micro lens fabrication by means of femtosecond two photon photopolymerization

7
Micro lens fabrication by means of femtosecond two photon photopolymerization Rui Guo 1 , Shizhou Xiao 1 , Xiaomin Zhai 1 , Jiawen Li 1 , Andong Xia 1 , 2* and Wenhao Huang 1* 1 Department of Precision Machinery and Precision Instrumentation, University of Science and Technology of China, Hefei 230026, P. R. China 2 State Key Laboratory of Molecular Reaction Dynamics, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, P. R. China [email protected] ; [email protected] Abstract: We report the fabrication of micro lens using an alternative annular scanning mode with continuous variable layer thickness by two- photon polymerization after multi-parameter optimization. Laser scanning mode and scanning pace parameter are optimized to achieve good appearance. As examples of the results, a 2 × 2 micro spherical lens array with diameter of 15 m and a micro Fresnel lens with diameter of 17 m are fabricated. Their optical performances are also tested. Compared to the conventional femtosecond two-photon fabrication, this work provides an alternative, effective and cheap processing method for the fabrication of micro optic device that requires arbitrary shape with high surface quality and small scale. ©2006 Optical Society of America OCIS codes: (220.4000) Microstructure fabrication; ( 320.7090) Ultrafast lasers; (350.3950) Micro-optics. References and links 1. T. J. Suleskiand and R. D. TeKolste,” Fabrication Trends for Free-Space Microoptics,” J. Lightwave Technol. 23, 633 (2005) 2. P. Nussbaum, R. Völkel, H. P. Herzig, M. Eisner and S. Haselbeck,“ Design, fabrication and testing of microlens arrays for sensors and Microsystems,“ Pure Appl. Opt. 6, 617 (1997) 3. S. Sugiyama, S. Khumpuang and G. Kawaguchi,” Plain-pattern to cross-section transfer (PCT) technique for deep x-ray lithography and applications,” J. Micromech. Microeng. 14,1399 (2004) 4. J. Yao, Z. Cui, F. Gao, Y. Zhang, Y. Guo, C. Du, H. Zeng and C. Qiu,” Refractive micro lens array made of dichromate gelatin with gray-tone photolithography,” Microelectron. Eng. 57-58, 729 (2001) 5. M. He, X. C. Yuan, N. Q. Ngo, J. Bu and S. H. Tao,” Single-step fabrication of a microlens array in sol–gel material by direct laser writing and its application in optical coupling,” J. Opt. A: Pure Appl. Opt. 6, 94 (2004). 6. S. Kawata, H. B. Sun, T. Tanaka and K. Takada,” Finer features for functional microdevices,” Nature 412, 697 (2001). 7. B. H. Cumpston, S. P. Ananthavel, S. Barlow, D. L. Dyer, J. E. Ehrlich, L. L. Erskine, A. A. Heikal, S. M. Kuebler, I. Y. S. Lee, D. McCord-Maughon, J. Qin, H. Röckel, M. Rumi, X. L. Wu, S. R. Marder and J. W. Perry,” Two-photon polymerization initiators for threedimensional optical data storage and microfabrication,” Nature 398, 51 (1999). 8. Z. W. Jiang, Y. J. Zhou, D. J. Yuan, W. H. Huang and A. D. Xia,” A Two-Photon Femtosecond Laser System for Three-Dimensional Microfabrication and Data Storage,” Chin. Phys. Lett. 20, 2126 (2003). 9. R. Guo, Z. Y. Li,Z. W. Jiang, D. J. Yuan, W. H. Huang and A. D. Xia,” Log-pile photonic crystal fabricated by two-photon photopolymerization,” J. Opt. A: Pure Appl. Opt. 7, 396 (2005) 10. H. B. Sun, M. Maeda, K. Takada, James W. M. Chon, M. Gu and S. Kawata,” Experimental investigation of single voxels for laser nanofabrication via two-photon photopolymerization,” Appl. Phys. Lett. 83, 819 (2003). 11. J. Serbin and B. Chichkov,“ High-resolution direct-write femtosecond laser technologies,“ in Solid State Laser Technologies and Femtosecond Phenomena, Jonathan A. C. Terry, W. Andrew Clarkson, eds. Proc. SPIE 5620,245-251 (2004). 12. V. R. Bhardwaj, P. B. Corkum, D. M. Rayner, C. Hnatovsky, E. Simova and R. S. Taylor,” Stress in femtosecond-laser-written waveguides in fused silica,” Opt.Lett. 29, 1312 (2004) (C) 2006 OSA 23 January 2006 / Vol. 14, No. 2 / OPTICS EXPRESS 810 #9879 - $15.00 USD Received 6 December 2005; revised 9 January 2006; accepted 14 January 2006

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Micro lens fabrication by means of femtosecond two photon photopolymerization

Rui Guo1, Shizhou Xiao1, Xiaomin Zhai1, Jiawen Li1, Andong Xia1,2* and Wenhao Huang1*

1Department of Precision Machinery and Precision Instrumentation, University of Science and Technology of China, Hefei 230026, P. R. China

2State Key Laboratory of Molecular Reaction Dynamics, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, P. R. China

[email protected]; [email protected]

Abstract: We report the fabrication of micro lens using an alternative annular scanning mode with continuous variable layer thickness by two-photon polymerization after multi-parameter optimization. Laser scanning mode and scanning pace parameter are optimized to achieve good appearance. As examples of the results, a 2 × 2 micro spherical lens array with diameter of 15 μm and a micro Fresnel lens with diameter of 17 μm are fabricated. Their optical performances are also tested. Compared to the conventional femtosecond two-photon fabrication, this work provides an alternative, effective and cheap processing method for the fabrication of micro optic device that requires arbitrary shape with high surface quality and small scale.

©2006 Optical Society of America

OCIS codes: (220.4000) Microstructure fabrication; ( 320.7090) Ultrafast lasers; (350.3950) Micro-optics.

References and links

1. T. J. Suleskiand and R. D. TeKolste,” Fabrication Trends for Free-Space Microoptics,” J. Lightwave Technol. 23, 633 (2005)

2. P. Nussbaum, R. Völkel, H. P. Herzig, M. Eisner and S. Haselbeck,“ Design, fabrication and testing of microlens arrays for sensors and Microsystems,“ Pure Appl. Opt. 6, 617 (1997)

3. S. Sugiyama, S. Khumpuang and G. Kawaguchi,” Plain-pattern to cross-section transfer (PCT) technique for deep x-ray lithography and applications,” J. Micromech. Microeng. 14,1399 (2004)

4. J. Yao, Z. Cui, F. Gao, Y. Zhang, Y. Guo, C. Du, H. Zeng and C. Qiu,” Refractive micro lens array made of dichromate gelatin with gray-tone photolithography,” Microelectron. Eng. 57-58, 729 (2001)

5. M. He, X. C. Yuan, N. Q. Ngo, J. Bu and S. H. Tao,” Single-step fabrication of a microlens array in sol–gel material by direct laser writing and its application in optical coupling,” J. Opt. A: Pure Appl. Opt. 6, 94 (2004).

6. S. Kawata, H. B. Sun, T. Tanaka and K. Takada,” Finer features for functional microdevices,” Nature 412, 697 (2001).

7. B. H. Cumpston, S. P. Ananthavel, S. Barlow, D. L. Dyer, J. E. Ehrlich, L. L. Erskine, A. A. Heikal, S. M. Kuebler, I. Y. S. Lee, D. McCord-Maughon, J. Qin, H. Röckel, M. Rumi, X. L. Wu, S. R. Marder and J. W. Perry,” Two-photon polymerization initiators for threedimensional optical data storage and microfabrication,” Nature 398, 51 (1999).

8. Z. W. Jiang, Y. J. Zhou, D. J. Yuan, W. H. Huang and A. D. Xia,” A Two-Photon Femtosecond Laser System for Three-Dimensional Microfabrication and Data Storage,” Chin. Phys. Lett. 20, 2126 (2003).

9. R. Guo, Z. Y. Li,Z. W. Jiang, D. J. Yuan, W. H. Huang and A. D. Xia,” Log-pile photonic crystal fabricated by two-photon photopolymerization,” J. Opt. A: Pure Appl. Opt. 7, 396 (2005)

10. H. B. Sun, M. Maeda, K. Takada, James W. M. Chon, M. Gu and S. Kawata,” Experimental investigation of single voxels for laser nanofabrication via two-photon photopolymerization,” Appl. Phys. Lett. 83, 819 (2003).

11. J. Serbin and B. Chichkov,“ High-resolution direct-write femtosecond laser technologies,“ in Solid State Laser Technologies and Femtosecond Phenomena, Jonathan A. C. Terry, W. Andrew Clarkson, eds. Proc. SPIE 5620,245-251 (2004).

12. V. R. Bhardwaj, P. B. Corkum, D. M. Rayner, C. Hnatovsky, E. Simova and R. S. Taylor,” Stress in femtosecond-laser-written waveguides in fused silica,” Opt.Lett. 29, 1312 (2004)

(C) 2006 OSA 23 January 2006 / Vol. 14, No. 2 / OPTICS EXPRESS 810#9879 - $15.00 USD Received 6 December 2005; revised 9 January 2006; accepted 14 January 2006

13. N. Takeshima, Y. Narita, S. Tanaka, Y. Kuroiwa, K. Hirao,” Fabrication of high-efficiency diffraction gratings in glass,” Opt.Lett. 30, 352 (2005)

14. Q. Z. Zhao, J. R. Qiu, X. W. Jiang, E. W. Dai, C. H. Zhou and C. S. Zhu,” Direct writing computer-generated holograms on metal film by an infrared femtosecond laser,” Opt. Express 13, 2089 (2005).

15. H. B. Sun, S. Matsuo and H. Misawa,” Three-dimensional photonic crystal structures achieved with two-photon-absorption photopolymerization of resin,” Appl. Phys. Lett. 74, 786 (1999).

16. K. Kaneko, H. B. Sun, X. M. Duan and S. Kawata,” Submicron diamond-lattice photonic crystals produced by two-photon laser nanofabrication,” Appl. Phys. Lett. 83, 2091 (2003).

17. M. Deubel, G. von Freymann, M. Wegener, S. Pereira, K. Busch and C. M. Soukoulis,“ Direct laser writing of three-dimensional photonic-crystal templates for telecommunications,“ Nature Mater. 3, 444 (2004).

18. J. Serbin, A. Ovsianikov, and B. Chichkov,” Fabrication of woodpile structures by two-photon polymerization and investigation of their optical properties,” Opt.Express 12, 5221 (2004).

19. H. F. Jiu, H. H. Tang, J. L. Zhou, J. Xu, Q. J. Zhang, H. Xing, W. H. Huang and A. D. Xia,”Sm(DBM)3Phen -doped poly(methyl methacrylate) for three-dimensional multilayered optical memory,” Opt. Lett. 30, 774 (2005)

20. J. Kato, N. Takeyasu, Y. Adachi, H. B. Sun, and S. Kawata,” Multiple-spot parallel processing for laser micronanofabrication,” Appl. Phys. Lett. 86, 044102 (2005)

21. M. Straub, M. Ventura, and M. Gu,” Multiple Higher-Order Stop Gaps in Infrared Polymer Photonic Crystals,” Phys. Rev. Lett. 91, 043901 (2003)

22. N. Takeshima, Y.Narita, T. Nagata, S. Tanaka and K. Hirao,” Fabrication of photonic crystals in ZnS-doped glass,” Opt.Lett. 30,537 (2005)

23. K. Takada, H. B. Sun, and S. Kawata,” Improved spatial resolution and surface roughness in photopolymerization-based laser nanowriting,” Appl. Phys. Lett. 86, 071122 (2005).

24. R. Guo,S. Z. Xiao,H. Xing,W. H. Huang and A. D. Xia,” Effect of overlapping degree to surface roughness in a femtosecond laser micro fabrication,” Photonics China 6, 9 (2004)

25. W . Watanabe, D. Kuroda, K . Itoh and J. Nishii,” Fabrication of Fresnel zone plate embedded in silica glass by femtosecond laser pulses,” Opt. Express 10, 9 78 (2002).

26. M. Born and E. Wolf, Principles of Optics (Cambridge, 1999). 27. H. Nishihara and T. Suhara,Micro Fresnel Lenses Progress in Optics(Elsevier Science, Amsterdam, 1987). 28. D. F. Vanderwerf,” Ghost-image analysis of Fresnel lens doublet,” in Stray Radiation in Optical Systems,

Robert P. Breault, eds. Proc. SPIE 1331, 143-157 (1990)

1. Introduction

Recent years micro-optics is gaining increasing attention in many fields, such as micro imaging, micro focus, beam shaping and so on [1]. Nowadays, micro lens are fabricated by various kinds of techniques, such as the photoresist reflow (PR) technique [2], moving pattern (MR) lithography [3], gray-tone (GT) photolithography [4] and laser direct writing (LDW) [5]. PR and MR techniques could fabricate cheap micro lens array, but it is difficult to achieve micro lens with arbitrary shape such as aspheric lens. GT technique has the ability to make designed shape micro lens, while it required special encoded photolithographic patterns, which would increase the cost greatly. Classical LDW could not meet the demand of micro lens with complex shape and fine structure down to micro- and nano- meter scales.

Tightly focused femtosecond laser, which can induce two-photon polymerization, has been considered as one of the potential techniques for three-dimensional micro- and nano- fabrications [6-9]. An 120 nm or higher resolution has been reported in ref. [10] by means of the femtosecond two-photon polymerization. This technology is suitable for the fabrication of three-dimensional and arbitrary designed microstructures. More importantly, it could directly locate the microstructure on a target object such as at the end of optical fiber [11]. Several micro optical devices such as waveguide in fused silicon, diffraction gratings in glass and holograms by ablation of metal film were reported by femtosecond laser microfabrication technology [12-14]. A three-dimensional photonic crystal, one of the most important micro optic devices, was also achieved by femtosecond laser two photon polymerization [9, 15-18]. However, for the practical application of micro optical devices such as micro lens, high quality surface profile and smoothness are generally required. Therefore, considering the complexity of the spherical surface, it is still a challenging theme to develop new method for the fabrication of the arbitrary 3D microstructure with high quality surface smoothness by two-photon polymerization.

(C) 2006 OSA 23 January 2006 / Vol. 14, No. 2 / OPTICS EXPRESS 811#9879 - $15.00 USD Received 6 December 2005; revised 9 January 2006; accepted 14 January 2006

Unlike the parallel linear scanning mode employed for femtosecond two-photon polymerization [8,9,15,17,18], in this paper, we manage to employ the femtosecond two-photon polymerization for fabricating micro lens and micro optical element by using the annular scanning mode cooperated with continuous variable scanning spaces. During experiments, some detail processing parameters such as the scanning mode and scanning step space, which may cause big affect to the fabricated micro lens qualities, are optimized through both theoretical simulations and experimental tests. A micro spherical lens array of 2 × 2 with each lens diameter of 15 μm, and a micro Fresnel lens with diameter of 17 μm are obtained. Finally, the optical properties of fabricated micro optics are also tested.

2. Materials and methods

A Ti: sapphire pulsed laser oscillator with 80 fs, 80 MHz, 400 mW and 800 nm was used for two-photon polymerization. The setup for the fabrication is used as reported as before [8,9,19]. A commercial resin (SCR500, JSR), which can produce two-photon polymerization induced by focused 800 nm femtosecond laser effectively, is used for fabrication [6]. The refractive indexes of the resin before and after polymerization are about 1.52 and 1.53, respectively [20]. The femtosecond laser intensity dependent two-photon polymerization with SCR-500 resin has been reported in our previous works [8]. A resolution of 200 nm in lateral direction and 700 nm in longitudinal direction has been achieved before in our lab [8,9].

The test system for optical property is equipped with a microscope, as shown in Fig. 1. The focus length of lens A is 10 cm, and the object plane was set on the focal plane. The micro lens sample was about 20 cm away from the lens A. An inverted microscope was used for the detection. Their positions can also be readout from the CCD attached microscope.

3. Results and discussion

Femtosecond two-photon microfabrication technology is a kind of laser direct writing process, which generally employs the parallel linear scanning mode during the fabrication, and has been proved to be very suitable for most of three-dimensional microstructures such as log-pile photonic crystal [9,15,17,18,21,22]. As micro lens is spherical symmetry structure, it is hard to get a real circle shape with high quality surface profile using the conventional parallel linear scanning mode. In fact, during parallel linear scanning, it can be imagined that the width of the fabricated line will make not only the deformation of circle dimension but also some protrusion on the edge of the lens surface as shown in Fig. 2(a). Unlike the conventional line scanning mode for the femtosecond two-photon fabrication [8,9,15,17,18], in order to obtain smooth-shaped micro lens with better optical performance by two-photon polymerization, some important fabrication parameters for a new scanning method should be analyzed first. In this work, we adopt an annular scanning path. Annular scanning path is able to ensure the lens’s orbicular symmetry and radial consistency as shown in Fig. 2(b). In order to obtain smooth-shaped micro lens with good optical property, besides the annular scanning mode, two other fabrication parameters are also important for obtaining high quality of micro optics, which should be taken into account during fabrication. They are: layer space and lateral step pace.

As a structure realized through point-by-point process, the fabrication lateral step pace is reasonably considered as the prime factor to affect the surface quality [23,24]. Surface roughness is one of the important parameter of the surface quality. To show the surface roughness of fabricated surface after two-photon polymerization, a series of structures of

10μm×10μm surface are polymerized with different lateral step space as shown in Fig. 3. The experimental results are evaluated by WYKO NT1000 (Veeco Metrology Group). The surface roughness (Ra) falls rapidly from 90 to 15 nm as the step pace decreased from 400 to 100 nm. The ideal surface roughness (Ra) 15 nm generated with 100 nm lateral step space is suitable for micro optic device.

(C) 2006 OSA 23 January 2006 / Vol. 14, No. 2 / OPTICS EXPRESS 812#9879 - $15.00 USD Received 6 December 2005; revised 9 January 2006; accepted 14 January 2006

Fig. 1 The diagram of the evaluation system for micro lens optical performance.

Considering that the three dimensional structure of micro lens is fabricated by a layer-by- layer process, we give further analysis on the layer space Delta Z used for fabrication, which also influences on micro lens quality. It is found that a fixed constant layer space Delta Z would introduce serious bench especially at the top part of the lens during fabrication, as shown in Fig. 2(b). In order to offset the surface bench and ameliorate the device’s quality, we adopt dynamically Delta Z to ensure the bench effect is under control according to the scanning space lx. The height of layer i can be followed by the formulation 2 2 1/ 2[ ( ) ]iZ R i lx= − ⋅ , in which lx is the lateral scanning step. The optimization result is shown in Fig. 2(c), where the simulated result shows better surface smoothness of micro lens.

Fig. 2. Simulations of microfabrication scanning method effects on micro lens surface quality: (a) parallel linear scanning method with fixed constant Delta Z, (b) annular scanning method with a fixed constant Delta Z, (c) annular scanning method with a dynamical Zi, where

2 2 1/ 2[ ( ) ]iZ R i lx= − ⋅ .

After detail optimization, we employ laser fabrication parameters as follows: Laser

excitation power 24 GW/cm2, 40× objective (NA = 0.85), the lateral scanning step x= 0.2 μm, single-step exposure time 25 ms. The SEM measurement of Micro lens is shown in Fig. 4(a)

(a) (b) (c)

(C) 2006 OSA 23 January 2006 / Vol. 14, No. 2 / OPTICS EXPRESS 813#9879 - $15.00 USD Received 6 December 2005; revised 9 January 2006; accepted 14 January 2006

and Fig. 4(b). The curve shown in Fig. 4(c) indicates the change of Delta Z according to the layer number i. The diameter of each lens is 15 μm, the height is 8 μm, and the center-to-center distance between two adjacent lenses is 15 μm.

Fig. 3. The correlation of the lateral scanning step (lx) effect with the surface roughness (Ra), The data are gotten by 50× WYKO NT1000 (Veeco Metrology Group).

Fig. 4. Scanning electron microscope images of micro lens fabricated with optimized parameters: (a), (b) 2×2 array micro spherical lens and its close-up view. (c) The curve indicates the alternation of Delta Z according to layer number i. (d-g) micro Fresnel lens fabricated with different lateral scanning step: 100 nm, 200 nm, 300 nm and 400 nm, respectively. For each Fresnel lens, the thickness is 2μm, and the diameter is 17 μm. The refractive index is 1.53. The number of zones is 3 and their radiuses are 5.0 μm, 7.1 μm and 8.7 μm, respectively.

Comparing with micro spherical lens, Fresnel lens requires finer and more complex

surface structure. Fresnel zone plate lens had been obtained with a diameter of 400 μm and a 7 μm size focus spot by the femtosecond laser microfabrication technology [25]. Generally micro Fresnel lens with small scale and better imaging ability can be applied in integrated

(d) (e) (f) (g)

(a) (c) (b)

(C) 2006 OSA 23 January 2006 / Vol. 14, No. 2 / OPTICS EXPRESS 814#9879 - $15.00 USD Received 6 December 2005; revised 9 January 2006; accepted 14 January 2006

optic devices. Based on the similar simulation and optimization, a micro Fresnel lens is also fabricated with smaller scale diameter and better imaging ability. Simply, we optimize the fabrication parameter by using the annular scanning mode with the lateral step pace. A series of Fresnel lens with different lateral step paces of 0.1 μm, 0.2 μm, 0.3 μm and 0.4 μm are fabricated, as shown in Fig. 4(d-g). It is found that the step pace parameters show great influence to the Fresnel lens quality. The fabrication parameters are optimized as follows: Laser excitation power 16 GW/cm2, numerical aperture (NA) of the focus lens 1.25(100×, oil), single-step exposure time 25ms and lateral step pace parameter 0.1 μm. The best Fresnel lens fabricated was shown in Fig. 4(d) with the optimized parameters. The diameter is 17 μm. The total fabrication takes about 8 min.

Fig. 5. The optical performance of micro lens: (a) the simulated and experimental focus intensity distribution of the micro spherical lens, the focus length is detected about 60 μm (b) The simulated and experimental diffractive intensity distribution of micro Fresnel lens, the detected image was obtained at a distance of 350 μm away from the lens, (c) and (d) are the detection of micro Fresnel lens imaging ability, (c) the real ghost image of “USTC”, detected at about 250 μm behind the Fresnel lens, (d) the spurious ghost image of “USTC”, detected at about 200 μm before the Fresnel lens.

To determine the quality of the fabricated micro lens, we try to test their optical

performance. During the test of the optical performance of the fabricated micro lens, the focus and images are tested on the image plane for the micro spherical lens and Fresnel lens, respectively. The experimental results are shown in Fig. 5(a) and Fig. 5(b). For simulation, we set the object plane as a function of I (x,y)=σ(x,y). The simulation intensity distributions of the two micro lenses are given based the Huygens-Fresnel principle and Kirchhoff’s diffraction theory [26],

(b)

(a) (c)

(d)

(C) 2006 OSA 23 January 2006 / Vol. 14, No. 2 / OPTICS EXPRESS 815#9879 - $15.00 USD Received 6 December 2005; revised 9 January 2006; accepted 14 January 2006

2 20 0

0 0 0 02

( ) ( ), , exp{ [1 ]}

2s

x x y yi(x y) (x y ) ikz dx dy

z zλ− −

= − +∫∫E E (1)

Where E(x0,y0)is the wave front after the micro lens’ modulation:

0 0 0 0, ( , ) exp( )lens lens lens(x y ) H x y A iϕ= ⋅ = ⋅ ⋅in inE E E (2)

Alens and φ lens are the modulation of the lens. As the designed lens are used for wave phase

retarding, theφ lens give most action designed as:

2 2 1/ 2spherical 0( ) ( )r k R rϕ = ⋅ − (3)

2 2 1/ 20( ) ( ( ) )fresnel r k f f rϕ = − + (4)

for micro spherical lens and Fresnel lens [27], respectively. Taking 0 2 /k π λ= and f is the

designed focal length, and considering 2( , ) ( , )I x y x y= E , the focusing intensity distributions

are obtained by a computer simulation as shown in Fig. 5(a) and Fig. 5(b). The experimental and the simulated results agree well with each other. The micro spherical lens’ focus length is measured about 60 μm. Meanwhile, the imaging ability of the micro Fresnel lens is also tested. Generally, except for real ghost images formed by refraction of the lens, it also generates spurious ghost images from its surface reflection [28]. The real ghost images are formed along the optical axis between the lens and the primary focus position, whereas the virtual ghost images are generated primarily in the outer regions of the lens. During the experiments, Letters “USTC” which have 2 cm×2 cm per letter are placed at the object plane. The real ghost image was detected about 250 μm behind the lens as shown in Fig. 5(c) and a virtual ghost image was observed about 200 μm before the lens as shown in Fig. 5(d). It agrees well with the result of theory analysis.

4. Conclusions

In summary, we employ the femtosecond two-photon microfabrication technology to the fabrication of micro lens with an alternative annular scanning mode with continuous variable layer thickness in this paper. After the optimization of some important processing parameters good micro spherical and Fresnel lens around 15 μm scale are obtained. The optical performance tests of the fabricated micro lens indicate that the technology is suitable for spherical micro optical devices fabrication. Furthermore, our works can be helpful not only for improving the spherical surface roughness during two-photon microfabrication but also for reducing the processing time for a specific spherical device. Meanwhile, continuous variable layer space scanning could lead to the gradual refractive index changes, which may probably bring some specific optical application based on the refractive optics. For the application in micro optical device fabrication, this technique may also be useful in mold fabrication that can copy micro lens in numbers. By using parallel fabrication as reported in ref. 20, the fabrication efficiency will be improved greatly.

Acknowledgments

This work is supported by NSFC (Nos. 50275140 and 50335050), “863” project (MEMS2003AA404-050), State Key Project for Fundamental Research (2003CB716900), and National Center for Nanoscience and Technology of China. It should be mentioned here that we have noticed by referee a related work published recently in Appl. Phys. Lett. (2005, 87, 154108), which described the subregional slicing for fabricating arbitrary 3D microstructures. Their results indicated that the subregional slicing method is also successful for 3D arbitrary microstructure.

(C) 2006 OSA 23 January 2006 / Vol. 14, No. 2 / OPTICS EXPRESS 816#9879 - $15.00 USD Received 6 December 2005; revised 9 January 2006; accepted 14 January 2006