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Line-shaped femtosecond pulses are well-suited to large-area machining with high throughput in laser cutting, peeling, and grooving of materials. First, we demonstrated the single-shot fabrication of a line structure in a glass surface using a line-shaped pulse generated by a holographic cylindrical lens displayed on a liquid-crystal spatial light modulator. We found the line structure was uniform and smooth near the ends because of the ability to precisely control the intensity distribution and to achieve single-shot fabrication. Second, we demonstrated a line-shaped beam deformed three-dimensionally for showing the potential of holographic line-shaped beam processing. Third, we demonstrated laser peeling of an indium tin oxide film. We found that little debris around the fabricated area was observed, because the debris was removed by the beam itself. Last, we demonstrated laser grooving of stainless steel. We found the swelling of the surface included upwardly growing nanogratings, although many line-shaped pulse irradiations were given. The swelling was caused by the depositions of the debris on the top of the nanogratings.
© 2015 Optical Society of America
1. Introduction
Femtosecond laser pulses formed in a desired spatial pattern by a computer-generated hologram (CGH) displayed on a spatial light modulator (SLM) can perform material laser processing with high throughput and high light-use efficiency [1–3]. This holographic femtosecond laser processing technique has been applied to two-photon polymerization [4–6], the fabrication of optical waveguides in transparent materials [7,8], the fabrication of volume phase gratings in polymers [9], the fabrication of high-aspect-ratio nanochannels [10], cell transfection [11], and laser cleaning [12]. In addition to the spatial shaping of laser pulses, spatial control of polarization has been applied to surface nanostructuring [13] for control of tribological properties [14], wettability [15], reflectance [16], and retardance [17]. Furthermore, the use of an SLM allows focus control in response to shape variations and deformation of the target, as well as adaptive wavefront control for 3D precision internal micro-structuring [18].
For wide-area fabrication in realistic applications, a linear beam offers considerably higher processing throughput than an ordinary Gaussian beam, because the total length of the beam scanning area is reduced, and the throughput is not restricted by mechanical limits. In addition to these quantitative advantages, line-beam processing has qualitative advantages, including the absence of artifacts derived from the scanning of a focused beam, as well as less debris, because the direction of the flying debris is parallel to the scanning direction of the beam, and part of the debris is removed by the laser beam itself [12]. A holographic laser processing technique based on an SLM can be applied to variable, three-dimensional beam shaping according to the target surface structure.
In the work described in this paper, we demonstrated material laser processing using line-shaped femtosecond pulses. In Sec. 2, we describe the method of designing a computer-generated hologram for producing a line-shaped beam. In Sec. 3, we describe the experimental setup. In Sec. 4, we present the experimental results of line-shaped beam processing of a glass surface, laser peeling of an indium tin oxide (ITO) film, and laser grooving of stainless steel. In Sec. 5, we conclude our paper.
2. Generation of a line-shaped beam
The phase distribution of a holographic cylindrical lens for generating a line-shaped beam is described by
where w(x) is a window function with a value from 0 to 1, k = 2π/λ is the wave number, and f is the focal length. In this experiment, f was set to 2000 mm. Figure 1 shows three types of holographic cylindrical lenses that we used, having different w(x), and their computational reconstructions at different propagation distances from the SLM. When w(x) was a rectangular function given bythe diffraction image was a long line-shaped beam, but oscillations due to diffraction caused by the edges of the aperture were present, as shown in Fig. 1(a). A smoothly decaying window function suppresses diffraction from the edges. We used a Hann window, which is a well-known window function, described byWe also used a modified form of the Akaike window, described bywhere is the Akaike window. As a result, the diffraction pattern also decayed smoothly toward the outside without any oscillations, as shown in Figs. 1(b) and 1(c), respectively. We adopted whann for comparison, because it is one of the window functions that is frequently used. It gave a shorter line-shaped beam and a gentle decay of the intensity. In a number of experimental trials for making a better function, the window function wmak produced a line-shaped beam which was long and showed smooth changes.
Fig. 1 Phase distributions of holographic cylindrical lenses with (a) wrect, (b) whann, and (c) wmak, and their computational reconstructions at different distances from the SLM.
3. Experimental setup
Figure 2 shows the holographic femtosecond laser processing system. It was mainly composed of an amplified femtosecond laser system (Coherent, Micra and Legend Elite), a liquid-crystal-on-silicon SLM (LCOS-SLM; Hamamatsu Photonics, X10468-02), laser processing optics, and a personal computer (PC). The femtosecond pulses with a center wavelength of λc = 800 nm, a spectral width of 30 nm FWHM, a pulse duration of 35 fs, a variable repetition frequency from 1 Hz to 1 kHz controlled by Pockels cell in the laser system, and linear polarization were collimated, and the collimated beam illuminated the SLM. The beam was diffracted by a computer-generated hologram, in this study a holographic cylindrical lens, displayed on the SLM and was transformed to a desired pattern (a line-shaped beam) at the desired plane. The line-shaped beam was imaged on the sample through a reduction optics with a magnification M of 1.24 × 10−2 constructed from a 60 × objective lens (OL) with a numerical aperture NA = 0.85 (focal length f = 3.09 mm) and a lens (f = 250 mm). The diameter of a beam incident on the pupil of an objective lens should be less than the pupil size because the diffraction on the edges of the objective lens appears in the hologram reconstruction. In our setup, the diameter of the incident beam was set to be less than the pupil size of the objective lens. The light use efficiency was 25% at the focal plane of the cylindrical lens, 18% in front of the OL, and 8% behind the OL. The irradiation energy E was the total pulse energy of the diffracted beam at the sample plane, which was obtained as follows. First, the ratio between the energy split off by the DM and the energy at the sample plane was estimated. The total irradiation pulse energy was continuously monitored using a power meter, as the product of the energy split off by the DM and this ratio. To observe laser processing of the sample, the sample was illuminated with a halogen lamp (HL), and a charge coupled device (CCD) image sensor captured images of the sample via a dichroic mirror (DM) and an infrared (IR) cut filter. The sample surface after laser processing was observed with a laser confocal microscope (OLS4000, Olympus) and a scanning electron microscope (SEM; FE-SEM S-4500, Hitachi).
The shorter width of the line-shaped beam was calculated from dshort = 1.22λc/NA. The longer width of the beam, dlong = SSLMM, where SSLM is the size of the SLM, is noted for reference, although each line beam had a different length. In the experiments, dshort = 1.1 µm and dlong = 165 µm for λ = 800 nm, NA = 0.85, SSLM = 13.3 μm, and M = 1.24 × 10−2. A dimension of the line-shaped beam is calculated to be 181.5 μm2 (1.1 μm × 165 μm). On the other hand, the dimension of the focused Gaussian beam is 1.21 μm2 (1.1 μm × 1.1 μm). Therefore, the processing area rate using the line-shaped beam is 150 times larger than that using the focused Gaussian beam. The length of the line-shaped beam depends on both the irradiation beam width and the size of holographic cylindrical lens. If the beam width is smaller than the size of cylindrical lens, the length depends on the beam width. In our experiment, the beam width is sufficiently larger than the size of cylindrical lens. Therefore, the length of the line-shaped beam only depends on the size of cylindrical lens.
4. Experimental results
4.1 Line-shaped processing of glass
Figure 3 shows SEM images of the area processed with a line-shaped femtosecond laser pulse. The target material was super white crown glass (B270, Schott). Laser processing was performed using holographic cylindrical lenses with window functions wrect, whann, and wmak, while changing the irradiation energy. In the case of wrect, as shown in Fig. 3(a), the fabricated line structure had large variations like those of the intensity distribution (Fig. 1(a)). In the case of whann, as shown in Fig. 3(b), the length of the fabricated line structure was increased from center of the structure toward each end with increasing the pulse energy. This results corresponded to the intensity profile in Fig. 1(b). The use of wmak formed a line structure with a length of 42 µm and a width of 0.58 µm when E = 4.9 µJ, as shown in Fig. 3(c). The ablation threshold in the case of Figs. 3(a) to 3(c) were 4.3, 1.4 and 2.7 μJ, respectively. The line structure was very sharp and smooth, exhibited little thermal damage and a small amount of debris, and had the same width near the ends, as shown in Fig. 4(a). The aspect ratio reached 72. Furthermore, the line-shaped beam had linear polarization, but no nanograting was found because the processing was performed in a single shot. Ordinarily, in line fabrication using femtosecond laser pulses, a scanning Gaussian beam is used, and the beam must be carefully set to have a circular polarization, because it is well-known that repeated irradiation of a femtosecond laser pulse with a directional polarization, including elliptical and linear polarizations, forms nanogratings, as shown in Fig. 4(b).
Fig. 3 SEM images of structure processed using holographic cylindrical lenses with window functions (a) wrect, (b) whann, and (c) wmak, respectively. E means the irradiation energy on the sample plane.
Fig. 4 (a) SEM images of structure processed using the line-shaped beam with wmak. Processing was performed with E = 7.1 μJ, a repetition frequency of 1 Hz, and a scanning speed of 2 μm/s. (b) SEM images of structure processed using a scanning focused Gaussian beam. Processing was performed with E = 142 nJ, a repetition frequency of 1 kHz, and a scanning speed of 60 μm/s.
Figure 5 shows the length of the line structure versus the irradiation energy. When wmak was used, the length changed more steeply with increasing irradiation energy compared with the case where whann was used. This is also evidence that the line-shaped beam generated by the holographic cylindrical lens with wmak had high uniformity over a wide area, because the slope depends on the spatial uniformity of the line-shaped beam, in addition to the nonlinearity of the ablation phenomenon.
Fig. 5 Length of fabricated structure versus irradiation energy. The filled triangles and circles show the results obtained with whann and wmak, respectively.
As a unique example showing the potential of holographically-generated line-shaped beam processing, we demonstrated a line-shaped beam deformed three-dimensionally. The beam was calculated by
which has a focal length depending on x. f(x) in Eq. (5) is the sine function described as f(x) = f0 + 100 × sin (0.58x), where f0 is 1000 mm, x is from 0.00 to 10.89 mm. As an example, a curved structure was formed in glass using a holographic cylindrical lens with different focal lengths at different positions, as shown in Fig. 6(a). Figure 6(b) shows a side view of the structure observed by a transmission optical microscope. The processing was performed in a single shot with E = 24 μJ. By using this technique, line-shaped beam processing can be performed on a curved surface.Fig. 6 Three-dimensional line structure fabricated by single-shot pulse irradiation inside glass. (a) Phase distribution of holographic cylindrical lens with different focal lengths. (b) Transmission optical microscope side view image of the fabricated structure.
4.2 Laser peeling of ITO film
Figure 7 shows wide laser peeling using a line-shaped beam generated by a holographic cylindrical lens. The sample was an ITO film with a thickness of 10 nm coated on a glass substrate. Figure 7(a) shows the holographic cylindrical lens with wmak. Figure 7(b) shows the optical reconstruction and its profile captured at the focal plane. The aspect ratio of the line-shaped beam was 52. In a comparison of the aspect ratio between the fabricated line structure (Fig. 3(c)) and the optical reconstruction, these values roughly agreed although there is a nonlinearity of laser processing. The laser peeling shown in Fig. 7(c) was performed with E = 2.7 μJ, a pulse repetition frequency of 50 Hz, and a sample scanning speed of Vscan = 2 μm/s. The threshold power for laser peeling of the ITO membrane, Eth(ITO), was 0.5 μJ, which was one-sixth of the ablation threshold of the super white crown glass (Eth(glass) = 3.0 μJ). When E = 1.4 μJ, at a scanning speed of Vscan = 25 μm/s or higher (an irradiation interval of 500 nm), the ITO membrane was partially removed in the form of a periodical grating; that is, the peeling operation failed. When E = 2.7 μJ (10% smaller than the ablation threshold of the glass), at Vscan = 25 μm/s the ITO membrane was completely removed. When E > Eth(glass), the glass substrate was damaged, as expected. In summary, suitable conditions for laser peeling were an energy E slightly smaller than the ablation threshold of the substrate and an irradiation interval smaller than the width along the short axis (0.58 µm in this experiment). We found that little debris around the fabricated area was observed. The debris was removed by the beam itself.
Fig. 7 (a) Phase distribution of holographic cylindrical lens and (b) its optical reconstruction. (c) Line-shaped laser peeling of ITO membrane on glass substrate.
4.3 Laser grooving on stainless steel
Figures 8(a) and 8(b) show laser grooving on stainless steel, showing a confocal microscope image and the profile, respectively. The grooving was performed using a line-shaped beam with E = 1.1 μJ, a pulse repetition frequency of 1 kHz, and Vscan = 20 μm/s, generated by a holographic cylindrical lens with wmak. In this case, linearly polarized light was used, and nanogratings with 500 nm spacing were observed, as shown in Fig. 8(c). When linearly polarized light in the perpendicular direction was used, we observed a nanograting oriented in the perpendicular direction, as shown in Fig. 8(d). When circularly polarized light was used, no nanograting was formed, as shown in Fig. 8(e).
Fig. 8 (a) Laser grooving of stainless steel with a line-shaped beam and (b) its depth profile. (c)–(e) SEM images of the structures processed with various polarizations.
From confocal microscope observation (Fig. 9), the depth of the grooves had a non-monotonic dependency on E and Vscan of the 1 kHz pulses with a linear polarization, although as a general prediction, the depth should increase with increasing E and decreasing Vscan. When E was set to the ablation threshold of stainless steel for a single shot, Eth(stainless) = 1.1 μJ, at Vscan = 3 μm/s, a maximum depth of 6.1 μm was measured, but at Vscan = 2 μm/s, the depth reduced due to debris. At higher Vscan, a decrease in the depth and a decrease in the rate of change of depth with increasing E were observed. When 0.4Eth(stainless) < E < 0.8Eth(stainless), at Vscan = 30 and 50 μm/s, swelling on the laser irradiation area was observed, as shown in Fig. 10(a). The laser irradiation area had a rough surface and a maximum height of 0.8 µm. This result was obtained with E = 0.7 μJ and Vscan = 50 μm/s. From the SEM image shown in Fig. 10(b), we found that the swelling included upwardly growing nanogratings with an interval of ~500 nm. The debris on the top of the nanogratings contributed to the swelling of the surface.
Fig. 10 (a) Confocal laser microscope and (b) SEM images of laser grooving of stainless steel using a line-shaped beam with E = 0.7 μJ and a scan speed of 50 μm/s. The grayscale bar indicates heights of −1.1 to 1.8 μm.
5. Conclusion
We proposed a material laser processing technique using line-shaped femtosecond pulses. The line-shaped femtosecond pulses were generated by a holographic cylindrical lens displayed on a spatial light modulator. We demonstrated single-shot fabrication of a line structure on a glass surface. The line structure had a uniform width near its ends, because the intensity distribution was well-controlled by adequate selection of a window function for the holographic cylindrical lens. We also demonstrated laser peeling of an ITO film and laser grooving of stainless steel with line-shaped pulses. Little debris was observed around the structure in the laser peeling of an ITO film. In the laser grooving of stainless steel, we found the swelling of the surface included upwardly growing nanogratings because of the deposition of the debris on the top of the nanogratings.
Femtosecond laser processing is usually considered to have low throughput, but the use of holographically-generated line-shaped femtosecond pulses is highly promising for large-area machining with high throughput in laser cutting, peeling, and grooving of materials. Furthermore, the use of a spatial light modulator allows processing of target materials having non-planar, three-dimensional shapes.
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