Formation of photonic structures in Sm2+-doped aluminosilicate glasses through phase separation

https://doi.org/10.1016/j.jnoncrysol.2006.03.024Get rights and content

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 · 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.

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], [2], [3], [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 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], [9], [10], [11], [12], [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.

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.

Section snippets

Sample preparation and characterization

A sol–gel method including phase separation was used to fabricate macroporous 5AlO3/2 · 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 · 6H2O were used as the sources of inorganic components. Al(OC4H9)3 was mixed with

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

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 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

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

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.

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