Correlation between atomic structure, structural nanoinhomogeneity, and quadratic optical nonlinearity in glasses of the K2O-TiO2-P2O5 system

Abstract

The methods of small angle synchrotron radiation scattering, vibration spectroscopy, and second harmonic generation are applied to glasses of the K 2 O -TiO 2 -P 2 O 5 system near the stoichiometry of potassium titanyl-phosphate to demonstrate that at the initial stage of phase separation, while glass remains x-ray amorphous and clear, it can possess quadratic optical nonlinearity. The emergence of nonlinearity is facilitated by the formation of nanoheterogeneities in glass whose structure resembles the structural pattern of a nonlinear-optical crystal. A correlation established between the structural specifics of x-ray amorphous glass and its quadratic optical nonlinearly suggests the advisability of describing the short-range and medium-range orders of glasses at the glass-formation boundary in the context of quasicrystallite models.The needs of optoelectronic engineering has stimulated an increasing number of studies dedicated to new nonlinear-optic (NLO) materials, such as crystals, ceramics, clear glass ceramics, and lately glass as well. Glass is a centrosymmetric (CS) medium and, accordingly, does not have even-order optical nonlinearity. The transparency and stable properties of oxide glasses, the possibility of gradual variation of their properties and imparting virtually any shape to glass elements, and their low cost and technological availability attract the attention of researchers, who attempt to induce quadratic optical nonlinearity (QON) in glass [1 -5].One of the methods for inducing QON is homogeneous or heterogeneous nucleation of non-centro-symmetric crystals in a glass volume. It is possible to achieve the second optical harmonic generation (SHG) in such nano-or microstructured glasses while retaining the transparency of the medium. Due to a relatively low concentration and a random orientation of nanocrystals, the SHG signal is significantly weaker than in monocrystals of the same composition. Therefore, to produce nanostructured glasses with a high QON it is advisable, first, to select glass compositions and heat-treatment schedules ensuring the maximum formation of a NLO crystalline phase in glass and, second, to endeavor to achieve the closest possible coordination between the refractive indexes of the matrix glass and crystals, which is especially important when crystals grow to submicron sizes, in which case media with QON comparable to that of monocrystals can be obtained. The possibility of significantly intensifying NLO effects by extruding nanostructural fiberglass and the prospects of the controlled development of complex surface and volumetric NLO structures separated by a clear matrix with a coordinated refractive index has generated even greater interest in glass as an active medium.Both above specified conditions are frequently satisfied in homogeneous nucleation, where the initial glass has a composition similar to that of the crystalline phase emerging in this glass. Therefore, the K 2 O -TiO 2 -P 2 O 5 system (KTP) can be regarded as a promising glass-forming system for nano...