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Optics Express

Optics Express

  • Editor: Michael Duncan
  • Vol. 12, Iss. 17 — Aug. 23, 2004
  • pp: 3947–3953
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Simple fabrication of diffraction gratings by two-beam interference method in highly photosensitive hybrid sol-gel films

Dong Jun Kang, Jin-Ki Kim, and Byeong-Soo Bae  »View Author Affiliations


Optics Express, Vol. 12, Issue 17, pp. 3947-3953 (2004)
http://dx.doi.org/10.1364/OPEX.12.003947


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Abstract

Sol-gel hybrid materials containing a large quantity of photoactive molecules exhibited large changes in both refractive index and volume by UV exposure. The materials were used for the fabrication of diffraction gratings using the two-beam interference method. With this technique, we could simply fabricate the diffraction gratings and easily control the grating periods. The diffraction effects and efficiencies of gratings rely heavily on the UV doses and the fabricated diffraction gratings showed a good diffraction performance.

© 2004 Optical Society of America

1. Introduction

Diffraction gratings are the important component for wavelength dispersion, conversion, signal processing and modulation in integrated optics [1

1. V. Weiss, A. A. Friesem, and V. A. Krongauz, “Holographic recording and all-optical modulation in photochromic polymers,” Opt. Lett. 18, 1089–1091 (1993). [CrossRef] [PubMed]

3

3. A Yariv and M. Nakamura, “Periodic structures for integrated optics,” IEEE J. Quantum Electron. QE-13, 233–253 (1977). [CrossRef]

]. Thus, the study of fabricating diffraction gratings has been intensively performed using a variety of methods, including a photo-mask, electron beam lithography, etching techniques, and holographic interference [4

4. J. T. Rantala, R. S. Penner, S. Honkanen, J. Vähäkangas, M. Fallahi, and N. Peyghambarian, “Negative tone hybrid sol-gel materials for electron-beam lithography,” Thin Solid Films 345, 185–187 (1999). [CrossRef]

6

6. H. J. Jiang, X.-C. Yuan, Y. Zhou, Y. C. Chan, and Y. L. Lam, “Single-step fabrication of diffraction gratings on hybrid sol-gel glass using holographic interference lithography,” Opt. Commun. 185, 19–24 (2000). [CrossRef]

]. Contact imprinting using a photo-mask is not suitable for the fabrication of gratings with a submicrometer period due to the diffraction limit between the mask and the samples. The lithography, which includes the etching process, is rather complex and needs several steps to reveal the precise surface structure. However, holography can produce fine patterns and it is easy to control the period of gratings. Thus, the interference method using beam holography has great potential for the fabrication of gratings with fine periods.

The materials as well as the fabrication processes are crucial factors in manufacturing the optical elements. Thus, the photo-polymers such as azo polymers [7

7. N. Zettsu, T. Ubukata, T. Seki, and K. Ichimura, “Soft Crosslinkable Azo Polymer for Rapid Surface Relief Formation and Persistent Fixation,” Adv. Mater. 13, 1693–1697 (2001). [CrossRef]

, 8

8. N. K. Viswanathan, D. Y. Kim, S. Bian, J. Williams, W. Liu, L. Li, L. Samuelson, J. Kumar, and S. K. Tripathy, “Surface relief structures on azo polymer films,” J. Mater. Chem. 9, 1941–1955 (1999). [CrossRef]

] and the sol-gel hybrid (SGH) materials [9

9. X.-C. Yuan, W. X. Yu, N. Q. Ngo, and W. C. Cheong, “Cost-effective fabrication of microlenses on hybrid sol-gel glass with a high-energy beam-sensitive gray-scale mask,” Opt. Express 10, 303–308 (2002), http://www.opticsexpress.org/abstract.cfm?URI=OPEX-10-7-303 [CrossRef] [PubMed]

12

12. W. Yu and X. -. Yuan, “Variable surface profile gratings in sol-gel glass fabricated by holographic interference,” Opt. Express 11, 1925–1930 (2003), http://www.opticsexpress.org/abstract.cfm?URI=OPEX-11-16-1925 [CrossRef] [PubMed]

] have received much attention because the patterns could be fabricated quickly. Recently, the photosensitive sol-gel hybrid (SGH) materials, doped with the large contents of photoactive molecule, were found to exhibit a larger refractive index and volume change by UV exposure [13

13. O. H. Park, S. J. Kim, and B. S. Bae, “Photochemical Reactions in Fluorinated Sol-Gel Hybrid Materials Doped with a Photolocking Agent for Direct Micropatterning,” J. Mater. Chem. 14, 1749–1753 (2004). [CrossRef]

15

15. B. S. Bae, O. H Park, R Charters, B Luther-Davies, and G. R. Atkins, “Direct laser writing of selfdeveloped waveguides in benzyl dimethyl ketal doped sol-gel hybrid glass,” J. Mater. Res. 16, 3184–3187 (2001). [CrossRef]

]. The photosensitivity of these materials could be increased by the simple addition of photoactive molecules. Also, these materials have the advantage of simple single-step photo-patterning due to the simultaneous changes in both refractive index and volume. Thus, these photosensitive SGH materials are good candidates for the simple fabrication of diffraction gratings.

In this paper, we reported on the simple fabrication of diffraction gratings with various periods by two-beam interference method using highly photosensitive SGH materials. In particular, we investigated the effect of the UV doses on the photosensitivity of the SGH materials and the relationships between the photosensitive properties upon UV exposure, the diffraction effects and efficiencies of gratings.

2. Experiment

The changes in the refractive index and thickness of the films before and after illumination with the He-Cd laser were measured using a prism coupler (Metricon 2010) at a wavelength of 632.8 nm. The images of the fabricated diffraction gratings were investigated using optical microscopy and atomic force microscope (AFM, Park Scientific Instruments, Autoprobe 5 M). The diffraction effects were monitored using a CCD camera and the diffraction efficiencies were measured by the Littrow configuration using a He-Ne laser with the wavelength of 632.8 nm in transmission mode.

3. Results and discussion

Figure 1 shows the changes of refractive index (a) and thickness (b) depending on the UV doses of He-Cd laser with the power density of 0.55 mW/cm2. In order to measure the changes of refractive index and thickness of UV exposed films, the films were baked at 150°C for 5 hours after UV exposure and then the changes in refractive index and thickness of the films were measured. The changes in the refractive index and the film thickness increase in proportional to the UV dose. When the UV dose increases, the refractive index and thickness changes continued to increase and then became saturated.

Fig. 1. Refractive index changes (a) and thickness changes (b) in UV exposed and baked SGH film depending on UV doses.

Fig. 2. Illustration of two-beam interference system (a) and the schematic illustration of the interference beam illuminated by the two-beam system (b).

Diffraction gratings were fabricated in photosensitive SGH films using the two-beam interference method. In order to use the high photosensitivity of SGH films, the wet films before heat treatment were used for the fabrication of diffraction gratings. Figure 2 shows the illustration of the two-beam interference system (a) and the schematic illustration of the interference beam illuminated by the two-beam system (b). The He-Cd laser is split into two beams by a half mirror (HM) and then the respective beam is condensed onto a sample surface to form straight interference fringes after passing through beam expander (L1), a 10 µm diameter pinhole (P) and collimating lens (L2). When this system is used for the fabrication of diffraction gratings, the grating period is represented by the equation Λ=λ/2sinθ, where λ is the wavelength of the He-Cd laser (325 nm), and θ is the incident beam angle on the SGH films. We can control the spacing of the fringe by changing the incidence angle of the two beams (θ=θ12), which can fabricate the diffraction gratings with the various periods in two-beam interference system.

Fig. 3. Optical micrographs of diffraction gratings with periods of 5 µm (a), 3 µm (b), and 1 µm (c) fabricated with the two-beam interference system (photo-fabrication: before baking, optical micrographs investigation: after baking).

In order to investigate the images of the fabricated diffraction gratings on the wet films, the films with the fabricated diffraction gratings were baked at 150°C for 5 hours and then the images of the fabricated diffraction gratings on the films were investigated by using optical microscope and AFM. Figure 3 shows the optical micrograph of the diffraction gratings with periods of 5 µm (a), 3 µm (b), and 1 µm (c) fabricated with the two-beam interference system in highly photosensitive SGH films. The diffraction grating patterns were very homogeneous regardless of the grating pitch.

Fig. 4. AFM images of diffraction gratings with periods of 600 nm (a) and 500 nm (b) fabricated with the two-beam interference system (photo-fabrication: before baking, AFM investigation: after baking).

Fig. 5. CCD images of diffraction effects of the gratings with the period of 5 µm depending on exposure time (Diffraction effects in-situ measurement: before baking).

In order to investigate the effects of the UV doses on the diffraction of gratings, we measured in-situ the diffraction effects of gratings with increasing the exposure time of the photo-fabrication of the gratings. The diffraction of the gratings was measured by using the He-Ne laser beam (λ=632.8 nm) and a CCD camera. The He-Ne laser beam was located in the front of samples and the CCD camera connected to a computer was placed at the back of the samples. Figure 5 shows the diffraction effects of the gratings with the period of 5 µm depending on the exposure time from a CCD camera. As the exposure time increased, the diffraction effects were much clearer and the gratings showed the strongest diffraction effects in around 15 minutes. After the 15 min. irradiation, the diffraction effects were not enhanced.

Fig. 6. Diffraction efficiencies of gratings with the period of 0.7 µm depending on exposure time (Diffraction efficiencies measurement: after baking).

For the more detailed analysis of the relationship between the diffraction effects and the UV doses, the first-order diffraction efficiencies (η: ratio of diffracted power to incident power) of the gratings printed upon the photosensitive hybrid films were measured at a wavelength of 632.8 nm. The films with the fabricated diffraction gratings were baked and then the diffraction efficiencies of the fabricated diffraction gratings on the films were investigated. Figure 6 shows the diffraction efficiencies of the gratings with the period of 0.7 µm depending on the exposure time in the transmission mode. The maximum diffraction efficiency of 11.57 % was attained during the irradiation. The irradiation time of the maximum diffraction efficiency was around 15min., which agreed well with the optimum exposure time for obtaining the good-patterned gratings and the strongest diffraction effects shown in Fig. 4 and 5. The decrease in η was appeared since the maximum diffraction efficiency. One of the origins of this decrease in η at the irradiation time longer than 15 min could be due to the line width broadening of the gratings, which results in the decrease of the grating depth and the refractive index changes between the high lines and the low lines in the sinusoidal gratings. A long irradiation time could also cause the distortion of the grating shape or surface, which could be another reason for the decrease in η. Consequently, the diffraction effects and efficiencies of gratings were heavily dependent on the UV exposure time and would be decreased by much UV doses. Thus, the optimum UV doses in photo-fabrication of the diffraction gratings using photosensitive SGH materials are the most important factors for obtaining the stronger diffraction effects of gratings.

4. Conclusion

In conclusion, the photosensitive SGH materials containing the large contents of photoactive molecule exhibited very high photosensitivity with large changes in both the refractive index and the volume by UV exposure. With our high photosensitive SGH materials, we simply fabricated the diffraction gratings and easily controlled the grating periods using two-beam interference method. The diffraction effects and efficiencies of gratings depended heavily on the contents of the UV dose and the fabricated diffraction gratings showed a good diffraction performance.

Acknowledgments

This work has been financially supported from the Sol-Gel Innovation Project (SOLIP) funded by Ministry of Commerce, Industry & Energy (MOCIE) in Korea.

References and links

1.

V. Weiss, A. A. Friesem, and V. A. Krongauz, “Holographic recording and all-optical modulation in photochromic polymers,” Opt. Lett. 18, 1089–1091 (1993). [CrossRef] [PubMed]

2.

T. Huang and K. H. Wagner, “Photoanisotropic incoherent-to-coherent optical conversion,” Appl. Opt. 32, 1888–1900(1993). [CrossRef] [PubMed]

3.

A Yariv and M. Nakamura, “Periodic structures for integrated optics,” IEEE J. Quantum Electron. QE-13, 233–253 (1977). [CrossRef]

4.

J. T. Rantala, R. S. Penner, S. Honkanen, J. Vähäkangas, M. Fallahi, and N. Peyghambarian, “Negative tone hybrid sol-gel materials for electron-beam lithography,” Thin Solid Films 345, 185–187 (1999). [CrossRef]

5.

P. Äyräs, J. T. Rantala, S. Honkanen, S. B. Memdes, and N. Peyghambarian, “Diffraction gratings in sol-gel films by direct contact printing using a UV-mercury lamp,” Opt. Commun. 162, 215–218 (1999). [CrossRef]

6.

H. J. Jiang, X.-C. Yuan, Y. Zhou, Y. C. Chan, and Y. L. Lam, “Single-step fabrication of diffraction gratings on hybrid sol-gel glass using holographic interference lithography,” Opt. Commun. 185, 19–24 (2000). [CrossRef]

7.

N. Zettsu, T. Ubukata, T. Seki, and K. Ichimura, “Soft Crosslinkable Azo Polymer for Rapid Surface Relief Formation and Persistent Fixation,” Adv. Mater. 13, 1693–1697 (2001). [CrossRef]

8.

N. K. Viswanathan, D. Y. Kim, S. Bian, J. Williams, W. Liu, L. Li, L. Samuelson, J. Kumar, and S. K. Tripathy, “Surface relief structures on azo polymer films,” J. Mater. Chem. 9, 1941–1955 (1999). [CrossRef]

9.

X.-C. Yuan, W. X. Yu, N. Q. Ngo, and W. C. Cheong, “Cost-effective fabrication of microlenses on hybrid sol-gel glass with a high-energy beam-sensitive gray-scale mask,” Opt. Express 10, 303–308 (2002), http://www.opticsexpress.org/abstract.cfm?URI=OPEX-10-7-303 [CrossRef] [PubMed]

10.

W. X. Yu, X.-C. Yuan, N. Q. Ngo, W. X. Que, W. C. Cheong, and K.V. Koudriachov, “Single step fabrication of continuous micro-optical elements in hybrid sol-gel glass by laser direct writing,” Opt. Express 10, 443–448 (2002), http://www.opticsexpress.org/abstract.cfm?URI=OPEX-10-10-443 [CrossRef] [PubMed]

11.

D. J. Kang, J. U. Park, B. S. Bae, J. Nishii, and K. Kintaka, “Single-Step Photo Patterning of Diffraction Gratings in Highly Photosensitive Hybrid Gel Films,” Opt. Express 11, 1144–1148 (2003), http://www.opticsexpress.org/abstract.cfm?URI=OPEX-11-10-1144 [CrossRef] [PubMed]

12.

W. Yu and X. -. Yuan, “Variable surface profile gratings in sol-gel glass fabricated by holographic interference,” Opt. Express 11, 1925–1930 (2003), http://www.opticsexpress.org/abstract.cfm?URI=OPEX-11-16-1925 [CrossRef] [PubMed]

13.

O. H. Park, S. J. Kim, and B. S. Bae, “Photochemical Reactions in Fluorinated Sol-Gel Hybrid Materials Doped with a Photolocking Agent for Direct Micropatterning,” J. Mater. Chem. 14, 1749–1753 (2004). [CrossRef]

14.

O. H. Park, J. I. Jung, and B. S. Bae, “Photoinduced Condensation of Sol-Gel Hybrid Glass Films Doped with Benzildimethylketal,” J. Mater. Res. 16, 2143–2148 (2001). [CrossRef]

15.

B. S. Bae, O. H Park, R Charters, B Luther-Davies, and G. R. Atkins, “Direct laser writing of selfdeveloped waveguides in benzyl dimethyl ketal doped sol-gel hybrid glass,” J. Mater. Res. 16, 3184–3187 (2001). [CrossRef]

16.

A. H. O. Kärkkäinen, J. M. Tamkin, J. D. Rogers, D. R. Neal, O. E. Hormi, G. E. Jabbour, J. T. Rantala, and M. R. Descour, “Direct photolithographic deforming of organomodified siloxane films for micro-optics fabrication,” Appl. Opt. 41, 3988 (2002). [CrossRef] [PubMed]

OCIS Codes
(160.6060) Materials : Solgel
(220.4610) Optical design and fabrication : Optical fabrication
(230.1950) Optical devices : Diffraction gratings
(260.3160) Physical optics : Interference

ToC Category:
Research Papers

History
Original Manuscript: July 19, 2004
Revised Manuscript: August 3, 2004
Published: August 23, 2004

Citation
Dong Jun Kang, Jin-Ki Kim, and Byeong-Soo Bae, "Simple fabrication of diffraction gratings by two-beam interference method in highly photosensitive hybrid sol-gel films," Opt. Express 12, 3947-3953 (2004)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-12-17-3947


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References

  1. V. Weiss, A. A. Friesem and V. A. Krongauz, �??Holographic recording and all-optical modulation in photochromic polymers,�?? Opt. Lett. 18, 1089-1091 (1993). [CrossRef] [PubMed]
  2. T. Huang and K. H. Wagner, �??Photoanisotropic incoherent-to-coherent optical conversion,�?? Appl. Opt 32, 1888-1900(1993). [CrossRef] [PubMed]
  3. Yariv, A and M. Nakamura, �??Periodic structures for integrated optics,�?? IEEE J. Quantum Electron. QE- 13, 233-253 (1977). [CrossRef]
  4. J. T. Rantala, R. S. Penner, S. Honkanen, J. Vähäkangas, M. Fallahi, and N. Peyghambarian, �??Negative tone hybrid sol-gel materials for electron-beam lithography,�?? Thin Solid Films 345, 185-187 (1999). [CrossRef]
  5. P. �?yräs, J. T. Rantala, S. Honkanen, S. B. Memdes, and N. Peyghambarian, �??Diffraction gratings in solgel films by direct contact printing using a UV-mercury lamp,�?? Opt. Commun. 162, 215-218 (1999). [CrossRef]
  6. H. J. Jiang, X.-C. Yuan, Y. Zhou, Y. C. Chan, and Y. L. Lam, �??Single-step fabrication of diffraction gratings on hybrid sol-gel glass using holographic interference lithography,�?? Opt. Commun. 185, 19-24 (2000) [CrossRef]
  7. N. Zettsu, T. Ubukata, T.Seki, and K. Ichimura, �??Soft Crosslinkable Azo Polymer for Rapid Surface Relief Formation and Persistent Fixation,�?? Adv. Mater. 13, 1693-1697 (2001). [CrossRef]
  8. N. K. Viswanathan, D. Y. Kim, S. Bian, J. Williams, W. Liu, L. Li, L. Samuelson, J. Kumar and S. K. Tripathy, �??Surface relief structures on azo polymer films,�?? J. Mater. Chem. 9, 1941-1955 (1999). [CrossRef]
  9. X.-C. Yuan, W. X. Yu, N. Q. Ngo, and W. C. Cheong, �??Cost-effective fabrication of microlenses on hybrid sol-gel glass with a high-energy beam-sensitive gray-scale mask,�?? Opt. Express 10, 303-308 (2002), <a href=" http://www.opticsexpress.org/abstract.cfm?URI=OPEX-10-7-303">http://www.opticsexpress.org/abstract.cfm?URI=OPEX-10-7-303</a> [CrossRef] [PubMed]
  10. W. X. Yu, X.-C. Yuan, N. Q. Ngo, W. X. Que, W. C. Cheong, and K.V. Koudriachov, �??Single step fabrication of continuous micro-optical elements in hybrid sol-gel glass by laser direct writing,�?? Opt..Express 10, 443-448 (2002), <a href="http://www.opticsexpress.org/abstract.cfm?URI=OPEX-10-10-443">http://www.opticsexpress.org/abstract.cfm?URI=OPEX-10-10-443</a> [CrossRef] [PubMed]
  11. D. J. Kang, J. U. Park, B. S. Bae, J. Nishii and K. Kintaka, "Single-Step Photo Patterning of Diffraction Gratings in Highly Photosensitive Hybrid Gel Films," Opt. Express 11, 1144-1148 (2003), <a href="http://www.opticsexpress.org/abstract.cfm?URI=OPEX-11-10-1144">http://www.opticsexpress.org/abstract.cfm?URI=OPEX-11-10-1144 [CrossRef] [PubMed]
  12. W. Yu and X. -. Yuan, "Variable surface profile gratings in sol-gel glass fabricated by holographic interference," Opt. Express 11, 1925-1930 (2003), <a href= "http://www.opticsexpress.org/abstract.cfm?URI=OPEX-11-16-1925">http://www.opticsexpress.org/abstract.cfm?URI=OPEX-11-16-1925</a> [CrossRef] [PubMed]
  13. O. H. Park, S. J. Kim and B. S. Bae, "Photochemical Reactions in Fluorinated Sol-Gel Hybrid Materials Doped with a Photolocking Agent for Direct Micropatterning," J. Mater. Chem. 14, 1749-1753 (2004). [CrossRef]
  14. O. H. Park, J. I. Jung, and B. S. Bae, "Photoinduced Condensation of Sol-Gel Hybrid Glass Films Doped with Benzildimethylketal,�?? J. Mater. Res. 16, 2143-2148 (2001). [CrossRef]
  15. B. S. Bae, O. H Park, R Charters, B Luther-Davies, and G. R. Atkins, �??Direct laser writing of selfdeveloped waveguides in benzyl dimethyl ketal doped sol-gel hybrid glass,�?? J. Mater. Res. 16, 3184-3187 (2001 [CrossRef]
  16. A. H. O. Kärkkäinen, J. M. Tamkin, J. D. Rogers, D. R. Neal, O. E. Hormi, G. E. Jabbour, J. T. Rantala, and M. R. Descour, �??Direct photolithographic deforming of organomodified siloxane films for microoptics fabrication,�?? Appl. Opt. 41, 3988 (2002). [CrossRef] [PubMed]

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