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Second-order optical nonlinearity in thermally poled Pyrex borosilicate glass
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Optics Communications 281 (2008) 1263–1267 www.elsevier.com/locate/optcom
Second-order optical nonlinearity in thermally poled Pyrex borosilicate glass Honglin An *, Simon Fleming Optical Fibre Technology Centre, University of Sydney, 206 National Innovation Centre, Australian Technology Park, Eveleigh, NSW 1430, Australia Received 4 October 2007; received in revised form 26 October 2007; accepted 26 October 2007
Abstract The thermal poling method was utilized to create second-order optical nonlinearity in Pyrex borosilicate glass. The distribution and amplitude of the induced nonlinearity were characterized with second harmonic microscopy. The induced optical nonlinearity was found in a thin layer around 1.9 lm under the anode surface with a magnitude as high as 0.24 pm/V, comparable to that observed in fused silica samples. SEM observation of the cross-section of the poled glass region, after it had been etched in diluted hydrofluoric acid for several minutes, revealed an etched trench, 1.8 lm under the anode edge and 0.3 lm in width; while in post-annealed samples, no such etched trench could be observed. The effect of poling voltage on the magnitude of the induced nonlinearity was also studied, where the results showed that higher poling voltage resulted in higher nonlinearity with a threshold of 0.9 kV. 2007 Elsevier B.V. All rights reserved. Keywords: Thermal poling; Optical glass; Second harmonic generation
1. Introduction Pristine silicate optical glasses do not possess secondorder optical nonlinearity (SON) due to their structural inversion symmetry. This structural symmetry can be broken by a thermal poling method, which can induce a large SON of the order of 1 pm/V in fused silica [1]. The underlying mechanism is now generally believed to be the creation of a strong space-charge field which is formed through the migration of mobile charges (mainly alkali ions) and subsequently frozen in when the poled sample is cooled down to room-temperature with the poling voltage still applied [2,3]. Silicate glasses with this high level of SON, in addition to their proven excellent optical transparency, high laser-damage threshold and ready availability, are promising materials for optical applications such as second harmonic (SH) generation, electro-optic modulation and switching, and naturally have attracted intense
*
Corresponding author. Tel.: +61 02 9351 1983; fax: +61 02 9351 1911. E-mail address: [email protected] (H. An).
0030-4018/$ - see front matter 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2007.10.098
research attention. While much effort has been directed at increasing the SON magnitude and characterizing its spatial profile, the SON stability still remains a very important issue, especially for long-term practical applications of optical devices based on these nonlinear optical glasses. The SON stability is directly related to the space-charge recombination process, which is governed by charge (ion or electron) diffusion and field-driven migration in the poled glasses. As a result, optical glasses with higher activation energy for charge migration should favor more stable SON. Experimental results have demonstrated a more stable SON in borosilicate glasses, which have a higher activation energy for charge migration and can be effectively utilized to confine the SON distribution to a borosilicate layer [4,5]. Results from room-temperature corona-poled silicate thin films show a similar conclusion [6]. We have already reported our results of inducing large SON in Schott D263 borosilicate glass [7]. Corning Pyrex glass, another borosilicate glass, is scientifically interesting to consider since it contains even fewer alkali ions, which are known to play important roles in the SON creation process. It is also technologically important since, as silicon
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H. An, S. Fleming / Optics Communications 281 (2008) 1263–1267
thermal expansion matching glass, it has been extensively used to produce glass wafers and substrates for use in integrated optics and micro-electrical–mechanical systems. In this paper, we report our result of thermal poling on this Pyrex glass. Both the distribution and the magnitude of the induced SON were measured. The distribution of mobile alkali and alkaline-earth ions was characterized with energy dispersive X-ray spectrometry (EDS) in conjunction with scanning electron microscopy (SEM). The effect of poling voltage on the magnitude of the induced SON was also studied. 2. Experimental results The borosilicate glass samples used in this experiment were commercially available 0.4 mm thick Corning Pyrex plates with a composition (by weight) of 80.6% SiO2, 13.0% B2O3, 2.3% Al2O3, 4.0% Na2O, and 0.1% K2O and a volume electrical resistivity of 106.1 ohm cm at 250 C. Due to its relatively low electrical resistivity, during the poling process, the voltage had to be applied step by step to avoid ‘‘thermal runaway’’ and dielectric breakdown [8]. In most cases, the voltage was increased from 1.5 to 1.8 kV with a step of 0.1 kV every 3 or 5 min. The poling temperature was in the range of 250–280 C. After 1.8 kV was reached, the voltage was kept on for a fixed time and then the heater on which the glass plate was placed was switched off with the poling voltage still applied, allowing the sample to cool down naturally to room-temperature, at which point the voltage was switched off. For specimen preparation for SH microscopy, the poled samples were adhered together with epoxy and sectioned into small blocks to expose the poled cross-sections, which were finally polished to an optical finish. For SEM, the polished cross-section was further etched in 10% (by volume) hydrofluoric (HF) acid solution for several min before being coated for SEM inspection. The spatial distribution and magnitude of the SON in the poled borosilicate glass samples were characterized by the SH microscopy described in detail elsewhere [9], with Y-cut crystalline quartz (d11 = 0.45 pm/V) as reference. Briefly, the SH microscopy was conducted with an inverted Leica DMIRBE microscope equipped with a Leica TCS2MP confocal system and Coherent Verdi-Mira tunable pulsed titanium sapphire laser. An excitation wavelength of 830 nm was used, with pulses in the 100– 200 fs range. The microscope is also equipped with dual photomultiplier transmitted light detectors, receiving the two-photon fluorescence signal (505–650 nm) in channel 1 (in some cases where there was a lack of fluorescence signal, a 543 nm helium–neon laser light was used to detect the transmission image of samples) and SH signal (405– 425 nm) in channel 2. Digital images from channels 1 and 2 can be acquired simultaneously and overlaid together for profile analysis by accompanying software. The optical resolution of the SH microscopy was estimated to be around 0.4 lm when using a 40· objective lens.
Within the range of chosen poling conditions, the observed SON was found to be always confined to a narrow layer beneath the anode surface. A typical SH result is shown in Fig. 1. The sample was poled at 260 C with the poling voltage applied from 1.2 to 1.8 kV with a step of 0.1 kV every 5 min; at 1.8 kV the sample was poled for another 5 min before the heater was switched off. For SON profile analysis, on the overlay image, a line-scan crossing the anode surface and the SON layer was conducted and the resulting SH signal profile is obtained, as shown in Fig. 2. It is found that the distance between the anode surface and the centre of this SON layer was within a range of 1.8–1.9 lm, which is larger than that observed in the Schott D263 sample [7]. As the poling voltage and poling time are very similar for the two cases, the reason for this difference is believed to be due to the lower concentrations of alkali ions in the Pyrex glass samples. Also note that the poling temperature for the Schott D263 samples
Fig. 1. Micrograph of the SH signal from the cross-section of the poled Pyrex sample. The image size is 93.75 lm · 93.75 lm. Inset: schematic diagram of the sample cross-section (in the x–y plane) under observation.
Fig. 2. Spatial profile of the SH signal along the poling direction in the poled sample.
H. An, S. Fleming / Optics Communications 281 (2008) 1263–1267
was even higher (300 C) than that (260 C) for the Pyrex samples. If all the other parameters except the temperature were the same, we would expect a SON layer to be formed further away from the anode surface in the Schott D263 sample than in the Pyrex sample. This was not the case and this result can be explained within the space-charge field model [2]: during poling, mobile cations (mainly alkali ions) migrate towards the cathode, leaving behind a cationdepleted region, often called depletion region, in which a strong space-charge field and a resultant optical nonlinearity will be formed. Ignoring possible external charge injection to the anode glass (as is reasonable in the case of short duration poling),pthe width of this depletion region can be ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi calculated to be 2eV =eN 0 , where e is the dielectric permittivity, V is the poling voltage, and N0 is the number density of alkali ions (mainly Na+ ions). It is obvious that, under similar poling conditions, the SON layer in the Pyrex, which contains fewer alkali ions than the Schott D263, will be farther away from the anode surface. To test the effect of poling voltage on the SON formation, we poled a group of samples at 260 C. The voltages were applied in steps: from 1.5 to 1.8 kV with a step of 0.1 kV every 5 min; at 1.8 kV for another 15 min for a total poling time of 30 min. For lower voltages than 1.5 kV the full voltage was applied in one step once the temperature had reached 260 C. Higher voltages were also tried but usually resulted in surface arcing, so the highest voltage was limited to 1.8 kV. The result is shown in Fig. 3. As expected, the magnitude of the SON, d33, increased with higher poling voltages. In all cases, the SON was distributed in a narrow layer, whose width varied little for different poling voltages. In the space-charge field model, higher local SON means higher local frozen-in electric field. Since the observable SH signal was only found within this narrow layer, it is concluded that a strong frozen-in electric field was distributed within this layer. As most of the applied voltage drops across this narrow region, the magni-
Fig. 3. Magnitude of the induced SON as a function of the applied poling voltage.
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tudes of the average frozen-in electric field and the resultant effective SON d 33 ¼ vð2Þ =2 ¼ 3Efrozen vð3Þ =2, where v(3) is the intrinsic third-order susceptibility of the poled glass, will be linearly proportional to the applied voltage. Therefore, higher SON is expected if the problem of surface arcing can be overcome by adopting bigger glass plates. Poling at lower-temperature may also help prevent arcing to allow higher poling voltages to be applied. For soft glasses like soda lime and Pyrex, lower-temperature poling than normal poling conditions for fused silica will result in smaller thermal current and may bring further benefit, not only decreasing the risk of dielectric breakdown but also improving the stability of the induced SON, according to recent experimental results on a soda lime glass where the thermal poling induced current was controlled to be small in magnitude and slow in increasing rate to induce stable SON [10]. At 0.9 kV, the effective SON was less than 0.01 pm/V, suggesting a threshold, only above which did the induced SON become substantial. This result is in agreement with previous results [7,11]. Different poling conditions were tested in an attempt to obtain higher SON. A SON as large as 0.24 pm/V was obtained in samples poled with a two-stage procedure: first, the sample was poled at 280 C with the poling voltage increased from 1.0 to 1.5 kV with a step of 0.1 kV per minute; then, the temperature was dropped to 250 C with the poling voltage increased from 1.5 to 1.8 kV with a step of 0.1 kV every 3 min. The SH micrograph is shown in Fig. 4, in which the SH result from a fused silica sample (1 mm thick GE124 plate), poled at 280 C and 3.5 kV for 30 min, was also displayed for comparison. It can be seen that the SH signal from the Pyrex sample is almost of the same magnitude as that from the fused silica sample.
Fig. 4. Micrograph of the SH signal from the cross-section parallel to the poling direction of a block comprising a poled Pyrex and a GE124 fused silica sample. The image size is 46.875 lm · 46.875 lm.
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H. An, S. Fleming / Optics Communications 281 (2008) 1263–1267
The existence of a frozen-in electric field can alter the etching rate of silicate glasses in hydrofluoric acid (HF) [12–14]. To measure with higher precision the spatial profile of the induced SON in poled samples with SEM, some samples were poled under the same condition as those samples for Fig. 1 and subsequently etched in 10% (in volume) diluted HF at 22 C for 90 s. A typical result is shown in Fig. 5. An etched trench can be clearly observed under the anode surface. The trench layer is 0.3 lm wide and 1.7–1.9 lm (typically 1.77 lm) away from the anode edge. This result is in excellent consistence with that from the SH measurement, as shown in Fig. 1. The poled region was easy to identify. After poling, some precipitates appeared on the cathode surface, a similar phenomenon to that found in Schott D263 borosilicate glass and soda lime glass but in lesser degree [15,16]. The cathode surface of the poled samples was observed under a light microscope with a typical result shown in Fig. 6. To probe any possible crystalline phases, an X-ray diffraction (XRD) measurement was conducted on the cathode
Fig. 5. SEM micrograph of the etched cross-section of the anode side of a poled Pyrex sample.
Fig. 7. Distribution of chemical elements under the anode of a poled Pyrex glass sample. The vertical dashed line represents the position of the anode surface.
side of the poled sample by use of a Shimadzu 6000 diffractometer with Cu Ka radiation. The diffraction intensity of the sample was measured by a standard h 2h scan with a scan step of 0.02. No peaks corresponding to any crystalline phases were detected. The presence of such precipitates was believed to be due to the migration of mobile ions, Na+ ions in particular, from the bulk of the poled glass to the cathode surface. As it has been discussed [7], during poling, Na+ ions can be driven to the cathode side, cross the glass surface and be reduced at the cathode glass surface by electrons. The resultant Na atoms become sodium dioxide after being oxidized by the oxygen in the ambient air, causing the appearance of the precipitates. To determine the new spatial distribution of relevant ions during poling process, EDS in conjunction with SEM was conducted with line-scans along the direction of applied electric field across the cross-sections of the poled region of a sample poled under the same condition as the sample for Fig. 4. The result is shown in Fig. 7. The element Si was measured as a reference as it is immobile at the thermal poling temperature of 300 C. From inspection of Fig. 7, it is clear that Na+ ions have been depleted in a region 3.3 lm beneath the anode surface. In contrast, K+ ions were first depleted in the immediate region beneath the anode and then accumulated with a peak at 1.8 lm beneath the anode but still within the Na-depleted region. These results are similar to those observed in our previous borosilicate glass [7]. 3. Conclusions
Fig. 6. Light microscopy micrograph showing precipitates on the cathode surface of the poled Pyrex sample.
In summary, we have thermally poled Pyrex borosilicate glass at 250–280 C with voltages up to 1.8 kV. In all cases, the induced SON has been found to be confined to a narrow layer, 0.3 lm wide and <2.0 lm beneath the anode surface. Above a certain threshold, the magnitude of the
H. An, S. Fleming / Optics Communications 281 (2008) 1263–1267
induced SON is proportional to the applied poling voltages, and the largest SON obtained has reached 0.24 pm/ V for a poling voltage of 1.8 kV. Higher poling voltages should be able to induce larger SON. During the poling process, substantial compositional changes have been found in the near-anode region of the poled glass, with Na+ ions depleted from a region 3.3 lm beneath the anode and K+ ions first depleted from the immediate region beneath the anode and then accumulated at 1.8 lm beneath the anode surface. On the cathode glass surface, some precipitates can be observed; a result believed to be due to the Na+ ions crossing the cathode glass surface, being reduced there, and subsequently oxidized by oxygen in air. Acknowledgements This research was supported under the Australian Research Council’s Discovery funding scheme (project number DP0774404). The authors acknowledge the facilities as well as scientific and technical assistance from staff in the NANO Major National Research Facility at the Electron Microscope Unit, the University of Sydney.
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References [1] R.A. Myers, N. Mukherjee, S.R.J. Brueck, Opt. Lett. 16 (1991) 1732. [2] P.G. Kazansky, P.St.J. Russell, Opt. Commun. 110 (1994) 611. [3] T.G. Alley, S.R.J. Brueck, R.A. Myers, J. Non-Cryst. Solids 242 (1998) 165. [4] O. Deparis, C. Corbari, P.G. Kazansky, K. Sakaguchi, Appl. Phys. Lett. 84 (2004) 4857. [5] H. An, S. Fleming, Electron. Lett. 41 (2005) 953. [6] S. Horinouchi, H. Imai, H. Yamasaki, K. Fukao, G.J. Zhang, T. Kinoshita, K. Mito, H. Hirashima, K. Sasaki, J. Nonlinear Opt. Phys. Mater. 5 (1996) 189. [7] H. An, S. Fleming, Appl. Phys. Lett. 89 (2006) 181111. [8] F.C. Garcia, I.C.S. Carvalho, E. Hering, W. Margulis, B. Lesche, Appl. Phys. Lett. 72 (1998) 3252. [9] H. An, S. Fleming, G. Cox, Appl. Phys. Lett. 85 (2004) 5819. [10] A.L. Moura, M.T. de Araujo, M.V.D. Vermelho, J.S. Aitchison, J. Appl. Phys. 100 (2006) 033509. [11] Y. Yamamoto, H. Nasu, T. Hashimoto, K. Kamiya, J. Non-Cryst. Solids 281 (2001) 198. [12] W. Margulis, F. Laurell, Opt. Lett. 21 (1996) 1786. [13] T.G. Alley, S.R.J. Brueck, Opt. Lett. 23 (1998) 1170. [14] P. Blazkiewicz, W. Xu, D. Wong, S. Fleming, J. Opt. Soc. Am. B 19 (2002) 870. [15] M. Qiu, F. Pi, G. Orriols, M. Bibiche, J. Opt. Soc. Am. B 15 (1998) 1362. [16] H. An, S. Fleming, J. Opt. Soc. Am. B 23 (2006) 2303.
Optics Communications 281 (2008) 1263–1267 www.elsevier.com/locate/optcom
Second-order optical nonlinearity in thermally poled Pyrex borosilicate glass Honglin An *, Simon Fleming Optical Fibre Technology Centre, University of Sydney, 206 National Innovation Centre, Australian Technology Park, Eveleigh, NSW 1430, Australia Received 4 October 2007; received in revised form 26 October 2007; accepted 26 October 2007
Abstract The thermal poling method was utilized to create second-order optical nonlinearity in Pyrex borosilicate glass. The distribution and amplitude of the induced nonlinearity were characterized with second harmonic microscopy. The induced optical nonlinearity was found in a thin layer around 1.9 lm under the anode surface with a magnitude as high as 0.24 pm/V, comparable to that observed in fused silica samples. SEM observation of the cross-section of the poled glass region, after it had been etched in diluted hydrofluoric acid for several minutes, revealed an etched trench, 1.8 lm under the anode edge and 0.3 lm in width; while in post-annealed samples, no such etched trench could be observed. The effect of poling voltage on the magnitude of the induced nonlinearity was also studied, where the results showed that higher poling voltage resulted in higher nonlinearity with a threshold of 0.9 kV. 2007 Elsevier B.V. All rights reserved. Keywords: Thermal poling; Optical glass; Second harmonic generation
1. Introduction Pristine silicate optical glasses do not possess secondorder optical nonlinearity (SON) due to their structural inversion symmetry. This structural symmetry can be broken by a thermal poling method, which can induce a large SON of the order of 1 pm/V in fused silica [1]. The underlying mechanism is now generally believed to be the creation of a strong space-charge field which is formed through the migration of mobile charges (mainly alkali ions) and subsequently frozen in when the poled sample is cooled down to room-temperature with the poling voltage still applied [2,3]. Silicate glasses with this high level of SON, in addition to their proven excellent optical transparency, high laser-damage threshold and ready availability, are promising materials for optical applications such as second harmonic (SH) generation, electro-optic modulation and switching, and naturally have attracted intense
*
Corresponding author. Tel.: +61 02 9351 1983; fax: +61 02 9351 1911. E-mail address: [email protected] (H. An).
0030-4018/$ - see front matter 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2007.10.098
research attention. While much effort has been directed at increasing the SON magnitude and characterizing its spatial profile, the SON stability still remains a very important issue, especially for long-term practical applications of optical devices based on these nonlinear optical glasses. The SON stability is directly related to the space-charge recombination process, which is governed by charge (ion or electron) diffusion and field-driven migration in the poled glasses. As a result, optical glasses with higher activation energy for charge migration should favor more stable SON. Experimental results have demonstrated a more stable SON in borosilicate glasses, which have a higher activation energy for charge migration and can be effectively utilized to confine the SON distribution to a borosilicate layer [4,5]. Results from room-temperature corona-poled silicate thin films show a similar conclusion [6]. We have already reported our results of inducing large SON in Schott D263 borosilicate glass [7]. Corning Pyrex glass, another borosilicate glass, is scientifically interesting to consider since it contains even fewer alkali ions, which are known to play important roles in the SON creation process. It is also technologically important since, as silicon
1264
H. An, S. Fleming / Optics Communications 281 (2008) 1263–1267
thermal expansion matching glass, it has been extensively used to produce glass wafers and substrates for use in integrated optics and micro-electrical–mechanical systems. In this paper, we report our result of thermal poling on this Pyrex glass. Both the distribution and the magnitude of the induced SON were measured. The distribution of mobile alkali and alkaline-earth ions was characterized with energy dispersive X-ray spectrometry (EDS) in conjunction with scanning electron microscopy (SEM). The effect of poling voltage on the magnitude of the induced SON was also studied. 2. Experimental results The borosilicate glass samples used in this experiment were commercially available 0.4 mm thick Corning Pyrex plates with a composition (by weight) of 80.6% SiO2, 13.0% B2O3, 2.3% Al2O3, 4.0% Na2O, and 0.1% K2O and a volume electrical resistivity of 106.1 ohm cm at 250 C. Due to its relatively low electrical resistivity, during the poling process, the voltage had to be applied step by step to avoid ‘‘thermal runaway’’ and dielectric breakdown [8]. In most cases, the voltage was increased from 1.5 to 1.8 kV with a step of 0.1 kV every 3 or 5 min. The poling temperature was in the range of 250–280 C. After 1.8 kV was reached, the voltage was kept on for a fixed time and then the heater on which the glass plate was placed was switched off with the poling voltage still applied, allowing the sample to cool down naturally to room-temperature, at which point the voltage was switched off. For specimen preparation for SH microscopy, the poled samples were adhered together with epoxy and sectioned into small blocks to expose the poled cross-sections, which were finally polished to an optical finish. For SEM, the polished cross-section was further etched in 10% (by volume) hydrofluoric (HF) acid solution for several min before being coated for SEM inspection. The spatial distribution and magnitude of the SON in the poled borosilicate glass samples were characterized by the SH microscopy described in detail elsewhere [9], with Y-cut crystalline quartz (d11 = 0.45 pm/V) as reference. Briefly, the SH microscopy was conducted with an inverted Leica DMIRBE microscope equipped with a Leica TCS2MP confocal system and Coherent Verdi-Mira tunable pulsed titanium sapphire laser. An excitation wavelength of 830 nm was used, with pulses in the 100– 200 fs range. The microscope is also equipped with dual photomultiplier transmitted light detectors, receiving the two-photon fluorescence signal (505–650 nm) in channel 1 (in some cases where there was a lack of fluorescence signal, a 543 nm helium–neon laser light was used to detect the transmission image of samples) and SH signal (405– 425 nm) in channel 2. Digital images from channels 1 and 2 can be acquired simultaneously and overlaid together for profile analysis by accompanying software. The optical resolution of the SH microscopy was estimated to be around 0.4 lm when using a 40· objective lens.
Within the range of chosen poling conditions, the observed SON was found to be always confined to a narrow layer beneath the anode surface. A typical SH result is shown in Fig. 1. The sample was poled at 260 C with the poling voltage applied from 1.2 to 1.8 kV with a step of 0.1 kV every 5 min; at 1.8 kV the sample was poled for another 5 min before the heater was switched off. For SON profile analysis, on the overlay image, a line-scan crossing the anode surface and the SON layer was conducted and the resulting SH signal profile is obtained, as shown in Fig. 2. It is found that the distance between the anode surface and the centre of this SON layer was within a range of 1.8–1.9 lm, which is larger than that observed in the Schott D263 sample [7]. As the poling voltage and poling time are very similar for the two cases, the reason for this difference is believed to be due to the lower concentrations of alkali ions in the Pyrex glass samples. Also note that the poling temperature for the Schott D263 samples
Fig. 1. Micrograph of the SH signal from the cross-section of the poled Pyrex sample. The image size is 93.75 lm · 93.75 lm. Inset: schematic diagram of the sample cross-section (in the x–y plane) under observation.
Fig. 2. Spatial profile of the SH signal along the poling direction in the poled sample.
H. An, S. Fleming / Optics Communications 281 (2008) 1263–1267
was even higher (300 C) than that (260 C) for the Pyrex samples. If all the other parameters except the temperature were the same, we would expect a SON layer to be formed further away from the anode surface in the Schott D263 sample than in the Pyrex sample. This was not the case and this result can be explained within the space-charge field model [2]: during poling, mobile cations (mainly alkali ions) migrate towards the cathode, leaving behind a cationdepleted region, often called depletion region, in which a strong space-charge field and a resultant optical nonlinearity will be formed. Ignoring possible external charge injection to the anode glass (as is reasonable in the case of short duration poling),pthe width of this depletion region can be ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi calculated to be 2eV =eN 0 , where e is the dielectric permittivity, V is the poling voltage, and N0 is the number density of alkali ions (mainly Na+ ions). It is obvious that, under similar poling conditions, the SON layer in the Pyrex, which contains fewer alkali ions than the Schott D263, will be farther away from the anode surface. To test the effect of poling voltage on the SON formation, we poled a group of samples at 260 C. The voltages were applied in steps: from 1.5 to 1.8 kV with a step of 0.1 kV every 5 min; at 1.8 kV for another 15 min for a total poling time of 30 min. For lower voltages than 1.5 kV the full voltage was applied in one step once the temperature had reached 260 C. Higher voltages were also tried but usually resulted in surface arcing, so the highest voltage was limited to 1.8 kV. The result is shown in Fig. 3. As expected, the magnitude of the SON, d33, increased with higher poling voltages. In all cases, the SON was distributed in a narrow layer, whose width varied little for different poling voltages. In the space-charge field model, higher local SON means higher local frozen-in electric field. Since the observable SH signal was only found within this narrow layer, it is concluded that a strong frozen-in electric field was distributed within this layer. As most of the applied voltage drops across this narrow region, the magni-
Fig. 3. Magnitude of the induced SON as a function of the applied poling voltage.
1265
tudes of the average frozen-in electric field and the resultant effective SON d 33 ¼ vð2Þ =2 ¼ 3Efrozen vð3Þ =2, where v(3) is the intrinsic third-order susceptibility of the poled glass, will be linearly proportional to the applied voltage. Therefore, higher SON is expected if the problem of surface arcing can be overcome by adopting bigger glass plates. Poling at lower-temperature may also help prevent arcing to allow higher poling voltages to be applied. For soft glasses like soda lime and Pyrex, lower-temperature poling than normal poling conditions for fused silica will result in smaller thermal current and may bring further benefit, not only decreasing the risk of dielectric breakdown but also improving the stability of the induced SON, according to recent experimental results on a soda lime glass where the thermal poling induced current was controlled to be small in magnitude and slow in increasing rate to induce stable SON [10]. At 0.9 kV, the effective SON was less than 0.01 pm/V, suggesting a threshold, only above which did the induced SON become substantial. This result is in agreement with previous results [7,11]. Different poling conditions were tested in an attempt to obtain higher SON. A SON as large as 0.24 pm/V was obtained in samples poled with a two-stage procedure: first, the sample was poled at 280 C with the poling voltage increased from 1.0 to 1.5 kV with a step of 0.1 kV per minute; then, the temperature was dropped to 250 C with the poling voltage increased from 1.5 to 1.8 kV with a step of 0.1 kV every 3 min. The SH micrograph is shown in Fig. 4, in which the SH result from a fused silica sample (1 mm thick GE124 plate), poled at 280 C and 3.5 kV for 30 min, was also displayed for comparison. It can be seen that the SH signal from the Pyrex sample is almost of the same magnitude as that from the fused silica sample.
Fig. 4. Micrograph of the SH signal from the cross-section parallel to the poling direction of a block comprising a poled Pyrex and a GE124 fused silica sample. The image size is 46.875 lm · 46.875 lm.
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H. An, S. Fleming / Optics Communications 281 (2008) 1263–1267
The existence of a frozen-in electric field can alter the etching rate of silicate glasses in hydrofluoric acid (HF) [12–14]. To measure with higher precision the spatial profile of the induced SON in poled samples with SEM, some samples were poled under the same condition as those samples for Fig. 1 and subsequently etched in 10% (in volume) diluted HF at 22 C for 90 s. A typical result is shown in Fig. 5. An etched trench can be clearly observed under the anode surface. The trench layer is 0.3 lm wide and 1.7–1.9 lm (typically 1.77 lm) away from the anode edge. This result is in excellent consistence with that from the SH measurement, as shown in Fig. 1. The poled region was easy to identify. After poling, some precipitates appeared on the cathode surface, a similar phenomenon to that found in Schott D263 borosilicate glass and soda lime glass but in lesser degree [15,16]. The cathode surface of the poled samples was observed under a light microscope with a typical result shown in Fig. 6. To probe any possible crystalline phases, an X-ray diffraction (XRD) measurement was conducted on the cathode
Fig. 5. SEM micrograph of the etched cross-section of the anode side of a poled Pyrex sample.
Fig. 7. Distribution of chemical elements under the anode of a poled Pyrex glass sample. The vertical dashed line represents the position of the anode surface.
side of the poled sample by use of a Shimadzu 6000 diffractometer with Cu Ka radiation. The diffraction intensity of the sample was measured by a standard h 2h scan with a scan step of 0.02. No peaks corresponding to any crystalline phases were detected. The presence of such precipitates was believed to be due to the migration of mobile ions, Na+ ions in particular, from the bulk of the poled glass to the cathode surface. As it has been discussed [7], during poling, Na+ ions can be driven to the cathode side, cross the glass surface and be reduced at the cathode glass surface by electrons. The resultant Na atoms become sodium dioxide after being oxidized by the oxygen in the ambient air, causing the appearance of the precipitates. To determine the new spatial distribution of relevant ions during poling process, EDS in conjunction with SEM was conducted with line-scans along the direction of applied electric field across the cross-sections of the poled region of a sample poled under the same condition as the sample for Fig. 4. The result is shown in Fig. 7. The element Si was measured as a reference as it is immobile at the thermal poling temperature of 300 C. From inspection of Fig. 7, it is clear that Na+ ions have been depleted in a region 3.3 lm beneath the anode surface. In contrast, K+ ions were first depleted in the immediate region beneath the anode and then accumulated with a peak at 1.8 lm beneath the anode but still within the Na-depleted region. These results are similar to those observed in our previous borosilicate glass [7]. 3. Conclusions
Fig. 6. Light microscopy micrograph showing precipitates on the cathode surface of the poled Pyrex sample.
In summary, we have thermally poled Pyrex borosilicate glass at 250–280 C with voltages up to 1.8 kV. In all cases, the induced SON has been found to be confined to a narrow layer, 0.3 lm wide and <2.0 lm beneath the anode surface. Above a certain threshold, the magnitude of the
H. An, S. Fleming / Optics Communications 281 (2008) 1263–1267
induced SON is proportional to the applied poling voltages, and the largest SON obtained has reached 0.24 pm/ V for a poling voltage of 1.8 kV. Higher poling voltages should be able to induce larger SON. During the poling process, substantial compositional changes have been found in the near-anode region of the poled glass, with Na+ ions depleted from a region 3.3 lm beneath the anode and K+ ions first depleted from the immediate region beneath the anode and then accumulated at 1.8 lm beneath the anode surface. On the cathode glass surface, some precipitates can be observed; a result believed to be due to the Na+ ions crossing the cathode glass surface, being reduced there, and subsequently oxidized by oxygen in air. Acknowledgements This research was supported under the Australian Research Council’s Discovery funding scheme (project number DP0774404). The authors acknowledge the facilities as well as scientific and technical assistance from staff in the NANO Major National Research Facility at the Electron Microscope Unit, the University of Sydney.
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