Article pubs.acs.org/JPCC
Influence of the Devitrification Mechanism on Second Harmonic Generation Efficiency and Transparency in Ba2NaNb5O15 Nanostructures Esther Fanelli,*,† Claudio Giannetti,‡ Antonio Aronne,† Stefania Pagliara,‡ Serena Esposito,§ and Gabriele Ferrini‡ †
Dipartimento di Ingegneria dei Materiali e della Produzione, Università di Napoli Federico II, Piazzale V. Tecchio, I-80125 Napoli, Italy ‡ Interdisciplinary Laboratories for Advanced Materials Physics (i-LAMP) and Dipartimento di Matematica e Fisica, Università Cattolica del Sacro Cuore, Via dei Musei, 41, I-25121 Brescia, Italy § Dipartimento di Ingegneria Meccanica, Strutture, Ambiente e Territorio, Università di Cassino, Via G. di Biasio, 43, I-0304, Cassino (Fr), Italy 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.
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INTRODUCTION Glasses do not show second-order optical nonlinearity, but structural modifications can break the centrosymmetry of the glass network and achieve a nonzero second-order nonlinear optical susceptibility, χ(2). Depending on how the structural modifications are induced, either the glass surface or the whole glass volume is modified. Examples of surface modifications are those induced by thermal poling, when an intense electric field is applied to the glass. In this case an alkali-depleted layer near the anode surface is formed, showing nonlinear optical activity. The width of the nonlinear layer, as well as the χ(2) values, change according to the glass composition from 3 to 7 μm and from 0.6 to 8.0 pm/ V, respectively.1−4 Structural changes involving the whole glass volume can be obtained subjecting the glass to a proper heat treatment at temperatures above the glass transition, by means of two main mechanisms: phase separation and nanocrystallization. These mechanisms can act also at the same time; in this way transparent nanostructured glasses exhibiting second harmonic generation (SHG) have been obtained in the potassium niobium−silicate (KNS) system.5 It was shown that the 23K2O·27Nb2O5·50SiO2 and 20K2O·25Nb2O5·55SiO2 initial © 2012 American Chemical Society
glasses exhibit SHG efficiencies of about 6 orders of magnitude smaller than the SHG from a phase-matched β-BaB2O4 (BBO) crystal and that this SHG activity is of bulk origin.5 It was also shown that SHG efficiencies exhibit a maximum in correspondence of the early stages of nanostructuring, as a consequence of nanosized inhomogeneities segregation within the amorphous matrix.5 In addition, glasses with composition 16.7K2O·16.7Nb2O5·66.7SiO2 and 25K2O·25Nb2O5·50SiO2 have shown SHG efficiencies about 1−3 times that of α-SiO2 (quartz) in their transparent nanostructured form, while much higher SHG efficiencies were found for the nontransparent ferroelectric glass ceramics obtained by their crystallization.6 It is worth to note that the origin of the SHG activity in the nanostructured glass state was related to a combination of third-order nonlinearity with spatial modulation of linear polarizability.7 A similar situation has been reported also in nanostructured glasses containing centrosymmetric crystals (K2Te4O9) in the glass matrix.8 In these materials, the origin of the SHG was ascribed to the abrupt change of the optical Received: July 5, 2012 Revised: October 18, 2012 Published: November 30, 2012 26874
dx.doi.org/10.1021/jp3066585 | J. Phys. Chem. C 2012, 116, 26874−26880
The Journal of Physical Chemistry C
Article
Na 2O·27Nb2O5·50SiO2 (BaNaNS), were prepared from reagent grade KNO3, NaNO3, BaCO3, Nb2O5, and SiO2. Well-mixed batches calculated to yield 60 g of glass were melted for 1.5 h at 1500 °C in Pt crucible. Melts were quickly poured on a brass plate preheated at 400 °C. All glasses were optically transparent, brown colored, and without any crystalline inclusions. The thermal behavior of the glasses was studied by differential thermal analysis (DTA). DTA curves were recorded on bulk or powdered (
