Measurements of Second-Harmonic Generation and the Variations in the Free and Clamped Values of the Dielectric Constants and Electro-Optic Coefficients in Barium Sodium Niobate

Abstract

This work is primarily concerned with the measurements of some of the properties of crystals of barium sodium niobate (BSN) which are significantly related to the degree of alignment of the ferroelectric domains. We point out that many of the previous measurements of the low‐frequency half‐wave voltage (HWV) might have been in error because of imperfect alignment (poling). The variations observed in these HWV measurements can be related, in principle, to a connection between the low‐frequency resistivity in different regions of the crystal and the degree of poling in those regions. For instances in which we observed variations in the free HWV we could demonstrate that such effects were not related to stoichiometric (compositional) variations within crystals, or from one crystal to another. In fact stoichiometric variations appeared to play no role in determining the outcome of our measurements of the low‐ and high‐frequency electro‐optic (EO) coefficients and dielectric constants. We present values for the low‐frequency HWV and high‐frequency principal axis EO coefficients in a perfectly poled sample and give a detailed discussion of the second‐harmonic generation (SHG) tests which were used to establish the single‐domain quality. Our measurements of the free dielectric constant ${\epsilon }_3^T$ showed only a slight relationship to the degree of poling. The high‐frequency value was critically related to poling; this can be explained by the size and nature of the domains present in unpoled samples. The significant fact is that imperfect poling is connected with resistive and dielectric variations. With the application of an electric field, such variations cause unknown field distributions to exist in the poled and unpoled regions of a crystal. Thus, even when optical propagation occurs in a completely poled region, EO measurements can be quite inaccurate. From our SHG measurements we have deduced a parameter which is very useful in evaluating crystal quality for parametric oscillation (PO) and we have obtained corroboration of the value of this parameter from our parametric fluorescence (PF) measurements. In the Appendix we examine the SHG power, the HWV, and their relationship in a multidomain crystal.