Second harmonic generation by lithium metasilicate grains in photocrystallized glass

The present work explores the formation of lithium metasilicate (Li2SiO3) crystalline grains in a photosensitive glass similar to Foturan® and the nonlinear optical characteristics displayed by the formed composite material. For partial crystallization of the glass, we applied the conventional three-step procedure: UV irradiation, primary heat treatment, and secondary annealing at higher temperature. The formation of crystalline Li2SiO3 was confirmed by Raman spectroscopy. The specimens exhibit quadratic optical nonlinearity without poling. The measured patterns of second optical harmonic radiation of the fabricated specimens demonstrate a two-lobe shape, characteristic of μm-sized second harmonic sources. Simulations in accordance with the nonlinear Rayleigh–Gans–Debye scattering model allowed us to estimate the average grain sizes: ∼1.0 μm after 20 min of secondary anneal, ∼2.7 μm after 40 min, and ∼4.7 μm after 60 min. The UV exposure duration shows a weak influence on the grain size. The combination of photolithography defining UV-irradiated regions with annealing-controlled crystallite size appears to be a promising approach for the fabrication of multi-element structures capable of generating second harmonics with a desired radiation pattern.