1. Introduction

Recently, polymers incorporating azodyes or containing azo units chemically or physically have attracted a great deal of attention and become promising photonic-based materials, such as optical storage materials, optical switch materials, optical holography materials, and nonlinear optics materials. [1-3] The desirable properties of these materials are attributed to the photoisomerization of the azodye groups accompanied by reorientation of azobenzene groups through photochemical trans-cis-trans isomerization circles under photo irradiation. [4-6]. The orientation induces a difference of refractive index parallel and perpendicular to the polarization of the incident light. Especially, the stable stored information in the materials can be erased by heating or optical method, which for the circularly polarized light can also be used to randomize the orientation of the azodye groups. After the stored information being erased thoroughly, the sample can be rewritten. Hence extensively studies have been made of azobenzene-doped photochromic materials, where small molecular weight azobenzene compounds are dispersed in polymer binders, [7-8]. by many research groups due to their promising photonic applications.

Organic-inorganic hybrid materials have been extensively investigated and indicated that the optical properties of them can be largely enhanced as compared with organic polymer materials. Ormosil-based organic-inorganic hybrid materials have been studied as a promising system for photonic applications in recent years. [9-10]. This is because organic groups are integrated in the glass and the bulky organic components fill the pores between the inorganic oxide chains. Making a thick hybrid thin film with a few microns thickness can be easily deposited by a single spin-coating processing and the low processing temperatures may enable direct integration with semiconductor sources, detectors and other optoelectronic components. In addition, the introduction of organic groups into an inorganic network improves mechanical properties, leading to the easier processing of thin film and the modification of an inorganic network structure with organic groups gives larger space for the isomerization of organic photoactive molecules as compared to inorganic glasses [11]. Therefore, organic-inorganic hybrids are thus anticipated as desirable materials for photonic applications, which can trap organic molecules [12-14]. In this paper, we report, for the first time to our knowledge, on the photoresponsive and waveguide properties of the TiO₂/γ-glycidoxy-propyltrimethoxysilane and methyltrimethoxysilane organic-inorganic hybrid materials doped with azobenzene small molecules for optical storage and optical switching applications.

2. Experimental study

4-hydroxyazobenzene is commercial compound, 4-(4-hydroxybutyloxy)azobenzene was synthesized from 4-hydroxyazobenzene as follows: To a 100 ml round bottom flask fitted with a condenser and self-balanced funnel, 15 mmol 4-hydroxyazobenzene, 26.8 mmol K₂CO₃ and 25 ml dimethyl sulphoxide (DMSO) were added. To a 250 ml self-balance funnel, 24.2 mmol 4-chlorobutynol-1 and 12 ml DMSO were added. The solution in the funnel was then added drop-wise to the flask while the flask was stirred and heated by an oil bath to 125°C. The reaction time was about 3 hrs and the reaction mixture was filtered. The filtrate was added to a 5-fold volume of water. The precipitate formed was filtered and recrystallized by methanol and hexane mixed solvents twice. TiO₂/γ-glycidoxypropyltrimethoxysilane (GLYMO) and methyltrimethoxysilane (MTES) hybrid organic-inorganic materials of molar composition 20TiO₂-40GLYMO-40MTES were prepared by a sol-gel technique in an organic-inorganic hybrid system. Here, the molar composition with 20TiO₂-40GLYMO-40MTES was chosen as matrix materials in this paper, considering that a high-quality waveguide film with high transparency and low propagation loss can be obtained from the hybrid material with the molar composition [15]. In the preparation of TiO₂/GLYMO-MTES hybrid material sol, details of preparation processing were described elsewhere. [15]. The azobenzene compound as synthesized was added to the TiO₂/GLYMO-MTES sol-gel hybrid solution in the weight ratio of 0.5% of the sol-gel hybrid solution. The final mixture was stirred for about 30 h at room temperature. We used quartz glass as substrates and they were ultrasonically cleaned in acetone and ethanol, respectively, rinsed with the de-ionized water and dried in pure N₂ gas. Two layers of the sol-gel film were spun onto the substrate at 3000 rpm for 35 seconds. The film-coated samples were then dried in vacuum oven for 10 hrs at the temperatures of 60 and 80°C. It should be mentioned that all reactions and manipulations of the sol preparation were carried out under dry nitrogen environment due to the extreme moisture sensitivity of the titanium alkoxides.

The photo-responsive measurements of the hybrid films were carried out by a Shimadzu UV-2501 PC UV-Vis spectrophotometer after irradiated by UV light for various time intervals. The UV irradiation light was produced by a Driel Instrument 66901 500 Watts mercury lamp through a filter centered at 370 nm with a circular cooling water system to maintain the lamp temperature at about 5°C. The UV irradiation light intensity was 2.0 mW/cm². The refractive index, the thickness, the propagation mode, and the propagation loss properties of the planar waveguide films were measured for both transverse electric (TE) and transverse magnetic (TM) polarization by an m-line apparatus (Metricon 2010) based on the prism coupling technique.

3. Results and discussion

 

Fig. 1. Modes of the hybrid planar waveguide film at the wavelength of 1539 nm

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The morphology and the roughness of the hybrid films were examined with a Digital Instruments Nanoscope IIIa AFM by means of a tapping mode. The results indicate that all films heated at different temperatures have a dense, porous-free, and smooth morphology. The value of the root mean square (RMS) surface roughness of the hybrid films was examined over a 10 μm × 10 μm areas and the surface roughness of the film obtained under the present processing condition is sufficiently small for photonic applications. For example, the surface roughness of the films heated at 60 and 80°C is 0.348 and 0.329 nm, respectively.

Figure 1 shows the propagation modes of the hybrid planar waveguide film dried at 80°C based on the prism coupling technique at the wavelength of 1539 nm. From a theory in Ref.16, the refractive index and the thickness of the hybrid waveguide film can be determined from the measured effective refractive indices values of both TE and TM modes. For TE modes, the results obtained are 1.5072 and 2.4μm, while for TM modes, they are 1.5063 and 2.5μm, respectively. The optical propagation loss of the hybrid planar waveguide film at 1539 nm is evaluated and is 0.78dB/cm. Similarly, the planar waveguide properties of the hybrid planar waveguide film dried at 60°C was also measured. For TE modes, the refractive index and the thickness are 1.5071 and 2.5 μm, while for TM modes, they are 1.5061 and 2.6 μm, respectively. The optical propagation loss at 1539 nm is 0.80dB/cm typically. As compared with the hybrid film heated at 80°C, it was noted that with increase heat treatment temperature, the refractive index of the film increases and the thickness drops. The organic solvents and compounds appear to evaporate and decompose with higher heat treatment temperature, which is possible to lead to a structure change of the hybrid film [15]. As known, for a guided wave propagating in a planar medium, the local refractive index fluctuations in the volume of the waveguide and the deviations from a perfectly plane geometry at the waveguide-cladding boundaries contribute to the losses. In addition, nonuniform hydrolysis and condensation of the trinary alkoxides mixture undoubtedly result in the big scattering loss. The homogeneity of a sol-gel glass synthesized from a mixture of more than two alkoxide precursors can be affected by the relative rates of homocondensation and heterocondensation. Silicon alkoxides hydrolyze rather slowly, and acid or base catalysis has to be employed frequently for accelerating the reaction quite effectively. However, titanium alkoxides hydrolyze at a much faster rate, and as a result, in mixtures of titanium and silicon alkoxides, it is still possible that a heterogeneous network containing Ti-rich and Si-rich domains may be formed in this system.

 

Fig. 2. UV absorbance changes of the hybrid film heated at the temperature of 60°C and upon irradiation (a) at UV light (370 nm) with 5s time interval from 0s to 45s and 15s time interval from 45s to 105s; (b) at Vis light (410 nm) with 5s time interval from 0s to 45s and 15s time interval from 45s to 90s

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Figures 2(a) and 2(b) show the UV-visible absorption spectral changes of the hybrid films heated at the temperature of 60°C after irradiation with non-polarized UV (370 nm) light, and subsequent irradiation with visible (410 nm) light. It can be seen from the Fig. 2 that an irradiation of the azodye-doped hybrid film with 370 nm light at room temperature causes a decrease in the absorbance about 352 nm due to the photoisomerization of the trans-form to the cis-form. When photoirradiation is stopped after the reaction system have reached the photostationary state, the electronic absorption of the hybrid film gradually recovers to the original one due to the backward thermal isomerization from the cis-form to the trans-form. The backward isomerization of the hybrid film from the cis-form to the trans-form can also achieve by irradiation with visible light of 410 nm. It can conclude that the maximum absorbance change caused by irradiation with UV light of 370 nm is about 0.2. These results can be further understood by the following: as we know, at room temperature azobenzene chromospheres are expected to be in the more stable trans state in the dark. It can be seen from the absorption spectra of the Fig. 2(a) in the range of 300-600 nm that there have been a major absorption peak at 352 nm and a weak one at 437 nm, which are related to the π-π ^* electronic transition of the trans isomers and n-π ^* electronic transition of the cis isomers. When the hybrid film is irradiated by the non-polarized UV light of 370 nm, azobenzene chromospheres undergoes a trans-cis photoisomerization process. The intensity of the absorption peak at 352 nm gradually decreases with increase irradiation time until an equilibrium state is reached. At the same time, the weak peak at 437 nm becomes more pronounced. The equilibrium or photostationary state is reached in about 105 seconds under the present irradiation conditions. Fig. 2(b) shows that when the hybrid film is irradiated by the visible light of 410 nm, azobenzene chromospheres undergoes a cis-trans photoisomerization process. The π-π ^* absorption of the trans isomers gradually increases and the n-π ^* absorption of the cis isomers decreases, showing a partial back-conversion of the absorption spectra. In addition, it was also found that the isomerization process could be reversed thermally. When the irradiated samples were thermally heated at a certain temperature, the intensity of the absorption peak around 352 nm increased steadily, until reaching the starting value before the irradiation, thus confirming the reversibility of the photo-thermal cis-trans isomerization process.

A similar trans-cis-trans photoisomerization process was also observed for the hybrid film heated at the temperature of 80°C, but the absorbance change is relatively smaller than that of the hybrid film heated at 60°C. It is probably related to the evaporation and decomposition of the organic matters including azodye inside the hybrid film due to higher heat treatment temperature, which probably leads to a little structure change of the hybrid film including appearance of some pin hole inside the film. The predominant photochemistry of azodye incorporating in hybrid organic-inorganic sol-gel matrices involves reversible photoisomerization from the thermally stable trans structure to the cis-form; the backward occurs both photochemically and thermally. When the forward and backward reaction rates are equal, the reaction system is said to be in a photostationary state as follows.

The reversibility of the trans-cis-trans photoisomerization of the hybrid films was studied by alternating irradiation of the non-polarized UV light (370 nm) and visible light (410 nm). The samples studied were dark-adapted before measurements and then being irradiated by UV light. After the photostationary state was reached, then the samples were irradiated by visible light immediately until another photostatioary state was observed again. The process was repeated for at least three irradiation cycles. At the end of each irradiation, the π-π ^* absorption of the trans isomers, which correspond to the photostationary state, was recorded, and the results are shown in Fig. 3, which shows all-optical switching effect of the hybrid film heated at 60°C at room temperature. It can be seen from the Fig. 3 that the trans-cis photoisomerization of the hybrid film shows a good reversibility within the irradiation cycles investigated. Considering that the photostationary state reached by irradiation of the visible light is a mixture of the trans and cis isomers, the absorbance does not completely recover to the original dark-adapted state due to the exist of the trans isomers only. Obviously, there is no apparent loss of photo-reversibility over the cycles investigated as the absorbance is essentially independent of irradiation cycles. It can be concluded that the as-prepared hybrid film exhibits good optical switching property and the switching behavior itself is reversible and reproducible. Similar results were also obtained for the hybrid film heated at 80°C. These azodye-doped hybrid films show only very little decay during the photochemical trans-form to the cis-form process and the thermal cis-form to trans-form recovery process. The results obtained from Fig. 3 indicate that the as prepared hybrid film should be good candidates for optical data storage and optical switching. It is also expected that a much shorter response time for the transition of the trans-cis photoisomerization process may be reached by using a laser pulse irradiation source instead of UV light source [17-18]. It can be concluded based on above these results that the as prepared hybrid matrix in this paper as compared with those organic-inorganic azodye-doped matrices reported in Refs. [12-14] has better optical waveguide properties and enable to allow directly integrating on the same chip the optical storage or optical switch devices with semiconductor sources, detectors, waveguide devices, and other optoelectronic components.

 

Fig. 3. Optical switching characteristics of the hybrid film heated at 60°C, photoinduced by the UV light of 370 nm and visible light of 410 nm alternately (light intensity, UV: 2 mW/cm², Vis: 3 mW/cm²)

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4. Conclusions

In conclusion, a new organic-inorganic hybrid material doped with azobenzene small molecules for optical data storage and optical switch applications has been prepared by the low temperature sol-gel process. A detailed analysis on photoresponsive properties based on the trans-cis-trans photoisomerization process from the hybrid films dried at different temperatures has been carried out and optical switch characteristics of the hybrid films have been demonstrated. Our results show that azobenzene has been successfully incorporated into silicon oxide matrices using the same 4,4’-dihydroxyazobenzene as starting materials by the sol-gel technique. In addition, the planar waveguide properties of the hybrid films, which include the refractive index, the film thickness, the propagation modes, and the propagation loss, have also been investigated. The present study shows that the as prepared hybrid materials are promising candidates for materials for photonic applications.