New method for visualizing atom-thin boron nitride layers

The Sum Frequency Microscope Unveils Secrets

At the Fritz Haber Institute, a microscope was recently developed that uses a trick from nonlinear optics to tackle the light-shy h-BN. In the so-called phase-resolved sum frequency microscopy, which employs this innovation, two laser beams are combined—one in the mid-infrared range and one in the visible range. This then produces a so-called sum frequency signal in the sample.

Schematic representation of the newly developed SFG microscope, recently developed at the Fritz Haber Institute (FHI) and used to image layers of hexagonal boron nitride (h-BN). Its crystal structure can also be traced in this way.(Image: FHI) — Schematic representation of the newly developed SFG microscope, recently developed at the Fritz Haber Institute (FHI) and used to image layers of hexagonal boron nitride (h-BN). Its crystal structure can also be traced in this way.
(Image: FHI)

Through resonant excitation, the h-BN crystal lattice begins to vibrate, and the measured sum frequency signal becomes very intense, allowing not only large sample areas of 100 × 100 square micrometers (1.6 × 10⁻⁵ square inches) to be imaged in less than a second, but also making the crystal orientation visible. It was observed in h-BN that the 2D layers grow as triangular domains and effectively have zigzag edges made of nitrogen. This, the researchers believe, makes the material ideal for novel optoelectronic components.

More Contrasting Insights Than With AF Microscopes

The newly developed microscope offers clear advantages compared to conventional examination methods. First and foremost, it can now make optically transparent materials visible through optical microscopy. The microscope images also have much higher contrast than conventional AFM images (Atomic-scanning Force Microscope = atomic force microscopy). The signal amplification through vibrational resonance enables "live imaging" of h-BN—including real-time information on crystal orientation. The new microscope thus enables the controlled fabrication of van der Waals structures, in which individual 2D layers are stacked. The authors anticipate that the new microscope has a promising future for the non-invasive and label-free investigation of a wide range of stacked 2D materials as well as their combinations with anisotropic molecular arrangements.

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Date: 08.12.2025

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