Light in the lowest symmetry crystals – emergent light-matter interactions.

Light offers a powerful tool in both understanding the properties of materials as well as creating new types of sensing, communication, and computation technologies. When light coherently couples to charge in a material this leads to the formation of polaritons - quasiparticles which inherit some of the properties of the charge and some of the properties of light. The properties of the material, and in particular the structure, have enormous power to control the properties of the polaritons formed. In this talk, we will explore how materials with low crystalline symmetry can give rise to new phenomena, and how these emergent interactions can be probed. Notably, we will introduce shear polaritons, which emerge in the monoclinic and triclinic lattices which have non-diagonal dielectric tensors and introduce a twist into light. We will discuss how far- and near-field based techniques can be used to probe these materials and their dispersion, by explicitly mapping out the 3D angular dispersion of the modes. As examples of this characterization will talk about two materials; anisotropic β-Ga₂O₃ and ReSe₂, which support low symmetry phononic and excitonic modes respectively. For β-Ga₂O₃ these phonon polaritons show symmetry breaking which are unique to shear polaritons and show their distinction from conventional polaritonic modes. For ReSe₂ we show that at low temperatures they exhibit a non-diagonal dielectric tensor in the near infrared, which could form the basis of new polarized near infrared light sources. These results have important implications for photonic technologies, as well as potentially in understanding new twisted and topological material systems.