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Abstract
Manipulation of femtosecond laser induced microstructures in glass by tuning the laser polarization has great potential in optics. Here we report two different polarization-dependent microstructures and their evolution with pulse repetition rate in an aluminosilicate glass induced by femtosecond laser irradiation. A V-shaped crack oriented parallel to the laser polarization plane is induced at the bottom of modified regions by pulses operated at 200 kHz, 1030 nm, and 300 fs. Further increasing the pulse repetition rate to 500 kHz leads to the formation of a dumbbell-shaped structure, which is elongated perpendicularly to the laser polarization, at the top of the modified region. The size of the coloration area and the dumbbell-shaped structure can be controlled by tuning the pulse duration. Further investigation indicates that higher numerical apertures are in favor of the presence of the polarization effects in femtosecond laser irradiation. The possible mechanism responsible for the formation of the two microstructures is discussed. These results could be helpful for understanding of ultrafast laser interaction with glass.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Femtosecond laser micro-nano machining of materials has manifested its great capability in both scientific research and industry in recent decades [1–5]. Among these attractive applications, femtosecond laser processing of transparent materials is of particular interest due to its ultra-high precision and three-dimensional processing ability [6–8]. Glass is a kind of isotropic transparent material with special properties, such as homogeneity, reversibility, no fixed melting point and extraordinary optical properties [9]. These properties make it important in modern optics including applications as mirrors, lens, and gain media for laser pulsing. Femtosecond laser micromachining of glass makes it more versatile, which has opened new frontiers in different applications. For example, femtosecond laser induced local refractive index change in glass can function as optical waveguides [10,11], the induced self-organized nanogratings are widely employed in micro optical elements/devices and optical storage [12,13], and the tailored surface nanostructures endow the material surface antireflective or super-hydrophobic property [14,15].
Aluminosilicate glass is a kind of multicomponent glass widely used in fiber amplifiers [16], liquid crystal display substrates [17], and radioactive waste glasses [18], due to their small coefficients of thermal expansion, good chemical durability and good thermo-mechanical properties. Femtosecond laser micromachining of aluminosilicate glass is thus fascinating for their great potentials in the as-mentioned research field. However, there are still few investigations of femtosecond laser micromachining of this type of glass up to now. About a decade ago, Kazansky et al. observed anomalous anisotropic photosensitivity of an aluminosilicate glass irradiated by linear polarized femtosecond laser. They anticipated that the photosensitivity of such isotropic medium can be controlled by employing mutual orientation of the polarization plane and the pulse front tilt of a laser [19]. A few years later, Zhang et al. demonstrated that a dumbbell-shaped defect structure and bubble aggregation can be induced in the glass by tuning the irradiation time, and there orientations are controllable by simply varying the polarization plane of the laser [20]. The as mentioned two studies reveal the effects of laser polarization on the induced internal structures in aluminosilicate glass. Nevertheless, the laser parameters employed in these experiments were very limited, for example, the repetition rate and the pulse duration were fixed. Therefore, it would be important to investigate the structural evolution by change of the laser parameters, which is helpful for the understanding of the formation mechanism of the observed phenomena and the exploring of new structures.
Previous studies show that laser parameters including pulse repletion rate (PRR), polarization, pulse energy, and pulse duration have great impacts on laser induced structures in other glasses [3,21]. In this paper, we investigate the effects of PRR, polarization, pulse energy, and pulse duration on the modification in aluminosilicate glass by femtosecond laser irradiation. The results show that different types of polarization–dependent modifications can be induced at different PRRs. It is possible to control the size of polarization-dependent modification by tuning the pulse duration. Moreover, the numerical aperture (NA) of the focusing objective lens and the pulse energy play important roles in the revealing of polarization effect in the modification. A brief discussion on the possible mechanism of the evolution of polarization-dependent modifications with the PRR is proposed.
2. Experimental section
The sample studied in this work is an aluminosilicate glass (glass composition 64SiO2-17Al2O3-5B2O3-15CaO, wt%) with dimensions of 20×5×5 mm3, which was well polished (surface roughness ∼20 nm) and mounted onto XYZ translation stage controlled by a computer. The sample was irradiated with the mode-locked Yb:KGW ultrafast laser system (Pharos, Light Conversion Ltd.) operating at a central wavelength of 1030 nm (photon energy ∼1.2 eV) and delivering pulses with repetition rates from 100-500 kHz. The pulse duration ranging from 300 fs to 1 ps with the pulse energy varying from 0.04 µJ to 1µJ was employed. The linearly polarized Gaussian beam, was focused into the glass sample around 100 µm below the surface via a 0.55 or 0.65 NA-objective lens. The beam spot size in the focus was estimated to be ∼2.28µm for 0.55 NA and ∼1.6µm for 0.65 NA using the formula 2ω0 = 1.22 λ / NA. The incident average fluence (defined as the pulse energy divided by the focal area) is calculated to be ∼0.0068–0.27J/cm2 depending on pulse energy. The pulse energy and polarization were controlled by an attenuator and a half-wave plate, respectively. The laser beam profile was analyzed both before and after the laser focus using a laser beam profiler (ML3743, Metrolux), which showed good Gaussian distribution of intensity in both cases, indicating no asymmetry of intensity distribution of the laser beam.
Optical images of the laser-modified regions were captured with a CCD camera attached to a Nikon microscope (Eclipse 80i) (transmission mode). The modified areas were characterized with a quantitative birefringence measurement system (CRi Abrio mounted on an Olympus microscope BX51) operating at 546 nm wavelength. Raman spectroscopy characterization of the laser-modified region was carried out by a laser confocal Raman spectrometer (Renishaw, inVia) with a 532 nm laser as the excitation source. All the measurements were carried out at room temperature.
3. Results and discussion
A series of locations in the sample were irradiated with various combination of experimental parameters (PRR, polarization, pulse duration, and laser energy). The parameters are summarized in Table 1. Figure 1(a)‒(e) show the modified regions irradiated by 1 million pulses operated at PRR ranging from 100 kHz to 500 kHz. For each PRR, two orthogonal polarizations are applied. The N.A. of the focusing lens is 0.65 and the pulse duration is fixed at 300 fs. The top view and side view of the modified regions are collected by optical microscopy. Obviously, the modified regions show a concentric shape on the whole in top views with their sizes changed with PRR. The region diameters are plotted in Fig. 1(f), which show an approximate linear increase as a function of PRR. This is due to the enhanced thermal accumulation effect with increasing PRR as more energy can be deposited in the focus in unit time, leading to considerable temperature elevation in the focus [21–23]. More features can be identified in the center of the modified regions from top views. At PRR of 100 kHz, the region modified at 0° polarization angle shows a lighter color compared to that modified at 90° polarization angle, as shown in Fig. 1(a). When the PRR is tuned to 200 kHz and 300 kHz, no obvious difference can be identified between 0° and 90°. However, nonuniform structures begin to appear in the center when the PRR is raised to 400 kHz. A brown spot oriented towards different direction formed in the center of the modified region. The orientation of the brown spots seems polarization dependent, which needs further clarification. With PRR raised to 500 kHz, a dump-bell shaped structure emerges, with its elongation direction perpendicular to the laser polarization plane.
Fig. 1. Optical microscope images (bright field) of the structures induced by femtosecond pulses operated at various repetition rates in the glass: 100 kHz (a), 200 kHz (b), 300 kHz (c), 400 kHz (d), 500 kHz (e), and the diameters of the modified regions as a function of repetition rates (f). Each spot was irradiated by 1 million pulses with pulse energy of 1 µJ, the blue arrows indicate laser polarization plane for each spot (horizontal arrow represents 0° and vertical arrow represents 90°), the inserts in (a) and (b) are magnified pictures of the bottom of the corresponding modified regions. The red cycle denotes the way of diameter measurement. The yellow circles/ellipses indicate the location of the inhomogeneous structure. Laser beam propagated from top in the side view pictures. The scale bar is 20 µm. The N.A. of the focusing lens is 0.65 and the pulse duration is fixed at 300 fs.
Table 1. Single factor experiment on laser induced polarization dependent structures
More details about the features of the modified regions are obtained in the side views. The inserts in Fig. 1(a) and (b) are magnified pictures of the bottom of the modified regions viewed from side. There are visible differences lie in the bottom structures formed at 0° polarization and 90° polarization in both 100 kHz and 200 kHz experiments. At 100 kHz, the tip formed with 0° polarization seems smaller than that formed with 90° polarization, which coincides with the color contrast in top views. At 200 kHz, A V-shaped structure is formed at the bottom under both polarizations. Interestingly, although no difference can be distinguished in the top view pictures, the V-shaped structure formed at 0° polarization shows a wider opening compared to that at 90° polarization. The sizes of these bottom structures in 100 kHz and 200 kHz experiments are around 2∼4 µm, which is slightly bigger than the size of the laser focus. At 300 kHz, no notable difference can be identified between the structures induced by the two orthogonal polarizations, which is in coincidence with the top views. At 400 kHz, the coloration at the bottom is considerably enhanced with both polarizations. In addition, two slightly colored spots emerge at the top of the modified regions with one offset right and the other offset left, coinciding with the orientation of the brown spots in the top views. The side view pictures indicate that brown spots are located at the top of the modified regions. When the PRR is raised to 500 kHz, the coloration area at the top is more enhanced than that at the bottom, as opposed to that formed at 400 kHz. In addition, the dumbbell-shaped structures observed in top views are also located at the top of the modified regions, as a part of the coloration area. The size of the brown spot formed at 400 kHz is ∼7 µm and that of the ‘dumbbell’ formed at 500 kHz is ∼15µm, which are much bigger than the size of the focus.
Since the bottom structures formed by 0° polarization and 90° polarization are different when the PRRs are100 kHz and 200 kHz, it is essential to verify whether they are polarization dependent. As the V-shaped structure formed at 200 kHz provides better contrast, pulses operated at 200 kHz with various polarization angles were applied for irradiation. The N.A. of the focusing lens is 0.65 and the pulse duration is fixed at 300 fs. Figure 2 shows the top and the side views of the regions modified by pulses polarized along 0°, 30°, 60° and 90°, respectively. To capture the features of the bottom structures, the imaging focus plane were set at the bottom when the optical microscope images were taken from top. As demonstrated, there is a black line with a break in the middle of the region where the laser propagates. Apparently, the black lines always tend to orient parallel to the polarization plane. The side views show that the rotation of the V-shaped crack follows the rotation direction of the polarization plane. Therefore, it is reasonable to regard the bottom structure as a V-shaped crack with its orientation controlled by the laser polarization plane. The left part of the V-shaped crack seems always longer than the right counterpart, which could be resulted from a little tilt of laser beam with respect to the sample surface.
Fig. 2. Top view pictures (top row) and side view pictures of the regions induced by pulses operated at 200 kHz with polarization angle of (a) 0°, (b) 30°, (c) 60° and (d)90°, respectively. The top view pictures were taken by setting the imaging focus plane at the bottom of the modified region to reveal the details of the bottom structure. The N.A. of the focusing lens is 0.65 and the pulse duration is fixed at 300 fs.The scale bar is 10 µm. The images were captured in bright field.
In contrast to the V-shaped crack, the elongation direction of the dumbbell-shaped structure is perpendicular to the laser polarization plane. The contrast indicates that the formation mechanisms of the two kind of polarization-dependent microstructures could be different. Based on the above observations, a tentative evolution process with PRR could be described as follows. At relatively low pulse repetition rates (100 and 200 kHz), the heat accumulation effect is relatively weak. The laser induced transient stress waves could lead to the cracking in the focus [24]. Similar phenomenon is observed at 100 kHz while the induced structure is much vaguer than the V-shaped structure. At moderate repetition rate (300 kHz), as the thermal accumulation effect is enhanced, it could lead to the suppression of the polarization effect, and no polarization-dependent features can be observed. At higher repetition rates (400 and 500 kHz), the heat accumulation effect is further enhanced, which will gradually lead to the formation of the coloration area at the top of the modified region. The colored area will highly increase the absorption at the top and result in the formation of more defects and color centers. Due to the different reflection coefficients for s and p-polarization on the interface between the colored area and the glass matrix, a lower temperature field elongated perpendicular to the laser polarization plane will form. As the absorption at the top continue to rise, the temperature there may become too high to the stable existence of defects and color centers. Then the defects and color centers will tend to drift to the lower temperature field, and form a dumbbell-shaped structure elongated perpendicular to the laser polarization plane [20]. Therefore, the domination of laser polarization on the orientation of the dumbbell-shaped structure is likely realized by affecting the local temperature field accompanied by some positive feedback mechanism. However, the full explanation on the observed evolution of laser polarization effect with PRR required complicated calculations and more systematic experiments.
Manipulation of the orientation of the dumbbell-shaped structure can be realized by tuning the linear polarization plane of the laser, as shown in Fig. 3(a), which is in analogy to the nanogratings formed in other glasses [25–27]. To better understand the formation mechanism of the dumbbell-shaped structure, it would be necessary to clarify the stress distribution around it. The birefringence signals in the three regions are collected by a quantitative birefringence measurement system. Figure 3(b) shows the mapping of optical retardance value in the modified regions on the horizontal plane, which reflects the birefringence intensity caused by stress. It is noted that the stress birefringence in the three modified regions shows very similar circular distributions, i.e. a ring-shaped maximum intensity at the margins, which declines monotonically toward the center. Figure 3(c) shows the slow-axis distribution of the corresponding stress birefringence demonstrated in Fig. 3(b). Clearly, a radial distribution of the slow-axis at the maximum birefringence ring is observed, which indicates that the stress gradient is along radial direction. The results indicate that the central area of the modified region is rarefied while the outer ring is compressed after irradiation, which is in agreement with the observation by Shimizu et al. [23].The results also suggest that the stress distribution around the dumbbell-shaped structure is not visibly affected by its orientation, which implies that the influence of laser polarization on the stress distribution is negligible.
Fig. 3. (a) Optical microscope pictures of the top views of the three regions irradiated by polarizations denoted as 0°, 45°, 90°; (b) distribution of birefringence in the three corresponding regions in (a); (c) distribution of slow axis in the three corresponding regions in (a); (d) Raman spectra of the glass matrix and different spots in the modified region. The N.A. of the focusing lens is 0.65 and the pulse duration is fixed at 300 fs.
To further analyze the structural change in the colored region, Raman spectroscopy was performed. The blue curve in Fig. 3(d) is collected from the unirradiated glass matrix (spot A), showing three broad bands centered at 556, 792, and 1099 cm-1, respectively. The band near 550 cm-1 can be assigned to the bending vibration modes of Si(Al)-O-Si(Al) bridges in glass [28], the band near 800 cm-1 is usually assigned to the cage motion of Si–O stretching vibrations [29], and the band near 1100 cm-1 is assigned to the antisymmetric stretching mode of Si–O–Si group involving mainly oxygen motion along the Si–Si direction [30]. When the spectrum was collected from the colored region, two new sharp peaks appeared at 174 and 238 cm-1. Moreover, the intensities of the two peaks increase as the coloration became stronger. Generally, the presence of a sharp peak in the Raman spectrum of glass is due to the occurrence of phase separation and crystallization. The increased intensities are likely due to an increased local order of the lattice structure at different location. The Raman spectra indicate that the dumbbell-shaped structure (spot D) experienced the same kind of modification as the rest part of the colored region. This result implies that the coloration at the top of the modified region is possibly due to the formation of crystalline phase. The anisotropic reflection of s and p-polarization components of the laser on the interface results in an elongated lower temperature field perpendicular to the laser polarization plane, which provides suitable temperature for the growth of crystal nucleus.
Pulse duration is another important parameter in ultrafast laser processing of transparent materials because the thermal effect is positively correlated with the pulse duration [31]. Experiment has also been carried out to unearth the influence of pulse duration on the polarization effect. Here the N.A. of the focusing lens is 0.65 and the pulse energy is fixed at 1 µJ. Figure 4 shows the structures induced by 500 kHz pulses with pulse duration ranging from 300 fs to 1000 fs. The dumbbell-shaped structures are formed in all the modified regions. In addition, the dumbbell-shaped structure along with the coloration region extend considerably with increasing pulse duration. As is known, longer pulse duration generates stronger thermal effect during irradiation, and the heat can accumulate under irradiation of 500 kHz pulses because the time interval between two adjacent pulses is as short as 2×10−6 s. The heat produced by former pulses is unable to fully disperse out of the focus, leading to a continuous temperature elevation within the inner region. The steep temperature gradient can drive more alkaline earth ions (Ca2+) to migrate and lead to the disruption of glass network at the location where Ca2+ ions aggregate, which may result in phase separation and even crystallization. The EPMA analysis showed that Ca2+ ions tended to migrate to the top and bottom of the modified regions [20], which explains why the coloration occurred at both the top and the bottom. However, the coloration at the top is much stronger than that at the bottom. This phenomenon could be explained as follows. At initial stage, more Ca2+ ions were driven to the bottom than that to the top due to the gravity and light momentum, which results in coloration more quickly at the bottom. As the irradiation goes on, more and more Ca2+ ions migrate to the top and coloration begins to occur. Once a slight coloration appears at the top, the absorbance of the coloration area will increase significantly, leading to a decreased laser intensity at the bottom. The enhanced absorbance at the top will further facilitate the coloration, and vice versa. This process further reduces the laser intensity at the bottom and hinders the growth of the coloration at the bottom. As a result, the coloration at the top is much stronger than that at the bottom. This could also be the reason why the polarization effect leading to the formation of the dumbbell-shaped structure presented at the top.
Fig. 4. Various regions modified by linear polarized pulses with pulse duration ranging from 300 fs to 1000 fs. The pulse energy is 1 µJ, N.A. is 0.65, and repetition rate is 500 kHz. The images were captured in bright field.
Further experiment has been conducted to verify the influence of the focusing objective lens on the revealing of polarization effect. Two objective lens with 0.65 NA and 0.55 NA respectively are employed for comparison. The pulse duration is fixed at 300 fs. Figures 5(a) and 5(b) show the top view and the side view of the structures induced by the 0.55 NA objective with pulse energy varying from 0.4 to 1.5 µJ. Notably, at low pulse energies (0.4 and 0.6 µJ), the coloration slightly occurred at the bottom center of the modified region. When the pulse energy pulse energy is increased to 1 and 1.5 µJ, the shape of the colored area becomes a ring surrounding the inner boundary of the modified region, slightly above the center viewed from side. However, no polarization dependent structures can be identified. Interestingly, with a 0.65 NA objective, the polarization-dependent dumbbell-shaped structure, surrounded by a circular coloration ring, gradually appears when the laser energy is increased to 1 µJ and above, as shown in Fig. 5(c). Another distinct difference is that the location of the colored areas at 1 and 1.5 µJ are at the top of the modified region in the side view by 0.65 NA objective. In the previous work, the dumbbell-shaped structure was induced with a 0.8 NA objective [20]. Therefore, two conclusions may be deduced from the observations. Firstly, objectives with higher NA are in favor of the formation of the dumbbell-shaped structure, which means that the polarization effect is more obvious when the laser beam is focused with objectives with high NA. Secondly, there is a certain fluence threshold for the formation of the dumbbell-shaped structure, and it is ∼ 50 J/cm2 for the 0.65 NA objective in current situation. In other words, the polarization effect does not appear in the multicomponent glass until the laser intensity density reaches a certain threshold.
Fig. 5. Optical microscope images (bright field) of the modified regions by different pulses energies: top view (a) and side view (b) of the structures induced by use of a 0.55 N.A. objective lens, top view (c) and side view (d) of the structures induced by use of a 0.65 N.A. objective lens. The scale bar is 20 µm. The pulse duration is fixed at 300 fs.
4. Conclusions
In conclusion, we observed the formation of two kinds of polarization-dependent microstructures and their evolution with pulse repetition rate in a homogenous aluminosilicate glass by linear polarized femtosecond laser irradiation. A V-shaped crack with the size close to the laser focus is formed at the bottom center of modified region and oriented parallel to the laser polarization plane after irradiation by 200 kHz pulses. A dumbbell-shaped structure oriented perpendicular to laser polarization plane is formed at the top center after irradiation with 500 kHz pulses provided appropriate conditions. Raman spectra analysis indicates that the dumbbell-shaped structure and the coloration area could be related to crystallization or color centers generation by laser irradiation. Stretching pulse duration to ∼1 ps can facilitate the formation of color centers and extend the size of the dumbbell-shaped structures. Moreover, comparison experiment reveals that objective lens with higher NA and a certain fluence threshold (∼50 J/cm2) are favored in revealing the polarization effect. The observed phenomenon reveals complicated physical processes, which could be helpful for the understanding of ultrafast laser interaction with glass.
Funding
Natural Science Foundation of Guangdong Province (501200050); National Natural Science Foundation of China (11704079, 11774071, 11874125); Science and Technology Program of Guangzhou (201804010451, 201904010104); State Key Laboratory of Luminescence and Applications (SKLA-2019-08).
Acknowledgments
The authors sincerely thank Prof. Peter G. Kazansky for providing the laboratory conditions.
Disclosures
The authors declare no conflicts of interest.
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