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Second harmonic generation of thermally poled ZnCl2 or ZnBr2–B2O3–TeO2 glasses and its mechanism
Journal of Non-Crystalline Solids 357 (2011) 1013–1015
Contents lists available at ScienceDirect
Journal of Non-Crystalline Solids j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / j n o n c r y s o l
Second harmonic generation of thermally poled ZnCl2 or ZnBr2–B2O3–TeO2 glasses and its mechanism Hiroyuki Nasu ⁎, Tomohiro Ito, Yuki Nagaike, Sachio Ninagawa, Tomohide Hatasa, Daisuke Hirota, Kenichi Hasegawa, Tadanori Hashimoto, Atsushi Ishihara Division of Chemistry for Materials, Faculty of Engineering, Mie University, 1577 Kurimamachiya-machi, Tsu 514-8507, Japan
a r t i c l e
i n f o
Article history: Received 8 March 2010 Received in revised form 1 November 2010 Available online 13 December 2010 Keywords: Second harmonic generation; Inorganic glass; Halogen-containing glass; Thermal poling; Cathode side polishing
a b s t r a c t Second harmonic generation (SHG) was observed from halide, in specific, chlorine or bromine-containing tellurite glasses. The magnitude of SHG increased with increasing halides concentration and poling time. Although the mechanism of SHG is still controversial, it is generally accepted that the idea of the deficiency of cations near the anode side cause SHG. However, in the preset oxyhalide glass system without any singly positive charge cation, the polishing of the anode side showed no effect on SHG. On the other hand, the polishing of the cathode side significantly decreased the magnitude of SHG in the present alkali-free oxyhalide glasses. Therefore, the SHG mechanism in the present oxyhalide glass systems is made evident in which the movement of anions results in an anion-deficient layer near the cathode side. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Second harmonic generation (SHG) is very useful in changing the frequency of incident light. Several single crystals are commercially available for this purpose. However, technological requirements for carefully controlled crystal growth often lead to high production cost for these single crystals. On the other hand, inorganic glasses have been proved to be chemically and/or physically stable, have shown high transparency, low manufacturing cost, ease of fabrication into waveguides and/or fibers, and a large capacity for accepting dopants, and are considered to be suitable as optical materials with high performance in current and future photonics systems. Theoretically speaking, substances that have centrosymmetry in their light wavelength spectra should not cause SHG, and thus it had been believed that inorganic glasses should not show SHG because of their optical isotropic nature. However, since Sasaki and Ohmori [1] reported SHG from a GeO2–SiO2 fiber, extensive studies have been carried out on SHG from inorganic glasses. In particular, Myers et al. [2] reported SHG from bulk silica glass induced by thermal poling that is, applying a high dc voltage at elevated temperatures. Thereafter, many researchers have studied thermal poling and have found SHG from various kinds of glasses [3–5]. The mechanism causing SHG is still a controversial issue. Some cases are well interpreted by the alignment of dipoles, and the others are well explained by the effect of induced electric fields (Edc) and third-order
⁎ Corresponding author. Tel.: +81 592321211x3842; fax: +81 592 31 2252. E-mail address: [email protected] (H. Nasu). 0022-3093/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2010.11.043
optical nonlinear susceptibility (χ(3)) such as χ(2) = 3Edc·χ(3), where χ(2) is the second-order optical nonlinear susceptibility. In the latter case, a deficiency of cations at the anode side has been observed, and polishing of the anode side decreased SHG intensity. Finally, no SHG pattern was detected at a certain depth of polishing. On the other hand, no influence on SHG has been observed for polishing of the cathode side [5]. In order to determine whether the deficiency of ions really results in SHG, the ion deficiency of the cathode side induced by migration of anions must be investigated. However, no study has so far been carried out on SHG phenomena in anion-migrating system expect few studies [6]. Therefore, this paper reports the SHG phenomena in halide-doped glasses and discusses the SHG mechanism. 2. Experimental procedure Analytical grade, commercially available ZnCl2, ZnBr2 (Nakarai Tesque), TeO2 (Kojundo Kagaku) and B2O3 (Nakarai Tesque) were used. The batch composition was xZnCl2·80TeO2·(20 − x)B2O3 (mol%) (x = 0, 5, 10, 15) and xZnBr2·80TeO2·(20− x)B2O3 (mol%) (x = 0,5 10); hereafter, they are called TB, 5ZTB, 10ZTB, 15ZTB, 5ZBTB and 10ZBTB, respectively. The total amount of each batch was 12 g. After mixing well, the batches were placed into an alumina crucible and melted at 900 °C for 30 min in the globe box filled with nitrogen in an electric furnace. Subsequently, the melts were poured onto an iron plate preheated to 100 °C and sandwiched with another iron plate. Then, the quenched bodies were annealed at 365 °C for 1 h. In order to measure SHG, all the glasses presently obtained were polished to optical grade and so that both surfaces were parallel using CeO2 powder. The sample thickness was about 1 mm.
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H. Nasu et al. / Journal of Non-Crystalline Solids 357 (2011) 1013–1015
For homogeneous poling treatment, gold film electrodes were deposited on both sides of the glass samples by vacuum evaporation; and the size of the electrodes was about 1 cm2. Optical transmission spectra of the samples were measured from 300 to 800 nm by UV–VIS-NIR spectrometry (Shimadzu UV-3100). The poling was carried out at 310–350 °C of various dc voltages of up to 2 kV for ZnCl2-containing glasses and 1.5 kV for ZnBr2-containing glasses both for various time durations with a stainless-steel holder placed in the electric furnace in air. After a certain time, the glasses were cooled to room temperature while continuously applying voltage. After the poling treatment, the gold electrodes were wiped off by acetone. Second harmonic generation was measured from a Maker fringe pattern. The light source used was a Q-switched Nd:YAG laser with a wavelength of 1064 nm , a pulse width of 10 ns of the wavelength and a 10 Hz frequency. Y-cut quartz was used as the standard to compare the derived SHG intensity.
poling. Thus, we determined that 1 h poling is appropriate to compare SHG intensity after 1 h poling. Fig. 2 depicts the influence of poling temperature on SHG intensity for ZnCl2 containing glasses. The increase in the poling temperature increases SHG intensity. The similar dependence of poling temperature could be seen in ZnBr2 containing glass, but the weakness of the durability for applying voltage of ZnBr2 containing glasses disturbed the investigation for applied voltage dependence of SHG. The polishing of the anode side of the ZnCl2 containing glass had no influence on SHG intensity and fringe pattern appearance. On the other hand, 5–10 μm of cathode side polishing, shown by the broken line in Fig. 3(A), of 10ZTB glass resulted in a markedly decreased SHG intensity. Further by 5–10 μm polishing diminished the fringe pattern shown on the dotted line. The similar influences are seen for 10ZBTB glass, the polishing was significant for SHG by polishing cathode side, but no influence in SHG can be found by polishing the anode side as shown in Fig. 3(B) and (C). Further, all of the samples were fully transparent in visible and near IR region. Thus, absorption is not necessary to the measurements.
3. Results 4. Discussion First of all, we discuss the influence of ZnCl2 and ZnBr2 contents on SHG intensity. Fig. 1(A) and (B) shows SHG intensity as a function of ZnCl2 or ZnBr2 content after poling at 310 °C at 2 kV for 1 h and at 300 °C at 1 kV for 1 h, respectively. As one can see, SHG intensity linearly increases with ZnCl2 or ZnBr2 content, and that of glass with 10 mol% addition of ZnCl2 increases to a value twice as large as that of ZnCl2-free glass. The addition of ZnBr2 showed a similar effect in increasing ZnBr2 content and the linear increase with the content. One of the important points was that the increase was more drastic in ZnBr2 glasses. With respect to poling time, although SHG intensity increased with increasing poling time initially, SHG intensity saturated after 1 h
Fig. 1. SH intensity as a function of (A) ZnCl2 content after poling at 310 °C at 2 kV for 1 h and (B) ZnBr2 content at 300 °C at 1 kV for 1 h.
The mechanism of SHG for thermally poled glasses is still controversial. There are two main proposals on the mechanism. One is the alignment of dipoles in the glasses3) and the other is the following process expressed as ð2Þ
χ
ð3Þ
= 3χ Edc ;
ð1Þ
where χ(2) and χ(3) are second- and third-order nonlinear optical susceptibilities, respectively, and Edc is the dc electric field resulting from the migration of ions [4]. In the latter case, the migration of cations has been explored and a cation-deficient anode side was confirmed. As a matter of fact, the polishing of the anode side markedly decreased the SHG intensity, while no influence of cathode side polishing of the oxide glasses was found. This is interpreted as being due to the migration of alkali and/or alkaline earth ions to the cathode side resulting in the cation-deficient anode side causing Edc. The migration takes place for negatively charged ions as well as positive ions. However, from Eq. (1) the dc electric field resulting from the thermal poling can be considered as mainly causing SHG. The highly charged, large anions like oxygen ions can be considered as repulsing each other, and widely and relatively uniform dispersed in the poled glasses. Thus, a similar discussion is possible for the present oxyhalide glasses. Specifically, the rapid movement of anions of the cathode side may result in the development of electrical neutrality through the present glasses and disturbing the dispersion of the highly charged heavy cations, and made the thin anion-deficient layer
Fig. 2. SH intensity as a function of poling temperature for ZnCl2 containing glasses after poling at 310 °C at 2 kV.
H. Nasu et al. / Journal of Non-Crystalline Solids 357 (2011) 1013–1015
1015
near the cathode cause SHG. This is presumed in ZnCl2 containing glasses in Ref. [6] and made sure by the similar phenomena seen for the heavier bromine ions containing oxyhalide glasses. As pointed out in Ref. [6], we firstly reported the possibility of the formation of anion-deficient region near the cathode electrode. The similar phenomena caused in ZnBr2 containing glasses mean that the most movable ions whichever they are positively or negatively charged form the SHG layer. However, the forming region of SHG depends on the sign of the charge. The positively charged ions form the SHG layer near the anode, on the other hand, the negatively charged ions form it near the opposite cathode side, even for the heavy bromide ions. This series of the work was briefly reported in Ref [6], but the final conclusion is made by this paper. The increase of the influence of halide content on SHG intensity can be attributed to the increase of halide content. Further, the increase of dependence of the kinds of halide on SHG intensity may result from the heavier bromine ions relatively moved in short distance caused the larger dc electric field for compensating the applied high voltage. 5. Conclusion We successfully observed SHG from thermally poled ZnCl2 and ZnBr2 containing tellurite glasses. The intensity of SHG increased as the halides content was increased for both systems. With respect to the SHG mechanism, the polishing of anode side has no effect on SHG, on the other hand, the polishing of cathode side significantly weakened SHG, although the SHG layer has been reported to form in the anode side for halide-free glasses. It can be explained that the SHG layer formed the opposite charged ions, that is, singly and negative charged halide ions. This series of work on zinc halides containing glasses reveals that the movements of negative ions, even heavy bromide ions, are possible, and that the aniondeficient layer results in SHG layer. References [1] Y. Sasaki, Y. Ohmori, Appl. Phys. Lett. 39 (1981) 466. [2] R.A. Myers, N. Mukherjee, S.J. Brueck, App. Opt. Lett. 16 (1991) 1732. [3] H. Nasu, H. Okamoto, A. Mito, J. Matsuoka, K. Kamiya, Jpn. J. Appl. Phys. 32 (1993) L406. [4] M. Qiu, VilasecaR. , CojocaruC. , MartorellJ. , MizunamiliT. , Appl. Phys. 88 (2000) 4666. [5] K. Tanaka, A. Narasaki, K. Hirao, N. Soga, J. Non-Cryst. Solids 203 (1996) 49. [6] NasuH. , NagaikeY. , TakedaH. , HashimotoT. , KamiyaK. , Jpn. J. Appl. Phys. 44 (2005) L943.
Fig. 3. (A)SH intensity for no polishing (solid line), 5–10 μm of anode side polishing (broken line) and polishing 5–10 μm cathode side (dotted line) of cathode side for ZnCl2 containing glasses poled at 310 °C at 2 kV for 1 h. (B) SH intensity for no polishing (before) (solid line), and after 50 μm polishing anode surface (broken line) for ZnBr2 containing glasses poled 300 °C at 1 kV. (C) SH intensity for no polishing (before) (solid line),after 50 μm polishing cathode surface (broken line) and after 100 μm polishing cathode surface (dotted line) for ZnBr2 containing glasses poled 300 °C at 1 kV.
Contents lists available at ScienceDirect
Journal of Non-Crystalline Solids j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / j n o n c r y s o l
Second harmonic generation of thermally poled ZnCl2 or ZnBr2–B2O3–TeO2 glasses and its mechanism Hiroyuki Nasu ⁎, Tomohiro Ito, Yuki Nagaike, Sachio Ninagawa, Tomohide Hatasa, Daisuke Hirota, Kenichi Hasegawa, Tadanori Hashimoto, Atsushi Ishihara Division of Chemistry for Materials, Faculty of Engineering, Mie University, 1577 Kurimamachiya-machi, Tsu 514-8507, Japan
a r t i c l e
i n f o
Article history: Received 8 March 2010 Received in revised form 1 November 2010 Available online 13 December 2010 Keywords: Second harmonic generation; Inorganic glass; Halogen-containing glass; Thermal poling; Cathode side polishing
a b s t r a c t Second harmonic generation (SHG) was observed from halide, in specific, chlorine or bromine-containing tellurite glasses. The magnitude of SHG increased with increasing halides concentration and poling time. Although the mechanism of SHG is still controversial, it is generally accepted that the idea of the deficiency of cations near the anode side cause SHG. However, in the preset oxyhalide glass system without any singly positive charge cation, the polishing of the anode side showed no effect on SHG. On the other hand, the polishing of the cathode side significantly decreased the magnitude of SHG in the present alkali-free oxyhalide glasses. Therefore, the SHG mechanism in the present oxyhalide glass systems is made evident in which the movement of anions results in an anion-deficient layer near the cathode side. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Second harmonic generation (SHG) is very useful in changing the frequency of incident light. Several single crystals are commercially available for this purpose. However, technological requirements for carefully controlled crystal growth often lead to high production cost for these single crystals. On the other hand, inorganic glasses have been proved to be chemically and/or physically stable, have shown high transparency, low manufacturing cost, ease of fabrication into waveguides and/or fibers, and a large capacity for accepting dopants, and are considered to be suitable as optical materials with high performance in current and future photonics systems. Theoretically speaking, substances that have centrosymmetry in their light wavelength spectra should not cause SHG, and thus it had been believed that inorganic glasses should not show SHG because of their optical isotropic nature. However, since Sasaki and Ohmori [1] reported SHG from a GeO2–SiO2 fiber, extensive studies have been carried out on SHG from inorganic glasses. In particular, Myers et al. [2] reported SHG from bulk silica glass induced by thermal poling that is, applying a high dc voltage at elevated temperatures. Thereafter, many researchers have studied thermal poling and have found SHG from various kinds of glasses [3–5]. The mechanism causing SHG is still a controversial issue. Some cases are well interpreted by the alignment of dipoles, and the others are well explained by the effect of induced electric fields (Edc) and third-order
⁎ Corresponding author. Tel.: +81 592321211x3842; fax: +81 592 31 2252. E-mail address: [email protected] (H. Nasu). 0022-3093/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2010.11.043
optical nonlinear susceptibility (χ(3)) such as χ(2) = 3Edc·χ(3), where χ(2) is the second-order optical nonlinear susceptibility. In the latter case, a deficiency of cations at the anode side has been observed, and polishing of the anode side decreased SHG intensity. Finally, no SHG pattern was detected at a certain depth of polishing. On the other hand, no influence on SHG has been observed for polishing of the cathode side [5]. In order to determine whether the deficiency of ions really results in SHG, the ion deficiency of the cathode side induced by migration of anions must be investigated. However, no study has so far been carried out on SHG phenomena in anion-migrating system expect few studies [6]. Therefore, this paper reports the SHG phenomena in halide-doped glasses and discusses the SHG mechanism. 2. Experimental procedure Analytical grade, commercially available ZnCl2, ZnBr2 (Nakarai Tesque), TeO2 (Kojundo Kagaku) and B2O3 (Nakarai Tesque) were used. The batch composition was xZnCl2·80TeO2·(20 − x)B2O3 (mol%) (x = 0, 5, 10, 15) and xZnBr2·80TeO2·(20− x)B2O3 (mol%) (x = 0,5 10); hereafter, they are called TB, 5ZTB, 10ZTB, 15ZTB, 5ZBTB and 10ZBTB, respectively. The total amount of each batch was 12 g. After mixing well, the batches were placed into an alumina crucible and melted at 900 °C for 30 min in the globe box filled with nitrogen in an electric furnace. Subsequently, the melts were poured onto an iron plate preheated to 100 °C and sandwiched with another iron plate. Then, the quenched bodies were annealed at 365 °C for 1 h. In order to measure SHG, all the glasses presently obtained were polished to optical grade and so that both surfaces were parallel using CeO2 powder. The sample thickness was about 1 mm.
1014
H. Nasu et al. / Journal of Non-Crystalline Solids 357 (2011) 1013–1015
For homogeneous poling treatment, gold film electrodes were deposited on both sides of the glass samples by vacuum evaporation; and the size of the electrodes was about 1 cm2. Optical transmission spectra of the samples were measured from 300 to 800 nm by UV–VIS-NIR spectrometry (Shimadzu UV-3100). The poling was carried out at 310–350 °C of various dc voltages of up to 2 kV for ZnCl2-containing glasses and 1.5 kV for ZnBr2-containing glasses both for various time durations with a stainless-steel holder placed in the electric furnace in air. After a certain time, the glasses were cooled to room temperature while continuously applying voltage. After the poling treatment, the gold electrodes were wiped off by acetone. Second harmonic generation was measured from a Maker fringe pattern. The light source used was a Q-switched Nd:YAG laser with a wavelength of 1064 nm , a pulse width of 10 ns of the wavelength and a 10 Hz frequency. Y-cut quartz was used as the standard to compare the derived SHG intensity.
poling. Thus, we determined that 1 h poling is appropriate to compare SHG intensity after 1 h poling. Fig. 2 depicts the influence of poling temperature on SHG intensity for ZnCl2 containing glasses. The increase in the poling temperature increases SHG intensity. The similar dependence of poling temperature could be seen in ZnBr2 containing glass, but the weakness of the durability for applying voltage of ZnBr2 containing glasses disturbed the investigation for applied voltage dependence of SHG. The polishing of the anode side of the ZnCl2 containing glass had no influence on SHG intensity and fringe pattern appearance. On the other hand, 5–10 μm of cathode side polishing, shown by the broken line in Fig. 3(A), of 10ZTB glass resulted in a markedly decreased SHG intensity. Further by 5–10 μm polishing diminished the fringe pattern shown on the dotted line. The similar influences are seen for 10ZBTB glass, the polishing was significant for SHG by polishing cathode side, but no influence in SHG can be found by polishing the anode side as shown in Fig. 3(B) and (C). Further, all of the samples were fully transparent in visible and near IR region. Thus, absorption is not necessary to the measurements.
3. Results 4. Discussion First of all, we discuss the influence of ZnCl2 and ZnBr2 contents on SHG intensity. Fig. 1(A) and (B) shows SHG intensity as a function of ZnCl2 or ZnBr2 content after poling at 310 °C at 2 kV for 1 h and at 300 °C at 1 kV for 1 h, respectively. As one can see, SHG intensity linearly increases with ZnCl2 or ZnBr2 content, and that of glass with 10 mol% addition of ZnCl2 increases to a value twice as large as that of ZnCl2-free glass. The addition of ZnBr2 showed a similar effect in increasing ZnBr2 content and the linear increase with the content. One of the important points was that the increase was more drastic in ZnBr2 glasses. With respect to poling time, although SHG intensity increased with increasing poling time initially, SHG intensity saturated after 1 h
Fig. 1. SH intensity as a function of (A) ZnCl2 content after poling at 310 °C at 2 kV for 1 h and (B) ZnBr2 content at 300 °C at 1 kV for 1 h.
The mechanism of SHG for thermally poled glasses is still controversial. There are two main proposals on the mechanism. One is the alignment of dipoles in the glasses3) and the other is the following process expressed as ð2Þ
χ
ð3Þ
= 3χ Edc ;
ð1Þ
where χ(2) and χ(3) are second- and third-order nonlinear optical susceptibilities, respectively, and Edc is the dc electric field resulting from the migration of ions [4]. In the latter case, the migration of cations has been explored and a cation-deficient anode side was confirmed. As a matter of fact, the polishing of the anode side markedly decreased the SHG intensity, while no influence of cathode side polishing of the oxide glasses was found. This is interpreted as being due to the migration of alkali and/or alkaline earth ions to the cathode side resulting in the cation-deficient anode side causing Edc. The migration takes place for negatively charged ions as well as positive ions. However, from Eq. (1) the dc electric field resulting from the thermal poling can be considered as mainly causing SHG. The highly charged, large anions like oxygen ions can be considered as repulsing each other, and widely and relatively uniform dispersed in the poled glasses. Thus, a similar discussion is possible for the present oxyhalide glasses. Specifically, the rapid movement of anions of the cathode side may result in the development of electrical neutrality through the present glasses and disturbing the dispersion of the highly charged heavy cations, and made the thin anion-deficient layer
Fig. 2. SH intensity as a function of poling temperature for ZnCl2 containing glasses after poling at 310 °C at 2 kV.
H. Nasu et al. / Journal of Non-Crystalline Solids 357 (2011) 1013–1015
1015
near the cathode cause SHG. This is presumed in ZnCl2 containing glasses in Ref. [6] and made sure by the similar phenomena seen for the heavier bromine ions containing oxyhalide glasses. As pointed out in Ref. [6], we firstly reported the possibility of the formation of anion-deficient region near the cathode electrode. The similar phenomena caused in ZnBr2 containing glasses mean that the most movable ions whichever they are positively or negatively charged form the SHG layer. However, the forming region of SHG depends on the sign of the charge. The positively charged ions form the SHG layer near the anode, on the other hand, the negatively charged ions form it near the opposite cathode side, even for the heavy bromide ions. This series of the work was briefly reported in Ref [6], but the final conclusion is made by this paper. The increase of the influence of halide content on SHG intensity can be attributed to the increase of halide content. Further, the increase of dependence of the kinds of halide on SHG intensity may result from the heavier bromine ions relatively moved in short distance caused the larger dc electric field for compensating the applied high voltage. 5. Conclusion We successfully observed SHG from thermally poled ZnCl2 and ZnBr2 containing tellurite glasses. The intensity of SHG increased as the halides content was increased for both systems. With respect to the SHG mechanism, the polishing of anode side has no effect on SHG, on the other hand, the polishing of cathode side significantly weakened SHG, although the SHG layer has been reported to form in the anode side for halide-free glasses. It can be explained that the SHG layer formed the opposite charged ions, that is, singly and negative charged halide ions. This series of work on zinc halides containing glasses reveals that the movements of negative ions, even heavy bromide ions, are possible, and that the aniondeficient layer results in SHG layer. References [1] Y. Sasaki, Y. Ohmori, Appl. Phys. Lett. 39 (1981) 466. [2] R.A. Myers, N. Mukherjee, S.J. Brueck, App. Opt. Lett. 16 (1991) 1732. [3] H. Nasu, H. Okamoto, A. Mito, J. Matsuoka, K. Kamiya, Jpn. J. Appl. Phys. 32 (1993) L406. [4] M. Qiu, VilasecaR. , CojocaruC. , MartorellJ. , MizunamiliT. , Appl. Phys. 88 (2000) 4666. [5] K. Tanaka, A. Narasaki, K. Hirao, N. Soga, J. Non-Cryst. Solids 203 (1996) 49. [6] NasuH. , NagaikeY. , TakedaH. , HashimotoT. , KamiyaK. , Jpn. J. Appl. Phys. 44 (2005) L943.
Fig. 3. (A)SH intensity for no polishing (solid line), 5–10 μm of anode side polishing (broken line) and polishing 5–10 μm cathode side (dotted line) of cathode side for ZnCl2 containing glasses poled at 310 °C at 2 kV for 1 h. (B) SH intensity for no polishing (before) (solid line), and after 50 μm polishing anode surface (broken line) for ZnBr2 containing glasses poled 300 °C at 1 kV. (C) SH intensity for no polishing (before) (solid line),after 50 μm polishing cathode surface (broken line) and after 100 μm polishing cathode surface (dotted line) for ZnBr2 containing glasses poled 300 °C at 1 kV.