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Photochromic gratings in sol–gel films containing diazo sulfonamide chromophore
Optical Materials 27 (2005) 1637–1641 www.elsevier.com/locate/optmat
Photochromic gratings in sol–gel films containing diazo sulfonamide chromophore Stanisław Kucharski *, Ryszard Janik Institute of Organic and Polymer Technology, Wrocław University of Technology, 50-370 Wrocław, Poland Received 10 December 2003; accepted 25 March 2004 Available online 25 December 2004
Abstract The photochromic sol–gel hybrid materials were prepared by incorporation of an azo chromophore containing sulfonamide fragment into polysiloxane cross-linked network. The materials were used to form transparent films on glass by spin-coating and/or casting. The reversible change of refraction index of the films on illumination with white light was observed by ellipsometry. The experiments with two beam coupling (TBC) and four wave mixing (4 WM) arrangement with green or blue laser beams as writing beams showed formation of a diffraction grating. The diffraction efficiency of the first order was 0.025–0.038 which yielded refraction index modulation in the range of up to 0.0066. 2004 Elsevier B.V. All rights reserved. PACS: 42.40.Eq; 42.70.JK Keywords: Photochromic grating; Sol–gel films; Holographic recording; Azo sulfonamide dye
1. Introduction Sol–gel processing is a versatile procedure for making advanced materials listed between ceramics and organic– inorganic hybrids. The starting materials used in the preparation of the ÔsolÕ are usually inorganic metal salts, siloxides or metal organic compounds such as alkoxides [1,2]. In a typical sol–gel process, the precursor is subjected to a series of hydrolysis and polymerization reactions to form a colloidal suspension, or sol. Processing of the sol makes it possible to obtain materials differing in forms, like xerogels, dense ceramics, aerogels, organic fibers and thin dense films. Organically modified sol–gels became interesting materials showing hybrid properties between two classes of the materials. The sol–gel processing can be used to incorporate organic compounds as trapped in a matrix
*
Corresponding author. E-mail address: [email protected] (S. Kucharski).
0925-3467/$ - see front matter 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2004.03.031
(guest host system) or being chemically bound to volume structure. The first type of material can incorporate organic dyes absorbing in IR or a visible region used as dopants to enhance photonic or spectroscopic [3–7], or photovoltaic properties [5]. Relatively easy process to obtain sol–gel where organic fragments are bound to the material structure by chemical bond lead to a class of materials named ORMOCERS or ORMOSILS [8– 14] and the advantage of this synthetic route was so required transparency of the materials. As expectancy for practical applications of the sol–gel materials was focused on optoelectronics, the authors tried to build organic functional fragments into the gel structure. The gels showing three-dimensional cross-linked structure were found to contain a free space in the matrix that made the organic fragments possible to move within cavities as a reaction to external field [15,16]. This resulted in a series of investigations on nonlinear optical properties [7,17], photorefractive [17–20] and photochromic [21–24] properties of the organically modified sol–gel structures. The functional properties of the
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S. Kucharski, R. Janik / Optical Materials 27 (2005) 1637–1641
Fig. 1. Chemical formulas of dye intermediates.
structures were created by the presence of such dyes as: spirooxazine [24], cyanine dyes [20], and Disperse Red [17,18,23–26], an azo dye with a nitro group. The transformation of the functional group under influence of light or/and electric field emerged as an opportunity to write and store information in a form of surface or volume diffraction grating developed by interaction of interfering laser beams with the material. The purpose of our work was to obtain transparent sol–gel films on solid support showing photochromic properties owing to incorporation of the sulfonamide containing azobenzene dye shown in Fig. 1. Our goal was to establish conditions for modulation of the refractive index of the film, and for writing holographic gratings by two-beam coupling and four wave mixing.
2. Materials and methods 2.1. Dye-1 {4-((E)-{4-[(2-hydroxyethyl)(methyl)amino]phenyl}diazenyl)-N-(5-methylisoxazol-3-yl)benzenesulfonamide} was synthesized in our laboratory by coupling reaction of diazonium salt derived from sulfamethoxazole (SIGMA) and N-methyl-N-(2-hydroxyethyl)aniline [mp = 211–213 C]; 1H NMR (DMSO-d6): d (ppm) 2.31 (s, 3H), 3.10 (s, 3H), 3.58 (m, 4H), 4.80 (br.s, 1H), 6.18 (s, 1H), 6.87 (d, 2H), 7.80 (d, 2H), 7.90 (d, 2H), 7.99 (d, 2H), 11,54 (s, 1H); UV–Vis(DMSO) kmax = 465 nm, e = 32 310 dm3 mol1 cm1 (lg e = 4.51)]. 2.2. Sol–gel procedure and film preparation The reagents were: tetraethoxysilane (TEOS, Aldrich), 3-isocyanatopropyltriethoxysilane (ICPTS, Aldrich) and chromophoric azo dye: 4-((E)-{4[(2-hydroxyethyl)-(methyl)amino]phenyl}diazenyl)-N(5-methylisoxazol-3-yl)benzenesulfonamide (DYE-1) [27]. The dye (0.6 mmol) and anhydrous pyridine (0.026 mol) was placed in a glass tube and sonificated to dissolve the dye. ICPTS (0.72 mmol) was added into
the tube and kept 24 h at 80 C preserving from moisture access. Into this solution was then added: TEOS (1.7 mmol), anhydrous ethanol (0.043 mol) and diluted HCl (0.036 mmol in 8.33 mmol water) and the content was sonificated (1 h) at room temperature. The solution so obtained was filtered using 0.2 lm filter and used for deposition of the sol on glass plates. The plates were kept at room temperature to let the volatile matter to evaporate and densification of the films took place at 80 C (2 h) or at 120 C (1 h). This procedure made it possible to obtain transparent hard films on glass support. 2.3. Two beam coupling Two p-polarized laser beams of 10 mW each from green laser diode (k = 532 nm) were focused on sol–gel film in a typical two-beam arrangement. The angle between the beams was ca. 7.7 and the sample was positioned normal to the bisection line. A diffraction beam was monitored using a Hamamatsu multiplier tube coupled with the PC computer. 2.4. Four wave mixing The arrangement for four waves mixing was set by using two writing beams from a green laser diode as mentioned above in two beam coupling, or by using a blue laser diode (k = 485 nm) of ca. 2.4 mW power. The beams were split symmetrically as above. The reading beam was an s-polarized beam of 632 nm laser diode. The s-polarization of reading beam was used to easy filter it from counter-propagating writing beam. 2.5. Ellipsometry Ellipsometer EL X-02C of DRE Ellipsometerbau GmbH (Germany) was used for the determination of film thicknesses and refractive indices. A linearly polarized laser beam of 3 mW (k = 632.8 nm) at incident angle of 70 was used in measurements.
S. Kucharski, R. Janik / Optical Materials 27 (2005) 1637–1641
3. Results and discussion The preparation of sol–gel films with using DYE-1 as a chemically bound photochromic fragment of the structure was fully justified as this dye was found to undergo trans–cis isomerization via its diazo group [29]. The maximum absorption band of the stable trans form lies at ca. 465 nm in DMSO solution whereas the cis form, which appears during illumination, has its maximum absorption band at ca. 385 nm. The spectrum of the addition product of the dye and ICPTS named ICPTS + DYE-1 in a solution reminds that of the DYE-1 with maximum absorption at ca. 460 nm. The absorption spectrum of the hardened gel structure deposited by spin coating onto a glass plate shows small blue shifting of the absorption band which can be seen at 450 nm (Fig. 2). The absorption band remains broad enough in its tail part to be able to absorb light emitted by a 532 nm or a 485 nm laser. Linear absorption coefficient determined with 10 mW laser at 532 nm for cast films was ca. 0.095 m1 whereas that obtained with blue laser at 485 nm, was more than twice as much. The change of absorption spectrum on illumination of the spin-coated film with white light was evident (Fig. 2). The power of the light beam was ca. 20 mW/cm2 and the exposition time was up to 1 min. The spectra were recorded immediately after illumination was switched off. The chromophore contained in the sol–gel structure is chemically bound to Si–O–Si cross-linked network. Blue shift of the maximum absorption band of the chromophore in sol–gel may be ascribed to the hydrogen bonds which have been formed between the azo groups and silanol groups present in the material [27,28]. The films deposited on glass plates showed reversible spectral change on illumination even with white light, so it might be assumed that chromophore groups had enough space in the material network to undergo trans–cis–trans isomerization; the first stage as a result of excitation with
Absorbance [arb. units]
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1: before illumination 2: after illumination 2
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0.04
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350
400
450
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600
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Wavelength [nm] Fig. 2. The UV–Vis spectra of the spin-coated film.
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light while the second one, as a result of thermal relaxation. The change of the nature of the chromophore should lead to changes of material dielectric properties which, as a consequence, should be manifested in a change of refractive index [29]. A good approach to evidence such phenomenon is an experimental arrangement with using crossed laser beams which interfere within material yielding dark and bright domains of micrometer dimension. The TBC tests were carried out on sol–gel films in configuration in which p-polarized beams (k = 532 nm) were used and the angle between them was 7.7. The sample was situated normal to the bisector line between beams. On switching the laser beams on, symmetrical diffraction spots appeared. The laser beams acted primarily as writing beams, and as interference became a reality, each beam appeared to be a reading beam of the diffraction grating formed. The time needed to reach maximum intensity of the diffraction signal was within 10–15 s. As the green laser light was heavily absorbed, only films of ca. 5 lm thickness were used in TBC experiments. The geometry of the writing beam resulted in grating fringe spacing, K, defined as K¼
k 2n sin h2
ð1Þ
where n is refraction index, h is angle between beams. Taking k = 532 nm, n = 1.43 (determined by ellipsometry), h = 7.7, one obtains K = 2.77 lm. Diffraction efficiency, g, can be determined as a ratio between intensity of diffracted (Id) and transmitted (It) beams and can be related to refraction index modulation Dn by following equation Id PdDn g¼ ¼ sin2 ð2Þ It þ Id k cos h0 where d is film thickness and h 0 is diffraction beam angle. For the given sample thickness it may be assumed that Raman–Nath criteria of diffraction are fulfilled. The g value determined from TBC for first order diffraction was 0.038 and the value of refraction index modulation calculated from Eq. (2) was 0.0066. In 4 WM experiment the writing beams were the same as in TBC and a reading beam (k = 632 nm) was practically not absorbed by the film. Contrary to writing beams it was s-polarized. The rise and decay of diffraction signal for a 10 lm film is shown in Fig. 3. The decay of the signal was caused by shutting down one of the writing beam. The measured diffraction efficiency was 0.036 which yielded change of refractive index, Dn, equal to 0.0038. Replacement of green laser diode by a blue one was also successful in generation of diffraction grating. The latter was much weaker nevertheless the diffraction efficiency of ca. 2.5% was achieved. Fig. 4 shows a dynamics of growing and decay of the diffraction signal.
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S. Kucharski, R. Janik / Optical Materials 27 (2005) 1637–1641 1.4
Diffraction signal [arb. units]
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Fig. 3. Diffraction signal vs. time in 4 WM. WB-writing beams (532 nm), WB on-writing beams on, WB off-writing beams off.
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intensity grating, population grating, absorption grating, refractive index grating.
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trans form absorbs light polarized parallel to its dipole moment, and this causes transformation to cis group. It could be observed when the film was illuminated with polarized light and the spectrum was recorded also with polarized light. For example, using p-polarized light for illumination yielded nearly no change of the UV–Vis spectrum when it was recorded with s-polarized arrangement [30]. The cis group being in a dark place re-isomerizes by thermal relaxation to trans form changing direction of the dipole moment to perpendicular one towards polarization plane. This photochemical transformation of the dye may lead to diffraction gratings of several kinds as for example:
20
25
30
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40
Time [s] Fig. 4. Diffraction signal vs. time in 4 WM using 485 nm writing beams. WB on-writing beams on, 1 WB off-one writing beam off. RB off-reading beam off.
Analyzing the growth and disappearance of the diffraction signal (Fig. 3) one comes to the conclusion that the growing period of the signal is relatively short being in the range of 10–15 s. This signifies that this process is faster as compared with that of polymeric methacrylate film [30], and also with photorefractive materials [31]. The time of signal decay up to nearly initial level was also within 10–15 s. In the case of blue writing beams the system reacted even faster. It is to mention that the power of the blue laser diode was as low as 1/8 that of the green laser, and the absorption of beam at k = 485 nm was stronger as compared with that at k = 532 nm. Cutting one writing beam off caused the diffraction signal to drop to a certain level and it was no initial level recorded before writing operation had been started. It is possible that a kind of interaction with reading beam took place. Photochromic behavior of an azo dye under illumination involves several consecutive processes initiated by the photochemical reaction of the diazo group. The
In the case of low Tg linear methacrylates containing azo substituents in side chains, the trans–cis isomerization is accompanied by material movement. The latter is slow process as compared to isomerization and as it also takes part in formation of diffraction pattern, the process of growth of diffraction signal is up to 1 min, whereas the thermal relaxation can last several hours [29,30]. The intermediate ICPTS + DYE-1 is a tri-functional monomer which in polycondensation yields a threedimensional network of the polysiloxane type. This tendency was even enhanced by using TEOS as a comonomer, a four-functional intermediate. The reaction between ICPTS and DYE-1 is nearly quantitative, so that there is practically no free DYE-1 in the form of guest molecules in the system. The azo dye in sol–gel is chemically bound within cross-linked structure which hinders the material movement and, as the process of growth and decay of diffraction signal is relatively fast, one may come to the conclusion that the isomerization itself plays predominant role in formation of the diffraction grating in the materials in question. This indicates that material movement within cross-networked gel structure is seriously hindered and only the chromophore fragments have sufficient free space around themselves to change configuration upon illumination and thermal relaxation. Additional investigations of trans–cis isomerization were carried out by ellipsometry. The measurements were made in a continuous way and unpolarized white light was periodically switched on the sample. A reversible process was observed and recorded by change of ellipsometric parameters. The simulation procedure to determine refractive index (real and imaginary), showed decrease of refractive index after illumination. This fact could be documented for the sample of ca. 160 nm thickness (Table 1).
S. Kucharski, R. Janik / Optical Materials 27 (2005) 1637–1641 Table 1 Change of refractive index on illumination (nr: real part, k: imaginary part) Before illumination
nr 1.4301
In stationary state, after illumination with white light (1 min, 2 mW/cm2) k 0.0412
nr 1.4287
k 0.0410
The difference of the real part of the refractive index of the sample was Dnr = 0.0014. This value is smaller than that obtained in TBC or 4 WM but it was determined for the sample whose thickness was a fraction of the wavelength used. The most important observation was that the reversibility of refractive index change on illumination (and relaxation) was relatively fast and reversible. 4. Conclusions The sulfonamide containing azo chromophore can be simply incorporated into a sol–gel structure by chemical bond. The materials so obtained can be used for formation of thin transparent films on solid support. The films were found to show photochromic properties owing to trans–cis isomerization of the chromophore. This effect could be utilized for generation of reversible diffraction pattern by TBC and 4 WM. The diffraction efficiency in Raman–Nath regime with using green laser beams was up to 0.038, yielding refraction index modulation in the range of up to 0.0066. Acknowledgments This work was supported by a grant of Center for Advanced Materials and Nanotechnology of Wrocław University of Technology, Wrocław, Poland. References [1] C.J. Brinker, G.W. Scherer, Sol–Gel Science, Academic Press, Boston, 1990.
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[2] G. Kickelbick, Prog. Polym. Sci. 21 (2003) 373. [3] P. Proposito, M. Casalboni, F. De Matteis, R. Pizzoferato, Superficies Vacio 9 (1999) 85. [4] E.M. Yeatman, E.J.C. Dawney, J. Sol–Gel Sci. Technol. 8 (1997) 1007. [5] M. Graetzel, J. Sol–Gel Sci. Technol. 22 (2001) 7. [6] K. Maruszewski, A. Sidorowicz, A. Ulatowska, H. Podbielska, W. Streˆk, J. Mol. Struct. 450 (1998) 193. [7] I. Wang, P.L. Baldeck, E. Botzung, N. Sanz, A. Ibanez, Opt. Mater. 21 (2002) 569. [8] K.-H. Haas, S. Amberg-Schwab, K. Rose, G. Schottner, Surf. Coat. Technol. 111 (1999) 72. [9] K.-H. Haas, S. Amberg-Schwab, K. Rose, Thin Solid Films 351 (1999) 198. [10] J. Habsuda, G.P. Simon, Y.B. Cheng, D.G. Hewitt, D.A. Lewis, H. Toh, Polymer 43 (2002) 4123. [11] J. Habsuda, G.P. Simon, Y.B. Cheng, D.G. Hewitt, D.R. Diggins, H. Toh, F. Cser, Polymer 43 (2002) 4627. [12] O. Sopera, C. Croutxe-Barghorn, D.J. Lougnot, New J. Chem. 25 (2001) 1006. [13] L.C. Klein, C.L. Beaudry, A.B. Wojcik, M. Mandanas, Ceram. Trans. 81 (1998) 273. [14] U. Streppel, P. Dannberg, C. Wa¨chter, A. Bra¨uer, L. Fro¨hlich, R. Houbertz, M. Popall, Opt. Mater. 21 (2003) 475. [15] B. Dunn, J.I. Zink, Chem. Mater. 9 (1997) 2280. [16] S. Yano, K. Iwata, K. Kurita, Mater. Sci. Eng. C 6 (1998) 75. [17] J. Reyes-Esqueda, B. Darracq, J. Garcia-Macedo, M. Canva, M. Blanchard-Desce, F. Chaput, K. Lahlil, J.P. Boilot, A. Brun, Y. Levy, Opt. Commun. 198 (2001) 207. [18] B. Darracq, F. Chaput, K. Lahlil, J.P. Boilot, Y. Levy, V. Alain, L. Ventelon, M. Blanchard-Desce, Opt. Mater. 9 (1998) 265. [19] S. Wu, F. Zeng, F. Li, J. Li, React. Funct. Polym. 46 (2001) 225. [20] A.V. Vannikov, A.D. Grishina, B.I. Shapiro, L.Y. Pereshivko, T.V. Krivenko, V.V. Savelyev, V.I. Berendyaev, R.W. Rychwalski, Chem. Phys. 287 (2003) 261. [21] L. Hou, H. Schmidt, Mater. Lett. 27 (1996) 215. [22] Y.-G. Mo, R.O. Dillon, P.G. Snyder, T.E. Tiwald, Thin Solid Films 355–356 (1999) 1. [23] D.H. Choi, K.J. Cho, Y.K. Cha, S.J. Oh, Bull. Kor. Chem. Soc. 21 (2000) 1222. [24] D.H. Choi, H.T. Hong, W.G. Jun, K.Y. Oh, Opt. Mater. 21 (2002) 373. [25] I.G. Marino, D. Bersani, P.P. Lottici, Opt. Mater. 15 (2000) 175. [26] I.G. Marino, D. Bersani, P.P. Lottici, Opt. Mater. 15 (2001) 279. [27] J.C. Crano, R.J. Giglielmetti, Organic Photochromic and Thermochromic Compounds, Vol. 2, Kluwer, New York, 1999. [28] S. Kawata, Y. Kawata, Chem. Rev. 100 (2000) 1777. [29] E. Ortyl, S. Kucharski, Cent. Eur. J. Chem. 2 (2003) 137. [30] S. Kucharski, R. Janik, P. Hodge, D. West, K. Khand, J. Mater. Chem. 12 (2002) 449. [31] H.S. Nalva, S. Miyata, Nonlinear Optical Properties of Organic Molecules and Polymers, CRC Press, 1997.
Photochromic gratings in sol–gel films containing diazo sulfonamide chromophore Stanisław Kucharski *, Ryszard Janik Institute of Organic and Polymer Technology, Wrocław University of Technology, 50-370 Wrocław, Poland Received 10 December 2003; accepted 25 March 2004 Available online 25 December 2004
Abstract The photochromic sol–gel hybrid materials were prepared by incorporation of an azo chromophore containing sulfonamide fragment into polysiloxane cross-linked network. The materials were used to form transparent films on glass by spin-coating and/or casting. The reversible change of refraction index of the films on illumination with white light was observed by ellipsometry. The experiments with two beam coupling (TBC) and four wave mixing (4 WM) arrangement with green or blue laser beams as writing beams showed formation of a diffraction grating. The diffraction efficiency of the first order was 0.025–0.038 which yielded refraction index modulation in the range of up to 0.0066. 2004 Elsevier B.V. All rights reserved. PACS: 42.40.Eq; 42.70.JK Keywords: Photochromic grating; Sol–gel films; Holographic recording; Azo sulfonamide dye
1. Introduction Sol–gel processing is a versatile procedure for making advanced materials listed between ceramics and organic– inorganic hybrids. The starting materials used in the preparation of the ÔsolÕ are usually inorganic metal salts, siloxides or metal organic compounds such as alkoxides [1,2]. In a typical sol–gel process, the precursor is subjected to a series of hydrolysis and polymerization reactions to form a colloidal suspension, or sol. Processing of the sol makes it possible to obtain materials differing in forms, like xerogels, dense ceramics, aerogels, organic fibers and thin dense films. Organically modified sol–gels became interesting materials showing hybrid properties between two classes of the materials. The sol–gel processing can be used to incorporate organic compounds as trapped in a matrix
*
Corresponding author. E-mail address: [email protected] (S. Kucharski).
0925-3467/$ - see front matter 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2004.03.031
(guest host system) or being chemically bound to volume structure. The first type of material can incorporate organic dyes absorbing in IR or a visible region used as dopants to enhance photonic or spectroscopic [3–7], or photovoltaic properties [5]. Relatively easy process to obtain sol–gel where organic fragments are bound to the material structure by chemical bond lead to a class of materials named ORMOCERS or ORMOSILS [8– 14] and the advantage of this synthetic route was so required transparency of the materials. As expectancy for practical applications of the sol–gel materials was focused on optoelectronics, the authors tried to build organic functional fragments into the gel structure. The gels showing three-dimensional cross-linked structure were found to contain a free space in the matrix that made the organic fragments possible to move within cavities as a reaction to external field [15,16]. This resulted in a series of investigations on nonlinear optical properties [7,17], photorefractive [17–20] and photochromic [21–24] properties of the organically modified sol–gel structures. The functional properties of the
1638
S. Kucharski, R. Janik / Optical Materials 27 (2005) 1637–1641
Fig. 1. Chemical formulas of dye intermediates.
structures were created by the presence of such dyes as: spirooxazine [24], cyanine dyes [20], and Disperse Red [17,18,23–26], an azo dye with a nitro group. The transformation of the functional group under influence of light or/and electric field emerged as an opportunity to write and store information in a form of surface or volume diffraction grating developed by interaction of interfering laser beams with the material. The purpose of our work was to obtain transparent sol–gel films on solid support showing photochromic properties owing to incorporation of the sulfonamide containing azobenzene dye shown in Fig. 1. Our goal was to establish conditions for modulation of the refractive index of the film, and for writing holographic gratings by two-beam coupling and four wave mixing.
2. Materials and methods 2.1. Dye-1 {4-((E)-{4-[(2-hydroxyethyl)(methyl)amino]phenyl}diazenyl)-N-(5-methylisoxazol-3-yl)benzenesulfonamide} was synthesized in our laboratory by coupling reaction of diazonium salt derived from sulfamethoxazole (SIGMA) and N-methyl-N-(2-hydroxyethyl)aniline [mp = 211–213 C]; 1H NMR (DMSO-d6): d (ppm) 2.31 (s, 3H), 3.10 (s, 3H), 3.58 (m, 4H), 4.80 (br.s, 1H), 6.18 (s, 1H), 6.87 (d, 2H), 7.80 (d, 2H), 7.90 (d, 2H), 7.99 (d, 2H), 11,54 (s, 1H); UV–Vis(DMSO) kmax = 465 nm, e = 32 310 dm3 mol1 cm1 (lg e = 4.51)]. 2.2. Sol–gel procedure and film preparation The reagents were: tetraethoxysilane (TEOS, Aldrich), 3-isocyanatopropyltriethoxysilane (ICPTS, Aldrich) and chromophoric azo dye: 4-((E)-{4[(2-hydroxyethyl)-(methyl)amino]phenyl}diazenyl)-N(5-methylisoxazol-3-yl)benzenesulfonamide (DYE-1) [27]. The dye (0.6 mmol) and anhydrous pyridine (0.026 mol) was placed in a glass tube and sonificated to dissolve the dye. ICPTS (0.72 mmol) was added into
the tube and kept 24 h at 80 C preserving from moisture access. Into this solution was then added: TEOS (1.7 mmol), anhydrous ethanol (0.043 mol) and diluted HCl (0.036 mmol in 8.33 mmol water) and the content was sonificated (1 h) at room temperature. The solution so obtained was filtered using 0.2 lm filter and used for deposition of the sol on glass plates. The plates were kept at room temperature to let the volatile matter to evaporate and densification of the films took place at 80 C (2 h) or at 120 C (1 h). This procedure made it possible to obtain transparent hard films on glass support. 2.3. Two beam coupling Two p-polarized laser beams of 10 mW each from green laser diode (k = 532 nm) were focused on sol–gel film in a typical two-beam arrangement. The angle between the beams was ca. 7.7 and the sample was positioned normal to the bisection line. A diffraction beam was monitored using a Hamamatsu multiplier tube coupled with the PC computer. 2.4. Four wave mixing The arrangement for four waves mixing was set by using two writing beams from a green laser diode as mentioned above in two beam coupling, or by using a blue laser diode (k = 485 nm) of ca. 2.4 mW power. The beams were split symmetrically as above. The reading beam was an s-polarized beam of 632 nm laser diode. The s-polarization of reading beam was used to easy filter it from counter-propagating writing beam. 2.5. Ellipsometry Ellipsometer EL X-02C of DRE Ellipsometerbau GmbH (Germany) was used for the determination of film thicknesses and refractive indices. A linearly polarized laser beam of 3 mW (k = 632.8 nm) at incident angle of 70 was used in measurements.
S. Kucharski, R. Janik / Optical Materials 27 (2005) 1637–1641
3. Results and discussion The preparation of sol–gel films with using DYE-1 as a chemically bound photochromic fragment of the structure was fully justified as this dye was found to undergo trans–cis isomerization via its diazo group [29]. The maximum absorption band of the stable trans form lies at ca. 465 nm in DMSO solution whereas the cis form, which appears during illumination, has its maximum absorption band at ca. 385 nm. The spectrum of the addition product of the dye and ICPTS named ICPTS + DYE-1 in a solution reminds that of the DYE-1 with maximum absorption at ca. 460 nm. The absorption spectrum of the hardened gel structure deposited by spin coating onto a glass plate shows small blue shifting of the absorption band which can be seen at 450 nm (Fig. 2). The absorption band remains broad enough in its tail part to be able to absorb light emitted by a 532 nm or a 485 nm laser. Linear absorption coefficient determined with 10 mW laser at 532 nm for cast films was ca. 0.095 m1 whereas that obtained with blue laser at 485 nm, was more than twice as much. The change of absorption spectrum on illumination of the spin-coated film with white light was evident (Fig. 2). The power of the light beam was ca. 20 mW/cm2 and the exposition time was up to 1 min. The spectra were recorded immediately after illumination was switched off. The chromophore contained in the sol–gel structure is chemically bound to Si–O–Si cross-linked network. Blue shift of the maximum absorption band of the chromophore in sol–gel may be ascribed to the hydrogen bonds which have been formed between the azo groups and silanol groups present in the material [27,28]. The films deposited on glass plates showed reversible spectral change on illumination even with white light, so it might be assumed that chromophore groups had enough space in the material network to undergo trans–cis–trans isomerization; the first stage as a result of excitation with
Absorbance [arb. units]
0.08
1
1: before illumination 2: after illumination 2
0.06
0.04
0.02
0.00 300
350
400
450
500
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600
650
700
Wavelength [nm] Fig. 2. The UV–Vis spectra of the spin-coated film.
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light while the second one, as a result of thermal relaxation. The change of the nature of the chromophore should lead to changes of material dielectric properties which, as a consequence, should be manifested in a change of refractive index [29]. A good approach to evidence such phenomenon is an experimental arrangement with using crossed laser beams which interfere within material yielding dark and bright domains of micrometer dimension. The TBC tests were carried out on sol–gel films in configuration in which p-polarized beams (k = 532 nm) were used and the angle between them was 7.7. The sample was situated normal to the bisector line between beams. On switching the laser beams on, symmetrical diffraction spots appeared. The laser beams acted primarily as writing beams, and as interference became a reality, each beam appeared to be a reading beam of the diffraction grating formed. The time needed to reach maximum intensity of the diffraction signal was within 10–15 s. As the green laser light was heavily absorbed, only films of ca. 5 lm thickness were used in TBC experiments. The geometry of the writing beam resulted in grating fringe spacing, K, defined as K¼
k 2n sin h2
ð1Þ
where n is refraction index, h is angle between beams. Taking k = 532 nm, n = 1.43 (determined by ellipsometry), h = 7.7, one obtains K = 2.77 lm. Diffraction efficiency, g, can be determined as a ratio between intensity of diffracted (Id) and transmitted (It) beams and can be related to refraction index modulation Dn by following equation Id PdDn g¼ ¼ sin2 ð2Þ It þ Id k cos h0 where d is film thickness and h 0 is diffraction beam angle. For the given sample thickness it may be assumed that Raman–Nath criteria of diffraction are fulfilled. The g value determined from TBC for first order diffraction was 0.038 and the value of refraction index modulation calculated from Eq. (2) was 0.0066. In 4 WM experiment the writing beams were the same as in TBC and a reading beam (k = 632 nm) was practically not absorbed by the film. Contrary to writing beams it was s-polarized. The rise and decay of diffraction signal for a 10 lm film is shown in Fig. 3. The decay of the signal was caused by shutting down one of the writing beam. The measured diffraction efficiency was 0.036 which yielded change of refractive index, Dn, equal to 0.0038. Replacement of green laser diode by a blue one was also successful in generation of diffraction grating. The latter was much weaker nevertheless the diffraction efficiency of ca. 2.5% was achieved. Fig. 4 shows a dynamics of growing and decay of the diffraction signal.
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S. Kucharski, R. Janik / Optical Materials 27 (2005) 1637–1641 1.4
Diffraction signal [arb. units]
WB off
WB off
1.2 1.0 0.8 0.6 0.4 0.2 0.0
WB on
WB on 0
10
20
30
40
0
50
60
70
Time [s]
Diffraction signal [arb. units]
Fig. 3. Diffraction signal vs. time in 4 WM. WB-writing beams (532 nm), WB on-writing beams on, WB off-writing beams off.
0.8
WB on
WB on
0.6 0.5 0.4 1 WB off
0.2 0.1
RB off
0.0 -5
0
5
10
15
• • • •
intensity grating, population grating, absorption grating, refractive index grating.
WB on
0.7
0.3
trans form absorbs light polarized parallel to its dipole moment, and this causes transformation to cis group. It could be observed when the film was illuminated with polarized light and the spectrum was recorded also with polarized light. For example, using p-polarized light for illumination yielded nearly no change of the UV–Vis spectrum when it was recorded with s-polarized arrangement [30]. The cis group being in a dark place re-isomerizes by thermal relaxation to trans form changing direction of the dipole moment to perpendicular one towards polarization plane. This photochemical transformation of the dye may lead to diffraction gratings of several kinds as for example:
20
25
30
35
40
Time [s] Fig. 4. Diffraction signal vs. time in 4 WM using 485 nm writing beams. WB on-writing beams on, 1 WB off-one writing beam off. RB off-reading beam off.
Analyzing the growth and disappearance of the diffraction signal (Fig. 3) one comes to the conclusion that the growing period of the signal is relatively short being in the range of 10–15 s. This signifies that this process is faster as compared with that of polymeric methacrylate film [30], and also with photorefractive materials [31]. The time of signal decay up to nearly initial level was also within 10–15 s. In the case of blue writing beams the system reacted even faster. It is to mention that the power of the blue laser diode was as low as 1/8 that of the green laser, and the absorption of beam at k = 485 nm was stronger as compared with that at k = 532 nm. Cutting one writing beam off caused the diffraction signal to drop to a certain level and it was no initial level recorded before writing operation had been started. It is possible that a kind of interaction with reading beam took place. Photochromic behavior of an azo dye under illumination involves several consecutive processes initiated by the photochemical reaction of the diazo group. The
In the case of low Tg linear methacrylates containing azo substituents in side chains, the trans–cis isomerization is accompanied by material movement. The latter is slow process as compared to isomerization and as it also takes part in formation of diffraction pattern, the process of growth of diffraction signal is up to 1 min, whereas the thermal relaxation can last several hours [29,30]. The intermediate ICPTS + DYE-1 is a tri-functional monomer which in polycondensation yields a threedimensional network of the polysiloxane type. This tendency was even enhanced by using TEOS as a comonomer, a four-functional intermediate. The reaction between ICPTS and DYE-1 is nearly quantitative, so that there is practically no free DYE-1 in the form of guest molecules in the system. The azo dye in sol–gel is chemically bound within cross-linked structure which hinders the material movement and, as the process of growth and decay of diffraction signal is relatively fast, one may come to the conclusion that the isomerization itself plays predominant role in formation of the diffraction grating in the materials in question. This indicates that material movement within cross-networked gel structure is seriously hindered and only the chromophore fragments have sufficient free space around themselves to change configuration upon illumination and thermal relaxation. Additional investigations of trans–cis isomerization were carried out by ellipsometry. The measurements were made in a continuous way and unpolarized white light was periodically switched on the sample. A reversible process was observed and recorded by change of ellipsometric parameters. The simulation procedure to determine refractive index (real and imaginary), showed decrease of refractive index after illumination. This fact could be documented for the sample of ca. 160 nm thickness (Table 1).
S. Kucharski, R. Janik / Optical Materials 27 (2005) 1637–1641 Table 1 Change of refractive index on illumination (nr: real part, k: imaginary part) Before illumination
nr 1.4301
In stationary state, after illumination with white light (1 min, 2 mW/cm2) k 0.0412
nr 1.4287
k 0.0410
The difference of the real part of the refractive index of the sample was Dnr = 0.0014. This value is smaller than that obtained in TBC or 4 WM but it was determined for the sample whose thickness was a fraction of the wavelength used. The most important observation was that the reversibility of refractive index change on illumination (and relaxation) was relatively fast and reversible. 4. Conclusions The sulfonamide containing azo chromophore can be simply incorporated into a sol–gel structure by chemical bond. The materials so obtained can be used for formation of thin transparent films on solid support. The films were found to show photochromic properties owing to trans–cis isomerization of the chromophore. This effect could be utilized for generation of reversible diffraction pattern by TBC and 4 WM. The diffraction efficiency in Raman–Nath regime with using green laser beams was up to 0.038, yielding refraction index modulation in the range of up to 0.0066. Acknowledgments This work was supported by a grant of Center for Advanced Materials and Nanotechnology of Wrocław University of Technology, Wrocław, Poland. References [1] C.J. Brinker, G.W. Scherer, Sol–Gel Science, Academic Press, Boston, 1990.
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