Abstract
We report on space-selective crystallization of congruent and polar Sr2TiSi2O8 crystals in a stoichiometric SrO-TiO2-SiO2 glass induced by (1030 nm, 300 fs) femtosecond laser irradiation. This allows us to compare with non-congruent laser-induced crystallization of polar LiNbO3 in non-stoichiometric Li2O-Nb2O5-SiO2 glass and gain information on the mechanism of nanocrystals orientation with the laser polarization that we pointed out previously. Using scanning electron microscopy (SEM), second harmonic generation (SHG), and electron backscattered diffraction (EBSD), we studied the laser-induced crystallization according to the laser processing parameters (pulse energy, pulse repetition rate, scanning speed). We found (1) a domain where the laser track is filled with crystals not perfectly textured (low energy), (2) a domain where an amorphous volume remains surrounded by a crystallized shell (high energy). This arises from Sr out-diffusion and may give rise to the crystallization of both SrTiO3 and Sr2TiSi2O8 phases at low speed. In the one-phase domain (at higher speed), the possibility to elaborate a tube with a perfect Fresnoite texture is found. A significant difference in size and morphology whereas the crystallization threshold remains similar is discussed based on glass thermal properties. Contrarily to Li2O-Nb2O5-SiO2 (LNS) glass, no domain of oriented nanocrystallization controlled by the laser polarization has been found in SrO-TiO2-SiO2 (STS) glass, which is attributed to the larger crystallization speed in STS glass. No nanogratings have also been found that is likely due to the congruency of the glass.
Highlights
Materials functionalized by non-linear crystals exhibiting second harmonic generation (SHG)effect have gained rapidly increasing attention due to their wide potential applications in various optical devices over the past two decades [1,2,3]
We can be surprised to see that the lowest pulse energy for crystallization in STS is to the one for LNS glass (0.12–0.2 W for STS and 0.16 W for LNS glass), whereas the nucleation rate is close to the one for LNS glass (0.12–0.2 W for STS and 0.16 W for LNS glass), whereas the nucleation known to be much faster for STS glass [8]
About the size of the heat-affected volume: if we compare the size of the heat-affected volume between STS and LNS glasses, we find that the width and the length of the HAV are much larger for STS glass
Summary
Materials functionalized by non-linear crystals exhibiting second harmonic generation (SHG)effect have gained rapidly increasing attention due to their wide potential applications in various optical devices over the past two decades [1,2,3]. Several methods, including thermal gradient [4], ultrasonic surface treatment [5], and continuous-wave laser irradiation [6,7], have been proposed to facilitate a controlled crystallization. These methods do not allow the control of the crystals’ space distribution in volume, whereas it is a key point for the applications in integrated optics. Femtosecond (fs) laser has been proved as a powerful tool to induce three-dimensional crystallization inside glasses. In 2000, Miura et al [9] were one of the first to use an 800 nm fs laser to induce β-BaB2 O4 crystal inside 47.5BaO-5Al2 O3 -47.5B2 O3 glass
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