g; BMIMBF4, 1.0g; Irg184, 0.2 g), (b) sample 35 (PEGDMA, 3.4 g; Epoxy L20/Hardener EPH161, 0.5 g, w/w = 4: 1; Irg184, 0.1 g, BMIMBF4, 1.0g).

#### **3.2 Fabricating optic diffuser using photopolymerizable holographic materials**

Optic diffusers are key optic elements in liquid crystal displays (LCDs) which spread the incident light from sources over a wide angle to prevent light sources from being seen directly by viewers and to keep the brightness uniform over the entire display area. Generally, the diffusers can be classified into two types: the particle-diffusing type or the surface-relief type. Particle diffusers rely on the transparent beads inside the plastic films of plates to scatter light [64-66]. The distribution of diffusing beads in the diffuser is nonuniform, which affects the performance of diffusion light. The surface-relief diffusers scatter the light by the microstructures thereon, e. g. microlens diffuser [67,68], random phase diffuser [69], deterministic diffractive diffuser [70] and holographic diffuser [71-77]. Much research has been focused on holographic diffusers, which were produced via exposure of the film of photopolymerizable holographic material to collimated light through a diffuser

the monomer is reduced, monomers in the dark and unexposed regions of the material diffuse to the exposed region. Due to the diffusion-controlled polymerization in the presence of some ionic liquids, the diffraction efficiencies increase continually during the hologram formation. This gives rise to higher diffraction efficiencies and bigger refractive index modulations compared to the other composites, whose diffraction efficiencies first increase, then drop sharply because of the polymerization rate controlled polymerization. On the other hand, the characters of ionic liquid have important effect on the properties of photopolymerizable holographic materials. For example, although both [BMIM][BF4] and [BMIM][PF6] can improve the sensitivity of the materials, only the former gave rise to high diffraction efficiency. This may be due to the different solubility property, viscosity or

Additionally, we have also looked after the morphology of the gratings with scanning electron microscopy. In absence of polymer binder, sample 2 gave rise to a homogeneous grating, where we can not find any obvious phase separation (Figure 9a). But in the presence of epoxy resin, an obvious phase separation occurs which forms droplets of approx. 1.5 µm on the grating (Figure 9b). Possibly these are due to the different solubilization of polymers in ionic liquids [60,61]. Phase separation is often seen in the

Fig. 9. SEM micrographs of the gratings. (a) sample 2 (PEGDMA, 4.0g; BMIMBF4, 1.0g; Irg184, 0.2 g), (b) sample 35 (PEGDMA, 3.4 g; Epoxy L20/Hardener EPH161, 0.5 g, w/w = 4:

**3.2 Fabricating optic diffuser using photopolymerizable holographic materials** 

Optic diffusers are key optic elements in liquid crystal displays (LCDs) which spread the incident light from sources over a wide angle to prevent light sources from being seen directly by viewers and to keep the brightness uniform over the entire display area. Generally, the diffusers can be classified into two types: the particle-diffusing type or the surface-relief type. Particle diffusers rely on the transparent beads inside the plastic films of plates to scatter light [64-66]. The distribution of diffusing beads in the diffuser is nonuniform, which affects the performance of diffusion light. The surface-relief diffusers scatter the light by the microstructures thereon, e. g. microlens diffuser [67,68], random phase diffuser [69], deterministic diffractive diffuser [70] and holographic diffuser [71-77]. Much research has been focused on holographic diffusers, which were produced via exposure of the film of photopolymerizable holographic material to collimated light through a diffuser

holographic polymer dispersed liquid crystal (H-PDLC) [62,63].

polarity of ionic liquids [27].

1; Irg184, 0.1 g, BMIMBF4, 1.0g).

source or mask. Comparing to other diffusers, the holographic diffusers have unique properties, such as controllable diffusion angle, directional property, volume refractive index variation and high transmittance. Hologram materials such as silver halide sensitized gelatine [72], dichromated gelatine [73], photopolymer [74-76] and azobenzene polymer [77] have been used to fabricate the diffusers. The properties of source diffuser or mask and holographic medium have important effects on the diffuser.

As we discuss in the 3.1 section, ionic liquids can be used as additives to increase the sensitivity, the diffraction efficiency and the resolution of photopolymerizable holographic materials. Interestingly, there is strong dark diffusion of the monomers during the polymerization process. In this section, we described the applications of ionic liquidsphotopolymerizable holographic materials in fabricating optic diffusers via lithographic writing process. A textured mask with random 2 – 15 µm apertures was used as the mask. The films of the materials were exposed to collimated UV light through the mask. Figure 10 illustrates the lithographic writing process. The film of ionic liquids-photopolymerizable holographic materials exposes to the UV light. During the exposure to the UV light, the monomers in the bright region were polymerized. Due to the reduction of the monomer concentration in the bright region, the monomers in the dark region diffuse to the bright region to form gradient structure with volume refractive index variation.

Fig. 10. Illustration of the lithographic writing process.

Symmetric and asymmetric diffusers with directional diffusion properties were both fabricating based on the ionic liquids-photopolymerizable holographic materials. For instance, using BMIMBF4 as additive (sample 2, Table 1), the optic diffusers were obtained successfully with directional diffusion properties. The transmittance values varied from 7- 57% within the measured angle (Figure 11a). In comparison, the composites without ionic liquids only afforded a transmittance film. Figure 11b, c show the photos of the diffusion patterns using 633 nm wavelength laser incident to the diffuser (a) a commercial particletype diffuser and (b) the diffuser fabricated with sample 2. Comparing to the particle-type diffuser, our diffuser can scatter the light more uniformly and effectively.

Ionic Liquids in Photopolymerizable Holographic Materials 15

The cross section of the diffuser was examined with optical microscopy. The modulation of the refractive index was visible as shown in Figure 13 of the fiber structure. The surface of the diffuser was analysed with scanning electron microscopy (SEM). Figure 14 (a, b) shows the surface image of the diffuser based on sample 2. The pattern of the mask has been successfully recorded to form a surface-relief structure. Interestedly, there were many particles in a range tens to hundreds of nanometers on the surface, which possibly arise from phase separation of BMIMBF4 in the bulk during polymerization. After that, we examined the cross section by SEM and found that most nanoparticles appeared in the region near both surfaces and that the bulk was more homogeneous [Figure 14 (c, d)]. The nanoparticles may function as particulate scatterers due to the low refractive index of n =

Fig. 13*.* The cross section optic images of the diffusers based on sample 2. (a) symmetric

Fig. 14. The SEM images of the diffuser based on sample 2. (a) The surface image in 2000 × magnification. (b) The surface image in 6000 × magnification. (c) The cross section near the

mask region. (d) The cross section near the substrate.

1.422 of BMIMBF4, compared to n = 1.463 of PEGDMA.

diffuser, (b) asymmetric diffuser.

Fig. 11. a) The direct transmittance (%) versus the sample tilt angle (degree) of the symmetric diffuser. The different composites may be seen from the different symbol of the skeleton. Sample 1: PEGDMA, 4.0 g; Irg184, 0.2 g; Sample 2 : PEGDMA, 4.0g; BMIMBF4, 1.0g; Irg184, 0.2 g. b) Photo of the diffusion pattern of the commercial particle-type diffuser. c) Photo of the diffusion pattern of the diffuser fabricated with sample 2.

The transmittance can be adjusted by changing the concentration of ionic liquids. Increasing the concentration of ionic liquids led to more haze as shown in Figure 12a. The materials can also be used to fabricate asymmetric diffusers. The films were exposed to a 60° angle to provide the asymmetric diffusers with directional diffusion property (Figure 12b). The characteristics of ionic liquids have an important influence on the diffusion properties of the diffusers. For instance, using 1-butyl-3-methyl-imidazolium hexafluorophosphate (BMIMPF6) as additive (Table 1, sample 3), which only led to 4% diffraction efficiency in the thin hologram, however representing strong diffusion during the polymerization process. It led to a diffuser with a transmittance value variable from 50% to 79% within the measured angle. Although it had a high diffraction efficiency using polymerizable ionic liquids as additive (Table 1, sample 33, 34), it also led to a diffuser with bad diffusion properties.

Fig. 12. The direct transmittance (%) versus the sample tilt angle (degree) of the symmetric diffuser. The different composites may be seen from the different symbol of the skeleton.

b

b

c)

 sample 1 sample 2

symmetric diffuser. The different composites may be seen from the different symbol of the skeleton. Sample 1: PEGDMA, 4.0 g; Irg184, 0.2 g; Sample 2 : PEGDMA, 4.0g; BMIMBF4, 1.0g; Irg184, 0.2 g. b) Photo of the diffusion pattern of the commercial particle-type diffuser. c) Photo of the diffusion pattern of the diffuser fabricated with sample 2.

The transmittance can be adjusted by changing the concentration of ionic liquids. Increasing the concentration of ionic liquids led to more haze as shown in Figure 12a. The materials can also be used to fabricate asymmetric diffusers. The films were exposed to a 60° angle to provide the asymmetric diffusers with directional diffusion property (Figure 12b). The characteristics of ionic liquids have an important influence on the diffusion properties of the diffusers. For instance, using 1-butyl-3-methyl-imidazolium hexafluorophosphate (BMIMPF6) as additive (Table 1, sample 3), which only led to 4% diffraction efficiency in the thin hologram, however representing strong diffusion during the polymerization process. It led to a diffuser with a transmittance value variable from 50% to 79% within the measured angle. Although it had a high diffraction efficiency using polymerizable ionic liquids as additive (Table 1, sample 33, 34), it also led to a diffuser with bad diffusion properties.


Sample tilt angle (degree) Fig. 11. a) The direct transmittance (%) versus the sample tilt angle (degree) of the

Fig. 12. The direct transmittance (%) versus the sample tilt angle (degree) of the

symmetric diffuser. The different composites may be seen from the different symbol of

a

Direct Transmittance (%)

the skeleton.

The cross section of the diffuser was examined with optical microscopy. The modulation of the refractive index was visible as shown in Figure 13 of the fiber structure. The surface of the diffuser was analysed with scanning electron microscopy (SEM). Figure 14 (a, b) shows the surface image of the diffuser based on sample 2. The pattern of the mask has been successfully recorded to form a surface-relief structure. Interestedly, there were many particles in a range tens to hundreds of nanometers on the surface, which possibly arise from phase separation of BMIMBF4 in the bulk during polymerization. After that, we examined the cross section by SEM and found that most nanoparticles appeared in the region near both surfaces and that the bulk was more homogeneous [Figure 14 (c, d)]. The nanoparticles may function as particulate scatterers due to the low refractive index of n = 1.422 of BMIMBF4, compared to n = 1.463 of PEGDMA.

Fig. 13*.* The cross section optic images of the diffusers based on sample 2. (a) symmetric diffuser, (b) asymmetric diffuser.

Fig. 14. The SEM images of the diffuser based on sample 2. (a) The surface image in 2000 × magnification. (b) The surface image in 6000 × magnification. (c) The cross section near the mask region. (d) The cross section near the substrate.

Ionic Liquids in Photopolymerizable Holographic Materials 17

In summary, we investigated the influence of ionic liquids on photopolymerizable holographic materials. Although not all of the ionic liquids can be used as additives for photopolymerizable holograms, we found that imidazolium, pyridium and phosphonium based ionic liquids with proper counter anions, such as BMIMBF4, OMIMBF4, BMIMNTf2, BPMBF4, OPMBF4, OPMNTf2, Bu4PBF4, C32H68PCl etc, can be used as additives to improve the properties of the materials. The sensitivity, resolution and the diffraction efficiency of the materials were increased efficiently. More interestingly, it presented strong dark diffusion of the monomers during polymerization process due to the diffusion controlled polymerization in the presence of some ionic liquids. Polymerizable ionic liquids were also used as additives in the holographic materials. High diffraction efficiencies were obtained as well. The photopolymerizable holographic materials have shown the potential application in fabricating optic diffuser for LCD. The symmetric and asymmetric diffusers with directional properties were successfully produced via lithographic recording method. The diffusion property can be regulated by changing the concentration of ionic liquid. The fiber structure, the surface-relief structure and the formation of nanoparticles lead to the

Ionic liquids are often named as so-called green solvents. However, "Greenness" of ionic liquids depends strongly on the structure. It is necessary to mention that ionic liquids exist as a component after the formation of the hologram. Low or no toxicity of ionic liquids is required for the actual application. Ionic liquids are designable. Our results are helpful for designing eco-friendly and successional holographic materials. Further researches on the application of ionic liquids in organic-inorganic nanocomposites and cationic ring-opening

The authors thank the Stiftung Europrofession, the State of Saarland and the Fonds der Chemischen Industrie for financial support. We thank Dr. Peter könig and Dr. Peter Rogin

[1] P. Hariharan, *Optical Holography: principles, techniques, and applications.* Cambridge

[3] T. J. Trout, J. J. Schmieg, W. J. Gambogi, A. M. Weber, (1998) *"Optical Photopolymers:* 

[4] D. J. Lougnot, in: *Radiat. Curing Polym. Sci. Technol*. Vol. 3 (Eds. J. P. Fouassier, J. F.

[5] M. L. Calvo, P. Cheben, *Fundamentals and advances in holographic materials for optical data* 

[6] J. R. Lawrence, F. T. O'Neill, J. T. Sheridan, (2001) "Photopolymer holographic recording

*storage, in Advances in information optics and photonics* (Eds. A. T. Friberg, R.

[2] P. Hariharan, *Basic of Holography*. Cambridge University press, 2002.

*Design and Applications," Adv. Mater.*, 10 , 1219-1224.

Rabek), Elsevier London, New York, 1993, pp. 65.

Dändliker), SPIE Press, Bellingham, 2008, Chapter 15.

**4. Conclusion** 

directional diffusion property of the diffuser.

**5. Acknowledgments** 

for useful discussions.

University press, 1996.

material," *Optic*, 112, 449-463.

**6. References** 

polymerization holographic materials are in progress.

For comparison, the diffuser with bad diffusion properties was also observed with optic microscopy and scanning electron microscopy. The fiber structure was successfully formed, but there was no uniform phase separation during the polymerization process as shown the optic images and SEM images in Figure 15.

Fig. 15. a) The cross section optic images of the diffusers based on sample 3. b) The cross section optic images of the diffusers based on sample 19. (c) The SEM image in 2000 × magnification of the diffuser based on sample 3. (d) The SEM image in 2000 × magnification of the diffuser based on sample 19.

Most tested composites with ionic liquid as additive formed the fibre structure successfully, which indicates the volume refraction index variation. The diffusioncontrolled polymerization in the presence of ionic liquid was beneficial for the formation of the fibre structure. Generally, during the lithographic process, the monomers in the bright region were polymerized. Due to the decreasing of the monomer concentration in the bright region, the monomers in the dark region diffuse to the bright region and polymerize to form the fibre structure. The properties of ionic liquids have an important effect on the diffuser. For instance, BMIMBF4 afforded the better diffusion property than BMIMPF6 and BVIMBF4. One of the reasons is possibly due to the better formation of nanoparticles for the former. Thus, a forming mechanism for the diffuser with good diffusion properties can be proposed, which (during the exposure) leads to photopolymerization of the monomers in the immediate area exposed to ultra-violet light, accompanied and followed by diffusion of monomers from the unexposed regions into the exposed regions. By further polymerization the fibre structure is formed and a phase separation of ionic liquid is observed leading to the formation of nanoparticles. The fibre structure, the surface-relief structure and the formation of nanoparticles altogether are responsible for the directional diffusion property of the diffuser.

### **4. Conclusion**

16 Holograms – Recording Materials and Applications

For comparison, the diffuser with bad diffusion properties was also observed with optic microscopy and scanning electron microscopy. The fiber structure was successfully formed, but there was no uniform phase separation during the polymerization process as shown the

Fig. 15. a) The cross section optic images of the diffusers based on sample 3. b) The cross section optic images of the diffusers based on sample 19. (c) The SEM image in 2000 × magnification of the diffuser based on sample 3. (d) The SEM image in 2000 × magnification

Most tested composites with ionic liquid as additive formed the fibre structure successfully, which indicates the volume refraction index variation. The diffusioncontrolled polymerization in the presence of ionic liquid was beneficial for the formation of the fibre structure. Generally, during the lithographic process, the monomers in the bright region were polymerized. Due to the decreasing of the monomer concentration in the bright region, the monomers in the dark region diffuse to the bright region and polymerize to form the fibre structure. The properties of ionic liquids have an important effect on the diffuser. For instance, BMIMBF4 afforded the better diffusion property than BMIMPF6 and BVIMBF4. One of the reasons is possibly due to the better formation of nanoparticles for the former. Thus, a forming mechanism for the diffuser with good diffusion properties can be proposed, which (during the exposure) leads to photopolymerization of the monomers in the immediate area exposed to ultra-violet light, accompanied and followed by diffusion of monomers from the unexposed regions into the exposed regions. By further polymerization the fibre structure is formed and a phase separation of ionic liquid is observed leading to the formation of nanoparticles. The fibre structure, the surface-relief structure and the formation of nanoparticles altogether are

responsible for the directional diffusion property of the diffuser.

optic images and SEM images in Figure 15.

of the diffuser based on sample 19.

In summary, we investigated the influence of ionic liquids on photopolymerizable holographic materials. Although not all of the ionic liquids can be used as additives for photopolymerizable holograms, we found that imidazolium, pyridium and phosphonium based ionic liquids with proper counter anions, such as BMIMBF4, OMIMBF4, BMIMNTf2, BPMBF4, OPMBF4, OPMNTf2, Bu4PBF4, C32H68PCl etc, can be used as additives to improve the properties of the materials. The sensitivity, resolution and the diffraction efficiency of the materials were increased efficiently. More interestingly, it presented strong dark diffusion of the monomers during polymerization process due to the diffusion controlled polymerization in the presence of some ionic liquids. Polymerizable ionic liquids were also used as additives in the holographic materials. High diffraction efficiencies were obtained as well. The photopolymerizable holographic materials have shown the potential application in fabricating optic diffuser for LCD. The symmetric and asymmetric diffusers with directional properties were successfully produced via lithographic recording method. The diffusion property can be regulated by changing the concentration of ionic liquid. The fiber structure, the surface-relief structure and the formation of nanoparticles lead to the directional diffusion property of the diffuser.

Ionic liquids are often named as so-called green solvents. However, "Greenness" of ionic liquids depends strongly on the structure. It is necessary to mention that ionic liquids exist as a component after the formation of the hologram. Low or no toxicity of ionic liquids is required for the actual application. Ionic liquids are designable. Our results are helpful for designing eco-friendly and successional holographic materials. Further researches on the application of ionic liquids in organic-inorganic nanocomposites and cationic ring-opening polymerization holographic materials are in progress.

#### **5. Acknowledgments**

The authors thank the Stiftung Europrofession, the State of Saarland and the Fonds der Chemischen Industrie for financial support. We thank Dr. Peter könig and Dr. Peter Rogin for useful discussions.

#### **6. References**


Ionic Liquids in Photopolymerizable Holographic Materials 19

[26] P. Kubisa, (2004) "Application of ionic liquids as solvents for polymerization

[27] P. Kubisa, (2005) "Ionic liquids in the synthesis and modification of polymers." *J.* 

[28] P. Kubisa, (2009) "Ionic liquids as solvents for polymerization process-progress and

[29] J. Lu, F. Yan, J. Texter, (2009) "Advanced applications of ionic liquids in polymer

[30] T. Erdmenger, C. Guerrero-Sanchez, J. Vitz, R. Hoogenboom, U. S. Schubert, (2010)

[31] A. J. Carmichael, D. M. Haddleton, S. A. F. Bo, K. R. Seddon, (2000) "Copper(I)

[32] S. Harrisson, S. R. Mackenzie, D. M. Haddleton, (2002) "Unprecedented solvent-

[33] S. Harrisson, S. R. Mackenzie, D. M. Haddleton, (2003) "Pulsed laser polymerization in

[34] J. Kadokawa, Y. Iwasaki, H. Tagaya, (2002) "Ring-Opening Polymerization of Ethylene

[35] T. Biedron, P. Kubisa, (2004) "Cationic polymerization of styrene in a neutral ionic

[36] R. Marcilla, M. Geus, D. Mecerreves, C. J. Duxbury, C. E. Koning, A. Heise, (2006) "Enzymatic polyester synthesis in ionic liquids." *Euro. Polym. J*. 42, 1215-1221. [37] C. Guerrro-Sanchez, M. Lobert, R. Hoogenboom, U. S. Schubert, (2007) "Microwave-

[38] E. Naudin, H. A. Ho, S. Branchaud, L. Breau, D. Belanger, (2002) "Electrochemical

[39] N. Li, J. Lu, Q. Xu, X. Xia, L. Wang, (2007) "Reverse atom transfer radical

[40] A. J. Carmichael, D. M. Heddleton, S. A. F. Bon, K. R. Seddon, (2000) "Copper(I)

[41] R. M. Lau, F. Van Rantwijk, K. R. Seddon, (2000) "Lipaze catalyzed reactions in ionic

Chlorostannate Melts." *Macromolecular Rapid Commun*, 23, 757-760.

liquid*." J. Polym. Sci. Part A: Polym. Chem.* 42, 3230-3235.

ionic liquids." *J. Phys. Chem. B.* 106, 10585-1-593.

"Recent developments in the utilization of green solvents in polymer chemistry."

mediated living radical polymerisation in an ionic liquid." *Chem. Commun.*, 1237-

induced acceleration of free-radical propagation of methyl methacrylate in ionic

an ionic liquid: strong solvent effects on propagation and termination of methyl

Carbonate Catalyzed with Ionic Liquids: Imidazolium Chloroaluminate and

assisted homogeneous polymerizations in water-soluble ionic liquids: an alternative and green approach for polymer synthesis." *Macromolecular Rapid* 

polymerization and characterization of poly(3-(4-fluorophenyl)thiophene) in pure

polymerization of MMA via immobilized catalysts in imidazolium ionic liquids." *J.* 

mediated living radical polymerization in an ionic liquid." *Chem. Commun.,* 1237-

processes." *Prog. Polym. Sci*., 29, 3-12.

*Polym. Sci. Part A, Polym. Chem.*, 43, 4675-4683.

challenges." *Prog. Polym. Sci.,* 34, 1333-1347.

science." *Prog. Polym. Sci.,* 34, 431-448.

liquids." *Chem. Commun*., 2850-2851.

*Commun*, 28, 456-464.

1238.

*Appl. Polym. Sci.*, 103, 3915-3919.

liquids." *Org. Lett.,* 2, 4189-4191.

methacrylate." *Macromolecules*, 36, 5072-5075.

*Chem. Soc. Rev.*, 39, 2217-3333.

1238.


[7] N. Suzuki, Y. Tomita, (2004), *"Silica-Nanoparticle-Dispersed Methacrylate Photopolymers* 

[8] C. Sanchez, M. J. Escuti, C. Van Heesch, C. W. M. Bastiaansen, D. J. Broer, J. Loos, R.

[9] F. Del Monte, O. Martinez, J. A. Rodrigo, M. L. Calvo, P. Cheben, (2006) "A

[10] C. Bräuche, H. Anneser, (1990) "Holographic spectroscopy and holographic

[11] N. Suzuki, Y. Tomita, T, Kojima, (2002) "Holograhic recording in TiO2 nanoparticledispersed methacrylate photopolymer films." *Appl. Phys. Lett.*, 81, 4121-4123. [12] W. S. Kim, Y. Jeong, J. Park, (2006) "Nanoparticle-indused refractive index modulation of organic-inorganic hybrid photopolymer." *Opt. Express*, 14, 8967-8973. [13] T. J. Trentler, J. E. Boyd, V. L. Colvin, (2002) "Epoxyl resin-photopolymer composites

[14] S. Martin, P. Leclere, Y. Renotte, V. Toal, Y. Lion, (1995) "Characterisation of an

[15] L. M. Goldenberg, O. V. Sakhno, T. N. Smirnova, P. Helliwell, V. Chechik, J. Stumpe,

[16] P. Cheben, M. L. Calvo, (2001) "A photopolymerizable glass with diffraction efficiency

[17] M. Ortuno, E. Fernandez, S. Gallego, A. Belendz, I. Pascual, (2007) "New

[20] L. J. Tucker, M. B. Sponsler, (2006) "Trithiocarbonate-mediated free-radical

[21] Acrylic anhydride was used as an additive to increase the "film speed" in hologram

[22] P. Wasserscheid, T. Welton, *Ionic Liquids in Synthesis*, Wiley VCH, Weinheim,

[23] B. Kirchner, *Top Curr Chem: Ionic liquid*, Vol. 290, Spinger-Verlag Berlin Heidelberg,

[24] P. Wasserscheid, W. Keim, (2000) "Ionic Liquids—New "Solutions" for Transition

[25] M. Freemantle, *An Introduction to ionic liquids*, Royal Society of Chemistry, 2010.

near 100% for holographic storage." *Appl. Phys. Lett.*, 78, 1490-1492.

acrylamide-based dry photopolymer holographic recording material." *Opt. Eng.,* 

(2008) "Holographic composites with gold nanoparticles: nanoparticles promote

photopolymer holographic recording material with sustainable design." *Optic* 

photopolymerization: improved uniformity in hologram recording." *Appl. Opt.,* 45,

Nussbaumer, (2005) "TiO2 Nanoparticle–Photopolymer Composites for Volume

volume holographic sol-gel material with large enhancement of dynamic range by incorporation of high refractive index species." *Adv. Mater.,* 18 ,

information recording in polymer matrices." *Lasers in Polymer Sci. Technol. Appl.,* 3,

*with Net Diffraction Efficiency Near 100%."* Appl. Opt., *43, 2125-2129.* 

Holographic Recording." *Adv. Funct. Mater.,* 15, 1623-1629.

for volume holography." *Chem. Mater.*, 12, 1431-1438.

polymer segregation." *Chem. Mater.*, 20, 4619-4627.

[18] B. M. Monroe, W. K. Smotherg, U.S Patent 4942112, Jun. 17, 1990.

material. A. M. Weber, U.S. Patent 5013632, May 7, 1991.

Metal Catalysis." *Angew. Chem Int. Ed.,* 39, 3772-3789.

2014-2017.

pp.181-210.

33, 3942–3946.

6973-6976.

2009.

Germany, 2003.

*Express*, 15, 12425-12435.

[19] K. Frank, CA Patent 2571951, Dec. 29, 2005.


Ionic Liquids in Photopolymerizable Holographic Materials 21

[59] H. Sillescu, D. Ehlich, in: *Lasers in Polymer Sci. Technol. Appl*., Vol 3, (Eds. J. P. Fouassier,

[60] P. Snedden, A. I. Copper, K. Scott, N. Winterton, (2003) "Cross-linked polymer-ionic

[61] N. Winterton, (2006) "Solubilization of polymers by ionic liquids." *J. Mater. Chem*., 16,

[62] R. L. Sutherland, L. V. Natarajan, V. P. Tondiglia, (1993) "Bragg gratings in an acrylate

[63] M. J. Escuti, P. Kossyrev, G. P. Crawford, (2000) "Expanded viewing-angle reflection

[64] G. H. Kim, (2005) "A PMMA composite as an optic diffuser in a liquid crystal display

[65] G. H. Kim, W. J. Kim, S. M. Kim, and J. G. Son, (2005) "Analysis of thermo-physical

[66] G. H. Kim, J. H. Park, (2007) "A PMMA optical diffuser fabricated using an

[67] S. I. Chang, J. B. Yoon, (2004) "Shape-controlled, high fill-factor microlens arrays

[68] S. I. Chang, J. B. Yoon, H. Kim, J. J. Kim, B. K. Lee, and D. H. Shin, (2006) "Microlens

[69] E. E. Garcia-Guerrero, E. R. Mendez, H. M. Escamilla, T. A. Leskova, and A. A.

[70] M. Parikka, T. Kaikuranta, P. Laakkonen, J. Lautanen, J. Tervo, M. Honkanen, M.

[71] C. Gu, J. R. Lien, F. Dai, and J. Hong, (1996) "Diffraction properties of volume

[72] S. I. Kim, Y. S. Choi, Y. N. Ham, C. Y. Park, and J. M. Kim, (2003) "Holographic

[73] S. Wadle, D. Wuest, J. Cantalupo, and R. S. Lakes, (1994) "Holographic diffusers," *Opt.* 

[74] S. Wadle, and R. S. Lakes, (1994) "Holographic diffusers: polarization effects," *Opt.* 

[75] C. Joubert, B. Loiseaux, A. Delboulbe, and J. P. Huignard, (1997) "Phase volume

polymer consisting of periodic polymer-dispersed liquid-crystal planes." *Chem.* 

from diffuse holographic-polymer dispersed liquid crystal films." *Appl. Phys. Lett*.,

and optical properties of a diffuser using PET/PC/PBT copolymer in LCD

fabricated by a 3D diffuser lithography and plastic replication method," *Opt.* 

array diffuser for a light-emitting diode backlight system." *Opt. Lett*., 31, 3016-

Maradudin, (2007) "Design and fabrication of random phase diffuser for extending

Kuittinen, and J. Turuner, (2001) "Deterministic diffractive diffusers for displays,"

diffuser by use of a silver halide sensitized gelatine process." *Appl. Opt.*, 42, 2482-

holographic optical components for high-brightness single-LCD projectors," *Appl.* 

J. F. Rabek) CRC-Press Inc., Boca Raton, Florida, 1990, pp. 211-226.

liquid composite materials." *Macromolecules*, 36, 4549-4556.

backlighting unit (BLU)." *Eur. Poly. J*., 41, 1729-1737.

electrospray method." *Appl. Phys. A,* 86, 347-351.

the depth of focus," *Opt. Express*, 15, 910-923.

holographic diffuser." *J. Opt. Soc. Am. A*, 13, 1704-1711.

backlight units." *Display*, 26, 37-43.

4281-4293.

*Mater*., 5, 1533-1538.

*Express*, 12, 6366-6371.

*Appl. Opt*., 40, 2239-2246.

3018.

2491.

*Eng*., 33, 213-218.

*Eng*., 33, 1084-1088.

*Opt.*, 36, 4761-4764.

77, 4262-4264.


[42] Y. S. Vygodskii, O. A. Mel'nik, A. S. Shaplov, E. L. Lozinskaya, I. A. Malyshkina, N. D.

[43] M. P. Scott, C. S. Brazel, M. G. Benton, J. W. Mays, J. D. Holbrey, R. D. Rogers, (2002)

[44] M. P. Scott, M. Rahman, C. S. Brazel, (2003) "Application of ionic liquids as low-

[45] [45] P. Snedden, A. Cooper, K. Scott, (2003) "Cross-linked polymer-ionic liquid

[46] L. Zhu, C. Y. Huang, Y. H. Patel, J. Wu, S. V. Malhotra, (2006) "Synthesis of porous

[47] J. Fuller, A. C. Breda, R. T. Carlin, (1997) "Ionic liquid-polymer gel electrolytes." *J.* 

[48] P. Wang, S. M. Zakeeruddin, I. Exnar, M. Gratzel, (2002) "High efficiency dye-

[49] T. Fukushima, K. Asaka, A. Kosaka, T. Aida, (2005) "Fully plastic actuator through

[50] H. Ohno, (2007) "Design of ion conductive polymers based on ionic liquids." *Macromol* 

[51] D. Batra, D. N. T. Hay, M. A. Firestone, (2007) "Formation of a biomimetic, liquid-

[52] J. Tang, M. Radosz, Y. Shen, (2008) "Poly(ionic liquid)s as optically transparent

[53] H. Lin, P. W. Oliveira, M. Veith, (2008) "Ionic liquid as additive to increase sensitivity,

[54] H. Lin, P. W. Oliveira, M. Veith, M. Gros, I. Grobelsek, (2009) "Optic diffusers based on

[55] H. Lin, P. W. Oliveira, M. Veith, (2011) "Application of ionic liquids in photopolymerizable holographic materials." *Opt. Mater.,* 33, 759-762. [56] X. Creary, E. D. Wilis, (2005) "Preparation of 1-butyl-3-methylimidazolium

[57] G. W. Stroke, *An introduction to coherent optics and Holography*, Academic Press, New

[58] S. A. Chesnokov, M. Y. Zakharina, A. S. Shaplov, Y. V. Chechet, E. I. Lozinskaya, O. A.

Mel'nik, Y. S. Vygodskii, G. A. Abakumov, (2008) "Ionic liquids as catalytic additives for the acceleration of the photopolymerization of poly(ethylene glycol

microwave-absorbing materials." *Macromolecules*, 41, 493-496.

tetrafluoroborate." *Organic Syntheses*, 82 , 166-169.

dimethacrylate)s." *Polym. Int.*, 57, 538-545.

volatility plasticizers for PMMA." *Eur. Polym. J.* 39, 1947-1953.

composite materials." *Macromolecules*, 36, 4549-4556.

*Macromol Rapid Commun.*, 27, 1306-1311.

*Electrochem Soc.*, 144, L67-69.

*Chem. Commun.*, 2972-2973.

44, 2410-2413.

34, 1150-1153.

York, 1969, pp. 225.

*Symp.*, 249, 551-556.

*Mater.,* 19, 1279-1287.

*Appl. Phys. Lett.*, 93, 141101.

*Polym. Sci. A.*, 49, 256-261.

*Commum.*, 1370-1371.

Gavrilova, (2007) "Synthesis and ionic conductivity of polymer ionic liquids."

"Application of ionic liquids as plasticizers for poly(methyl methacrylate)." *Chem.* 

polyurea with room-temperature ionic liquids via interfacial polymerization."

sensitized nanocrystalline solar cells based on ionic liquid polymer gel electrolyte."

layer-by-layer casting with ionic-liquids-based bulky gel." *Angew. Chem. Int. Ed.*,

crystalline hydrogel by self-assembly and polymerization of an ionic liquid." *Chem.* 

resolution, and diffraction efficiency of photopolymerizable hologram material."

photopolymerizable hologram material with an ionic liquid as additive." *Opt. Lett.*,


**2** 

 *México* 

**Norland Optical Adhesive 65**®

*2Universidad Michoacana de San Nicolás de Hidalgo* 

J.C. Ibarra1, L. Aparicio-Ixta2, M. Ortiz-Gutiérrez2 and C.R. Michel1

Research on photosensitive materials is an active field where the main goal is to find materials with desirable characteristics for optical data storage. Some of these special characteristics are high sensibility, high resolution and wide spectral range, low cost, among others (Smith, 1975). For this purpose many kinds of materials that for this purpose, such as silver halide, photoresist, dichromated gelatin, photopolymers, thermal recording materials, photothermoplastics, photocromics, and photorefractive crystals (Bjelkhangen & Thompson, 1996; Hariharan, 1980; Kang et al., 2004; Koustuk, 1999) have been used. The most widely

Photopolymers have excellent holographic characteristics, such as high refraction index modulation, real time recording, low cost, etc. The response on these materials depends of parameters such as incident beam intensity, monomers concentration, polymerization velocity, humidity, temperature, thickness of the sample, etc. (Adhami et al., 1991; Gallego et al., 2005; Gleeson, et al., 2005). Recent papers show that photopolymer's thickness is of great importance (Neipp et al., 2003; Ortuño, et al., 2003). The spectral sensibility of these materials can be easily modified if the photopolymers are mixed with dyes such as crystal

Some photopolymers employed in optical storage are given in (K. & M. Budinski, 1999; Fernandez et al., 2006; Ibarra & Olivares, 2006; Leclere et al., 1995; Naydenova et al., 2006). One of these polymers is an adhesive called Norland Optical Adhesive 65® (NOA 65®). (Pinto & Olivares, 2002) and co-workers report that they have used NOA 65® in its natural form to record computer generated Fourier holograms using microlithography techniques. Recently (Aleksejeva & Teteris, 2010), the photopolymers NOA 60, NOA 61, NOA 63, NOA 65 and NOA 68 were studied as materials for fabrication of volume gratings, they recorded transmission and reflection diffraction gratings and used a He–Cd laser of 325nm line,

In this work a study became of the holographic material composed by Norland Optical adhesive 65 (NOA 65) mixed with crystal violet dye (CV) was made. In this material we recorded transmission real time phase holographic gratings and Fourier holograms. obtaining diffraction efficiency of 1.85% using a light beam at wavelength 598 nm from a He-Ne laser was obtained. The gratings were recorded changing parameters such as

**1. Introduction** 

used at present are photopolymers.

violet (Luna et al., 1997,1998; Ortiz et al. 2007).

obtaining diffraction efficiency >80%.

**as Holographic Material** 

*1CUCEI, Universidad de Guadalajara* 

