Nanoporous-template-modulated azopolymers for enhancing reversible photo-transformation

Lihong Kang; Shencheng Fu; Xintong Zhang; Xinnong Wang; Jiarui Wu; Shuangyan Liu; Ruiya Ji; Xiuxiu Han; Yichun Liu; Jinhuan Li

doi:10.1364/OSAC.1.000477

1. Introduction

Effective utilization of photons has promoted the evolution of information technology. Especially for holography technology, it can be applied in advanced display [1–3] and the next-generation data storage with huge capacity, high-density and fast response speed in the current Big-Data-Era [4–7]. Compared with bit-mode recording, holography uses the full information of light, i.e., phase and amplitude, which can record the whole information of objects. Commonly, Holographic Versatile Disc presents large-volume optical storage ability (~6 TB) [8], which is much higher than that of Blu-ray Disc whose storage capacity is only 50 GB. Developing a suitable recording medium for holographic storage is the key to high-density information storage. Azopolymer materials based on the advantages of photochromic properties, low cost, self-processing ability and ease of access, have become one of the most commonly photo-energy transformation materials [9–11]. Applications of azopolymers have been investigated in holographic memory, optical switches, waveguides, and updatable holographic display for their high refractive-index-modulation ability [12–17].

Commonly, large refractive index modulation for azopolymers can be obtained benefiting from cis-orientation and trans-cis deformation of molecule in repeatable cycles by single visible irradiation [16–18]. For coherent excitation, however, the trans-molecules at bright regions of holographic fringes can be changed to the cis-ones, which can induce wriggling of the polymer framework and form lager free volume [19,20]. Thus, the trans-molecules at dark regions of holographic fringes tend to move to the bright one, continuing to be involved in the photo-chemical reaction. The molecule movement may result in a phase-shifted surface relief grating [21]. However, with negative effect, the polymer chain may also twine with the chromophore molecules to induce the fluctuant surface of the whole sample, i.e., the volume shrinkage, which affects the stability of holographic signal in reading process, even weakens the diffraction efficiency of holographic storage under long-term exposure [22,23].

In order to solve the problem, inorganic nanoparticles as a guest was doped in an organic host matrix to reduce shrinkage rate [24–31] and modulate refractive index [32–34]. The oxide nanoparticles such as (TiO₂, ZrO₂ and SiO₂) or the noble metal nanoparticles (such as Au and Ag) were incorporated in azopolymer to enhance the diffraction efficiency [35–40]. However, the surface modification of inorganic nanoparticles is rather difficult to be developed for better adapting to the organic-inorganic hybrid system. The homogeneous and transparent properties are still the challenge for azopolymer based hybrid systems [37–40]. Additionally, the entanglement of the polymer backbone hinders the reversible conversion from cis- to trans-form. Therefore, the behavior of photoinduced reversible transformation of azopolymers is suppressed and erasable holographic memory for the system is also difficult to be achieved. Hence, it is urgent to find a simple and effective way to realize the combination of inorganic and organic materials at nanoscale so as to better suppress the volume shrinkage of the azopolymers effectively and to improve the erasable performance of the hybrid films.

In this paper, TiO₂ nanoporous films deposited with azopolymer, as a new organic-inorganic nano-hybrid structure, are designed and fabricated. A phenomenological dynamic model is proposed to well fit the holographic kinetics of the hybrid films. Erasable hologram reconstruction is also achieved in the system.

2. Experimental

2.1 Film preparation

Poly(Disperse-Red-19-p-phenydiacryate), i.e. PDR₁₉, was commercially available as a guest role from Sigma-Aldrich without further purification. Its chemical structure is shown in Fig. 1

Fig. 1 Chemical structure and photo-transformation of PDR₁₉.

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. TiO₂ nanoporous films as a host role were prepared on glass slides by dip-coating from a solution of TiO₂ NPs (STS-01, 0.4 mol/L, Ishihara Sangyo) and PEO20-PP070-PPO20 block copolymer (20 g/L) (STS-01, 0.4 mol/L, Ishihara Sangyo) in an water-ethanol mixture solvent and annealing at 450°C to remove the polymer, as shown in Fig. 2(a) and 2(b)

Fig. 2 (a, b) Fabrication process of TiO₂ nanoporous films. (c) Obtaining of PDR₁₉/TiO₂(n). (d–f) Preparation of PDR₁₉/THF solution. (g) Photograph of PDR₁₉/TiO₂ films on the “NENU” printed paper.

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, respectively. The obtained TiO₂ porous films have high transparency. The thickness of the TiO₂ porous film was controlled by the times (n) of dip-coating. Only one side of the nanostructure was preserved treated by hydrofluoric acid dissolution so as to avoid interference in optical absorption and diffraction. PDR₁₉ (1 mg) was dissolved in tetrahydrofuran (THF, 1 mL) as shown Fig. 2(d). The mixed solution was magnetic stirring for about 48 hours [Fig. 2(e)], then placed at room temperature (300 K) for two hours to obtain the supernatant clear solution [Fig. 2(f)]. The mixed solution was finally drop-coated on TiO₂ porous film [Fig. 2(c)]. After drying at room temperature (300 K) for about ten hours in darkness, the nano-hybrid films, abbreviated as PDR₁₉/TiO₂(n), were formed [Fig. 2(g)]. For comparison, the mixed solution of PDR₁₉/THF was also drop-coated on glass slides to obtain a pure PDR₁₉ film (abbreviated as pure PDR₁₉) after drying at room temperature (300 K) for about ten hours in darkness. The resultant PDR₁₉ film thickness was 203 ± 5 nm, measured by a step profiler (KLA-Tencor). The optical properties of the samples were characterized by UV-Vis spectrophotometer.

Figures 3(a) and 3(c)

Fig. 3 Top-viewed and cross-sectional SEM images TiO₂ (a, c) and PDR₁₉/TiO₂(3) (b, d) films. (e) Size distribution histograms and cumulative percentage of volume fraction of pore size on the surface of TiO₂(3) film derived from SEM photographs.

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show the top-viewed and cross-sectional scanning electron microscope (SEM) images of the bare surface and cross section (via mechanical cutting) of TiO₂(3) film on FTO substrate, respectively. For comparison, Figs. 3(b) and 3(d) present the similar SEM images of PDR₁₉/TiO₂(3) film on FTO substrate after spraying gold. The average thickness for the TiO₂(3) film is 553 nm and that for PDR₁₉/TiO₂(3) film is 664 nm. It can be concluded that the pure PDR₁₉ with the average thickness of 111 nm exists on the TiO₂ foam. Nano-scale pores with uniform distribution for the TiO₂(3) film were observed from the top-view of SEM. While the surface of PDR₁₉/TiO₂(3) film shows much more flatness after the PDR₁₉ deposition.

We characterized the size distribution for the pore on the surface of TiO₂ film by use of the software of nanomeasure, which can measure the size of every pore in SEM images, and carried out statistics of the calculated results. The average pore size is ~7.9 nm, which is much smaller than the length of polymer chain. The cumulative volume fraction (%) was also calculated which is defined as the population percentage of the total nanopores for the statistical ones of which the sizes are below the given value. The pore size (<10 nm) occupied a volume fraction [Fig. 3(e)] is ~90%.

2.2 Optical setup

Optical setup for polarization holographic recording is shown in Fig. 4

Fig. 4 Optical setup for holographic recording system. BS, beam splitter; M, mirror; RP, retardation plate; L, lens.

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. The interfering beams (p-polarized, 473 nm) generated from a blue laser as writing beams, and a red beam (s-polarized, 671 nm) as probe one. Half-wave plates were used to adjust the polarization state of the writing beams and the reading beams. In the case of p and s polarization, the electric-field vector oscillates in the plane of incidence and perpendicular to it, respectively. The intersection angle between the writing beams was set as 10°. The diameters of all the laser beams were 0.3 cm, so the irradiation area is 0.07 cm². The power of each blue laser beam was 10 mW, and the power of the red laser beam was set as 0.5 mW to minimize the destructive effect of readout radiation which can in principle also lead to photochemical interactions. The light intensity diffracted from holographic gratings was registered on a photodiode interfaced with a computer. Diffraction efficiency is defined as the ratio between intensities of the first–order diffracted beam and the incident beam after passing through the sample. Erasable hologram reconstruction was captured by a CMOS camera after adding a group of lens in one of the writing beams and subsequently using a shutter to control “on” or “off” of the other blue beam.

3. Results and discussion

3.1 Absorption spectra

Figure 5

Fig. 5 Absorption spectra in the UV-Vis region (350–900 nm) of the PDR₁₉/THF solution, pure PDR₁₉, PDR₁₉/TiO₂(1), PDR₁₉/TiO₂(3) and PDR₁₉/TiO₂(6) films. The inset graph presents the dependence of maximum absorbance and absorption peak position on dip-coating times of titania (n). The dot line is the guide for eyes.

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shows absorption spectra of PDR₁₉/TiO₂(n) films, pure PDR₁₉ film and PDR₁₉/THF solution. The main peak located below 400 nm is related to the π–π* electronic transitions of trans-isomers of the azobenzene moieties. On the other hand, there is a shoulder peak around 470 nm corresponding to the n–π* electronic transition of cis-isomer. In visible region, the absorption peak position of PDR₁₉/TiO₂(1,3) films is near to that of PDR₁₉/THF solution. However, the absorption peak of the pure PDR₁₉ film appears with a red-shift of ~17 nm to the former. While the absorption peak of PDR₁₉/TiO₂(6) just appears in the middle of those for liquid- and solid- forms of PDR₁₉. Besides, the loading capacity of azopolymers is increased after introducing TiO₂ matrices and tends to be saturated with further increasing the dip-coating times n, as shown in the inset graph of Fig. 5.

Commonly, absorption peak shift is related to the environmental change of the chromospheres [41]. After deposition of azopolymer into the titania nanoporous film, azo polymer macromolecules were segmented into many narrow spaces. Steric hindrance effect is supposed to be the most important factor in photochromic molecules transformation. The distribution of PDR₁₉ molecules in the TiO₂(1) porous structure is similar to that in THF solution, where PDR₁₉ molecules are dispersed uniformly in the matrices. Increasing the dip-coating times (n) of TiO₂ may induce the interleaving of multilayer networks which affects the invasion of dye molecules. The amount of the chromophore participating in aggregation is thus increased, resulting in red-shift of absorption spectra of the hybrid films and further approaching to that of the pure PDR₁₉.

The height of the absorption peak may quantify the amount of dye in solid matrices [42]. Absorption coefficient can be expressed as α = Aln10/d, where A is the absorbance value, d is the sample thickness. As the measured thickness and the absorbance at 473 nm of the pure PDR₁₉ are 203 ± 5 nm and 0.158, respectively, the corresponding absorption coefficient was calculated to be 17900 ± 450 cm⁻¹ at 473 nm. As the absorbance of TiO₂ at 473 nm is so small that it can be ignored, the photo-sensitivity of the nano-hybrid films at the blue region is only ascribed to the organic dye. Based on the determined absorption coefficient and the measured absorbance of PDR₁₉/TiO₂(n), the effective adsorbed thicknesses of PDR₁₉ in TiO₂(1), TiO₂(3) and TiO₂(6) matrices are determined to be 308 ± 8 nm, 626 ± 16 nm and 817 ± 21 nm, respectively. The thickness of PDR₁₉ on the top of TiO₂(3) foam is about 111 nm, so the thickness of PDR₁₉ penetrating into the TiO₂ foam is determined to be about 515 nm. It was confirmed by SEM that one dip-coating operation can increase ~180 nm for the thickness of TiO₂. Obviously, the hybrid system can be described by “Double Layer Model”: the bottom layer is the TiO₂ porous film loading with azopolymers; the upper layer is the pure azopolymers, which cannot enter the inorganic framework. When the dip-coating times increase, the adsorption ability of the nanoporous film with organic dyes begins to decrease, which can be also explained as the interleaving of multilayer networks affecting the invasion of dye molecules. This is in accordance with the result of the red-shift of absorption peak for the PDR₁₉/TiO₂(6) film.

3.2 Holographic dynamics

Holographic dynamics in pure PDR₁₉, PDR₁₉/TiO₂(1), PDR₁₉/TiO₂(3) and PDR₁₉/TiO₂(6) films were measured using two coherent blue beams (473 nm, both 10 mW) with p-polarization states, as shown in Fig. 6

Fig. 6 Temporal evolution of the first-order diffraction efficiency in p-p recording configuration for pure PDR₁₉, PDR₁₉/TiO₂(1), PDR₁₉/TiO₂(3) and PDR₁₉/TiO₂(6) films.

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. It was noticed that the highest diffraction efficiency of the pure PDR₁₉, PDR₁₉/TiO₂(1), PDR₁₉/TiO₂(3) and PDR₁₉/TiO₂(6) are 0.038%, 0.065%, 0.137%, 0.11%, respectively. The diffraction efficiency of PDR₁₉/TiO₂(3) increased about 3.6 folds than that of the pure PDR₁₉. Similar to the results of absorption spectra, diffraction efficiency can be also enhanced by introducing TiO₂ matrices and by further increasing their thickness. Besides, it was found that the diffraction efficiency of the pure PDR₁₉ film undergo the process of increasing firstly and decrease later in the temporal evolution. However, the holographic grating of PDR₁₉/TiO₂(n) films show a trend of durable growth. These results indicate that the nanoporous TiO₂ structure plays a key role in weakening the volume shrinkage of the azopolymers. For the s-s or s-p polarization configurations, similar results were obtained. However, the diffraction efficiencies of PDR₁₉/TiO₂(3) for the two polarization configurations (maximum value, 0.057% and 0.091%, respectively) are lower than that of the p-p one.

Diffraction efficiency can be calculated as a sum of diffraction efficiencies resulting from the refractive index (Δn) gratings and the light diffraction on the absorption (Δα) [43]. A phenomenological model describing that the inorganic nanoporous structure inhibits the volume shrinkage of azopolymers is expressed as followed,

η_{(t)} = {\frac{2 π d}{λ} Δ n_{\max} [1 - \exp (- R_{r} t)] \exp (- R_{r e} t)}^{2} + \sin^{2} {\frac{d}{2} Δ α_{\max} [1 - \exp (- R_{a} t)] \exp (- R_{a e} t)},

where Δn_max is the maximum refractive index modulation, R_r and R_re are the recording and erasing rate constants for the refractive index gratings, Δα_max is the maximum absorption grating amplitude, R_a and R_ae are the recording and erasing rate constants of the absorption grating, respectively, λ is the wavelength of the probe beam (671 nm in the case), d the thickness modulation. Grating amplitudes of refractive index and absorption can be determined via grating translation technique [44]. Here, rate constants of gratings were obtained by fitting parameters to the holographic kinetics results (Table 1

Table 1. Kinetic Parameters Obtained by Fitting to the Holographic Recording Experiments of Pure PDR₁₉, PDR₁₉/TiO₂(1, 3 and 6) Films

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). It was indicated that recording rate constant of absorption grating for PDR₁₉/TiO₂(6) (9 × 10⁻³ s⁻¹) is much higher than those of the others (~10⁻⁴ s⁻¹) which can be explained as the highest adsorbed amount of PDR₁₉ dyes in the TiO₂(6) film. While erasure rate constant of absorption grating decreases via introducing TiO₂ nanoporous structures and further increasing the dip-coating times, which again demonstrate the inhibiting role of TiO₂ template on azopolymer volume shrinkage.

For the optimized sample, PDR₁₉/TiO₂(3), the effect of the recording light intensity on the diffraction efficiency was also investigated. As shown in Figs. 7(a) and 7(b)

Fig. 7 (a) Diffraction Efficiency of PDR_19/TiO₂(3) film with different recording powers (5 mW, 8 mW, 10 mW, 12 mW and 15 mW). (b) Dependence of maximum diffraction efficiency of PDR₁₉/TiO₂(3) on writing light powers.

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, the maximum diffraction efficiency increases firstly and then decreases with increasing recording light power. When the recording light power is 10 mW, the highest and stable diffraction efficiency can be obtained. Kinetic parameters obtained by fitting to the holographic recording experiments of the PDR₁₉/TiO₂(3) film with different recording powers (5 mW, 8 mW, 10 mW, 12 mW and 15 mW) are listed in Table 2

Table 2. Kinetic Parameters Obtained by Fitting to the Holographic Recording Experiments of PDR₁₉/TiO₂(3) Film with Different Recording Light Powers

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. Recording rate constant for holographic dynamics is also found to be decreased if further increasing writing light powers. We confirmed that increasing light power in TiO₂ nanoporous films can enhance light-scattering range in the film (not shown here). The stable form of dye molecules are thus disrupted in the dark regions of holographic fringes when applying high writing powers. Grating contrast is weakened accordingly.

3.3 Erasable hologram

To further explore the framework role of TiO₂ porous structure on holographic dynamics, the single visible beam erasing experiment was carried out. When the diffracted light intensity of the recorded grating reaches the maximum value at 4000 s, one of the blue writing beams was turned off. As shown in Fig. 8(a)

Fig. 8 Time dependence of the first-order diffraction efficiency in writing and erasing processes for pure PDR₁₉ and PDR₁₉/TiO₂(3) films with (p-p) polarization configurations. (b) Molecular distribution in PDR₁₉/TiO₂(3) and pure PDR₁₉ films before and after the coherent lights irradiation.

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, the diffraction efficiency of the PDR₁₉/TiO₂(3) film with 664 nm decreases from 0.137% to 0.008% until the erasing process proceeds at 20000 s. The diffraction efficiency for the organic-inorganic hybrid film exhibits a drastic decline by 94%. While for the pure PDR₁₉ film, the diffraction efficiency only decreases from 0.035% to 0.018%. The decline amplitude is 48%. The visible light erasing ability for PDR₁₉/TiO₂(3) is approximately 1.96 times higher than that of the pure PDR₁₉. The contrast shows that the TiO₂ nanoporous structure plays a spatial restrictions role for azopolymer molecules, which can inhibit the entanglement of the polymer backbone and promote the conversion from cis- to trans-molecules under the irradiation of the blue light. So the erasing effect is enhanced. Besides, multiple writing-erasing recycling experiments were performed and compared between the PDR₁₉/TiO₂(3) and pure PDR₁₉, as shown in the inset graph in Fig. 8(a). Multiple writing-erasing operations can be well carried out on PDR₁₉/TiO₂(3), while little change was found for pure PDR₁₉. The rewriting ability of PDR₁₉/TiO₂(3) also benefits from the free transformation of azo molecules in the nanoporous structures. In fact, the erasure process can be optimized increasing power density of writing beams or modulating polarization-state of erasing beams.

The photo-transformation kinetics of the azopolymer in solution, in solid state and in nanoporous template under exactly the same illumination conditions (473 nm, 10 mW; 671 nm, 0.005 μW) was also performed. We recorded transmittance at 671 nm versus time under the alternated illumination of blue (473 nm) pumping light. Azopolymers in the nanoporous template present the highest optical modulation ability; while time-accumulation effect of optical signal for the polymer in THF solution was observed, and little transmittance change was found in the pure azopolymer of solid-state. All the observations are in accordance with the holographic experimental results.

Figure 8(b) illustrates the possible mechanism for the erasing and rewriting of azopolymers upon coherent light irradiation. Before laser irradiation, azobenzene groups with trans-form are distributed uniformly in pure PDR₁₉ and PDR₁₉/TiO₂(3) films. Upon the coherent exposure, different molecular structures in bright and dark regions of interference fringes are produced. The trans-molecules at the bright region of the holographic fringes can be changed to the cis-ones. For the pure PDR₁₉ film, the entanglement of the polymer backbone inhibits partly the conversion from cis- to trans-molecules. However, TiO₂(n) film provide large amount of narrow regions for azo molecules, which tend to behave in optical conversion more independently. cis-molecules as excited form thus become free. Besides, the nanostructure of TiO₂ framework also provides many suitable sites for the cis-molecules to be reverted to the original state. So the high re-writability of holographic gratings is realizable in PDR₁₉/TiO₂(n) film.

3.4. Holographic image storage

The efficient information recording in the PDR₁₉/TiO₂(3) film provides possibility for clearly image storage. Figure 9

Fig. 9 (a) Writing process with two coherent blue lights for stored “Ruby” holograms in the PDR₁₉/TiO₂(3) and erasing process for the stored holograms in the PDR₁₉/TiO₂(3).

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shows the + 1 order diffracted images from the reconstruction beam in PDR₁₉/TiO₂(3) samples. It was found that the brightness of the image was increased in the recording process [Fig. 9(a)] and decreased until disappeared during the erasing process [Fig. 9(b)]. The recording and erasing process of holographic image for the PDR₁₉/TiO₂(3) film is agreed with the trend of diffraction efficiency in Fig. 8, which can further demonstrate the framework role of TiO₂ porous structure for PDR₁₉ in the process of optical information storage.

In fact, similar phenomenon was observed in spirooxazine based systems. Their aggregations effect was also weakened by deposition in TiO₂ or SiO₂ nano-template. Besides of the enhancement of photo-transformation via modulation of nanoporous template, diffraction efficiency of holographic grating in photochromic polymers can also be realized by annealing treatment above or below glass transition temperature [45–47]. Combination of nanoporous template modulation with annealing technology may accelerate photochromism process of the organic-inorganic hybrid system. Further investigation will be carried out.

4. Conclusions

In this paper, a new kind of photochromic system for optical transformation was obtained by depositing azopolymers into TiO₂ nanoporous films. The volume shrinkage rate of the azopolymer can be reduced by the supporting of TiO₂ framework. The stability and efficiency of storage are both improved in PDR₁₉/TiO₂(n) films. The highest diffraction efficiency can be achieved while the thickness of TiO₂ porous film is about 553 nm. Besides recyclable holographic recording can be obtained in PDR₁₉/TiO₂(n) systems, and the erasable hologram reconstruction of PDR₁₉/TiO₂(3) are achieved. This work puts a bright way to inhibit molecule aggregation in solid matrices and contributes to exploit updatable photo-transformation device.

Funding

National Natural Science Foundation of China (10974027, 31271442, 51372036, 51732003, 61007006); the 111 project (B13013); the Fundamental Research Funds for the Central Universities (2412017FZ011); and the Natural Science Foundation of JiLin Province of China (20180101218JC).

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Variable	Pure PDR₁₉	PDR₁₉/TiO₂(1)	PDR₁₉/TiO₂(3)	PDR₁₉/TiO₂(6)
R_r (s⁻¹)	5.0 × 10⁻⁴	5.6 × 10⁻⁴	6.0 × 10⁻⁴	6.7 × 10⁻⁴
R_re (s⁻¹)	9.1 × 10⁻⁴	7.7 × 10⁻⁴	3.8 × 10⁻⁴	1.1 × 10⁻⁴
R_a (s⁻¹)	4.7 × 10⁻⁴	4.3 × 10⁻⁴	3.8 × 10⁻⁴	9.0 × 10⁻³
R_ae(s⁻¹)	6.3 × 10⁻⁵	2.9 × 10⁻⁵	6.3 × 10⁻⁶	1.1 × 10⁻⁷

Variable	5 mw	8 mw	10 mw	12 mw	15 mw
R_r (s⁻¹)	2 × 10⁻⁴	3.3 × 10⁻⁴	6 × 10⁻⁴	5 × 10⁻⁴	3.2 × 10⁻⁴
R_re (s⁻¹)	1 × 10⁻³	5.3 × 10⁻⁴	3.8 × 10⁻⁴	3.8 × 10⁻⁴	5.4 × 10⁻⁴
R_a (s⁻¹)	6.6 × 10⁻⁴	5.9 × 10⁻⁴	3.8 × 10⁻⁴	4.3 × 10⁻⁴	5.9 × 10⁻⁴
R_ae (s⁻¹)	4.3 × 10⁻⁶	5.4 × 10⁻⁵	6.3 × 10⁻⁶	3.1 × 10⁻⁵	7.4 × 10⁻⁵

Variable	Pure PDR₁₉	PDR₁₉/TiO₂(1)	PDR₁₉/TiO₂(3)	PDR₁₉/TiO₂(6)
R_r (s⁻¹)	5.0 × 10⁻⁴	5.6 × 10⁻⁴	6.0 × 10⁻⁴	6.7 × 10⁻⁴
R_re (s⁻¹)	9.1 × 10⁻⁴	7.7 × 10⁻⁴	3.8 × 10⁻⁴	1.1 × 10⁻⁴
R_a (s⁻¹)	4.7 × 10⁻⁴	4.3 × 10⁻⁴	3.8 × 10⁻⁴	9.0 × 10⁻³
R_ae(s⁻¹)	6.3 × 10⁻⁵	2.9 × 10⁻⁵	6.3 × 10⁻⁶	1.1 × 10⁻⁷

Variable	5 mw	8 mw	10 mw	12 mw	15 mw
R_r (s⁻¹)	2 × 10⁻⁴	3.3 × 10⁻⁴	6 × 10⁻⁴	5 × 10⁻⁴	3.2 × 10⁻⁴
R_re (s⁻¹)	1 × 10⁻³	5.3 × 10⁻⁴	3.8 × 10⁻⁴	3.8 × 10⁻⁴	5.4 × 10⁻⁴
R_a (s⁻¹)	6.6 × 10⁻⁴	5.9 × 10⁻⁴	3.8 × 10⁻⁴	4.3 × 10⁻⁴	5.9 × 10⁻⁴
R_ae (s⁻¹)	4.3 × 10⁻⁶	5.4 × 10⁻⁵	6.3 × 10⁻⁶	3.1 × 10⁻⁵	7.4 × 10⁻⁵

Nanoporous-template-modulated azopolymers for enhancing reversible photo-transformation

Abstract

1. Introduction

2. Experimental

2.1 Film preparation

2.2 Optical setup

3. Results and discussion

3.1 Absorption spectra

3.2 Holographic dynamics

3.3 Erasable hologram

3.4. Holographic image storage

4. Conclusions

Funding

References

Cited By

Figures (9)

Tables (2)

Equations (1)

OSA Continuum