4.2. Applicability to a wide range of materials

The imaging principle of the Čerenkov SHG microscope lies in the sensitivity of the Čerenkov emission on the existence of the spatial variation of the second-order nonlinearity . Therefore, it can apply to any transparent materials with sharp variations. For example, in our experiment we have obtained high-quality images of ferroelectric domain patterns in LiNbO3, LiTaO3, KTiOPO4, and Sr0.28Ba0.72Nb2O6 crystals, as shown in Figure 11.

Figure 10. Domain structures imaged by Čerenkov SHG, taken with the focal plane of the fundamental beam located 10 mm inside the corresponding materials: (a) congruent LiNbO3 with 2D quasi-periodic domain structure and (b) as-grown Sr0.28Ba0.72Nb2O6 crystal.

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

photomultiplier are employed, respectively. Short-pass filters are used to prevent the transmit-

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

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 imaging principle of the Čerenkov SHG microscope lies in the sensitivity of the Čerenkov emission on the existence of the spatial variation of the second-order nonlinearity . Therefore, it can apply to any transparent materials with sharp variations. For example, in our experiment we have obtained high-quality images of ferroelectric domain patterns in LiNbO3,

Figure 10. Domain structures imaged by Čerenkov SHG, taken with the focal plane of the fundamental beam located 10 mm inside the corresponding materials: (a) congruent LiNbO3 with 2D quasi-periodic domain structure and (b) as-grown

ted fundamental beam from entering the detectors.

32 Ferroelectrics and Their Applications

4.1. High contrast and high spatial resolution

the very central part of the laser beam's focus.

4.2. Applicability to a wide range of materials

Sr0.28Ba0.72Nb2O6 crystal.

The advantages of this nonlinear Čerenkov imaging system include:

microscope is capable of imaging ferroelectric domains with high contrast.

LiTaO3, KTiOPO4, and Sr0.28Ba0.72Nb2O6 crystals, as shown in Figure 11.

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

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 these images.

### 4.3. Capability for 3D imaging

As we described above, the scanning of laser focus in the X-Y plane enables us to obtain twodimensional images of ferroelectric domains. Then if we stack a series X-Y plane images recorded at different depths inside the material, we can produce 3D images of domains. This is an advantage that cannot be met by the traditional domain imaging techniques. In Figure 12 we show a number of 3D images of ferroelectric domain patterns, which are formed in a congruent LiNbOb3 crystal [38]. From these images we can see how the initially circularshaped domains transform to hexagons with depth [Figure 12(b)], how defects were formed during the domain inversion process [Figure 12(c)], and how the neighboring domains merge to form a bigger one [Figure 12(d)]. Revealing these details is essential for a full understanding of domain inversion and growth processes. This is also very useful for improving the quality of ferroelectric domain patterns, which is critical for a wide range of future applications.

Kalinowski, Dr. Qian Kong, Dr. Wenjie Wang, Prof. Crina Cojocaru, and Prof. Joes Trull for

Nonlinear Optical Effects at Ferroelectric Domain Walls http://dx.doi.org/10.5772/intechopen.77238 35

\*

1 Laser Physics Centre, Research School of Physics and Engineering, Australian National

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their valued contributions to this work.

There is no conflict of interest for this work.

, Wieslaw Krolikowski1,2 and Yan Sheng<sup>1</sup>

2 Science Program, Texas A&M University at Qatar, Doha, Qatar

tronics Letters. 1991;27:2040-2042. DOI: 10.1049/el:19911263

Physics. 2012;84:119-156. DOI: 10.1103/RevModPhys.84.119

126805. DOI: 10.1103/PhysRevLett.107.126805

10.1126/science.246.4936.1400

10.1038/ncomms2990

\*Address all correspondence to: yan.sheng@anu.edu.au

475-481. DOI: 10.1103/PhysRev.17.475

Conflict of interest

Author details

University, Canberra, Australia

10.1109/68.275441

Xin Chen<sup>1</sup>

References
