In situ spectroscopic measurement of transmitted light related to defect formation in SiO₂ during femtosecond laser irradiation

We measured real-time spectra of light transmitted through SiO₂ specimens during irradiation of amplified ultrashort laser with a fluence of 540 mJ/cm². The real-time spectra exhibit a peak at around 400 nm, which significantly depends on the irradiation time. The observation and identification of defects were performed by measurements of electron spin resonance (ESR). Both dependences of the peak at around 400 nm on irradiation time and laser power are in good agreement with those of the ESR signal intensity of positively charged oxygen vacancies (E^′ center). This strong correlation shows that self-trapped excitons are created followed by the formation of the E^′ center and finally that of ESR inactive centers, namely, oxygen-deficiency centers. © 2003 American Institute of Physics.