- Email: [email protected]
Formation of photonic structures in Sm2+-doped aluminosilicate glasses through phase separation
Journal of Non-Crystalline Solids 352 (2006) 2496–2500 www.elsevier.com/locate/jnoncrysol
Formation of photonic structures in Sm2+-doped aluminosilicate glasses through phase separation Koji Fujita a
a,*
, Shunsuke Murai a, Kazuki Nakanishi b, Kazuyuki Hirao
a
Department of Material Chemistry, Graduate School of Engineering, Kyoto University, Nishikyo-ku, Katsura, Kyoto 615-8510, Japan b Department of Chemistry, Graduate School of Science, Kyoto University, Sakyo-ku, Kitashirakawa, Kyoto 606-8502, Japan Available online 26 May 2006
Abstract Sm2+-doped Al2O3–SiO2 glasses with three-dimensionally interconnected macroporous morphology have been prepared via the alkoxide-derived sol–gel process containing poly(ethylene oxide) and SmCl3 Æ 6H2O. The macroporous morphology is obtained by concurrently inducing the phase separation and sol–gel transition. When the macroporous aluminosilicate glasses doped with Sm2+ are irradiated with a visible light laser at the wavelength of 488 nm, a hole or a dip appears in the plot of fluorescence intensity versus the incident angle of laser beam, indicating that the valence state of Sm2+ is spatially modulated through the interference of multiply scattered light. The hole profile can be controlled by adjusting the macroporous morphology. Ó 2006 Elsevier B.V. All rights reserved. PACS: 81.20.Fw; 81.05.Rm; 42.25.Dd; 42.25.Hz Keywords: Luminescence; Aluminosilicates; Rare-earths in glasses; Sol–gel, aerogel and solution chemistry
1. Introduction Various interesting effects due to the interference of multiply scattered light have been observed in dielectrically disordered materials, where the indices of refraction vary on length scales of the order of the wavelength of light. For instance, the interference between counterpropagating waves in dielectrically disordered structures gives rise to enhanced backscattering. This phenomenon is known as coherent backscattering or weak localization [1–4]. Later, many interference effects were recognized such as the spatial correlations in the intensity of light transmitted through dielectrically disordered media [5]. These experiments were carried out on optically passive media. Some attempts have been also made to extend the field of study to active media. For example, the multiple scattering of light in the presence of amplification leads to laser-like emission without a cavity [6,7]. The mirrorless laser action is *
Corresponding author. Tel.: +81 75 383 2432; fax: +81 75 383 2420. E-mail address: [email protected] (K. Fujita).
0022-3093/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2006.03.024
called a random laser, which is achieved in a number of physical forms including powdered laser crystals and high-gain organic dyes in combination with strongly scattering media. On the other hand, the multiple scattering of light in the presence of photobleaching results in an optical memory effect [8]. The experimental situation is realized in strongly scattering media combined with photoreactive species, such as Sm2+ or fulgide [8–13]. When such a material is irradiated with monochromatic light, the interference of multiply scattering light causes the spatial modulation of optical absorbance through photobleaching. Since the interference effect depends on the wavelength and incident angle of the monochromatic light, a dip or a hole is burned in the frequency and wave-vector domains. The hole-burning effect obviously stems from the grating formation on a macroscopic length scale due to the interference of multiply scattering light, and thereby, the chemical and/or physical state of ions or molecules can be manipulated in a small volume inside a medium. This phenomenon is thus applicable to high-density optical storage, in which data information is recorded as three-dimensional random interference patterns.
K. Fujita et al. / Journal of Non-Crystalline Solids 352 (2006) 2496–2500
So far, most studies on multiple light scattering in dielectrically disordered media have been performed for fine particles, including powders and colloidal suspensions, and the scattering properties were controlled by changing the density and size of particles. However, monolithic structures are more favorable than fine particles for the practical use. Pore formation is a very promising technique for tailoring the scattering strength as well as for obtaining monolithic scattering media. Recently, we have prepared macroporous SiO2 monoliths by inducing phase separation in alkoxy-derived sol–gel systems, and showed that the scattering strength can be tuned through the change in the macroporous morphology [14]. Also, we have succeeded in incorporating Sm2+ ions into macroporous Al2O3–SiO2 glass systems utilizing the sol–gel method including phase separation [15,16]. It is expected that the capability of tailoring the scattering strength in optically active media extends the possibility of photonic applications such as random lasers or optical memories. Here, we report on the occurrence of a hole-burning effect by the interference of multiply scattered light in macroporous Al2O3–SiO2 glasses doped with Sm2+, where the photoionization of Sm2+ is used as the photobleaching process to produce the hole [8,9,12,13]. In particular, the tunability of hole properties through the change in macroporous morphology is demonstrated. 2. Experimental 2.1. Sample preparation and characterization A sol–gel method including phase separation was used to fabricate macroporous 5AlO3/2 Æ 95SiO2 (in mol%) glasses containing nominally 3 wt% Sm2O3 [16]. The choice of the glass system comes from the fact that Sm2+ ions can be homogeneously incorporated into SiO2 glass codoped with Al2O3, in contrast to the case of pure SiO2 glass [17,18]. Tetramethoxysilane, Si(OCH3)4, and aluminum sec-butoxide, Al(OC4H9)3, and SmCl3 Æ 6H2O were used as the sources of inorganic components. Al(OC4H9)3 was mixed with sec-butanol with a volume ratio of 1:2 to lower the viscosity. Poly(ethylene oxide) (PEO) with an average molecular weight of 10 000 was used as the polymer component to induce the phase separation. The starting compositions of the samples are listed in Table 1. Nitric acid was utilized as the catalyst for the hydrolysis and condensation. Gel samples were prepared by mixing a
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2 M nitric acid aqueous solution containing PEO and SmCl3 Æ 6H2O with a mixture of Si(OCH3)4, Al(OC4H9)3, and sec-butanol in an ice bath, as we reported previously [16]. After stirring vigorously for 30 min, the resultant transparent solution was poured into a glass container. Gelation was carried out at 40 °C in the sealed container. The wet gel thus obtained was aged for 24 h and dried at 60 °C to induce the evaporation of the solvent. Heat treatment was performed at 800 or 1000 °C for 2 h in air to remove PEO completely and also to obtain the sintered gel. To reduce Sm3+ into Sm2+ ions, the sintered gel heattreated at 800 °C in air were reheated at 1000 °C for 30 min under a flowing reducing gas of 50 vol.% N2, 47.5 vol.% Ar, and 2.5 vol.% H2. It was ascertained by X-ray diffraction analysis with CuKa radiation that the heat-treated samples were amorphous. A scanning electron microscope (SEM; Hitachi, S-2600N) was used to observe the macroscopic morphology of the heat-treated gel. 2.2. Optical measurements Fluorescence spectra were measured at room temperature with a fluorescence spectrophotometer (Hitachi, 850). A 488 nm line from a cw Ar+ laser (Spectra Physics, Stabilite 2017) was used as the excitation source. The 488-nm Ar+ laser was also used for measurements of hole-burning effects. A single laser beam was utilized to burn and subsequently to probe the hole. First, the sample was irradiated with the writing laser beam for 20 min to burn the hole through the photobleaching of Sm2+. The burning power was 30 mW, and the beam diameter on the sample was about 2 mm. Then, the laser beam was attenuated by a factor of 103 and used as the reading beam to probe the hole. The intensity of Sm2+ fluorescence was plotted against the incident angle of laser beam. The sample was set on a stage and rotated by a stepping motor to change the angle between the incident light and the sample. The fluorescence was detected with a photomultiplier tube combined with glass filters, and the signal was recorded using a lock-in amplifier controlled by a personal computer. For the purpose of evaluating the scattering strength of samples, coherent backscattering (CBS) experiments were performed so that the transport mean free path of light, that is, the average length after which the propagation direction of the light was randomized by scattering, was obtained. A collimated beam of the Ar+ laser (488 nm) was reflected by a beam splitter and incident on the sample
Table 1 Starting compositions of samples (unit: g) Sample
Si(OCH3)4
Al(OC4H9)3
sec-Butanol
HNO3a
H2O
PEO
SmCl3 Æ 6H2O
P085 P090 P095 P100
4.90 4.90 4.90 4.90
0.42 0.42 0.42 0.42
0.70 0.70 0.70 0.70
2.10 2.10 2.10 2.10
8.48 8.48 8.48 8.48
0.85 0.90 0.95 1.00
0.13 0.13 0.13 0.13
a
60% Aqueous solution.
K. Fujita et al. / Journal of Non-Crystalline Solids 352 (2006) 2496–2500
surface with a small angle from the normal incidence. The scattered light around the backscattered direction was collected by a lens with 10-cm focal length and detected using a charge-coupled device placed at the focal point. 3. Results Fig. 1 depicts SEM photographs of specimens heat-treated at 800 °C in air and reheated at 1000 °C under reducing atmosphere. All the specimens exhibit the bicontinuous morphology of gel skeleton and pores. The size of pores and skeleton became smaller as the PEO content was increased. The mercury porosimetry measurements revealed sharp pore size distributions in all the specimens [16]. Fluorescence spectra of P085 heat-treated under different conditions are shown in Fig. 2. For the macroporous glass heat-treated at 1000 °C in air, fluorescence peaks corresponding to the 4G5/2 ! 6H5/2,7/2,9/2 transitions of Sm3+ are observed at around 565, 603, and 649 nm, respectively. On the other hand, fluorescence lines characteristic of Sm2+ appear for the macroporous glass heat-treated at 800 °C in air and reheated at 1000 °C under a reducing atmosphere. The fluorescence peaks near 684, 700, and 728 nm are assigned to the 5D0 ! 7F0,1,2 transitions of Sm2+, respectively [16–18]. Fig. 3(a) illustrates relevant energy levels of Sm2+ in the present Al2O3–SiO2 glass system. Since the crystal field strength for Sm2+ is relatively weak, the lowest energy level of 4f55d states is higher than 5D0 level. Hence, the excitation into 4f55d bands with a 488-nm light causes the nonradiative relaxation to 5D0 state accompanied by the sharp
Sm 3+ : 4G5/2 Emission intensity (arb. units)
2498
J=7/2
λex = 488 nm
6H J
J=9/2
J=5/2 in air Sm 2+: 5D0
7F J
J=0 J=1 J=2 in Ar/H2
500
600
700 Wavelength (nm)
800
Fig. 2. Fluorescence spectra of 5AlO3/2 Æ 95SiO2 glass (P085) heat-treated at 1000 °C in air and 5AlO3/2 Æ 95SiO2 glass (P085) heat-treated at 800 °C in air and subsequently reheated at 1000 °C under reducing atmosphere. The excitation wavelength (kex) was 488 nm.
fluorescence due to the 4f–4f transitions as shown in Fig. 2. Time evolution of Sm2+ fluorescence near 683 nm under the laser irradiation is depicted in Fig. 3(b). As the irradiation time is increased, the fluorescence intensity is gradually decreased as a result of the photobleaching ascribed to the photoionization of Sm2+ to Sm3+ [18]. In Fig. 4(a), the difference signal of fluorescence intensity before and after the irradiation with an Ar+ laser is plotted for P085, P090, P095, and P100 against the incident angle of laser beam. The laser irradiation was performed for 20 min at the incident angle of 0 mrad. For all of the macroporous
Fig. 1. SEM images of 5AlO3/2 Æ 95SiO2 gels heat-treated at 800 °C in air and subsequently reheated at 1000 °C under reducing atmosphere: (a) P085, (b) P090, (c) P095, and (d) P100. Bars = 10 lm.
K. Fujita et al. / Journal of Non-Crystalline Solids 352 (2006) 2496–2500
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(a) Emission intensity (arb. units)
P100
P095
P090
P085
-40
-20
0
20
40
Angle (mrad)
glasses, holes, or dips, can be observed in the plot of fluorescence intensity versus incident angle. The hole width increases in the order of P085 < P090 < P095 < P100. Fig. 4(b) shows coherent backscattering (CBS) results for P085, P090, P095, and P100. The measurements were performed for the macroporous glasses without Sm2+. CBS is observed as an increase in the reflected intensity at the exact backscattering direction, which produces a cone in the plot of backscattered intensity versus scattering angle. The increase in the backscattered intensity originates from the constructive interference of time-reversed counterpropagating waves [1–3]. In non-absorbing media, the width of cone reflects the lateral spread of light in a disordered medium and is proportional to k/l, where k is the wavelength of light, and l is the transport mean free path [4]. In other words, the broader CBS cone means the smaller l, or higher scattering strength. We evaluated from theoretical fits to the CBS data [4] that l = 5.6 lm for P085, l = 4.0 lm for P090, l = 3.1 lm for P095, and l = 2.5 lm for P100. This result indicates that the scattering strength is higher in the specimen with finer macroporous morphology. A similar relation between l and gel morphology has been reported for macroporous SiO2 glasses prepared via the sol–gel process incorporating phase separation [14]. 4. Discussion 4.1. Macropore formation and morphology control The macroporous morphology as shown in Fig. 1 is formed via the development of a transient structure of phase separation induced during the hydrolysis and polycondensation of alkoxides and the subsequent freezing of the structure by the sol–gel transition [19]. The variation
(b) Backscattered intensity (arb. units)
Fig. 3. (a) Energy levels of Sm2+ in 5AlO3/2 Æ 95SiO2 glass. (b) Time evolution of the fluorescence intensity due to the 5D0–7F0 transition of Sm2+ near 683 nm under irradiation with 488-nm laser light (30 mW). The measurements were performed for 5AlO3/2 Æ 95SiO2 glass (P085) heattreated at 800 °C in air and subsequently reheated at 1000 °C under a reducing atmosphere.
P100 P095 P090 P085
-60
-40
-20
0 Angle (mrad)
20
40
60
Fig. 4. (a) Hole profiles of P085, P090, P095, and P100 as a function of the incident angle of laser beam. The measurements were carried out on the 4f6 ! 4f55d transition of Sm2+ using a 488-nm line of an Ar+ laser. (b) CBS cones of P085, P090, P095, and P100 without Sm2+. An Ar+ laser with a wavelength of 488 nm was used as the light source. The solid curves represent theoretical fits from diffusion approximation [4].
of macroporous morphology with PEO content reflects the phase-separation tendency, because the starting composition remains constant except for the amount of PEO that induced the phase separation. In the present system containing PEO, the phase separation is driven by a repulsive interaction between solvent mixtures and PEO adsorbed on alkoxide-derived oligomers, and the phase-separation tendency exhibits a maximum against the PEO concentration, depending on the distribution of PEO between inorganic oligomers and solvents [19]. The PEO concentrations employed herein are higher than the PEO concentration of maximum phase-separation tendency, so the phase separation becomes less with increasing the PEO concentration. As a result, the finer bicontinuous structures are obtained due to the later onset of phase separation. After drying of the wet gel, the phase with PEO and inorganic components becomes the gel skeleton, and the phase composed mainly of solvents turns into macropores.
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K. Fujita et al. / Journal of Non-Crystalline Solids 352 (2006) 2496–2500
4.2. Photoreaction of Sm2+ in macroporous aluminosilicate glasses It is evident from Fig. 2 that some Sm3+ ions are reduced to Sm2+ ions in macroporous Al2O3–SiO2 glasses upon heating under reducing conditions. As reported by Nogami and Abe [17,18], the fraction of Sm2+ to Sm3+ in sol–gelderived Al2O3–SiO2 glasses depends on the glass composition; the reduction of Sm3+ to Sm2+ takes place more readily when the glasses contain a larger number of Al3+ ions. In the present case, the glass composition is fixed at 5AlO3/2 Æ 95SiO2 (in mol%), so it can be assumed that the Sm2+ content is fairly constant throughout the samples. When the photobleaching as depicted in Fig. 3(b) occurs in transparent media, holes are never detected in the plot of fluorescence intensity versus incident angle of the laser beam. The combination of the photobleaching with multiple light scattering in macroporous glasses can produce the holes as shown in Fig. 4(a), indicating that the mechanism of hole formation relies on the interference of multiply scattered light [8–13]. It is anticipated that the angular width of hole profile is inversely proportional to the lateral spread of light in the medium, since the light waves injected at different points on the incident plane interfere inside the medium to induce the spatial modulation of the valence state of Sm2+. The behavior of light diffusion is affected by scattering and absorption, and hence, the hole profile is controlled by both the parameters. The influence of optical absorption on the hole profile is presumably small due to the low sample-to-sample variation in Sm2+ content as mentioned above. On the other hand, the theoretical analysis of the CBS cone, as demonstrated in Fig. 4(b), indicates that the light scattering strength increases in the order of P085 < P090 < P095 < P100. Thus, the variation of hole width as shown in Fig. 4(a) is ascribed to the change in the strength of scattering rather than the amount of absorption. The strong scattering in the macroporous glass with fine morphology such as P100 suppresses the propagation of scattered lights inside the medium, which in turn leads to the broadening of the angular profile of hole as well as the CBS cone. Quantitative analysis of hole shape should definitely be performed in future to understand more deeply the light propagation in macroporous glasses. 5. Conclusion We have prepared macroporous Al2O3–SiO2 glasses doped with Sm2+ using the sol–gel method accompanied
by the phase separation, and observed the hole-burning effect based on the interference of multiply scattered light. The laser irradiation on the 4f6 ! 4f55d transition of Sm2+ resulted in holes, or dips, in the plot of fluorescence intensity versus incident angle of the laser beam, and the hole profile varied depending on the macroporous morphology. CBS measurements indicated that the scattering strength was increased as the macroporous morphology became fine. The variation of the hole shape is qualitatively explained in terms of the change in the scattering strength. Acknowledgements This study was financially supported by the Industrial Technology Research Program (No. 04A25023) from New Energy and Industrial Technology Development Organization (NEDO) of Japan, and the Grand-in-Aid for Scientific Research (A) (No. 15206072) from the Ministry of Education, Culture, Sports, Science, and Technology (MEXT) of Japan. K.F. also thanks the Asahi Glass Foundation. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19]
Y. Kuga, A. Ishimaru, J. Opt. Soc. Am. A 1 (1984) 831. M.P. van Albada, A. Lagendijk, Phys. Rev. Lett. 55 (1985) 2692. P. Wolf, G. Maret, Phys. Rev. Lett. 55 (1985) 2696. E. Akkermans, P. Wolf, R. Maynard, Phys. Rev. Lett. 56 (1986) 1471. S. Feng, C. Kane, P.A. Lee, A.D. Stone, Phys. Rev. Lett. 61 (1988) 834. N.M. Lawandy, R.M. Balachandran, A.S.L. Gomes, E. Sauvain, Nature 368 (1994) 436. H. Cao, Y.G. Zhao, S.T. Ho, E.W. Seelig, Q.H. Wang, R.P.H. Chang, Phys. Rev. Lett. 82 (1999) 2278. A. Kurita, Y. Kanematsu, M. Watanabe, K. Hirata, T. Kushida, Phys. Rev. Lett. 83 (1999) 1582. A. Kurita, Y. Kanematsu, M. Watanabe, K. Hirata, T. Kushida, J. Lumin. 87–89 (2000) 986. A. Kurita, Y. Kanematsu, T. Kushida, T. Sagaisaka, Y. Yokoyama, Mol. Cryst. Liq. Cryst. 344 (2000) 205. M. Tomita, T. Ito, S. Hattori, Phys. Rev. B 64 (2001) 180202. K. Fujita, Y. Ohashi, K. Hirao, Opt. Lett. 28 (2003) 567. K. Fujita, Y. Ohashi, K. Hirao, Trans. J. Non-Cryst. Solids 345&346 (2004) 407. S. Murai, K. Fujita, K. Nakanishi, K. Hirao, Jpn. J. Appl. Phys. Part 1 43 (2004) 5359. K. Fujita, S. Murai, Y. Ohashi, K. Nakanishi, K. Hirao, Chem. Lett. 33 (2004) 1120. S. Murai, K. Fujita, K. Nakanishi, K. Hirao, J. Phys. Chem. B 108 (2004) 16670. M. Nogami, Y. Abe, J. Appl. Phys. 80 (1996) 409. M. Nogami, Y. Abe, Phys. Rev. B 56 (1997) 182. K. Nakanishi, J. Porous Mater. 4 (1997) 67.
Formation of photonic structures in Sm2+-doped aluminosilicate glasses through phase separation Koji Fujita a
a,*
, Shunsuke Murai a, Kazuki Nakanishi b, Kazuyuki Hirao
a
Department of Material Chemistry, Graduate School of Engineering, Kyoto University, Nishikyo-ku, Katsura, Kyoto 615-8510, Japan b Department of Chemistry, Graduate School of Science, Kyoto University, Sakyo-ku, Kitashirakawa, Kyoto 606-8502, Japan Available online 26 May 2006
Abstract Sm2+-doped Al2O3–SiO2 glasses with three-dimensionally interconnected macroporous morphology have been prepared via the alkoxide-derived sol–gel process containing poly(ethylene oxide) and SmCl3 Æ 6H2O. The macroporous morphology is obtained by concurrently inducing the phase separation and sol–gel transition. When the macroporous aluminosilicate glasses doped with Sm2+ are irradiated with a visible light laser at the wavelength of 488 nm, a hole or a dip appears in the plot of fluorescence intensity versus the incident angle of laser beam, indicating that the valence state of Sm2+ is spatially modulated through the interference of multiply scattered light. The hole profile can be controlled by adjusting the macroporous morphology. Ó 2006 Elsevier B.V. All rights reserved. PACS: 81.20.Fw; 81.05.Rm; 42.25.Dd; 42.25.Hz Keywords: Luminescence; Aluminosilicates; Rare-earths in glasses; Sol–gel, aerogel and solution chemistry
1. Introduction Various interesting effects due to the interference of multiply scattered light have been observed in dielectrically disordered materials, where the indices of refraction vary on length scales of the order of the wavelength of light. For instance, the interference between counterpropagating waves in dielectrically disordered structures gives rise to enhanced backscattering. This phenomenon is known as coherent backscattering or weak localization [1–4]. Later, many interference effects were recognized such as the spatial correlations in the intensity of light transmitted through dielectrically disordered media [5]. These experiments were carried out on optically passive media. Some attempts have been also made to extend the field of study to active media. For example, the multiple scattering of light in the presence of amplification leads to laser-like emission without a cavity [6,7]. The mirrorless laser action is *
Corresponding author. Tel.: +81 75 383 2432; fax: +81 75 383 2420. E-mail address: [email protected] (K. Fujita).
0022-3093/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2006.03.024
called a random laser, which is achieved in a number of physical forms including powdered laser crystals and high-gain organic dyes in combination with strongly scattering media. On the other hand, the multiple scattering of light in the presence of photobleaching results in an optical memory effect [8]. The experimental situation is realized in strongly scattering media combined with photoreactive species, such as Sm2+ or fulgide [8–13]. When such a material is irradiated with monochromatic light, the interference of multiply scattering light causes the spatial modulation of optical absorbance through photobleaching. Since the interference effect depends on the wavelength and incident angle of the monochromatic light, a dip or a hole is burned in the frequency and wave-vector domains. The hole-burning effect obviously stems from the grating formation on a macroscopic length scale due to the interference of multiply scattering light, and thereby, the chemical and/or physical state of ions or molecules can be manipulated in a small volume inside a medium. This phenomenon is thus applicable to high-density optical storage, in which data information is recorded as three-dimensional random interference patterns.
K. Fujita et al. / Journal of Non-Crystalline Solids 352 (2006) 2496–2500
So far, most studies on multiple light scattering in dielectrically disordered media have been performed for fine particles, including powders and colloidal suspensions, and the scattering properties were controlled by changing the density and size of particles. However, monolithic structures are more favorable than fine particles for the practical use. Pore formation is a very promising technique for tailoring the scattering strength as well as for obtaining monolithic scattering media. Recently, we have prepared macroporous SiO2 monoliths by inducing phase separation in alkoxy-derived sol–gel systems, and showed that the scattering strength can be tuned through the change in the macroporous morphology [14]. Also, we have succeeded in incorporating Sm2+ ions into macroporous Al2O3–SiO2 glass systems utilizing the sol–gel method including phase separation [15,16]. It is expected that the capability of tailoring the scattering strength in optically active media extends the possibility of photonic applications such as random lasers or optical memories. Here, we report on the occurrence of a hole-burning effect by the interference of multiply scattered light in macroporous Al2O3–SiO2 glasses doped with Sm2+, where the photoionization of Sm2+ is used as the photobleaching process to produce the hole [8,9,12,13]. In particular, the tunability of hole properties through the change in macroporous morphology is demonstrated. 2. Experimental 2.1. Sample preparation and characterization A sol–gel method including phase separation was used to fabricate macroporous 5AlO3/2 Æ 95SiO2 (in mol%) glasses containing nominally 3 wt% Sm2O3 [16]. The choice of the glass system comes from the fact that Sm2+ ions can be homogeneously incorporated into SiO2 glass codoped with Al2O3, in contrast to the case of pure SiO2 glass [17,18]. Tetramethoxysilane, Si(OCH3)4, and aluminum sec-butoxide, Al(OC4H9)3, and SmCl3 Æ 6H2O were used as the sources of inorganic components. Al(OC4H9)3 was mixed with sec-butanol with a volume ratio of 1:2 to lower the viscosity. Poly(ethylene oxide) (PEO) with an average molecular weight of 10 000 was used as the polymer component to induce the phase separation. The starting compositions of the samples are listed in Table 1. Nitric acid was utilized as the catalyst for the hydrolysis and condensation. Gel samples were prepared by mixing a
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2 M nitric acid aqueous solution containing PEO and SmCl3 Æ 6H2O with a mixture of Si(OCH3)4, Al(OC4H9)3, and sec-butanol in an ice bath, as we reported previously [16]. After stirring vigorously for 30 min, the resultant transparent solution was poured into a glass container. Gelation was carried out at 40 °C in the sealed container. The wet gel thus obtained was aged for 24 h and dried at 60 °C to induce the evaporation of the solvent. Heat treatment was performed at 800 or 1000 °C for 2 h in air to remove PEO completely and also to obtain the sintered gel. To reduce Sm3+ into Sm2+ ions, the sintered gel heattreated at 800 °C in air were reheated at 1000 °C for 30 min under a flowing reducing gas of 50 vol.% N2, 47.5 vol.% Ar, and 2.5 vol.% H2. It was ascertained by X-ray diffraction analysis with CuKa radiation that the heat-treated samples were amorphous. A scanning electron microscope (SEM; Hitachi, S-2600N) was used to observe the macroscopic morphology of the heat-treated gel. 2.2. Optical measurements Fluorescence spectra were measured at room temperature with a fluorescence spectrophotometer (Hitachi, 850). A 488 nm line from a cw Ar+ laser (Spectra Physics, Stabilite 2017) was used as the excitation source. The 488-nm Ar+ laser was also used for measurements of hole-burning effects. A single laser beam was utilized to burn and subsequently to probe the hole. First, the sample was irradiated with the writing laser beam for 20 min to burn the hole through the photobleaching of Sm2+. The burning power was 30 mW, and the beam diameter on the sample was about 2 mm. Then, the laser beam was attenuated by a factor of 103 and used as the reading beam to probe the hole. The intensity of Sm2+ fluorescence was plotted against the incident angle of laser beam. The sample was set on a stage and rotated by a stepping motor to change the angle between the incident light and the sample. The fluorescence was detected with a photomultiplier tube combined with glass filters, and the signal was recorded using a lock-in amplifier controlled by a personal computer. For the purpose of evaluating the scattering strength of samples, coherent backscattering (CBS) experiments were performed so that the transport mean free path of light, that is, the average length after which the propagation direction of the light was randomized by scattering, was obtained. A collimated beam of the Ar+ laser (488 nm) was reflected by a beam splitter and incident on the sample
Table 1 Starting compositions of samples (unit: g) Sample
Si(OCH3)4
Al(OC4H9)3
sec-Butanol
HNO3a
H2O
PEO
SmCl3 Æ 6H2O
P085 P090 P095 P100
4.90 4.90 4.90 4.90
0.42 0.42 0.42 0.42
0.70 0.70 0.70 0.70
2.10 2.10 2.10 2.10
8.48 8.48 8.48 8.48
0.85 0.90 0.95 1.00
0.13 0.13 0.13 0.13
a
60% Aqueous solution.
K. Fujita et al. / Journal of Non-Crystalline Solids 352 (2006) 2496–2500
surface with a small angle from the normal incidence. The scattered light around the backscattered direction was collected by a lens with 10-cm focal length and detected using a charge-coupled device placed at the focal point. 3. Results Fig. 1 depicts SEM photographs of specimens heat-treated at 800 °C in air and reheated at 1000 °C under reducing atmosphere. All the specimens exhibit the bicontinuous morphology of gel skeleton and pores. The size of pores and skeleton became smaller as the PEO content was increased. The mercury porosimetry measurements revealed sharp pore size distributions in all the specimens [16]. Fluorescence spectra of P085 heat-treated under different conditions are shown in Fig. 2. For the macroporous glass heat-treated at 1000 °C in air, fluorescence peaks corresponding to the 4G5/2 ! 6H5/2,7/2,9/2 transitions of Sm3+ are observed at around 565, 603, and 649 nm, respectively. On the other hand, fluorescence lines characteristic of Sm2+ appear for the macroporous glass heat-treated at 800 °C in air and reheated at 1000 °C under a reducing atmosphere. The fluorescence peaks near 684, 700, and 728 nm are assigned to the 5D0 ! 7F0,1,2 transitions of Sm2+, respectively [16–18]. Fig. 3(a) illustrates relevant energy levels of Sm2+ in the present Al2O3–SiO2 glass system. Since the crystal field strength for Sm2+ is relatively weak, the lowest energy level of 4f55d states is higher than 5D0 level. Hence, the excitation into 4f55d bands with a 488-nm light causes the nonradiative relaxation to 5D0 state accompanied by the sharp
Sm 3+ : 4G5/2 Emission intensity (arb. units)
2498
J=7/2
λex = 488 nm
6H J
J=9/2
J=5/2 in air Sm 2+: 5D0
7F J
J=0 J=1 J=2 in Ar/H2
500
600
700 Wavelength (nm)
800
Fig. 2. Fluorescence spectra of 5AlO3/2 Æ 95SiO2 glass (P085) heat-treated at 1000 °C in air and 5AlO3/2 Æ 95SiO2 glass (P085) heat-treated at 800 °C in air and subsequently reheated at 1000 °C under reducing atmosphere. The excitation wavelength (kex) was 488 nm.
fluorescence due to the 4f–4f transitions as shown in Fig. 2. Time evolution of Sm2+ fluorescence near 683 nm under the laser irradiation is depicted in Fig. 3(b). As the irradiation time is increased, the fluorescence intensity is gradually decreased as a result of the photobleaching ascribed to the photoionization of Sm2+ to Sm3+ [18]. In Fig. 4(a), the difference signal of fluorescence intensity before and after the irradiation with an Ar+ laser is plotted for P085, P090, P095, and P100 against the incident angle of laser beam. The laser irradiation was performed for 20 min at the incident angle of 0 mrad. For all of the macroporous
Fig. 1. SEM images of 5AlO3/2 Æ 95SiO2 gels heat-treated at 800 °C in air and subsequently reheated at 1000 °C under reducing atmosphere: (a) P085, (b) P090, (c) P095, and (d) P100. Bars = 10 lm.
K. Fujita et al. / Journal of Non-Crystalline Solids 352 (2006) 2496–2500
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(a) Emission intensity (arb. units)
P100
P095
P090
P085
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-20
0
20
40
Angle (mrad)
glasses, holes, or dips, can be observed in the plot of fluorescence intensity versus incident angle. The hole width increases in the order of P085 < P090 < P095 < P100. Fig. 4(b) shows coherent backscattering (CBS) results for P085, P090, P095, and P100. The measurements were performed for the macroporous glasses without Sm2+. CBS is observed as an increase in the reflected intensity at the exact backscattering direction, which produces a cone in the plot of backscattered intensity versus scattering angle. The increase in the backscattered intensity originates from the constructive interference of time-reversed counterpropagating waves [1–3]. In non-absorbing media, the width of cone reflects the lateral spread of light in a disordered medium and is proportional to k/l, where k is the wavelength of light, and l is the transport mean free path [4]. In other words, the broader CBS cone means the smaller l, or higher scattering strength. We evaluated from theoretical fits to the CBS data [4] that l = 5.6 lm for P085, l = 4.0 lm for P090, l = 3.1 lm for P095, and l = 2.5 lm for P100. This result indicates that the scattering strength is higher in the specimen with finer macroporous morphology. A similar relation between l and gel morphology has been reported for macroporous SiO2 glasses prepared via the sol–gel process incorporating phase separation [14]. 4. Discussion 4.1. Macropore formation and morphology control The macroporous morphology as shown in Fig. 1 is formed via the development of a transient structure of phase separation induced during the hydrolysis and polycondensation of alkoxides and the subsequent freezing of the structure by the sol–gel transition [19]. The variation
(b) Backscattered intensity (arb. units)
Fig. 3. (a) Energy levels of Sm2+ in 5AlO3/2 Æ 95SiO2 glass. (b) Time evolution of the fluorescence intensity due to the 5D0–7F0 transition of Sm2+ near 683 nm under irradiation with 488-nm laser light (30 mW). The measurements were performed for 5AlO3/2 Æ 95SiO2 glass (P085) heattreated at 800 °C in air and subsequently reheated at 1000 °C under a reducing atmosphere.
P100 P095 P090 P085
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Fig. 4. (a) Hole profiles of P085, P090, P095, and P100 as a function of the incident angle of laser beam. The measurements were carried out on the 4f6 ! 4f55d transition of Sm2+ using a 488-nm line of an Ar+ laser. (b) CBS cones of P085, P090, P095, and P100 without Sm2+. An Ar+ laser with a wavelength of 488 nm was used as the light source. The solid curves represent theoretical fits from diffusion approximation [4].
of macroporous morphology with PEO content reflects the phase-separation tendency, because the starting composition remains constant except for the amount of PEO that induced the phase separation. In the present system containing PEO, the phase separation is driven by a repulsive interaction between solvent mixtures and PEO adsorbed on alkoxide-derived oligomers, and the phase-separation tendency exhibits a maximum against the PEO concentration, depending on the distribution of PEO between inorganic oligomers and solvents [19]. The PEO concentrations employed herein are higher than the PEO concentration of maximum phase-separation tendency, so the phase separation becomes less with increasing the PEO concentration. As a result, the finer bicontinuous structures are obtained due to the later onset of phase separation. After drying of the wet gel, the phase with PEO and inorganic components becomes the gel skeleton, and the phase composed mainly of solvents turns into macropores.
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4.2. Photoreaction of Sm2+ in macroporous aluminosilicate glasses It is evident from Fig. 2 that some Sm3+ ions are reduced to Sm2+ ions in macroporous Al2O3–SiO2 glasses upon heating under reducing conditions. As reported by Nogami and Abe [17,18], the fraction of Sm2+ to Sm3+ in sol–gelderived Al2O3–SiO2 glasses depends on the glass composition; the reduction of Sm3+ to Sm2+ takes place more readily when the glasses contain a larger number of Al3+ ions. In the present case, the glass composition is fixed at 5AlO3/2 Æ 95SiO2 (in mol%), so it can be assumed that the Sm2+ content is fairly constant throughout the samples. When the photobleaching as depicted in Fig. 3(b) occurs in transparent media, holes are never detected in the plot of fluorescence intensity versus incident angle of the laser beam. The combination of the photobleaching with multiple light scattering in macroporous glasses can produce the holes as shown in Fig. 4(a), indicating that the mechanism of hole formation relies on the interference of multiply scattered light [8–13]. It is anticipated that the angular width of hole profile is inversely proportional to the lateral spread of light in the medium, since the light waves injected at different points on the incident plane interfere inside the medium to induce the spatial modulation of the valence state of Sm2+. The behavior of light diffusion is affected by scattering and absorption, and hence, the hole profile is controlled by both the parameters. The influence of optical absorption on the hole profile is presumably small due to the low sample-to-sample variation in Sm2+ content as mentioned above. On the other hand, the theoretical analysis of the CBS cone, as demonstrated in Fig. 4(b), indicates that the light scattering strength increases in the order of P085 < P090 < P095 < P100. Thus, the variation of hole width as shown in Fig. 4(a) is ascribed to the change in the strength of scattering rather than the amount of absorption. The strong scattering in the macroporous glass with fine morphology such as P100 suppresses the propagation of scattered lights inside the medium, which in turn leads to the broadening of the angular profile of hole as well as the CBS cone. Quantitative analysis of hole shape should definitely be performed in future to understand more deeply the light propagation in macroporous glasses. 5. Conclusion We have prepared macroporous Al2O3–SiO2 glasses doped with Sm2+ using the sol–gel method accompanied
by the phase separation, and observed the hole-burning effect based on the interference of multiply scattered light. The laser irradiation on the 4f6 ! 4f55d transition of Sm2+ resulted in holes, or dips, in the plot of fluorescence intensity versus incident angle of the laser beam, and the hole profile varied depending on the macroporous morphology. CBS measurements indicated that the scattering strength was increased as the macroporous morphology became fine. The variation of the hole shape is qualitatively explained in terms of the change in the scattering strength. Acknowledgements This study was financially supported by the Industrial Technology Research Program (No. 04A25023) from New Energy and Industrial Technology Development Organization (NEDO) of Japan, and the Grand-in-Aid for Scientific Research (A) (No. 15206072) from the Ministry of Education, Culture, Sports, Science, and Technology (MEXT) of Japan. K.F. also thanks the Asahi Glass Foundation. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19]
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