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Spatially selected crystallization in glass by YAG laser irradiation
Journal of Non-Crystalline Solids 345&346 (2004) 127–131 www.elsevier.com/locate/jnoncrysol
Spatially selected crystallization in glass by YAG laser irradiation Tsuyoshi Honma a, Yasuhiko Benino a, Takumi Fujiwara a, Ryuji Sato b, Takayuki Komatsu a,* a
Department of Chemistry, Nagaoka University of Technology, 1603-1 Kamitomioka-cho, Nagaoka 940-2188, Japan b Department of Materials Engineering, Tsuruoka National College of Technology, Tsuruoka 997-8511, Japan Available online 12 September 2004
Abstract A continuous wave (cw) Nd:YAG laser operating at a wavelength of 1064 nm irradiated Sm2O3- and Dy2O3-doped glasses under various irradiation conditions in the systems of Ln2O3–BaO–B2O3 (Ln:Sm and Dy), and non-linear optical b-BaB2O4 (b-BBO) crystals lines are formed at the surface of glass substrates. It is proposed from the azimuthal dependence of second harmonic intensity that b-BBO crystal lines might be single crystals with the c-axis orientation along the laser scanning direction. The photoluminescence spectra indicate that Sm3+ ions are incorporated into b-BBO crystal lines. The present study proposes that cw Nd:YAG laser irradiation to glass containing Sm3+ and Dy3+ is a novel technique for a spatially selected crystallization in glass. 2004 Elsevier B.V. All rights reserved. PACS: 42.65.Ky; 42.70.Nq; 81.05.Kf; 81.05.Pj
1. Introduction Laser irradiation of glass materials has received much attention, because this technique has been regarded as a new processing technique for spatially selected structural modification and/or crystallization in glass [1–4]. Recently, Sato et al. [5] have reported that crystal dots consisting of the Sm2Te6O15 phase are induced by irradiation of a continuous wave (cw) Nd:YAG laser operating at a wavelength of k = 1064 nm in RO–Sm2O3– TeO2 (R:Mg, Ba) glasses. Since Nd:YAG laser is more conventional compared with other kinds of lasers such as short-wavelength excimer laser, CO2 gas laser and femtosecond pulsed laser, it is of interest to check the effect of Nd:YAG laser irradiation on various glass systems and to clarify the mechanism of YAG laserinduced crystallization. Honma et al. [6–8] have found that the irradiation of Nd:YAG laser with k = 1064 nm *
Corresponding author. Tel.: +81 258 47 9313; fax: +81 258 47 9300. E-mail address: [email protected] (T. Komatsu). 0022-3093/$ - see front matter 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2004.08.009
induces the formation of crystal dots and lines showing a second harmonic generation (SHG) in Sm2O3–Bi2O3– B2O3 glasses. Tanaka et al. [9] have applied the YAG laser irradiation technique to Sm2O3–BaO–B2O3 glasses and found that b-BaB2O4 (b-BBO) crystal dots showing SHGs are formed. It has been proposed that cw Nd:YAG laser irradiations to glass containing Sm3+ 6 ions cause continuous f–f transitions (6F9/2 H5/2) in 3+ Sm and continuous electron–phonon couplings (i.e., non-radiative relaxation), consequently inducing thermal effects [5]. That is, in the YAG laser irradiation processing, local regions centered around Sm3+ ions are selectively heated. This technique might be, therefore, called selective (samarium) atom heat processing. In this study, we apply the YAG laser irradiation technique to Sm2O3 (or Dy2O3)–BaO–B2O3 glasses and try to fabricate non-linear optical b-BBO crystal lines. In particular, we examine whether b-BBO crystal lines written by YAG laser irradiation are single-crystal lines or polycrystal lines. We choose b-BBO crystals by the following reasons: b-BBO developed by Chen et al. [10] is one of the most important non-linear optical
T. Honma et al. / Journal of Non-Crystalline Solids 345&346 (2004) 127–131
crystals in laser optics, and its crystal structure is wellknown [11]. Its remarkable features include a large second-order optical non-linearity of d11 = 2.22 pm/V, the wide optical transmittance range (190–3500 nm) and a high laser damage threshold [12].
2. Experimental procedure The glass compositions examined in this study are 10Sm2O3 Æ 40BaO Æ 50B2O3 and 10Dy2O3 Æ 45BaO Æ 45B2O3. Commercial powders of reagent grade (99.9% purity) Sm2O3, Dy2O3, BaCO3, B2O3 were melted in a platinum crucible at 1200 C for 40 min in an electric furnace. The melts were poured on to an iron plate and pressed to a thickness of about 1.5 mm by another iron plate. The glass transition, Tg, and crystallization onset, Tx, temperatures were determined using differential thermal analysis (DTA) (Seiko Instruments Inc.; TG/ DTA320) at a heating rate of 10 K/min, in which glass powders with the particle size of 30–100 lm were used. Optical absorption spectra were taken in the wavelength range of 190–3200 nm on Shimadzu UV-3150 spectrometer. The glasses were mechanically polished to a mirror finish with CeO2 powders. A cw Nd:YAG laser with k = 1064 nm irradiated the surface of the glasses using an objective lens (20 and 60·), and the sample stage was automatically moved during laser irradiations to write crystal lines. The crystal lines written by YAG laser irradiations were observed with a polarization optical microscope. The formation of the b-BBO crystalline phase was confirmed using X-ray diffraction (XRD) analysis (CuKa radiation) at room temperature. The second harmonic (SH) intensity of crystal lines was measured by using a fundamental wave of Q-switched Nd:YAG laser with k = 1064 nm as a laser source, in which linearly polarized fundamental laser beams were introduced into crystal lines perpendicularly and the azimuthal dependence of SHG signals was measured by rotating the sample. A y-cut a-quartz (0.6 mm thickness) crystal plate was used as a reference of SH intensity. Micro-Raman and photoluminescence spectra of b-BBO crystal lines were measured with a three-dimensional spatially resolved laser microscope (Tokyo Instruments. Co. Nanofinder) operated at Ar+ (488 nm) laser.
(10Dy:BBO glass) shows Tg = 554 ± 2 C and Tx = 682 ± 2 C. We confirmed that the crystalline phase obtained by a conventional heat treatment in an electric furnace is assigned to only the b-BaB2O4(ICDD# 850914) phase. The optical absorption spectra at room temperature are shown in Fig. 1, giving the absorption 6 coefficients at 1064 nm of 7.9 ± 0.2 cm1 (6F9/2 H5/2) 1 6 for 10Sm:BBO glass and of 4.5 ± 0.2 cm ( F9/2 6 H15/2) for 10Dy:BBO glass. 10Sm:BBO glass has an optical absorption coefficient of 7.3 ± 0.2 cm1 even at 250 C. In order to write crystal lines easily and stably in the glasses, the following techniques were used. At first, to avoid the fracture of glasses due to the thermal shock during laser irradiations, the glasses were heated at about 150 C using a hotplate. Secondly, to form the crystal nuclei of b-BBO easily, b-BBO crystal grains with a diameter of 5–30 lm, which were prepared by a conventional powder sintering method, were set onto the surface of the glasses, and then, YAG laser irradiated the crystal grains on the surface. Through continuous YAG laser irradiations, b-BBO grains were first dissolved (incorporated) into the interior of the glasses, and then crystalline dots were formed. Finally, the glasses were moved automatically at the speeds of 3–8 lm/s. The suitable YAG laser powers were 0.6 W for 10Sm:BBO glass and 0.9 W for 10Dy:BBO glass. By applying the above techniques and procedures, crystal lines with a width of approximately 5 lm and with
40 10Dy:BBO glass 30
4.5 cm-1
20
Absorption coefficient / cm—1
128
10
0 40 10Sm:BBO glass 30 7.9 cm-1
20
3. Results 10
3.1. Formation of b-BaB2O4 crystal line by YAG laser irradiation 10Sm2O3 Æ 40BaO Æ 50B2O3 glass (designated here as 10Sm:BBO glass) has the values of Tg = 570 ± 2 C and Tx = 702 ± 2 C, and 10Dy2O3 Æ 45BaO Æ 45B2O3
0
500
1000
1500
2000
Wavelength /nm Fig. 1. Optical absorption spectra for 10Sm2O3 Æ 40BaO Æ 50B2O3 (10Sm:BBO) and 10Dy2O3 Æ 45BaO Æ 45B2O3 (10Dy:BBO) glasses.
T. Honma et al. / Journal of Non-Crystalline Solids 345&346 (2004) 127–131
129
(110)
Intensity /arb. units
10Sm2O3-40BaO-50B2O3
crystallized glass (750°C, 60min )
crystal lines
20
30
40 50 2θ /deg.
60
70
Fig. 3. XRD patterns for a crystal line array obtained by YAG laser irradiation in 10Sm2O3 Æ 40BaO Æ 50B2O3 (10Sm:BBO) glass and for the crystallized glass formed by a conventional heat treatment.
Fig. 2. Polarization optical microphotographs for b-BBO crystal lines written by scanning YAG laser irradiations (laser power 0.7 W, scanning speeds 8 lm/s) in 10Sm:BBO glass; (a) observed from the YAG laser irradiated direction, (b) observed from the cross-section of the sample.
desired lengths (up to 50 mm) were stably written in the glasses. As an example, polarization optical microphotographs for the lines written by YAG laser irradiations in 10Sm:BBO glass are shown in Fig. 2. We can observe the uniform retardation in the crystal lines. Similar results were obtained in the case of 10Dy:BBO glass. These results indicate that b-BBO crystal lines written in 10Sm:BBO and 10Dy:BBO glasses are extremely homogeneous. 3.2. XRD and Raman analyses A crystal line array of 15 lines was prepared by scanning YAG laser irradiation, where the length of each crystal line is 10 mm and a distance between lines (pitch) is 50 lm. The XRD pattern for such a crystal line array is shown in Fig. 3. The peak corresponding to the (1 1 0) plane in b-BBO is confirmed, suggesting that b-BBO crystals are highly oriented in the lines. The microRaman spectra for the 10Sm:BBO glass and crystal line are shown in Fig. 4. The Raman spectrum for the crystal line is consistent with the spectrum for b-BBO single crystals reported [13,14]. Several peaks are observed at 385, 483, 599, 621, 637, 667, 788, 1528 and 1543 cm1
Fig. 4. Micro-Raman scattering spectra for (a) glassy part and (b) crystal line in 10Sm:BBO sample. The polarization optical microphotograph for the b-BaB2O4 crystal lines is also shown.
in the spectrum for the crystal line. These peaks are attributed to the stretching mode of [B3O6]3 meta-borate ring. 3.3. Second harmonic generation from crystal lines Fig. 5 shows the second harmonic wave images (the intensities of green light with a wavelength of 532 nm) observed from a crystal line written in 10Dy:BBO glass with a second harmonic microscope pumped with
130
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Q-switched YAG laser operating k = 1064 nm. It is seen that the SH intensity is almost uniform over a whole line at each angle (angle: between linearly polarized incident laser and a crystal line). The azimuthal dependence of the SHG signals for b-BBO crystal lines is also shown in Fig. 6, in which the SH intensity is measured by changing the angle between linearly polarized incident laser and b-BBO crystal line. The maximum SH intensities are observed at the rotation angles of 0 and 180, and the zero intensities are located at 90 and 270, i.e., twofold angular dependence as the crystal line was rotated. The same experiment was carried out for the y-cut b-BBO single crystal (plate with 0.5 mm thickness) obtained commercially, and it was found that the azimuthal dependence of the SHG signals for y-cut b-BBO single crystal is almost the same as that (Fig. 6) for b-BBO crystal lines written by YAG laser irradiation. Fig. 7. Photoluminescence spectra of Sm3+ for (a) glassy part, (b) crystallized glass obtained by heat treatment in an electric furnace, and (c) crystal line written by YAG laser irradiation in 10Sm:BBO glass. The excited wavelength is 488 nm (Ar+ laser).
3.4. Photoluminescence spectra for Sm3+ ions
Fig. 5. Second harmonic wave images (532 nm) at different polarization angles for b-BBO crystal line formed in 10Dy:BBO glass.
4. Discussion
Intensity /arb. units
10Sm:BBO 10Dy:BBO
1.0
0.5
0 0
60
120
180
240
300
Fig. 7 shows the photoluminescence spectra of 10Sm:BBO precursor glass and b-BBO crystal line. It is seen that the photoluminescence peaks corresponding to Sm3+ are observed in both glass and crystal line. We could not confirm photoluminescence peaks corresponding to Sm2+. The photoluminescence intensities for crystal lines are much higher than those for the glass, and the peak splitting, so-called Stark splitting, are observed in crystal lines. This means that Sm3+ ions are incorporated into the b-BBO crystal structure.
360
Rotation angle /deg.
Fig. 6. Azimuthal dependence of SHG signals for the crystal lines written by YAG laser irradiation in 10Sm:BBO and 10Dy:BBO glasses. The solid line is a fitting curve by cos4 h.
It is obvious from the results shown in Figs. 2–7 that b-BBO crystal lines showing SHGs are formed in 10Sm2O3 Æ 40BaO Æ 50B2O3 and 10Dy2O3 Æ 45BaO Æ 45B2O3 glasses by Nd:YAG laser irradiation. Furthermore, the present study demonstrates that b-BBO crystals in the lines are highly oriented along the laser scanning direction. As shown in Fig. 6, the maximum SH intensities are observed at the rotation angles of 0 and 180, and the zero intensities are located at 90 and 270, i.e., twofold angular dependence as the crystal line was rotated. For a y-cut b-BBO, the effective second order non-linear coefficient, deff (i.e., SH intensity measured with the analyzer parallel to the polarization direction of a normally incident fundamental beam) is simply given by the following equation
T. Honma et al. / Journal of Non-Crystalline Solids 345&346 (2004) 127–131
d eff ¼ d 11 cos2 h:
ð1Þ
131
5. Conclusion
4
The solid line in Fig. 6 represents the fitting by cos h curve. It can be seen that the observed SH intensity maxima and minima resemble the theoretical prediction. The structure of b-BBO is made up of [B3O6]3 anionic hexagonal groupings slightly distorted with threefold axis, and planer [B3O6]3 units are stacked along the c-axis direction [10,11]. It is, therefore, considered that planner [B3O6]3 units in b-BBO with the c-axis orientation in the lines written by YAG laser irradiations are also stacked along the laser scanning direction (crystal growth direction). Since the polarization of [B3O6]3 units is the origin of second-order optical non-linearity in b-BBO and is present within the plane (parallel), a SHG should be observed when the direction of the polarization of incident YAG laser is consistent with that of [B3O6]3 units, i.e., parallel. In this study, linearly polarized YAG laser has been used, and thus we can expect that a strong SHG is induced at the incident angle of 0 and 180. The results shown in Fig. 6 are good agreement with the above prediction. Furthermore, it should be pointed out again that the azimuthal dependence of SHG signals for b-BBO crystal lines written by YAG laser irradiations in 10Sm:BBO and 10Dy:BBO glasses is almost the same as that for commercially available y-cut b-BBO single crystal. Consequently, it is concluded that b-BBO crystal lines written by YAG laser irradiations in 10Sm:BBO and 10Dy:BBO glasses might be single-crystal lines with the c-axis orientation along the line growth direction. The present study also demonstrates that Sm3+ ions are incorporated into the b-BBO crystal lines. In the selective (samarium) atom heat processing, cw Nd:YAG laser are absorbed by Sm3+ in glass through f–f transitions and the surrounding of Sm3+ is heated through a non-radiative relaxation. This might mean a high mobility (diffusion) of Sm3+ in the YAG laser irradiated part. Sole et al. [15] tried to grow b-BBO single-crystals in several BaB2O4–Na2O–Nd2O3 solutions and found that the presence of Nd2O3 stabilizes the low-temperature form b-BBO phase and Nd3+-doped b-BBO single crystals are grown. Considering the coordination number (eightfold BaO8) and ionic radius of Ba2+ ions in b-BBO, Sm3+ ions might substitute Ba2+ sites in the structure of b-BBO crystal lines written by YAG laser irradiations [15]. If Sm3+ ions are substituted for Ba2+ ions, the charge compensation must be fulfilled by the generation of defects such as Ba2+ site vacancies. The discussion on the charge compensations in Sm3+ doped b-BBO will be the next stage in this study.
We succeeded in writing non-linear optical b-BBO crystal lines by cw Nd:YAG laser irradiation in barium borate glasses containing Sm2O3 or Dy2O3. It was proposed from XRD analysis, micro-Raman analysis, polarization optical microphotographs and SHG measurements that the crystal lines might be b-BBO single crystals with the c-axis orientation along the laser scanning direction. It was also clarified from photoluminescence spectra that Sm3+ ions are incorporated into bBBO crystal lines.
Acknowledgments This work was supported from the Grant-in-Aid for Scientific Research from the Ministry of Education, Science, Sport, and Culture, Japan, and by the 21st Century Center of Excellence (COE) Program in Nagaoka University of Technology. One of the authors (T.H.) would like to thank the Sasakawa Scientific Resarch Grant from The Japan Science Society for the partial financial support to this work.
References [1] K.O. Hill, B. Malo, F. Bilodeau, D.C. Johnson, J. Albert, Appl. Phys. Lett. 62 (1993) 1035. [2] K. Miura, J. Qiu, H. Inouye, T. Mitsuya, K. Hirao, Appl. Phys. Lett. 71 (1997) 3329. [3] T. Fujiwara, M. Takahashi, A.J. Ikushima, Appl. Phys. Lett. 71 (1997) 1032. [4] T. Fujiwara, R. Ogawa, Y. Takahashi, Y. Benino, T. Komatsu, J. Nishii, Phys. Chem. Glasses 43C (2002) 213. [5] R. Sato, Y. Benino, T. Fujiwara, T. Komatsu, J. Non-Cryst. Solids 289 (2001) 228. [6] T. Honma, Y. Benino, T. Fujiwara, R. Sato, T. Komatsu, Opt. Mater. 20 (2002) 27. [7] T. Honma, Y. Benino, T. Fujiwara, R. Sato, T. Komatsu, J. Ceram. Soc. Jpn. 110 (2002) 398. [8] T. Honma, Y. Benino, T. Fujiwara, T. Komatsu, R. Sato, Appl. Phys. Lett. 82 (2003) 892. [9] H. Tanaka, T. Honma, Y. Benino, T. Fujiwara, T. Komatsu, J. Phys. Chem. Solids 64 (2003) 1179. [10] C. Chen, Y. Wu, R. Li, J. Cryst. Growth 99 (1990) 790. [11] R. Fro¨hlich, Zeitschrift fu¨r Kristallographie 168 (1984) 109. [12] D. Eimel, L. Davis, S. Velsko, E.K. Graham, A. Zalkin, J. Appl. Phys. 62 (1987) 1968. [13] T. Bogang, W. Guozhen, X. Ryiya, Spectrochim. Acta 43 (1987) 65. [14] S. Hong, B. Wu, Opt. Eng. 34 (1995) 1738. [15] R. Sole, V. Nikolov, M.C. Pujol, J. Gavalda, X. Ruiz, J. Massons, M. Aguilo, F. Diaz, J. Cryst. Growth 207 (1999) 104.
Spatially selected crystallization in glass by YAG laser irradiation Tsuyoshi Honma a, Yasuhiko Benino a, Takumi Fujiwara a, Ryuji Sato b, Takayuki Komatsu a,* a
Department of Chemistry, Nagaoka University of Technology, 1603-1 Kamitomioka-cho, Nagaoka 940-2188, Japan b Department of Materials Engineering, Tsuruoka National College of Technology, Tsuruoka 997-8511, Japan Available online 12 September 2004
Abstract A continuous wave (cw) Nd:YAG laser operating at a wavelength of 1064 nm irradiated Sm2O3- and Dy2O3-doped glasses under various irradiation conditions in the systems of Ln2O3–BaO–B2O3 (Ln:Sm and Dy), and non-linear optical b-BaB2O4 (b-BBO) crystals lines are formed at the surface of glass substrates. It is proposed from the azimuthal dependence of second harmonic intensity that b-BBO crystal lines might be single crystals with the c-axis orientation along the laser scanning direction. The photoluminescence spectra indicate that Sm3+ ions are incorporated into b-BBO crystal lines. The present study proposes that cw Nd:YAG laser irradiation to glass containing Sm3+ and Dy3+ is a novel technique for a spatially selected crystallization in glass. 2004 Elsevier B.V. All rights reserved. PACS: 42.65.Ky; 42.70.Nq; 81.05.Kf; 81.05.Pj
1. Introduction Laser irradiation of glass materials has received much attention, because this technique has been regarded as a new processing technique for spatially selected structural modification and/or crystallization in glass [1–4]. Recently, Sato et al. [5] have reported that crystal dots consisting of the Sm2Te6O15 phase are induced by irradiation of a continuous wave (cw) Nd:YAG laser operating at a wavelength of k = 1064 nm in RO–Sm2O3– TeO2 (R:Mg, Ba) glasses. Since Nd:YAG laser is more conventional compared with other kinds of lasers such as short-wavelength excimer laser, CO2 gas laser and femtosecond pulsed laser, it is of interest to check the effect of Nd:YAG laser irradiation on various glass systems and to clarify the mechanism of YAG laserinduced crystallization. Honma et al. [6–8] have found that the irradiation of Nd:YAG laser with k = 1064 nm *
Corresponding author. Tel.: +81 258 47 9313; fax: +81 258 47 9300. E-mail address: [email protected] (T. Komatsu). 0022-3093/$ - see front matter 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2004.08.009
induces the formation of crystal dots and lines showing a second harmonic generation (SHG) in Sm2O3–Bi2O3– B2O3 glasses. Tanaka et al. [9] have applied the YAG laser irradiation technique to Sm2O3–BaO–B2O3 glasses and found that b-BaB2O4 (b-BBO) crystal dots showing SHGs are formed. It has been proposed that cw Nd:YAG laser irradiations to glass containing Sm3+ 6 ions cause continuous f–f transitions (6F9/2 H5/2) in 3+ Sm and continuous electron–phonon couplings (i.e., non-radiative relaxation), consequently inducing thermal effects [5]. That is, in the YAG laser irradiation processing, local regions centered around Sm3+ ions are selectively heated. This technique might be, therefore, called selective (samarium) atom heat processing. In this study, we apply the YAG laser irradiation technique to Sm2O3 (or Dy2O3)–BaO–B2O3 glasses and try to fabricate non-linear optical b-BBO crystal lines. In particular, we examine whether b-BBO crystal lines written by YAG laser irradiation are single-crystal lines or polycrystal lines. We choose b-BBO crystals by the following reasons: b-BBO developed by Chen et al. [10] is one of the most important non-linear optical
T. Honma et al. / Journal of Non-Crystalline Solids 345&346 (2004) 127–131
crystals in laser optics, and its crystal structure is wellknown [11]. Its remarkable features include a large second-order optical non-linearity of d11 = 2.22 pm/V, the wide optical transmittance range (190–3500 nm) and a high laser damage threshold [12].
2. Experimental procedure The glass compositions examined in this study are 10Sm2O3 Æ 40BaO Æ 50B2O3 and 10Dy2O3 Æ 45BaO Æ 45B2O3. Commercial powders of reagent grade (99.9% purity) Sm2O3, Dy2O3, BaCO3, B2O3 were melted in a platinum crucible at 1200 C for 40 min in an electric furnace. The melts were poured on to an iron plate and pressed to a thickness of about 1.5 mm by another iron plate. The glass transition, Tg, and crystallization onset, Tx, temperatures were determined using differential thermal analysis (DTA) (Seiko Instruments Inc.; TG/ DTA320) at a heating rate of 10 K/min, in which glass powders with the particle size of 30–100 lm were used. Optical absorption spectra were taken in the wavelength range of 190–3200 nm on Shimadzu UV-3150 spectrometer. The glasses were mechanically polished to a mirror finish with CeO2 powders. A cw Nd:YAG laser with k = 1064 nm irradiated the surface of the glasses using an objective lens (20 and 60·), and the sample stage was automatically moved during laser irradiations to write crystal lines. The crystal lines written by YAG laser irradiations were observed with a polarization optical microscope. The formation of the b-BBO crystalline phase was confirmed using X-ray diffraction (XRD) analysis (CuKa radiation) at room temperature. The second harmonic (SH) intensity of crystal lines was measured by using a fundamental wave of Q-switched Nd:YAG laser with k = 1064 nm as a laser source, in which linearly polarized fundamental laser beams were introduced into crystal lines perpendicularly and the azimuthal dependence of SHG signals was measured by rotating the sample. A y-cut a-quartz (0.6 mm thickness) crystal plate was used as a reference of SH intensity. Micro-Raman and photoluminescence spectra of b-BBO crystal lines were measured with a three-dimensional spatially resolved laser microscope (Tokyo Instruments. Co. Nanofinder) operated at Ar+ (488 nm) laser.
(10Dy:BBO glass) shows Tg = 554 ± 2 C and Tx = 682 ± 2 C. We confirmed that the crystalline phase obtained by a conventional heat treatment in an electric furnace is assigned to only the b-BaB2O4(ICDD# 850914) phase. The optical absorption spectra at room temperature are shown in Fig. 1, giving the absorption 6 coefficients at 1064 nm of 7.9 ± 0.2 cm1 (6F9/2 H5/2) 1 6 for 10Sm:BBO glass and of 4.5 ± 0.2 cm ( F9/2 6 H15/2) for 10Dy:BBO glass. 10Sm:BBO glass has an optical absorption coefficient of 7.3 ± 0.2 cm1 even at 250 C. In order to write crystal lines easily and stably in the glasses, the following techniques were used. At first, to avoid the fracture of glasses due to the thermal shock during laser irradiations, the glasses were heated at about 150 C using a hotplate. Secondly, to form the crystal nuclei of b-BBO easily, b-BBO crystal grains with a diameter of 5–30 lm, which were prepared by a conventional powder sintering method, were set onto the surface of the glasses, and then, YAG laser irradiated the crystal grains on the surface. Through continuous YAG laser irradiations, b-BBO grains were first dissolved (incorporated) into the interior of the glasses, and then crystalline dots were formed. Finally, the glasses were moved automatically at the speeds of 3–8 lm/s. The suitable YAG laser powers were 0.6 W for 10Sm:BBO glass and 0.9 W for 10Dy:BBO glass. By applying the above techniques and procedures, crystal lines with a width of approximately 5 lm and with
40 10Dy:BBO glass 30
4.5 cm-1
20
Absorption coefficient / cm—1
128
10
0 40 10Sm:BBO glass 30 7.9 cm-1
20
3. Results 10
3.1. Formation of b-BaB2O4 crystal line by YAG laser irradiation 10Sm2O3 Æ 40BaO Æ 50B2O3 glass (designated here as 10Sm:BBO glass) has the values of Tg = 570 ± 2 C and Tx = 702 ± 2 C, and 10Dy2O3 Æ 45BaO Æ 45B2O3
0
500
1000
1500
2000
Wavelength /nm Fig. 1. Optical absorption spectra for 10Sm2O3 Æ 40BaO Æ 50B2O3 (10Sm:BBO) and 10Dy2O3 Æ 45BaO Æ 45B2O3 (10Dy:BBO) glasses.
T. Honma et al. / Journal of Non-Crystalline Solids 345&346 (2004) 127–131
129
(110)
Intensity /arb. units
10Sm2O3-40BaO-50B2O3
crystallized glass (750°C, 60min )
crystal lines
20
30
40 50 2θ /deg.
60
70
Fig. 3. XRD patterns for a crystal line array obtained by YAG laser irradiation in 10Sm2O3 Æ 40BaO Æ 50B2O3 (10Sm:BBO) glass and for the crystallized glass formed by a conventional heat treatment.
Fig. 2. Polarization optical microphotographs for b-BBO crystal lines written by scanning YAG laser irradiations (laser power 0.7 W, scanning speeds 8 lm/s) in 10Sm:BBO glass; (a) observed from the YAG laser irradiated direction, (b) observed from the cross-section of the sample.
desired lengths (up to 50 mm) were stably written in the glasses. As an example, polarization optical microphotographs for the lines written by YAG laser irradiations in 10Sm:BBO glass are shown in Fig. 2. We can observe the uniform retardation in the crystal lines. Similar results were obtained in the case of 10Dy:BBO glass. These results indicate that b-BBO crystal lines written in 10Sm:BBO and 10Dy:BBO glasses are extremely homogeneous. 3.2. XRD and Raman analyses A crystal line array of 15 lines was prepared by scanning YAG laser irradiation, where the length of each crystal line is 10 mm and a distance between lines (pitch) is 50 lm. The XRD pattern for such a crystal line array is shown in Fig. 3. The peak corresponding to the (1 1 0) plane in b-BBO is confirmed, suggesting that b-BBO crystals are highly oriented in the lines. The microRaman spectra for the 10Sm:BBO glass and crystal line are shown in Fig. 4. The Raman spectrum for the crystal line is consistent with the spectrum for b-BBO single crystals reported [13,14]. Several peaks are observed at 385, 483, 599, 621, 637, 667, 788, 1528 and 1543 cm1
Fig. 4. Micro-Raman scattering spectra for (a) glassy part and (b) crystal line in 10Sm:BBO sample. The polarization optical microphotograph for the b-BaB2O4 crystal lines is also shown.
in the spectrum for the crystal line. These peaks are attributed to the stretching mode of [B3O6]3 meta-borate ring. 3.3. Second harmonic generation from crystal lines Fig. 5 shows the second harmonic wave images (the intensities of green light with a wavelength of 532 nm) observed from a crystal line written in 10Dy:BBO glass with a second harmonic microscope pumped with
130
T. Honma et al. / Journal of Non-Crystalline Solids 345&346 (2004) 127–131
Q-switched YAG laser operating k = 1064 nm. It is seen that the SH intensity is almost uniform over a whole line at each angle (angle: between linearly polarized incident laser and a crystal line). The azimuthal dependence of the SHG signals for b-BBO crystal lines is also shown in Fig. 6, in which the SH intensity is measured by changing the angle between linearly polarized incident laser and b-BBO crystal line. The maximum SH intensities are observed at the rotation angles of 0 and 180, and the zero intensities are located at 90 and 270, i.e., twofold angular dependence as the crystal line was rotated. The same experiment was carried out for the y-cut b-BBO single crystal (plate with 0.5 mm thickness) obtained commercially, and it was found that the azimuthal dependence of the SHG signals for y-cut b-BBO single crystal is almost the same as that (Fig. 6) for b-BBO crystal lines written by YAG laser irradiation. Fig. 7. Photoluminescence spectra of Sm3+ for (a) glassy part, (b) crystallized glass obtained by heat treatment in an electric furnace, and (c) crystal line written by YAG laser irradiation in 10Sm:BBO glass. The excited wavelength is 488 nm (Ar+ laser).
3.4. Photoluminescence spectra for Sm3+ ions
Fig. 5. Second harmonic wave images (532 nm) at different polarization angles for b-BBO crystal line formed in 10Dy:BBO glass.
4. Discussion
Intensity /arb. units
10Sm:BBO 10Dy:BBO
1.0
0.5
0 0
60
120
180
240
300
Fig. 7 shows the photoluminescence spectra of 10Sm:BBO precursor glass and b-BBO crystal line. It is seen that the photoluminescence peaks corresponding to Sm3+ are observed in both glass and crystal line. We could not confirm photoluminescence peaks corresponding to Sm2+. The photoluminescence intensities for crystal lines are much higher than those for the glass, and the peak splitting, so-called Stark splitting, are observed in crystal lines. This means that Sm3+ ions are incorporated into the b-BBO crystal structure.
360
Rotation angle /deg.
Fig. 6. Azimuthal dependence of SHG signals for the crystal lines written by YAG laser irradiation in 10Sm:BBO and 10Dy:BBO glasses. The solid line is a fitting curve by cos4 h.
It is obvious from the results shown in Figs. 2–7 that b-BBO crystal lines showing SHGs are formed in 10Sm2O3 Æ 40BaO Æ 50B2O3 and 10Dy2O3 Æ 45BaO Æ 45B2O3 glasses by Nd:YAG laser irradiation. Furthermore, the present study demonstrates that b-BBO crystals in the lines are highly oriented along the laser scanning direction. As shown in Fig. 6, the maximum SH intensities are observed at the rotation angles of 0 and 180, and the zero intensities are located at 90 and 270, i.e., twofold angular dependence as the crystal line was rotated. For a y-cut b-BBO, the effective second order non-linear coefficient, deff (i.e., SH intensity measured with the analyzer parallel to the polarization direction of a normally incident fundamental beam) is simply given by the following equation
T. Honma et al. / Journal of Non-Crystalline Solids 345&346 (2004) 127–131
d eff ¼ d 11 cos2 h:
ð1Þ
131
5. Conclusion
4
The solid line in Fig. 6 represents the fitting by cos h curve. It can be seen that the observed SH intensity maxima and minima resemble the theoretical prediction. The structure of b-BBO is made up of [B3O6]3 anionic hexagonal groupings slightly distorted with threefold axis, and planer [B3O6]3 units are stacked along the c-axis direction [10,11]. It is, therefore, considered that planner [B3O6]3 units in b-BBO with the c-axis orientation in the lines written by YAG laser irradiations are also stacked along the laser scanning direction (crystal growth direction). Since the polarization of [B3O6]3 units is the origin of second-order optical non-linearity in b-BBO and is present within the plane (parallel), a SHG should be observed when the direction of the polarization of incident YAG laser is consistent with that of [B3O6]3 units, i.e., parallel. In this study, linearly polarized YAG laser has been used, and thus we can expect that a strong SHG is induced at the incident angle of 0 and 180. The results shown in Fig. 6 are good agreement with the above prediction. Furthermore, it should be pointed out again that the azimuthal dependence of SHG signals for b-BBO crystal lines written by YAG laser irradiations in 10Sm:BBO and 10Dy:BBO glasses is almost the same as that for commercially available y-cut b-BBO single crystal. Consequently, it is concluded that b-BBO crystal lines written by YAG laser irradiations in 10Sm:BBO and 10Dy:BBO glasses might be single-crystal lines with the c-axis orientation along the line growth direction. The present study also demonstrates that Sm3+ ions are incorporated into the b-BBO crystal lines. In the selective (samarium) atom heat processing, cw Nd:YAG laser are absorbed by Sm3+ in glass through f–f transitions and the surrounding of Sm3+ is heated through a non-radiative relaxation. This might mean a high mobility (diffusion) of Sm3+ in the YAG laser irradiated part. Sole et al. [15] tried to grow b-BBO single-crystals in several BaB2O4–Na2O–Nd2O3 solutions and found that the presence of Nd2O3 stabilizes the low-temperature form b-BBO phase and Nd3+-doped b-BBO single crystals are grown. Considering the coordination number (eightfold BaO8) and ionic radius of Ba2+ ions in b-BBO, Sm3+ ions might substitute Ba2+ sites in the structure of b-BBO crystal lines written by YAG laser irradiations [15]. If Sm3+ ions are substituted for Ba2+ ions, the charge compensation must be fulfilled by the generation of defects such as Ba2+ site vacancies. The discussion on the charge compensations in Sm3+ doped b-BBO will be the next stage in this study.
We succeeded in writing non-linear optical b-BBO crystal lines by cw Nd:YAG laser irradiation in barium borate glasses containing Sm2O3 or Dy2O3. It was proposed from XRD analysis, micro-Raman analysis, polarization optical microphotographs and SHG measurements that the crystal lines might be b-BBO single crystals with the c-axis orientation along the laser scanning direction. It was also clarified from photoluminescence spectra that Sm3+ ions are incorporated into bBBO crystal lines.
Acknowledgments This work was supported from the Grant-in-Aid for Scientific Research from the Ministry of Education, Science, Sport, and Culture, Japan, and by the 21st Century Center of Excellence (COE) Program in Nagaoka University of Technology. One of the authors (T.H.) would like to thank the Sasakawa Scientific Resarch Grant from The Japan Science Society for the partial financial support to this work.
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