Speaker
Description
Strong light-matter coupling in polar materials gives rise to phonon polaritons – hybrid quasiparticles with mixed photonic and vibrational character. Here, we show that polar nanocrystals exhibit Raman scattering that depends sensitively on their structure and electromagnetic environment. This spectral dispersion enables refractive index sensing in the mid-infrared via visible-wavelength Raman spectroscopy and draws parallels with molecular systems under vibrational strong coupling. Importantly, Raman scattering emerges only under nanoscale confinement of phonon polaritons, within a particle size range of ∼ 40 – 400 nm, bounded by volume limitations at smaller sizes and polariton coherence constraints at larger scales. In optimal structures, self-hybridization arises between localized phonon modes and surface phonon polaritons hosted by the same nanoparticle. Finally, we show that depositing only a few nanometers of alumina (∼ 10−4 of the free-space wavelength) enables tuning and complete suppression of surface phonon polaritons in this system, as probed by Raman scattering. Our findings establish polar nanocrystal Raman spectroscopy as a versatile probe of mid-infrared polaritonics and open routes to optical sensing inaccessible to surface- and tip-enhanced Raman scattering as well as noble-metal and graphene plasmonic platforms [1].
ACKNOWLEDGEMENT
This work was supported by Swedish Research Council (VR project, grant No. 2022-03347), Chalmers Area of Advance Nano, 2D-TECH VINNOVA competence center (Ref. 2024-03852), Olle Engkvist foundation (grant No. 211-0063), and the Knut and Alice Wallenberg Foundation (KAW, grant No. 2019.0140).
REFERENCES
[1] Zograf et al. arXiv, arXiv:2603.25895