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Virtual Journal for Biomedical Optics

Virtual Journal for Biomedical Optics

| EXPLORING THE INTERFACE OF LIGHT AND BIOMEDICINE

  • Editor: Gregory W. Faris
  • Vol. 5, Iss. 3 — Feb. 10, 2010
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UV waveguides light fabricated in fluoropolymer CYTOP by femtosecond laser direct writing

Yasutaka Hanada, Koji Sugioka, and Katsumi Midorikawa  »View Author Affiliations


Optics Express, Vol. 18, Issue 2, pp. 446-450 (2010)
http://dx.doi.org/10.1364/OE.18.000446


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Abstract

We have fabricated optical waveguides inside the UV-transparent polymer, CYTOP, by femtosecond laser direct writing for propagating UV light in biochip applications. Femtosecond laser irradiation is estimated to increase the refractive index of CYTOP by 1.7 × 10−3 due to partial bond breaking in CYTOP. The waveguide in CYTOP has propagation losses of 0.49, 0.77, and 0.91 dB/cm at wavelengths of 632.8, 355, and 266 nm, respectively.

© 2010 OSA

1. Introduction

2. Experiments

To fabricate the optical waveguides in this study, the refractive index of CYTOP was modified using a commercial femtosecond laser system: a 800-mW regenerative amplified laser with an emission wavelength of 775 nm, a pulse duration of 150 fs, and a repetition rate of 1 kHz. The detailed configuration has been described elsewhere [11

11. K. Sugioka, Y. Cheng, and K. Midorikawa, “Three-dimensional microcmachining of glass using femtosecond laser for lab-on-a-chip device manufacture,” Appl. Phys., A Mater. Sci. Process. 81(1), 1–10 (2005). [CrossRef]

]. The pulse energy was adjusted using a polarizer and a neutral density filter. The 6-mm beam width of the output laser was reduced to 3 mm by passing it through an aperture to improve the beam quality. The beam was focused using a × 20 microscope objective with a numerical aperture (NA) of 0.46. A 0.5-mm-wide slit was placed before the objective lens to improve the cross-sectional shape of the optical waveguide [12

12. 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 photostructurable glass by use of a femtosecond laser,” Opt. Lett. 28(1), 55–57 (2003). [CrossRef] [PubMed]

]. Optical waveguides were fabricated by translating the CYTOP samples perpendicular to the laser beam axis using a computer-controlled xyz-stage with a resolution of 1 µm. The laser scanning speed was varied in the range from 0.1 to 3.0 mm/s and the laser energy was varied between 3 and 11 µJ/pulse before the slit. After the slit, this laser energy range decreased to 0.65 to 2.5 µJ/pulse because only 22% of the light passed through the slit. The whole fabrication process was recorded using a CCD camera and displayed on a PC monitor. To make 500-µm-thick CYTOP films, commercial CYTOP solution was placed in a programmable furnace and the temperature of the furnace was increased to 230 °C in 30 hrs. The film was then cut and mechanochemically polished along the cut edges to achieve an optical finish. The mode profile, the induced refractive index change, and the propagation loss were measured to optically characterize the waveguides. The mechanism for the refractive index increase was investigated by X-ray photoelectron spectroscopy (XPS) measurements.

To optimize the conditions for fabricating waveguides inside the CYTOP samples, we first conducted systematic experiments for various writing conditions. Figure 1
Fig. 1 Cross-sectional optical microscope images of waveguides written in CYTOP by femtosecond laser irradiation at (a) various laser pulse energies with a laser scanning speed of 1500 µm/s and (b) various laser scanning speeds with a laser pulse energy of 0.9 µJ/pulse.
shows microscope images of the cross-sectional profiles of waveguides fabricated under different writing conditions. Figure 1 shows that optical waveguides with no laser-induced damage were written at pulse energies before the objective lens in the range 0.8 to 2.0 µJ/pulse at a laser scanning speed of 1.5 mm/s and at a pulse energy of 0.9 µJ/pulse for laser scanning speeds between 1.5 and 3.0 mm/s. The cross-sectional profiles of the written waveguides are longitudinally elliptical in shape; this could be rectified by employing a narrower slit. A refractive index increase could not be induced below a pulse energy of 0.8 µJ/pulse or above a scanning speed of 3.0 mm/s, while above a pulse energy of 2.0 µJ/pulse or below a scanning speed of 1.5 mm/s, laser-induced damage may be generated inside the CYTOP sample (see Fig. 1). Based on these results, a laser energy of 0.9 µJ/pulse and a scanning speed of 1.5 mm/s were selected for the subsequent experiments.

To measure the profiles of guided light beams, a 632.8-nm-wavelength He-Ne laser beam was coupled to one facet of the written waveguide by a × 50 objective lens (NA: 0.55). The near-field profile and the intensity distribution profile were obtained by imaging the output beam at another facet of the waveguide (see Fig. 2
Fig. 2 (a) Near-field pattern of He-Ne laser beam transmitted by a waveguide and (b) its intensity distribution.
).

The intensity profile in Fig. 2 demonstrates that the waveguide essentially functions as a single-mode waveguide at a wavelength of 632.8 nm. We confirmed that it also functions as a single-mode waveguide even at 355, and 266 nm wavelengths. The full-width at half-maximum of the intensity profile is approximately 3.4 µm in the y-direction and 6.5 µm in the z-direction. The refractive index change (Δn) induced by laser irradiation was estimated by measuring the NA of the 5-mm-long waveguide using the method of Homoelle et al [13

13. D. Homoelle, S. Wielandy, A. L. Gaeta, N. F. Borrelli, and C. Smith, “Infrared photosensitivity in silica glasses exposed to femtosecond laser pulses,” Opt. Lett. 24(18), 1311–1313 (1999). [CrossRef]

]. The 632.8-nm-wavelength He-Ne laser beam was coupled to the waveguide and the NA of the cone of the guided light that exited from the waveguide was measured in the far field. The NA of the waveguide was measured to be 0.07. Assuming a step index change, Δn was estimated to be 1.7 × 10−3.

The propagation loss at various wavelengths was evaluated by fabricating waveguides with five different lengths. Much care was taken to assure that identical coupling conditions were achieved for all five waveguides. 632.8, 355, and 266 nm wavelength CW lasers were coupled to the waveguide using an objective lens ( × 50, NA: 0.55 for 632.8 nm; × 15, NA: 0.32 for 355 nm and 266 nm) to determine the optical losses. Several measurements were taken at each waveguide and the average of these measurements was adopted. Figure 3
Fig. 3 Linear fit of optical loss data for different length waveguides at wavelengths of (a) 632.8, (b) 355, and (c) 266 nm.
shows the throughput data for the optical loss as a function of the waveguide length. The data for each wavelength were fitted with a linear equation. The gradient of the linear fit represents the propagation loss and the y-axis intercept gives the coupling loss.

From Fig. 3, the coupling losses are 0.3 dB for 632.8 nm, 1.1 dB for 355 nm, and 1.7 dB for 266 nm. The coupling losses are caused by either a mismatch between the size of the beam and that of the waveguide or misalignment between the waveguide and the focused CW laser beam. The propagation losses are estimated to be 0.49, 0.77, and 0.91 dB/cm for wavelengths of 632.8, 355, and 266 nm. For biochip applications, the propagation losses obtained for the two UV wavelengths are acceptable due to the short waveguide length in biochips.

3. Discussion

When focused femtosecond laser pulses with low energies are introduced to PMMA or PTFEP, partial bond breaking in the polymer is induced; this causes the volume of the polymer to contract, which increases its refractive index [14

14. S. Sowa, W. Watanabe, T. Tamaki, J. Nishii, and K. Itoh, “Symmetric waveguides in poly(methyl methacrylate) fabricated by femtosecond laser pulses,” Opt. Express 14(1), 291–297 (2006). [CrossRef] [PubMed]

]. A similar mechanism is thought to be responsible for the observed increase in the refractive index of CYTOP. XPS measurements were conducted to characterize changes in the chemical bonding states in irradiated regions of CYTOP. The XPS analysis sample was prepared by line-by-line femtosecond laser irradiation on the CYTOP surface at a pitch of 10 µm for the same experimental conditions as those used for fabricating the optical waveguides; unirradiated CYTOP was used for the control. Figure 4(a)
Fig. 4 (a) Chemical bonding structure of unirradiated CYTOP. C1s XPS spectra of (b) unirradiated and (c) irradiated CYTOP.
shows the chemical bonding structure of unirradiated CYTOP and Figs. 4(b) and (c) show XPS spectra of C1s for the CYTOP surface before and after femtosecond laser irradiation, respectively. Figures 4(b) and (c) reveal that the relative area ratios of the peaks (CF2O:CFO and CF2:CF) are 24:59:17 before femtosecond laser irradiation and are 19:63:18 after laser irradiation. This reveals that laser irradiation increases the CFO and CF2 bond concentrations, while the CF2O bond concentration is reduced by laser irradiation. This suggests that femtosecond laser irradiation induces partial bond breaking of the CO and CF bonds in the CF2O bond (labeled (3) in Fig. 4(a)). Although the detailed changes in the chemical bonding structure of femtosecond laser irradiated CYTOP still remain unclear, we suspect that the partial bond breaking of CF and CO bonds by the femtosecond laser, which would cause the volume to contract, may contribute to the increase in the refractive index of CYTOP.

4. Conclusion

In conclusion, we have fabricated UV waveguides in CYTOP by femtosecond laser direct writing. The refractive index increase in the core was estimated to be 1.7 × 10−3, which might be caused by partial bond breaking in the CF2O bonds in CYTOP due to the femtosecond laser irradiation. The waveguides functioned as single-mode waveguides at 632.8, 355, and 266 nm wavelengths. Propagation losses of 0.49, 0.77, and 0.91 dB/cm were obtained at wavelengths of 632.8, 355, and 266 nm, respectively. The fabricated waveguides presented here will be very useful for UV analysis of samples and UV-induced stimulation of biocells in microbiochips.

References and links

1.

W. Watanabe, S. Sowa, T. Tamaki, K. Itoh, and J. Nishii, “Three-Dimensional Waveguides Fabricated in Poly(methyl methacrylate),” Jpn. J. Appl. Phys. 45(29), L765–L767 (2006). [CrossRef]

2.

C. R. Mendonca, L. R. Cerami, T. Shih, R. W. Tilghman, T. Baldacchini, and E. Mazur, “Femtosecond laser waveguide micromachining of PMMA films with azoaromatic chromophores,” Opt. Express 16(1), 200–206 (2008). [CrossRef] [PubMed]

3.

S. O. Konorov, A. B. Fedotov, and A. M. Zheltikov, “Three dimensional reversible laser micromachining with subnanojoule femtosecond pulses based on two-photon photochromism,” Appl. Phys. B 76, 707 (2003).

4.

R. M. Vazquez, R. Osellame, D. Nolli, C. Dongre, H. van den Vlekkert, R. Ramponi, M. Pollnau, and G. Cerullo, “Integration of femtosecond laser written optical waveguides in a lab-on-chip,” Lab Chip 9(1), 91–96 (2009). [CrossRef] [PubMed]

5.

P. Schulze, M. Ludwig, F. Kohler, and D. Belder, “Deep UV laser-induced fluorescence detection of unlabeled drugs and proteins in microchip electrophoresis,” Anal. Chem. 77(5), 1325–1329 (2005). [CrossRef] [PubMed]

6.

H. M. Verkade, T. Teli, L. V. Laursen, J. M. Murray, and M. J. O’Connell, “A homologue of the Rad18 postreplication repair gene is required for DNA damage responses throughout the fission yeast cell cycle,” Mol. Gen. Genet. 265, 993 (2001).

7.

Seehttp://www.agc.co.jp/english/chemicals/shinsei/cytop/cytop.htm

8.

C. Anolick, J. A. Hrivnak, and R. C. Wheland, “Soluble Perfluoropolymers,” Adv. Mater. 10(15), 1211–1214 (1998). [CrossRef]

9.

Y. Hanada, K. Sugioka, and K. Midorikawa, “Selective cell culture on UV transparent polymer by F2 laser surface modification,” Appl. Surf. Sci. 255(24), 9885–9888 (2009). [CrossRef]

10.

K. Obata, K. Sugioka, N. Shimazawa, and K. Midorikawa, “Fabrication of microchip based on UV transparent polymer for DNA electrophoresis by F2 laser ablation,” Appl. Phys., A Mater. Sci. Process. 84(3), 251–255 (2006). [CrossRef]

11.

K. Sugioka, Y. Cheng, and K. Midorikawa, “Three-dimensional microcmachining of glass using femtosecond laser for lab-on-a-chip device manufacture,” Appl. Phys., A Mater. Sci. Process. 81(1), 1–10 (2005). [CrossRef]

12.

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 photostructurable glass by use of a femtosecond laser,” Opt. Lett. 28(1), 55–57 (2003). [CrossRef] [PubMed]

13.

D. Homoelle, S. Wielandy, A. L. Gaeta, N. F. Borrelli, and C. Smith, “Infrared photosensitivity in silica glasses exposed to femtosecond laser pulses,” Opt. Lett. 24(18), 1311–1313 (1999). [CrossRef]

14.

S. Sowa, W. Watanabe, T. Tamaki, J. Nishii, and K. Itoh, “Symmetric waveguides in poly(methyl methacrylate) fabricated by femtosecond laser pulses,” Opt. Express 14(1), 291–297 (2006). [CrossRef] [PubMed]

OCIS Codes
(250.5460) Optoelectronics : Polymer waveguides
(320.7090) Ultrafast optics : Ultrafast lasers
(350.3390) Other areas of optics : Laser materials processing
(250.4745) Optoelectronics : Optical processing devices

ToC Category:
Optoelectronics

History
Original Manuscript: September 17, 2009
Revised Manuscript: November 6, 2009
Manuscript Accepted: November 17, 2009
Published: January 4, 2010

Virtual Issues
Vol. 5, Iss. 3 Virtual Journal for Biomedical Optics

Citation
Yasutaka Hanada, Koji Sugioka, and Katsumi Midorikawa, "UV waveguides light fabricated in fluoropolymer CYTOP by femtosecond laser direct writing," Opt. Express 18, 446-450 (2010)
http://www.opticsinfobase.org/vjbo/abstract.cfm?URI=oe-18-2-446


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References

  1. W. Watanabe, S. Sowa, T. Tamaki, K. Itoh, and J. Nishii, “Three-Dimensional Waveguides Fabricated in Poly(methyl methacrylate),” Jpn. J. Appl. Phys. 45(29), L765–L767 (2006). [CrossRef]
  2. C. R. Mendonca, L. R. Cerami, T. Shih, R. W. Tilghman, T. Baldacchini, and E. Mazur, “Femtosecond laser waveguide micromachining of PMMA films with azoaromatic chromophores,” Opt. Express 16(1), 200–206 (2008). [CrossRef] [PubMed]
  3. S. O. Konorov, A. B. Fedotov, and A. M. Zheltikov, “Three dimensional reversible laser micromachining with subnanojoule femtosecond pulses based on two-photon photochromism,” Appl. Phys. B 76, 707 (2003).
  4. R. M. Vazquez, R. Osellame, D. Nolli, C. Dongre, H. van den Vlekkert, R. Ramponi, M. Pollnau, and G. Cerullo, “Integration of femtosecond laser written optical waveguides in a lab-on-chip,” Lab Chip 9(1), 91–96 (2009). [CrossRef] [PubMed]
  5. P. Schulze, M. Ludwig, F. Kohler, and D. Belder, “Deep UV laser-induced fluorescence detection of unlabeled drugs and proteins in microchip electrophoresis,” Anal. Chem. 77(5), 1325–1329 (2005). [CrossRef] [PubMed]
  6. H. M. Verkade, T. Teli, L. V. Laursen, J. M. Murray, and M. J. O’Connell, “A homologue of the Rad18 postreplication repair gene is required for DNA damage responses throughout the fission yeast cell cycle,” Mol. Gen. Genet. 265, 993 (2001).
  7. See http://www.agc.co.jp/english/chemicals/shinsei/cytop/cytop.htm
  8. C. Anolick, J. A. Hrivnak, and R. C. Wheland, “Soluble Perfluoropolymers,” Adv. Mater. 10(15), 1211–1214 (1998). [CrossRef]
  9. Y. Hanada, K. Sugioka, and K. Midorikawa, “Selective cell culture on UV transparent polymer by F2 laser surface modification,” Appl. Surf. Sci. 255(24), 9885–9888 (2009). [CrossRef]
  10. K. Obata, K. Sugioka, N. Shimazawa, and K. Midorikawa, “Fabrication of microchip based on UV transparent polymer for DNA electrophoresis by F2 laser ablation,” Appl. Phys., A Mater. Sci. Process. 84(3), 251–255 (2006). [CrossRef]
  11. K. Sugioka, Y. Cheng, and K. Midorikawa, “Three-dimensional microcmachining of glass using femtosecond laser for lab-on-a-chip device manufacture,” Appl. Phys., A Mater. Sci. Process. 81(1), 1–10 (2005). [CrossRef]
  12. 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 photostructurable glass by use of a femtosecond laser,” Opt. Lett. 28(1), 55–57 (2003). [CrossRef] [PubMed]
  13. D. Homoelle, S. Wielandy, A. L. Gaeta, N. F. Borrelli, and C. Smith, “Infrared photosensitivity in silica glasses exposed to femtosecond laser pulses,” Opt. Lett. 24(18), 1311–1313 (1999). [CrossRef]
  14. S. Sowa, W. Watanabe, T. Tamaki, J. Nishii, and K. Itoh, “Symmetric waveguides in poly(methyl methacrylate) fabricated by femtosecond laser pulses,” Opt. Express 14(1), 291–297 (2006). [CrossRef] [PubMed]

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