Richard I. Blackwell

Large refractive index change in silicone-based and non-silicone-based hydrogel polymers induced by femtosecond laser micro-machining

A Ti:Sapphire femtosecond laser with a pulse energy of 1.3 nJ at a 93 MHz repetition rate has been used to micro-machine optical gratings inside several silicone-based and non-silicone-based hydrogel polymers. By measuring the diffraction efficiency of the gratings at 632.8 nm, we find as large as 0.06+/- 0.005 average refractive index change within the irradiated area.

Femtosecond laser micromachining of waveguides in silicone-based hydrogel polymers.

By tightly focusing 27 fs laser pulses from a Ti:sapphire oscillator with 1.3 nJ pulse energy at 93 MHz repetition rate, we are able to fabricate optical waveguides inside hydrogel polymers containing approximately 36% water by weight. A tapered lensed fiber is used to couple laser light at a wavelength of 632.8 nm into these waveguides within a water environment. Strong waveguiding is observed due to large refractive index changes. A large waveguide propagation loss is found, and we show that this is caused by surface roughness which can be reduced by optimizing the waveguides.

Micro-Raman spectroscopy of refractive index microstructures in silicone-based hydrogel polymers created by high-repetition-rate femtosecond laser micromachining

Micro-Raman spectroscopy was used to study silicone-based hydrogel polymers after being modified by 800 nm, 27 fs laser pulses from a Ti:sapphire oscillator at 93 MHz 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 +0.06±0.005 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.

Characterization of index changes in silicone- and nonsilicone-based hydrogel polymers induced by femtosecond micromachining

Diffraction gratings and optical waveguides are micro-machined inside hydrogel polymers containing up to 80% water using a 93 MHz Ti:Sapphire femtosecond laser with 27 fs pulses. Index changes as large as +0.06 are observed.

Raman spectroscopic study of silicone-based hydrogel polymers with large index changes induced by femtosecond laser micromachining

Raman spectroscopy has been used to study the femtosecond laser micromachined regions inside silicone-based hydrogel polymers. No significant changes in the Raman spectrum are observed even when index change is as large as +0.06.