Analysis of defects patterned by femtosecond pulses inside KBr and SiO2 glass

Colour centres in KBr and defects in silica glass were formed by focused femtosecond laser pulses. It is shown that under simple laser exposure, KBr develops a similar colouration as that achieved with electron and ion bombardment or high-energy X-ray irradiation. The three-dimensional (3D) character of direct laser writing in the volume of KBr allows a new level of control in the spatial arrangement of colour centres and defects. Five different colour centres were identified in KBr through the absorption spectrum; they have different charge and vacancy distribution configurations. The densities of the V- and F-centres were estimated to be 3.9 × 1019 and 3.4 × 1019 cm−3 using Smakula’s formula. In silica, a high density of paramagnetic E′ centres ~1.9 × 1020 cm−3 was determined by quantitative electron spin resonance spectroscopy. Birefringence due to colour centres and laser-induced defects was measured using Stokes polarimetry. In the case of colour centres in KBr, retardation in excess of 0.05 (Formula presented.) was determined throughout the visible spectrum from 400 to 800 nm. The use of polariscopy for analysis of high-pressure and high-temperature phase formation induced by 3D laser structuring is discussed