- Email: [email protected]
High-quality crystal growth of organic nonlinear optical crystal DAST
Journal of Crystal Growth 237–239 (2002) 2104–2106
High-quality crystal growth of organic nonlinear optical crystal DAST F. Tsunesada*, T. Iwai, T. Watanabe, H. Adachi, M. Yoshimura, Y. Mori, T. Sasaki Department of Electrical Engineering, Osaka University, 2-1 Yamadaoka Suita, Osaka 565-0871, Japan
Abstract Methods for growing high-quality nonlinear optical 4-dimethylamino-N-methyl-4-stilbazolium tosylate (DAST) crystals have been developed. In conventional growth methods, it was difficult to control the position of spontaneous nucleation and the nucleation period. Difficulty in the nucleation and growth positioning was effectively solved by the slope nucleation method (SNM), and the nucleation period could be controlled by the laser irradiation method (LIM). In this study, we could obtain a high-quality DAST crystal by combining SNM and LIM. This crystal had a full-width at half-maximum (FWHM) of the X-ray diffraction rocking curve of 18.7 arcsec, compared with 50–130 arcsec FWHM for conventional crystals. r 2002 Elsevier Science B.V. All rights reserved. PACS: 42.70.M; 81.10.A; 42.70.J Keywords: A1. Nucleation; A1. Supersaturated solutions; A1. X-ray diffraction; A2. Growth from solutions; B1. Organic compounds; B2. Nonlinear optic materials; B3. Nonlinear optical
1. Introduction Organic nonlinear optical (NLO) materials have been intensely investigated due to their potentially high nonlinearities and rapid electro-optic effect response compared to inorganic NLO materials. For example, 4-dimethylamino-N-methyl-4-stilbazolium tosylate (DAST) crystal [1] has an extremely large electro-optic coefficient of r111 ¼ 7778 pm/V [2]. DAST crystals can be expected to be used for optical-sampling and *Corresponding author. Tel.: +81-6-6879-7707; fax: +81-66879-7708. E-mail address: [email protected] (F. Tsunesada).
electro-optical (EO) sampling devices through frequency conversion or EO effects. However, DAST crystals are not used for these applications in spite of their excellent properties because it is difficult to grow crystals with sufficient quality. It is important to grow high-quality, sufficiently large crystals for practical applications. Generally, organic crystals are grown by obtaining seed crystals from spontaneous nucleation then growing a single crystal by using that seed crystal. In this process, we must remove the seed crystal from the solution and fix it onto a seed rod. Furthermore, it is not easy to grow high-quality crystals. It may be more convenient to grow crystals directly from spontaneously nucleated crystals. However, it is impossible to control the
0022-0248/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 0 1 ) 0 2 2 6 6 - 7
F. Tsunesada et al. / Journal of Crystal Growth 237–239 (2002) 2104–2106
2105
position and period of nucleation by this method. To grow high-quality crystals, it is essential to solve these problems. We have therefore developed a new technique applying both the slope nucleation method and the laser irradiation method. The former determines the position of nucleation and growth; the latter controls the associated period. By using this technique, we could obtain highquality DAST crystals.
2. Slope nucleation method Because DAST crystals tend to become planar crystals with the (0 0 1) face as the largest facet, spontaneous nuclei are grown with their (0 0 1) facets contacting the bottom of the growth vessel. Additionally, spontaneously generated nuclei closely contact each other because of uncertainty in the position, which increases the possibility of DAST crystals becoming poly crystals. Ideally, a crystal should grow without its (0 0 1) facet ever contacting the vessel. To achieve this, we investigated the slope nucleation method (SNM). The key point of SNM is that a Teflon plate with grooves is inserted into the growth vessel. Except for this, the process is the same as in the conventional spontaneous nucleus growth. The crystal can grow at the grooves while standing erect (Fig. 1). By using this method, we could obtain many crystals simultaneously in one process. To evaluate the crystallinity, we measured the FWHM of the XRD rocking curve and found it to be 20.2–57.5 arcsec. The result indicates that the slope nucleation method is more effective for obtaining high-quality DAST crystals than the conventional methods, as the FWHM of crystals grown by traditional methods is 50–130 arcsec.
3. Improvement by combining the laser irradiation method with SNM By introducing SNM, we could grow higher quality DAST crystals than by the conventional growth method. Nevertheless, the quality was not sufficient for practical applications, such as EO sampling devices. Although SNM is a useful
Fig. 1. Growth process in the slope nucleation method (SNM) for DAST crystals. (1) Spontaneous nuclei in the solution condensed onto the Teflon plate. (2) Slipping of DAST crystals on the Teflon Plate. (3) DAST crystals erected on the grooves. (4) Continuous growth of DAST crystals on the grooves.
method for controlling the growth position of spontaneously nucleated DAST crystals, it cannot control the period of nucleation. Normally, in DAST solution, spontaneous nucleation occurs at temperatures below the saturation point (i.e., the highly supersaturated region). The nuclei generated spontaneously in a highly supersaturated solution grow rapidly after nucleation, leading to poor crystallinity. If the nuclei can be spontaneously generated in a less supersaturated solution by triggering nucleation, the crystal would grow slowly and would be expected to have high crystallinity. Various external factors can trigger nucleation. We succeeded in inducing nucleation in a less supersaturated solution by laser irradiation. We report the results of this experiment below. We first stirred the solution and kept it above the saturation point for several hours. We next cooled the solution to slightly below the saturation point and stopped stirring. While maintaining that temperature, we exposed the solution to a
2106
F. Tsunesada et al. / Journal of Crystal Growth 237–239 (2002) 2104–2106
example, DAST crystals became polycrystals when exposed to excessive laser irradiation. Reducing the laser energy resulted in more effective generation of single crystals. Fig. 3 shows a typical DAST crystal grown at a cooling rate of 0.11C/ day for 4 days. This crystal exhibits a FWHM of 18.7 arcsec, which indicates that the crystallinity of DAST crystals grown by this technique exceeds that of crystals grown from normal spontaneous nucleation in highly supersaturated solutions. Fig. 2. Relationship between the degree of supersaturation and the time until nucleation could be observed after laser irradiation.
4. Conclusion
Fig. 3. DAST crystal grown by new technique.
Q-switched Nd:YAG laser beam with a wavelength of 1064 nm. The measured pulse of this laser was 23 ns, and the size of spot at the focus was 1.67 104 cm2. We next varied the degree of supersaturation, number of laser pulses, and laser energy. Fig. 2 shows the relation between the degree of supersaturation and the time until nucleation could be observed after laser irradiation. Without laser irradiation, spontaneous nucleation usually does not occur in supersaturated solutions of o20%. However, we were surprised to find that laser irradiation could induce nucleation in such a low-supersaturation region. The number of nucleated crystals and their quality strongly depend on the laser irradiation. For
High-quality DAST crystals have been grown by the slope nucleation method that effectively controls the position of crystal growth after spontaneous nucleation. However, the growth from the spontaneous nucleation increased the growth speed because the nucleation occurred in a highly supersaturated solution. To generate nucleation intentionally in a low-supersaturation solution, we combined laser light (1064 nm) irradiation of the solution with the slope nucleation method. The DAST crystal obtained by this technique had a full-width at half-maximum (FWHM) of the X-ray diffraction rocking curve of 18.7 arcsec, compared with 50–130 arcsec FWHM for the conventional crystals. By thus controlling the growth conditions thus we could obtain high-quality DAST crystals.
References [1] H. Nakanishi, H. Matsuda, S. Okada, M. Kato, Res. Soc. Int. Mtg. Adv. Mater. 1 (1989) 97. . [2] F. Pan, G. Knofle, Ch. Bosshard, S. Follonier, R. Spreiter, M.S. Wong, P. Gunter, . Appl. Phys. Lett. 69 (1996) 13.
High-quality crystal growth of organic nonlinear optical crystal DAST F. Tsunesada*, T. Iwai, T. Watanabe, H. Adachi, M. Yoshimura, Y. Mori, T. Sasaki Department of Electrical Engineering, Osaka University, 2-1 Yamadaoka Suita, Osaka 565-0871, Japan
Abstract Methods for growing high-quality nonlinear optical 4-dimethylamino-N-methyl-4-stilbazolium tosylate (DAST) crystals have been developed. In conventional growth methods, it was difficult to control the position of spontaneous nucleation and the nucleation period. Difficulty in the nucleation and growth positioning was effectively solved by the slope nucleation method (SNM), and the nucleation period could be controlled by the laser irradiation method (LIM). In this study, we could obtain a high-quality DAST crystal by combining SNM and LIM. This crystal had a full-width at half-maximum (FWHM) of the X-ray diffraction rocking curve of 18.7 arcsec, compared with 50–130 arcsec FWHM for conventional crystals. r 2002 Elsevier Science B.V. All rights reserved. PACS: 42.70.M; 81.10.A; 42.70.J Keywords: A1. Nucleation; A1. Supersaturated solutions; A1. X-ray diffraction; A2. Growth from solutions; B1. Organic compounds; B2. Nonlinear optic materials; B3. Nonlinear optical
1. Introduction Organic nonlinear optical (NLO) materials have been intensely investigated due to their potentially high nonlinearities and rapid electro-optic effect response compared to inorganic NLO materials. For example, 4-dimethylamino-N-methyl-4-stilbazolium tosylate (DAST) crystal [1] has an extremely large electro-optic coefficient of r111 ¼ 7778 pm/V [2]. DAST crystals can be expected to be used for optical-sampling and *Corresponding author. Tel.: +81-6-6879-7707; fax: +81-66879-7708. E-mail address: [email protected] (F. Tsunesada).
electro-optical (EO) sampling devices through frequency conversion or EO effects. However, DAST crystals are not used for these applications in spite of their excellent properties because it is difficult to grow crystals with sufficient quality. It is important to grow high-quality, sufficiently large crystals for practical applications. Generally, organic crystals are grown by obtaining seed crystals from spontaneous nucleation then growing a single crystal by using that seed crystal. In this process, we must remove the seed crystal from the solution and fix it onto a seed rod. Furthermore, it is not easy to grow high-quality crystals. It may be more convenient to grow crystals directly from spontaneously nucleated crystals. However, it is impossible to control the
0022-0248/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 0 1 ) 0 2 2 6 6 - 7
F. Tsunesada et al. / Journal of Crystal Growth 237–239 (2002) 2104–2106
2105
position and period of nucleation by this method. To grow high-quality crystals, it is essential to solve these problems. We have therefore developed a new technique applying both the slope nucleation method and the laser irradiation method. The former determines the position of nucleation and growth; the latter controls the associated period. By using this technique, we could obtain highquality DAST crystals.
2. Slope nucleation method Because DAST crystals tend to become planar crystals with the (0 0 1) face as the largest facet, spontaneous nuclei are grown with their (0 0 1) facets contacting the bottom of the growth vessel. Additionally, spontaneously generated nuclei closely contact each other because of uncertainty in the position, which increases the possibility of DAST crystals becoming poly crystals. Ideally, a crystal should grow without its (0 0 1) facet ever contacting the vessel. To achieve this, we investigated the slope nucleation method (SNM). The key point of SNM is that a Teflon plate with grooves is inserted into the growth vessel. Except for this, the process is the same as in the conventional spontaneous nucleus growth. The crystal can grow at the grooves while standing erect (Fig. 1). By using this method, we could obtain many crystals simultaneously in one process. To evaluate the crystallinity, we measured the FWHM of the XRD rocking curve and found it to be 20.2–57.5 arcsec. The result indicates that the slope nucleation method is more effective for obtaining high-quality DAST crystals than the conventional methods, as the FWHM of crystals grown by traditional methods is 50–130 arcsec.
3. Improvement by combining the laser irradiation method with SNM By introducing SNM, we could grow higher quality DAST crystals than by the conventional growth method. Nevertheless, the quality was not sufficient for practical applications, such as EO sampling devices. Although SNM is a useful
Fig. 1. Growth process in the slope nucleation method (SNM) for DAST crystals. (1) Spontaneous nuclei in the solution condensed onto the Teflon plate. (2) Slipping of DAST crystals on the Teflon Plate. (3) DAST crystals erected on the grooves. (4) Continuous growth of DAST crystals on the grooves.
method for controlling the growth position of spontaneously nucleated DAST crystals, it cannot control the period of nucleation. Normally, in DAST solution, spontaneous nucleation occurs at temperatures below the saturation point (i.e., the highly supersaturated region). The nuclei generated spontaneously in a highly supersaturated solution grow rapidly after nucleation, leading to poor crystallinity. If the nuclei can be spontaneously generated in a less supersaturated solution by triggering nucleation, the crystal would grow slowly and would be expected to have high crystallinity. Various external factors can trigger nucleation. We succeeded in inducing nucleation in a less supersaturated solution by laser irradiation. We report the results of this experiment below. We first stirred the solution and kept it above the saturation point for several hours. We next cooled the solution to slightly below the saturation point and stopped stirring. While maintaining that temperature, we exposed the solution to a
2106
F. Tsunesada et al. / Journal of Crystal Growth 237–239 (2002) 2104–2106
example, DAST crystals became polycrystals when exposed to excessive laser irradiation. Reducing the laser energy resulted in more effective generation of single crystals. Fig. 3 shows a typical DAST crystal grown at a cooling rate of 0.11C/ day for 4 days. This crystal exhibits a FWHM of 18.7 arcsec, which indicates that the crystallinity of DAST crystals grown by this technique exceeds that of crystals grown from normal spontaneous nucleation in highly supersaturated solutions. Fig. 2. Relationship between the degree of supersaturation and the time until nucleation could be observed after laser irradiation.
4. Conclusion
Fig. 3. DAST crystal grown by new technique.
Q-switched Nd:YAG laser beam with a wavelength of 1064 nm. The measured pulse of this laser was 23 ns, and the size of spot at the focus was 1.67 104 cm2. We next varied the degree of supersaturation, number of laser pulses, and laser energy. Fig. 2 shows the relation between the degree of supersaturation and the time until nucleation could be observed after laser irradiation. Without laser irradiation, spontaneous nucleation usually does not occur in supersaturated solutions of o20%. However, we were surprised to find that laser irradiation could induce nucleation in such a low-supersaturation region. The number of nucleated crystals and their quality strongly depend on the laser irradiation. For
High-quality DAST crystals have been grown by the slope nucleation method that effectively controls the position of crystal growth after spontaneous nucleation. However, the growth from the spontaneous nucleation increased the growth speed because the nucleation occurred in a highly supersaturated solution. To generate nucleation intentionally in a low-supersaturation solution, we combined laser light (1064 nm) irradiation of the solution with the slope nucleation method. The DAST crystal obtained by this technique had a full-width at half-maximum (FWHM) of the X-ray diffraction rocking curve of 18.7 arcsec, compared with 50–130 arcsec FWHM for the conventional crystals. By thus controlling the growth conditions thus we could obtain high-quality DAST crystals.
References [1] H. Nakanishi, H. Matsuda, S. Okada, M. Kato, Res. Soc. Int. Mtg. Adv. Mater. 1 (1989) 97. . [2] F. Pan, G. Knofle, Ch. Bosshard, S. Follonier, R. Spreiter, M.S. Wong, P. Gunter, . Appl. Phys. Lett. 69 (1996) 13.