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OPTICS LETTERS / Vol. 35, No. 7 / April 1, 2010
Fabrication of microfluidic channels with a circular cross section using spatiotemporally focused femtosecond laser pulses Fei He,1,2 Han Xu,1 Ya Cheng,1,4 Jielei Ni,1,2 Hui Xiong,1 Zhizhan Xu,1,5 Koji Sugioka,3 and Katsumi Midorikawa3 1
State Key Laboratory of High Field Laser Physics, Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, P.O. Box 800-211, Shanghai 201800, China 2 Graduate School of the Chinese Academy of Sciences, Beijing 100039, China 3 Laser Technology Laboratory, RIKEN-Advanced Science Institute, Hirosawa 2-1, Wako, Saitama 351-0198, Japan 4
[email protected] 5
[email protected] Received January 7, 2010; accepted February 19, 2010; posted March 5, 2010 (Doc. ID 122439); published March 31, 2010 We report on the fabrication of hollow microfluidic channels with a circular cross-sectional shape embedded in fused silica by spatiotemporally focusing the femtosecond laser beam. We demonstrate both theoretically and experimentally that the spatiotemporal focusing of femtosecond laser beam allows for the creation of a three-dimensionally symmetric spherical intensity distribution at the focal spot. © 2010 Optical Society of America OCIS codes: 140.3390, 140.7090, 160.2750, 270.4180, 350.3390.
Nowadays, femtosecond laser micromachining has been proved to be a highly attractive solution for three-dimensional (3D) microfabrication in transparent materials [1–12]. For instance, as building blocks of microfludic systems, microchannels can be fabricated inside fused silica by femtosecond laser direct write followed by wet chemical etching. Generally, the single-path transverse write method (i.e., the writing direction perpendicular to the laser propagation direction) intrinsically results in elliptical cross sections, which is unfavorable for many practical applications, such as writing optical waveguides or fabricating microchannels with circular cross sections. Although a juxtaposing multiple scan method can create controllable cross-sectional patterns, it increases the complexity and may lead to the degradation of the resolution [2]. Several beam shaping methods have been proposed for improving the aspect ratio of the transverse profile while each has pros and cons. For instance, employing a focusing objective with a higher numerical aperture (NA) can improve the axial resolution, but it also shortens the working distance. By shaping of the input laser beams with either a pair of cylindrical lenses [3] or a narrow slit [4,7] placed before the objective lens, one can obtain optical waveguides or microchannels with circular cross sections. However, both of these methods would require adjustment of the orientation of the slit or the cylindrical lens pair during the scanning of the laser beam if the microstructure is not purely one-dimensional. To overcome this problem, a crossed-beam irradiation method was proposed for obtaining an isotropic 3D spatial resolution [10]. However, this scheme requires the precise alignment of two objective lenses, namely, the two lenses must share a common focus and the laser pulses passing through the two lenses must arrive at the common focus simultaneously. These requirements cause an additional complexity for the implementation of the 0146-9592/10/071106-3/$15.00
technique. More recently, we proposed a glass drawing method for fabricating microchannels with circular cross sections in fused silica [12]. However, this technique is effective only for channels oriented nearly along the drawing direction. In this Letter, we attempt to offer a different solution to control the cross section by temporal focusing of the femtosecond laser pulses. The core of this technique is to separate the spectral components of the femtosecond pulses in space before the pulses enter the objective lens. Temporal focusing occurs because the spatial overlapping of different frequency components only happens around the focus, leading to the shortest pulse duration and, consequently, the highest peak intensity. This will facilitate an improved axial resolution of the femtosecond laser microfabrication because the peak intensity will decrease rapidly due to the broadening of the pulse duration when moving away from the geometric focal spot. The details of this technique can be found in [13]. The schematic of our experimental setup is shown in Fig. 1. Our chirped pulse amplification system delivers 40 fs laser pulses at a center wavelength of 800 nm with a spectral bandwidth of ⬃30 nm and polar-
Fig. 1. (Color online) Schematic of the experimental setup: i, ␥, d are the incident angle, the first order diffractive angle, and the distance between the gratings, respectively. © 2010 Optical Society of America
April 1, 2010 / Vol. 35, No. 7 / OPTICS LETTERS
ized along the Y direction, which has been described in detail elsewhere [12]. In this experiment, the input laser beam size is controlled by an adjustable circular aperture. A pair of parallel gratings (1200 line/mm, blazing at 800 nm) is used to separate the spectral components of the incident pulse in the X coordinate. The distance between the gratings is set to be ⬃180 mm and the incident angle is ⬃45° in our experiment. To precompensate the chirp induced by all the optics such as the neutral density (ND) filters, objective lens, and grating pair, the input pulse is positively prechirped by adjusting the compressor in the amplifier. The best precompensation of the dispersion can be achieved when the strongest ionization (i.e., the brightest plasma) of the air was observed at the focus of the objective. The power of the laser pulse is adjusted by the ND filters before being focused by a 20⫻ objective lens 共NA= 0.46兲 into the glass sample. The personal-computer-controlled XYZ stage has a resolution of 1 m. First, we show theoretically how to control the aspect ratio of the intensity distribution at the focus. For a laser beam with a typical waist of 2W0 = 5 mm, the intensity distribution of the laser beam directly focused by the objective can be obtained using Eq. (1) in [4]. As shown in Fig. 2(a), the intensity distribution turns out to be elliptical in this case. However, with the temporal focusing scheme, a quite different intensity distribution can be obtained as shown in Figs. 2(b) and 2(c). The field of a spatially dispersed pulse can be expressed as
冋
A1共x,y, 兲 = A0 exp −
再
⫻exp −
共 − 0兲 2 ⍀2
册
关x − ⌬x共兲兴2 + y2 2W02
冎
,
共1兲
where A0 is a field amplitude, 0 is the carrier frequency, 冑2 ln 2⍀ is the FWHM of the frequency spectrum of the pulse, W0 is the incident beam waist before the grating pair, and ⌬x共兲 ⬇ ␣共 − 0兲 is the displacement of each spectral component [13]. According to the experimental parameters, the spectral bandwidth is set to be 30 nm and the beam waist is 2W0 = 3 mm in our simulation. As for our configuration, ␣ = d0 cos i / 共0 cos3 ␥兲, where d is the distance between the gratings; i and ␥ are the incident
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angle and the first-order diffractive angle, respectively; is the groove density of the gratings; and 0 is the carrier wavelength. Using the slow varying envelope approximation, the field after the objective can be written as
冉
A2共x,y, 兲 = A1共x,y, 兲exp − ik
x2 + y2 2f
冊
,
共2兲
where k = 2c / 0 (c is the light propagation velocity in vacuum) and f is the focal length of the objective. Under the paraxial approximation, the laser field near the focus can be obtained by the use of the Fresnel diffraction formula below: A3共x,y,z, 兲 =
exp共ikz兲 iz
冋
冕冕
⫻exp ik
⬁
A 2共 , , 兲
−⬁
共x − 兲2 + 共y − 兲2 2z
册
dd . 共3兲
The intensity distribution in the time domain can thus be obtained by performing an inverse Fourier transform of A3共x , y , z , 兲 as follows: I共x,y,z,t兲 = 兩A3共x,y,z,t兲兩2 =
冏冕
⬁
−⬁
A3共x,y,z, 兲exp共− it兲d
冏
2
.
共4兲
Combining Eqs. (1)–(4), the intensity distributions of the focused spatially dispersed beam in both the XZ and YZ planes can be obtained, as shown in Figs. 2(b) and 2(c). One can clearly see that a nearly spherical intensity distribution has been obtained with the temporal focusing. It should be specifically noted that the threshold intensity of the femtosecond laser fabrication of transparent materials not only strongly depends on the peak intensity but can also be affected by the ultrashort laser pulse duration [14]. In addition, the axial resolution would also be affected by the order of the optical nonlinearity of the interaction between the femtosecond laser pulse and the transparent material (e.g., the nth order optical nonlinearity means simultaneous absorption of n photons). Therefore, since the order of the optical nonlinearity of the interaction between the femtosecond laser and fused
Fig. 2. (Color online) Numerically calculated laser intensity distributions at the focus produced by an objective lens (a) without and (b), (c) with temporal focusing technique in XZ and YZ planes, respectively.
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OPTICS LETTERS / Vol. 35, No. 7 / April 1, 2010
silica is not clearly defined in our experiment [15], quantitative reproduction of the experimental results by theoretical simulation is difficult. However, for a practical application, the axial resolution of femtosecond laser writing with temporally focused pulses can be continuously controlled simply either by changing the incident beam size using the circular aperture or by increasing the spatial chirp of the laser pulse by adjusting the distance between the two gratings, because the pulse width stretching factor—which is defined as the ratio of the pulse duration at the back aperture of the objective to that measured at the focus—can be analytically obtained as ␣⍀ / W0 [13]. To experimentally demonstrate the control of the cross-sectional aspect ratio, we fabricate 3D microchannels in fused silica by femtosecond laser direct write. In our experiment, U-shaped channel patterns with a length of 2 mm are first inscribed 300 m beneath the surface at a constant speed of 50 m / s. To show that controllable cross-sectional shapes can be obtained for microchannels oriented in arbitrary directions without rotating any optical elements during the laser irradiation, we fabricate two groups of channels aligned in both the X and Y directions as illustrated in Fig. 3(a). To continuously vary the aspect ratio, the beam size and the average power of the femtosecond laser are gradually increased from 2 to 5 mm and from 2 to 4 mW, respectively. After the laser irradiation, the sample is subjected to a 150 min etching in a solution of 10% HF diluted with water in an ultrasonic bath. The microchannels are formed after all the areas modified by the femtosecond laser are completely removed. It is noted that the group of channels oriented along the X direction is formed somewhat faster than that along the Y direction due to the polarization-selective etching [9]. We examine the cross sections of the channels by cutting and polishing the sample. It is found that the axial resolution of the fabrication can be continuously controlled by changing the input beam size. This is consistent with the above theoretical analysis. The aspect ratios of the cross section gradually increase
Fig. 3. (Color online) (a) Schematic of the 3D microfluidic channels fabricated using the temporal focusing method. The red arrows denote the translation directions of the stage. (b)–(i) Optical micrographs of cross section of microfluidic channels. The beam sizes and laser powers are 2 mm and 4 mW in (b), (f); 3 mm and 3.5 mW in (c), (g); 4 mm and 2 mW in (d), (h); and 5 mm and 2 mW in (e), (i).
from 0.8 to 1.7, as shown in Figs. 3(b)–3(i). In particular, for a laser power of 3.5 mW and a beam size of ⬃3 mm, the channels oriented in both the X and Y directions exhibit a nearly circular cross section as shown in Figs. 3(c) and 3(g). These results clearly suggest that a spherical focal spot with isotropic resolution in a 3D space has been achieved with the temporally focused femtosecond pulses. To summarize, we demonstrate the control of the cross section of the microchannels using the temporal focusing technique. In addition, nonlinear selffocusing before the focus, which frequently results in elongated structures (particularly in highly nonlinear materials such as polymers and crystals), can be avoided with this technique. Last, but not the least, this method is also expected to be useful for fabricating circularly symmetric optical waveguides regardless of the translation direction of the femtosecond laser beam. This work is supported by the National Basic Research Program of China (grant no. 2006CB80600) and National Natural Science Foundation of China (NSFC) (grant no. 10974213). Y. Cheng acknowledges the support of 100 Talents Program of the Chinese Academy of Sciences (CAS), Shanghai Pujiang Program, and National Outstanding Youth Foundation. References 1. A. Marcinkevičius, S. Juodkazis, M. Watanabe, M. Miwa, S. Matsuo, H. Misawa, and J. Nishii, Opt. Lett. 26, 277 (2001). 2. Y. Bellouard, A. Said, M. Dugan, and P. Bado, Opt. Express 12, 2120 (2004). 3. R. Osellame, S. Taccheo, M. Marangoni, R. Ramponi, P. Laporta, D. Polli, S. D. Silvestri, and G. Cerullo, J. Opt. Soc. Am. B 20, 1559 (2003). 4. Y. Cheng, K. Sugioka, K. Midorikawa, M. Masuda, K. Toyoda, M. Kawachi, and K. Shihoyama, Opt. Lett. 28, 55 (2003). 5. M. Masuda, K. Sugioka, Y. Cheng, N. Aoki, M. Kawachi, K. Shihoyama, K. Toyoda, H. Helvajian, and K. Midorikawa, Appl. Phys. A 76, 857 (2003). 6. Y. Cheng, K. Sugioka, and K. Midorikawa, Opt. Lett. 29, 2007 (2004). 7. M. Ams, G. Marshall, D. Spence, and M. Withford, Opt. Express 13, 5676 (2005). 8. R. Stoian, C. Mauclair, A. M. Blondin, N. Huot, E. Audouard, J. Bonse, A. Rosenfeld, I. V. Hertel, J. Laser Micro/Nanoeng. 4, 45 (2009). 9. C. Hnatovsky, R. S. Taylor, E. Simova, V. R. Bhardwaj, D. M. Rayner, and P. B. Corkum, Opt. Lett. 30, 1867 (2005). 10. K. Sugioka, Y. Cheng, K. Midorikawa, F. Takase, and H. Takai, Opt. Lett. 31, 208 (2006). 11. Y. Liao, J. Xu, Y. Cheng, Z. Zhou, F. He, H. Sun, J. Song, X. Wang, Z. Xu, K. Sugioka, and K. Midorikawa, Opt. Lett. 33, 2281 (2008). 12. F. He, Y. Cheng, Z. Xu, Y. Liao, J. Xu, H. Sun, C. Wang, Z. Zhou, K. Sugioka, K. Midorikawa, Y. Xu, and X. Chen, Opt. Lett. 35, 282 (2010). 13. G. Zhu, J. Howe, M. E. Durst, W. Zipfel, and C. Xu, Opt. Express 13, 2153 (2005). 14. R. R. Gattass and E. Mazur, Nat. Photonics 2, 219 (2008). 15. A. Tien, S. Backus, H. Kapteyn, M. Murnane, and G. Mourou, Phys. Rev. Lett. 82, 3883 (1999).
