Origin of the Anisotropic Beer-Lambert Law from Dichroism and Birefringence in $\beta$-Ga$_2$O$_3$

TL;DR

This paper explains the anisotropic Beer-Lambert law in $eta$-Ga$_2$O$_3$ by analyzing how dichroism and birefringence influence polarization and absorption at different depths, revealing the interplay of excitonic effects and optical anisotropy.

Contribution

It provides a detailed polarization-based analysis linking anisotropic absorption to dichroism and birefringence in $eta$-Ga$_2$O$_3$, elucidating the physical origin of the anisotropic Beer-Lambert law.

Findings

Absorption coefficients depend on polarization and wavelength.

Polarization shifts toward the least absorbing direction with depth.

Critical penetration depth correlates with maximum ellipticity.

Abstract

The anisotropic optical absorption edge of $\beta$-Ga$_2$O$_3$ follows a modified Beer-Lambert law having two effective absorption coefficients. The absorption coefficient of linearly polarized light reduces to the least absorbing direction beyond a critical penetration depth, which itself depends on polarization and wavelength. To understand this behavior, a Stokes vector analysis is performed to track the polarization state as a function of depth. The weakening of the absorption coefficient is associated with a gradual shift of linear polarization to the least absorbing crystallographic direction in the plane, which is along the a-exciton within the (010) plane or along the b-exciton in the (001) plane. We show that strong linear dichroism near the optical absorption edge causes this shift in $\beta$-Ga$_2$O$_3$, which arises from the anisotropy and spectral splitting of the physical absorbers i.e., excitons. The linear polarization shift is accompanied by a variation in the ellipticity due to the birefringence of $\beta$-Ga$_2$O$_3$. Analysis of the phase relationship between the incoming electric field to that at a certain depth reveals the phase speed as an effective refractive index, which varies along different crystallographic directions. The critical penetration depth is shown to be correlated with the depth at which the ellipticity is maximal. Thus, the anisotropic Beer-Lambert law arises from the interplay of both the dichroic and birefringent properties of $\beta$-Ga$_2$O$_3$.