Heterogenous nucleation at twin boundaries in a VO₂ thin film revealed by nano-optical spectroscopy

Symmetry lowering phase transitions allow each unit cell of a derived phase to assume one of several variants of the crystallographic orientation. As a result self-organized twin-domains can form on mesoscopic length-scales. Polarized microscopy is an established approach to characterize the local crystallographic orientation of such domains. However, the inherent limitations of diffraction forbid the observation of nanotextured domain patterns with conventional microscopy. Here, I discuss an accurate approach to model the frequency dependent nano-optical contrast in low symmetry crystalline structures. Comparison between the model calculations and our experimental data show good agreement for two expected twin-domain orientations in a VO₂ thin film. In our model, the nano-optical contrast between domains stems from the anisotropic character of phonon-polaritons. Thus our approach may enable fingerprint assignment of the local crystallographic orientation in nanotextured domains. Co-located nano-optical images at distinct infrared frequencies directly reveal real-space correlations between the twin-domain nanotexturing and locations where metallic domains emerge at the onset of the transition region.