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The dynamic process of material ejection and shock wave evolution during one single femtosecond laser pulse ablation of aluminum target in water and air is experimentally investigated by employing pump-probe technique. Shadowgraphs and digital holograms with high temporal resolution are recorded, which intuitively reveal the characteristics of femtosecond laser ablation in the water-confined environment. The experimental result indicates that the liquid significantly restrict the diffusion of the ejected material, and it has a considerable effect on the attenuation of the shock wave. In addition, the expansion Mach wave generated by the ultrasonic expansion of the shock wave is observed.
© 2015 Optical Society of America
1. Introduction
Femtosecond laser ablation is a complex process involving various mechanisms. In particular, the ablation behavior not only depends on the parameters of the laser pulse, but also depends on properties of matter and the environment of ablation. In recent years, it is found that ablation in water has some attractive characteristics, including the higher ablation rate [1,2], higher damage thresholds [3], significant confinement of the movement of the plasma plume [4], etc., some of which could be beneficial to laser machining or synthesis of nanoparticles [4,5] and so on.
Due to the attractive advantages of laser processing by in water confined environment, many works have been performed to investigate the femtosecond laser ablation behavior [2,3,6–8]. However, previous researches of femtosecond laser ablation in water mainly focus on the morphology of the target surface after ablation, while not investigating the ablation process.
In this work, we employ the pump-probe technique to record the dynamic process of femtosecond laser ablation of aluminum in water and in ambient air, applying shadowgraphy and digital holography on material ejection and shock wave evolution, which intuitively present the ablation behavior. According to the amplitude and phase information of shock wave and material ejection, we compare the ablation behaviors in water and in air.
2. Experimental setup
A Ti:sapphire femtosecond laser amplifier (HP-Spitfire, Spectra-Physics Inc.) is employed to generate a single 50 fs laser pulse with a central wavelength of 800 nm (see experimental setup in Fig. 1). The output laser radiation is divided into a pump and a probe radiation. The pump radiation is focused by a 10 × microscope objective (NA = 0.25) on the silica glass sample mounted on a 3-axis combo translation stage, and the probe radiation is frequency doubled by a BBO crystal as the probe beam, which is then divided into the object beam and the reference beam by BS2. DL1 and DL2 are used for obtaining adjustable delay times between pump and probe pulse. The 4f system composed of lens L1 (f = 1.5 cm) and L2 (f = 30 cm) is used to image the object on the CCD (MINTRON 1881EX with a total pixel number of 576 × 768 and a pixel size of 8.3 μm × 8.3 μm). A bandpass filter at 400 nm together with some neutral density filters are used to prevent the residual 800 nm light and the fluorescence generated during the ablation from entering the CCD. In our experiment, the time-resolved recording of the dynamic process of laser ablation is based on the pump-probe technique, in which a pump beam and a probe beam is used to excite and probe the dynamic process respectively. The dynamic process of laser ablation is repeated several times, and the probe beam is employed to detect the dynamic process at each time with different time delays. Based on the pump-probe technique, the shadowgraphs or the digital holograms of dynamic process of laser ablation can be recorded [9–11]. In particular, the digital holograms can be recorded by adding the reference beam (dashed blue lines in Fig. 1) to interfere with the object beam on the CCD recording plane, while the shadowgraphs can be recorded by blocking the reference beam.
Fig. 1 (a) Experimental setup. BS: beam splitter; DL: delay lines; O: objective; L: lenses. (b) Schematic of target position. (c) Shadowgraph of laser induced breakdown in water.
In order to avoid the unwanted water break down before the laser pulse irradiates the target surface, the sample is placed on the edge (Z1) of the break down region, as shown in Figs. 1(b) and 1(c). According to the shadowgraphs of Fig. 1(c), the distance between the edge (Z1) and center (Z0) of the water break down area is measured to be 37.2 μm. Consequently, the radius of the light spot at Z1 is calculated to be 4.67 μm according to the propagation law of a Gaussian beam, where the beam quality factor M2 is 1.5. For the laser with pulse energy of 1.08 μJ in our experiment, the laser fluence on the target surface is calculated to be 1.58 J/cm2, which is greater than the ablation threshold of aluminum in air (~0.1 J/cm2 [12]).
3. Results and discussion
By employing the setup mentioned above, the time-resolved shadowgraphs of femtosecond laser ablation of aluminum in water and in air are recorded, as shown in Fig. 2. It can be seen that a half-hemisphere shock wave front is generated, which expands gradually. In addition, it is noticed that the ejected material is confined in a small region near the laser irradiation area in case of a water confined environment, and a region with reduced transmittance close to the target surface is observed, due to scattering and absorption. The little gray level in this region indicates the strong absorption and scattering of the probe radiation by the ejected material, and consequently indicates that density of the ejected material is very high. The compression of the ambient medium during the confinement of plasma expansion in water could generate the shock wave with a higher pressure compared with that in air [1].
Fig. 2 Shadowgraphs of femtosecond laser ablation of aluminum in water (a) and in air (b). The parameters of the laser pulse in water and in air are the same. The laser fluence on the target surface is estimated to be 1.58 J/cm2.
While in the case of ablation in air, a region with little gray level close to the target surface can be also found at time delays between 8.5 and 18.3 ns, as shown in Fig. 2(b). However, with the increase of delay time, the transmissivity near the target surface increase gradually. After ionization ceases, emitted vapor and particles form the dust-like ejected material near the target surface, which is with a lower density compared with that in water. The decrease of the density of the ejected material near the target surface in the air indicates that the ejected material can propagate relatively freely inside the shock wave front compared with the case of ablation in water.
The dynamics of femtosecond laser ablation of aluminum in the air were also reported in previous works [9,13,14]. The behavior of material ejection depends on the laser fluence. In particular, it is reported in [9,13,14] that the contact front could formed due to the compression of the ambient air by the ejected material with high momentum when the laser fluence is high enough. While in this work, since the laser fluence is not such high, the compression of the air by the ejected material is not strong enough, and thus the contact front can hardly be found in Fig. 2.
The ejected material could contain particles with different sizes, and the particle size distribution depends on the laser fluence. The higher laser fluence could generate the particles with greater size. In our experiment, the laser fluence is significantly above the ablation threshold, which indicates the possibility of generate particles with relatively big size. The dust like ejected material close to the target surface, which appears grainy and with low propagation speed, could probably be liquid droplets. The generation of droplets is the characteristic of the occurrence of the critical point phase separation (CPPS) [15]. The temperature is a key parameter for the occurrence of the thermal mechanism of CPPS, and we need to calculate the temperature to further identity the occurrence of CPPS. Based on the two-temperature model [16] with one dimension, the lattice temperature of aluminum is calculated by employing implicit finite difference method considering the medium with depth below 1 μm, and the result is shown in Fig. 3. It can be seen in Fig. 3 that the aluminum lattice at the depth of 282 nm can be heated up to the thermodynamic critical temperature of aluminum of 5720 K in 50 ps, which implies that CPPS could take place on the target surface in our experiment. The supercritical material with temperature higher than thermodynamic critical temperature cools down owing to the expansion during the material ejection. The CPPS could take place near the critical temperature when the supercritical material passes through the critical point to enter the unstable zone, leading to the ejection of liquid droplets [15]. Besides, the nanoparticles could also be contained in the ejected material, which should mainly distribute further away from the target surface compared with the liquid droplets due to the higher speed. Further work is needed to experimentally clarify the nature of the ejected material in detail.
In addition, the Mach cone can be observed in Fig. 2(a), which is attributed to thecomplex hydrodynamic effects off the high speed flow during the propagation of the shock wave. Similar phenomena, including the half-hemispherical shock wave and V-shape Mach cone, were also observed in the nanosecond laser ablation of solids in the water [17].
According to the time-resolved shadowgraphs in Fig. 2, we also investigate the evolution of shock wave in water and in air. Figure 4 shows the radius of the shock wave front at different time delays. It can be seen that, the radius of the shock wave front increases approximately linearly between 5.3 ns and 53.6 ns. Therefore, we perform linear fitting, and the correlation coefficient of linear regression is found to be 0.99869, which indicates the velocity of the shock wave almost keeps constant between 5.3 ns and 53.6 ns. Furthermore, the velocity of the shock wave is calculated to be 1618 m/s according to linear fitting, which is close to the sound velocity (1482 m/s). Besides, the radius of the shock wave is measured to be 13.7 μm at 5.3 ns, and thus the average velocity of the shock wave before 5.3 ns is assuming a linear proportionality about 2585 m/s, which is considerably greater than the sound velocity. This means that the magnitude of the supersonic shock wave in the water attenuate rapidly to the magnitude close to the sound acoustic wave within the time scale of nanosecond, which is attributed to the fact that the kinetic energy of the shock wave can convert fast to the energy of ambient water.
Fig. 4 Radius of the shock wave front at different time delays for the ablations in air and in water.
For the case of ablation in air, it can be seen in Fig. 2 that the increase of the radius of shock wave front with the delay time is highly nonlinear. We fit the data according to the to Sedov’s blast wave theory [18,19], in which the radius of the shock wave front in the air is given by
where is the coefficient close to 1, E the energy that generates the shock wave, t the time measured from the moment of explosion, and ρ the density of the unperturbed air in front of the shock wave. It is calculated that the energy that generates the shock wave is about 0.258 μJ, which is 24% of the laser pulse energy (1.08 μJ).According to the fitting curve in Fig. 4, the propagation velocity of the shock wave in air decreases fast from 2668 m/s to 1270 m/s from 5.3 ns to18.3 ns, and then decreases very slowly from 1270 m/s to 666 m/s from 18.3 ns to 53.6 ns. According to Fig. 4, it is calculated that the average propagating velocity of the shock wave front between 18.3 ns and 53.6 ns is about 790 m/s, which is still considerably greater than the sound velocity in the air (342 m/s at 20°C). In addition, it can be seen that the propagation velocity of the shock wave in air is greater than that in the water at short time delays, while it becomes less than that in the water at larger time delays. This is attributed to the fact that the sound velocity in water is greater than that in air, and the dissipation of the shock wave energy to the ambient medium depends on the difference between the velocity of the shock wave and the velocity of the sound wave. When the velocity of the shock wave is close to the sound velocity, the dissipation of the shock wave energy is slow. So the propagation velocity of the shock wave in water keeps approximately constant when it decreases to the value close to the sound velocity. While the propagation velocity of the shock wave in air is much greater than the sound velocity, and thus the fast dissipation of the shock wave energy to the ambient air decreases the propagation velocity of the shock wave in the air lower than that in the water after 10 ns.
In order to obtain the phase information related to the ablation process, we also we also record the time-resolved digital holograms corresponding to the shadowgraphs in Fig. 2, by adding the reference beam (the dashed line Fig. 1(a)) to interfere with the object beam. Figure 5 shows the phase difference maps outside the target during the ablation process in water and in air, which provide the phase information of the shock wave evolution and the material ejection. A positive or negative value of the phase difference refers to a refractive index of less than or greater than the refractive index of the undisturbed medium, respectively.
Fig. 5 Phase difference maps of femtosecond laser ablation of aluminum in water (a) and in air (b). The parameters of the laser pulse in water and in air are the same. The laser fluence on the target surface is 1.58 J/cm2.
For ablation in water, it can be seen in Fig. 5(a) that the phase difference inside the shock wave front is negative, which corresponds to the refractive index greater than that of the undisturbed water, and consequently corresponds to the compressed water. However, it can be seen that the phase difference near the target surface become positive at large time delays, which could be mainly contributed by the ejection of neutral liquid droplets or vapor, which is confined near the laser irradiation area by water.
For ablation in the air, it can be seen from Fig. 5(b) that there is rim with the negative phase shift outside the shock wave front, which corresponds to the compressed air with the refractive index greater than 1. In addition, there is a big region with the positive phase shift (corresponds to the refractive index less than 1) as high as several rads inside the shock wave front, which indicates high density free electrons generated by the mechanism of ionization. This indicates that the ionized material can diffuse inside the shock wave front freely. Besides, it can be seen that the phase difference near the target surface is slightly below 0, which corresponds to the refractive index slightly greater than 1, and thus corresponds to the neutral ejected material [20] with low density.
In addition, the Mach cone can be easily seen in the phase difference map of the ablation in water shown in Fig. 5(a), and it can be deduced that the refractive index inside the Mach cone and outside the shock wave front is greater than that of the undisturbed medium according to the higher phase shift, which corresponds to the water with lower density disturbed by the Mach wave. This indicates an expansion Mach wave generated in our case, which decrease the density of the affected medium. While for ablation in the air, the Mach cone can be hardly seen due to the lower magnitude of disturbance of refractive index in the air.
4. Conclusion
The dynamic process of femtosecond laser ablation of aluminum in water and in air has been investigated by employing pump-probe imaging technique obtaining time-resolved intensity and phase information related to the ablation process. It is found that the behaviors of material ejection and shock wave evolution in the case of ablation in water and in air are different. In particular, the ejected material is confined in a small region near the target surface in the water-confined environment, while the ejected material can propagate away from target with low resistance inside the shock wave front in the air. In addition, the liquid has a considerable effect on decrease of the shock wave velocity, and it can decrease the velocity of the shock wave close to the sound velocity within nanoseconds decelerating slowly afterwards. Furthermore, a V-shaped Mach cone propagating on both sides of the half-hemispherical shock wave front is observed, which is generated by the complex hydrodynamic process during the supersonic expansion of the shock wave front.
Acknowledgment
This work is supported by the National Natural Science Foundation of China (No. 61405140 and 61227010), and the National Instrumentation Program (No. 2013YQ030915).
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