Complex refractive index of crystalline quartz particles from UV to thermal infrared

Abstract

Crystalline quartz is an important element of many materials and one of the major compounds of mineral dust. Thus, knowledge of its optical properties is essential for many applications, in particular for remote sensing techniques. However, despite the fact that crystalline quartz has been an object of study for many years using various experimental approaches, its optical properties and in particular its complex refractive indices (CRI) remain uncertain. The focus of this investigation is the determination of a new set of CRI of crystalline quartz particles retrieved from a methodology which has already demonstrated its efficiency on pure amorphous materials and volcanic ashes. The experimental set-up allows recording simultaneously size distribution (from 15 nm to 20 µm) of airborne particles and extinction spectra over a wide continuous spectral range from thermal infrared (650 cm−1) to UV (40,000 cm−1) with high spectral resolution (up to 0.5 cm−1). By associating these measurements and a numerical procedure coupling scattering theories, the single subtractive Kramers-Kronig relation, and an optimal estimation method in an iterative process, we precisely determine the real and imaginary parts of the CRI of crystalline SiO2 with mean values of the uncertainties of 2.5% and 1.7%, respectively. Moreover, the use of these CRIs allows to simulate an extinction spectrum of another quartz sample very well, which shows their suitability as well as the good reproducibility of the whole process.