TIME-RESOLVED STUDY OF LASER-INDUCED COLOR-CENTERS 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 femtosecon d laser pulse. Using a conventional pump-probe technique, we have meas ured the absorptions at 219 nm (5.66 eV) and at 240 nm (5.16 eV) as fu nctions of time, at two different temperatures (10 and 300 K). The ris e time of the absorption is measured to be 150 (+/-50) fs. It is indep endent of probe wavelength and sample temperature. The absorption coef ficients are similar at both probe wavelengths and the values at room temperature are about twice these at 10 K. We attribute the onset of t he absorption to the ultra-fast formation of self-trapped excitons (ST Es). The STEs recombine radiatively at a low temperature. At room temp erature, we observe a cumulative effect. This demonstrates that, at 30 0 K, some of the STEs are converted to permanent colour centres, which we tentatively identify as neutral oxygen vacancies.