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Optics Express

  • Editor: Michael Duncan
  • Vol. 14, Iss. 24 — Nov. 27, 2006
  • pp: 11901–11909
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Large refractive index change in silicone-based and non-silicone-based hydrogel polymers induced by femtosecond laser micro-machining

Li Ding, Richard Blackwell, Jay F. Künzler, and Wayne H. Knox  »View Author Affiliations


Optics Express, Vol. 14, Issue 24, pp. 11901-11909 (2006)
http://dx.doi.org/10.1364/OE.14.011901


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Abstract

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 632.8nm, we find as large as 0.06±0.005 average refractive index change within the irradiated area.

© 2006 Optical Society of America

1. Introduction

Femtosecond laser micro-machining has attracted increasing interest due to its unique 3D structuring capabilities in transparent materials. Many different kinds of structures including optical waveguides, light couplers and 3D optical storage inside glasses [1–12

1. K. M. Davis, K. Miura, N. Sugimoto, and K. Hirao, “Writing waveguides in glass with a femtosecond laser,” Opt. Lett. 21, 1729–1731 (1996). [CrossRef] [PubMed]

] and photoresist polymers such as PMMA and photopolymerizable resin [13–15

13. S. Maruo, O. Nakamura, and S. Kawata, “Three-dimentional microfabrication with two-photon-absorbed photopolymerization,” Opt. Lett. 22, 132–134 (1997). [CrossRef] [PubMed]

] have been created. When femtosecond laser pulses of sufficient energy are tightly focused in the bulk of a transparent material, the high intensity at the focus causes a nonlinear absorption (typically multi-photon absorption) and leads to a range of modifications due to optical breakdown or structural and chemical changes in the material. Several research groups have studied the refractive index change within the irradiated area in glass and polymeric materials and found values in the range of 1×-2 to 1×-4 [1–12

1. K. M. Davis, K. Miura, N. Sugimoto, and K. Hirao, “Writing waveguides in glass with a femtosecond laser,” Opt. Lett. 21, 1729–1731 (1996). [CrossRef] [PubMed]

, 15

15. A. Zoubir, C. Lopez, M. Richardson, and K. Richardson, “Femtosecond laser fabrication of tubular waveguides in poly(methyl methacrylate),” Opt. Lett. 29, 1840–1842 (2004). [CrossRef] [PubMed]

]. Since the absorption is strongly nonlinear, the index change inside the bulk materials is generally localized in the focal volume, leaving the surrounding area unaffected. By translation of the bulk sample with respect to the laser focal point, fabrication of different 3D structures can be realized. Recently, much larger index of refraction changes have been produced by femtosecond pulse irradiation in glasses that have been doped with semiconductor materials [16

16. N. Takeshima, Y. Kuroiwa, Y. Narita, S. Tanaka, and K. Hirao, “Fabrication of a periodic structure with a high refractive-index difference by femtosecond laser pulses,” Opt. Express 12, 4019–4024 (2004). [CrossRef] [PubMed]

, 17

17. N. Takeshima, Y. Narita, S. Tanaka, Y. Kuroiwa, and K. Hirao, “Fabrication of high-efficiency diffraction gratings in glass,” Opt. Lett. 30, 352–354 (2005). [CrossRef] [PubMed]

]. In those reports, the large index increase was attributed to a phase separation, leading to formation of semiconductor microcrystallites.

Femtosecond lasers used in micro-machining can be operated over a wide range of parameters, from low-repetition-rates (10 Hz to 1MHz) with pulse energies of microjoules to millijoules [1–6

1. K. M. Davis, K. Miura, N. Sugimoto, and K. Hirao, “Writing waveguides in glass with a femtosecond laser,” Opt. Lett. 21, 1729–1731 (1996). [CrossRef] [PubMed]

], to high-repetition-rates (>1MHz) femtosecond lasers with pulse energies typically of the order of nanojoules [7–12

7. A. M. Streltsov and N. F. Borrelli, “Fabrication and analysis of a directional coupler written in glass by nanojoule femtosecond laser pulses,” Opt. Lett. 26, 42–43 (2001). [CrossRef]

]. For low-repetition-rate femtosecond lasers, the excitation density is usually well above the material nonlinear absorption threshold, and because the thermal diffusion time is much shorter than the time interval between the adjacent laser pulses, usually each pulse is responsible for the change of the focal volume and the modification region is often within the focal volume. Under some focusing conditions, these high energy pulses may lead to micro-machined structures with asymmetric modification profiles. In the other hand, when high-repetition-rate femtosecond lasers are used, heat accumulation plays a more important role in the laser-material interaction. In this case, the thermal diffusion time is longer than the time interval between the laser pulses, and the absorbed laser energy can accumulate within the focal volume and increase the laser-material interaction temperature [18

18. S. M. Eaton, H. Zhang, P. R. Herman, F. Yoshino, L. Shah, J. Bovatsek, and A. Y. Arai, “Heat accumulation effects in femtosecond laser-written waveguides with variable repetition rate,” Opt. Express 13, 4708–4716 (2005). [CrossRef] [PubMed]

, 19

19. C. B. Schaffer, J. F. Garcia, and E. Mazur, “Bulk heating of transparent materials using a high-repetitionrate femtosecond laser,” App. Phys. A 76, 351–354 (2003). [CrossRef]

]. Usually as a result, the size of the modification region will increase and a more symmetric structure may be formed. For high-repetition rate femtosecond lasers, generally the pulse energy is much lower than that of the low-repetition-rate femtosecond lasers. Since material nonlinear multi-photon absorption is highly peak powerdependent, at low pulse energies it is possible that only the center part of the pulse has enough power to exceed the nonlinear absorption threshold and introduce modifications. Therefore with the combined effects of multi-photon absorption and heat accumulation, the structures that high-repetition-rate, low-pulse-energy femtosecond lasers create can still be of the order of the focus size, or within the focal volume [7

7. A. M. Streltsov and N. F. Borrelli, “Fabrication and analysis of a directional coupler written in glass by nanojoule femtosecond laser pulses,” Opt. Lett. 26, 42–43 (2001). [CrossRef]

, 9

9. K. Minoshima, A. M. Kowalevicz, I. Hartl, E. P. Ippen, and J. G. Fujimoto, “Photonic device fabrication in glass by use of nonlinear materials processing with a femtosecond laser oscillator,” Opt. Lett. 26, 1516–1518 (2001). [CrossRef]

].

2. Experiment setup

The femtosecond laser micro-machining experiment setup is shown in Fig. 1. The laser source is a Kerr-lens mode-locked Ti:Sapphire laser (K-M Labs) pumped by 4 W of green light from a frequency-doubled Nd:YVO4 laser. The laser generates pulses of 300 mW average power, 30-fs pulsewidth and 93MHz repetition rate at 800 nm. The measured average laser power at the objective focus on the material is about 120mW, corresponding to a pulse energy of about 1.3 nJ. We use two SF10 prisms in a double-pass configuration at 37.5cm separation distance between the prisms to compensate the dispersion of the microscope objective. A collinear autocorrelator using 3rd order harmonic generation is employed to measure the pulse width at the objective focus. A concave mirror pair is added into the optical path in order to adjust the beam size to optimally fill the objective aperture. We used third order surface harmonic generation (THG) autocorrelation to characterize the pulse width at the focus of the high-numerical-aperture objectives because of its simplicity, high signal to noise ratio and lack of material dispersion that second harmonic generation (SHG) crystals usually introduce [20

20. D. Meshulach, Y. Barad, and Y. Silberberg, “Measurement of ultrashort optical pulses by third-harmonic generation,” J. Opt. Soc. Am. B 14, 2122–2125 (1997). [CrossRef]

, 21

21. J. A. Squier, D. N. Fittinghoff, C. P. J. Barty, K. R. Wilson, M. Muller, and G. J. Brakenhoff, “Characterization of femtosecond pulses focused with high numerical aperture optics using interferometric surface-third-harmonic generation,” Opt. Commun. 147, 153–156 (1998). [CrossRef]

]. The THG signal is generated at the interface of air and an ordinary cover slip (Corning No. 0211 Zinc Titania glass), and measured with a photomultiplier and a lock-in amplifier. We tested a set of different high-numerical-aperture objectives while carefully adjusting the separation distance between the two prisms and the amount of glass inserted. We found that a nearly transform-limited 30-fs duration pulse can be obtained at the focus of a 60X 0.70NA Olympus LUCPlanFLN long-working-distance objective. Three Newport VP-25XA linear servo stages with 100 nm resolution form a 3D scanning platform which is controlled and programmed by computer. Since they are DC servo-motor driven stages, they can move smoothly between adjacent steps. In addition, a CCD camera along with a monitor is used beside the objective to real-time monitor the micro-machining process.

Fig. 1. Experimental setup for femtosecond micro-machining

3. Experiment result and discussion

Three different polymers have been initially studied in our experiments. They are amorphous hydrogels with degradation temperature of about 250 °C. PV2526-164 is a silicone-containing hydrogel that can absorb 36% water by weight. RD1817 is a hydrogel based on a copolymer of 2-hydroxyethyl methacrylate and N-1-vinyl-pyrrolidinone that absorbs 80% water by weight, and its refractive index when hydrated is very close to the index of water. HEMA B is a hydrogel based on 2-hydroxyethyl methacrylate that absorbs 38% water by weight. Its refractive index is the highest among these three materials. Table 1 shows the parameters of the three polymers. We measured the refractive indices in a hydrated state by ellipsometry. Figure 2 shows the transmission spectra of these materials measured by an Ocean Optics HR4000 Spectrometer.

Table 1. Parameters of different hydrogel polymers

table-icon
View This Table
Fig. 2. Transmission spectra of PV2526-164, RD1817 and HEMA B

The hydrogel samples are mounted horizontally on the scanning platform, and the femtosecond laser is focused with a high-numerical-aperture objective through a cover slip inside the sample (Fig. 3).

Fig. 3. Samples are placed in a hydrating solution between two microscope slides and irradiated with 30 fs pulses. The sample holder is translated continuously.

The hydrogel samples are maintained in a solution (Bausch and Lomb “Renu” solution) in room temperature to maintain their water-content during micro-machining and subsequent optical measurements. The thickness of these hydrogel samples in the solution is about 700µm, and the laser is typically focused ~100µm beneath the top surface of the sample. Periodic gratings structures were fabricated with a scanning speed of 0.4µm/s in an X-Y plane perpendicular to the laser beam. Figure 4 shows the differential interference contrast (DIC) and phase contrast (PC) images of plan views of the gratings that we made inside these three materials. We use an Olympus BX51 Model microscope for these pictures.

Figs. 4. (a) and (b). (a). and 4 (b). DIC and PC images of a grating in PV2526-164
Figs. 4. (c) and (d) (c). and 4 (d). DIC and PC images of a grating in RD1817
Figs. 4. (e) and (f) (e). and 4. (f). DIC and PC images of a grating in HEMA B

These images show periodic structures with a 5-µm spacing were fabricated inside the samples. The structures are difficult to see in bright-field microscope, indicating that these structures exhibit very low scattering loss. We estimate that the width of the grating lines is approximately 1 µm, which is significantly smaller than the laser focus diameter 2.5 µm that we measured using a knife-edge method in air. We investigated the cross sectional images of the lines by cutting the material with a razor blade mounted on a xyz translation stage perpendicular to the lines. Figure 5(a) shows a DIC image of the cross section of a grating line formed inside the PV2526-164 polymer. We notice that the cross section has an elliptical shape. From these photos, we estimate that the thickness of the line is approximately 3 µm. In a transverse writing configuration, the laser modification region is typically found to be extended in the longitudinal direction [22

22. Y. Cheng, K. Sugioka, K. Midorikawa, M. Masuda, K. Toyoda, M. Kawachi, and K. Shihoyama, “Control of the cross-sectional shape of a hollow microchannel embedded in photo structurable glass by use of a femtosecond laser,” Opt. Lett. 28, 55–57 (2003). [CrossRef] [PubMed]

]. In particular, we note that spherical aberration caused by the refractive index mismatch between the air, cover slip and polymer can strongly influence the cross section [23

23. G. Zhou, M. J. Ventura, and M. Gu, “Photonic bandgap properties of void-based body-centered-cubic photonic crystals in polymer,” Opt. Express 13, 4390–4395 (2005). [CrossRef] [PubMed]

]. The Olympus focusing objective that we use has a variable cover slip correction covering a variation in thickness ranging from 0.1 to 1.3 millimeters. By carefully adjusting the cover slip correction of the objective, we are able to compensate the spherical aberration of the focusing laser beam even when the focus is deep inside of the bulk polymer materials, to achieve the result shown in Fig. 5(a). In contrast, Fig. 5(b) shows the cross section of a line formed inside PV2526-164 without any spherical aberration correction, where the longitudinal axis is 10 microns long.

Fig. 5. Cross section of one grating line formed inside PV2526-164 (a) with spherical aberration correction, (b) without spherical aberration correction

We investigated these gratings by focusing an unpolarized He-Ne laser beam with a wavelength of 632.8nm on these gratings and monitoring the diffraction pattern. The diffraction angles showed good agreement with the diffraction equation:

mλ=dsinθ
(1)

where m is the diffraction order, λ is the wavelength of the incident laser beam which here is 632.8nm, and d is the grating period. Figure 6 shows the diffraction patterns of these three gratings.

Fig. 6. (a). Diffraction pattern of the grating fabricated in PV2526-164
Fig. 6. (b). Diffraction pattern of the grating fabricated in RD1817
Fig. 6. (c). Diffraction pattern of the grating fabricated in HEMA B

Diffracted light from 0th order to 2nd order can be easily seen in these images. The diffraction efficiency of the grating can be measured, and since the efficiency is a function of the refractive index change, we can use it to calculate the refractive index change in the laser irradiation region. Considering the grating as a phase grating, its transmittance function can be written as:

t(x0,y0)=(eiϕ2eiϕ1)rect(x0a)*1dcomb(x0d)+eiϕ1
(2)

where a is the grating linewidth, d is the groove spacing, ϕ 2 and ϕ 1 are the phase delays through the lines and ambient region respectively: ϕ2=2π×(n+Δn)×bλ and ϕ1=2π×n×bλ . b is the thickness of the grating line, n is the average refractive index of the material, Δn is the average refractive index change in the grating lines, and λ is the incident light wavelength of the measurement (632.8nm). Here, the grating linewidth we are using is 1 µm and the thickness is 3 µm. We assume a uniform (‘top-hat’ shape) index change within the lasereffected region. The convolution theorem can be used to calculate the spectrum of the grating such as:

T(fx,fy)=F{t(x0,y0)}=(eiϕ2eiϕ1)asinc(afx)comb(dfx)δ(fy)+eiϕ1δ(fx,fy)
(3)

Then, the intensity distribution of the grating diffraction pattern is:

I(x,y)=(1λz)2×[(eiϕ2eiϕ1)adm=sinc(amd)δ(xλzmd,yλz)+eiϕ1δ(xλz,yλz)]2
(4)

From this formula, the intensities of the 0th, 1st and 2nd order diffraction light are:

I0=(1λz)2×[(ei2π×(n+Δn)×bλei2π×n×bλ)ad+ei2π×n×bλ]2
(5)
I1=(1λz)2×[(ei2π×(n+Δn)×bλei2π×n×bλ)adsinc(ad)]2
(6)

and

I2=(1λz)2×[(ei2π×(n+Δn)×bλei2π×n×bλ)adsinc(2ad)]2
(7)

By comparing the light intensities of 1st, 2nd and 0th diffraction orders, we can determine the refractive index change within the grating lines. Figure 7 shows the ratio of intensity of the 1st diffraction order to 0th order of the grating in PV2526-164 is 0.1374, and the corresponding refractive index change determined by the analysis is about 0.06. Using the ratio of intensity of the 2nd diffraction order to 0th order of the grating, the calculated refractive index change is virtually identical. We note that several factors could affect this result, such as the accuracy of measurement of the different diffraction order intensities, and the measurements of grating linewidth and thickness. To reduce measurement error of the diffraction order intensities, we usually took several measurements and averaged the results. In principle, the refractive index change is not a uniform top-hat function, rather we suspect that it is a gradient-index structure on a small scale. From the cross-section pictures (Fig. 5), we can see that the affected region is reasonably well-defined. For the purposes of the present investigation, we assume that the index profile is uniform, and considering our measurement accuracy of the parameters as discussed, we calculate an error bar on the refractive index change of±0.005. Using the same method, we determine the average refractive index change in RD1817 and HEMA B are 0.05±0.005 and 0.03±0.005.

Fig. 7. Using 1st order diffraction efficiency to calculate refractive index change in PV2526-164

4. Discussion and conclusions

The index of refraction changes that we observe in these experiments are much larger than those that have been observed in ordinary glasses or other polymer materials. We can speculate on the origin of the optical effects. We believe that the action of the femtosecond pulses is principally to cause a phase separation similar to that discussion by Takeshima [16

16. N. Takeshima, Y. Kuroiwa, Y. Narita, S. Tanaka, and K. Hirao, “Fabrication of a periodic structure with a high refractive-index difference by femtosecond laser pulses,” Opt. Express 12, 4019–4024 (2004). [CrossRef] [PubMed]

, 17

17. N. Takeshima, Y. Narita, S. Tanaka, Y. Kuroiwa, and K. Hirao, “Fabrication of high-efficiency diffraction gratings in glass,” Opt. Lett. 30, 352–354 (2005). [CrossRef] [PubMed]

], except that instead of the phase separation causing a precipitation of semiconductor microcrystallites, in our case, there may be a densification of the polymer material such that water is locally excluded from the line regions, leading to a large change in refractive index. We note that, in principle, the index change could be positive or negative, and the diffraction grating experiments would not elucidate the difference, however if the change is due to a material densification, we would expect the change to be positive. We have performed preliminary experiments on waveguide fabrication in these materials under nearly identical conditions, and these results suggest that the index change is positive. These results will be reported elsewhere.

We found large refractive index change in hydrogel polymers induced by a high-repetition rate, low-pulse-energy femtosecond laser. The modified regions are as small as 1µm×3µm, and the average refractive index changes as large as 0.06±0.005 at room temperature are observed. We are further investing fabrication of more complex three-dimensional optical structures and the dependence of the refractive index change on the different pulse energy and temperature, and these results will be reported in the future.

References and links

1.

K. M. Davis, K. Miura, N. Sugimoto, and K. Hirao, “Writing waveguides in glass with a femtosecond laser,” Opt. Lett. 21, 1729–1731 (1996). [CrossRef] [PubMed]

2.

E. N. Glezer, M. Milosavljevic, L. Huang, R. J. Finlay, T. H. Her, J. P. Callan, and E. Mazur, “Threedimensional optical storage inside transparent materials,” Opt. Lett. 21, 2023–2025 (1996). [CrossRef] [PubMed]

3.

D. Homoelle, S. Wielandy, Alexander L. Gaeta, N. F. Borrelli, and Charlene Smith, “Infrared photosensitivity in silica glasses exposed to femtosecond laser pulses,” Opt. Lett. 24, 1311–1313 (1999). [CrossRef]

4.

W. Watanabe, T. Asano, K. Yamada, K. Itoh, and J. Nishii, “Wavelength division with three-dimensional couplers fabricated by filamentation of femtosecond laser pulses,” Opt. Lett. 28, 2491–2493 (2003). [CrossRef] [PubMed]

5.

R. Osellame, N. Chiodo, G. D. Valle, S. Taccheo, R. Ramponi, G. Cerullo, A. Killi, U. Morgner, M. Lederer, and D. Kopf, “Optical waveguide writing with a diode-pumped femtosecond oscillator,” Opt. Lett. 29, 1900–1902 (2004). [CrossRef] [PubMed]

6.

Y. Nasu, M. Kohtoku, and Y. Hibino, “Low-loss waveguides written with a femtosecond laser for flexible interconnection in a planar light-wave circuit,” Opt. Lett. 30, 723–725 (2005). [CrossRef] [PubMed]

7.

A. M. Streltsov and N. F. Borrelli, “Fabrication and analysis of a directional coupler written in glass by nanojoule femtosecond laser pulses,” Opt. Lett. 26, 42–43 (2001). [CrossRef]

8.

C. B. Schaffer, A. Brodeur, J. F. Garcia, and E. Mazur, “Micromachining bulk glass by use of femtosecond laser pulses with nanojoule energy,” Opt. Lett. 26, 93–95 (2001). [CrossRef]

9.

K. Minoshima, A. M. Kowalevicz, I. Hartl, E. P. Ippen, and J. G. Fujimoto, “Photonic device fabrication in glass by use of nonlinear materials processing with a femtosecond laser oscillator,” Opt. Lett. 26, 1516–1518 (2001). [CrossRef]

10.

K. Minoshima, A. M. Kowalevicz, E. P. Ippen, and J. G. Fujimoto, “Fabrication of coupled mode photonic devices in glass by nonlinear femtosecond laser materials processing,” Opt. Express 10, 645–652 (2002). [PubMed]

11.

Alexander M. Streltsov and Nicholas F. Borrelli, “Study of femtosecond-laser-written waveguides in glasses,” J. Opt. Soc. Am. B 19, 2496–2504 (2002). [CrossRef]

12.

R. Osellame, N. Chiodo, V. Maselli, A. Yin, M. Zavelani-Rossi, G. Cerullo, P. Laporta, L. Aiello, S. De Nicola, P. Ferraro, A. Finizio, and G. Pierattini, “Optical properties of waveguides written by a 26 MHz stretched cavity Ti:sapphire femtosecond oscillator,” Opt. Express 13, 612–620 (2005). [CrossRef] [PubMed]

13.

S. Maruo, O. Nakamura, and S. Kawata, “Three-dimentional microfabrication with two-photon-absorbed photopolymerization,” Opt. Lett. 22, 132–134 (1997). [CrossRef] [PubMed]

14.

G. Witzgall, R. Vrijen, E. Yablonovitch, V. Doan, and B. J. Schwartz, “Single-shot two-photon exposure of commercial photoresist for the production of three-dimentional structures,” Opt. Lett. 23, 1745–1747 (1998). [CrossRef]

15.

A. Zoubir, C. Lopez, M. Richardson, and K. Richardson, “Femtosecond laser fabrication of tubular waveguides in poly(methyl methacrylate),” Opt. Lett. 29, 1840–1842 (2004). [CrossRef] [PubMed]

16.

N. Takeshima, Y. Kuroiwa, Y. Narita, S. Tanaka, and K. Hirao, “Fabrication of a periodic structure with a high refractive-index difference by femtosecond laser pulses,” Opt. Express 12, 4019–4024 (2004). [CrossRef] [PubMed]

17.

N. Takeshima, Y. Narita, S. Tanaka, Y. Kuroiwa, and K. Hirao, “Fabrication of high-efficiency diffraction gratings in glass,” Opt. Lett. 30, 352–354 (2005). [CrossRef] [PubMed]

18.

S. M. Eaton, H. Zhang, P. R. Herman, F. Yoshino, L. Shah, J. Bovatsek, and A. Y. Arai, “Heat accumulation effects in femtosecond laser-written waveguides with variable repetition rate,” Opt. Express 13, 4708–4716 (2005). [CrossRef] [PubMed]

19.

C. B. Schaffer, J. F. Garcia, and E. Mazur, “Bulk heating of transparent materials using a high-repetitionrate femtosecond laser,” App. Phys. A 76, 351–354 (2003). [CrossRef]

20.

D. Meshulach, Y. Barad, and Y. Silberberg, “Measurement of ultrashort optical pulses by third-harmonic generation,” J. Opt. Soc. Am. B 14, 2122–2125 (1997). [CrossRef]

21.

J. A. Squier, D. N. Fittinghoff, C. P. J. Barty, K. R. Wilson, M. Muller, and G. J. Brakenhoff, “Characterization of femtosecond pulses focused with high numerical aperture optics using interferometric surface-third-harmonic generation,” Opt. Commun. 147, 153–156 (1998). [CrossRef]

22.

Y. Cheng, K. Sugioka, K. Midorikawa, M. Masuda, K. Toyoda, M. Kawachi, and K. Shihoyama, “Control of the cross-sectional shape of a hollow microchannel embedded in photo structurable glass by use of a femtosecond laser,” Opt. Lett. 28, 55–57 (2003). [CrossRef] [PubMed]

23.

G. Zhou, M. J. Ventura, and M. Gu, “Photonic bandgap properties of void-based body-centered-cubic photonic crystals in polymer,” Opt. Express 13, 4390–4395 (2005). [CrossRef] [PubMed]

OCIS Codes
(050.1950) Diffraction and gratings : Diffraction gratings
(160.5470) Materials : Polymers
(320.7110) Ultrafast optics : Ultrafast nonlinear optics

ToC Category:
Ultrafast Optics

History
Original Manuscript: August 8, 2006
Revised Manuscript: September 25, 2006
Manuscript Accepted: November 18, 2006
Published: November 27, 2006

Virtual Issues
Vol. 1, Iss. 12 Virtual Journal for Biomedical Optics

Citation
Li Ding, Richard Blackwell, Jay F. Kunzler, and Wayne H. Knox, "Large refractive index change in silicone-based and non-silicone-based hydrogel polymers induced by femtosecond laser micro-machining," Opt. Express 14, 11901-11909 (2006)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-14-24-11901


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References

  1. K. M. Davis, K. Miura, N. Sugimoto, and K. Hirao, "Writing waveguides in glass with a femtosecond laser," Opt. Lett. 21, 1729-1731 (1996). [CrossRef] [PubMed]
  2. E. N. Glezer, M. Milosavljevic, L. Huang, R. J. Finlay, T. H. Her, J. P. Callan, and E. Mazur, "Three-dimensional optical storage inside transparent materials," Opt. Lett. 21, 2023-2025 (1996). [CrossRef] [PubMed]
  3. D. Homoelle, S. Wielandy, AlexanderL. Gaeta, N. F. Borrelli, and Charlene Smith, "Infrared photosensitivity in silica glasses exposed to femtosecond laser pulses," Opt. Lett. 24, 1311-1313 (1999). [CrossRef]
  4. W. Watanabe, T. Asano, K. Yamada, K. Itoh, and J. Nishii, "Wavelength division with three-dimensional couplers fabricated by filamentation of femtosecond laser pulses," Opt. Lett. 28, 2491-2493 (2003). [CrossRef] [PubMed]
  5. R. Osellame, N. Chiodo, G. D. Valle, S. Taccheo, R. Ramponi, G. Cerullo, A. Killi, U. Morgner, M. Lederer, and D. Kopf, "Optical waveguide writing with a diode-pumped femtosecond oscillator," Opt. Lett. 29, 1900-1902 (2004). [CrossRef] [PubMed]
  6. Y. Nasu, M. Kohtoku, and Y. Hibino, "Low-loss waveguides written with a femtosecond laser for flexible interconnection in a planar light-wave circuit," Opt. Lett. 30, 723-725 (2005). [CrossRef] [PubMed]
  7. A. M. Streltsov, and N. F. Borrelli, "Fabrication and analysis of a directional coupler written in glass by nanojoule femtosecond laser pulses," Opt. Lett. 26, 42-43 (2001). [CrossRef]
  8. C. B. Schaffer, A. Brodeur, J. F. Garcia, and E. Mazur, "Micromachining bulk glass by use of femtosecond laser pulses with nanojoule energy," Opt. Lett. 26,93-95 (2001). [CrossRef]
  9. K. Minoshima, A. M. Kowalevicz, I. Hartl, E. P. Ippen, and J. G. Fujimoto, "Photonic device fabrication in glass by use of nonlinear materials processing with a femtosecond laser oscillator," Opt. Lett. 26,1516-1518 (2001). [CrossRef]
  10. K. Minoshima, A. M. Kowalevicz, E. P. Ippen, and J. G. Fujimoto, "Fabrication of coupled mode photonic devices in glass by nonlinear femtosecond laser materials processing," Opt. Express 10, 645-652 (2002). [PubMed]
  11. Alexander M. Streltsov, and Nicholas F. Borrelli, "Study of femtosecond-laser-written waveguides in glasses," J. Opt. Soc. Am. B 19,2496-2504 (2002). [CrossRef]
  12. R. Osellame, N. Chiodo, V. Maselli, A. Yin, M. Zavelani-Rossi, G. Cerullo, P. Laporta, L. Aiello, S. De Nicola, P. Ferraro, A. Finizio, and G. Pierattini, "Optical properties of waveguides written by a 26 MHz stretched cavity Ti:sapphire femtosecond oscillator," Opt. Express 13, 612-620 (2005). [CrossRef] [PubMed]
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