Nonlinear optical properties (SHG, THG) of N-(4-nitrophenyl)-l-prolinol doped alumina film prepared by a sol-gel process

ELSEVIER

Thin Solid Films 283 (1996) 221-225

Nonlinear optical properties (SHG, THG) of N-(4-nitrophenyl)-Lprolinol doped alumina film prepared by a sol-gel process Yosuke Hosoya ", Shinji Ohsugi b, Shinzo Muto b, Yoichi Kurokawa " * aDepartment of Engineering Science, Faculty of Engineering, Tohoku University.Sendai 980, Japan b Department of Electrical Engineering and Computer Science, Facultyof Engineering, Yamanashi University, Takeda, Kofu 400, Japan Received 15 August 1995; accepted 24 November 1995

Abstract

Orientationally ordered microcrystallites of N-(4-nitrophenyl)-L-prolinol (NPP) were formed by incorporating NPP molecules in an alumina film that possesses a card pack structure which is derived througha sol-gel process. The resulting alumina film exhibitsenhancement of second and third order nonlinear optical effects. The optimum conditions necessary to prepare the NPP-doped film were determinedfrom measured enhancement. Keywords: Aluminium oxide; Amorphous materials; Optical properties; Second harmonic

1. Introduction

There have been many investigations of nonlinear optical (NLO) organic materials [ 1 ]. However, it is generally difficult to grow NLO molecules in a macroscopic large crystalline form. Therefore, transparent films doped with NLO materials appear to have promise for integrated optical and waveguide applications. The techniques to enhance NLO properties are not yet well established. The most practical method is to apply an electric field to a glassy polymer film doped with NLO molecules [2]. In this method, the doped film is heated to near the glass transition temperature, where the dopants are mobile and susceptible to orientation in an applied electric field. Then the film is cooled in the electric field to fix the polar alignment. The orientation of dopant allows enhancement of second harmonic generation (SHG). However, the SHG intensity decays with time due to the relaxation of the matrix [2]. On the other hand, it is reported that the intercalation of NLO molecules into a host layer leads to an orientated arrangement of the molecules [ 3 ]. But the intercalation is limited and the enhancement is small. Several approaches have been taken to prepare hybrid gels comprised of organic and inorganic components for optical materials [4]. 2-Methyl-4-nitroaniline (MNA) was incorporated in SiO2-polymethyl methacrylate. The composite retained optical quality at higher temperatures and was more resistant to abrasion [ 5 ]. * Corresponding author. 0040-60901961515.00 © 1996 Elsevier Science S.A. All rights reserved SSDI0040-6090(95 )08501-7

In a previous paper, it was shown that a poled MNA-doped alumina film enhanced SHG intensity [6]. It was also found that there occurred an intensity decay with time of a lower magnitude than that observed in a polymer film. Also, it was previously reported that an SHG active alumina film was obtained by aligning the dopant N-(4-nitrophenyl)-L-prolinol (NPP) in the matrix using a sol-gel process [7]. In the present work, the authors ha ve attempted to optimize the SHG enhancement of an NPP doped alumina film by varying the preparation condition of the sol-gel method, the THG activity of such films was also investigated. The effects of the alumina matrix on alignment of NPP microerystallites are also discussed.

2. Experimental The route to preparing the alumina sol was the same as that previously reported [6]. A hydrous alumina precipitate was obtained by reaction of an aqueous AICI3 solution (hexahydrate salt was used) with an NH3 containing solution. The white precipitate was then filtered and washed with puie water. It was then peptized with acetic acid under reflux at 80 °C for 8 h. Alumina sol has an iso-electric point at about pH 9.5. Therefore, a cationic surface active reagent must be used to solubilize the NLO organic substance into the sol, because an anionic reagent causes rapid aggregation of the sol. NPP was chosen as a dopant because of its potential

222

Y.Hosoya etal. I Thin Solid Films 283 (1996) 221-225

utility to exhibit NLO strong behavior [ 8]. The powder SHG efficiency for NPP is 50 ~ 100 times greater than that for urea and 5 times larger than MNA, though it is difficult to grow in bulk form. The mixture of sol, cetyltrimethylammonium bromide (CTAB) as a surface active reagent, and NPP was prepared at a fixed proportion by weight. The mixed sol solution was converted into a transparent gel film by slow dehydration at room temperature or 70 °C, and then the gel film was annealed at 100 or 130 °C. The doped film has a uniform yellow-orange color which pales slightly as the NPP loading is lowered. The absorption spectra of NPP solution or NPP doped alumina film were recorded using a Shimadzu UV-2200 spectrometer. X-ray diffraction measurements were taken using CuKa radiation with a Ni filter (30 kV, 20 mA). The second order effective NLO coefficient, d~ff,and third order NLO susceptibility, X t3), of the NPP-doped alumina films (NPP/AI203) were determined by the Maker fringe method [9]. For SHG and THG measurements, the experimental equipment including a Q-switched Nd-YAG laser was similar to that previously used by us [ 10]. The laser system is schematically shown in Fig. 1. The laser beam was split into two beams, one to the sample and the other to monitor the fundamental input power. The sample was set on a rotating stage. The path length of the sample and the reference was varied by rotating the angle 0, the signal was monitored as a function of the interaction length to obtain the fringe. From these fringes, the SHG coefficient or the thirdorder nonlinear susceptibility could be determined by tomp: "ing the signal intensity and the coherence length with the reference. For a reference, we used the value of dr| (quartz) •0.50 pm V - t ( = 1.2× 10 -9 esu) for SHG and X(3)(NaCl) = 8 . 0 × 10 -23 m2V -2 (5.7× 10 - ' s esu) for THG. The values of refractive index were determined by interpolation by applying Sellmeir's dispersion equation to the values obtained using Abbe's refractometer at some laser wavelengths (He.-Cd laser:441.6 nm, He-No laser: 543,594, 632 nm, Nd-YAG laser: 1064 nm). They were used for the fitting of the SHG and THG Maker fringe curves.

3. Results and discussion Nitroaniline derivatives are among the most widely investigated organic substance in NLO materials. NPP is a pnitroaniline-like substance which has a ~'-electronic aromatic system involving donor (prolinol) and accepter (nitro) substituent groups and its properties relate to those of MNA analogs [ 111. Therefore, NPP displays an intense intramolecular charge transfer absorption band in the UV-VIS range, as shown in Fig. 2. It shows the characteristic band which is dependent on the polarity of solvent, as shown. The absorption is blue-shifted as the polarity of the medium decreases. It can be seen from the figure, that the polarity around the NPP in the doped film is between that of H20 and EtOH, that is, NPP exists in the polar hydrogen-bonding solid state environment. This means that NPP molecule forms a microcrystal, in which the chirality and the hydrogen bonding group of NPP induce a non-centrosymmetric type of packing that is significant for the enhancement of NLO properties. The doped film does not absorb the second or third order harmonic wavelengths generated upon irradiation with the 1064 nm Nd-YAG fundamental. CTAB was used to solve NPP into the sol. However, the addition of a large amount of NPP or CTAB to the sol reduces the strength of the doped filth. F ~r such a case, the doped film becomes cloudy when the crystallization of NPP proceeds. Optimizing the microstructure of a doped film to yield tile maximum NLO properties involves control of the AI~O3:CTAB:NPP component ratio and annealing conditions. Annealing of the doped film causes the migration of NPP within the matrix, and then cooling results in its aggregation and crystallization. The extent of this behavior depends on the above factors. From previous preliminary investigation [7], it is considered that an appropriate component ratio of NPP:CTAB is 1:2 ~ 6 by weight to obtain a homogeneous transparent film. Fig. 3(a) and (b) show XRD patterns of doped films. The doped film without annealing shows no XRD peaks. This may indicate that NPP molecules are finely distributed in the

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wt.%; a number of smaller crystallites than the case of 100 °C are then formed upon cooling. At lower loading, NPP is not saturated at 130 °C, NPP crystallization is ordinarily promoted on cooling and large crystallites are formed, as for material derived at i00 °C. Therefore, the optimum amount of NPP loading to form a ( 101 ) orientation shifts to the lower loading side compared to samples annealed at 100 °C. As shown in Fig. 4, the fringe pattern, that is, the dependence of SHG intensity on the incident angle of the fundamental beam of the doped film, was in good agreement with the calculation from the theoretical expression of Jerphagov and Kurtz [9]. Fig. 5 shows the NPP composition on den-of the doped film at a fixed ratio of NPP:CTAB = 1:2.5. The behavior is in fair agreement with that of ( 101 ) peak in Fig. 3. The higher the intensity of the ( 101 ) peak, the SHG becomes more intense. The d~rrincreases with increasing NPP and then decreases with further loading. SHG enhancement requires NLO crystallite to be noncentrosymmetric structure. In the case of NPP, the plate-like crystal, in which the mean plane of the molecule is adjacent to the crystallographic (101) plane, optimizes molecular arrangement. On the other hand, the host, boehmite alumina,

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definite ratio of CTAB/NPP= 1:2.5. (a) doped films annealed in air at 100 *Cfor 2 h, (b) dopedfilmsannealedin air at 130 *Cfor 9.h. CTAB micelle. By annealing the film at 100 °C, which is a lower temperature than the melting point ( 116 --, ! 17 °C) of NPP and then cooling to room temperature, the NPP microcrystalite grew by an Ostwald ripening effect and gave the XRD peak. In this process, the gel film shrinks by thermal dehydration and the micelles are highly distorted, which may induce the NPP molecules to crystallize. It seems that NPP microerystallites in the film exhibit a preferred crystalline (101) orientation of an ortho-rhombic structure. Its X-ray peak intensity does not necessarily increase with loading. The highest peak is obtained in the doped film of NPP:3.5 wt.%, CTAB:8.75 wt.%. NPP tends to take a plate crystal form. Therefore, NPP molecules can be expected to be stacked lying parallel to alumina layers, which will be described later. Upon treating the doped film at the higher temperature of 130 °C, which is above the melting point of NPP, the XRD peak as a function of NPP loading is different from that derived at 100 °C. The appearance of the peak as a whole is weak compared to Fig. 3(a) and the peak height reaches a maximum at lower loading than the film treated at 100 °C. This can be ascribed to the fact that more NPP becomes melted at 130 °C, and molecularly distributed into the doped film. NPP is supersaturated in a film doped at higher loading than NPP:-" 3

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224

K Hosoya et al. / Thin Solid Films 283 (1996) 221-225

is generally characterized by a card-pack structure, though our alumina is the more amorphous 'pseudo' boehmite. Therefore, it can be predicted that the alumina layer has an effect on the NPP molecule to induce stacking and form the plate-like microcrystal lying parallel to the alumina layer. Such an effect will depend upon the proportions of NPP and CTAB. In the case of lower loading of NPP, the field within the alumina layer can effectively act on the thermally activated NPP and induce the orientation of NPP. But, too much thermal activation of NPP causes fracture of the microstructure of alumina. As a result, the NPP arrangement becomes more random, and the dipole moment of the molecule is cancelled out, which causes SHG intensity to taper off at the higher loading, as shown in Fig. 5. Recently, an interest has arisen in third-order NLO materials such as semiconductor particle doped glass, polysilane or polydiacetylene [ 1]. In the case of macromolecules, X(3), third-order nonlinear susceptibility, increases with an increase of the length of the ~r-electron conjugated chain. Investigations of low-molecular weight NLO materials are rarely found in the literature because they have lower susceptibilities compared to NLO macromolecules. Third order NLO properties can be observed even in optically isotropic materials in which the dipole moment is cancelled out, and it has been thought that third-order properties are independent of molecular orientation. However, it was recently reported that ,,(c3) depends on the conformation within the overall structure in the case of polydiacetylene [ 12]. Fig. 6 shows the typical fringe patterns of THG intensity. The observed THG does not reach to absolute zero, as shown. This is due to THG from the matrix itself and the influence of the absorption loss in THG. The film annealed at 100 °C gives a larger effect than that annealed at 130 °C. This corresponds to the behavior of SHG intensity in which the ordered doped film gives the higher intensity. Fig. 7 shows the dependence of X (3) on the NPP composition. The X TM values of films which are gelled at room temperature and annealed at 100 or 130 °C, increase with NPP loading at low composition, and decrease through a maximum at 2.5 ~ 3.5 wt.% NPP. As shown in Fig. 3, this behavior corresponds to the intensity of the NPP (101) peak as in the case of SHG. - NPP/AItOs film J NPP:3wtX.CTAB:?.SatZ / path length L=II5pB

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The higher degree of NPP molecule orientated, the more intense THG is observed. THG was also investigated for the films gelled at 70 °C and annealed at 100 °C for 2 h as shown in Fig. 7. These films gave no XRD peak, and a relatively homogeneous dispersion of NPP molecule was indicated. The X(3) of these films also increased with NPP loading, but the maximum which is observed in the case of the former samples (gelled at room temp., annealed at 100 or 130 °C) is not observed and the intensity is one third of that of the former samples. At higher loading, more than 4 wt.%, it seems that the X(3) do not depend on preparation conditions but depend on N I P content alone. This can be attributed to the random arrangement of NPP caused by over-loading, as described in the case of SHG. The analogy between the behavior of SHG and THG for the doped film may be explained by the cascade process of SHG. In the range of the ordered NPP doped film, an enhancement of THG is possibly not due to a direct THG process (oJ + ~o+ co- 3= 3 ~ 3.5 m 2 V -e ( = ~ 2.5 X 10- ,3 esu) were obtained for SHG and THG, respectively. There are no data which can be directly compared with these coefficients. The SHG coefficient is comparable to powder SHG coefficients ofMNA ( ~ 8 × 10 -s esu) and p-nitroaniline intercalated into/3-cyclodextrin ( ~ 10 -s esu) [ 13]. However, the third-order susceptibility is much lower than that of polydiacetylene which has the highest coefficient ( ~ 10-m, esu) among organic NLO materials [ 14]. The degradation of the NLO properties or crystallite orientation in time was not observed. In this NPP doped alumina, NPP molecule is fixed in microcrystal of itself and the microcrystal is physically fixed in rigid alumina layer. Therefore, 4x10-21 I '

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Y. Hosoya et al. /Thin Sofid Films 283 (1996) 221-225

the orientation is not relaxed and NLO properties are preserved.

4. Conclusion NPP (N-(4-nitrophenyl)-L-prolinol) has been orientationally doped into an alumina film using a sol-gel method without applying an electric field. Resultant films showed enhancements of SHG and THG. Enhancements are due to alignment of NPP microcrystallites induced by a formation of an oriented alumina film in which the plate-shaped pseudoboehmite alumina particles are stacked in a card-pack structure.

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[3] S. Cooper and P.K. Dutta, J. Phys. Chem., 94 (1990) 114. [4] B. Dunn and J.I. Zink, J. Mater. Chem., 1 ( 1991 ) 903; L.C. Klein, Annu. Rev. Mater. Sci., 23 (1993) 437. [5] T.M. Che, R.V. Comey, G. Khanarian, R.A. Keosiuan and M. Borzo, J. Non-Cryst. Solids, 102 (1988) 280. [6] Y. Kobayashi, S. Muto, A. Matsuzaki and Y. Kurokawa, Thin Solid Films, 213 (1992) 126. [7] Y. Hosoya, S. Muto, T. Ohsugi and Y. Kurokawa, Thin Solid Films, 256 (1995) 4. [8] J. Zyss, J.F. Nicoud and M. Coquillay, J. Chem. Phys., 81 (1984) 4160. [9] S.K. Kurtz, in F.T. Arcchi and E.O. Schulz-Dubois (eds.), Laser Handbook, Vol. I, North-Holland, 1972, p. 923; J. Jerphagov and S.K. Kurtz, Phys. Rev. B, 1 (1970) 1739. [10] S. Muto, T. Okada and H. Itoh, Proc. SPIE, Nonlinear Optical Properties of Organic Materials I/1, Vol. 2025, 1993 p. 436; S. Muto and M. Kumimura, Kogaku (Optics), 20 ( 1991 ) 164 (in Japanese). [ 11 ] L. Ledoux, J. Zyss, A. Migus, J. Etchepare, G. Grillon and A. Antonetti, Appi. Phys. Len.o 48 (1986) 1564. [ ! 2 ] C. Halvorson, T.W. Hagler, D. Moses, Y. Kan and A.J. Heeger, Synth. Met., 55-57 (1993) 3961. [ 13] D.F. Eaton, A.G. Anderson, W. Tom and Y. Wang, J. Am. Chem. Soc., 109 (1987) 1886. [14] S. Etemad and Z.G. Soos, in R.J.H. Clark and R.E. Hester (eds.), Spectroscopy of Advanced Materials, Wiley, New York, 1991, p. 87.