Quantification of surface contamination on optical glass via sensitivity-improved calibration-free laser-induced breakdown spectroscopy

Highlights: • Detection of manufacturing-induced trace contaminants in optical glass. • Depth-resolved analysis of contaminants. • Trace element quantification via a sensitivity-improved calibration-free LIBS approach. • Observation of correlation between surface contamination and index of refraction. • Verification of high measuring accuracy by reference measurements. We report quantitative analysis of manufacturing-induced trace contaminants of optical glass surfaces by laser-induced breakdown spectroscopy (LIBS). Therefore, spectra recorded with an echelle spectrometer coupled to a gated detector were analysed using a calibration-free LIBS approach based on the calculation of the spectral radiance of a plasma in local thermodynamic equilibrium. The measurements were carried out in experimental conditions that enable both accurate modelling of plasma emission and high sensitivity for trace element analysis. Depth-resolved measurements were performed by recording spectra for successive laser pulses applied to the same irradiation sites. Validated via inductively coupled plasma atomic emission spectroscopy for the bulk glass composition, the measurements evidence a surface contamination that originates from polishing during glass manufacturing. The measured penetration depths of the contaminants are discussed in the frame of the underlying mechanisms of surface contamination and related to the changes of the optical properties evidenced by ellipsometric measurements. Demonstrated here for optical glass, the sensitivity-improved calibration-free LIBS approach can be used to quantify contaminations of surfaces in all kind of technological applications.