Optical three-dimensional profiling of charged domain walls in ferroelectrics by Cherenkov second-harmonic generation

Cherenkov second-harmonic generation (CSHG) is a powerful tool for three-dimensional domain wall profiling in ferroic bulk crystals. Here, we apply this noninvasive technique for tracking head-to-head charged domain walls (CDWs) across millimeter-thick ferroelectric single-crystalline lithium niobate. CSHG sensitively reveals the inclination $\alpha >0$ of any such CDW with a superb optical resolution. Moreover, we deduce fully charged head-to-head CDWs ($\alpha$ = 90${}^{\circ }$) to be much rougher and to show protrusions, domain inclusions, and novel topologies. Our findings provide insight into the mechanisms of electron transport and charge trapping in CDWs, as is mandatory for their use in prospective nanoelectronic devices.

Figure 1

(a) Experimental setup used for laser scanning Cherenkov second-harmonic imaging (CSHG). (b) Setup and sample configuration for surface analysis through PFM and c-AFM measurements.

Figure 2

Type I sc-LNO sample showing (a) an $xy$ image of the random domain configuration as generated through poling, (b) the 3D domain wall (DW) constitution as rendered from a stack of 150 sectional $xy$ images, (c) the single domain slope for a 5% Mg doped sc-LNO, and (d) three sectional images recorded at different depths of ferroelectric DWs in an sc-LNO that has been repoled repeatedly; as a result, a fully enclosed domain cavity is generated [see center area in the middle (55 $\mu$m) picture].

Figure 3

Fully charged domain wall in type II sc-LNO samples. (a) Schematic of this sample type, with arrows indicating the intrinsic ferroelectric polarization; (b–d) sectional $xy$ scans as obtained by CSHG, c-AFM, and PFM, respectively, displaying the DW roughness of approximately 3 $\mu$m rms; (e) 3D CSHG image of the head-to-head CDW generated from a stack of 150 $xy$ images of $100\times 100$ $\mu {\mathrm{m}}^2$ each; (f) 3D rendered CSHG similar to (e) displaying a torus-like topological inclusion (tunnel).