J Sol-Gel Sci Technol (2010) 56:1–6 DOI 10.1007/s10971-010-2264-z
ORIGINAL PAPER
Third-order nonlinear and planar waveguide properties of TiO2/ organically modified silane hybrid films doped with different disperse red 1 content M. Sun • Wen Xiu Que • Tian Xi Gao C. H. Kam
•
Received: 27 March 2010 / Accepted: 4 June 2010 / Published online: 15 June 2010 Ó Springer Science+Business Media, LLC 2010
Abstract TiO2/organically modified silane organic– inorganic hybrid films doped with different disperse red 1 contents are prepared by combining a low-temperature sol– gel method and a spin-coating process. Effects of the disperse red 1 content on the third-order nonlinear and the planar waveguide properties of the hybrid films are also studied by a z-scan technique and a prism coupling technique. Results indicate that the nonlinear refractive index of the hybrid films is negative, whose magnitude is of the order of 10-8 esu for the measured samples. It is suggested that both the thermal effect and the photoisomerization process contribute to the third-order nonlinear optical properties of the hybrid films jointly. It is also found that the refractive index and the thickness of the hybrid films decrease with an increase of the temperature as the independent variable. In addition to, optical absorption properties of the hybrid films are characterized by UV–Visible spectroscopy. These results indicate that the as-prepared hybrid films are promising candidates for photonic applications. Keywords Sol–gel method Disperse red 1 Hybrid thin film Third-order nonlinear Planar waveguide
M. Sun W. X. Que (&) T. X. Gao Electronic Materials Research Laboratory, School of Electronic and Information Engineering, Xi’an Jiaotong University, Xi’an 710049, Shaanxi, People’s Republic of China e-mail:
[email protected] C. H. Kam School of Electrical and Electronic Engineering, Nanyang Technological University, Nanyang Avenue, Singapore 639798, Singapore
1 Introduction As a family of dye molecules, azobenzene is well-known for their cis-trans photoisomerization with respect to the N=N double bond. Especially in this family, azobenzene dyes such as disperse red 1 (DR1) have been widely used as a dopant in polymeric thin films for its large optical nonlinearities [1–4], which are interesting for applications in optical limiting and optical switching [5–8]. However, most photoactive organic materials may not be stable in ambient environment for long periods of time and exhibit low-mechanical strength, as well as relative low optical transparency as compared to that of an inorganic oxide. Thus optically homogeneous and transparent organic– inorganic hybrid sol–gel materials containing organic components can be considered as desirable photo-based materials for photonic applications [9–12]. Due to the intrinsic properties of these materials, especially, incorporation of azobenzene dye organic molecular groups into inorganic materials, these hybrid material systems have been widely applied for optical switching and optical storages in recent years [13–15]. Therefore, organic–inorganic hybrid materials have been extensively investigated as desirable material systems for photonic and optical waveguide applications since they can trap organic molecules. Especially, waveguide film configuration with an enough thick and low loss based on these organic–inorganic hybrid systems can be easily obtained by a single spin-coating process and a low heat treatment temperature [16–19]. In this paper, based on a modified sol–gel technique and a spin-coating technique, the DR1-doped TiO2/organically modified silane (ormosil) organic–inorganic hybrid films containing the matrix of SiO2–TiO2 were prepared for optical storage and optical switching potential applications. Effects of the DR1 azodye contents on the third-order
123
2
J Sol-Gel Sci Technol (2010) 56:1–6
nonlinear optical properties of the hybrid films were investigated by a z-scan technique. Furthermore, the planar waveguide properties such as the refractive index and the film thickness with the temperature as the independent variable and the absorption properties of the as-prepared planar waveguide films are also studied by a prism coupling technique and a UV–Visible absorption spectroscopy.
variable, the propagation modes, and the propagation loss of the hybrid films doped with DR1 azodye molecules were measured for transverse electric polarization by an m-line apparatus (Metricon 2010) based on the prism coupling technique. Absorption spectra of the hybrid films deposited on slide glasses were carried out by a JASCO V-570 UV/ VISNIR Spectrometer.
2 Experimental
3 Results and discussion
The DR1 azodye-doped organic–inorganic hybrid material sols were prepared by three solutions, which were individually prepared. For solution I, c-glycidoxypropyltrimethoxysilane (GLYMO) was mixed with ethanol and de-ionized water at a molar ratio of 1:4:4. After being stirred for about 30 min, 0.01 mol hydrochloric acid (HCl, 37 wt% in water) was added into the solution to catalyze the hydrolysis, followed the solution was further stirred for about 60 min in air. Similarly, for solution II, methyltrimethoxysilane (MTES) was mixed with ethanol and deionized water at a molar ratio of 1:4:4 added with 0.01 mol hydrochloric acid (37 wt. % in water) as the catalyst, and the mixture solution was stirred for about 60 min in air. For solution III, tetrabutyl titanate (Ti (OC4H9)4, used as TiO2 source) was added to acetylacetone at a molar ratio of 1:4 under a nitrogen environment and the solution was stirred until homogenization was reached. The three solutions were then mixed to have a molar ratio of 40GLYMO40MTES-20TiO2 as a matrix. The final mixture was stirred for a few hours at room temperature. The commercial DR1 azodye compound was then added to the TiO2/GLYMO/ MTES sol–gel hybrid matrix solution in the different weight ratio of the hybrid solution, which includes 0.2, 0.5, 1, and 1.5%. The final mixture solution was stirred for about one to two days at room temperature again. Silicon, silica-on-silicon and 24 mm 9 24 mm slide glass were used as substrates and they were ultrasonically cleaned in acetone and ethanol, respectively, rinsed with the de-ionized water and dried. Following the common practice for spin-coating, a 0.22-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 3,000 rpm for 35 s. Then the film-coated samples were directly baked at different temperatures for 10 min in air. Effects of the DR1 azodye contents on the third-order nonlinear optical properties of the hybrid films were studied by a z-scan technique employing an Nd:YAG pulsed laser at 532 nm wavelength. The incident energy, waist radius, and pulse width of the laser are 30 lJ, 19 lm and 7 ns, respectively. Dependence of the refractive index and the film thickness with the temperature as the independent
Figure 1 shows a dependence of the refractive index and the thickness of the hybrid films doped with different DR1 contents on the baking temperature at the wavelength of 633 nm based on the prism coupling technique. It can be seen that the thickness and the refractive index of the hybrid films decrease with the increase of the baking temperature from room temperature to 200 °C. It is probably related to more dense film due to higher baking temperature and a possible structural change of the hybrid film due to a thermal initiated orientation of the DR1 small molecules [20], which result in a decrease of the refractive index of the hybrid film. It is also noted that both the refractive index and the thickness of the hybrid films rise with increase the doping of the DR1 content, that is to say, the hybrid films doped with higher DR1 content have higher the refractive index and the film thickness values. There has been no reasonable explanation for this phenomenon, but we suspect that it is still probably related to the structural and compositional change of the hybrid film due to the addition of much more DR1 small molecular groups. Furthermore, the changes of the thickness and the refractive index of the hybrid film doped with DR1 of 1 wt. % and obtained at room temperature with the temperature as the independent variable at the wavelength of 633 nm were also observed as shown in Fig. 2. It can be seen that both the refractive index and the film thickness drop gradually during increase the temperature as the independent variable. Indicating that the dynamic structure of the hybrid film may occur some changes during a continuous heating process, which should be attributed to the orientation of the DR1 small molecules groups inside the hybrid film. Table 1 gives a change of the refractive index with the heating temperature (dn/dT) of the hybrid film at different wavelengths of incident laser, which is obtained by fitting the refractive-temperature curve of the temperature as the independent variable. Results indicate that the obtained dn/dT is negative for all the samples measured. Interestingly, these results should be used to understand the third-order nonlinearity of the hybrid films, which will be further discussed in the following section. Figure 3 shows the propagation modes of the hybrid planar film deposited on a silica-on-silicon substrate (the
123
J Sol-Gel Sci Technol (2010) 56:1–6
3 Table 1 Dependence of dn/dT of the hybrid films doped with DR1 of 1% and baked at different temperatures on the wavelength of incident laser Wavelength (nm) Temperature (°C) 25
Fig. 1 a Dependence of the refractive index of the hybrid films doped with different DR1 contents on the baking temperature; b Dependence of the thickness of the hybrid films doped with different DR1 contents on the baking temperature
Fig. 2 Dependence of the thickness and the refractive index of the hybrid film doped with DR1 of 1% and obtained at room temperature on the temperature as the independent variable
50
100
150
633
-2.97E-4 -2.57E-4 -2.16E-4 -2.84E-4
1312
-2.45E-4 -2.56E-4 -2.48E-4 -2.60E-4
thickness of the silica buffer is 500 nm) by a single spincoating process and doped with DR1 of 1%, which was measured by the prism coupling technique at the wavelength of 1538 nm. It can be observed from Fig. 3, which shows the transverse electric (TE) mode and transverse magnetic (TM) mode, that the hybrid film obtained under present conditions only can support one TE or TM mode to be guided at the wavelength of 1538 nm, and the other dips observed in the intensity spectra are attributed to substrate modes due to their index smaller than the value of the substrate refractive index. It should be mentioned here that the refractive index and the thickness of the hybrid planar waveguide film as shown in Figs. 1 and 2 are all determined by the measured effective refractive index values of both TE and TM modes. The optical propagation losses of the hybrid planar waveguide film at the wavelength of 1538 nm are also evaluated and typically for TE and TM mode are 0.94 and 0.97 dB/cm, respectively. The relative high propagation losses of the hybrid films should be related to the local refractive index fluctuations in the volume of the waveguide film and the deviations from a perfectly plane geometry at the waveguide-cladding boundaries. Another key factor to affect the losses is the non-uniform hydrolysis and condensation of the trinary alkoxides mixture of more than two alkoxide precursors,
Fig. 3 Modes of the hybrid planar waveguide film doped with DR1 of 1% and baked at 50 °C at the wavelength of 1538 nm
123
4
J Sol-Gel Sci Technol (2010) 56:1–6
Fig. 4 UV absorption spectra of the hybrid films doped with different DR1 contents and baked at the temperature of 50 °C
which result from the different relative rates of homocondensation and heterocondensation of the silicon alkoxides and the titanium alkoxides. Thus, it is possible that a heterogeneous network containing T-rich and Si-rich domains may be formed in this system, which becomes a scattering area to the incident light and leads to a greater loss. Figure 4 shows the absorption spectra of the hybrid films doped with different DR1 contents from 0.2 to 1.5% and baked at a temperature of 50 °C. It can be seen that the DR1-doped hybrid films have two absorption peaks at about 325 and 500 nm, respectively, which should be attributed to electronic transition of the DR1 molecules. It can be observed that the absorbance at the peaks of 325 and 500 nm increases with increase of the doping content of the DR1 molecule obviously. For instance, the absorbance at 500 nm increases from 0.09 to 0.295 when the DR1 content is doped from 0.2 to 1.5%. The third-order nonlinear optical properties of the hybrid films baked at 50 °C and doped with different DR1 contents were studied by using the z-scan technique with an Nd: YAG pulsed laser at the wavelength of 532 nm as shown in Figs. 5 and 6, which the solid lines and dotted lines correspond to the fitting curves and experimental curves, respectively. The thickness of the hybrid films for non-linear optical measurement is about 1.0 lm. The open-aperture z-scan curve of the hybrid film doped with DR1 of 0.2% as shown in Fig. 5b shows an absorptive valley and is ascribed to the two-photon absorption process [21] since it meets the condition of Eg/ 2 \ hv \ Eg. The nonlinear absorption coefficient is measured and has an order of 104 cm/GW. It can be also observed from the Fig. 5 that the peak of the transmittance has been suppressed greatly, while the valley of the transmittance has been enhanced due to the nonlinear absorption. According to the theory as reported in Reference [22], the nonlinear optical properties of the measured
123
Fig. 5 a Closed-aperture z-scan curve of the hybrid film doped with DR1 of 0.2%; b Open-aperture z-scan trace of the hybrid film doped with DR1 of 0.2%
sample can be induced only near the focus with the strong intensity. If the nonlinear coefficient of the sample is negative, the sample reveals a self-defocusing effect like a divergent lens and results in the collimation of the light in the z-direction in the front of the focus. Thus the far-field light beam becomes narrow and the light transmittance increases, which make the z-scan curve show the peak. However, due to the self-defocusing effect of the sample behind the focus, it results in the divergence of the light beam, thus the light transmittance decreases and makes the z-scan curve show the valley. Conversely, if the nonlinear coefficient of the sample is positive, the z-scan curve would reveal the valley, which precedes the peak. According to these theories, the closed-aperture z-scan trace as shown in Fig. 5a indicates that the nonlinear coefficient of the hybrid film doped with DR1 azodye of 0.2% is negative, since the peak precedes the valley. Figure 6 shows closed-aperture z-scan fitting curves of the hybrid films doped with different DR1 contents. It can be
J Sol-Gel Sci Technol (2010) 56:1–6
5
suppose that this phenomenon also exists in the sample irradiated by the non-polarized light as long as the light intensity is strong enough. Thus, the azobenzene molecules like liquid crystalline phase are correlated closely and the orientation of these azobenzene molecules may have a uniform effect on the respond to the external disturbance. Therefore, it can be concluded from above these results and discussion that various physical mechanisms are attributed to the contribution of the v(3). In a word, both thermal effect and photo-induced isomerization contribute to the larger nonlinear photo-responsive properties of the DR1doped hybrid films.
4 Conclusions Fig. 6 Closed-aperture z-scan traces of the hybrid films doped with different DR1 contents
Table 2 Third-order nonlinear optical refractive index and susceptibility of the hybrid films doped with different DR1 contents and baked at 50 °C Azobenzene content
0.2%
0.5%
1%
Re v(3) (esu)
-1.751E-8
-1.9E-8
-5.33E-8
b (cm/GW)
4.5E ? 4
4.8E ? 4
8.6E ? 4
observed that the valley of the hybrid film doped with DR1 of 1% is deeper than those of the hybrid films doped with DR1 of 0.2 and 0.5% as shown in Fig. 6. The nonlinear refractive index (Re v(3)) and nonlinear absorption coefficient (b) of the hybrid films baked at 50 °C and doped with different DR1 contents are presented in Table 2. The results in Table 2 indicate that the magnitude of the nonlinear refractive index is of the order of 10-8 esu and the absolute value of the nonlinear refractive index and the third-order susceptibility increase gradually with increase the doping of the DR1 content for all the measured samples. It should be stressed here that the existence of the third-order nonlinear properties of the as-prepared hybrid films doped with DR1 molecular groups is probably related to the following aspects. Firstly, the as-prepared hybrid film has an obvious absorption peak at about 532 nm as shown in Fig. 4, which may result in accumulating heat under a single pulse of laser. That is to say, a heat-induced third-order nonlinear effect occurs in our hybrid film, in fact, it can be further understood by the result and discussion of dn/dT obtained from Fig. 2 as given in Table 1. Secondly, azobenzene molecule could rotate to surround the double-bond irradiated by the linearly polarized light, which results in the trans-cis photoisomerization. In the reversible process of the photoisomerization, the azobenzene molecules are reoriented and become anisotropic. We
TiO2/GLYMO/MTES hybrid films doped with DR1 molecular groups for photonic applications have been derived by the low-temperature sol–gel spin-coating process from the inorganic–organic hybrid system. The thirdorder nonlinear optical properties and planar waveguide properties of the as-prepared hybrid films doped with different DR1 contents have been studied by the z-scan technique and the prism coupling technique. Results indicate that the magnitude of the nonlinear refractive index is of the order of 10-8 esu for all the measured samples, in addition, both the absolute value of the nonlinear refractive index and the third-order susceptibility increase with the increase of the DR1 content. It has been suggested that both the thermal effect and the photo-induced isomerization contribute to the large nonlinear photo-responsive properties of the hybrid films doped with DR1 molecular groups. These results indicate that the DR1 molecular groups have been successfully incorporated into silicon oxide matrices by the sol–gel technique. The change of the thickness and refractive index of the hybrid films with the temperature as the independent variable has also been investigated. Acknowledgments This work was supported by the National Natural Science Foundation of China under Grant No. 90923012 and 60477003, and the Scientific Research Foundation for the Returned Overseas Chinese Scholars, Education Ministry of China (2007).
References 1. Qian Guodong, Zu Jifeng, Guo Jiayu, Si Jinhai, Wang Mingquan, Hirao K (2006) Chem Phys Lett 421:101 2. Sekkat Z, Dumont M (1992) Appl Phys B 54:486 3. Pu Hongting, Liu Ling, Jiang Weichun, Li Xinwan, Chen Jianping (2008) J Appl Poly Sci 108:1378 4. Gayathri C, Ramalingam A (2008) Optik 119:409–414 5. Rangel-Rojo R, Yamada S, Matsuda H, Yankelevich D (1998) Appl Phys Lett 72:1021
123
6 6. Rosso V, Loicq J, Renotte Y, Lion Y (2004) J Non-Cryst Solids 342:140 7. Rosalyn PDS, Senthil S, Kannan P, Vinitha G, Ramalingam A (2007) J Phys and Chem Solids 68:1812 8. Gomy C, Schmitzer AR (2007) Org Lett 9:3865 9. Chaput F, Boilot JP, Riehl D, Le´vy Y (1994) J Sol–Gel Sci Technol 2:779 10. Yuan X-C, Yu WX, Ngo NQ, Cheong WC (2002) Opt Express 10:303 11. Luo XS, Zha CJ, Luther-davies Barry (2004) J Sol–Gel Sci Technol 32:297 12. Jeong SH, Jang W–H, Moon JH (2004) Thin Solid Films 466:204 13. Jiang XL, Li L, Kumar J, Kim DY, Shivshankar V, Tripathy SK (1996) Appl Phys Lett 68:2618 14. Sharma A, Dokhanian M, Kassu A (2005) Opt Lett 30:501
123
J Sol-Gel Sci Technol (2010) 56:1–6 15. Alam MZ, Ohmachi T, Ogata T, Nonaka T, Kurihara S (2006) Opt Mater 29:365 16. Innocenzi P, Battaglin G, Guglielmi M, Signorini R, Bozio R, Maggini M (2000) J Non-Cryst Solids 265:68 17. Ribeiro SJL, Messaddeq Y, Goncalves RR, Ferrari M, Montagna M, Aegerter AM (2000) Appl Phys Lett 77:3502 18. Que WX, Zhou Y, Lam YL, Chan YC, Kam CH (2000) Thin Solid Films 358:16 19. Que WX, Jia CY, Sun M, Sun Z, Wang LL, Zhang ZJ (2008) Opt Express 16:3490 20. Nunzi JM (2004) Lasers and Electro-Optics Society 2:723 21. De Salvo R, Said AA, Hagan DJ, Van Stryland EW, Sheik-Bahae M (1996) IEEE J Quantum Electron 32:1324 22. Sheik-Bahae M, Said AA, Van Stryland EW (1989) Opt Lett 14:955