Sum-Frequency Spectro-Microscopy to Image Infrared Materials Excitations

Niclas Mueller Fritz-Haber Institute

Nonlinear optical microscopy and spectroscopy are powerful tools to characterize interfaces and lower-dimensional materials. Here, I show two examples how we use infrared + visible sum-frequency generation to image mid-infrared materials excitations with wide-field optical microscopy. The techniques provide combined spatial and spectral information. 1. We visualize the propagation patterns of infrared phonon polaritons in a metasurface of silicon carbide (Fig. 1a-c) [1]. Through a combination of microscopy and spectroscopy, we observe the hybridization and strong coupling of propagating and localized polaritons. 2. We visualize monolayers of hexagonal boron nitride on an insulating substrate (Fig. 1d). This material is usually optically invisible because of its large band gap. Resonant infrared excitation of phonons and heterodyne sum-frequency imaging enable us to image, both, its topography and crystal orientation.

[1] Niemann, Mueller et al. Advanced Materials 36, 2312507 (2024)