(Invited) Raman Spectroscopy of Confined Hyperbolic and Surface Phonon-Polaritons in 2D Materials
Authors: Francoeur Sebastien 
Published: July 2021

Phonon-polaritons are quasiparticles resulting from the strong coupling between polar phonons and the electromagnetic field. This hybrid matter-light quasiparticle exhibits remarkable characteristics enabling new or enhanced nanophotonics functionalities. Polaritons in 2D materials are particularly promising: the natural anisotropy leads to a hyperbolic frequency dispersion and enables for example sub-wavelength imaging, low-loss propagation, spontaneous emission enhancement, and extreme field confinement. In addition, 2D heterostructures allow unprecedented opportunities for polariton dispersion engineering.
Due to the large momentum mismatch between polaritons and free space propagating fields, polariton measurements require either near-field coupling to its evanescent wave or a direct coupling to the underlying polar oscillation. So far, polaritons in 2D materials have been largely studied using the near-field coupling mediated by an AFM tip. This technique has been extensively applied to h-BN, where imaging of polaritons allows determining the dispersion relation, propagative behaviour, and confinement of polaritons. In contrast, Raman scattering probes polar phonons directly. However, polaritonic effects are manifest at very low wavenumber and can be observed only in a forward scattering configuration, which has made until now this important and ubiquitous characterization techniques ineffective for the study of polaritons in 2D materials.
In this work, we demonstrate for the first time that Raman spectroscopy in a backscattering configuration is in fact an extraordinarily powerful and versatile tool for polariton studies in 2D materials. Usually forbidden, they can be readily observed due selection rule relaxation occurring in thin samples and the deep sub-wavelength confinement typical of hyperbolic materials. For the demonstration, we used gallium selenide owing to its strong polar resonances, its two hyperbolic regions and its nested reststrahlen structure.
Polariton Raman scattering spectra are both calculated and experimentally measured, allowing an unambiguous identification of the polaritons involved. Measurements as function of incident angle directly reveal the dispersion of the upper extraordinary and surface polaritons. The dependence of polariton frequencies on sample thickness is reported and agrees very well with the calculated values, as long as the silicon oxide and silicon substrate on which the sample is deposited are included in the calculation. This demonstrates that polaritons are quite sensitive to their dielectric environment.
Excitation near the 1s-exciton leads to a strong exaltation of the upper extraordinary polariton and the otherwise unobserved lower ordinary polariton. This resonant enhancement allows probing subtle effects related to propagation and confinement along lateral directions. The scattering efficiency, initially isotropic for large samples, becomes polarized along the weaker confinement direction. In addition, the polariton frequency becomes polarization dependent and is highest along the strongly confined direction. From these results, we find low propagation losses and a propagation figure of merit of γ-1 ≥ 39 is found. This value is among the highest reported and establishes GaSe as a promising 2D materials.
More significantly, these results demonstrate that Raman spectroscopy, a simple, rapid, non-invasive and widely available technique, can effectively probe confined polariton states. Compatible with high spatial imaging and energy resolution, Raman spectroscopy will undoubtedly accelerate the development of a wider variety of polaritonic materials and complex 2D heterostructures for mid-infrared nanophotonics applications.