Microchannel fabrication in silica glass by ... - ulaval ULAVAL

IOP PUBLISHING

JOURNAL OF MICROMECHANICS AND MICROENGINEERING

doi:10.1088/0960-1317/18/3/035039

J. Micromech. Microeng. 18 (2008) 035039 (4pp)

Microchannel fabrication in silica glass by femtosecond laser pulses with different central wavelengths Quan Sun, Ali Saliminia, Francis Théberge, Réal Vallée and See Leang Chin Centre d’Optique, Photonique et Laser, Department of Physics, Université Laval, G1V 0A6 Québec, Canada E-mail: [email protected]

Received 15 October 2007, in final form 21 December 2007 Published 20 February 2008 Online at stacks.iop.org/JMM/18/035039 Abstract A newly developed tunable visible femtosecond laser source was used to fabricate microchannels in silica glass. For comparison, femtosecond laser pulses from a Ti-sapphire laser at 800 nm as well as from a femtosecond optical parametric amplifier at 1.3 µm were also employed to fabricate microchannels. We found that it is much more efficient to drill micro-channels using the visible pulses because of their lower damage threshold. The quality of the cross section of the microchannel is also better with this visible femtosecond laser source because of the high beam quality of the visible pulses.

technique developed by Li et al [7, 9], and microchannels with constant diameter could be easily achieved with their technique.

1. Introduction Microchannel fabrication in optical materials has been the subject of numerous studies recently due to its high potential for the development of integrated micro-optics and biochip devices [1–3]. Femtosecond laser sources have been extensively used for laser-assisted microfabrication due to its versatility, namely with respect to direct 3D-writing of structures in solid materials [2–4]. In fact, the nonlinear multiphoton absorption process arising from the irradiation by intense femtosecond pulses ensures that the effective interaction zone inside the bulk of the transparent medium is located in a confined neighborhood of the focus. Such unique property of the femtosecond pulses enables one to precisely micromachine inside various transparent materials. Many research groups have studied microchannel fabrication by use of femtosecond laser pulses and their applications in the microfluidic system [3, 5–10]. In particular, 3D microfluidic structures in a single glass chip have been demonstrated by femtosecond laser direct writing for biochemical analysis [3 and references therein]. Normally, a bulk transparent sample is first irradiated by femtosecond laser pulses. After irradiation, microchannels are produced by selective chemical etching of the irradiated sample with diluted hydrofluoric (HF) acid. Alternatively, microchannels have also been fabricated by femtosecond laser with a water-assisted 0960-1317/08/035039+04$30.00

Nowadays, the laser wavelength for femtosecond laserassisted microchannels fabrication is mostly around 800 nm. Our newly developed tunable visible femtosecond laser source [11] provides a new tool for microchannels fabrication with the help of chemical etching. This new visible femtosecond laser source has excellent mode quality (M2 beam quality factor: 1.01) and high-energy stability (rms energy fluctuation: 1.8%). A central wavelength of 580 nm was used in our experiments. For comparison, pulses at the central wavelength of 800 nm from a Ti:sapphire femtosecond laser and 1.3 µm from a femtosecond optical parametric amplifier (OPA) were also used in our experiments. For the first time, high quality channels were obtained with high efficiency using the new visible femtosecond source as compared to the two other longer wavelengths. The high efficiency is due to the effect of shorter wavelength producing a higher damage than longer wavelengths at the same pulse power [12]. The spatial high quality occurs naturally because of the high beam quality of the new light source due to filamentation with self-spatial filtering [11, 13]. 1

© 2008 IOP Publishing Ltd

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J. Micromech. Microeng. 18 (2008) 035039

Quan Sun et al

Figure 1. Schematic of the experimental setup for writing the channels to be etched. Three femtosecond laser sources were used (central wavelengths were 580 nm, 800 nm, and 1.3 µm respectively) (NDF: neutral density filter). (This figure is in colour only in the electronic version)

Figure 2. Optical microscope images of the structures written at 800 nm after 1 h etching in a 10% HF acid solution in ultrasonic bath. The incident pulse energy increased from top to bottom.

2. Experiment In our experiments, three femtosecond sources were used to fabricate micro-channels: (1) a Ti:sapphire chirped pulse amplification laser system (Spitfire, Spectra-Physics) providing 1 kHz laser pulses at 800 nm with 2 mJ maximum pulse energy at ∼43 fs (measured by a single shot autocorrelator); (2) an OPA pumped by this 800 nm beam emitting tunable pulses between 1.1 µm and 2.4 µm; (3) a visible femtosecond laser source which we recently developed based on four-wave mixing inside the filament in air of the 800 nm pulse mixed with the infrared pulse from the OPA [11]. The central wavelength of the visible pulse was tunable in the range of 480–650 nm. In this paper, the visible femtosecond source was set at 580 nm to fabricate microchannels, and its pulse duration was measured to be ∼30 fs (obtained from a new technique based upon imaging the two-photon fluorescence distribution generated by the laser pulse propagation inside a dispersive dye solution [14]). The pulse duration of the infrared beam at 1.3 µm from the OPA was measured using the same two-photon fluorescence technique and was found to be ∼36 fs. Figure 1 presents the schematic layout of the fabrication setup. For each of these three femtosecond sources, the laser beam was focused into the bulk of a silica glass plate (Corning 7980-UV) with a microscope objective of 22.5 mm focal length (6.3×, NA = 0.20). The 4.2 mm thick silica glass sample with well-polished surfaces was mounted on a motorized stage. During the writing, the sample was translated at a speed 20 µm s−1 along the beam propagation axis, z, as shown in figure 1. In this parallel writing scheme, the polarization of the incident laser was always perpendicular to the writing direction for all three sources so that we could avoid different polarization effects on chemical etching as recently reported [5, 6]. The input pulse energy was controlled by a variable neutral density filter (NDF) inserted before the objective, and measured before the objective. The transmittance of the objective for the three wavelengths we used were similar, ∼85%. For each laser wavelength, we wrote a series of structures with different pulse energies. After writing, the front and rear surfaces were re-polished and then the sample was immersed in a diluted 10% HF acid solution and inserted in an ultrasonic bath for 1 h. The use of an ultrasonic bath enhances the debris removal and also contributes to refresh the HF acid inside the microchannels, thus increasing the etching efficiency. After etching, we verified the samples with an optical microscope

Figure 3. Optical images of the structures produced at 580 nm after 1 h etching in a 10% HF acid solution in ultrasonic bath. The incident pulse energy increased from top to bottom.

(Leica DM LM) and compared the results for writing with the three laser wavelengths.

3. Results and discussion First, we present the results when 800 nm femtosecond laser pulses were used. The optical images of the etched regions are shown in figure 2. For relatively low pulse energies, weak etching occurred within a few tens of micrometers from the front surface. On the other hand, when the pulse energy reached 5 µJ, a ∼700 µm long microchannel was observed with an entrance diameter of the order of 30 µm. By further increasing the pulse energy, we noted that the length of the microchannels was not really sensitive to this parameter, which is in agreement with previous observations reported in the literature [8]. From figure 2, the threshold for producing long microchannels was inferred to be lying between 3 and 5 µJ, which is near the damage threshold in fused silica for our experimental conditions [15]. As the microchannels fabricated by other groups [5, 6, 8] with this etching-assisted technique, the microchannels we got were tapered. Because the etching always starts from the end surfaces, and then goes inside gradually. Figure 3 shows the optical images of the microchannels fabricated in silica glass by use of 580 nm femtosecond pulses after 1 h etching. As seen, we have obtained long microchannels with pulse energies ranging from 0.7 µJ to 3 µJ. A comparison between the results shown in figures 2 and 3 clearly indicates that the threshold for long channel formation at 580 nm, which was below 0.7 µJ, is considerably lower than that at 800 nm. Note that the channel length also depends on the etching condition. That is, we were able to obtain very long (several millimeters) microchannels by increasing the chemical etching time. We note that the white structures appearing in figure 3 are due to imaging. These 2


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Figure 4. Comparison of the structures fabricated with the three laser wavelengths under different pulse energies while keeping the same laser peak power (35 MW). Left: optical images of the structures after 1 h etching in a 10% HF acid solution in an ultrasonic bath. Right: optical images (DIC mode) of the structures before etching. From top to bottom, the laser wavelengths are 580 nm, 800 nm and 1.3 µm, respectively.

(a)

(b)

Figure 5. Typical cross section of the microchannels with the writing wavelength of 580 nm (a) and 800 nm (b). The pulse energy is 0.7 µJ and 5.0 µJ, respectively.

Keldysh parameters γ [18] for the three wavelengths would be larger than 1 so that multiphoton absorption would be the dominating process here. Now, multiphoton absorption strongly depends on the ratio between the band-gap and the laser wavelength [19]. In our case, the band-gap is fixed, while the laser wavelength is varied. At the wavelength of 800 nm and 1.3 µm, the number of photons (the order of multiphoton absorption, k) required to overcome the band-gap (Eg ∼ 9 eV) are six and ten, respectively, whereas at 580 nm, only four photons are required. The rate of multiphoton absorption markedly depends on k as R ∝ σk I k , where σ k and I are the multiphoton absorption cross section and laser intensity, respectively. Thus, considering that σ k increases very rapidly with decreasing k, the threshold for achieving sufficiently high electron density corresponding to structural damage should occur at lower intensities or pulse energies for shorter excitation wavelengths. So it is easier for 580 nm femtosecond pulses to achieve microchannels at low pulse power, while for 800 nm pulses, it will occur at relative higher pulse power. And due to the much higher damage threshold corresponding to the 1.3 µm femtosecond pulses, no etched microchannel was obtained in our experiments. Although the 1.3 µm femtosecond pulses are not suitable for microchannel fabrication, they have proven themselves very reliable and efficient for the writing of low-loss waveguides. In fact, our previous work of waveguide writing in silica glass with femtosecond pulses from an optical parametric amplifier at 1.5 µm shows a much broader range of writing parameters and waveguides with better characteristics in comparison with those fabricated at 800 nm [20]. On the other hand, as demonstrated in this paper shorter wavelength (i.e. visible) femtosecond pulses are much more convenient for drilling micro-channels, because of their lower damage threshold. This should obviously be taken into account in the development of systems for the large-scale fabrication of photonic devices involving both optical waveguides and microfluidic components. We also compared the cross section of the microchannels fabricated by 800 nm and 580 nm femtosecond pulses. As shown in figure 5, the cross section of the microchannel fabricated at 580 nm is more circular than that at 800 nm. In other words, the quality of cross section at 580 nm is better than that at 800 nm, which is the result of the excellent mode quality of our visible femtosecond beam compared to the 800 nm beam. As the measurements reported in our previous

white structures can also be found in figure 2, though they look much weaker. This is because the images in figure 3 were not taken at the same time as those in figure 2 and thus the contrast of the images are different. We also tried to fabricate microchannels with the 1.3 µm femtosecond laser beam. Surprisingly, by using 1.3 µm laser pulse energy up to 24 µJ, no microchannel could be observed under the microscope, whereas only superficial etching, i.e. traces shorter than those appearing on top of figure 2, was observed for higher input energies. To further illustrate the etching behavior at different wavelengths, we compared the fabricated channels for the same input peak power of 35 MW. The results after 1 h etching are shown in figure 4 where it clearly appears that the visible femtosecond pulses are more efficient for microchannel fabrication than the two others. Indeed, the 1.3 µm laser beam did not produce microchannel and at 800 nm, the etching depth was only 30 µm long whereas for the 580 nm laser beam the length of the etched microchannel was about 640 µm i.e. 20 times longer than that produced by the 800 nm pulses. The aspect ratio (the channel length divided by the channel entrance diameter) was about 22:1 and 1:1 for 580 nm and 800 nm pulses, respectively. The corresponding differential interference contrast (DIC) microscope images of the irradiated samples before etching are shown on the right-hand side of figure 4. These images confirm that the long-etched microchannels written at 580 nm result from the damaged tracks having a non-uniform void-like morphology. However, at 800 nm the written track is indeed a waveguide with smooth and moderate index change thus suffering a weak chemical etching. For the 1.3 µm pulses, no visible modification was observed from the DIC image at the input pulse energy of 1.3 µJ corresponding to 35 MW peak power. These observations can be analyzed based on the dependence of the silica glass damage threshold on writing wavelength. Under the focusing conditions prevailing in our experiment (f = 22.5 mm), the filamentation process dominates [16, 17] and is accompanied by intensity clamping [11, 13, 17], i.e., the intensity cannot increase further even for higher pulse energies. Although the value of this clamped intensity in glass has not been precisely established so far, it is believed to be of the order of 1013 W cm−2 and should be lower than the clamped intensity in air which is of the order of 5 × 1013 W cm−2 [13, 17]. Accordingly, the calculated 3


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References

paper, the M2 quality factor of our tunable visible (580 nm in this experiment) beam was 1.01, close to the theoretical limit of 1, while the quality factor for the 800 nm beam was M2 = 1.21 [11]. This excellent mode quality of our visible femtosecond beam is due to the spatial self-filtering and intensity clamping processes during the filamentation of femtosecond pulses [11, 13].

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4. Conclusion In conclusion, we have demonstrated the dependence of the microchannels fabricated in fused silica glass using chemical etching on the femtosecond laser wavelength. Under the same writing and etching conditions, the visible femtosecond pulses have proven to be much more efficient for microchannel fabrication at relative low pulse energy as compared to 800 nm and 1.3 µm femtosecond pulses. Also, the high mode quality of our new tunable visible femtosecond laser source enables us to get a circular cross section of the microchannel.

Acknowledgments This work was supported by the Natural Sciences and Engineering Research Council of Canada (NSERC), Defence R&D Canada—Valcartier (DRDC—Valcartier), Le Fonds Québécois de la Recherche sur la Nature et les Technologies (FQRNT), Canada Research Chairs (CRC), Canada Foundation for Innovation (CFI) and Canadian Institute for Photonic Innovations (CIPI). The authors appreciate the technical support of M Martin and M D’Auteuil. We also acknowledge the fruitful discussion with Dr W Liu and Mr F Liang.

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