Investigation of Surface Phonon Polariton Modes in 4H-SiC Structures

Abstract

One of the fundamental challenges in nanophotonics is the diffraction limit, which prevents light from being confined and controlled in dimensions smaller than about half its wavelength. This limitation has long been a serious limitation to achieving control of light at the nanoscale. One promising solution to overcome this problem is surface polaritons. These are hybrid excitations that arise from the interaction of photons with collective electronic or ionic oscillations in materials and allow light to be confined and guided in dimensions much smaller than the diffraction limit. Phonon-polaritons (PhPs) are hybrid photon-phonon waves found in polar dielectric materials. Unlike surface plasmons, surface phonon polaritons (SPhPs) have very low losses in the mid-infrared to terahertz range, without compromising their strong electromagnetic field confinement. The realization of nanoscale light control requires a detailed study of surface phonon polaritons (SPhPs), how they are excited, propagated, and interact with other excitations in solids. Despite all the progress in the field of nanoscale light control, many questions remain that indicate the need for further investigation of SPhPs. In this thesis, I investigate SPhPs in 4H-SiC and introduce a new method for detecting and reconstructing SPhP eigenmodes using confocal Raman microscopy. We show that, contrary to previous expectations, Raman-active phonons couple to localized SPhPs, allowing for the three-dimensional reconstruction of SPhP eigenmodes using visible and near-infrared light. This approach provides a powerful tool for imaging nanophotonic modes. While localized SPhPs have been extensively studied, the experimental observation of traveling SPhPs in isotropic nanostructured dielectric material has remained elusive. Here, I present and characterize traveling SPhPs in one-dimensional gratings, showing that their group velocity, propagation length, and confinement can be tuned through geometry. This discovery offers a new pathway for developing SPhP-based interconnects in nanophotonic circuitry. Finally, this dissertation investigates the interaction between free carriers and optical phonons in doped 4H-SiC epitaxial layers. While the coupling of longitudinal optical phonons (LO) with plasmons in polar semiconductors is well known and widely studied in the scientific literature, the results obtained from the Raman studies of this research reveal a new and unreported phenomenon. Specifically, it was observed that by changing the doping concentration in 4H-SiC epitaxial layers, the frequency of the phonon mode E₁(TO) undergoes a shift. This result is against the common understanding that transverse optical phonons are unable to couple with free carriers. The discovery of this phenomenon not only challenges the conventional idea but also opens new opportunities for controlling the phonon–electron interaction in polar semiconductors. Such a capability could enable the development of new concepts in engineering the optical and electronic responses of materials at the nanoscale. These findings advance the fundamental physics of light-matter interactions and open new opportunities for designing next-generation nanophotonic devices in sensing, communications, and quantum technologies.