Synthesis of LaF3 nanocrystals by laser-induced Nd3+ atom heat processing in oxyfluoride glasses

https://doi.org/10.1016/j.matchemphys.2008.07.062Get rights and content

Abstract

Oxyfluoride glasses with the compositions of 33SiO2–21Al2O3–33NaF–9LaF3–4Nd2O3xErF3 (mol%), x = 0 and 0.5, are prepared using a conventional melt-quenching technique, and the laser-induced crystallization method using continuous-wave (cw) diode lasers with a wavelength of λ = 795 nm is applied to form LaF3 crystals on the glass surface. It is found from X-ray diffraction analyses and micro-photoluminescence spectra that Nd3+-, Er3+-doped LaF3 crystals with a diameter of ∼23 nm are patterned by laser irradiations with a laser power of 1.8 W and a laser scanning speed of 10 μm/s. It is proposed that the lines patterned by laser irradiations are consisted of the composite of Nd3+-doped LaF3 nanocrystals and the SiO2-based oxide glassy phase. It is suggested that the combination of cw diode laser (λ = 795 nm) irradiations and the non-radiated relaxation process in the ff transition level of 4F5/2  4I9/2 of Nd3+ ions is effective in the spatially selected local patterning of fluoride crystals in oxyfluoride glasses with Nd3+ ions.

Introduction

Transparent oxyfluoride-based crystallized glasses (glass-ceramics) consisting of fluoride nanocrystals have received much attention [1], [2], [3], [4], [5], [6], [7], [8]. In such materials, for instance, rare-earth (RE3+) ions are incorporated into fluoride nanocrystals being embedded in SiO2-based oxide glass matrices, and the drawbacks to chemical and thermal stabilities in fluoride materials have been considered to be improved. Usually, oxyfluoride-based crystallized glasses are fabricated through well-controlled heat treatments in an electric furnace and desired fluoride nanocrystals are formed in the interior of glasses.

On the other hand, laser irradiations of glasses have also received much attention, because of the possibility for spatially selected local structural modifications such as refractive index changes or crystallizations in glasses. We developed laser-induced crystallization techniques in glasses, i.e., rare-earth atom heat (REAH) processing and transition metal atom heat (TMAH) processing methods, and have succeeded in patterning crystal lines consisting of nonlinear or ferroelectric crystals such as β-BaB2O4, Ba2TiGe2O8, and LiNbO3 [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19]. Goutaland et al. [20] demonstrated that the crystallization of tellurite and germanate-based glasses can be achieved by a combination of heat treatments at the temperatures below the crystallization temperature and irradiations of continuous-wave (cw) ultraviolet (UV) laser (Argon laser with a wavelength of λ = 244 nm). It is, therefore, of interest to apply laser-induced crystallization techniques to oxyfluoride glasses and to pattern optical functional fluoride crystals. Such patternings would induce other potentials for optical device applications of oxyfluoride glasses.

In this study, we apply the REAH processing technique to oxyfluoride glasses to pattern Nd3+-, Er3+-doped LaF3 fluoride crystal lines on the glass surface, in which cw diode lasers with λ = 795 nm are irradiated onto oxyfluoride glasses of SiO2–Al2O3–NaF–LaF3–Nd2O3–ErF3. Very recently, we succeeded in patterning crystal dots and lines consisting of fluoride nanocrystals such as CaF2 by using the TMAH processing technique (i.e., Nd:YAG lasers with λ = 1064 nm or Yb:YVO4 lasers with λ = 1080 nm as laser sources and Ni2+ ions are used as laser absorbers) [21]. In the present study, we propose that Nd3+ ions can act as good laser absorbers and consequently the laser energies absorbed by Nd3+ ions are transferred effectively to lattice vibrations through the non-radiative relaxation process to induce the crystallization of LaF3 crystals. Mekhlouf et al. [22] have reported the local crystallization of LaF3 nanocrystals in an oxyfluoride glass using a combination technique of UV laser (λ = 244 nm) irradiations and heat treatments at temperatures below the glass transition temperature. Gonzalez-Perez et al. [23] reported that cw Argon laser irradiations can achieve successfully a local crystallization in Tm3+- and Yb3+-co-doped oxyfluoride glasses.

Section snippets

Experimental

Several studies on the fabrication and optical properties of transparent oxyfluoride-based crystallized glasses consisting of LaF3 nanocrystals have been reported so far [2], [3], [4], [5], [6], [7]. In this study, we developed two glasses for the patterning of lines consisting of RE3+ (Nd3+, Er3+)-doped LaF3 crystals by laser irradiations, i.e., 33SiO2–21Al2O3–33NaF–9LaF3–4Nd2O3xErF3 glasses (mol%) with x = 0 and 0.5. The glasses were prepared using a conventional melt-quenching method.

Glass formation and crystallization in electric furnace

The XRD measurements for the melt-quenched samples were carried out, and only halo patterns without any sharp peaks were observed. The DTA patterns for the melt-quenched samples with the compositions of 33SiO2–21Al2O3–33NaF–9LaF3–4Nd2O3xErF3 (mol%) with x = 0 and 0.5 are shown in Fig. 1, in which the samples with x = 0 (no ErF3) and x = 0.5 (0.5ErF3) are designated here as Glass A and Glass B, respectively. In the DTA patterns, the samples showed endothermic peaks due to the glass transition. The

Conclusions

The oxyfluoride glasses of 33SiO2–21Al2O3–33NaF–9LaF3–4Nd2O3xErF3 (mol%), x = 0 and 0.5, were prepared using a conventional melt-quenching technique, and cw diode lasers with λ = 795 nm were irradiated onto the glass surface to pattern crystal lines consisting of LaF3 crystals. It was found from X-ray diffraction analyses and micro-photoluminescence spectra that Nd3+-, Er3+-doped LaF3 crystals with a diameter of ∼23 nm were patterned by laser irradiations with a power of 1.8 W and a scanning speed of

Acknowledgment

This work was supported from the Grant-in-Aid for Scientific Research from the Ministry of Education, Science, Sports, Culture and Technology, Japan.

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