Morphology design of highly oriented nonlinear optical Ba2TiSi2O8 crystals at the glass surface by crystallization in reduced atmosphere

The crystallization behavior of 40BaO–20TiO2–40SiO2 glass (mol%) in a reduced atmosphere (7%H2/93%Ar) is examined to develop a new approach for the design and control of the morphology of nonlinear optical fresnoite (Ba2TiSi2O8) crystals. It is found that the glass exhibits surface crystallization when heated in a reduced atmosphere, resulting in c-axis orientations of Ba2TiSi2O8 crystals at the glass surface and enhanced second harmonic intensities. The formation of Ti3+ ions at the surface, through the reduction of Ti4+ ions in the reduced atmosphere, is confirmed from electron spin resonance spectra. It is clarified that the presence of Ti3+ ions at the surface induces the prominent change in the crystallization behavior from bulk nanocrystallization to surface crystallization of stoichiometric fresnoite.