Fabrication, structure and optical properties of lanthanum boron germanium pentoxide: A model transparent ferroelectric glass-ceramic (TFGC)

Traditionally, glasses are used as optically passive media for transmitting light. New developments in glass materials focus on incorporating new functionalities, such as electro-optic effect, photo-refraction, optical amplification, piezo-optical effect etc. Examples of such material systems include the emerging class of transparent ferroelectric glass-ceramics (TFGC), which are glasses with ferroelectric crystalline phase. LaBGeO5 is a model TFGC system, which forms glass easily and devitrifies congruently into a ferroelectric phase throughout the sample volume. We have fabricated LaBGeO5 TFGC samples with controlled microstructure by a two-step heat treatment process. During devitrification the density of glass increases; on molecular scale, triangular BO2O- structural units that are found only in the glassy state transform to four-coordinated BO4- structural units. The ferroelectncity of the crystallites is the key issue that determines the optical activity of TFGC. It is affected by stresses at the glass-crystallite interfaces originating from the thermal expansion mismatch and the ferroelectnc transformation of crystallites below the glass-transition temperature of LaBGeO5. A dynamic state of stresses results from the interaction of interfacial stresses with the ferroelectric domain structure of crystallites. A simple technique of monitoring interfacial stresses and ferroelectric state of crystallites from optical transmission and second harmonic generation measurements is described. In-situ state of the composite structure is established with the help of common techniques like scanning electron microscopy, electron backscattered diffraction, and a novel technique, piezoresponse force microscopy. This information is employed to relate the overall nonlinear optical response to the microstructure of TFGC. Finally, the understanding developed from the investigation of LaBGeO 5 TFGC is exploited in a novel method for fabricating laser-assisted tailored single-crystal architecture, which opens the possibility of forming active elements of integrated optics inexpensively in a glass substrate.