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Shape-controlled microlens arrays fabricated by diffuser lithography
Microelectronic Engineering 87 (2010) 1420–1423
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Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee
Shape-controlled microlens arrays fabricated by diffuser lithography Jie-Ming Kang a, Mau-Kuo Wei a,*, Hung-Yi Lin b, Juin-Haw Lee c, Hoang-Yan Lin c, Jen-Hui Tsai b, Tung-Chuan Wu b a
Department of Materials Science and Engineering, National Dong Hwa University, Hualien 974, Taiwan, Republic of China Mechanical and Systems Research Laboratories, Industrial Technology Research Institute, Hsinchu 310, Taiwan, Republic of China c Graduate Institute of Photonics and Optoelectronics, Department of Electrical Engineering, National Taiwan University, Taipei 106, Taiwan, Republic of China b
a r t i c l e
i n f o
Article history: Received 14 September 2009 Received in revised form 12 November 2009 Accepted 13 November 2009 Available online 1 December 2009 Keywords: Microlens Diffuser Photolithography Exposure dose Gap distance
a b s t r a c t In this study, we used diffuser lithography to fabricate microlens arrays with the height smaller or larger than the radius of curvature of microlenses. The experimental results showed that the fill factor of the duplicated microlens arrays can be adjusted not only by the gap distance of clear holes on the photomask but also by the UV exposure dose. The surface morphology of the duplicated microlenses can also be altered by the exposure dose, the gap distance, and the thickness of the photomask. For example, a spherical microlens array having a fill factor very near 100% could be fabricated by using a 10-lm gap distance and an exposure dose of 300 mJ/cm2. Crown Copyright Ó 2009 Published by Elsevier B.V. All rights reserved.
1. Introduction Microlens arrays had been widely used in the applications of fiber coupling [1], light gathering [2], and light extraction [3,4]. Therefore, many manufacture processes had been developed, such as photoresist melting (or thermal-reflow) [5,6], ink-jet printing [7,8], laser micromachining [9,10], proximity printing [11,12], gray-scale photolithography [13], and diffuser lithography [14,15]. Only spherical microlenses can be produced when using either the thermal-reflow or the ink-jet printing methods. Though spherical microlens arrays can also be made through proximity printing, the aspect ratio of microlenses is normally less than unity due to light diffraction near the edge of clear patterns on the photomask. Microlenses with complicated shapes can be easily fabricated by using either laser micromachining or gray-scale photolithography. However, the processing time is too long and the surface of microlenses is too rough for using laser micromachining. Besides, the gray-scale photomask costs extremely high. Therefore, Chang et al. [14,15] developed the diffuser lithography to fabricate both spherical and aspherical microlens arrays. Compared to a traditional photolithography, only an additional diffuser sheet is needed for the diffuser lithography. Thus, diffuser
* Corresponding author. Tel.: +886 3 8634221; fax: +886 3 8634200. E-mail address: [email protected] (M.-K. Wei).
lithography possesses many advantages, such as simple process, shape control of microlenses, and compatible to IC process. In this study, the influence of process parameters on the surface morphology of duplicated microlenses had been systematically studied. These process parameters included UV exposure dose, the gap distance between two neighboring clear holes on the photomask, and the thickness of the photomask. 2. Experimental methods Microlens arrays were fabricated by the combination of photolithography and molding techniques, as shown in Fig. 1. First, a 4inch (1 0 0) test-grade, p-typed silicon wafer was used as a substrate. Second, the photoresist AZ P4620 (from Clariant K. K. Technology Production Dept.) was spun on the wafer. The thickness of the photoresist was around 18 lm after soft baking. Third, a diffuser film BS-702 (from Taiwan Keiwa Inc.) was put on a photomask. There were clear hole-array patterns, having a diameter of 10 lm and different gap distances in the range of 8–30 lm, on the photomask. This photomask was then put on the photoresist to execute the soft-contact-mode printing using different UV exposure doses (200–800 mJ/cm2). Fourth, concave microlens arrays were formed after developing the photoresist. Fifth, liquid polydimethylsiloxane (PDMS, trade mark Sylgrad 184A) mixed with its hardener (trade mark Sylgrad 184B) was poured onto the wafer and put in an oven to be thermally cured at 65 °C for 4 h. Finally, a
0167-9317/$ - see front matter Crown Copyright Ó 2009 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2009.11.146
J.-M. Kang et al. / Microelectronic Engineering 87 (2010) 1420–1423
1421
bertian light source, when UV parallel light rays transmit just through this diffuser layer. That is, the intensity of light rays in any direction is equal. 3.1. Thickness of photomask
Fig. 1. Schematic process flow chart for making microlens arrays.
free-standing PDMS film with the duplicated convex microlens arrays on the film surface was formed after separating the film from the photoresist. The surface morphology of these duplicated microlenses was recorded by a scanning electron microscope (Hitachi, S3500).
3. Results and discussion The diffuser film BS-702 is composed of a polyester substrate, a light-diffusing layer and an adhesive layer. The light-diffusing layer is made of organic spherical particles with diameters smaller than tens of micrometers [16]. As shown in Fig. 2, they become a Lam-
Fig. 2. Schematic propagation of parallel light rays translating through a diffuser film and a photomask.
Here we consider two extremely different cases in diffuser lithography. If the thickness of photomask approaches zero, the intensity profile of exposure light on the photoresist layer should be spherical. The duplicated microlenses would thus be spherical. If the thickness of photomask approaches infinity, the intensity profile of exposure light on the photoresist layer would be the shape of a top-hat. The duplicated microstructures would be disks. In this topic, two different thicknesses of photomask were used: 5 and 10 mm. When a photomask, having a clear-hole-array pattern of diameter and gap distance of 10 and 20 lm respectively, was put on a 18-lm thick photoresist layer, followed by exposing a UV dose of 300 mJ/cm2, the surface morphology of the duplicated microlens array from this developed photoresist mold was illustrated in Fig. 3. It can be seen that the duplicated microlenses using the thinner photomask (5 mm) look more spherical, but look like a top-hat shape when using a thick photomask (10 mm). This result confirms the above illustration. Besides, the duplicated microlenses using the thinner photomask had a big lens height due to a large exposure dose. 3.2. Exposure dose In general, positive photoresist have an absorptivity of around 40%. The photoresist molecules are ruptured after the attack of photo-induced radicals or acids. The photoresist is then bleached after the scission of the molecules. That is, the photoresist molecules are transparent to the UV light. Therefore, the depth of developed photoresist patterns increases with increasing the exposure dose, until it reaches the thickness of the photoresist. Fig. 4 shows the surface morphology of the duplicated microlens arrays when using the same gap distance of 15 lm, but under different UV exposure doses. The thickness of the used photomask was 5 mm. When the UV exposure dose was smaller than 200 mJ/ cm2, no patterns were formed after developing the photoresist. When the UV exposure dose increased to 300 mJ/cm2, the duplicated microlenses were spherical and greater than a hemisphere. As the exposure dose increased, the duplicated microlenses turned to cone-like microstructures. Besides, the diameter of base region and the height of microlenses increased with increasing the UV exposure dose. As the exposure dose reached 600–700 mJ/cm2, the base-connected cone-like microlens arrays were formed. Therefore, the UV exposure dose cannot only alter the surface morphology of the duplicated microlenses, but also change the fill factor of microlens array.
Fig. 3. The microphotographs of the duplicated microlenses when using the same exposure dose and gap distance of 300 mJ/cm2 and 20 lm respectively, but different thicknesses of photomask.
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Fig. 4. The microphotographs of the duplicated microlenses when using the same gap distance of 15 lm but different exposure doses.
Fig. 5. The microphotographs of the duplicated microlenses when using the same exposure dose of 200 mJ/cm2 but different gap distances.
J.-M. Kang et al. / Microelectronic Engineering 87 (2010) 1420–1423
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4. Conclusions In this study, the UV exposure dose, the gap distance between two neighboring holes on the photomask, and the thickness of the photomask in the diffuser lithography had been systematically studied. When the UV exposure dose was smaller than 200 mJ/cm2, no patterns were formed after the development of photoresist. Spherical microlens arrays can be fabricated by using small UV exposure doses (200–300 mJ/cm2) and large gap distances (15– 30 lm). But cone-like microlenses can be manufactured by using large UV exposure doses and small gap distances. Microlens arrays having high fill factor can be made by using large exposure doses or small gap distances. Acknowledgements Fig. 6. Morphology of one fabricated microlens array (gap distance: 10 lm, exposure dose: 300 mJ/cm2, tilt angle: 60°).
3.3. Gap distance For the light-extraction purpose, high fill factor of microlens array is wanted. The reasonable way to increase the fill factor is to decrease the gap distance of microlenses. Fig. 5 depicts the surface morphology of the duplicated microlens arrays when using the same exposure dose of 200 mJ/cm2, but different gap distances of clear holes on the photomask. The thickness of the photomask was 5 mm. As the gap distance of clear holes on the photomask decreased, the gap distance of duplicated microlenses also decreased, but the fill factor of microlenses increased. As the gap distance of clear holes on the photomask approached 8 lm, the base regions of microlenses contacted each other. It can also be seen that the height of the duplicated microlenses increased with increasing the gap distance of clear holes on the photomask. Besides, the microlenses get more spherical with increasing the gap distance. The reason for these results is not clear. In summary, high fill-factor microlens arrays could be made by adjusting the proper process parameters of diffuser lithography. For example, a microlens array having a fill factor close to 100% (Fig. 6) could be obtained by using the UV exposure dose of 300 mJ/cm2 and a 10-lm gap distance of clear holes on the photomask.
The authors gratefully acknowledge the financial support given by the National Science Council of the Republic of China under the projects of No. NSC 98-2221-E-259-003 and NSC 98-2622-E-259001-CC3. References [1] J.J. Yang, C.-F. Chen, Y.-S. Liao, Opt. Commun. 281 (2008) 474–479. [2] H. Mutoh, IEEE Trans. Electron Devices 50 (2003) 19–25. [3] J.-H. Lee, Y.-H. Ho, K.-Y. Chen, H.-Y. Lin, J.-H. Fang, S.-C. Hsu, J.-R. Lin, M.-K. Wei, Opt. Express 16 (2008) 21184–21190. [4] M.-K. Wei, H.-Y. Lin, J.-H. Lee, K.-Y. Chen, Y.-H. Ho, C.-C. Lin, C.-F. Wu, H.-Y. Lin, J.-H. Tsai, T.-C. Wu, Opt. Commun. 281 (2008) 5625–5632. [5] S. Galixto, M. Ornelas-Rodriguez, Opt. Lett. 24 (1999) 1212–1214. [6] M.-K. Wei, J.-H. Lee, H.-Y. Lin, Y.-H. Ho, K.-Y. Chen, C.-C. Lin, C.-F. Wu, H.-Y. Lin, J.-H. Tsai, T.-C. Wu, J. Opt. A: Pure Appl. Opt. 10 (2008) 055302. [7] D.L. MacFarlane, V. Narayan, J.A. Tatum, W.R. Cox, T. Chen, D.J. Hayes, IEEE Photonics Technol. Lett. 6 (1994) 1112–1114. [8] F.-C. Chen, J.-P. Lu, W.-K. Huang, IEEE Photonics Technol. Lett. 21 (2009) 648– 650. [9] A.Y. Smuk, N.M. Lawandy, J. Appl. Phys. 87 (2000) 4026–4030. [10] H. Takao, H. Miyagami, M. Okoshi, N. Inoue, Jpn. J. Appl. Phys. 44 (2005) 1808– 1811. [11] C.-P. Lin, H. Yang, C.-K. Chao, J. Micromech. Microeng. 13 (2003) 748–757. [12] T.-H. Lin, H. Yang, C.-K. Chao, S.-Y. Hung, Microsyst. Technol. 15 (2009) 1255– 1261. [13] W. Yu, X.-C. Yuan, Opt. Express 11 (2003) 2253–2258. [14] S.-I. Chang, J.-B. Yoon, Opt. Express 12 (2004) 6366–6371. [15] S.-I. Chang, J.-B. Yoon, H. Kim, J.-J. Kim, B.-K. Lee, D.H. Shin, Opt. Lett. 31 (2006) 3010–3018. [16] H.-Y. Lin, J.-H. Lee, M.-K. Wei, C.-L. Dai, C.-F. Wu, Y.-H. Ho, H.-Y. Lin, T.-C. Wu, Opt. Commun. 275 (2007) 464–469.
Contents lists available at ScienceDirect
Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee
Shape-controlled microlens arrays fabricated by diffuser lithography Jie-Ming Kang a, Mau-Kuo Wei a,*, Hung-Yi Lin b, Juin-Haw Lee c, Hoang-Yan Lin c, Jen-Hui Tsai b, Tung-Chuan Wu b a
Department of Materials Science and Engineering, National Dong Hwa University, Hualien 974, Taiwan, Republic of China Mechanical and Systems Research Laboratories, Industrial Technology Research Institute, Hsinchu 310, Taiwan, Republic of China c Graduate Institute of Photonics and Optoelectronics, Department of Electrical Engineering, National Taiwan University, Taipei 106, Taiwan, Republic of China b
a r t i c l e
i n f o
Article history: Received 14 September 2009 Received in revised form 12 November 2009 Accepted 13 November 2009 Available online 1 December 2009 Keywords: Microlens Diffuser Photolithography Exposure dose Gap distance
a b s t r a c t In this study, we used diffuser lithography to fabricate microlens arrays with the height smaller or larger than the radius of curvature of microlenses. The experimental results showed that the fill factor of the duplicated microlens arrays can be adjusted not only by the gap distance of clear holes on the photomask but also by the UV exposure dose. The surface morphology of the duplicated microlenses can also be altered by the exposure dose, the gap distance, and the thickness of the photomask. For example, a spherical microlens array having a fill factor very near 100% could be fabricated by using a 10-lm gap distance and an exposure dose of 300 mJ/cm2. Crown Copyright Ó 2009 Published by Elsevier B.V. All rights reserved.
1. Introduction Microlens arrays had been widely used in the applications of fiber coupling [1], light gathering [2], and light extraction [3,4]. Therefore, many manufacture processes had been developed, such as photoresist melting (or thermal-reflow) [5,6], ink-jet printing [7,8], laser micromachining [9,10], proximity printing [11,12], gray-scale photolithography [13], and diffuser lithography [14,15]. Only spherical microlenses can be produced when using either the thermal-reflow or the ink-jet printing methods. Though spherical microlens arrays can also be made through proximity printing, the aspect ratio of microlenses is normally less than unity due to light diffraction near the edge of clear patterns on the photomask. Microlenses with complicated shapes can be easily fabricated by using either laser micromachining or gray-scale photolithography. However, the processing time is too long and the surface of microlenses is too rough for using laser micromachining. Besides, the gray-scale photomask costs extremely high. Therefore, Chang et al. [14,15] developed the diffuser lithography to fabricate both spherical and aspherical microlens arrays. Compared to a traditional photolithography, only an additional diffuser sheet is needed for the diffuser lithography. Thus, diffuser
* Corresponding author. Tel.: +886 3 8634221; fax: +886 3 8634200. E-mail address: [email protected] (M.-K. Wei).
lithography possesses many advantages, such as simple process, shape control of microlenses, and compatible to IC process. In this study, the influence of process parameters on the surface morphology of duplicated microlenses had been systematically studied. These process parameters included UV exposure dose, the gap distance between two neighboring clear holes on the photomask, and the thickness of the photomask. 2. Experimental methods Microlens arrays were fabricated by the combination of photolithography and molding techniques, as shown in Fig. 1. First, a 4inch (1 0 0) test-grade, p-typed silicon wafer was used as a substrate. Second, the photoresist AZ P4620 (from Clariant K. K. Technology Production Dept.) was spun on the wafer. The thickness of the photoresist was around 18 lm after soft baking. Third, a diffuser film BS-702 (from Taiwan Keiwa Inc.) was put on a photomask. There were clear hole-array patterns, having a diameter of 10 lm and different gap distances in the range of 8–30 lm, on the photomask. This photomask was then put on the photoresist to execute the soft-contact-mode printing using different UV exposure doses (200–800 mJ/cm2). Fourth, concave microlens arrays were formed after developing the photoresist. Fifth, liquid polydimethylsiloxane (PDMS, trade mark Sylgrad 184A) mixed with its hardener (trade mark Sylgrad 184B) was poured onto the wafer and put in an oven to be thermally cured at 65 °C for 4 h. Finally, a
0167-9317/$ - see front matter Crown Copyright Ó 2009 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2009.11.146
J.-M. Kang et al. / Microelectronic Engineering 87 (2010) 1420–1423
1421
bertian light source, when UV parallel light rays transmit just through this diffuser layer. That is, the intensity of light rays in any direction is equal. 3.1. Thickness of photomask
Fig. 1. Schematic process flow chart for making microlens arrays.
free-standing PDMS film with the duplicated convex microlens arrays on the film surface was formed after separating the film from the photoresist. The surface morphology of these duplicated microlenses was recorded by a scanning electron microscope (Hitachi, S3500).
3. Results and discussion The diffuser film BS-702 is composed of a polyester substrate, a light-diffusing layer and an adhesive layer. The light-diffusing layer is made of organic spherical particles with diameters smaller than tens of micrometers [16]. As shown in Fig. 2, they become a Lam-
Fig. 2. Schematic propagation of parallel light rays translating through a diffuser film and a photomask.
Here we consider two extremely different cases in diffuser lithography. If the thickness of photomask approaches zero, the intensity profile of exposure light on the photoresist layer should be spherical. The duplicated microlenses would thus be spherical. If the thickness of photomask approaches infinity, the intensity profile of exposure light on the photoresist layer would be the shape of a top-hat. The duplicated microstructures would be disks. In this topic, two different thicknesses of photomask were used: 5 and 10 mm. When a photomask, having a clear-hole-array pattern of diameter and gap distance of 10 and 20 lm respectively, was put on a 18-lm thick photoresist layer, followed by exposing a UV dose of 300 mJ/cm2, the surface morphology of the duplicated microlens array from this developed photoresist mold was illustrated in Fig. 3. It can be seen that the duplicated microlenses using the thinner photomask (5 mm) look more spherical, but look like a top-hat shape when using a thick photomask (10 mm). This result confirms the above illustration. Besides, the duplicated microlenses using the thinner photomask had a big lens height due to a large exposure dose. 3.2. Exposure dose In general, positive photoresist have an absorptivity of around 40%. The photoresist molecules are ruptured after the attack of photo-induced radicals or acids. The photoresist is then bleached after the scission of the molecules. That is, the photoresist molecules are transparent to the UV light. Therefore, the depth of developed photoresist patterns increases with increasing the exposure dose, until it reaches the thickness of the photoresist. Fig. 4 shows the surface morphology of the duplicated microlens arrays when using the same gap distance of 15 lm, but under different UV exposure doses. The thickness of the used photomask was 5 mm. When the UV exposure dose was smaller than 200 mJ/ cm2, no patterns were formed after developing the photoresist. When the UV exposure dose increased to 300 mJ/cm2, the duplicated microlenses were spherical and greater than a hemisphere. As the exposure dose increased, the duplicated microlenses turned to cone-like microstructures. Besides, the diameter of base region and the height of microlenses increased with increasing the UV exposure dose. As the exposure dose reached 600–700 mJ/cm2, the base-connected cone-like microlens arrays were formed. Therefore, the UV exposure dose cannot only alter the surface morphology of the duplicated microlenses, but also change the fill factor of microlens array.
Fig. 3. The microphotographs of the duplicated microlenses when using the same exposure dose and gap distance of 300 mJ/cm2 and 20 lm respectively, but different thicknesses of photomask.
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Fig. 4. The microphotographs of the duplicated microlenses when using the same gap distance of 15 lm but different exposure doses.
Fig. 5. The microphotographs of the duplicated microlenses when using the same exposure dose of 200 mJ/cm2 but different gap distances.
J.-M. Kang et al. / Microelectronic Engineering 87 (2010) 1420–1423
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4. Conclusions In this study, the UV exposure dose, the gap distance between two neighboring holes on the photomask, and the thickness of the photomask in the diffuser lithography had been systematically studied. When the UV exposure dose was smaller than 200 mJ/cm2, no patterns were formed after the development of photoresist. Spherical microlens arrays can be fabricated by using small UV exposure doses (200–300 mJ/cm2) and large gap distances (15– 30 lm). But cone-like microlenses can be manufactured by using large UV exposure doses and small gap distances. Microlens arrays having high fill factor can be made by using large exposure doses or small gap distances. Acknowledgements Fig. 6. Morphology of one fabricated microlens array (gap distance: 10 lm, exposure dose: 300 mJ/cm2, tilt angle: 60°).
3.3. Gap distance For the light-extraction purpose, high fill factor of microlens array is wanted. The reasonable way to increase the fill factor is to decrease the gap distance of microlenses. Fig. 5 depicts the surface morphology of the duplicated microlens arrays when using the same exposure dose of 200 mJ/cm2, but different gap distances of clear holes on the photomask. The thickness of the photomask was 5 mm. As the gap distance of clear holes on the photomask decreased, the gap distance of duplicated microlenses also decreased, but the fill factor of microlenses increased. As the gap distance of clear holes on the photomask approached 8 lm, the base regions of microlenses contacted each other. It can also be seen that the height of the duplicated microlenses increased with increasing the gap distance of clear holes on the photomask. Besides, the microlenses get more spherical with increasing the gap distance. The reason for these results is not clear. In summary, high fill-factor microlens arrays could be made by adjusting the proper process parameters of diffuser lithography. For example, a microlens array having a fill factor close to 100% (Fig. 6) could be obtained by using the UV exposure dose of 300 mJ/cm2 and a 10-lm gap distance of clear holes on the photomask.
The authors gratefully acknowledge the financial support given by the National Science Council of the Republic of China under the projects of No. NSC 98-2221-E-259-003 and NSC 98-2622-E-259001-CC3. References [1] J.J. Yang, C.-F. Chen, Y.-S. Liao, Opt. Commun. 281 (2008) 474–479. [2] H. Mutoh, IEEE Trans. Electron Devices 50 (2003) 19–25. [3] J.-H. Lee, Y.-H. Ho, K.-Y. Chen, H.-Y. Lin, J.-H. Fang, S.-C. Hsu, J.-R. Lin, M.-K. Wei, Opt. Express 16 (2008) 21184–21190. [4] M.-K. Wei, H.-Y. Lin, J.-H. Lee, K.-Y. Chen, Y.-H. Ho, C.-C. Lin, C.-F. Wu, H.-Y. Lin, J.-H. Tsai, T.-C. Wu, Opt. Commun. 281 (2008) 5625–5632. [5] S. Galixto, M. Ornelas-Rodriguez, Opt. Lett. 24 (1999) 1212–1214. [6] M.-K. Wei, J.-H. Lee, H.-Y. Lin, Y.-H. Ho, K.-Y. Chen, C.-C. Lin, C.-F. Wu, H.-Y. Lin, J.-H. Tsai, T.-C. Wu, J. Opt. A: Pure Appl. Opt. 10 (2008) 055302. [7] D.L. MacFarlane, V. Narayan, J.A. Tatum, W.R. Cox, T. Chen, D.J. Hayes, IEEE Photonics Technol. Lett. 6 (1994) 1112–1114. [8] F.-C. Chen, J.-P. Lu, W.-K. Huang, IEEE Photonics Technol. Lett. 21 (2009) 648– 650. [9] A.Y. Smuk, N.M. Lawandy, J. Appl. Phys. 87 (2000) 4026–4030. [10] H. Takao, H. Miyagami, M. Okoshi, N. Inoue, Jpn. J. Appl. Phys. 44 (2005) 1808– 1811. [11] C.-P. Lin, H. Yang, C.-K. Chao, J. Micromech. Microeng. 13 (2003) 748–757. [12] T.-H. Lin, H. Yang, C.-K. Chao, S.-Y. Hung, Microsyst. Technol. 15 (2009) 1255– 1261. [13] W. Yu, X.-C. Yuan, Opt. Express 11 (2003) 2253–2258. [14] S.-I. Chang, J.-B. Yoon, Opt. Express 12 (2004) 6366–6371. [15] S.-I. Chang, J.-B. Yoon, H. Kim, J.-J. Kim, B.-K. Lee, D.H. Shin, Opt. Lett. 31 (2006) 3010–3018. [16] H.-Y. Lin, J.-H. Lee, M.-K. Wei, C.-L. Dai, C.-F. Wu, Y.-H. Ho, H.-Y. Lin, T.-C. Wu, Opt. Commun. 275 (2007) 464–469.