Abstract
The refractive index of a y-cut SiO2 crystal surface is reconstructed from orientation-dependent soft X-ray reflectometry measurements in the energy range from 45 to 620 eV. Owing to the anisotropy of the crystal structure in the (100) and (001) directions, a significant deviation of the measured reflectance at the Si L 2,3 and O K absorption edges is observed. The anisotropy in the optical constants reconstructed from these data is also confirmed by ab initio Bethe-Salpeter equation calculations for the O K edge. This new experimental data set expands the existing literature data for quartz crystal optical constants significantly, particularly in the near-edge regions.
Highlights
Silicon dioxide (SiO2) is very well known for its polymorphism (Bruckner, 1970)
In this paper we evaluate orientation-dependent reflection measurements with soft X-rays on a quartz crystal
The line plots at a fixed angle of incidence i reveal that the measured anisotropy at the absorption edges is not a measurement artifact, since the measurement uncertainty is within the smallest line width
Summary
Silicon dioxide (SiO2) is very well known for its polymorphism (Bruckner, 1970). One of its crystalline forms is called quartz and is classified into different types (I–IV) depending on the manufacturing process and the resulting impurities (Bruckner, 1970; Kitamura et al, 2007). The wavelength within the medium can shrink to the order of the lattice constant These anisotropies have been observed in various materials (Yu & Cardona, 1971; Pastrnak & Vedam, 1971; Letz et al, 2003) in the past and have been confirmed for quartz at the O K edge by X-ray absorption near-edge spectroscopy (Taillefumier et al, 2002). The real part 1 À and the imaginary part of the complex refractive index are often referred to as optical constants or the n&k values of a certain material This approximation is sufficient for photon energies above 30 eV and far away from the absorption edges (Henke et al, 1993). The optical constants at the absorption edges reconstructed from the soft X-ray reflectivity measurements confirm the theoretically expected anisotropy of quartz in the complex refractive index. The observed anisotropy at the O K edge is confirmed by ab initio simulations using OCEAN (obtaining core excitations from ab initio electronic structure and NIST BSE) calculations (Gilmore et al, 2015; Vinson et al, 2011)
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