Optical absorption and SHG in PMMA and SiO2-matrices doped with DO3 as function of poling time J. García-Macedo1A, A. Franco1, G. Valverde-Aguilar1, and L. Romero2 1
Departamento de Estado Sólido, Instituto de Física, Universidad Nacional Autónoma de México, México, D.F. 04510, México A conrresponding author e-mail:
[email protected] Fax number: +52 (55) 5622 5011, Telephone number: +52 (55) 5622 5103 Abstract- The orientation of non-linear dyes embedded in different matrices plays an important role on the optical properties of films. The order parameter is related with the dyes orientation through the optical absorption (OA). The answer of the dye to a corona poling treatment depends of the molecule-molecule and molecule-matrix interactions. Amorphous and nanostructured films of PMMA and SiO2 doped with the organic dye Disperse Orange 3 (DO3) were prepared. Sodium Dodecyl Sulfate (SDS) was used as template to perform a large-order lamellar nanostructure, detected by X-ray diffraction (XRD). The films were characterized by scanning electronic microscopy (SEM). OA and second harmonic generation (SHG) intensity measurements were carried out at several corona poling times. SHG measurements were registered at 60, 80, 100 and 120ºC. The experimental results are shown in plots of order parameter and SHG intensity against corona poling time. We fitted the OA and the SHG results as function of the corona poling time with a model developed by us, employing only one fitting parameter related to the matrix-chromophore interactions. The lamellar structure provides largest order parameter values. The most intense SHG signal was obtained in the SiO2:DO3 films with lamellar phase.
1. INTRODUCTION
In the last years a lot of attention has been put on the development of materials with well controlled nanostructures, due to their remarkable enhanced properties with respect to those materials without that ordered structure at the nanometric scale. Several kinds of properties in the materials have been improved by means of the incorporation of nanoparticles or nanostructures. In particular, the development of materials with very specific optical properties has experienced an important evolution due to the incorporation of nanoparticles in well organized nanostructures induced in the materials [1]. Metamaterials, photonic crystals and plasmonics are very important areas of the scientific research involved in the development of materials with new and interesting linear and non-linear optical features. Clear examples of materials whose optical properties are governed by their order at the nano-level scale are the nanostructured films doped with push-pull molecules. The collective orientation of the push-pull molecules inside the films changes the linear and non-linear optical responses of the films. One of the most striking optical properties of these films is observed when their chromophores are arranged in a non-centrosymmetrical distribution. When that happens, the films exhibit second-order non-linear optical properties detectable by second harmonic generation (SHG) measurements. A non-centrosymmetric
distribution of the orientation of the push-pull chromophores in a film can be attained by applying an intense external electric field across the film, like the well know case of the corona technique [2]. The orientation of the chromophores by corona poling is affected by local matrix-chromophore and chromophore-chromophore electrostatic interactions. The presence of nanostructures in the materials, in which the chromophores are embedded, helps to overcome some chromophores orientation difficulties related to the electrostatic interactions mentioned above. It means that the intensity of SHG, the speed of the chromophore orientation and the others parameters are expected to be controlled by means of the modification of the local environment around each one of the chromophores. That happens when the material is nanostructurated. In this work the materials were nanostructurated by means of the incorporation of an ionic surfactant during the film matrix formation step. The presence of nanostructures in the films was confirmed by X-ray diffraction (XRD) measurements. The effect of the nanostructures in the chromophores orientation was monitored by UV-vis optical absorption and SHG intensity measurements as function of corona poling time at several temperatures on amorphous and nanostructured films. The fitting of the experimental results by means of our model [3] helps to quantify the effects of the nanostructures on the materials SHG signal intensity, speed of the chomophores orientation and stability of the non-centrosymmetric molecular arrangement. 2. EXPERIMENTAL DETAILS 2.1 Synthesis of the materials. All the materials under study were doped with the push-pull chromophore 4-amino-4-nitrobenzene, better known as Disperse Orange 3 (DO3), which has a big permanent dipole moment equal to 2.47 x 10-29 C m. Four kinds of different materials were studied in this work. These materials can be classified in two main groups: amorphous and nanostructured. The aim of the paper is to detect the main effects of the nanostructures in the optical responses of the materials. Besides, the matrices of the films had two different compositions, one of them was polymethylmethacrylate (PMMA) and the other one was SiO2. The first one typically has weaker bonds than the second one, i.e., the chromophore orientation is easier in the first one at low temperatures. The PMMA films were made by mixing 80% in weight of tetrahydrofuran (THF) and 20% in weight of PMMA and DO3. From the total of PMMA and DO3, 95% in weight is PMMA and 5% in weight is DO3. The SiO2 films were made by sol-gel technique, mixing tetraethylortosilicate (TEOS), ethanol (EtOH), deionized water and DO3 in the next molar ratios: TEOS : DO3 : EtOH : H2O = 1 : 0.015 : 4 : 10. An ionic surfactant Sodium Dodecyl Sulfate (SDS) was added to the solutions in order to nanostructurate the films. The ratio of SDS is 5% in weight with respect to the total solution. The total solution was filtered and the deposition of the films was carried out by dip-coating at an extraction rate of 5 cm/min. 2.2 Corona poling technique. The orientation of the chromophores was done using the well documented corona poling technique. Our system basically consists on a silver needle positioned at 5 centimeters from a copper plate. The silver needle is orthogonal to the plate and both of them serve as electrodes. The sample is hold by the copper plate and there is a voltage between the electrodes equal to 6 kilovolts. The copper plate works as a heater too, and has a hole just under the sample where the light can pass through. The corona poling setup is described in detail elsewhere. 2.3 Second harmonic generation. The second harmonic generation was measured in situ by transmittance using a YAG:Nd laser at 1064 nm as the fundamental beam of light. This fundamental beam is collected and focused onto the sample by a convergent lens. Another lens, set after the corona poling system, collects all the light coming from the sample and sends it to a photomultiplier. The light passes through a color filter which blocks the fundamental beam and allows the transmission of the generated beam at 532 nm. In a previous work this
set-up is completely described. 3. RESULTS AND DISCUSSION XRD patterns confirmed the existence of nanostructures in the films with SDS, as well as the kind of the geometrical arrangement of the nanostructures (Figure 1). In both cases, PMMA and SiO2, the long-range order corresponds to a lamellar geometrical arrangement. The average distances between nanostructures were 3.85 nm for the samples with PMMA and 3.80 nm for the samples with SiO2.
Figure 1. XRD patterns obtained for (a) PMMA:DO3 and (b) SiO2:DO3 amorphous and nanostructured films. The films thicknesses were determined by means of a statistical analysis based on SEM images obtained from the samples as shown in Figure 2. It was found the films were very homogeneous and the corresponding average thicknesses were 9.63 µm for the PMMA:DO3 amorphous film, 5.64 µm for the PMMA:DO3 nanostructured film, 2.75 µm for the SiO2:DO3 amorphous film and 3.38 µm for the SiO2:DO3 nanostructured film.
Figure 2. SEM images obtained for amorphous and nanostructured films (a) PMMA:DO3 (top) and (b) SiO2:DO3 (bottom).
The UV-vis spectra of the samples shows that the maximum absorption is centered at 440 nm for all the samples, with or without SDS, as shown in Figure 3. It means there are not optical properties due directly to the surfactants. After corona poling the height of the maximum in the spectra decreases. The order parameter ρ relates de UV-vis spectra to the efficiency in the orientation of the chromophores through equation 1 [3, 4], A (1) ρ =1− ⊥ A where A⊥ is the film absorbance at 440 nm after poling and A is the corresponding absorbance before poling. It is remarkable that the nanostructured films exhibit larger order parameter.
Figure 3. UV-vis absorption spectra obtained for (a) PMMA:DO3 and (b) SiO2:DO3 amorphous and nanostructured films before corona poling. The ratio between the maximum spectrum height before and after corona poling determines the order parameter, i.e., it shows how easy the orientation of the chromophores is. It was found that the largest order parameter corresponds to the lamellar nanostructures. The SHG intensity signal dynamics is plotted as function of the corona poling time at several temperatures, as shown in Figure 4. The SHG vs. corona poling time spectra was fitted with a model developed by us, employing only one fitting parameter related to the matrix-chromophore interactions. The theoretical fit corresponds to the solid black line. These plots show a growth in the signal until a maximum plateau. The maximum value is directly related to the number of chromophores oriented non-centrosymmetrically, the largest value was obtained for the SiO2:DO3 nanostructured films. The speed of the orientation of the chromophores can be represented by a matrix-chromophore interactions parameter γ, as larger this parameter is slower the orientation is, too. The lowest value of this parameter was attained for the SiO2:DO3 nanostructured films. It means that the stability also is poor in these samples, in the sense that if the matrix-chromophore interactions are small then the chromophores easily lost their orientation.
Figure 4. Plots of SHG intensity vs. Corona poling time for (a) PMMA:DO3 and (b) SiO2:DO3 amorphous and nanostructured films at several temperatures. 4. CONCLUSIONS XRD patterns show a better long-range order of the SiO2 matrix than the PMMA one. The samples do not degrade with the experimental studies, as can be deduced from the cyclic measurements. The lamellar nanostructures give place to the largest order parameter values. The most intense SHG signal was obtained in the sample of SiO2 with lamellar nanostructure. The orientation and disorientation of the DO3 molecules was faster in the samples of SiO2 with lamellar nanostructure. It means that the nanostructures are a good option for modifying the dynamics of the chromophores orientation, which would allow increasing the efficiency of the SHG signal, the stability of the system and the speed for reaching the maximum SHG intensity. ACKNOWLEDGEMENTS The authors acknowledge the financial supports of CONACyT 79781, CONACyT 89584, NSF-CONACyT, PUNTA, ICyTDF and PAPIIT 116506-3. GVA is grateful for PUNTA postdoctoral fellowship. The authors are thankful to M. in Sc. Manuel Aguilar-Franco (XRD), Diego Quintero (preparation of the samples for SEM studies) and Roberto Hernández-Reyes (SEM) for technical assistance. REFERENCES 1. Franco A., J. A. García-Macedo, I. G. Marino and P. P. Lottici, “Photoinduced birefringence in nanostructured SiO2:DR1 Sol-Gel films”, J. of Nanosc. & Nanotechn., 8, 12, 6576–6583, 2008.
2. Franco A., G. Valverde-Aguilar, J. García-Macedo, "Optical absorption and second harmonic generation in SiO2:DR1 sol-gel films as function of poling time”, in Proceedings of SPIE, San Diego, USA, 2006, 633116-1 to 633116-12. 3. Franco A., G. Valverde-Aguilar, J. García-Macedo, M. Canva, F. Chaput and Y. Levy, “Modeling of the Second harmonic generation in SiO2 sol-gel films doped with nanoscopic DR1 molecules as function of the poling time”, Opt. Mater., 29, 1, 6-11, 2006. 4. Franco A., G. Valverde-Aguilar and J. García-Macedo, “Orientational dynamics of DR1 molecules in sol-gel films”, Opt. Mater., 29, 7, 814-820, 2007.
