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In this paper, self-organized microgratings are fabricated in SrTiO3 crystal just by scanning the focus of a tightly-focused linearly-polarized femtosecond laser beam to form a single line. The polarization direction of the laser beam is rotated by a λ/2 waveplate to check the effect of the polarization azimuth on the micrograting morphology. Fourier analyzing of the microscopic images of the microgratings indicates that the polarization plane azimuth of the laser beam does have influence on the microgratings in the aspects of groove orientation and groove spacing. A possible mechanism of polarization dependence is also proposed.
©2013 Optical Society of America
1. Introduction
Femtosecond laser has exhibited a powerful micro-processing ability in metals, semiconductors, dielectrics, polymers and etc. Specially, femtosecond laser is very suitable for three-dimensionally inscribing transparent dielectrics through multiphoton ionization mechanism owing to its ultrashort pulse duration and ultrahigh peak power. Up to now, a wide variety of micro-/nano-structures have been prepared in transparent materials, including voids, filaments, bubbles, nanochannels and periodic ripples [1–6]. Among these structures, self-organized structures attracted much more interest from researchers for their possible contributions to the light-material interaction theory as well as the parallel machining strategies. The most influential reports on self-organized structures should be traced back to the subwavelength ripples on the surface of compound semiconductors (A. Borowiec et al., 2003), the nanoscale light fingerprints in bulk of fused silica (Y. Shimotsuma et al., 2003) and the self-organized nanovoids along laser beam propagation axis inside fused silica (S. Kanehira et al., 2005) [6–8]. On the basis of these pioneering works, much significant progress has been made in the past ten years. V. R. Bhardwaj revealed that the two-dimensional nanofingerprint was in fact a three-dimensional array of nano-planes with their normals parallel to the laser polarization [9]. Shortly afterwards, this type of nanoplane array was found to have the pulse duration dependence, the form birefringence effect and the erasable property [10–12]. As for the self-organized voids possibly serving for multiple-focus processing, in addition to the method of tightly focusing of laser beam, they were verified to be also generated either through focusing a Gauss-Bessel beam or by focusing a truncated laser beam [13,14]. We also did some researches on the self-organized structures, including the self-formed microvoid array perpendicular to the laser propagation direction, the quasi-periodic void array grown oppositely to the laser propagation direction and the orientation-controllable self-organized microgratings [15–17]. This paper focuses on a further study of the self-organized micrograting which was reported to be rapidly formed by just scanning the laser focus along a line perpendicular to the laser propagation axis in our previous paper. This type of self-organized micrograting was previously proved to have its orientation controlled by the irradiation pulse number per unit scanning length. In this paper, a new feature of polarization dependence is found for these microgratings.
2. Experiments
The sample used in our experiment is a piece of four-facet-polished SrTiO3 crystal which has a cubic perovskite crystal structure and has no birefringence effect. A Ti:sapphire laser system launches pulses with a pulse duration of 120 fs and a wavelength of 800 nm at a repetition rate of 1 kHz. A variable neutral filter was used for continuously attenuating the laser power. As shown in Fig. 1, the linearly-polarized laser beam was tightly focused into the bulk of SrTiO3 crystal through an objective lens with a high numerical aperture of 0.9. Scanning of the laser focus in the samples was implemented by moving the motorized sample-mounting stage with respect to the stationary lens. Micromachining process was monitored by a real-time imaging system. The self-organized micrograting was induced by just scanning a line perpendicular to the laser beam propagation direction, as sketched in Fig. 1 where K, S, E respectively stand for the laser propagation direction, the scanning direction of laser focus and the polarization direction. The polarization azimuth angle φ can be continuously tuned by rotating a λ/2 waveplate inserted before the objective lens.
3. Results and discussions
The (100) crystalline plane of the SrTiO3 sample is chosen as the X-Y entrance plane of Fig. 1. In our experiment, three types of linear polarization states were used, which have the polarization azimuth angle φ of 0°, 45°and 90°respectively. The laser focus, situated at 100 μm beneath the crystal surface, was scanned at a velocity of 100 μm/s to form a single line along Y axis of Fig. 1. The obtained microstructures were observed from Y-Z plane. The different microstructures corresponding to the three different polarization states are displayed in Figs. 2(a)-2(c), where the laser beam horizontally propagated from left to right and scanning direction is vertically upward. Apparently, all the microstructures are quasi-regular self-organized microgratings as reported previously by us. Interestingly, the polarization state has negligible effects on the detailed structures of the microgratings that grooves of microgratings seem to tilt more heavily and to be self-assembled more densely with the increasing of polarization azimuth angle. In order to investigate the periodic structures in more detail, the images shown in Figs. 2(a)-2(c) were respectively subjected to a discrete two-dimensional Fourier transformation (2D-FT), the results of which are sequentially shown in Figs. 3(a)-3(c). Intuitively, there are mainly three spatial frequencies in every subfigure of Fig. 3 and the spatial frequency vector (gx, gy) is gradually counterclockwise rotated around the origin of the coordinate system when azimuth angle φ of polarization was increased from 0° in Fig. 3(a) to 90° in Fig. 3(c). A detailed analysis of Fig. 3 can obtain the spatial frequency vector (gx,gy), and then the corresponding spatial frequencies, spatial periods and groove orientation angles θ are listed in Table 1. Comparison of Table 1 and Fig. 2 can draw two conclusions. The first conclusion is that any of the microgratings in Fig. 2 can be divided from left to right into three sections with different grating parameters. Specifically, the groove spacing is on the whole gradually decreased and the groove orientation angle θ is slowly increased from the left to right section of the image. The second conclusion is that the laser beam with polarization azimuth angle φ of 0° and 90° respectively induced most dense and most sparse grooves and separately led to a largest and a smallest groove orientation angle, while the laser beam with polarization azimuth angle φ of 45° fabricated a micrograting with a groove spacing and a groove orientation angle located between the above extreme values.
Fig. 2 Self-organized microgratings induced by the polarization states with azimuth angle φ of 0° (a), 45° (b), 90° (c).
Fig. 3 Two-dimensional Fourier transformation analysis of the microscopic images in Figs. 2(a)-2(c).
Table 1. The grating parameters calculated by detailed analysis of the Fourier-transformed results of Fig. 3.
In our previous paper, we suggested that self-organized void arrays induced by the stationary irradiation of laser focus in the sample were building blocks for formation of self-organized microgratings [17]. Furthermore, to explain the unexpected tilted grooves of self-organized microgratings, a void-moving model based on the bubble motion phenomenon reported by W. Watanabe was proposed. W. Watanabe experimentally verified that even without moving the laser focus, the successive irradiation of multiple laser pulses moved a microscopic bubble against the laser propagation direction inside crystalline calcium fluoride and amorphous silica glass [18]. They considered that for a laser beam propagating from left to right, an additional pulse most easily induced a new microexplosion at the left interface of the pre-formed void where damage threshold was significantly decreased because of possible defects and color centers generated by the previous pulses, and then the debris of the new microexplosion immediately filled up the pre-formed void so that void seemed moved against laser propagation direction. Our void-moving model for formation of self-organized microgratings with tilted grooves is sketched in Fig. 4, where with laser focus scanned perpendicular to the laser propagation direction, two adjacent void arrays were formed sequentially and partially overlapped along scanning direction. In Fig. 4, the pre-formed void array and the post-formed void array are respectively denoted with red color and green color and the voids in every single void array are sequentially numbered with Arabic numbers. Based on results of W. Watanabe, it is easily understood that the post-formed green void array tends to shift horizontally against the laser propagation direction with respect to the pre-formed red void array. From Fig. 4, it is clear that the micrograting grooves are formed by connecting of voids in adjacent void arrays. Hence, the groove width is determined by the void diameter and the groove tilting is caused by the horizontal displacement distance of the void against the laser propagation direction. We previously reported that the groove orientation of microgratings was dependent on the scanning velocity of laser focus. According to our modeling of formation process of microgratings at different scanning velocities, for the scanning velocity of 100 μm/s employed here, the following four developing stages for the post-formed void array are necessary to form the micrograting with groove orientation angle θ between 0° and 90°, as shown in Fig. 4. Clearly, upon successive irradiation by the relatively high pulse number of 10shots/μm, the void in the post-formed green void array moves such a long distance that it is firstly detached from the void with the same Arabic number in the pre-formed red void array and then it catches, sticks on and finally surpasses the void labeled with neighboring smaller Arabic number in the pre-formed red void array. Therefore, the tilting degree of the groove orientation is in fact determined by the difference between the actual moving distance |AB| and the void-void spacing |CB|. From the previous reports on void arrays and the gx component of spatial frequency vector of microgratings listed in Table 1, it is sure that the void diameter and the void-void spacing both decreased from left section to right section of the micrograting. According to the results of W. Watanabe that the larger void generated with pulses of a higher energy moved a longer distance against laser propagation direction, we consider that although the larger void in the left section of micrograting moved at a longer distance of |AB| than the smaller void in the right section, the significantly larger void-void spacing |CB| on the left than on the right makes the effective distance |AC| contributing to the groove orientation is decreased from left to right. This explains that the groove orientation angle θ increased from left to right in every single micrograting.
Fig. 4 Schematic graph for the formation process of self-organized microgratings. The arrows labeled with K and S respectively stand for the laser propagation direction and the scanning direction.
Now we will pay more attention to the reason for the polarization dependence of microgratings. According to our void-moving model, morphology differences of the microgratings are directly related to the fine structure of the void array as building blocks. Therefore, the influence of laser polarization on self-organized void arrays must be firstly investigated. The results are shown in Figs. 5 (a)-5(b), where laser beam with a pulse energy of 60 μJ was focused 100 μm beneath the surface of SrTiO3 crystal for a stationary irradiation time of 1/125 s. It is clear that the void array induced with the laser beam of polarization azimuth angle of 0°has a larger void diameter and a larger void-void spacing relative to that for the laser beam with polarization azimuth angle of 90°. With regard to the void array formation, we once suggested that interface spherical aberration caused by the refractive index difference between air and sample is the possible mechanism. However, the axial multi-focus light field distribution caused by interface spherical aberration is only determined by the refractive index difference, the focal depth and the convergence angle of light and it does not have any relationship with azimuth angle of linear polarization. Therefore, why void array changes with polarization is a puzzling phenomenon. D. Liu et al. once reported that the bulk damage threshold of fused silica changed with the direction of linear polarization of light [19]. Their conclusion was further verified in our experiment conducted in fused silica for investigating polarization dependence of void array, as shown in Fig. 6. Figure 6 shows that with the pulse energy fixed, changing azimuth angle of linear polarization has pronounced effects on void array length. The intuitive polarization-dependent damage threshold here was also reported by Peter G. Kazansky as anisotropic photosensitivity of an isotropic alumosilicate glass [20]. Peter G. Kazansky et al. experimentally demonstrated that the laser-modified structures in isotropic medium depend not only on laser polarization but also on pulse front tilt value. They put forward that it is the mutual orientation of light polarization azimuth (LPA) and the pulse front tilt (PFT) caused the anisotropic photosensitivity. Because LPA and PFT are the attributes possessed by the laser beam alone, we believe that this mechanism is also applicable to fused silica glass and similarly isotropic cubic-crystal SrTiO3. Experimental facts in Fig. 6 inspired us to put forward a damage-threshold-related mechanism for the polarization dependence of void arrays in Fig. 5. As can be seen in Fig. 5(b), the void array induced at the polarization state of azimuth angle φ = 90° arranges so dense that adjacent voids nearly connects with each other. We infer that when polarization state of azimuth angle φ = 0° was used (Fig. 5(a)), the bulk threshold of SrTiO3 was much lower relative to the polarization state of φ = 90 °, so the same multi-focus distribution of light as that in Fig. 5(b) will generate larger voids in Fig. 5(a) so that adjacent two voids completely connected to merge into a single larger void. At the same time, due to the high transverse binding ability of the lateral bulk material, debris of the newly-formed larger void are preferentially ejected along the laser axis so that the newly-formed voids are separated with each other again by debris to form a larger-period void array rather than to be connected all together to form a long hollow microchannel. Merging of two close voids into one single larger void was once reported by W. Watanabe [21]. So the process suggested by us is reasonable. According to the above analysis, the larger void diameter and the larger void spacing in Fig. 5(a) possibly result from the merging of voids and the axial rearrangement of voids induced by the polarization-dependent lower bulk damage threshold. The larger void-void spacing in Fig. 5(a) explains the larger groove period in Fig. 2(a) relative to that in Fig. 2(c). Moreover, as the same with the analysis of the single micrograting, voids in Fig. 5(a) roughly have larger diameter than that in Fig. 5(b) and correspondingly move a longer distance |AB| against the laser propagation direction. However, the increasing amount of the moving distance |AB| of voids in Fig. 5(a) relative to Fig. 5(b) cannot go beyond the enhancement amount of void-void spacing |CB| so that the micrograting in Fig. 2 (a) has less tilted grooves than that in Fig. 2(c).
Fig. 5 Effect of polarization direction on the void array induced in the bulk of SrTiO3 crystal. Polarization azimuth angle φ is 0° (a) and 90°(b)..
Fig. 6 Polarization dependence of void arrays induced in fused silica by tightly focusing laser beam with a pulse energy of 35 μJ at a focal depth of 400 μm for a stationary irradiation time of 1/4 s.
4. Conclusion
In this paper, effects of the azimuth angle of linear polarization on the self-organized microgratings are investigated. It is found that with polarization azimuth angle changes from 0°, 45° to 90°, the micrograting has a gradually decreased groove period and the grooves of microgratings has an slowly increased degree of tilt. We propose that polarization-induced bulk damage threshold difference makes that the void arrays as building blocks of microgratings has a polarization-dependent fine structures further resulting in micrograting morphology difference. This research adds the polarization state with the irradiation pulse number per unit scanning length as diverse controlling factors for self-organized microgratings which have the potential to be used as micro-optical devices in the lab on the chip.
Acknowledgments
The authors greatly appreciate the experimental contribution from Mengdi Qian and Junyi Ye. Thanks for the financial support from the National Natural Science Foundation of China (Grant Nos. 61205128, 11102075 and 51132004) and the Research Foundation for Advanced Talents of Jiangsu University (No. 09JDG022).
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