Lithium niobate thin films shown in super resolution

Efficient nonlinear optical interactions rely on quasi-phase matching, which can be achieved in ferroelectric materials by periodically reversing the orientation of the ferroelectric domains. Periodically poled thin films of lithium niobate, a ferroelectric material, have garnered recent interest for use in photonic devices due to its chemical and physical stability, as well as its wide availability.

Second harmonic microscopy is a non-invasive technique for visualizing ferroelectric materials’ structures. While conventional microscopy detects variations in light properties, second harmonic microscopy detects variations in a sample’s ability to generate second harmonic light. Previously, second harmonic microscopy has been used to study lithium niobite in bulk but not thin film forms. Rüsing et al. report a comprehensive analysis of this technique in x-cut lithium niobate thin films.

Through both simulation and experiment of second harmonic microscopy, the authors describe the complex interplay of reflections, resonances, and phase matching in lithium niobate thin films. They found that this technique can determine the structure of poled domains in thin films with sub-diffraction-limited depth resolution on the order of tens of nanometers, and diffraction-limited lateral resolution comparable to other microscopy techniques.

Author Michael Rüsing said that photonic device fabrication can be greatly helped by using a non-destructive imaging method, like second harmonic microscopy, to complement other established techniques, such as diagnostic imaging after selective etching, or atomic force microscopy-based techniques.

A better understanding of second harmonic microscopy will allow it to become a useful tool for studying ferroelectric thin films and help researchers fabricate functional optical devices in thin films with more homogeneous domain structures over longer lengths and with shorter periods, making them more desirable for applications in wavelength conversion and quantum optics.

Source: “Second harmonic microscopy of poled x-cut thin film lithium niobate: Understanding the contrast mechanism,” by M. Rüsing, J. Zhao, and S. Mookherjea, Journal of Applied Physics (2019). The article can be accessed at https://doi.org/10.1063/1.5113727 .