Time-resolved study of laser-induced colour centres in SiO2

We report the investigation of the onset of an absorption band in the UV in crystalline quartz (alpha-SiO2) induced by an intense femtosecond laser pulse. Using a conventional pump-probe technique, we have measured the absorptions at 219 nm (5.66 eV) and at 240 nm (5.16 eV) as functions of time, at two different temperatures (10 and 300 K). The rise time of the absorption is measured to be 150 (+/-50) fs. It is independent of probe wavelength and sample temperature. The absorption coefficients are similar at both probe wavelengths and the values at room temperature are about twice these at 10 K. We attribute the onset of the absorption to the ultra-fast formation of self-trapped excitons (STEs). The STEs recombine radiatively at a low temperature. At room temperature, we observe a cumulative effect. This demonstrates that, at 300 K, some of the STEs are converted to permanent colour centres, which we tentatively identify as neutral oxygen vacancies.