Influence of the Devitrification Mechanism on Second Harmonic Generation Efficiency and Transparency in Ba₂NaNb₅O₁₅Nanostructures

Abstract

The crystallization behavior of two new stable glasses belonging to the BaO−K2O−Na2O−Nb2O5−SiO2 system, synthesized by the melt-quenching technique, was studied with the aim to obtain nanostructured glasses based on tungsten−bronze phases. Nanostructured samples with valuable second harmonic generation (SHG) activity were obtained starting from 8BaO·15Na2O·27Nb2O5·50SiO2 glass after suitable heat treatments performed in the glass transition range, by means of the bulk nucleation mechanism. The influence of the devitrification mechanism on the SHG efficiency was clarified with particular reference to the effect of structural modifications on the transparency of the nanostructured glasses. These samples are formed by Ba2NaNb5O15 nanocrystals (about 15 nm in size) randomly dispersed in a phase-separated amorphous matrix. Their SHG efficiency is partially impaired by the Rayleigh scattering originated by the nanocrystals. These nanostructured glasses can be useful when the generation efficiency is not an issue, as in the diagnostics for ultrafast lasers, due to the independence of the second harmonic generation from the polarization state of the laser light and no need for preferential material orientation

Tunable Raman Gain in Transparent Nanostructured Glass-Ceramic Based on Ba2NaNb5O15 †

“…In this case, the development of the new liquid phase occurs by nucleation and the subsequent growth of nuclei into droplets, whose size may be controlled by a proper heat treatment [ 43 ]. Depending on the composition of the initial glass and the heat treatment temperature, this process produces either two amorphous phases (amorphous nanostructuring) or the growth of a crystalline phase uniformly dispersed in the amorphous phase (crystalline nanostructuring), thus producing TGCs [ 44 , 45 , 46 ].…”

“…The optical efficiency of TGCs is related to scattering phenomena originated by nanocrystals, which are randomly distributed in the glassy matrix, acting as scattering centers [ 42 , 46 ]. Scattering is influenced by the difference in refractive indexes between the amorphous and crystalline phases, by the size and distribution of crystals in the glassy matrix [ 42 ], and by the crystallization mechanism in the active phase [ 42 , 44 , 45 , 46 ]. The high anisotropy of a non-centrosymmetric crystal usually induces surface nucleation, making bulk nucleation a precondition to obtain a transparent nanostructure with nonlinear optical (NLO) properties [ 46 ].…”

“…Scattering is influenced by the difference in refractive indexes between the amorphous and crystalline phases, by the size and distribution of crystals in the glassy matrix [ 42 ], and by the crystallization mechanism in the active phase [ 42 , 44 , 45 , 46 ]. The high anisotropy of a non-centrosymmetric crystal usually induces surface nucleation, making bulk nucleation a precondition to obtain a transparent nanostructure with nonlinear optical (NLO) properties [ 46 ]. Therefore, TGC were obtained using either bulk nucleating phases, such as NaNbO 3 [ 42 ], LiNbO 3 [ 47 ], Ba 2 NaNb 5 O 15 [ 46 ], or using suitable nucleating agents to promote bulk nucleation [ 42 ].…”

Formation of Ba2NaNb5O15 Crystal and Crystallization Kinetics in BaO–Na2O–Nb2O5–SiO2–B2O3 Glass

“…10 BNN has been used in second harmonic generation and electro-optic devices because of its high nonlinear coefficient and resistance to optical damage. 11,12 In spite of useful ferroelectricity and optical properties, a limitation exists in applying BNN crystals, caused by twinning defects and a lack of high quality crystallinity during the synthesizing process. It is naturally required to broaden the sample preparation method and the types of samples for the further development of BNN applications.…”

“…The ferroelectric orthorhombic Pba 2 structure BNN with the lattice parameters a = 12.425, b = 12.484, c = 3.997 Å at room temperature transforms to the ferroelectric tetragonal 4mm at 260 °C, and with further heating, to the paraelectric tetragonal 4/ mmm at 560 °C. , At room temperature, BNN has the useful values of dielectric constant ε = 250 and spontaneous polarization P = 0.40 C/m 2 , which can be compared with those of LiNbO 3 (ε = 80, P = 0.71 C/m 2 ), , LiTaO 3 (50, 0.5 C/m 2 ), BaTiO 3 (3000, 0.25 C/m 2 ) . BNN has been used in second harmonic generation and electro-optic devices because of its high nonlinear coefficient and resistance to optical damage. , …”