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.
© 2008 Optical Society of America
Near infrared femtosecond laser micromachining has been demonstrated to be a powerful microfabrication tool, attracting increasing interest in the fields of optics, semiconductors, biomedicine and microelectronics [1–4]. When femtosecond laser pulses are tightly focused within a transparent bulk material, nonlinear absorption can be induced in the focal volume and lead to highly localized energy deposition resulting a range of possible changes in material properties. This highly localized modification gives femtosecond laser micromachining a unique 3D microfabrication capability, and has been used to create volume optical storage [5, 6], gratings [7, 8], waveguides [1, 9–12], photonic bandgap structures [13–17] and microfluidic devices [4, 18].
Recently, this technology has been employed to fabricate photonic structures in various polymer materials. Many different mechanisms can be induced by femtosecond laser micromachining in polymer materials. For various photoresists or polymer resins, two-photon polymerization is the major mechanism that is induced by femtosecond laser micromachining [6, 13, 15–17, 19]. Monomers within these materials with lower degree of polymerization can be highly polymerized through this mechanism and thereby form polymer structures. Post-exposure treatments such as thermal bake are usually needed to realize full polymerization. Recent research has shown that photoinitiators or chromophores with large two-photon absorption (TPA) cross-sections can help to enhance polymerization rates during the femtosecond micromachining process [6, 17]. Femtosecond lasers are also employed to micromachine polymer materials that are fully polymerized such as poly(methylmethacrylate) (PMMA) and polystyrene (PS) [7, 9–11, 20–23]. Femtosecond laser-induced photochemical modifications such as polymer backbone cleavage and chain unzipping cause the refractive index (RI) changes in pure PMMA . Recently, PMMA doped with various dyes has also been studied for structural modifications and device fabrication with RI changes ranging from 10-4 to 10-3 through femtosecond laser micromachining [7, 11, 21, 25].
We recently reported large RI changes and waveguide fabrication induced by femtosecond laser micromachining in ophthalmologic hydrogel polymers [8, 12]. These hydrogel polymers are fully polymerized and, when hydrated, contain from 20 to 80% water. Because of their high optical clarity and high water and oxygen permeability, these hydrogel polymers have been widely used in making contact lenses and other ophthalmologic devices. In our previous experiments [8, 12] we were able to obtain large refractive index changes (up to +0.06) with high optical quality, however the maximum writing speeds were extremely slow (<1 µm/s), limiting practical applications. In this paper, we report studies of femtosecond laser micromachining in hydrogel polymers doped with Fluorescein and Coumarin dyes. We obtain much larger RI changes in doped hydrogel polymers, with laser scanning speeds enhanced by more than 1000x compared to undoped hydrogels. We quantitatively investigate the magnitudes of RI change induced by femtosecond laser micromachining with different laser scanning speeds and dye doping concentrations. We also investigate the critical role that water plays in realizing this enhancement.
2. Experimental Setup
Two typical kinds of ophthalmologic hydrogel polymers were studied in our experiments. Hydroxyethyl methacrylate (HEMA)-based hydrogel polymers are the second of five generations of ophthalmologic hydrogel materials . Although they were invented in 1960s, they are still widely used to make contact lenses. We studied HEMA-based hydrogel polymers with a trade name Akreos® (Bausch and Lomb) which has about 80% by weight HEMA. The water concentration of Akreos® is about 29% by weight. The others are Silicone-based hydrogel polymers, which represent the fifth and the most recent generation of ophthalmologic hydrogel materials . They have significantly improved oxygen permeability and are used to make ophthalmologic devices for long-term use with the trade name PureVision™ (Balafilcon A) from Bausch & Lomb. They contain tris(trimethylsiloxy)silyl propylvinyl carbamate (TPVC), N-vinyl pyrrolidone (NVP) and other silicone components. The water concentration of PureVision™ (Balafilcon A) is about 36% by weight. Figure 1 shows the molecular structures of these major compositions within these hydrogel polymers. During the sample doping process, Fluorescein or Coumarin 1 dye was first dissolved in an ethanol-water mixture and then solution-doped into the hydrogel polymer samples. The doping concentrations of Fluorescein and Coumarin 1 are prepared to be 0.17% and 16% respectively, which we experimentally determined gave roughly equal enhancements in writing speed. It has been reported that diluted Fluorescein and Coumarin solutions with 100µM concentration have 33–36 GM TPA cross section at 800 nm excitation wavelength, where GM (Goeppert - Meyer) is the TPA cross section unit equal to 10-50 cm4 s photon-1 mol-1 [27, 28]. Figure 2 shows the molecular structures of these two dyes.
We micromachined the samples using 27-fs, 800-nm laser pulses operating at a 93 MHz repetition rate from a Kerr-lens mode-locked Ti:Sapphire femtosecond laser oscillator (K-M Labs). The focusing objective was a 60X 0.70NA Olympus LUCPlanFLN long-working-distance microscope objective which could be adjusted to create a nearly diffraction-limited laser spot at the focal point. In the experiments, the average laser power at the focal point was set as 120 mW, corresponding to pulse energies of 1.3 nJ. One pair of SF10 prisms in a double-pass configuration was used in the optical path to pre-compensate dispersion and form nearly transform-limited pulses at the focus. The hydrogel samples were mounted on a 3D scanning platform formed by three Newport VP-25XA linear servo stages . Different scanning speeds ranging from 0.4 µm/s to 1mm/s with respect to the laser beam were tested. After the micromachining process, RI changes within the laser machined region were measured by differential interference contrast (DIC) microscopy that we calibrated by writing diffraction gratings and measuring diffraction efficiency .
3. Experiment results and discussion
The transmission spectra of 700µm-thick pure and doped hydrogel polymer samples were measured with an Ocean Optics HR4000 spectrometer (Fig. 3). Strong absorption around 400 nm is observed when pure hydrogel polymers are doped with these two dyes. All the samples are substantially transparent near 800 nm before and after doping.
Horizontal periodic grating structures were typically formed about 100–150 µm below the top surface within each sample writing at different speeds. During the micromachining process, fluorescence was observed at the laser focal point inside only the doped materials. Figure 4 and Table 1 show the measured RI changes as we varied the scanning speed in different samples. For all the samples, the observed RI change decreases as the scanning speed increases. The largest RI change induced in pure Akreos and pure Balafilcon A was about 0.03±0.005 and 0.06±0.005 respectively when scanning speed was slow. Visible optical damage consisting of primarily black carbon residue as determined by micro-Raman spectroscopy  was observed when the scanning speed was less than 2µm/s for pure Akreos, due to excess heat accumulation. For pure Balafilcon A, no visible damage was observed even when the scanning speed was as slow as 0.4µm/s. When the scanning speed was increased to >10 µm/s, RI changes for both pure Akreos and pure Balafilcon A dropped to less than 0.005, which is our detection limit for index change. In contrast, when the hydrogel polymers were doped with Fluorescein or Coumarin 1 dyes, optical breakdown thresholds could be easily reached if the scanning was slower than 40 µm/s, and optical damage and strong plasma luminescence could be observed during the micromachining process. The largest RI change in Akreos was 0.08±0.005 when doping with Coumarin dye, and 50 µm/s scanning speed was employed. The RI change was still as large as 0.025±0.005 even when the scanning speed was increased to 1mm/s. Different from Akreos, the largest RI change as 0.065±0.005 was obtained when Balafilcon was doped with Fluorescein. The RI change decreased to 0.015±0.005 at 1 mm/s scanning speed. Much faster scanning (>10 mm/s) speeds were also tested for these doped hydrogel polymers in the experiments, and RI changes as large as 0.01 were also observed. These results indicate that femtosecond laser micromachining writing speeds can be greatly improved in the doped hydrogel polymers compared to undoped hydrogels . As we can see from Figure 4, Coumarin 1 gave the larger enhancement with Akreos, and Fluorescein gave the largest enhancement with Balafilcon A. This may be related to the details of the solution doping process, or other factors. We are further investigating the origin of this.
Akreos samples doped with Fluorescein dye ranging from 0.0625% to 0.5% were also investigated. Figure 5 shows the measured RI changes induced by 120 mW average power femtosecond laser pulses with 1mm/s scanning speed. Four Fluorescein dye concentrations of 0.0625%, 0.125%, 0.25% and 0.5% were used. As large RI change of 0.065±0.005 could be induced in the Akreos hydrogel samples doped with 0.5% Fluorescein, RI change decreased rapidly when the dye concentration dropped. For Akreos sample doped with 0.0625% Fluorescein dye, RI change was barely detected. These results were consistent with the curve shown in Fig. 4(a) that no RI change was induced in pure Akreos when the scanning speed was higher than 100µm/s, indicating the important role that the dye plays in the hydrogel polymers in determining the femtosecond laser micromachining writing speed. For undoped hydrogels, the nonlinear absorption is due to either a weak two-photon absorption, or more generally a weak higher order multi-photon absorption process, and hence we obtain the slow micromachining speeds. For dye-doped hydrogels such as those employed here, however, two-photon absorption would be the dominant nonlinear absorption process under the 800 nm excitation conditions. We have carried out preliminary experiments to measure the nonlinear absorption. We measured the transmitted light ratio with the hydrogel sample located exactly in focus and out of focus. For undoped hydrogels, we observed nonlinear absorption in the range of a few percent, however for doped hydrogels, we observed nonlinear absorptions as high as 15%, indicating significant enhancement of the nonlinear absorption. The detailed intensity dependence of refractive index changes has been studied for glasses , and we are studying the full intensity dependence of our nonlinear absorption and index changes in order to elucidate the exact multiphoton contributions to the machining process, the results of which will be reported elsewhere. Figure 5 shows that for high scanning speeds, higher dye concentrations produce larger RI changes in the hydrogel. If the concentration is too high, (for instance, >2% Fluorescein-doping) the dye creates aggregates in the hydrogel polymers, changing the polymer’s properties and inducing enhanced irregular optical damage in the micromachining process.
We then studied the effect of water concentration. In our experiments, 0.5% Fluorescein doped Akreos samples with three different water concentrations were also prepared and tested, and the micromachining results were compared (Fig. 6). As we described above, for the normal Akreos samples with 29% of water concentration, RI changes as large as 0.065±0.005 were observed when the Fluorescein doping concentration was 0.5% and the scanning speed was 1 mm/s. Figure 7(a) shows the phase contrast (PC) image of several micromachined grating lines inside such a normal doped Akreos sample. No optical damage was observed along these lines, and the bright field pictures were clear. When the water concentration of the doped Akreos samples was decreased to 21%, scattered optical damage spots were created along the grating lines as shown in the PC image (Fig. 7(b)). Strong plasma light emission from the focus was observed when these damage spots were being formed during the micromachining process. These damage spots could be observed under the bright field (BF) microscope as black carbonized spots, while the other RI changed parts along the grating lines could not be observed under the BF microscope. The measured RI change within these grating lines was about 0.03±0.005. Although decreasing focused laser power or increasing scanning speed to >2 mm/s prevented inducing damage, the RI change induced in these low-water-concentration Akreos samples was less than 0.005. Fluorescein doped Akreos samples with much less water concentration (12%) were further tested. Figure 7(c) shows the micromachined results in these samples while other experimental parameters were kept as the same. While much more laser-induced damage spots were induced along the grating lines, the RI changed regions became undetectable. These RI changes are <0.005 as shown in Fig. 6. These results indicate that the water content within the polymers has a great influence the femtosecond laser micromachining process. Our recent theoretical calculations show that compared with solid dry polymers, water within the hydrogel polymers has higher heat capacity and lower heat diffusivity which can prevent rapid and large temperature increase of the polymer within the laser focal volume. The laser-induced thermal accumulation can induce additional crosslinking inside the polymer structures which densifies the polymer network, resulting in large refractive index changes . These results are also consistent with recent report on femtosecond laser micromachining of PMMA doped azoaromatic chromophores that low laser damage threshold (Epulse<0.2nJ) and small refractive index (Δn≈1×10-4) could be observed if only solid polymer material without any water was micromachined (for instance, as in Ref ).
In conclusion, we demonstrate large enhancement of the writing speed and large RI changes during femtosecond laser micromachining in HEMA-based and silicone-based ophthalmologic hydrogel polymers doped with Fluorescein or Coumarin 1 dyes. High-repetition-rate, low-pulse-energy, 800 nm-wavelength femtosecond laser pulses from a Ti:Sapphire oscillator were employed during the experiments. The dyes that we used are known to have large two-photon absorption cross-section at near 800 nm excitation wavelength, and this provides a mechanism for increasing the nonlinear absorption in the focus region, and therefore the micromachining efficiency. By optimizing the type and the concentration of dyes, as well as the concentration of water and the laser irradiation conditions, large index changes can been obtained with rapid scanning. Our results may find applications in several fields including ophthalmology, biomedical optics, microfluidics, biosensors, or manufacturing of customized refractive or diffractive devices requiring bio-compatible polymers.
This research was supported by grants from Bausch and Lomb, Inc. and the CEIS program #0833810 at the University of Rochester.
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