Near-field scanning optical microscopy of ferroelectric domain walls

Abstract

We have observed domain walls in ferroelectric LiTaO3 crystals using a polarization and phase sensitive near-field scanning optical microscope. The strain induced birefringence was used to observe the domain walls. The domain walls are measured to be 1 μm wide and show a variation of strain along the domain walls probably due to defects. These measurements allow an estimate of the birefringence at the domain wall of 6×10−5 and associated shear strain of 4×10−5.

Second-harmonic imaging of ferroelectric domain walls

“…These scatterers are likely to be related to strain-induced defects ͑dislocations͒ as suggested for 180°domains in LiTaO 3 crystals. 7,8 Stress and associated defects can also be introduced by polishing, and SH-related scattering inside the domains is clearly seen on the images ͑Fig. 4͒ obtained with sample N4 that has been polished.…”

“…These observations support our hypothesis about the domain walls, and even fit quantitatively if the thickness of the domain walls is taken to be ϳ1 m. Such a value has been reported for the domain walls in ferroelectric LiTaO 3 crystals measured with a linear polarization sensitive near-field microscope. 8 In summary, using second-harmonic imaging we have visualized 180°domain walls in two different ferroelectrics and investigated the origin of the imaging mechanism and the corresponding selection rules. It is shown that, with respect to SHG, the domain walls can be treated as a nearly isotropic nonlinear interface layer with depolarization properties.…”

“…6 The transition regions between neighboring domains, i.e., domain walls, is perhaps an even more interesting object for SHG microscopy: nonlinear optical properties of domain walls are largely unknown but progressively important ͑e.g., for QPM-based devices͒ with the decrease of distances between the walls ͑e.g., the spatial period of QPM structures͒. 180°domain walls in a LiTaO 3 single crystal have been imaged with linear optical methods, viz., by using unpolarized light 7 and polarization sensitive techniques, 7,8 suggesting that light is scattered by the walls and its polarization is modified due to strain-induced birefringence. 7,8 In this letter, we demonstrate that the domain walls in periodically poled ferroelectric KTiOPO 4 ͑KTP͒ and LiNbO 3 ͑LN͒ crystals can be imaged with SHG microscopy, and consider the origin of the selection rules in optical secondharmonic ͑SH͒ images.…”

“…180°domain walls in a LiTaO 3 single crystal have been imaged with linear optical methods, viz., by using unpolarized light 7 and polarization sensitive techniques, 7,8 suggesting that light is scattered by the walls and its polarization is modified due to strain-induced birefringence. 7,8 In this letter, we demonstrate that the domain walls in periodically poled ferroelectric KTiOPO 4 ͑KTP͒ and LiNbO 3 ͑LN͒ crystals can be imaged with SHG microscopy, and consider the origin of the selection rules in optical secondharmonic ͑SH͒ images. The experimental setup ͑the SHG microscope 3 ͒ used to image 180°domain walls represents a projection microscope in transmission with a powerful laser as a light source and a charge-coupled device ͑CCD͒ camera as a sensitive image detector ͓Fig.…”

Investigation of domain walls in periodically poled MgO:LiNbO3 by second harmonic imaging

“…These pinning defects are thought to be physical defects such as screw dislocations or localized variations of point defects [15] . During the poling process, the pinning points will change in curvature of the domain wall around them.…”