Niclas Mueller
Fritz-Haber-Institute of the Max-Planck-Society – Berlin, Germany
_______________________________________
Nonlinear optical spectroscopy and microscopy are powerful tools to characterize interfaces and lower-dimensional materials [1,2]. Here we introduce infrared + visible sumfrequency generation (SFG) imaging of hexagonal boron nitride (hBN) monolayers within a wide-field optical microscope. The technique enables us to image the topography and crystal orientation of chemical vapor deposition (CVD) grown hBN islands, which are usually optically invisible. A mid-infrared laser is used to resonantly excite the transverse optical phonon of hBN and a second visible laser then upconverts this excitation to generate a sumfrequency signal in the visible spectral range. This allows us to image the topography of hBN with a sub-diffractional spatial resolution that is set by the visible SFG wavelength. Heterodyning the generated SFG signals further enables us to obtain phase information [2,3] and thereby image the local crystal orientation of hBN. Finally, by azimuthally scanning the sample and using a rotational Fourier decomposition [4], we can separate different symmetry components of the SFG signal. We find that, while the signal is 3-fold symmetric in the interior of the hBN islands, their edges display 1-fold symmetry and have a different spectral response. Furthermore, regions with overlapping hBN islands that form twisted bilayers show a significantly enhanced SFG signal.
Figure 1. (a) SEM image of hBN monolayer islands grown with CVD on an Fe catalyst. (b) SFG spectrum showing the resonance of the hBN phonon. (c) SFG magnitude and (d) phase images of hBN monolayer islands showing the topography (c) and crystal orientation (d). All SFG images are recorded on an insulating glass substrate.
[1] Fellows et al. Nat. Commun. 15, 3161 (2024)
[2] Niemann, Mueller et al. Adv. Mater. 36, 2312507 (2024)
[3] Khan et al. Opt. Express. 31, 28792 (2023)
[4] Fellows et al. J. Phys. Chem. Lett. 15, 10849 (2024)
Email: nmueller@fhi-berlin.mpg.de