Optically anisotropic media: New approaches to the dielectric function, singular axes, microcavity modes and Raman scattering intensities

Abstract

magnified imageSix out of seven crystal systems are optically anisotropic and birefringent. We review recent insight that biaxial crystals generally exhibit four singular axes (or exceptional points) which can pairwise degenerate for special cases. Planar anisotropic microcavities are discussed as effectively biaxial systems and we predict exceptional points and demonstrate experimentally partially coalesced eigenstates. Also the general form of the dielectric function of anisotropic crystals based on individual dipole oscillators for phonon and electronic resonance is discussed. The impact of birefringence on Raman scattering intensities has been historically either ignored or modeled incorrectly. A recent theory for uniaxial and biaxial crystals explains experimental Raman scattering intensities for excitation off the principal directions without free parameters, allowing the unambiguous determination of the Raman tensor components. The above points are demonstrated and relevant in particular for the currently technologically important materials GaN, ZnO (uniaxial) and β‐Ga2O3 (biaxial). Formula for the Raman scattering intensity as a function of incoming and outgoing polarization and the Raman tensor viewed through birefringent crystal (calcite).magnified imageFormula for the Raman scattering intensity as a function of incoming and outgoing polarization and the Raman tensor viewed through birefringent crystal (calcite).