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Defect centers and room temperature persistent spectral hole burning in X-ray irradiated Eu3+-doped glass
Journal of Luminescence 96 (2002) 163–169
Defect centers and room temperature persistent spectral hole burning in X-ray irradiated Eu3+-doped glass Masayuki Nogami*, Tomotaka Ishikawa, Tomokatsu Hayakawa Department of Materials Science and Engineering, Nagoya Institute of Technology, Showa Nagoya, 466-8555 Japan Received 6 April 2001; received in revised form 20 July 2001; accepted 23 July 2001
Abstract We have investigated the room temperature persistent spectral hole burning of X-ray irradiated Eu3+-doped Al2O3aSiO2 glasses. The sol–gel-derived Eu3+-doped glass heated in air at 8001C and irradiated with X-ray exhibits the hole spectra at room temperature, while on the other hand no hole is formed in the glass heated at 10001C. We can attribute these differences to the different formation of the defect centers by the X-ray irradiation. The formation of the activated Eu3+ ions together with the oxygen defect centers plays an important role on the room temperature hole burning. The lifetime of the burnt hole is estimated to be B1.5 105 s at 200 K. r 2002 Elsevier Science B.V. All rights reserved. Keywords: Hole burning; Eu3+; Glass; X-ray; Sol–gel
1. Introduction The Sm2+- and Eu3+-doped glasses have been attracting much interest, because of their room temperature persistent spectral hole burning (PSHB) properties [1–3] and a great potential in a high-density frequency-selective optical memory [4,5]. The Sm2+ and Eu3+ ions have the same 4f 6 electron configuration and the hole spectra are observed on the excitation spectra of the 7F0-5D0 transition of these ions. Although many materials have been developed to display the PSHB property, the hole burning mechanism still remains *Corresponding author. Tel.: +81-52-735-5285; fax: +8152-735-5285. E-mail address: [email protected] (M. Nogami).
unclear. One proposed mechanism for the hole burning is the photo-induced charge transfer between the rare earth ions, that is, the ionization and reduction for the Sm2+ and Eu3+ ions, respectively, by laser irradiation [6,7]. The released electrons and holes are considered to be captured in the Sm3+ and Eu2+ ions, respectively. According to this reaction, the glasses should be prepared to dope the rare earth ions having different valences. For preparing the room temperature PSHB glasses, they are melted or heat-treated in hydrogen gas atmosphere in order to reduce the rare earth ions [1–3,7]. In contrast, we successfully prepared the Sm2+- and Eu3+-doped glasses using a sol–gel method and observed the PSHB at room temperature [8–10]. These sol–gel-derived glasses
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are treated by heating in hydrogen gas atmosphere or irradiating an X-ray, which resulted in the PSHB at room temperature. We have found that irrespective of whether the holes are burned at room temperature or not, it is dependent on the preparation conditions of glass before the X-ray irradiation [10]. Fig. 1 is an illustration of the excitation spectra after laser irradiation for burning. It is clear that the hole is only observed at room temperature in the glass heated in air at 8001C, followed by irradiation of X-ray. On the other hand, the hole is not observed in the glasses heated at 10001C followed by X-ray irradiation. We also found that in the sol–gel derived glasses, the room temperature PSHB is observed only in the Al3+ ion-containing glasses, and not in the SiO2 glass [9]. The Eu3+ ions are not necessarily reduced into Eu2+ by the X-ray irradiation [10]. From these results, we consider that the X-ray irradiation also contributes to the hole burning. Further investigations are necessary to study the X-ray irradiation on the hole burning and to discuss the hole burning reaction of the Eu3+ ions. In this work, we measure the optical absorption and electron spin resonance (ESR) spectra of the Eu3+-doped Al2O3aSiO2 glasses and relate the
Fig. 1. The hole burning spectra, burned and measured at 298 K, of Eu3+-doped 1Al2O3 9SiO2 glass heated in air at 8001C (a), followed by irradiating with the X-ray for 14 h (b). Curve (c) is for the glass heated at 10001C in air, followed by X-ray irradiation for 14 h.
PSHB property with the defect centers induced in the glass by the X-ray irradiation.
2. Experiments 1Al2O3 9SiO2 (mole ratio) glasses containing 1 wt% Eu2O3 were prepared by the hydrolysis of Si(OC2H5)4, Al(OC4Hsec 9 )3, and EuCl3 6H2O. The starting materials were commercially available and used as received. A detailed explanation of the gel preparation is given elsewhere [9,10]. The gel was heated to form the glass in air at 8001C and 10001C for 2 h. The X-ray irradiation was performed using the Cu Ka line (Rigaku RAD-B system) with 40 kV and 20 mA at room temperature for 14 h. The sample of B0.5 mm thickness was placed at a distance of 15 cm from the X-ray source. Optical absorption spectra were measured with a Jasco, U-Best spectrometer. The ESR measurement was performed using Jasco, JESFE ME3X spectrometer at room temperature. The g-values of the obtained ESR signals were calibrated by the utilization of diphenyl-picrylhydrazal (DPPH). The PSHB was observed on the excitation spectra of the 7F0-5D0 transition of Eu3+ by scanning a Rhodamine 6G dye laser over the 7 F0-5D0 transition while monitoring the fluorescence of the 5D0-7F2 transition at 619 nm. As previously reported, when the glasses are heated in air, a large amount of hydroxyl bonds remain in the glass and the spectral holes are burned by the rearrangement of the hydroxyl bonds surrounding the Eu3+ ions [11,12]. To eliminate the effect of the hydroxyl bonds on the hole burning, the glasses were heated in a vacuum at 8001C for 2 h before the X-ray irradiation. The hole was burned by irradiating with a laser with a power of B300 mW at a B2 mm diameter spot at the wavelength around the peak position of the 7F0-5D0 transition of Eu3+ ions for 30 min. Shown in Fig. 1 are the spectra, burned and measured at room temperature, for the glasses heated in air at 8001C and 10001C, followed by irradiating X-ray for 14 h.
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3. Results and discussion 3.1. Optical absorption and ESR spectra The glasses, obtained by heating in air, are transparent and colorless irrespective of whether they contain Eu2O3 or not. Fig. 2 shows the optical absorption spectra of the samples heated in air at 8001C ((a) and (b)) and the absorptivity change induced by the X-ray irradiation ((c)–(e)). The difference shown in the spectra is obtained by subtracting the absorption before irradiation from that after irradiation. The Al2O3aSiO2 glass has a strong absorption above 33 000 cm1 due to the fundamental absorption of the glass structure. In addition to this absorption, the Eu3+-doped Al2O3aSiO2 glass shows a broad absorption band peaking at around 45 000 cm1, which can be
Fig. 2. The optical absorption spectra ((a) and (b)) of 1Al2O3 9SiO2 glasses with (a) and without (b) Eu2O3 heated in air at 8001C and change in the absorption spectra ((c)–(e)) on irradiation of X-ray for glasses heated at 8001C (d), and 10001C (e). Curve (c) is for glass containing no Eu2O3. The spectral curves are vertically shifted for clarity and the scale corresponds to the absorption coefficient of 8 cm1.
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assigned to the existence of the Eu2+ ions as discussed later. The absorption peaks due to the Eu3+ ions are also observed at 465 and 395 nm in the visible wavelength region, the absorption coefficients of which are less than a few cm1 and do not appear in the same scale of Fig. 2. When the glasses are irradiated with the X-ray, large changes are observed in the absorption properties, which are strongly dependent on the glass composition and heating temperature of glass in air. The Al2O3aSiO2 glass containing no Eu3+ ions shows an induced absorption band peaking at 34 000 cm1 with shoulders around 27 000 and 37 000 cm1. When the Eu3+ ions are doped in the Al2O3aSiO2 glass, the above absorption intensity diminishes, instead, a strong absorption band peaking at 39 000 cm1 with a shoulder at around 32 000 cm1 is induced by the X-ray irradiation. On the other hand, the glass heated at 10001C becomes grayish by the X-ray irradiation and shows a broad band ranging from 13 000 to 45 000 cm1, which is quite different from that of the glass heated at 8001C. These changes in the absorption spectra suggest that the formation of some defect centers and breaking of the glass structure occur by the X-ray irradiation. We found no clear change in the absorption intensity of the Eu3+ ions. The ESR spectra of the samples treated in the same way were measured at room temperature, which are shown in Fig. 3. Note that the Eu3+doped Al2O3aSiO2 glass heated in air has an intense ESR signal between 0.13 and 0.25 T. The Eu2+ ions exhibit an ESR signal between 0.13 and 0.25 T [13], while the Eu3+ ions are non-magnetic and do not contribute to the ESR spectra. This result indicates that a part of the Eu3+ ions are reduced into Eu2+ in the Al2O3aSiO2 glass only by heating in air. After the X-ray irradiation, a sharp ESR signal is observed at around 0.34 T in addition to the signals of the Eu2+ ions. In order to check the effect of the glass components on the 0.34 T line, the Eu3+-doped SiO2 glass was irradiated with the X-ray to measure the ESR spectra, but no signal was observed at this position. This result and the comparison of the g-value with that of the free electron (g ¼ 2:0023), the signal at 0.34 T can be assigned to the hole
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Fig. 3. The ESR spectra of Eu3+-doped 1Al2O3 9SiO2 glasses heated in air at 8001C (a), followed by irradiating with X-ray (b). Curve (c) is for the glass heated at 10001C in air before X-ray irradiation. Curve (d) is for glass without Eu2O3, heated in air at 8001C, followed by X-ray irradiation.
centers trapped with oxygen bounded to the Al ions. Further, it is interesting to note, in Fig. 3, that the intensity of the Eu2+-signal changes little after the X-ray irradiation (see Fig. 3(b)). This strongly suggests that the Eu3+ ions hardly reduced into Eu2+ by the X-ray irradiation. Despite this, however, a strong optical absorption band in the range of 30 000–42 000 cm1 induced by the X-ray irradiation (Fig. 2(d)) cannot be explained from the ESR experiment. Hosono and Kawazoe measured the optical absorption and ESR spectra of the g-ray irradiated SiO2 glass containing 0.6 mol% Al2O3 and discussed that the induced absorption in the range of 12 000–50 000 cm1 can be attributed to the Alrelated defect centers such as AlaOHC and Al E0
centers [14]. The absorption spectrum of the glass that is heated at 10001C and irradiated with the Xray (Fig. 2(e)) resembles that of the g-ray irradiated glass, suggesting that the Al-related defect centers are formed in our X-ray irradiated Eu3+doped glass. Further, as shown in Figs. 2 and 3, we notice that the Eu3+ ions are partially reduced into Eu2+ by only heating in air atmosphere, but additional reduction does not take place by irradiating the X-ray. On the other hand, the origin of the absorption bands induced in the glass that is heated at 8001C and irradiated by the X-ray is not clear. Mackey and Nahum measured the optical absorption spectra of the Eu3+-doped silicate glasses after the X-ray irradiation and attributed the induced band at around 30 000 cm1 to the electron-trapped Eu3+ ions [15]. This center is considered to be different from the Eu2+ ions and designated as an [Eu3+] state. We have already reported the optical absorption in the Eu3+-doped phosphate glasses by the X-ray irradiation, where the electrons released from the POH bonds are considered to be captured in the nearest Eu3+ ions [16]. Accordingly, we can postulate that the induced absorption band between 39 000 and 32 000 cm1 in the X-rayirradiated glass can be attributed to the activated state of the Eu3+ ions. 3.2. Annealing of the optical absorption bands Changes induced by the X-ray irradiation are examined from the thermal relaxation of the optical properties. Fig. 4 shows intensity changes of the absorption bands in the ultraviolet wavelength region as a function of the heating temperature in air atmosphere. The absorption intensities are normalized to the intensities of glasses after the X-ray irradiation for 14 h. It is evident that the intensities of the bands decrease as the heat-treatment temperature increases, but the dependence of the intensity on temperature is quite different; the absorption intensities for the Eu3+doped Al2O3aSiO2 glass, prepared by heating at 10001C and then irradiating the X-ray for 14 h, rapidly decrease on heating and disappear above 4001C. On the other hand, for the Eu3+-doped Al2O3aSiO2 glass heated at 8001C, the absorption
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Fig. 4. Intensity of X-ray-induced optical bands as a function of heat treatment temperature for the Eu3+-doped 1Al2O3 9SiO2 glasses. Glasses are heated in air at 8001C (a) and 10001C (b) before the X-ray irradiation. Curve (c) is for glass containing no Eu2O3 heated in air at 8001C. The intensities are normalized to the intensities of glasses after the X-ray irradiation for 14 h.
intensity is only B40% of its initial value at 4001C, although it rapidly decreases with the increasing temperature up to 4001C. Further heating diminishes the intensity and it completely disappears for samples heated at 7001C. This result suggests that two kinds of defect color centers are induced in the glass heated at 8001C by the X-ray irradiation and they relax at different rates. 3.3. PSHB properties As shown in Fig. 1, it is pointed out that the room temperature PSHB is only observed in the Eu3+-doped Al2O3aSiO2 glass heated in air at 8001C and then X-ray irradiated, whereas no hole is burned in the glass heated at 10001C, even though the X-ray is irradiated for a prolonged period. Thus, the heat-treatment of glass in air before the X-ray irradiation is essential to burn the hole. We notice that there is no difference in the optical absorption and ESR spectra of glasses heated in air between 8001C and 10001C. However, once the X-ray is irradiated, remarkable difference is observed in the absorption spectra
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(see Fig. 2(d) and (e)). In the glass heated at 10001C, the Al-related defect centers are preferentially formed by the X-ray irradiation, but no change occurs in the Eu3+ ions. Also, it is apparent that the existence of this kind of Alrelated defect does not contribute to the hole burning. We can also point out that the existence of Eu2+ ions, which are formed in the present glass heated at 10001C, does not necessarily play an important role in the hole burning. On the other hand, the glass heated at 8001C shows a different spectral feature from that of the glass heated at 10001C by the X-ray irradiation. The formation of the room temperature PSHB is strongly related to the optical absorption in the energy ranging from 25 000 to 45 000 cm1. The absorption in this region is associated to the activated state of Eu3+ ions; that is, capturing the electrons donated from the AlaO defect centers. At present, we consider that the electron transfer between the Eu3+ ions and the excited Eu3+ or defect centers results in the formation of the PSHB. To further investigate the effect of the defect centers on the hole burning, the X-ray-irradiated glass was heated in air at various temperatures, followed by burning the hole at room temperature, the depth of which is plotted in Fig. 5 as a function of temperature. It is interesting to notice that the hole depth decreases as the heat-treatment temperature increases, but the hole with the depth of half before heating is observed after heating at 5001C. This hole depth dependence on temperature is the same as that observed for the absorption intensity (Fig. 4), strongly suggesting that the induced defect centers play an important role in the formation of the room temperature PSHB. Further study is required to clarify the role that the defect center plays on the hole burning. Fig. 6 shows the typical PSHB spectra, burned and measured at various temperatures, of glasses obtained by heating at 8001C in air followed by Xray irradiation. The width and depth of the burned hole are 2.5 cm1 FWHM and 39% of the total fluorescence intensity at the burning wave number, respectively, at 7 K. The burned hole became small as the burning temperature increased, but the hole was clearly observed even at room temperature.
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Fig. 5. Hole depth as a function of heat treatment temperature for the glass heated at 8001C, followed by the X-ray irradiation. Hole was burned at room temperature and hole depth was defined as the depth to the total fluorescence intensity at the burning position.
materials [2,17–19]. The hole-burnt state has a higher energy than the unburnt state and it relaxes across the activation barrier into the unburnt state. The PSHB spectra were measured at different times after burning at 200 K. During this process, all the products with barriers lower than the activation barrier corresponding to that time, recover the unburnt state, resulting in the erasure of the hole. Fig. 7 shows the hole area that remains as a function of time, where the hole area is normalized to unity at 5 min after burning at 200 K. It is evident that the hole area nonexponentially decreases with time. Assuming that the activation barrier has a Gaussian distribution and the relaxation rate from the burnt state to the unburnt one follows with the Arrhenius type equation, the activation barrier for the burnt hole can be estimated. Fitting the data shown in Fig. 7 to the theoretical equation, which is shown by a solid curve, gives the activation barrier as B0.7 eV, a high value to stabilize the hole at room temperature. The lifetime of the burnt hole is also estimated to be B1.5 105 s at 200 K. 4. Conclusions We have studied the room temperature PSHB of the Eu3+-doped Al2O3aSiO2 glasses by
Fig. 6. The hole burning spectra, burned and measured at 7, 77, 200, and 298 K, of Eu3+-doped 1Al2O3 9SiO2 glass heated in air at 8001C, followed by irradiating with the X-ray for 14 h.
Next, we study the stability of the PSHB property from the hole-erasure measurement. The holeerasure behavior has been studied in great detail both experimentally and theoretically in many
Fig. 7. The hole area as a function of time after burning laser is removed. The hole area is normalized to unity at 5 min after burning. The solid line is a fitting function based on Gaussian distribution.
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irradiating with the X-ray and found that the formation of the PSHB strongly depends on the heating temperature in the air of glass before Xray irradiation. The PSHB was observed in the glass heated at 8001C by irradiating with the Xray, while on the other hand no hole was formed in the glass heated at 10001C. These differences are attributed to the formation of different defect centers by the X-ray irradiation. The formation of the activated Eu3+ ions together with the oxygen defect centers plays an important role on the room temperature PSHB.
Acknowledgements This research was partly supported by a Grantin-Aid for Scientific Research (No. 13305048) from the Ministry of Education, Science, and Culture of Japan. References [1] K. Hirao, S. Todoroki, D.H. Cho, N. Soga, Opt. Lett. 18 (1993) 1586.
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[2] A. Kurita, T. Kushida, Opt. Lett. 19 (1994) 314. [3] K. Fujita, K. Hirao, N. Soga, Opt. Lett. 23 (1998) 543. [4] W.E. Moerner, Persistent spectral hole-burning: science and applications, in: Current Physics, Vol. 44, Springer, Berlin, 1988. [5] J. Friedrich, D. Haarer, Angew. Chem. Int. Ed. Engl. 23 (1984) 113. [6] A. Winnacker, R.M. Shelby, R.M. MacFarlane, Opt. Lett. 10 (1985) 350. [7] K. Fujita, K. Tanaka, K. Hirao, N. Soga, J. Opt. Soc. Am. B 15 (1998) 2700. [8] M. Nogami, Y. Abe, K. Hirao, D.H. Cho, Appl. Phys. Lett. 66 (1995) 2952. [9] M. Nogami, T. Hayakawa, T. Ishikawa, Appl. Phys. Lett. 75 (1999) 3072. [10] M. Nogami, T. Ishikawa, Phys. Rev. B. 63 (2001) 10 4205. [11] M. Nogami, Y. Abe, Appl. Phys. Lett. 71 (1997) 3465. [12] M. Nogami, T. Hayakawa, Phys. Rev. B 56 (1997) R14 235. [13] D.R. MacFarlane, P.J. Newman, J.D. Cashion, A. Edgar, J. Non-Cryst. Solids 256&257 (1999) 53. [14] H. Hosono, H. Kawazoe, Nucl. Instrum. Methods B 91 (1994) 395. [15] J.H. Mackey, J. Nahum, Phys. Chem. Glasses 9 (1968) 52. [16] M. Nogami, S. Ito, Phys. Rev. B 61 (2000) 14 295. [17] S.P. Love, C.E. Mungan, A.J. Sievers, J.A. Campbell, J. Opt. Soc. Am. B9 (1992) 794. [18] J. Zhang, S. Huang, J. Yu, Opt. Lett. 17 (1992) 1146. [19] H. Yugami, R. Yagi, S. Matsuo, M. Ishigame, Phys. Rev. B 53 (1996) 8283.
Defect centers and room temperature persistent spectral hole burning in X-ray irradiated Eu3+-doped glass Masayuki Nogami*, Tomotaka Ishikawa, Tomokatsu Hayakawa Department of Materials Science and Engineering, Nagoya Institute of Technology, Showa Nagoya, 466-8555 Japan Received 6 April 2001; received in revised form 20 July 2001; accepted 23 July 2001
Abstract We have investigated the room temperature persistent spectral hole burning of X-ray irradiated Eu3+-doped Al2O3aSiO2 glasses. The sol–gel-derived Eu3+-doped glass heated in air at 8001C and irradiated with X-ray exhibits the hole spectra at room temperature, while on the other hand no hole is formed in the glass heated at 10001C. We can attribute these differences to the different formation of the defect centers by the X-ray irradiation. The formation of the activated Eu3+ ions together with the oxygen defect centers plays an important role on the room temperature hole burning. The lifetime of the burnt hole is estimated to be B1.5 105 s at 200 K. r 2002 Elsevier Science B.V. All rights reserved. Keywords: Hole burning; Eu3+; Glass; X-ray; Sol–gel
1. Introduction The Sm2+- and Eu3+-doped glasses have been attracting much interest, because of their room temperature persistent spectral hole burning (PSHB) properties [1–3] and a great potential in a high-density frequency-selective optical memory [4,5]. The Sm2+ and Eu3+ ions have the same 4f 6 electron configuration and the hole spectra are observed on the excitation spectra of the 7F0-5D0 transition of these ions. Although many materials have been developed to display the PSHB property, the hole burning mechanism still remains *Corresponding author. Tel.: +81-52-735-5285; fax: +8152-735-5285. E-mail address: [email protected] (M. Nogami).
unclear. One proposed mechanism for the hole burning is the photo-induced charge transfer between the rare earth ions, that is, the ionization and reduction for the Sm2+ and Eu3+ ions, respectively, by laser irradiation [6,7]. The released electrons and holes are considered to be captured in the Sm3+ and Eu2+ ions, respectively. According to this reaction, the glasses should be prepared to dope the rare earth ions having different valences. For preparing the room temperature PSHB glasses, they are melted or heat-treated in hydrogen gas atmosphere in order to reduce the rare earth ions [1–3,7]. In contrast, we successfully prepared the Sm2+- and Eu3+-doped glasses using a sol–gel method and observed the PSHB at room temperature [8–10]. These sol–gel-derived glasses
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are treated by heating in hydrogen gas atmosphere or irradiating an X-ray, which resulted in the PSHB at room temperature. We have found that irrespective of whether the holes are burned at room temperature or not, it is dependent on the preparation conditions of glass before the X-ray irradiation [10]. Fig. 1 is an illustration of the excitation spectra after laser irradiation for burning. It is clear that the hole is only observed at room temperature in the glass heated in air at 8001C, followed by irradiation of X-ray. On the other hand, the hole is not observed in the glasses heated at 10001C followed by X-ray irradiation. We also found that in the sol–gel derived glasses, the room temperature PSHB is observed only in the Al3+ ion-containing glasses, and not in the SiO2 glass [9]. The Eu3+ ions are not necessarily reduced into Eu2+ by the X-ray irradiation [10]. From these results, we consider that the X-ray irradiation also contributes to the hole burning. Further investigations are necessary to study the X-ray irradiation on the hole burning and to discuss the hole burning reaction of the Eu3+ ions. In this work, we measure the optical absorption and electron spin resonance (ESR) spectra of the Eu3+-doped Al2O3aSiO2 glasses and relate the
Fig. 1. The hole burning spectra, burned and measured at 298 K, of Eu3+-doped 1Al2O3 9SiO2 glass heated in air at 8001C (a), followed by irradiating with the X-ray for 14 h (b). Curve (c) is for the glass heated at 10001C in air, followed by X-ray irradiation for 14 h.
PSHB property with the defect centers induced in the glass by the X-ray irradiation.
2. Experiments 1Al2O3 9SiO2 (mole ratio) glasses containing 1 wt% Eu2O3 were prepared by the hydrolysis of Si(OC2H5)4, Al(OC4Hsec 9 )3, and EuCl3 6H2O. The starting materials were commercially available and used as received. A detailed explanation of the gel preparation is given elsewhere [9,10]. The gel was heated to form the glass in air at 8001C and 10001C for 2 h. The X-ray irradiation was performed using the Cu Ka line (Rigaku RAD-B system) with 40 kV and 20 mA at room temperature for 14 h. The sample of B0.5 mm thickness was placed at a distance of 15 cm from the X-ray source. Optical absorption spectra were measured with a Jasco, U-Best spectrometer. The ESR measurement was performed using Jasco, JESFE ME3X spectrometer at room temperature. The g-values of the obtained ESR signals were calibrated by the utilization of diphenyl-picrylhydrazal (DPPH). The PSHB was observed on the excitation spectra of the 7F0-5D0 transition of Eu3+ by scanning a Rhodamine 6G dye laser over the 7 F0-5D0 transition while monitoring the fluorescence of the 5D0-7F2 transition at 619 nm. As previously reported, when the glasses are heated in air, a large amount of hydroxyl bonds remain in the glass and the spectral holes are burned by the rearrangement of the hydroxyl bonds surrounding the Eu3+ ions [11,12]. To eliminate the effect of the hydroxyl bonds on the hole burning, the glasses were heated in a vacuum at 8001C for 2 h before the X-ray irradiation. The hole was burned by irradiating with a laser with a power of B300 mW at a B2 mm diameter spot at the wavelength around the peak position of the 7F0-5D0 transition of Eu3+ ions for 30 min. Shown in Fig. 1 are the spectra, burned and measured at room temperature, for the glasses heated in air at 8001C and 10001C, followed by irradiating X-ray for 14 h.
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3. Results and discussion 3.1. Optical absorption and ESR spectra The glasses, obtained by heating in air, are transparent and colorless irrespective of whether they contain Eu2O3 or not. Fig. 2 shows the optical absorption spectra of the samples heated in air at 8001C ((a) and (b)) and the absorptivity change induced by the X-ray irradiation ((c)–(e)). The difference shown in the spectra is obtained by subtracting the absorption before irradiation from that after irradiation. The Al2O3aSiO2 glass has a strong absorption above 33 000 cm1 due to the fundamental absorption of the glass structure. In addition to this absorption, the Eu3+-doped Al2O3aSiO2 glass shows a broad absorption band peaking at around 45 000 cm1, which can be
Fig. 2. The optical absorption spectra ((a) and (b)) of 1Al2O3 9SiO2 glasses with (a) and without (b) Eu2O3 heated in air at 8001C and change in the absorption spectra ((c)–(e)) on irradiation of X-ray for glasses heated at 8001C (d), and 10001C (e). Curve (c) is for glass containing no Eu2O3. The spectral curves are vertically shifted for clarity and the scale corresponds to the absorption coefficient of 8 cm1.
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assigned to the existence of the Eu2+ ions as discussed later. The absorption peaks due to the Eu3+ ions are also observed at 465 and 395 nm in the visible wavelength region, the absorption coefficients of which are less than a few cm1 and do not appear in the same scale of Fig. 2. When the glasses are irradiated with the X-ray, large changes are observed in the absorption properties, which are strongly dependent on the glass composition and heating temperature of glass in air. The Al2O3aSiO2 glass containing no Eu3+ ions shows an induced absorption band peaking at 34 000 cm1 with shoulders around 27 000 and 37 000 cm1. When the Eu3+ ions are doped in the Al2O3aSiO2 glass, the above absorption intensity diminishes, instead, a strong absorption band peaking at 39 000 cm1 with a shoulder at around 32 000 cm1 is induced by the X-ray irradiation. On the other hand, the glass heated at 10001C becomes grayish by the X-ray irradiation and shows a broad band ranging from 13 000 to 45 000 cm1, which is quite different from that of the glass heated at 8001C. These changes in the absorption spectra suggest that the formation of some defect centers and breaking of the glass structure occur by the X-ray irradiation. We found no clear change in the absorption intensity of the Eu3+ ions. The ESR spectra of the samples treated in the same way were measured at room temperature, which are shown in Fig. 3. Note that the Eu3+doped Al2O3aSiO2 glass heated in air has an intense ESR signal between 0.13 and 0.25 T. The Eu2+ ions exhibit an ESR signal between 0.13 and 0.25 T [13], while the Eu3+ ions are non-magnetic and do not contribute to the ESR spectra. This result indicates that a part of the Eu3+ ions are reduced into Eu2+ in the Al2O3aSiO2 glass only by heating in air. After the X-ray irradiation, a sharp ESR signal is observed at around 0.34 T in addition to the signals of the Eu2+ ions. In order to check the effect of the glass components on the 0.34 T line, the Eu3+-doped SiO2 glass was irradiated with the X-ray to measure the ESR spectra, but no signal was observed at this position. This result and the comparison of the g-value with that of the free electron (g ¼ 2:0023), the signal at 0.34 T can be assigned to the hole
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Fig. 3. The ESR spectra of Eu3+-doped 1Al2O3 9SiO2 glasses heated in air at 8001C (a), followed by irradiating with X-ray (b). Curve (c) is for the glass heated at 10001C in air before X-ray irradiation. Curve (d) is for glass without Eu2O3, heated in air at 8001C, followed by X-ray irradiation.
centers trapped with oxygen bounded to the Al ions. Further, it is interesting to note, in Fig. 3, that the intensity of the Eu2+-signal changes little after the X-ray irradiation (see Fig. 3(b)). This strongly suggests that the Eu3+ ions hardly reduced into Eu2+ by the X-ray irradiation. Despite this, however, a strong optical absorption band in the range of 30 000–42 000 cm1 induced by the X-ray irradiation (Fig. 2(d)) cannot be explained from the ESR experiment. Hosono and Kawazoe measured the optical absorption and ESR spectra of the g-ray irradiated SiO2 glass containing 0.6 mol% Al2O3 and discussed that the induced absorption in the range of 12 000–50 000 cm1 can be attributed to the Alrelated defect centers such as AlaOHC and Al E0
centers [14]. The absorption spectrum of the glass that is heated at 10001C and irradiated with the Xray (Fig. 2(e)) resembles that of the g-ray irradiated glass, suggesting that the Al-related defect centers are formed in our X-ray irradiated Eu3+doped glass. Further, as shown in Figs. 2 and 3, we notice that the Eu3+ ions are partially reduced into Eu2+ by only heating in air atmosphere, but additional reduction does not take place by irradiating the X-ray. On the other hand, the origin of the absorption bands induced in the glass that is heated at 8001C and irradiated by the X-ray is not clear. Mackey and Nahum measured the optical absorption spectra of the Eu3+-doped silicate glasses after the X-ray irradiation and attributed the induced band at around 30 000 cm1 to the electron-trapped Eu3+ ions [15]. This center is considered to be different from the Eu2+ ions and designated as an [Eu3+] state. We have already reported the optical absorption in the Eu3+-doped phosphate glasses by the X-ray irradiation, where the electrons released from the POH bonds are considered to be captured in the nearest Eu3+ ions [16]. Accordingly, we can postulate that the induced absorption band between 39 000 and 32 000 cm1 in the X-rayirradiated glass can be attributed to the activated state of the Eu3+ ions. 3.2. Annealing of the optical absorption bands Changes induced by the X-ray irradiation are examined from the thermal relaxation of the optical properties. Fig. 4 shows intensity changes of the absorption bands in the ultraviolet wavelength region as a function of the heating temperature in air atmosphere. The absorption intensities are normalized to the intensities of glasses after the X-ray irradiation for 14 h. It is evident that the intensities of the bands decrease as the heat-treatment temperature increases, but the dependence of the intensity on temperature is quite different; the absorption intensities for the Eu3+doped Al2O3aSiO2 glass, prepared by heating at 10001C and then irradiating the X-ray for 14 h, rapidly decrease on heating and disappear above 4001C. On the other hand, for the Eu3+-doped Al2O3aSiO2 glass heated at 8001C, the absorption
M. Nogami et al. / Journal of Luminescence 96 (2002) 163–169
Fig. 4. Intensity of X-ray-induced optical bands as a function of heat treatment temperature for the Eu3+-doped 1Al2O3 9SiO2 glasses. Glasses are heated in air at 8001C (a) and 10001C (b) before the X-ray irradiation. Curve (c) is for glass containing no Eu2O3 heated in air at 8001C. The intensities are normalized to the intensities of glasses after the X-ray irradiation for 14 h.
intensity is only B40% of its initial value at 4001C, although it rapidly decreases with the increasing temperature up to 4001C. Further heating diminishes the intensity and it completely disappears for samples heated at 7001C. This result suggests that two kinds of defect color centers are induced in the glass heated at 8001C by the X-ray irradiation and they relax at different rates. 3.3. PSHB properties As shown in Fig. 1, it is pointed out that the room temperature PSHB is only observed in the Eu3+-doped Al2O3aSiO2 glass heated in air at 8001C and then X-ray irradiated, whereas no hole is burned in the glass heated at 10001C, even though the X-ray is irradiated for a prolonged period. Thus, the heat-treatment of glass in air before the X-ray irradiation is essential to burn the hole. We notice that there is no difference in the optical absorption and ESR spectra of glasses heated in air between 8001C and 10001C. However, once the X-ray is irradiated, remarkable difference is observed in the absorption spectra
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(see Fig. 2(d) and (e)). In the glass heated at 10001C, the Al-related defect centers are preferentially formed by the X-ray irradiation, but no change occurs in the Eu3+ ions. Also, it is apparent that the existence of this kind of Alrelated defect does not contribute to the hole burning. We can also point out that the existence of Eu2+ ions, which are formed in the present glass heated at 10001C, does not necessarily play an important role in the hole burning. On the other hand, the glass heated at 8001C shows a different spectral feature from that of the glass heated at 10001C by the X-ray irradiation. The formation of the room temperature PSHB is strongly related to the optical absorption in the energy ranging from 25 000 to 45 000 cm1. The absorption in this region is associated to the activated state of Eu3+ ions; that is, capturing the electrons donated from the AlaO defect centers. At present, we consider that the electron transfer between the Eu3+ ions and the excited Eu3+ or defect centers results in the formation of the PSHB. To further investigate the effect of the defect centers on the hole burning, the X-ray-irradiated glass was heated in air at various temperatures, followed by burning the hole at room temperature, the depth of which is plotted in Fig. 5 as a function of temperature. It is interesting to notice that the hole depth decreases as the heat-treatment temperature increases, but the hole with the depth of half before heating is observed after heating at 5001C. This hole depth dependence on temperature is the same as that observed for the absorption intensity (Fig. 4), strongly suggesting that the induced defect centers play an important role in the formation of the room temperature PSHB. Further study is required to clarify the role that the defect center plays on the hole burning. Fig. 6 shows the typical PSHB spectra, burned and measured at various temperatures, of glasses obtained by heating at 8001C in air followed by Xray irradiation. The width and depth of the burned hole are 2.5 cm1 FWHM and 39% of the total fluorescence intensity at the burning wave number, respectively, at 7 K. The burned hole became small as the burning temperature increased, but the hole was clearly observed even at room temperature.
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Fig. 5. Hole depth as a function of heat treatment temperature for the glass heated at 8001C, followed by the X-ray irradiation. Hole was burned at room temperature and hole depth was defined as the depth to the total fluorescence intensity at the burning position.
materials [2,17–19]. The hole-burnt state has a higher energy than the unburnt state and it relaxes across the activation barrier into the unburnt state. The PSHB spectra were measured at different times after burning at 200 K. During this process, all the products with barriers lower than the activation barrier corresponding to that time, recover the unburnt state, resulting in the erasure of the hole. Fig. 7 shows the hole area that remains as a function of time, where the hole area is normalized to unity at 5 min after burning at 200 K. It is evident that the hole area nonexponentially decreases with time. Assuming that the activation barrier has a Gaussian distribution and the relaxation rate from the burnt state to the unburnt one follows with the Arrhenius type equation, the activation barrier for the burnt hole can be estimated. Fitting the data shown in Fig. 7 to the theoretical equation, which is shown by a solid curve, gives the activation barrier as B0.7 eV, a high value to stabilize the hole at room temperature. The lifetime of the burnt hole is also estimated to be B1.5 105 s at 200 K. 4. Conclusions We have studied the room temperature PSHB of the Eu3+-doped Al2O3aSiO2 glasses by
Fig. 6. The hole burning spectra, burned and measured at 7, 77, 200, and 298 K, of Eu3+-doped 1Al2O3 9SiO2 glass heated in air at 8001C, followed by irradiating with the X-ray for 14 h.
Next, we study the stability of the PSHB property from the hole-erasure measurement. The holeerasure behavior has been studied in great detail both experimentally and theoretically in many
Fig. 7. The hole area as a function of time after burning laser is removed. The hole area is normalized to unity at 5 min after burning. The solid line is a fitting function based on Gaussian distribution.
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irradiating with the X-ray and found that the formation of the PSHB strongly depends on the heating temperature in the air of glass before Xray irradiation. The PSHB was observed in the glass heated at 8001C by irradiating with the Xray, while on the other hand no hole was formed in the glass heated at 10001C. These differences are attributed to the formation of different defect centers by the X-ray irradiation. The formation of the activated Eu3+ ions together with the oxygen defect centers plays an important role on the room temperature PSHB.
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