Periodic Ferroelectric Domain Structures Characterization by Scanning Near Field Optical Microscopy

In this work, Scanning Near Field Microscopy (SNOM) observation and characterization of quasi-periodic ferroelectric domains in rare earth doped LiNbO 3 , Nd doped Ba 2 NaNb 5 O 15 , and LiTaO 3 crystals are presented. The formation of ferroelectric domains has been induced either during the crystal growth procedure (LiNbO 3 and Ba 2 NaNb 5 O 15 ) or by applying pulsed electric field (LiTaO3). SNOM reflection, transmission and collection modes have been used indistinctly to characterize the ferroelectric domain structure. In the three crystals under study, an unexpected optical contrast associated to the presence of ferroelectric domains has been observed. In particular, working in the SNOM collection mode the optical contrasts reached a value closed to ∼ 80% in Ba 2 NaNb 5 O 15 and LiTaO 3 crystals. These high optical contrast values are explained considering that the ferroelectric domains pattern operates as a periodic array of planar waveguides (optical super-lattices). The invoked light-pipe mechanism has been assessed by beam propagation method (BPM) simulations of the light propagation through the structure, from which the refractive index modulations have been obtained. These values compare favorably with data obtained by light diffraction experiments under normal and Bragg incidence.