The anisotropy in the optical constants of quartz crystals for soft X-rays

TL;DR

This study measures and analyzes the anisotropic optical constants of quartz crystals in the soft X-ray range, revealing significant deviations at absorption edges and providing new experimental data validated by theoretical calculations.

Contribution

It presents the first polarization-dependent soft X-ray reflectometry measurements of quartz, revealing anisotropy in optical constants and expanding existing data with new near-edge region measurements.

Findings

Significant anisotropy observed at Si-L2,3 and O-K edges.

Reconstructed optical constants differ from isotropic models.

Experimental data validated by Bethe-Salpeter calculations.

Abstract

The refractive index of a y-cut SiO$_2$ crystal surface is reconstructed from polarization dependent soft X-ray reflectometry measurements in the energy range from 45 eV to 620 eV. Due to the anisotropy of the crystal structure in the (100) and (001) directions, we observe a significant deviation of the measured reflectance at the Si-L$_{2,3}$ and O-K absorption edges. The anisotropy in the optical constants reconstructed from these data is also confirmed by ab initio Bethe-Salpeter Equation calculations of the O-K edge. This new experimental dataset expands the existing literature data for quartz optical constants significantly, particularly in the near-edge regions.