Effects of pulse duration on overall temporal behavior of the bubble produced by nanosecond laser ablation in water

Full text: Liquid-phase laser ablation is expected to be applied to various applications, such as underwater in-situ elemental analysis and synthesis of nanoparticles in liquid. However, the mechanism is not fully clarified due to complex interactions among pulsed laser, solid, liquid, plasma and bubble induced in the process. To improve the performance of the elemental analysis and the synthesis of nanoparticles, we have to understand the behavior of the ablated species and the bubble, and their relationship should be also controlled. Plasma emission images and shadowgraph images have been observed to investigate the ablated species and the bubble. However, in the case of the shadowgraphy multiple events are required to capture the bubble dynamics during its whole lifetime. Therefore, it is difficult to evaluate the temporal behavior precisely since in our case the shotto-shot fluctuations are significant. Gregorčič et al. have used a laser-beam-transmittance probe (LBTP) to measure the whole bubble dynamics from a single pulse. In the case of LBTP the continuous-wave (CW) illumination probe is needed to illuminate the interaction area. The portion of the CW laser is scattered and deflected by the bubble. Thus, the transmitted light power, which is collected by a lens to a photodetector, decreases by increasing the bubble's diameter. In such a way, the whole bubble dynamics is detected from a single laser shot, which is especially important in the case of non-repeatable conditions due to the significant shot-to-shot fluctuations. In the present study, the behavior of the ablated species and the bubble induced by laser ablation in water are investigated by employing LBTP. We study the effects of laser-pulse duration, which could control the relationship between the ablated species and the bubble. A Q-switch Nd:YAG laser with the wavelength of 1064 nm, the pulse duration of 20 ns (short pulse) or 100 ns (long pulse), and the pulse energy of 6 mJ (fixed) was focused onto a Cu target in water. As a CW probe we used a He-Ne laser (632.8 nm) expanded by 10 times by using a beam expander. The probe passed through the bubble on the ablation spot in the direction parallel to the target surface. The transmitted probe light was collected by a lens into a photodiode. The photodiode signal was measured by an oscilloscope. In the case that we observed the signal immediately after the laser irradiation, the band-pass interference filter (632 ± 1.5 nm) was placed in front of the photodiode to avoid the detection of the plasma emission as well as the scattered light of the pulsed laser. The temporal profile of the pulsed laser was observed by another photodiode which detects the reflected light from a beam splitter that was placed on the laser pathway. Results and discussion - When a bubble starts to expand the LBTP signal decreases since less light is transmitted through the interaction area. In our experiments, the several negative peaks were obtained in the LBTP signal due to multiple oscillations of the bubble. Here, the amplitude and the duration of a single peak in LBTP signal decreases with the repetition. From peak widths, which were from ~10 μs to ~100 μs, the time of a single bubble oscillation can be obtained. The peaks attributed to the third and subsequent bubbles in the case of the short pulse had higher amplitude and were wider than that in the case of the long pulse, although the peaks attributed to the first bubble were almost the same for the both cases. This is probably because the ablated species remains in the bubble in the case of the long pulse compared to the case of the short pulse, which may enhance the viscosity of the bubble. Unexpectedly, a scattering signal with the width of ~1 μs was observed immediately after the laser irradiation. This is probably related to the ejection and dispersion of the ablated species into the water phase. The beginning of the signal was steep and the profile fluctuated shot-by-shot in the case of the short pulse compared to the case of the long pulse. This might indicate that relatively large amount of the ablated species are ejected into the water phase in the case of the short pulse in contrast to the case of the long pulse. This could affect the contents of the bubble and consequently the repetition behavior of the bubble might be changed as discussed above. (author)