Simultaneous characterization of the electro-optic, inverse-piezoelectric, and electro-absorptive effects in (Sr, Ba)Nb2O6 (SBN) thin films
Résumé
The electro-optic (EO) light modulation is a key function in light-wave technologies. It is mostly realized by exploiting the linear EO Pockels effect in ferroelectric bulk crystals like lithium niobate (LN) for primary example. Optimizing the performance of EO modulation has led to develop EO thin film waveguides. By using the Crystal Ion Slicing (CIS) and wafer bonding technique [1] or epitaxial thin film deposition [2], some research groups have thus succeeded in increasing the EO performance of LN modulators until a value close to the theoretical limit for LN. Any further significant improvement in the EO modulation performances can then only arise from changing LN for a material with better electro-optic properties. Beside LN, the ferroelectric materials which have been considered in the literature in view of preparing electro-optic thin films are mainly BaTiO3 (BT), (Ba,Sr)TiO3 (BST), (Pb,La)(Zr,Ti)O3 (PLZT), and (Sr,Ba)Nb2O6 (SBN). Electro-optic coefficients higher than that of LN crystal by more than two orders of magnitude have been reported for some of these films [3-6]. In addition to increasing drastically the EO modulation performances, the implementation of the Pockels EO effect in thin film waveguides opens up the path to the realization of electrically-tunable photonic crystal (PC) devices. The position and shape of a photonic band gap are dependent on the refractive index of the propagating medium and therefore can be electro-optically controlled. This EO control considerably extends the scope of PC structures potential functionalities, and should play a main role in the future deployment of the photonic technology. The great potential of EO thin films is linked to our ability to prepare highly EO and well-controlled thin films in the configurations required. A reliable method of characterizing the electric-field-induced response of a thin film is an essential tool for progressing in this field. The electric-field-induced response is a combination of inverse piezoelectric, electro-optic and electro-absorptive effects. In this presentation we propose a method which allows determining simultaneously these three contributions to the global effect experimentally observed. A specific feature of this method is its high level of self-consistency. The difficulties and reliability of electro-optic characterization are discussed. The method is used to characterize the electric-field-induced response of an epitaxial (Sr,Ba)Nb2O6 (SBN) thin film, which we prepared.