Raman Scattering from Hyperbolic Phonon-Polaritons in 2D Materials

Polaritons in two-dimensional (2D) materials have attracted attention in recent years given their potential for extreme optical field confinement. In particular, hyperbolic phononpolaritons (HPPs) have shown great potential for developing tunable infrared and THz nanophotonic devices with low propagation losses. An ever-growing list of methods are used to study and control these polaritons with an impressive level of precision, specificity, and sensitivity. These techniques rely on resonant coupling to the studied polaritons, therefore their applicability is limited to polaritons with energies above 500 cm−1 (60 meV) due to instrumentation availability. However, most 2D materials exhibit HPPs with energies below this limit, thus their study would require further development of current methods, or the introduction of a different approach. In this thesis, it is demonstrated for the first time that Raman spectroscopy is a convenient and powerful tool for the investigation of a wide range of strongly confined hyperbolic phonon-polariton states in 2D materials. Raman scattering is almost universally available in 2D materials research labs, effectively lifting the energy limitations imposed by current resonance-based techniques. For this demonstration, Gallium selenide (GaSe) was chosen as a prototypical material owing to its strong polar resonances, but the technique should translate to a large number of 2D materials. This thesis reports four important contributions, as listed below. The first contribution of this work is a thorough study of its photo-oxidation, conducted to determine ideal experimental conditions for preserving sample integrity. The reported results offer valuable insight into the photo-chemical processes involved in the rapid photo-oxidation of thin GaSe flakes, which is a common issue with 2D materials. The concomitant presence of oxygen, humidity and light is shown to trigger rapid oxidation, which is inhibited by removal of any of these three elements. Earliest signs of GaSe sample degradation are also reported as Ga2Se3 and Ga2O3 Raman modes at 155 cm−1 and 161 cm−1, respectively, and can be used to identify sample degradation or to select high quality samples. In this work, samples are prepared in an inert oxygen-free atmosphere and measured in vacuum optical cells to ensure optimal sample quality. The second and most significant original contribution of this thesis is the demonstration that sub-micron thin GaSe flakes exhibit strong polariton backscattering signal due to momentum conservation relaxation.