"ONLINE - Observation of phonon polaritons in multilayer hexagonal boron nitride films grown by chemical vapor deposition"

Who: Eugenio Calandrini, Nanooptics Group

Place: nanoGUNE webinar online

Date: Monday, 21 February 2022, 11:00

Observation of phonon polaritons in multilayer hexagonal boron nitride films grown by chemical vapor deposition 

Calandrini Eugenio 

Nanooptics, CIC nanoGUNE 

Hexagonal boron nitride (hBN) is a heavily studied van der Waals (vdW) material that is able to support highly confined phonon polaritons (PhP) in the mid-infrared spectral range. It inspired novel concepts in the field of nanoscale infrared imaging, field-enhanced infrared sensing and vibrational strong coupling. However, the typical approach for fabricating h-BN PhP devices relies on mechanical exfoliation of bulk crystals, viable only for proof-of-concept experiments due to its low yield. Although the interest in producing hBN and other vdW materials by other means is raising, PhPs on synthetized hBN were only reported on mono- and bilayers. PhP nanoresonators and other devices based on such thin layers, however, may be unpractical for real-world applications due to very small extinction cross sections. 

Here we present a combined scattering-type scanning near-field optical microscopy (s-SNOM) and far-field infrared spectroscopy study of wafer-scale hBN films of several nanometer thickness, which were grown by chemical vapor deposition (CVD) and transferred onto a Si/SiO2 substrate. 

We performed far-field reflection spectroscopy in order to extract the dielectric permittivity of the CVD grown hBN films and calculated the expected dispersion and lifetime of the PhPs. Interestingly, and in stark contrast to exfoliated hBN, s-SNOM imaging of the CVD grown material revealed random interference patterns all over the film that we can attribute to PhP scattering at randomly distributed defects in the hBN layer. To analyze these patterns and determine the polariton dispersion and lifetime, we developed a theoretical model and an experimental method. Finally, we designed and etched hBN nanoribbons that exhibit transverse PhP resonances with quality factors ~50, which are only a factor of about two smaller than that of PhP resoantors made of exfoliated hBN flakes . Our study shows the potential of CVD-grown hBN layers to be used for large-scale fabrication of PhP based resonators and devices, e.g. for infrared sensing applications. 


ONLINE nanoGUNE: Eugenio Calandrini, Nanooptics Group  - CIC nanoGUNE

When: Feb 21, 2022 11:00 AM 

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