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Abstract
Although the experimental setup needed for the plasma and cavitation bubble creation with the laser is relatively simple, enormous variety of physical processes take place in this experiment, covering the topics from uid dynamics, to heat and mass transfer, to chemical reactions, plasma physics and light emission. Laser induced plasma (LIP) shows a wide range of characteristics depending on the experimental parameters used for its creation. In addition to the complexity of the LIP itself, when such a plasma is created inside the liquid environment several extra factors come into play, making both the experimental and theoretical investigation highly challenging tasks. In a liquid environment, besides plasma formation due to the laser action, the shock wave and cavitation bubble are created, all of them sharing the input laser energy. Due to the interrelation of the phenomena occurring after laser induced breakdown (LIB) in liquid environment, shock wave, bubble and plasma emission, in order to get real insight into the physics of the processes, one need to study more than one eect at the same time.
In this study main focus was on the last two aspects of the LIB, i.e. cavitation bubble and plasma emission, while the subject of the shock wave emergence and propagation was also discussed in short. The mutual impact of these two extremely interesting phenomena, laser produced plasma and the cavitation bubble, is considered very important for numerous applications, the production of nanoparticles being one of the more popular recently. Cavitation bubble plays a crucial role in determining the size, shape and chemistry of the products of laser ablation in liquid. Although its importance have been recognized and many research groups are dealing with this topic, a lots of speculations about the role of the cavitation bubble in nanoparticles production still exist. Another point on which the opinions about the cavitation bubble are divergent regards the question of double pulse laser induced breakdown spectroscopy (DP-LIBS). Plasma-bubble relations in DP-LIBS are still not very well documented, while the relation between the plasma and bubble in single pulse laser induced breakdown spectroscopy (SP-LIBS) are almost completely missing.
In this work precisely those intermediate time scales in SP LIBS at which the bubble starts to appear and the plasma radiation is present, ranging from nanoseconds to millisecond, were studied by several experimental methods. Plasma was studied using the fast photography and optical emission spectroscopy (OES), the shock wave propagation was recorded with the Schlieren technique and cavitation bubble evolution was captured using shadowgraphy and probe beam techniques. In underwater single pulse laser ablation of alumina and aluminium, a long lasting optical emission of several hundreds of μs duration was detected for the first time. Duration of emission on the pure aluminium coincided with the first bubble cycle, while on the alumina further prolonging of optical emission was detected and it was explained through the material related properties. Based on the unique observations of the glowing particles inside the cavitation bubble on aluminium and thin glowing layer in contact with the target on alumina, the temperature of the bubble was estimated to be more than 1000 K throughout the whole bubble lifetime. In the early phases of the bubble evolution it’s temperature was determined from the molecular AlO bands and it was around 4000 K, which agrees well with the theoretical models.
