Dharmendra Jani

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.

Large enhancement of femtosecond laser micromachining speed in dye-doped hydrogel polymers

Ophthalmologic hydrogel polymers are doped with Fluorescein or Coumarin dyes prior to the femtosecond laser micromachining process. We find that the achievable micromachining writing speed can be greatly increased while maintaining large refractive index changes (up to +0.08). Compared with previous results in dye-doped polymers that do not contain water such as PMMA, we obtain much larger index changes and much faster writing speeds.

Optimization of femtosecond laser micromachining in hydrogel polymers

High-repetition-rate low-pulse-energy near-infrared femtosecond laser pulses from a Ti:sapphire oscillator were used to micromachine localized refractive index structures inside ophthalmologic hydrogel polymers. The relation between laser-induced refractive index modification and different laser micromachining conditions was investigated in both pure and dye copolymerized hydrogel polymers. We studied the nonlinear absorption enhancement of the laser energy induced by copolymerized dyes during the micromachining process and the effects on increasing the laser scanning speed. We discussed the wavelength dependence and the laser pulse energy dependence of the micromachining results in a laser operation wavelength range from 700 nm to 1000 nm. By changing the water concentration in pure and doped hydrogel polymers, we further investigated the critical role that water plays in the creation of large refractive index modifications in hydrogels without inducing optical breakdown or damage. A thermal model was used to explain the experimental results. By increasing nonlinear absorption in hydrogel polymers and optimizing femtosecond laser operation parameters, large refractive index modifications could be achieved with greatly increased laser micromachining speeds. In this paper, we discuss the optimization of material and laser parameters for the hydrogel material system.

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.

The Role of Nonlinear Absorption in Enhancement of Efficiency of Femtosecond Micromachining in Hydrogels

Nonlinear absorption of femtosecond pulses in pure and doped hydrogel polymers has been measured, explaining the significantly faster machining speeds in doped hydrogels during the femtosecond laser micromachining that we have observed in doped samples.

Femtosecond Laser Micromachining of Ophthalmologic Hydrogels with Two Photon Absorption Enhancement

Ophthalmologic hydrogel polymers are doped with fluorescein or coumarin dyes to enhance two photon absorption (TPA) during the femtosecond laser micromachining process. Consequently, micromachining speed can be significantly increased.