Our work has now been published in Advanced Materials. We demonstrate how sum-frequency generation (SFG) microscopy enables rapid imaging of hexagonal boron nitride (hBN) monolayers, which are typically optically invisible. By nonlinearly mixing mid-infrared and visible laser pulses, we achieve full crystallographic imaging through phase-resolved SFG microscopy. Resonant excitation of a phonon provides an 800-fold enhancement, making the technique highly sensitive.

The project emerged from a collaboration between the groups of Alex Paarmann, Martin Thämer, and Alex Fellows at FHI Berlin, and was carried out using the SFG microscope in Martin Thämer’s lab. It was great to be part of this team effort.