The Singular Optical Axes in Biaxial Crystals and Analysis of Their Spectral Dispersion Effects in $\beta$-Ga$_2$O$_3$

TL;DR

This paper classifies biaxial optical materials, provides analytical solutions for singular optical axes orientations, and analyzes their spectral dispersion effects in $eta$-Ga$_2$O$_3$, revealing complex spectral behaviors and symmetry properties.

Contribution

It offers explicit analytical solutions for singular optical axes in monoclinic and orthorhombic crystals and analyzes their spectral dispersion in $eta$-Ga$_2$O$_3$, including spectral symmetry points.

Findings

Identification of quasi-uniaxial symmetry around 7.23-7.33 eV

Discovery of triaxial spectral points at 8.14 eV and 8.37 eV

Spectral analysis of Stokes vectors near singular axes

Abstract

We classify and distinguish optically biaxial materials, which can have triclinic, monoclinic or orthorhombic crystal symmetry, by the degeneracy of the indices of refraction of their four singular optical axes (Windungsachsen) in the absorption regime. We provide explicit analytical solutions for angular orientations of the singular optical axes in monoclinic crystals and orthorhombic crystals. As a model material we analyze monoclinic gallia ($\beta$-Ga$_2$O$_3$) and discuss in detail the dispersion (i.e. the spectral variation of the angular position) of its singular optical axes. For a certain energy range ($E \approx 7.23$-$7.33$ eV) we find quasi-uniaxial symmetry. At two energies ($E \approx 8.14$ eV and $E \approx 8.37$ eV) we find triaxial spectral points for which one regular optical axis and two singular optical axes exist. Concurrently a Stokes analysis of the spectral dependence of the electrical field eigenvectors is made and discussed for various crystal orientations. For a singular optical axis $|S_3|=1$; for the two degenerate singular axes at the triaxial point the Stokes vector is undefined. For a certain energy ($E=6.59$ eV), the $\langle 010 \rangle$-orientation is close to a singular optical axis, $|S_3|=0.977$. The analysis provided here is prototypical for the treatment of the optical properties of optically biaxial functional materials in the absorption and gain regimes.