Nonlinear near-field imaging of ferroelectric domain structure in YMnO3

Ferroelectric materials are characterized by the spontaneous formation of local electric polarization in the form of domains. Controlling their spatial arrangement is the foundation of the ferroelectric random access memory and of several electro-optical and nanomechanical device applications. A particular need to understand the ferroelectric domain behavior has emerged recently when it was discovered that in magnetoelectric multiferroics the ferroelectric state can couple rigidly to a magnetically ordered state – either intrinsically or via a ferroelectric substrate with tunable lattice constant. In this regard YMnO3 has attracted much interest as an intrinsic ferroelectric.

Among optical methods second harmonic generation (SHG) is particularly sensitive to the ferroelectric state since the loss of inversion symmetry in the long-range electric ordering is the origin of the SH response, and therefore it couples directly to the ferroelectric ordering parameter.  In combination with scanning probe techniques SHG provides a powerful tool for domain imaging with ultrahigh spatial resolution and sensitivity.

We showed that the surface topology of nanoscale ferroelectric domains can be resolved in phase-sensitive second-harmonic generation (SHG) imaging by scattering-type scanning near-field microscopy (s-SNOM).

In our experimental implementation of SHG s-SNOM we take advantage of the polarization selective scattering of plasmonic Au scanning probe tips and their local near-field enhancement in the region below the tip apex, leading to SHG emission with nanometer spatial resolution.  This allows for selecting a specific second order nonlinear optical susceptibility tensor component sensitive to the ferroelectric polarization.

Phase and symmetry selective second-harmonic generation (SHG) s-SNOM. a) Tip-scattered near-field SHG imaging with self-homodyne detection of ferroelectric order in YMnO3. The interference of tip-scattered near-field with the far-field response of the sample itself serving as reference, provides relative local phase information. b) Far-field SHG anisotropy measurement in transmission is used for sample orientation and selection of the ferroelectric c(2)zxx tensor component.

YMnO3 has a single spontaneous polarization direction along the hexagonal crystallographic axis, which leads to the formation of 180° (antiparallel) ferroelectric domains.  Thus opposite domains are optically equivalent, with a SH-polarization of identical amplitude but a 180° relative phase. Imaging contrast between these antiparallel domains thus requires phase sensitive SHG detection. The discrimination of the optical phase difference between the tip-scattered near-field SHG polarizations induced in neighboring antiparallel ferroelectric domains is then obtained by constructive and destructive interference with the additional tip-scattered far-field signal from the sample serving as a local self-homodyne phase reference.

The figure shows the tip-scattered SHG near-field image of YMnO3 for three regions on the sample.  The selective probing of the nonzero χ (2)zxx tensor component leads to strong optical contrast (left panel) corresponding to the intrinsic ferroelectric domain structure in YMnO3. Cigar shaped domains elongated along the hexagonal crystallographic axis are observed, with typical dimensions on the order of 200 x 1000 nm. The elongation corresponds to the uniaxial crystal structure and reflects its electrostatic anisotropy.

Self-homodyne interferometric imaging of ferroelectric domains in YMnO3. Phase sensitive SHG s-SNOM images of YMnO3(100) for pinsout (left panels) and pinpout (right panels) polarization configuration for three representative regions of the sample. Contrast is obtained for pinsout probing c(2)zxx tensor component specific to the ferroelectric order. In the first panel the z-axis is rotated about 10° with respect to the horizontal scan direction. No significant contrast is obtained in pinpout with c(2)xxx = 0 for YMnO3. The signal is due to the broken symmetry of the tip itself with the pump and probe polarized parallel with respect to the tip axis.

The capability of SHG-s-SNOM for imaging ferroic order with sub-wavelength resolution can be applied to electric as well as magnetic structures. This makes the technique ideal for studies of the coupling between the magnetic and electric state including control of the magnetoelectric phase on nanoscopic length scales.

This research is in collaboration with the group of Prof. Manfred Fiebig (University of Bonn).

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