Analysis of defects patterned by femtosecond pulses inside KBr and SiO 2 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 × 10 19 and 3.4 × 10 19 cm −3 using Smakula’s formula. In silica, a high density of paramagnetic E ′ centres 1.9 × 10 20 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 π 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.