1. Introduction

As one knows well now, the trans-cis-trans photoisomerization process results in the reorientation of azodye molecules, which is promising and potential applications in various photonic devices, such as optical storage, optical switch, and nonlinear optics applications. Therefore, polymers containing azobenzene chromophore molecules or azobenzene-based photochromic polymers have been studied extensively by many research groups due to their promising photonic applications over the past ten years. [1–3]. Considering that most photoactive organic materials may not be stable in ambient environment for long periods of time, low-mechanical strength, and relative low optical transparency as compared to that of an inorganic oxide. Recently, optically homogeneous and transparent organic-inorganic hybrids containing organic components have been widely studied and indicated that the optical properties of them can be largely enhanced as compared with organic polymer materials [4–6]. The introduction of organic groups into an inorganic network improves mechanical properties, leading to the easier processing of thick 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 [7]. Therefore, organic-inorganic hybrids are thus anticipated as desirable materials for photonic applications, which can trap organic molecules. Especially, organic-inorganic hybrid waveguide film configuration become more important, they can be obtained at a low heat treatment temperature, which would allow directly integrating on the same chip waveguide device with other optoelectronic components. Therefore, including TiO₂/ormosil and ZrO₂/ormosil hybrid materials have been reported and studied by the sol-gel technique [8–10]. However, the germanosilicate glasses, which have high optical transmission in the visible and infrared range, as core dopant and silica as the substrate ensure that these waveguides will have virtually identical characteristics to single-mode fibers, indicating they are potentially ideal applications for integrated photonics devices.

It has been reported that the preparation and optical properties of the sol-gel GeO₂/ormosils hybrid planar waveguides in our previous work, [11,12] but we think that it is of more great interest and significance to study the doping of the planar hybrid waveguides doped with photochromic amorphous molecular material based on azobenzene for photonic applications. However, there have not been any reports of preparation and photonic properties of such azobenzene doped optical waveguide films so far. Here, we describe what we believe to be the first study on the photoresponsive and planar waveguide properties of GeO₂/γ-glycidoxypropyltrimethoxysilane low temperature sol-gel derived organic-inorganic hybrid materials containing azobenzene small molecules for optical storage and optical switch applications. In addition, to the best of our knowledge, the present study provides the first example of the surface relief pattern by heat-induced on such azobenzene-containing small molecular hybrid waveguide films.

2. Experimental Study

GeO₂/γ-glycidoxypropyltrimethoxysilane (GLYMO) hybrid organic-inorganic materials sols of molar composition 30GeO₂-70GLYMO were prepared by a sol-gel technique, details of preparation processing were described in our previous work [11]. The commercial 4-hydroxyazobenzene azobenzene compound was then added to the GeO₂/GLYMO sol-gel hybrid solution in the weight ratio of 5% of the sol-gel hybrid solution. The final mixture was stirred for about 25 h at room temperature. Quartz glass and silica-on-silicon were used as substrates and they were ultrasonically cleaned in acetone and ethanol, respectively, rinsed with the de-ionized water and dried in pure N ₂ gas. Following the common practice for spin-coating, a 0.1-micron-pore filter was attached to a syringe for removing foreign particles before the resultant solution was spin-coated onto a substrate. One layer of the sol-gel film was spun onto the substrate at 3500 rpm for 35 seconds. Then the film-coated samples were directly put in a furnace for about 15 minutes at different temperatures of 50, 100, 120, 150, and 200°C in air.

The thickness and the refractive index of the hybrid films were measured for transverse electric (TE) polarization by an m-line apparatus (Metricon 2010) based on the prism coupling technique at the wavelengths of 633. 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 362 nm with a circular cooling water system to maintain the lamp temperature at about 5°C. The UV irradiation light intensity was 2.0mW/cm². The surface relief patterns, which were induced by heating at a certain temperature, of the hybrid films were observed by a Digital Instruments Nanoscope IIIa AFM using the tapping mode.

3. Results and discussion

It was shown that dense and highly transparent azobenzene-containing small molecular GeO₂/GLYMO organic-inorganic hybrid waveguide films could be obtained below a low heat treatment temperature of 150°C. Light wave guiding was easily demonstrated when such a film was deposited on a silica-on-silicon substrate. Our experimental results also indicate that a crack-free hybrid film with a thickness of more than 10 μm could be obtained by a multiple spin-coating process and heated at 100°C.

The thickness and refractive index of the hybrid films heated at different heat treatment temperatures were also estimated. As expected, with increase the heat treatment temperature, the refractive index of the hybrid film increases and the thickness drops. That is to say, the film thickness becomes thinner and the refractive index becomes higher as the heat treatment temperature rises. For example, the thickness and refractive index for the hybrid film obtained at room temperature are 1.67 μm and 1.496, but for the hybrid film heated at 100°C, they are 1.62 μm and 1.499, respectively. It should be mentioned here that the thickness and refractive index of the hybrid film are not sensitive to the heat treatment temperature in the range between room temperature and 150°C. For example, the decrease of the film thickness is about 4.3 % and the increase of the refractive index is about 0.3%, respectively, when the heat treatment temperature is increased from room temperature to 150°C. However, the change in the film thickness and refractive index is more substantial when the heat treatment temperature is further increased from 150°C up to 200°C. It can be also obtained that within the heat treatment temperature range between room temperature and 200°C, the refractive index of the hybrid film can be varied from 1.496 to 1.505 at the wavelength of 633 nm.

Figure 1 shows the UV-visible absorption spectra of the hybrid films obtained at room temperature and heated at 120°C after irradiation with non-polarized UV (362 nm) light. As known, azobenzene chromophores are expected to be in the more stable trans state in the dark at room temperature. It can be seen from the absorption spectra of the hybrid film obtained at room temperature as shown in Fig. 1(a) that there have been an intense absorption peak at 350 nm corresponding to the π-π* electronic transition of the trans azobenzene side chain and a weak peak at about 435 nm which originates from the weak n-π* electronic transition of the cis isomers. When the hybrid film is irradiated by the non-polarized UV light of 362 nm, azobenzene chromophores induces trans-cis photoisomerization in the azobenzene side chains of the hybrid film. The process demonstrates itself in the absorption spectra as a reduction in the trans isomers π-π* peak and a concomitant increase in the weaker cis isomers n-π* peak. That means the intensity of the absorption peak at 350 nm gradually decreases with increase irradiation time until an equilibrium state is reached. At the same time, the weak peak at 435 nm becomes more pronounced. A similar trans-cis photoisomerization process was also observed for the hybrid film heated at 120°C as shown in Fig. 1(b), but the absorbance change is relatively smaller than that of the hybrid film obtained at room temperature. It is probably related to the evaporation and decomposition of the azobenzene dye 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 and surface structure on the hybrid film. It is noteworthy that the equilibrium or photostationary state is short much as compared with that of the hybrid film in Fig. 1(a) under the present irradiation conditions, that is to say, the photoisomerization in this hybrid film proceeds more quickly than the hybrid film obtained at room temperature, indicating the photoisomerization of the hybrid film heated at 120°C should be uniform photoreactions due to evaporation and decomposition of the organic solvents remaining inside the hybrid film. These results indicate that irradiation of the as-prepared hybrid film with the UV light of 362 nm at room temperature causes the decrease in the absorbance around 350 nm due to the photoisomerization of the trans-form to the cis-form. When irradiation is ceased after the reaction system has achieved the photostationary state, the electronic absorption spectra of the hybrid film gradually recovered to the original one due to the backward isomerization from the cis-form to the trans-form. In addition, it was also found that the isomerization process could be reversed thermally. When the irradiated hybrid film samples were thermally heated above 150°C, the intensity of the absorption peak around 350 nm increased steadily, until reaching the starting value before the irradiation, thus confirming the reversibility of the photo-thermal cis-trans isomerization process. Furthermore, the as-prepared hybrid film shows only very little decay during the photochemical trans-form to the cis-form process and the thermal cis-form to trans-form recovery process.

 

Fig. 1. UV-Vis absorbance changes of the hybrid film upon irradiation at UV of 362 nm and light intensity of 2 mW/cm². (a) Film obtained at room temperature, (b) Film heated at 120°C

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The morphologies of the hybrid films heated at different temperature were examined with a Digital Instruments Nanoscope IIIa AFM by means of a tapping mode. It was found that for those hybrid films heated below 50°C, a dense, porous-free, and smooth morphology can be obtained. But with increase heat treatment temperature, for example, as shown in Fig. 2, a blurry surface relief patterns can be observed when the hybrid film heated at 80°C. Furthermore, a clear surface relief patterns with regular periodicity can be seen on the surface of the hybrid film when the heat treatment temperature is increased up to 120°C. Then, evidently disappears above the heat treatment temperature of 150°C. It is noteworthy that the surface modulation observed in Fig. 3 provides a regularly spaced sinusoidal structure with a periodicity of about 14 nm, but the modulation depths are only a few nanometers, which is much smaller than that of the holographic surface relief structures induced by lasers. The modulation depth may be related the heat treatment temperature of the hybrid film and the structure of the patterns, but it is not clear enough at this stage. The AFM observation of the hybrid film heated at different temperatures indicates that the surface relief pattern easy retained even after a few months at room temperature in the dark. As known, a holographic surface relief grating (SRG) on azobenzene polymers upon irradiation of two interfering beams from a low-power laser has been extensively fabricated [6, 13–15]. Due to the cyclic trans-cis-trans photoisomerization of azobenzene in the side chain, mass transport is induced to form SRG which is determined by the distribution of the interfered light intensity. The mechanism leading to the formation of the surface relief pattern induced by heating at a certain temperature in our hybrid films is not well understood at this moment, but we suspect that it is probably related to material displacement caused by trans-cis cycling of the azobenzene chromophore or reorientation of the azobenzene groups leading to birefringence in the hybrid film due to a suitable heating induction. 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. [8–10]] enable to allow directly integrating on the same chip the optical storage or optical switch devices with waveguide devices or other optoelectronic components.

 

Fig. 2. AFM image of the surface relief patterns formed on azobenzene-containing hybrid film heated at 80°C

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Fig. 3. AFM image of the surface relief patterns formed on azobenzene-containing hybrid film heated at 120°C

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4. Conclusions

In conclusion, a new azobenzene-containing GeO₂/GLYMO hybrid material for photonic applications has been prepared by the sol-gel process from an organic-inorganic hybrid system. The hybrid films have showed photoresponsive properties of the trans-cis-trans photoisomerization by the irradiation of UV to verify its optical storage and switching characteristics. We have demonstrated that such a hybrid film can be used for surface relief structure induced by heating and the surface relief structure has can keep good stability. Note that the as prepared hybrid films used to form the surface relief patterns shown in Fig. 3 is the first example of such structure formation in such a low temperature organic-inorganic hybrid material induced by a certain heat treatment temperature. The results show that azobenzene has been incorporated into silicon oxide matrices by the sol-gel technique. In addition, the planar waveguide properties of the hybrid films doped with azodyes and heated at different temperatures have been investigated. The present study shows that the hybrid materials are promising candidates for materials photonic applications.