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

In situ Spectroscopic Measurement of Transmitted Light Related to Defect Formation in SiO2 during Femtosecond Laser Irradiation. N.Fukata, Y.Yamamoto, K.Murakami, M.Hase, M.Kitajima: Applied Physics Letters, 2003, 83[17], 3495-7. See also: Physica B, 2003, 340-342, 986-9