4.1. High contrast and high spatial resolution

A typical two-dimensional image of a ferroelectric domain structure obtained by the Čerenkov second harmonic microscope is shown in Figure 10(a). The quasi-periodic domain patterns, where the bright boundaries represent ferroelectric domain walls which facilitate stronger Čerenkov harmonic emissions, were clearly seen. Obviously the Čerenkov second harmonic microscope is capable of imaging ferroelectric domains with high contrast.

Figure 11. The Čerenkov SHG microscopy applies to a wide range of ferroelectric materials [18]. (a) Congruent LiNbO3 with 2D short-range-ordered domain structure [38]. (b) Stoichiometric LiTaO3 with 2D quasi-periodic domain structure [39]. (c) KTiOPO4 with 1D periodic domain structure [40]. (d) z-cut as-grown Sr0.28Ba0.72Nb2O6 crystal with naturally

Nonlinear Optical Effects at Ferroelectric Domain Walls http://dx.doi.org/10.5772/intechopen.77238 33

Figure 12. Three-dimensional visualization of inverted ferroelectric domains inside congruent LiNbO3 crystal by Čerenkov-type second harmonic generation laser scanning microscopy. (a) Domain distribution in the nonlinear photonic structure. (b) Transformation from the initially circular to hexagonally shaped domains. (c) Formation of a defect during the domain growth. (d) Merging of two initially separated ferroelectric domains. The ImageJ software was used to create

random domain structure at X-Y plane [41].

these images.

Figure 10(b) depicts the image of ferroelectric domain patterns obtained in an as-grown Sr0.28Ba0.72Nb2O6 crystal, which process naturally random domain structures in two dimensions. It is seen that the Čerenkov method offers an exceptional spatial resolution and even domain boundaries separated by less than 250 nm can be easily resolved. This is below the diffraction limit for the excitation laser wavelength of 820 nm, owing to the mechanism of nonlinear optical interaction, i.e., the Čerenkov second harmonic signal can only be excited in the very central part of the laser beam's focus.
