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Highly stable persistent spectral hole burning in Eu3+ ions doped oxy-fluoride glasses of 30CaF2–10Al2O3–60B2O3
Optical Materials 29 (2007) 1789–1792 www.elsevier.com/locate/optmat
Highly stable persistent spectral hole burning in Eu3+ ions doped oxy-fluoride glasses of 30CaF2–10Al2O3–60B2O3 Hailian Liang a, Hiromasa Hanzawa b, Takashi Horikawa a, Ken-ichi Machida b
a,*
a Center for Advanced Science and Innovation, Osaka University, 2-1 Yamadaoka, Suita, Osaka 565-0871, Japan Graduate School of Engineering Science, Osaka University, 1-3 Machikaneyama-cho, Toyonaka, Osaka 560-8531, Japan
Received 27 March 2006; accepted 6 October 2006 Available online 22 November 2006
Abstract High temperature persistent spectral hole burning up to room temperature has been observed in Eu3+ ions doped oxy-fluoride glasses with a composition of 30CaF2–10Al2O3–60B2O3 (mol%) melted in a reducing atmosphere. The hole stability was studied through lightinduced hole refilling and temperature cycling experiments. The burned holes survive thermal cycling to 300 K due to a high barrier height of 0.69 eV in the sample. Ó 2006 Published by Elsevier B.V. PACS: 42.70.a; 42.70.Ln; 71.23.k; 71.23.Cq; 71.55.i Keywords: Oxy-fluoride glass; Hole stability; Barrier height
1. Introduction Spectral hole burning is a high-resolution spectroscopy technique to study the structure in disordered solids [1]. Especially persistent spectral hole burning (PSHB) has attracted considerable research interests for its potential application in frequency domain optical storage with the possibility to improve the areal densities several orders of magnitude higher due to an additional use of a frequency dimension [2]. The storage capacity is limited by the ratio of inhomogeneous to homogeneous linewidth. Glasses are prevalent as the matrices for PSHB materials for its broad inhomogeneous linewidth originated from the disorder of their structure [3–5]. Some Pr3+, Nd3+, Er3+, Eu3+, Sm2+, and Tb3+ ions doped glasses showing PSHB properties have been reported [6–10]. For practical applications, materials showing high temperature and stable PSHB are desirable. The hole stability is evaluated by the spontaneous *
Corresponding author. Tel./fax: +81 6 6879 4209. E-mail address: [email protected] (K.-i. Machida).
0925-3467/$ - see front matter Ó 2006 Published by Elsevier B.V. doi:10.1016/j.optmat.2006.10.001
hole-filling process, which is due to a tunneling process or a thermally activated process from the burnt states to the unburnt ones [2,11]. In this study, Eu3+-doped oxy-fluoride glasses showing good PSHB properties at room temperature were investigated. Light-induced hole refilling and temperature cycling experiments were performed to evaluate the hole stability. 2. Experimental The nominal composition of the glass specimen was 30CaF2–10Al2O3–60B2O3 (mol%) doped with 5 wt% Eu2O3. The samples were prepared from reagent grade B2O3, Al2O3, CaF2, and Eu2O3. At first, the components of B2O3, Al2O3, and CaF2 were melted to prepare the undoped glass. To compensate for the vaporization losses of B2O3, an excess of 8 mol% B2O3 was added to the starting batches. After the mixture was melted at 1373 K in a platinum crucible for 30 min in air, the melt was poured onto a stainless steel mold to quench. To increase the homogeneousness and reduce some Eu3+ ions, the obtained
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transparent glasses were pulverized, mixed with 5 wt% Eu2O3 and remelted in a reducing atmosphere by using a RF furnace at 1373 K for 1 h. Finally, the glass sample was obtained by cooling the melt down to room temperature with the RF furnace in situ. The resulting glass, which was optically transparent and yellow in color, was polished for optical measurements. Luminescence spectra of Eu3+ ions at 10 K were recorded by a monochromator (Nikon, G-250) excited by a rhodamine dye laser. The PSHB was observed on the excitation spectra of the 7F0 ! 5D0 transition of the Eu3+ ions while monitoring the luminescence of 5D0 ! 7F2 at 612 nm of Eu3+ ions by a rhodamine 6G dye laser (Spectra Physics, 375) with a linewidth of 40 GHz pumped by a Nd:YVO4 green laser (Coherent, Verdi-5). The samples were placed in a cryostat system cooled by compressed helium gas. The PSHB spectra were recorded in a temperature range from 10 to 300 K. A photomultiplier tube (Hamamatsu Photonics, R649) combined with a monochromator (Nikon, G-250) was used to record the excitation spectra before and after hole burning.
Fig. 2. Excitation spectra for 7F0 ! 5D0 of the Eu3+ ions in the sample before and after laser irradiation for 20 min at 10 K.
3. Results and discussion 3.1. Emission spectra of 5D0 ! 7F2 of Eu3+ ions at 10 K The emission group of 5D0 ! 7F2 of Eu3+ ions doped glass melted in a reducing atmosphere under different excitation at 10 K is shown in Fig. 1. The shapes of 5 D0 ! 7F2 group emission indicate a strong dependence on the excitation wavelength. This is a manifestation of fluorescence line narrowing (FLN), which is caused by the difference in the crystal-field strength experienced by different sets of ions in the glass [12]. Materials exhibiting the FLN characteristic are promising candidates for hole burning. Thus, we characterized the hole spectra on the sample.
Fig. 3. Difference curves between the excitation spectra before and after laser irradiation in the sample at various temperatures: (a) 10 K, (b) 50 K, (c) 100 K, (d) 200 K, and (e) 300 K.
3.2. PSHB spectra The typical excitation spectra before and after hole burning of Eu3+ ions at 10 K in the glass are shown in Fig. 2. A hole is clearly observed at the burning wavelength after the sample is exposed to a 150 mW laser beam for 20 min. The hole depth and width are about 14% and 2.6 cm1, respectively. Fig. 3 shows the difference spectra of Eu3+ in the glasses after exposure to 150 mW laser irradiation for 10 min at 10, 50, 100, 200, and 300 K. With increasing burning temperature, the hole depth decreases and the hole width increases. When the temperature comes to 300 K, the hole depth decreases to 3.4% and the hole width increases to 4.05 cm1. 3.3. Light-induced hole refilling
Fig. 1. Fluorescence spectra of Eu3+ ions in the sample melted in a reducing atmosphere as a function of different excitation wavelength at 10 K.
Light-induced hole refilling studies were performed at 10 K. The burning power and burning time for each hole was 30 mW and 5 min, respectively. Five holes are clearly burned in the range from 578.4 to 579.4 nm as shown in Fig. 4. It reveals that the previously burned holes are par-
H. Liang et al. / Optical Materials 29 (2007) 1789–1792
Fig. 4. Multihole spectra of the Eu3+ ions in the sample. Holes were burned at five wavelengths in the order of numbers at 10 K.
tially filled by the increased fluorescence around the new hole. This behavior is attributed to the relaxation from the high-energy hole-burnt state to the low-energy unburnt one [13]. 3.4. Temperature cycling experiments In order to evaluate the thermal stability of the burned holes and to check the spontaneous hole refilling, temperature cycling experiments were also carried out. After the hole was burned at 10 K, the sample was heated to 30 K, kept there for 3 min, and followed by cooling down to 10 K. Then, the hole spectrum was measured again at 10 K. This process was repeated to the higher excursion temperature of 50, 80, 120, 150, 200, 250, and 300 K. The difference hole spectra measured at 10 K after each excursion process are shown in Fig. 5. With increasing temperature, the width and depth of the holes increase and decrease, respectively. The hole burned at 10 K is clearly observed after excursion temperature up to 300 K. The area of the holes is determined by a Lorentzian fitting of
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Fig. 6. The normalized hole area as a function of excursion temperature in the sample.
the hole spectra. The dependence of the hole area normalized to unity at 10 K on the excursion temperature is shown in Fig. 6. With increasing excursion temperature, the area of the holes gradually decreases. During the temperature cycling process, the spontaneous hole-filling takes place due to the relaxation from the hole-burnt state to the unburnt one across the activation barrier (VT). The barrier height that can be crossed is expressed as [14,15]: V T ¼ kT ln ðR0 sÞ where k is the Boltzmann constant, T is the excursion temperature in K, R0 is the attempt frequency, and s is the holding time at the excursion temperature. The normalized hole area, which is assumed to be proportional to the number of remained molecules after the hole-filling, is calculated according to the following equation [16]: R1 P ðV ÞdV AðT Þ ¼ R 1V T P ðV ÞdV V T ¼10 K where P(V) is the barrier height distribution. For our cases, the holding time of s is 180 s. The barrier height distribution is defined as the sum of three Gaussian functions. The heights of the three barriers are 0.02, 0.33, and 0.69 eV using the fitting parameter of R0 = 1012 s1 for our cases. The burned hole maintains at 300 K, because the hole-filling is prevented by the high barrier of 0.69 eV. This value is much higher than that for the Eu3+ ions doped silica and silicate glasses prepared by the sol– gel method [16,17]. 4. Conclusions
Fig. 5. (a) Difference spectrum at 10 K and (b–i) difference spectra at 10 K after excursion process to different cycling temperatures, (b) 30 K, (c) 50 K, (d) 80 K, (e) 120 K, (f) 150 K, (g) 200 K, (h) 250 K, and (i) 300 K.
Eu3+ ions doped oxy-fluoride glasses in composition of 30CaF2–10Al2O3–60B2O3 were prepared in a reducing atmosphere. PSHB spectrum was recorded at room temperature. Temperature cycling experiments reveal that the holes burned at 10 K show high thermal stability due to the high barrier height of 0.69 eV. The spectra burned at 10 K remain after excursion to 300 K.
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Acknowledgement This work was partially supported by a Grant-in-Aid for the Scientific Research (No.15205025) from the Ministry of Education, Culture, Sports, Science, and Technology of Japan. References [1] O. Sild, K. Haller, Zero-Phonon Lines and Spectral Hole Burning in Spectroscopy and Photochemistry, Springer-Verlag, Berlin, 1988. [2] W.E. Moerner, Persistent Spectral Hole Burning: Science and Applications, Springer, Berlin, 1988. [3] A. Kurita, T. Kushida, T. Izumitani, M. Matsukawa, Opt. Lett. 19 (1994) 314. [4] K. Fujita, K. Tanaka, K. Hirao, N. Soga, J. Opt. Soc. Am. B 15 (1998) 2700.
[5] N. Murase, Y. Kawasaki, A. Tomita, J. Lumin. 98 (2002) 301. [6] S. Todoroki, K. Hirao, N. Soga, J. Appl. Phys. 72 (1992) 5853. [7] J.R. Qiu, K. Nouchi, K. Miura, T. Mitsuyu, K. Hirao, J. Phys.: Condens. Matter 12 (2000) 5061. [8] K. Fujita, K. Tanaka, H. Sasaki, K. Hirao, J. Lumin. 86 (2000) 297. [9] Y. Sun, C.W. Thiel, R.L. Cone, R.W. Equall, R.L. Hutcheson, J. Lumin. 98 (2002) 281. [10] A. Osvet, S. Emelianova, R. Weissmann, V.I. Arbuzov, A. Winnacker, J. Lumin. 86 (2000) 323. [11] J. Zhang, S. Huang, J. Yu, Opt. Lett. 16 (1992) 1146. [12] C.R. Pulluru, R.R. Kalluru, B.R. Reddy, T.A. Konovalova, L.D. Kispert, Appl. Phys. Lett. 87 (2005) 091107. [13] H.P. You, M. Nogami, J. Appl. Phys. 95 (2004) 2781. [14] W. Ko¨hler, J. Friedrich, Phys. Rev. Lett. 59 (1987) 2199. [15] J. Zollfrank, J. Friedrich, T. Tani, Polymer 30 (1989) 231. [16] H. Hanzawa, D. Ueda, G. Adachi, K. Machida, Y. Kanematsu, J. Lumin. 94/95 (2001) 503. [17] M. Nogami, T. Hayakawa, Phys. Rev. B 56 (1997) R14235.
Highly stable persistent spectral hole burning in Eu3+ ions doped oxy-fluoride glasses of 30CaF2–10Al2O3–60B2O3 Hailian Liang a, Hiromasa Hanzawa b, Takashi Horikawa a, Ken-ichi Machida b
a,*
a Center for Advanced Science and Innovation, Osaka University, 2-1 Yamadaoka, Suita, Osaka 565-0871, Japan Graduate School of Engineering Science, Osaka University, 1-3 Machikaneyama-cho, Toyonaka, Osaka 560-8531, Japan
Received 27 March 2006; accepted 6 October 2006 Available online 22 November 2006
Abstract High temperature persistent spectral hole burning up to room temperature has been observed in Eu3+ ions doped oxy-fluoride glasses with a composition of 30CaF2–10Al2O3–60B2O3 (mol%) melted in a reducing atmosphere. The hole stability was studied through lightinduced hole refilling and temperature cycling experiments. The burned holes survive thermal cycling to 300 K due to a high barrier height of 0.69 eV in the sample. Ó 2006 Published by Elsevier B.V. PACS: 42.70.a; 42.70.Ln; 71.23.k; 71.23.Cq; 71.55.i Keywords: Oxy-fluoride glass; Hole stability; Barrier height
1. Introduction Spectral hole burning is a high-resolution spectroscopy technique to study the structure in disordered solids [1]. Especially persistent spectral hole burning (PSHB) has attracted considerable research interests for its potential application in frequency domain optical storage with the possibility to improve the areal densities several orders of magnitude higher due to an additional use of a frequency dimension [2]. The storage capacity is limited by the ratio of inhomogeneous to homogeneous linewidth. Glasses are prevalent as the matrices for PSHB materials for its broad inhomogeneous linewidth originated from the disorder of their structure [3–5]. Some Pr3+, Nd3+, Er3+, Eu3+, Sm2+, and Tb3+ ions doped glasses showing PSHB properties have been reported [6–10]. For practical applications, materials showing high temperature and stable PSHB are desirable. The hole stability is evaluated by the spontaneous *
Corresponding author. Tel./fax: +81 6 6879 4209. E-mail address: [email protected] (K.-i. Machida).
0925-3467/$ - see front matter Ó 2006 Published by Elsevier B.V. doi:10.1016/j.optmat.2006.10.001
hole-filling process, which is due to a tunneling process or a thermally activated process from the burnt states to the unburnt ones [2,11]. In this study, Eu3+-doped oxy-fluoride glasses showing good PSHB properties at room temperature were investigated. Light-induced hole refilling and temperature cycling experiments were performed to evaluate the hole stability. 2. Experimental The nominal composition of the glass specimen was 30CaF2–10Al2O3–60B2O3 (mol%) doped with 5 wt% Eu2O3. The samples were prepared from reagent grade B2O3, Al2O3, CaF2, and Eu2O3. At first, the components of B2O3, Al2O3, and CaF2 were melted to prepare the undoped glass. To compensate for the vaporization losses of B2O3, an excess of 8 mol% B2O3 was added to the starting batches. After the mixture was melted at 1373 K in a platinum crucible for 30 min in air, the melt was poured onto a stainless steel mold to quench. To increase the homogeneousness and reduce some Eu3+ ions, the obtained
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transparent glasses were pulverized, mixed with 5 wt% Eu2O3 and remelted in a reducing atmosphere by using a RF furnace at 1373 K for 1 h. Finally, the glass sample was obtained by cooling the melt down to room temperature with the RF furnace in situ. The resulting glass, which was optically transparent and yellow in color, was polished for optical measurements. Luminescence spectra of Eu3+ ions at 10 K were recorded by a monochromator (Nikon, G-250) excited by a rhodamine dye laser. The PSHB was observed on the excitation spectra of the 7F0 ! 5D0 transition of the Eu3+ ions while monitoring the luminescence of 5D0 ! 7F2 at 612 nm of Eu3+ ions by a rhodamine 6G dye laser (Spectra Physics, 375) with a linewidth of 40 GHz pumped by a Nd:YVO4 green laser (Coherent, Verdi-5). The samples were placed in a cryostat system cooled by compressed helium gas. The PSHB spectra were recorded in a temperature range from 10 to 300 K. A photomultiplier tube (Hamamatsu Photonics, R649) combined with a monochromator (Nikon, G-250) was used to record the excitation spectra before and after hole burning.
Fig. 2. Excitation spectra for 7F0 ! 5D0 of the Eu3+ ions in the sample before and after laser irradiation for 20 min at 10 K.
3. Results and discussion 3.1. Emission spectra of 5D0 ! 7F2 of Eu3+ ions at 10 K The emission group of 5D0 ! 7F2 of Eu3+ ions doped glass melted in a reducing atmosphere under different excitation at 10 K is shown in Fig. 1. The shapes of 5 D0 ! 7F2 group emission indicate a strong dependence on the excitation wavelength. This is a manifestation of fluorescence line narrowing (FLN), which is caused by the difference in the crystal-field strength experienced by different sets of ions in the glass [12]. Materials exhibiting the FLN characteristic are promising candidates for hole burning. Thus, we characterized the hole spectra on the sample.
Fig. 3. Difference curves between the excitation spectra before and after laser irradiation in the sample at various temperatures: (a) 10 K, (b) 50 K, (c) 100 K, (d) 200 K, and (e) 300 K.
3.2. PSHB spectra The typical excitation spectra before and after hole burning of Eu3+ ions at 10 K in the glass are shown in Fig. 2. A hole is clearly observed at the burning wavelength after the sample is exposed to a 150 mW laser beam for 20 min. The hole depth and width are about 14% and 2.6 cm1, respectively. Fig. 3 shows the difference spectra of Eu3+ in the glasses after exposure to 150 mW laser irradiation for 10 min at 10, 50, 100, 200, and 300 K. With increasing burning temperature, the hole depth decreases and the hole width increases. When the temperature comes to 300 K, the hole depth decreases to 3.4% and the hole width increases to 4.05 cm1. 3.3. Light-induced hole refilling
Fig. 1. Fluorescence spectra of Eu3+ ions in the sample melted in a reducing atmosphere as a function of different excitation wavelength at 10 K.
Light-induced hole refilling studies were performed at 10 K. The burning power and burning time for each hole was 30 mW and 5 min, respectively. Five holes are clearly burned in the range from 578.4 to 579.4 nm as shown in Fig. 4. It reveals that the previously burned holes are par-
H. Liang et al. / Optical Materials 29 (2007) 1789–1792
Fig. 4. Multihole spectra of the Eu3+ ions in the sample. Holes were burned at five wavelengths in the order of numbers at 10 K.
tially filled by the increased fluorescence around the new hole. This behavior is attributed to the relaxation from the high-energy hole-burnt state to the low-energy unburnt one [13]. 3.4. Temperature cycling experiments In order to evaluate the thermal stability of the burned holes and to check the spontaneous hole refilling, temperature cycling experiments were also carried out. After the hole was burned at 10 K, the sample was heated to 30 K, kept there for 3 min, and followed by cooling down to 10 K. Then, the hole spectrum was measured again at 10 K. This process was repeated to the higher excursion temperature of 50, 80, 120, 150, 200, 250, and 300 K. The difference hole spectra measured at 10 K after each excursion process are shown in Fig. 5. With increasing temperature, the width and depth of the holes increase and decrease, respectively. The hole burned at 10 K is clearly observed after excursion temperature up to 300 K. The area of the holes is determined by a Lorentzian fitting of
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Fig. 6. The normalized hole area as a function of excursion temperature in the sample.
the hole spectra. The dependence of the hole area normalized to unity at 10 K on the excursion temperature is shown in Fig. 6. With increasing excursion temperature, the area of the holes gradually decreases. During the temperature cycling process, the spontaneous hole-filling takes place due to the relaxation from the hole-burnt state to the unburnt one across the activation barrier (VT). The barrier height that can be crossed is expressed as [14,15]: V T ¼ kT ln ðR0 sÞ where k is the Boltzmann constant, T is the excursion temperature in K, R0 is the attempt frequency, and s is the holding time at the excursion temperature. The normalized hole area, which is assumed to be proportional to the number of remained molecules after the hole-filling, is calculated according to the following equation [16]: R1 P ðV ÞdV AðT Þ ¼ R 1V T P ðV ÞdV V T ¼10 K where P(V) is the barrier height distribution. For our cases, the holding time of s is 180 s. The barrier height distribution is defined as the sum of three Gaussian functions. The heights of the three barriers are 0.02, 0.33, and 0.69 eV using the fitting parameter of R0 = 1012 s1 for our cases. The burned hole maintains at 300 K, because the hole-filling is prevented by the high barrier of 0.69 eV. This value is much higher than that for the Eu3+ ions doped silica and silicate glasses prepared by the sol– gel method [16,17]. 4. Conclusions
Fig. 5. (a) Difference spectrum at 10 K and (b–i) difference spectra at 10 K after excursion process to different cycling temperatures, (b) 30 K, (c) 50 K, (d) 80 K, (e) 120 K, (f) 150 K, (g) 200 K, (h) 250 K, and (i) 300 K.
Eu3+ ions doped oxy-fluoride glasses in composition of 30CaF2–10Al2O3–60B2O3 were prepared in a reducing atmosphere. PSHB spectrum was recorded at room temperature. Temperature cycling experiments reveal that the holes burned at 10 K show high thermal stability due to the high barrier height of 0.69 eV. The spectra burned at 10 K remain after excursion to 300 K.
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Acknowledgement This work was partially supported by a Grant-in-Aid for the Scientific Research (No.15205025) from the Ministry of Education, Culture, Sports, Science, and Technology of Japan. References [1] O. Sild, K. Haller, Zero-Phonon Lines and Spectral Hole Burning in Spectroscopy and Photochemistry, Springer-Verlag, Berlin, 1988. [2] W.E. Moerner, Persistent Spectral Hole Burning: Science and Applications, Springer, Berlin, 1988. [3] A. Kurita, T. Kushida, T. Izumitani, M. Matsukawa, Opt. Lett. 19 (1994) 314. [4] K. Fujita, K. Tanaka, K. Hirao, N. Soga, J. Opt. Soc. Am. B 15 (1998) 2700.
[5] N. Murase, Y. Kawasaki, A. Tomita, J. Lumin. 98 (2002) 301. [6] S. Todoroki, K. Hirao, N. Soga, J. Appl. Phys. 72 (1992) 5853. [7] J.R. Qiu, K. Nouchi, K. Miura, T. Mitsuyu, K. Hirao, J. Phys.: Condens. Matter 12 (2000) 5061. [8] K. Fujita, K. Tanaka, H. Sasaki, K. Hirao, J. Lumin. 86 (2000) 297. [9] Y. Sun, C.W. Thiel, R.L. Cone, R.W. Equall, R.L. Hutcheson, J. Lumin. 98 (2002) 281. [10] A. Osvet, S. Emelianova, R. Weissmann, V.I. Arbuzov, A. Winnacker, J. Lumin. 86 (2000) 323. [11] J. Zhang, S. Huang, J. Yu, Opt. Lett. 16 (1992) 1146. [12] C.R. Pulluru, R.R. Kalluru, B.R. Reddy, T.A. Konovalova, L.D. Kispert, Appl. Phys. Lett. 87 (2005) 091107. [13] H.P. You, M. Nogami, J. Appl. Phys. 95 (2004) 2781. [14] W. Ko¨hler, J. Friedrich, Phys. Rev. Lett. 59 (1987) 2199. [15] J. Zollfrank, J. Friedrich, T. Tani, Polymer 30 (1989) 231. [16] H. Hanzawa, D. Ueda, G. Adachi, K. Machida, Y. Kanematsu, J. Lumin. 94/95 (2001) 503. [17] M. Nogami, T. Hayakawa, Phys. Rev. B 56 (1997) R14235.