Mechanism of micro-lens array formation by using four-beam interference lithography E. Stankevicius, M. Gedvilas, M. Garliauskas, G. Raciukaitis Center for Physical Sciences and Technology (CPST), Savanoriu av. 231, LT-02300 Vilnius, Lithuania e-mail: [email protected] Photo-polymerization processes induced by ultra-short pulsed lasers [1-3] have drawn a lot of attention in the past few years due to its precision and flexibility. The higher process efficiency can be achieved by the parallel processing technique using interference of several laser beams [4-7]. Depends on the interfering beams parameters (phase, angle between beams, number of beams, used wavelengths, etc) [6] different types of micro-structures are possible to form. Micro-structures fabricated by four-beam interference lithography have shape of the micro-pillar array [8]. The shape of fabricated micro-pillars strongly depends on the laser irradiation dose [6]. Top of the micro-pillars in a single-photon excitation case have a spherical shape, acting as micro-lenses (Fig. 1). Fig. 1 Micro-lenses fabricated by interference lithography by using the same exposure time (30 s) and different laser average power: a) ~ 470 mW, b) ~ 630 mW and c) ~ 930 mW. The distance between micro-lenses is ~ 60 µm. Laser processing parameters are: wavelength - 515 nm, repetition rate - 100 kHz. SEM images of the structures are tilted by 34 deg. Here, we will introduce rapid and flexible micro-lens array fabrication method by using four-beam interference lithography and a single-photon excitation technique. Also we will analyze the influence of the laser processing parameters (the average laser power and the irradiation time) to the shape of these micro-lenses and propose a model which explicitly treats the close relationship between the laser intensity, the irradiation time, period of intensity distribution and the diameter of the fabricated micro-lens. The proposed model is in good agreement with the experimental results and explains the mechanism of micro-lens formation. [1] S. Maruo, O. Nakamura and S. Kawata, "Three-dimensional microfabrication with two-photon-absorbed photopolymerization", Opt. Lett. 22, 132-134, 1997. [2] J. Serbin, A. Egbert, A. Ostendorf, B. N. Chichkov, R. Houbertz, G. Domann, J. Schulz, C. Cronauer, L. Fröhlich and M. Popall, "Femtosecond laser-induced two-photon polymerization of inorganic organic hybrid materials for applications in photonics", Opt. Lett. 28, 301-303, 2003. [3] R. Guo, S. Xiao, X. Zhai, J. Li, A. Xia and W. Huang, "Micro lens fabrication by means of femtosecond two photon photopolymerization", Opt. Express 14, 810-816, 2006. [4] M. Campbell, D. N. Sharp, M. T. Harrison, R. G. Denning and A. J. Turberfield, "Fabrication of photonic crystals for the visible spectrum by holographic lithography", Nature 404, 53-56, 2000. [5] T. Kondo, S. Matsuo, S. Juodkazis and H. Misawa, "Femtosecond laser interference technique with diffractive beam splitter for fabrication of three-dimensional photonic crystals", Appl. Phys. Lett. 79, 725-727, 2001. [6] E. Stankevicius, M. Gedvilas, B. Voisiat, M. Malinauskas and G. Raciukaitis, "Fabrication of periodic micro-structures by holographic lithography", Lith. J. Phys. 53, 227-237, 2013. [7] H. Misawa, T. Kondo, S. Juodkazis, V. Mizeikis and S. Matsuo, "Holographic lithography of periodic two- and three- dimensional microstructures in photoresist SU-8", Opt. Express 14, 7943-7953, 2006. [8] E. Stankevicius, M. Malinauskas, M. Gedvilas, B. Voisiat and G. Raciukaitis, "Fabrication of periodic micro-structures by multi- photon polymerization using the femtosecond laser and four-beam interference", Mat. Sci. (Medžiagotyra) 17, 244-248, 2011. a) b) c)

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Mechanism of micro-lens array formation by using four-beam interference lithography

E. Stankevicius, M. Gedvilas, M. Garliauskas, G. Raciukaitis

Center for Physical Sciences and Technology (CPST), Savanoriu av. 231, LT-02300 Vilnius, Lithuania

e-mail: [email protected] Photo-polymerization processes induced by ultra-short pulsed lasers [1-3] have drawn a lot of attention in the past few years due to its precision and flexibility. The higher process efficiency can be achieved by the parallel processing technique using interference of several laser beams [4-7]. Depends on the interfering beams parameters (phase, angle between beams, number of beams, used wavelengths, etc) [6] different types of micro-structures are possible to form. Micro-structures fabricated by four-beam interference lithography have shape of the micro-pillar array [8]. The shape of fabricated micro-pillars strongly depends on the laser irradiation dose [6]. Top of the micro-pillars in a single-photon excitation case have a spherical shape, acting as micro-lenses (Fig. 1).

Fig. 1 Micro-lenses fabricated by interference lithography by using the same exposure time (30 s) and different laser average power:

a) ~ 470 mW, b) ~ 630 mW and c) ~ 930 mW. The distance between micro-lenses is ~ 60 µm. Laser processing parameters are: wavelength - 515 nm, repetition rate - 100 kHz. SEM images of the structures are tilted by 34 deg.

Here, we will introduce rapid and flexible micro-lens array fabrication method by using four-beam interference lithography and a single-photon excitation technique. Also we will analyze the influence of the laser processing parameters (the average laser power and the irradiation time) to the shape of these micro-lenses and propose a model which explicitly treats the close relationship between the laser intensity, the irradiation time, period of intensity distribution and the diameter of the fabricated micro-lens. The proposed model is in good agreement with the experimental results and explains the mechanism of micro-lens formation. [1] S. Maruo, O. Nakamura and S. Kawata, "Three-dimensional microfabrication with two-photon-absorbed photopolymerization",

Opt. Lett. 22, 132-134, 1997. [2] J. Serbin, A. Egbert, A. Ostendorf, B. N. Chichkov, R. Houbertz, G. Domann, J. Schulz, C. Cronauer, L. Fröhlich and M. Popall,

"Femtosecond laser-induced two-photon polymerization of inorganic organic hybrid materials for applications in photonics", Opt. Lett. 28, 301-303, 2003.

[3] R. Guo, S. Xiao, X. Zhai, J. Li, A. Xia and W. Huang, "Micro lens fabrication by means of femtosecond two photon photopolymerization", Opt. Express 14, 810-816, 2006.

[4] M. Campbell, D. N. Sharp, M. T. Harrison, R. G. Denning and A. J. Turberfield, "Fabrication of photonic crystals for the visible spectrum by holographic lithography", Nature 404, 53-56, 2000.

[5] T. Kondo, S. Matsuo, S. Juodkazis and H. Misawa, "Femtosecond laser interference technique with diffractive beam splitter for fabrication of three-dimensional photonic crystals", Appl. Phys. Lett. 79, 725-727, 2001.

[6] E. Stankevicius, M. Gedvilas, B. Voisiat, M. Malinauskas and G. Raciukaitis, "Fabrication of periodic micro-structures by holographic lithography", Lith. J. Phys. 53, 227-237, 2013.

[7] H. Misawa, T. Kondo, S. Juodkazis, V. Mizeikis and S. Matsuo, "Holographic lithography of periodic two- and three-dimensional microstructures in photoresist SU-8", Opt. Express 14, 7943-7953, 2006.

[8] E. Stankevicius, M. Malinauskas, M. Gedvilas, B. Voisiat and G. Raciukaitis, "Fabrication of periodic micro-structures by multi-photon polymerization using the femtosecond laser and four-beam interference", Mat. Sci. (Medžiagotyra) 17, 244-248, 2011.

a) b) c)