Electro-optic Properties of DR1 Doped SiO 2 Organic/inorganic Guest-host Films

Transcription

1 CHEM. RES. CHINESE UNIVERSITIES 009, 5(5), 76 7 Electro-optic Properties of DR1 Doped SiO Organic/inorganic Guest-host Films LIU Shao-lin, YANG Han *, JIN Ru-long, YAN Zhao-xu and YI Mao-bin State Key Laboratory on Integrated Optoelectronics, College of Electronic Science and Engineering, Jilin University, Changchun 1001, P. R. China Abstract We made a comprehensive investigation on the electro-optic(eo) properties of hybrid sol-gel silica film doped with Dispersed Red 1 dye(dr1/sio ). An important finding is that the dipoles which don t orient along the direction of corona poling field in the films can be instantaneously poled by alternate modulation electrical field. This instant poling effect results in the EO coefficients dependent on the frequency and intensity of alternate modulation electric field, and some experiments have proved it. We also investigated the electro-optic coefficients and thickness of films at different stir temperatures(t s ) of start solution and at different number densities of DR1. Experimentally the DR1 number density was optimized to an order of 10 1 /cm and a large EO coefficient of γ = 7 pm/v for DR1/SiO film was measured by simple reflection technique at 1 khz frequencies of modulation electric field. These findings may benefit the practical application of DR1/SiO films in the field of EO modulator and EO probing. Keywords Electro-optic; DR1/SiO ; Instant poling; Number density; Frequency Article ID (009) Introduction Recently, organic nonlinear optical(nlo) materials have been extensively investigated for their large nonlinear optical coefficients and the easy formation of films. In particular, azo dye doped organic/ inorganic guest-host type electro-optic(eo) films have been extensively reported for their potential application in the field of EO modulators [1 4] and EO probing [5,6]. The electro-optic coefficients of those materials are affected by lots of parameters under preparation condition. Completely understanding the EO characteristics of the films would benefit their application in required field. In recent twenty years a great many papers investigating the nonlinear properties of azo dye doped organic/inorganic guest-host type materials had been published. In the papers [7 10], the affection of chromophore-chromophore electrostatic interactions on the EO coefficients of chromophore doped polymer materials was simulated, demonstrating that macroscopic optical nonlinearities are attenuated at a certain high number density of chromophores because of the enhanced electrostatic interactions. The dipole moment increase results in the reduction of number density at which the EO coefficients attain the maximum. The chromophore shape also effects the electrostatic interactions. The papers [11,1] depicted the dispersion of EO coefficients of poled azo-dye-doped polymer films over a wavelength range limited by the probing laser. The articles [1,14] developed a series of highly efficient, thermally and chemically stable chromophores, compared the EO coefficients of the films at different poling temperatures, and compared poling efficiencies of EO polymers of different loading densities and containing different substituents. The papers [15,16] reported the ultra-efficient poling of electro-optic polymers by means of an organically modified sol-gel cladding layer, resulting in EO coefficient enhancement. The articles [17 19] reported the transient properties of electro-optic films in poled situation, or under poling with alternating voltage field. Up to now, however, no papers have reported the dependence of electro-optic coefficients on the frequency and intensity of modulation electric field. In this work, we investigated these two cases in the DR1/SiO [DR1: C 16 H 18 N 4 O, and the molecular structure of DR1 is shown in Fig.1(A)] organic/inorganic guest-host films. We proposed a dipole molecular rotation model, which explains the electro-optic *Corresponding author. yanghan@jlu.edu.cn Received October 7, 008; accepted November 4, 008. Supported by the Chinese University Innovation Program from Ministry of Education of China(No ).

2 No.5 LIU Shao-lin et al. 77 performances by considering the force of external modulation electric field. Fig.1 Molecular structure of DR1(A) and DR1 molecular orientation form along the direction of corona poling field in DR1/SiO films(b) The azo molecules in a film orient their dipoles moment along the direction of corona poling field. Because of the electrostatic interactions, the orientation of molecules occurs merely in part, while the residual molecules don t orient completely. These molecules contribute to the EO coefficient by the instant poling effecting when the modulation electric field is applied. The molecular corona poling situation is shown in Fig.1(B). The EO coefficient γ is comprised of two parts: the corona poling γ p (by corona poling electric field) and the instant poling γ i (by modulation electric field), that is, γ = γ p + γ i (1) where γ p is related with the order parameter Ф, dictating the corona poled dipole at the glass temperature of silica films. These molecules maintain direction along the corona poling field by the SiO locking with the temperature going down. All the papers published so far have merely concerned this term. The γ i is induced by the residual molecular disorder, which exists even at the glassed temperature because of the electrostatic interactions and/or the damp of SiO matrix. Experimental.1 Sample Preparation The SiO films were prepared from the hydrolysis and condensation reaction of tetraethoxysilane [Si(OC H 5 ) 4, TEOS] with water under the acid condition. With the middle processes neglected, the total reaction is written as Si(OC H 5 ) 4 + H O SiO + 4C H 5 OH () With N,N-dimethylformamide[(CH ) NCHO, DMF] as the cosolvent, DR1 was doped in the sol. We prepared three types of DR1/SiO films under diffe- rent conditions to investigate their EO properties: (1) The composition molar ratio of DR1/DMF/ TEOS/H O/HCI was ( ):.:1:.:0.08. After mixing these compositions, the solution was stirred for half an hour to get a homogeneous sol at room temperature. Then the sol was spin-coated onto an ITO(indium-tin oxide) glass with a thickness of nm. The number density of DR1 in these films is /cm. Having been baked at 80 C for two hours to remove residual solvent, these films were corona poled under a high electric field over 5000 V/cm at 105 C for an hour to order the dipoles of DR1 molecules. Finally, a thin aluminum film was thermally evaporated on the poled film as an electrode and as the reflection mirror to measure electro-optic coefficients. () The molar number of DR1 was fixed at mol, and other compositions are the same as those of the films in the first case. The number density of DR1 in SiO was /cm. The stir temperature was changed in a range of 0 60 C. () Poled and unpoled DR1/SiO films with a number density /cm of DR1 were prepared. The EO properties of these two types of films were compared to prove that the instant poling effect was the dominant factor to cause the dependence of γ on the frequency of modulation electric field.. EO Coefficients Measurement The EO coefficients were measured by simple reflection technique [0,1]...1 Poled Films Usually, the approximation γ γ 1 is adopted to calculate the EO coefficients of poled polymer films, which have an mm symmetry. This is, however, not precise when the poling electric field is relatively high, and the orientational order parameter Ф is relatively large. In this case the value β=γ /γ 1 could be obviously larger than. From literature [], the orientational order parameter Ф=1 A /A 0, where A and A 0 are the absorbances of the poled and unpoled silica films at 500 nm, respectively, as shown in Fig.(A), and Ф is expressed as Ф=1+/u (/u)cothu () where u is the ratio of electrostatic dipole alignment energy to thermal energy, u=μe p /kt, here μ is the molecular ground-state dipole moment, E p is the poling electric field, k is the Boltzman coefficient, T is the

3 78 CHEM. RES. CHINESE UNIVERSITIES Vol.5 poling temperature, cothu is the hyperbolic cotangent function. By means of the absorption spectra of the polymer films before and after poling, the orientational order parameter was obtained, then the value of u was calculated via Eq.(). So the ratio of γ /γ 1 could be calculated by equation as follows: coth u coth u + coth u γ 1+ Φ u u u u β = = γ Φ coth u 1+ coth u u u u u (4) then γ is expressed as λ I m none sin α 1 n γ = + 1/ π IcVm ( no sin α) β ne with the use of the approximations n o n e n, the formula in simple reflection technique is modified as 1/ β λ ( n sin α) I m γ = (6) β 1 πn sin α IcVm where laser wavelength is λ=1. μm, the incidence angle α=45, and the refractive index n=1.48, as measured by ATR(attenuated total reflection). o ( n e sin α) 1/ ( n o 4 no sin α) 1/ 1 (5) β/(β 1) values are 1.4, 1.19, 1.18, and If the approximation γ γ 1 is used, the β/(β 1) is 1.5, which may be parted into , , 1. 1., , and corresponding to Φ from large to small values. In this paper the range for β/(β 1) chosen and to calculate the γ and the accuracy of measurement was improved by ca. 0%. In Fig.(B), the calculated r values are 6.5, 19, 0 and 7 pm/v, respectively, corresponding to Φ mentioned above... Unpoled Film The orientational order parameter Ф for unpoled film has no meaning. However, under alternate modulation electric field, the instant poling effect could cause the DR1 poling, which exhibits an instant mm symmetry. In this case it is proper to set the γ γ 1 (β=) approximation in Eq.(5). Results and Discussion.1 EO Coefficients and Thickness of DR1/SiO Films vs. DR1 Number Density Fig. Absorption spectra of DR1/SiO films(a) and EO coefficient vs. the orientational order parameter(b) Symbols and solid line indicate the measured and fitted results, respectively. The EO coefficient increases proximately linearly as the orientational order parameter Φ increases as shown in Fig.(B). The large Φ indicates that more DR1 molecules in the films orient their dipoles along the direction of poling field and contribute to the γ p. The Φ values of corresponding samples are 0.46, 0.56, 0.57 and 0.66, respectively, which are larger than that reported before. The β values calculated from Eq.(4) are 5.10, 6.19, 6.4, and 8., corresponding to Φ. From the theoretical calculation, the EO coefficient for poled dipolar chromophores in a polymer matrix is expressed in terms of an acentric order parameter cos θ as [7] Nβf ( ω1) f ( ω) f ( ω) γ = cos θ (7) 4 ne where N is the chromophore number density, β is the first hyperpolarizability of DR1, f(ω) is a local-field correction, n e is the extraordinary index of refraction, θ is the angle between the direction of dipole and the poling field, and the trigonal bracket denotes the average of cos θ. On the basis of according Langevin polynomials and neglecting the higher term, the acentric order parameter is expressed as

4 No.5 LIU Shao-lin et al. 79 f (0) Ep cos θ = (8) 5kT where μ is the dipole moment, k is the Boltzmann constant, and T is the poling temperature. The Eq.(7) suggests that the EO coefficient should increase linearly with DR1 number density. In the case of N<10 1 /cm, this is true as shown in Fig. (γ was measured by simple reflection technique at 1 khz frequency of modulating electric field). Once the N increases higher, the electrostatic interaction increases against the poling electric field. μ Since the DR1 doped in the SiO matrix as guest, the increase of its number density has little affection on the thickness of films as shown in Fig.. When the number density increases from N= /cm to N= /cm, the film thickness increases from 410 to 450 nm(measured by profiler). In other words, ten times increase of loading merely induces about 10% increase in thickness. So this effect is negligible compared with the effect of the other composites of films and the velocity of spin-coating of films.. EO Coefficients and Thickness of DR1/SiO Films vs. Stir Temperature of Start Solution Fig. EO coefficient and thickness of poled DR1/SiO films vs. the DR1 number density Triangles indicate the EO coefficient measured by simple reflection technique, dots indicate the thickness of films measured by profiler at same DR1 number density and solid line indicates the fitted results. The EO coefficients attain maximum at /cm, and attenuate gradually at the higher number density. The effect of electrostatic interaction on the EO coefficient can be interpreted by London and Harper s theory []. London has shown that the electrostatic interactions between chromophores are expressed as the sum of three terms, namely, orientation force, induction force, and dispersion force: 4 w 1 μ Iχ = + μ χ + (9) 6 R kt 4 where R is the average distance between chromophores, χ is the polarizability, and I is the ionization potential. With this consideration, the acentric order parameter is expressed as: μf (0) E p W cos θ = 1 L (10) 5kT kt where L(W/kT)=coth(W/kT) kt/w is the Langevin function. The second term in brackets represents the attenuation of the order parameter associated with chromophore-chromophore electrostatic interactions. For the chromophore DR1, most parameters in Eq.(9) are constant except R. When the number density increases, R reduces, W increases, as reduces acentric order parameter and causes the reduction of EO coefficient. Owing to the rapidness of reaction in solution, many factors influence the quality of sol-gel films and sequentially affect the EO coefficient γ of DR1/SiO. Besides the composition of start solution, the stir temperature(t s ) is a very important factor as shown in Fig.4(A). The γ values were measured by means of simple reflection technique at 1 khz frequency of modulation electric field. The optimum stir temperature ranges 0 60 C for poled DR1/SiO films. In this area, the average value of γ gradually increases vs. T s, as is indicated by the solid line in Fig.4(A). It could be attributed to the fact that DR1 is more homogeneously dissolved in solution under higher T s, and the hydrolysis reaction between TEOS and H O is promoted by higher T s, eventually resulting in fine DR1/SiO films and improving the poling efficiency Fig.4 EO coefficients of poled DR1/SiO films vs. the stir temperature(t s ) of start solution(a) and the thickness of DR1/SiO films prepared at different stir temperature of start solution(b) Triangles denote the EO coefficient of different samples, the diamonds denote the average of EO coefficient at a certain T s.

5 70 CHEM. RES. CHINESE UNIVERSITIES Vol.5 of DR1. On the other side, higher T s induces a longer temperature fall time of gel process, so it is more difficult to keep the characteristics of film uniform. That s why the γ becomes more uneven at higher T s. The films crack when the T s is under 0 C. The film becomes extremely inhomogeneous when T s is over 70 C, and even no desired film could be obtained. We have considered the hydrolysis reaction is not exhaustive, so stress induces the cracking of films under 0 C. When the stir temperature is beyond 70 C, the reaction in the solution may be more complex: quick vaporization of solvent and the probability of other components joining in the reaction, both of which degrade the film quality. The stir temperature of the start solution affects the physical properties of SiO matrix after spincoating, then affects the thickness of DR1/SiO as shown in Fig.4(B). The thickness of poled DR1/SiO film is 456 nm at 0 C and 415 nm at 60 C. High stir temperature facilitates the reaction, increases the solution fluidity and improves the solvent vaporization, leading to the reduction of the film thickness.. Order Relaxation with Time at Room Temperature obvious and the elapse time from initial value to a stable one is longer. We would attribute this to the fact that the quantity of DR1 molecules oriented their dipoles along the direction of poling field is bigger at larger Φ, so the repulsive effect of dipoles is more intensive and the locking effect of SO inorganic matrix on the much quantity of DR1 becomes more difficult..4 Instant Poling Effect of Dipole by Alternate Modulation Electric Field Under corona poling electric field, the dipole of most of DR1 molecules orient along the direction of corona poling field, however, for the electrostatic interaction and the damping of SiO matrix, partial numbers of DR1 molecules orient their dipoles in disorder. If the SiO matrix retains certain elastic, these disordered molecules could be reordered when the film is applied a modulation electric field. This can be modeled by a rod rotated in a damping SiO matrix under external force moments as shown in Fig.6. Commonly, the nonlinear optical coefficients decline with time even at room temperature after removal of the poling electric field because of the relaxation of the orientation of doped dye molecules. In poled DR1/SiO films, the rigidity of SiO inorganic matrix prevents DR1 molecules from rotating, which helps to achieve better temporally stable property as shown in Fig.5. The γ of DR1/SiO declines in the initial time but holds an almost stable value at the low level. In the case of larger r, the attenuation becomes more Fig.5 Orientation relaxation of poled DR1/SiO films with time at room temperature Blank or black symbols(star, triangle, square and dot) indicate the EO coefficients of different samples measured at different period, and solid lines indicate fitted results. Fig.6 Rotary moment of DR1 in modulation electric field The moments that DR1 felt consist of two terms: one is modulation electric field moment, T E = μf (0)sinθE m cos(ωt), where f (0) is located field factor, θ is the angle between the direction of the dipole and the modulated electric field, E m cos(ωt) is the alternate modulation electric field in simple reflection technique. The other is damping moment T ξ =ξdθ/dt, where ξ is the frictional coefficient of DR1 in SiO matrix. In terms of calculation of Stocks, the rotational frictional coefficient ξ=8πηa, where η is the viscidity of SiO matrix, a is the radius of a molecule [4]. Based on the theorem of rotation, the total rotational moment is equal to the rotational inertia of molecule multiplied by the angle acceleration: M=I(d θ/dt )= T E T ξ (11) Then we can get the rotational dynamic equation of dipole molecules in SiO matrix under a external electric field: I(d θ/dt )+ ξ(dθ/dt) =μf (0)sinθE m cos(ωt) (1) Eq.(1) is different from normal elastic harmonic equation by the nonlinear term sinθ, so it is hard to

6 No.5 LIU Shao-lin et al. 71 gain the analytic solution. This dynamic equation of dipole indicates that θ is a function of frequency ω and E m, the latter is the intensity of modulation electric field when the film is measured via simple reflection technique. The acentric order parameter cos θ is not only a function of poling electric field, but also a function of ω and E m. The EO coefficient is also related to ω and E m by Eq.(6). The prediction has been approved by the experimental results, as shown in Fig.7. The EO coefficients of poled films increase as the frequency decreases. Because the molecules rotate more closely following the modulation electric field at a lower frequency, the macroscopical EO coefficient γ i in Eq.(1) increases. These two samples were prepared at the stir temperature T s =0 C and 60 C, respectively. The DR1 number density is /cm. films is larger than that of the poled films, as shown in Fig.9. The reason has not been completely understood, but there is one thing that can be sure, that is, unpoled films contain more molecules to be instantaneously poled by modulation electric field compared with corona poled films. The simultaneous rotation of numerous molecules yields abundant heat, leading to a local temperature to rise, even causing the melting of the Al films on SiO. Fig.7 EO coefficient γ of poled DR1/SiO films as a function of frequency of modulation electric field in simple reflection technique lgf is common logarithm of frequency; symbols and solid line indicate the measured and fitted results, respectively. Since the γ i is proportional to the intensity of modulation electric field, the EO signal on the detector is proportional to the square intensity of modulation electric field. That is I m (V m /d), so the I m is expressed nonlinearly with the V m, as shown in Fig.8. In Fig.8(A), the DR1 number density in silica films is /cm, and in Fig.8(B), N is 10 times higher than that in Fig.8(A). Because of chromophorechromophore electrostatic interactions, the EO coefficient in Fig.8(B) is lower than that in Fig.8(A). The nonlinearity of the curve of I m -V m is larger than that in Fig.8(A). We would attribute the nonlinear enhancement in Fig.8(B) to the fact that it has more number of DR1 to be instantaneously poled, so the γ i is larger than that in Fig.8(A). The instant poling effect has also been proved by measuring the EO coefficients of unpoled films. It is surprising to find that the EO coefficient of unpoled Fig.8 EO signal amplitude I m vs. modulation voltage amplitude V m in Eq.(5) (A) DR1 number density N in the poled DR1/SiO film is /cm ; (B) DR1 number density N of the poled DR1/SiO film is /cm. The symbols and solid line indicate the measured and fitted results, respectively. Fig.9 EO coefficients of poled(a) and unpoled(b) DR1/SiO films as a function of frequency of modulation electric field mearsured by simple reflection technique lgf is common logarithm of frequency; the DR1 number density N is /cm ; the symbols and solid line indicate the measurement and fitted results, respectively. The phenomena have been investigated in microscope, shown as the inset of Fig.10. As the frequency falls, the molecular vibration is enhanced, at about 500 Hz, the Al begin to melt and bladders

7 7 CHEM. RES. CHINESE UNIVERSITIES Vol.5 appear. The bubbling surface results in the fall of reflection, as shown in Fig.10. alternate modulation electric field. The experimental result indicates that unpoled EO films may exhibit a very large EO effect by the instant poling effect. It is probable to prepare EO modulator or EO probe with unpoled EO films without the process of corner poling. Fig.10 Reflection beam intensity I c of unpoled DR1/SiO films in Eq.(5) vs. the frequency of modulation electric field Symbols and solid line indicate the measurement and fitted results, respectively; the insets are microscope pictures of surface of Al films in corresponding frequency band when applied the modulation electric field in simple reflection technique; lg f is common logarithm of frequency. 4 Conclusions This paper reports a comprehensive investigation on the electro-optic properties of hybrid sol-gel silica films doped with organic Disperse Red 1 dye (DR1/SiO ). (1) We firstly introduced the orientational order parameter Ф into the formula for corona poled films by simple reflection technique to improve the accuracy of theoretical calculation. () The EO coefficient of DR1/SiO films reaches the maximum at the DR1 number density N= /cm. Further increasing N of DR1, the EO coefficients start to attenuate by the electrostatic interactions among dipole molecules. The stir temperature increase within a range of 0 60 C improves the EO effect of silica film. () The dipole molecules that do not orient along the direction of corner poling field in the film can be instantaneously poled by alternate modulation electric field. This instant poling effect results in the EO coefficients dependent on the frequency and intensity of References [1] Taylor E. W., Nichter J. E., Nash F. D., et al., Appl. Phys. Lett., 005, 86, 011 [] Song R., Yick A., Steier W. H., Appl. Phys. Lett., 007, 90, [] Hwang S. J., Yu H. H., Wang J., Opt. Commun., 004,, 41 [4] Cui Y. J., Ren H. S., Yu J. C., et al., Dyes Pigments, 009, 81, 5 [5] Chen K. X., Zhang H. B., Zhang D. M., et al., Opt. Las. Technol., 00, 7, 449 [6] Liu H. F., Yi M. B., Zhang H. B., et al., Opt. Las. Technol., 005, 7, 410 [7] Harper A. W., Sun S., Dalton L. R., et al., J. Opt. Soc. Am. B, 1998, 15, 9 [8] Robinson B. H., Dalton L. R., J. Phys. Chem. A, 000, 104, 4785 [9] Pereverzev Y. V., Prezhdo O. V., Phys. Rev. E, 000, 6, 84 [10] Pereverzev Y. V., Prezhdo O. V., Dalton L. R., Chem. Phys. Chem., 004, 5, 181 [11] Singer K. D., Kuzyk M. G., Sohn J. E., J. Opt. Soc. Am. B, 1987, 4(6), 968 [1] Shi W., Ding Y. J., Mu X. D., et al., J. Opt. Soc. Am. B, 00, 19(), 15 [1] Ma H., Liu S., Luo J. D., et al., Adv. Funct. Mater., 00, 1(9), 565 [14] Chen L. J., Zhong Q., Cui Y. J., et al., Dyes Pigments, 008, 76, 195 [15] Derose C. T., Enami Y., Loychik C., et al., Appl. Phys. Lett., 006, 89, 1110 [16] Grote J. G., Hagen J. A., Zetts J. S., et al., J. Phys. Chem. B, 004, 108, 8584 [17] Wu J. W., J. Opt. Soc. Am. B, 1991, 8(1), 14 [18] Pedersen T. G., Jespersen K., Johansen P. M., et al., J. Opt. Soc. Am. B, 00, 19(11), 6 [19] Franco A., Guadalupe V. A., Jorge G. M., Opt. Mater., 007, 9, 814 [0] Teng C. C., Man H. T., Appl. Phys. Lett., 1990, 56, 174 [1] Shuto Y., Amano M., J. Appl. Phys., 1995, 77, 46 [] Page R. H., Jurich M. C., Reck B., et al., J. Opt. Soc. Am. B, 1990, 7, 19 [] London F., Trans. Faraday Soc., 197,, 8 [4] Felderhof B. U., Deutch J. M., J. Chem. Phys., 1975, 6(6), 91