Micro-Raman spectroscopy was used to study silicone-based hydrogel polymers after being modified by , laser pulses from a Ti:sapphire oscillator at repetition rate. When the irradiation conditions were below the optical breakdown threshold of the polymers, no significant changes in the Raman spectra and background fluorescence were observed even when refractive index changes as large as were observed. On the other hand, changes in the Raman spectra and fluorescence were easily detected when higher pulse energy was employed to induce visible optical damage in the hydrogel polymers. These results show that a significant refractive index modification, below the optical breakdown threshold in silicone-based hydrogel polymers, can be realized in the absence of any significant change in the Raman spectrum of polymer composition. A thermal model is presented to explain these results. It shows that high-repetition-rate laser pulses cause significant heat accumulation, which can induce additional cross-linking and densification in the polymer network, resulting in locally increased refractive index.
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