High-power laser interaction with matter leads toplasma formation with the generation of high-pressure shock waves [1,2]. Initially, the applicationof laser-induced high-pressure shock waves was con-fined to material processing. This involved a changein the material structure and its stress state to insti-gate dislocation and plastic deformations, etc. [3].High-density dislocations were utilized to achievehigher surface hardness and corrosion resistance,whereas plastic deformations were used to enhancethe fatigue strength of the materials employed incommercial use [4–8]. When laser-assisted shockwaves are generated in confined geometry, especiallyin liquids, the pressure is more than an order ofmagnitude as compared to that of direct ablation invacuum/gas, and the corresponding duration is 2–3times longer [9]. The laser–liquid interaction alsogenerates cavitation bubbles, which oscillate under
the hydrostatic pressure of the surrounding liquidand emit secondary shock waves during the growthand collapse phase of the bubble. A laser-producedshock wave in liquid has recently been used for steamlaser cleaning of chips in the semiconductor industry,processing of curved surfaces like turbine blades, andmaintenance of power plant reactors [10,11]. In thefield of biotechnology and medicine, the laser-induced plasma and emitted shock wave assists intissue cutting, dissection of cellular organelles, cytos-keletal filaments, etc. [12]. These are also useful inophthalmic surgery like posterior capsulotomy andin fragmentation of ureteric kidney stones via shockwave lithotripsy [13–15]. However, the expandedshock waves are not always desirable as they resultin collateral damage to adjacent tissues. Further, ca-vitation effects also cause severe dilation of cellularorganelles. It was observed that the focusing of a low-energy laser on surfaces of tissues minimizes thedamage [16,17]. In view of the complexity of thelaser-induced breakdown process, there is a need togain insight onto the dynamics of emitted shockwaves and explore the prospects of minimizing the
damaging effects of shock waves and cavitation bub-bles for optimum application.
In the present paper, we study the underlying dy-namics of laser-assisted shock waves at the target–liquid interface. There are several reports on studiesof laser-induced shock waves in liquids using opticalprobes [18–20]. Most of these analyses are limited tolaser-induced shock waves emanating from a focalspot in the liquid medium. We analyze the laser-induced breakdown process at the target–liquid in-terface for various focusing conditions. The changein focusing condition not only varies the energy den-sity but also alters the degree of ionization of the ir-radiated medium. Hence, it is interesting to see therelevance of the focusing condition on shock waveexpansion and cavitation effect at the target–liquidinterface.
The beam deflection setup (BDS) was used to mea-sure the shock wave velocity. The shock wave pres-sure is estimated from shock wave velocities at thetarget–liquid interface. This can be implementedto understand the physical conditions during thesynthesis of nanoparticles via the laser-inducedbreakdown at the target–liquid interface [21,22]. Es-timates of physical condition like pressure and tem-perature at the target–liquid interface can be used tostudy the growth rate and size of nanoparticles. This
accordingly assists in modulating the nanoparticleproperties for size-selective applications [23].
2. Experimental Setup
A high-power Q-switched Nd:YAG laser (Model HYL101, λ ¼ 532nm, energy ¼ 4mJ, pulse duration10ns) was focused onto a titanium target (purity99.7%) immersed in a liquid cell (dimension 10 cm×10 cm × 10 cm) filled with distilled deionized water(conductivity <1 μΩ−1). The BDS was assembled tostudy the shock wave dynamics during the laser-induced breakdown at the titanium–water interface.The schematic of the experimental setup is shown inFig. 1(a). A cwHe–Ne laser was aligned transverse tothe direction of Nd:YAG laser. The output of the He–Ne beam was passed through the breakdown regionand was detected by a photodiode (13 DSI 001), PD1.The photodiode is aligned to detect the maximumsignal corresponding to the center of the Gaussianbeam from the output of the He–Ne laser in the re-gion of breakdown. The formation of laser-producedplasma and shock waves, followed by cavitation bub-bles, results in change in the refractive index, whichin turn deflects the He–Ne beam. The deflection isregistered in the form of a dip (modulation) in PD1signal. As the plasma and the bubbles are over,the He–Ne beam comes back to its original pathand the photodiode to its initial maximum level.
Fig. 1. (Color online) (a) Schematic of BDS. (b), (c) Illustration of laser focusing condition.
The photodiode signal is displayed via a 50Ω termi-nator onto one of the channels of the digital storageoscilloscope (DSO), TektronixModel TDS 2012, inter-faced with the computer. The DSO is triggered by 4%of 4% reflection of the Nd:YAG laser beam frommicroscopic glass plates G1 and G2, detected by aphotodiode (PD2), and is also displayed simulta-neously on another channel of the DSO as referencepoint. To record the shock waves at different loca-tions with respect to the breakdown region, theHe–Ne laser and PD1 were moved simultaneouslyalong the axis of the source Nd:YAG laser. The timedelay between deflections incurred at different dis-tances is used to measure the velocities.
The experiment was conducted for three differentfocusing conditions onto a Ti target as illustrated inFigs. 1(b) and 1(c). These are locations of the target atþ10mm, 0mm, and −5mm from the laser focus.These focusing conditions are designated as A, B, andC, respectively, for further correspondence. The fo-cusing conditions were varied by changing the posi-tion of the lens by means of a micrometer screwattached with the lens mount. The estimated laserfluence at above-focus, focus, and below-focus werefound to be 1:4 J=cm2, 80 J=cm2, and 0:2J=cm2,respectively. The laser fluence is measured from theirradiation energy per pulse and laser spot size fall-ing onto the target [24]. The irradiation of the high-power laser ablates the titanium target, and thematerial from the target contaminates the surround-ing water molecules. To avoid any artifacts due to thecontaminated water, it was changed frequently dur-ing the course of experiment for all of these focusingconditions.
3. Results and Discussion
High-power laser irradiation inside a liquid mediumleads to plasma formation. The basic mechanism oflaser-induced plasma is highly complex and non-linear. The irradiation I (I ≥ 107 W=cm2) from apulsed laser onto a target–liquid interface generatesthe starting seed electrons via multiphoton ioniza-tion or the tunnelling effect [25–27]. For impure med-ia, free electrons also arise from the easily ionizableimpurities, which act as shallow donors. The seedelectrons in this case are generated via local heatingor absorption of one or two photons, which also con-tributes in lowering the threshold breakdown. Oncea free electron is created, it absorbs a photon via in-verse bremmsstrahlung in the course of collidingwith heavier species. With each collision it gains ki-netic energy. After bremmsstrahlung absorption ofseveral photons, the electron achieves sufficient ki-netic energy to create another free electron via im-pact ionization. This eventually leads to avalancheionization, which causes rapid excitation and ioniza-tion of the medium in the focal volume into plasmaformation.
Once the laser-induced hot plasma is formed, itgets confined by the surrounding liquid layer. As aresult, the plasma expansion gets delayed, and a
shock wave originates. When the laser pulse is over,recombination processes begins and the plasma is re-placed by vaporized fluid mass, which constitutescavitation bubbles within a few microseconds. Thecavitation bubble expands due to the inertia of thevaporized fluid mass and reaches a state wherethe inside bubble pressure becomes less than the hy-drostatic pressure of the surrounding liquid. Tomaintain the hydrostatic pressure, the bubble im-plodes and reaches a state where the pressure insideit becomes more than the hydrostatic pressure. Thebubble rebounds and emits the excess of energy as asecondary shock wave. Thus, with each bubble oscil-lation, a shock wave is emitted, and the bubble oscil-lation attenuates until all its energy is dissipatedinto the surrounding liquid.
The emission of laser-induced plasma, shockwaves, and cavitation bubbles locally changes thedensity and hence the refractive index. This changein refractive index is registered as deflection of theHe–Ne laser beam passing through the focal volumeof the source Nd:YAG laser [Fig. 1(a)]. The spatialprofile of the BDS traces are recorded at different dis-tances relative to the target–liquid interface. Thetarget–liquid interface is marked as 0mm, and therest of the positions of the BDS traces are with re-spect to the target–liquid interface. The completebeam deflection traces for the laser-focused condition[B, Fig. 1(c)] at 0mm from the target–water interfaceis shown in Fig. 2. Trace 1 is the Nd:YAG trigger sig-nal for reference. Trace 2 represents the completebeam deflection trace within the proximity of focus.The duration of first narrow negative peak is of theorder of 1 μs, and its deflected signal (∼2mV) is lesscompared to that of the cavitation bubbles (∼50mV).Therefore, the signals were recorded separately at anexpanded scale of 1 μs=division (div) as depicted inTrace 3. Possible candidates for the first negativenarrow peak (expanded in Trace 3) could be due tohigh energetic ions and electrons in plasma as wellas due to the shock waves. The electrons are limited
Fig. 2. (Color online) Complete trace of BDS for the tightlyfocused condition (B) at 0mm from the titanium–water interface.
to the interfacial region only, due to the electron–ionrecombination occurring immediately after the gen-eration of the plasma [28]. With the passage of time,the shock wave expands beyond the interfacial re-gion. The other discontinuities (visible in Trace 3) areascribed to the acoustic emissions due to the onset ofcavitation bubbles [29,30]. In addition, the broad dipin Trace 2 followed by the subsequent negative dipsis attributed to primary, secondary, and third-ordercavitation bubble oscillations, respectively.
The laser-assisted shock waves were also recordedfor various focusing conditions [Figs. 1(b) and 1(c)]during the laser-induced breakdown at the titanium–
water interface. The complete BDS trace for theabove-focus condition and the focus condition at 0mmfrom the target–liquid interface is illustrated in Fig. 3for comparison. At focused condition [B, Fig. 1(c)],maximum energy is deposited, and hot plasma isformed. As a consequence, cavitation effects are morevigorous (Trace 2) and higher-order bubble oscilla-tions are also pronounced. In contrast, for the above-focus [A, Fig. 1(c)] condition (Trace 3), weaker bubblesare formed with no other secondary bubbles. The ex-panded scale of the BDS trace for various focusingconditions is shown in Fig. 4. Each trace has been ta-ken after an average of 16 Nd:YAG laser shots. Theexperiment was repeated several times to check thereproducibility. Comparing the BDS trace of above-focus [Trace 2, Fig. 4(a)] with focus [Trace 2, Fig. 4(b)]conditions, it appears that the first negative peak dueto the shock wave is apparent, but no acoustic emis-sions are observed. In addition, for above-focus, theweak gradient in the photodiode signal after the firstnegative dip reflects the onset of weak cavitationbubble phenomena.
Some more interesting features are unveiled whenthe BDS traces for the below-focus condition are re-corded. From Fig. 4(c), it appears that there are twoshock waves, shock wave 1 and shock wave 2, apartfrom the third discontinuity before the onset of cavi-
tation bubbles. As we move away from the titanium–
water interface, it seems that shock wave 1 [shownby the red arrow, Fig. 4(c)] diverges out with an in-crease in distance from the interfacial region. How-ever, shock wave 2 [shown by the white arrow,Fig. 4(c)] converges toward the target with an in-crease in distance (Trace 1–7, 0–4mm) up to a dis-tance of 5mm (Trace 8) from the interface, beyondwhich it diverges out. This situation is rather allur-ing and can be explained from the geometry of laserablationatthebelow-focuscondition[Figs.1(b)and1(c)].
Fig. 3. (Color online) Comparison of beam-deflected signal forfocusing conditions focus (Trace 2) and above-focus (Trace 3).
For the below-focus condition, the titanium target isplaced 5mm below the focal spot of the Nd:YAG la-ser. The reference position for the titanium–waterinterface is 0mm, and the focal region of theNd:YAG laser is at ∼5mm above the titanium–
water interface. Hence, the water breakdownpredominantly takes place at 5mm from thetitanium–water interface. Thus, shock waves aregenerated in the pure water breakdown as wellas at the titanium–water interface. As a result, dualshock waves are generated, one from the target–water interface, shock wave 1, and the other fromwater breakdown, shock wave 2. It is observed that,with an increase in distance from the titanium–
water interface, the two shock waves approach eachother and overlap at 3mm from the titanium–waterinterface. Trace 5, recorded at 3mm from the tita-nium–water interface, shows the zone of overlap ofthese two shock waves. With a further increase indistance (Trace 8, 5mm), the probe He–Ne beamtouches the area of water breakdown region whereshock wave 2 becomes prominent and shock wave 1turns flaccid. The first peak at Trace 8 is shockwave 2 emanating from the water breakdown area,and the second peak resembles shock wave 1 evol-ving from the titanium–water interface. Trace 8 ismeasured at 5mm from the titanium–water inter-face. This position is approximately the positionfor water breakdown in accordance with Fig. 1(c).Once the water breakdown area is reached, a sharpfall in photodiode voltage is encountered due to exu-berant cavitation bubbles, and shock wave 1 fromthe titanium–water interface becomes weaker andmerges onto the cavitation bubbles so is not de-tected beyond these regions.
The spatial evolution of the shock wave velocitiesfrom the titanium–water interface for above-focus,focus, and below-focus conditions are shown inFigs. 5(a)–5(c), respectively. At the vicinity of the in-terfacial/focal region, the shock wave travels withsupersonic velocity. Away from the interfacial/focalregion, the velocity goes down to acoustic and subso-nic speed as loss mechanisms via heat conduction,diffusion, and recombination are more at these re-gions. The velocities at the interfacial region forabove-focus, focus, and below-focus are found to be2:4km=s, 6:4km=s, and 2km=s. In addition, for thebelow-focus condition, water breakdown also occurs.The shock wave velocity emanating from waterbreakdown is 6:5km=s. The shock wave pressure isestimated from the shock wave velocities usingNewton’s second law across a shock wave discontinu-ity given by [31]
P − P0 ¼ Usupρ; ð1Þwhere P and P0 are the shock wave and the hydro-static pressure, respectively,Us is the shock wave ve-locity, up is the particle velocity, and ρ is the densityof water before compression. The shock wave andparticle velocities are related through the equationof state and are empirically given by
Us ¼ Aþ Bup; ð2Þ
where A is the sound velocity and B is a constant,which are equal to 1:48km=s and 2.07, respectively,up to a pressure of 20kbars [32]. A higher-order ap-proximation up to 250kbars is given in the literature[33]. The estimated shock wave pressure for above-focus, focus, and below-focus conditions with thedistance from the target is shown in Figs. 6(a)–6(c),respectively. The shock wave pressure at thetitanium–water interface for above-focus, focus, andbelow-focus conditions is 1GPa, 18GPa, and 0:6GPa,respectively. The pressure experienced at the waterbreakdown region for the below-focus condition dueto shock wave 2 [Fig. 4(c)] is 18:3GPa. For above-focus and focus conditions, the maximum energy is
received at the target, and hence the shock wavepressure is maximum in the vicinity of target,whereas for the below-focus condition, most of the en-ergy is taken away by the breakdown of water.Hence, the shock wave pressure is minimum on thetarget for the below-focus condition. Thus, the altera-tion in focusing condition can modulate the pressureat the titanium–water interface. This is due to thechange in laser fluence and degree of liquid ioni-zation with variation in focusing conditions. Atthe tightly focused condition, fluence is very high(80 J=cm2), and extreme pressure is experienced.The prominent variation in pressure with a changein focusing condition has significant implicationsonto the synthesized nanoparticles prepared via la-ser-induced breakdown at the metal–water interface
[34,35]. The very high pressure (18GPa) increasesthe molecular interaction and enhances the probabil-ity of formation of thermodynamically stable oxideswith increased nanoparticle sizes. Besides low flu-ence ð1:4 J=cm2; 0:2 J=cm2Þ, induced low pressureð1GPa; 0:6GPaÞ may lead to growth of smaller kine-tically stable metal oxide nanoparticulates [36].Thus, the change in pressure at the target–liquid in-terfacial region can be used to modulate the phase,structure, and critical size of the synthesized nano-particulates.
4. Conclusion
In summary, the spatial and temporal evolution ofshock waves generated via high-power laser-inducedbreakdown at the titanium–water interface is stu-died. The focusing conditions are varied, and influ-ence onto the shock waves is interpreted. For theabove-focusing condition, the cavitation effect isminimized as evident from the deflected signal ofthe He–Ne laser. Hence, for clinical application, ir-radiation at the defocused condition may inhibitthe dilation of adjacent tissues. In addition, at thebelow-focus condition, dual shock waves originatingfrom the target–water interface and water break-down region are observed. However, the use oflaser-induced breakdown at the below-focus condi-tion for material processing and clinical applicationremains unexploited.
The shock wave velocities and pressure were alsoestimated. A high-pressure zone (18GPa) was ob-served for the tightly focused condition. These condi-tions may favor formation of thermodynamicallystable materials during laser-induced breakdownat the target–water interface, whereas for the defo-cused condition, pressure ≤1GPa is observed, whichcan be used for formation of kinetically favoredsmaller-sized nanoparticles.
References1. Y. B. Zel’dovich and Y. P. Raizer, Physics of Shock Waves and
High Temperature Hydrodynamics Phenomena (Academic,1967).
2. D. A. Hutchins, “Ultrasonic generation by pulsed lasers,”Phys. Acoust. 18, 21–123 (1988).
3. P. Forget and M. Jeandin, “Déformation à l’échelle cristallo-graphique d’alliages à base de nickel mono- et polycristallinspar choc laser en mode confine,” J. Phys. III France 5,1133–1144 (1995).
4. G. Banaś, H. E. Elsayed-Ali, F. V. Lawrence, Jr., and J. M.Rigsbee, “Laser shock-induced mechanical and microstructur-al modification of welded maraging steel,” J. Appl. Phys. 67,2380–2384 (1990).
5. B. P. Fairand, B. A. Wilcox, W. J. Gallagher, and D. N.Williams, “Laser shock-induced microstructural and mechan-ical property changes in 7075 aluminium,” J. Appl. Phys. 43,3893–3895 (1972).
6. R. Fabbro, P. Peyre, L. Berthe, and X. Scherpereel, “Physicsand applications of laser-shock processing,” J. Laser Appl.10, 265–279 (1998).
7. Y. Sano, M. Kimura, N. Mukai, M. Yoda, O. Minoru, andT. Ogisu, “Process and application of shock compression bynanosecond pulses of frequency-doubled Nd:YAG laser, ” Proc.SPIE 3888, 294–306 (2000).
8. Y. K. Zhang, C. L. Hu, L. Cai, J. C. Yang, and X. R. Zhang,“Mechanism of improvement on fatigue life of metal bylaser-excited shock waves,” Appl. Phys. A 72, 113–116(2001).
9. R. Fabbro, J. Fournier, P. Ballard, D. Devaux, and J. Virmont,“Physical study of laser-produced plasma in confined geome-try,” J. Appl. Phys. 68, 775–784 (1990).
10. A. Kruusing, “Underwater and water-assisted laser proces-sing: part 1—general features, steam cleaning and shockprocessing,” Opt. Lasers Eng. 41, 307–327 (2004).
11. C. Konagai, Y. Sano, and N. Aoki, “Underwater direct metalprocessing by high-power copper vapour laser,” in PulsedMetal Vapour Lasers, C. E. Little and N. V. Sabotinov, eds.(Kluwer Academic, 1996), pp. 371–376.
12. M. W. Berns, W. H. Wright, and R. W. Steubing, “Lasermicrobeam as a tool in cell biology,” Int. Rev. Cytol. 129,1–44 (1991).
13. D. R. Stager, Jr., X. Wang, D. R. Weakley, Jr., and J. Jelius,“The effectiveness of Nd:YAG laser capsulotomy for the treat-ment of posterior capsule opacification in children with acrylicintraocular lenses,” J. AAPOS 10, 159–163 (2006).
14. R. Hofmann and R. Hartung, “Laser-induced shock-wavelithotripsy of ureteric calculi,” World J. Urol. 7, 142–146(1989).
15. S. A. Batishche, “Features of gallstone and kidney stone frag-mentation by IR-pulsed Nd:YAG laser radiation,” Proc. SPIE2395, 94–97 (1995).
16. A. Vogel, P. Schweiger, A. Frieser, M. N. Asiyo, andR. Birngruber, “Intraocular Nd:YAG laser surgery: light–tissue interaction, damage range, and reduction of collateraleffects,” IEEE J. Quantum Electron. 26, 2240–2260 (1990).
17. M. K. Fallor and R. H. Hoft, “Intraocular lens damage asso-ciated with posterior capsulotomy: a comparison of intraocu-lar lens designs and four different Nd:YAG laser instruments,”J. Am. Intra Ocul. Implant Soc. 11, 564–567 (1985).
18. J. G. Fujimoto, W. Z. Lin, E. P. Ippen, C. A. Puliafito, and R. F.Steinert, “Time-resolved studies of Nd:YAG laser-inducedbreakdown,” Invest. Ophthalmol. Visual Sci. 26, 1771–1777(1985).
19. X. Chen, R.-Q. Xu, J.-P. Chen, Z.-H. Shen, L. Jian, and X.-W.Ni, “Shock-wave propagation and cavitation bubble oscillationby Nd:YAG laser ablation of a metal in water,” Appl. Opt. 43,3251–3257 (2004).
20. R. Petkovšek, J. Možina, and G. Močnik, “Optodynamic char-acterization of the shock waves after laser-induced breakdownin water,” Opt. Express 13, 4107–4112 (2005).
21. G. W. Yang, “Laser ablation in liquids: applications in thesynthesis of nanocrystals,” Prog. Mater. Sci. 52, 648–698(2007).
22. Q. X. Liu, C. X. Wang, W. Zhang, and G. W. Wang, “Immisciblesilver–nickel alloying nanorods growth upon pulsed-laser in-duced liquid/solid interfacial reaction,” Chem. Phys. Lett. 382,1–5 (2003).
23. C. X. Wang, P. Liu, H. Cui, and G. W. Yang, “Nucleation andgrowth kinetics of nanocrystals formed upon pulsed-laser ab-lation in liquid,” Appl. Phys. Lett. 87, 201913 (2005).
24. A. Nath, S. S. Laha, and A. Khare, “Effect of focusing condi-tions on synthesis of titanium oxide nanoparticles via laserablation in titanium–water interface,” Appl. Surf. Sci. 257,3118–3122 (2011).
25. L. V. Keldysh, “Ionization in the field of strong electromag-netic wave,” Sov. Phys. JETP 20, 1307–1314 (1965).
26. M. V. Ammosov, N. B. Delone, and V. P. Krainov, “Tunnel io-nization of complex atoms and atomic ions in an electromag-netic field,” Sov. Phys. JETP 64, 1191–1194 (1986).
27. J. Noack and A. Vogel, “Laser–induced plasma formation inwater at nanosecond to femtosecond time scales: calculationof thresholds, absorption coeffecients, and energy density,”IEEE J. Quantum Electron. 35, 1156–1167 (1999).
28. F. Docchio, P. Regondi, M. R. C. Capon, and J. Mellerio, “Studyof the temporal and spatial dynamics of plasmas induced inliquids by nanosecond Nd:YAG laser pulses. 1: Analysis ofthe plasma starting times,” Appl. Opt. 27, 3661–3668 (1988).
29. P. Zhong, I. Cioanta, S. Zhu, F. H. Cocks, and G. M. Preminger,“Effects of tissue constraint on shock wave-induced bubble ex-pansion in vivo,” J. Acoust. Soc. Am. 104, 3126–3129 (1998).
30. P. Zhong, I. Cioanta, F. H. Cocks, and G. M. Preminger,“Inertial cavitation and associated acoustic emission producedduring electrohydraulic shock wave lithotripsy,” J. Acoust.Soc. Am. 101, 2940–2950 (1997).
31. A. G. Doukas, A. D. Zweig, J. K. Frisoli, R. Birngruber, and T. F.Deutsch, “Non-invasive determination of shock wave pressuregenerated by optical breakdown,” Appl. Phys. B 53, 237–245(1991).
32. P. Harris and H. N. Presles, “Reflectivity of 5:8kbar shockfront in water,” J. Chem. Phys. 74, 6864–6866 (1981).
33. M. H. Rice and J. M. Walsh, “Equation of state of water to 250kilobars,” J. Chem. Phys. 26, 824–830 (1957).
34. F. Mafuné, J.-Y. Kohno, Y. Takeda, T. Kondow, and H. Sawabe,“Formation of gold nanoparticles by laser ablation in aqueoussolution of surfactant,” J. Phys. Chem. B 105, 5114–5120(2001).
35. C. X. Wang, Y. H. Yang, and G. W. Yang, “Thermodynamicalpredictions of nanodiamonds synthesized by pulsed-laser ab-lation in liquid,” J. Appl. Phys. 97, 066104 (2005).
36. V. R. Palkar, P. Ayyub, S. Chattopadhyay, and M. Multani,“Size induced structural transitions in Cu-O and Ce-O sys-tems,” Phys. Rev. B 53, 2167–2170 (1996).
