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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 18, Iss. 11 — May. 24, 2010
  • pp: 11159–11172
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Tuning spectral properties of ultrafast laser ablation plasmas from brass using adaptive temporal pulse shaping

M. Guillermin, A. Klini, J. P. Colombier, F. Garrelie, D. Gray, C. Liebig, E. Audouard, C. Fotakis, and R. Stoian  »View Author Affiliations


Optics Express, Vol. 18, Issue 11, pp. 11159-11172 (2010)
http://dx.doi.org/10.1364/OE.18.011159


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Abstract

Using automated laser pulse temporal shaping we report on enhancing spectral emission characteristics of ablation plasmas produced by laser irradiation of brass on ultrafast time scales. For different input irradiance levels, control of both atomic and ionic species becomes possible concerning the yield and the excitation state. The improved energy coupling determined by tailored pulses induces material ejection with lower mechanical load that translates into hot gas-phase regions with higher excitation degrees and reduced particulates.

© 2010 Optical Society of America

1. Introduction

The evolution of the ablated material, including the emissivity of the ejected species, depends on the amount and distribution of the energy stored in the material and it is drastically influenced by the laser fluence, wavelength, pulse duration, and the size of the irradiation spot. Improvements considering various criteria of plasma evolution and material transfer were seen by using multipulse sequences or post-irradiation plasma treatment [14–20

14. A. Semerok and C. Dutouquet, “Ultrashort double pulse laser ablation of metals,” Thin. Sol. Films 453–454, 501–505 (2004). [CrossRef]

]. As long as some information exists on the dynamics of phase transitions, specific pulse forms can be designed that exploit the transient material changes and the underlying thermal transport. However, considering the complexity of processes occurring between the initial excitation and the final structural transformation and their lack of coherence, the task of defining apriori the possible control factors becomes difficult. This indicates a strong requirement for more efficient improvement procedures, capable of optimizing ultrafast laser-induced processes even when the physical information at hand is rather scarce. Here beam engineering methods in the time domain can deliver a significant advantage.

Building on the above-described scenario we intend to show that by controlling the energy feedthrough on ultrafast scales [12

12. J. P. Colombier, P. Combis, A. Rosenfeld, I. V. Hertel, E. Audouard, and R. Stoian, “Optimized energy coupling at ultrafast laser-irradiated metal surfaces by tailoring intensity envelopes: Consequences for material removal from Al samples,” Phys. Rev. B 74, 224106/1–16 (2006). [CrossRef]

, 21

21. R. Stoian, A. Mermillod-Blondin, N. M. Bulgakova, A. Rosenfeld, I. V. Hertel, M. Spyridaki, E. Koudoumas, P. Tzanetakis, and C. Fotakis, “Optimization of ultrafast laser generated low-energy ion beams from silicon targets,” Appl. Phys. Lett. 87, 124105/1–3 (2005). [CrossRef]

, 22

22. M. Guillermin, C. Liebig, F. Garrelie, R. Stoian, A.-S. Loir, and E. Audouard, “Adaptive control of femtosecond laser ablation plasma emission,” Appl. Surf. Sci. 255, 5163–5166 (2009). [CrossRef]

] particular thermodynamic states can be achieved in the expanding material, carrying also an improved spectral signature. Using automated temporal pulse shaping integrated in adaptive loops excitation rates can be optimized to control the excitation degree and, to a certain extent, the emission characteristics of the ablation plume. Largely employed in coherent control studies for molecular dissociation and bond breaking [23

23. A. Assion, T. Baumert, M. Bergt, T. Brixner, B. Kiefer, V. Seyfried, M. Strehle, and G. Gerber, “Control of Chemical Reactions by Feedback-Optimized Phase-Shaped Femtosecond Laser Pulses,” Science 282, 919–922 (1998). [CrossRef] [PubMed]

], the technique is particularly powerful in regulating incoherent material transformations on mesoscopic to macroscopic scales characteristics to laser ablation. The effect was previously analyzed for silicon and aluminium samples [12

12. J. P. Colombier, P. Combis, A. Rosenfeld, I. V. Hertel, E. Audouard, and R. Stoian, “Optimized energy coupling at ultrafast laser-irradiated metal surfaces by tailoring intensity envelopes: Consequences for material removal from Al samples,” Phys. Rev. B 74, 224106/1–16 (2006). [CrossRef]

, 21

21. R. Stoian, A. Mermillod-Blondin, N. M. Bulgakova, A. Rosenfeld, I. V. Hertel, M. Spyridaki, E. Koudoumas, P. Tzanetakis, and C. Fotakis, “Optimization of ultrafast laser generated low-energy ion beams from silicon targets,” Appl. Phys. Lett. 87, 124105/1–3 (2005). [CrossRef]

, 22

22. M. Guillermin, C. Liebig, F. Garrelie, R. Stoian, A.-S. Loir, and E. Audouard, “Adaptive control of femtosecond laser ablation plasma emission,” Appl. Surf. Sci. 255, 5163–5166 (2009). [CrossRef]

], indicating kinetic and excitation control upon regulated laser radiation. It will be extended here to multicomponent metallic alloys (brass) [24–27

24. A. De Giacomo, M. Dell’Aglio, O. De Pascale, R. Gaudiuso, R. Teghil, A. Santagata, and G. P. Parisi, “ns- and fs-LIBS of copper-based-alloys: A different approach,” Appl. Surf. Sci. 253, 7677–7681 (2007). [CrossRef]

] in view of the technologic and scientific interest posed by multicomponent samples in laser ablation and analytics experiments. Two main issues will be followed here: relative excitation of various species and absolute excitation and ionization degrees in the plume. We note that excitation and ionization control may significantly impact on current PLD and LIBS techniques.

The paper is organized as follows. The experimental section describes the methodological approach and the utilized instruments. The following sections describe the use of designer pulses and adaptive optimization procedures in different fluence regimes, intended to improve specific features of various emission products from ablated brass. These include, as indicated above, absolute and relative spectral emission yields, and nanoparticle ejection. The results of various temporal excitation pulse forms in thin film deposition techniques, particularly their morphologies, are accompanying the spectral emissivity study. Hydrodynamic simulations are used to illuminate specific thermodynamic influences leading to spectral control of the ablation products. Effects are identified in the probability of liquid nanoparticle ejection and the temperature of the plasma front. A conclusion section summarizes the results.

2. Experiment

Brass (composition 62%Cu and 38%Zn) samples were irradiated with ultrashort near infrared Ti:Sapphire laser pulses (Fourier transform-limited duration 150 fs) generated by an 1 kHz, 800 nm oscillator-amplifier laser system. The choice was motivated by the different thermal and atomic properties of the components (e.g. higher phase transition points for Cu and higher ionization potential for Zn). The samples were placed in a vacuum chamber at a pressure of 10-5 Pa and irradiated at oblique incidence (45°) with p-polarized laser radiation down to an elliptical spot of approximately 7740μm2. The irradiation sequence contains 45 pulses in a row at a repetition rate of 1 kHz. A programmable temporal pulse shaping unit can generate arbitrary wave forms in a time window of 15 ps, inducing corresponding plasma plumes. The technique relies on spectral phase modulation via complex filtering of pulse frequency components in dispersive systems equipped with pixelated liquid-crystal spatial light modulators. Using a phase-only approach, the energy in the laser sequence is always conserved. The resulting pulses were characterized by a second order background free intensity cross-correlation technique using a short reference pulse retrieved from the laser beam before the shaping procedure [12

12. J. P. Colombier, P. Combis, A. Rosenfeld, I. V. Hertel, E. Audouard, and R. Stoian, “Optimized energy coupling at ultrafast laser-irradiated metal surfaces by tailoring intensity envelopes: Consequences for material removal from Al samples,” Phys. Rev. B 74, 224106/1–16 (2006). [CrossRef]

]. The spectral emissivity of the laser ablation plume was detected and quantified by a 300 to 1200 lines/mm grating spectrometer equipped with a time-gated intensified charge coupled device (ICCD) camera. An optical system, similar to that described in Ref. [22

22. M. Guillermin, C. Liebig, F. Garrelie, R. Stoian, A.-S. Loir, and E. Audouard, “Adaptive control of femtosecond laser ablation plasma emission,” Appl. Surf. Sci. 255, 5163–5166 (2009). [CrossRef]

], collects the emitted light and focuses it on the entrance of an optical fiber coupled with the spectrometer in such a way that light is gathered from the most intense region of the plasma, close to the sample surface, as represented in Fig. 1(a). The optical signal corresponds thus to a time and space averaged signal of neutral or ionic emission. The spectral information is analyzed and, based on a maximization criteria of various spectral lines, a figure of merit (fitness) is assigned. The recorded spectrum corresponds to a recording time of 300 ns at a delay of 100 ns with respect to the pulse arrival and serves then as feedback for the closed loop created between the detection system and the pulse tailoring unit. The feedback loop is driven by an evolutionary strategy that manipulates discrete spectral phase masks on the modulator (and therefore the corresponding pulse forms) using genetic propagators to maximize the experimental output; an approach similar to the one used in Ref. [21

21. R. Stoian, A. Mermillod-Blondin, N. M. Bulgakova, A. Rosenfeld, I. V. Hertel, M. Spyridaki, E. Koudoumas, P. Tzanetakis, and C. Fotakis, “Optimization of ultrafast laser generated low-energy ion beams from silicon targets,” Appl. Phys. Lett. 87, 124105/1–3 (2005). [CrossRef]

]. The optimization result is a laser pulse that provides the best fitness spectrum with respect to the imposed constraints discussed later in the text.

Fig. 1. (a) Experimental two-dimensional images of the plume. (b) Typical spectra of the brass plasma under ultrashort pulse laser irradiation. The acquired spectra correspond to the plasma core that propagates towards the right side. (c) Spectral assignment of the main Cu-I, Zn-I, Cu-II, Zn-II lines [28] based on Grotrian diagrams.

The irradiation procedure produces typical spectra of the plasma core as shown in Fig. 1(a,b) in the range of 300 – 700 nm. The spectral assignment indicates transitions schematically represented in Fig. 1(c) with complete spectral evaluation given in [28

28. M. Guillermin, “Study of the femtosecond laser ablation plume, control and optimization of processes,” PhD Thesis, Université Jean Monnet, Saint Etienne (2009) (http://tel.archives-ouvertes.fr/tel-00395196/en/).

]. The lines employed in this study were chosen to have a variety of upper levels, some of them more susceptible to be collisionally pumped and thus sensitive to the electronic temperature. In addition to integrated signals, we focus therefore respectively on the neutral transitions around 330 nm and on the ionic transitions located around 490 nm.

Fig. 2. (a) Neutral and (b) ionic integrated spectral emission intensities in the spectral region 300–700 nm for various pulse shapes as a function of the incident laser fluence. SP, LP, DP sequences were used (see text for details). Different behavioral domains can be defined, corresponding to different average fluence regimes (LF-low fluence, MF-moderate fluence, HF-high fluence).

3. Results

3.1. Material removal with designed laser pulses

A first spectral diagnosis is facilitated by irradiation with various input energies while employing short pulse (SP) laser radiation (corresponding to the Fourier transform-limited value of 150 fs), as the initial energy deposition defines to a great extent the follow-up behavior. As a function of the incident laser fluence the integrated neutral Cu-I and Zn-I signals or, respectively, ionic Cu-II, Zn-II emissions detected in the range of 300–700 nm show a non-uniform behavior depicted in Fig. 2(a,b), indicating several regions of interest. All these regions are well situated in the ablation regime above the modification threshold of F = 0.16 J/cm2 (measured by the spot regression method) and, as well, above the plasma detection threshold of F = 0.5 J/cm2. While improving the signal-to-noise ratio, the chosen fluences with rather elevated values with respect to the ablation threshold help in keeping the eventual elemental fractionation effects at high number of pulses per site at a low level [29

29. C. C. Garcia, H. Lindner, A. von Bohlen, C. Vadlab, and K. Niemax, “Elemental fractionation and stoichiometric sampling in femtosecond laser ablation,” J. Anal. At. Spectrom. 23, 470–478 (2008). [CrossRef]

]. The low fluence region located between the observed plasma generation threshold of 0.5 J/cm2 and a value of 1.5 J/cm2 average fluence corresponds to a plasma with a dominant component consisting of neutral species, increasing in yield in the moderate fluence range and beginning to slow down its increase upon the onset of strong ionization (2.5 J/cm2). The spectroscopic temperature for the neutrals determined using Boltzmann regression on several Cu-I lines vary little with the fluence and stabilizes at a value of approximately 4000 K. This corresponds to a spatially and temporally averaged value and can only be used as a qualitative and relative property of different irradiation regimes. The increasing fluence is then inducing a higher yield of excited species by developing stronger thermal gradients and a higher volume of ablated material. In this case the fluence serves as an indicative parameter for establishing particular evolution regimes that will be discussed below.

In the respective regimes we have made an attempt to analyze the effect of a pulse shape on the ablation characteristics by employing simple designer pulses. This involves usually stretched pulses and double pulse sequences as simpler assumptions may be made regarding their influence on the material evolution. Attempting to modify the spectral signatures of atomic and ionic species, the underlying idea is to exploit the electronic relaxation and its effect on heat diffusion, the lattice heating dynamics on the scale of electron-phonon coupling, together with the potential generation of a liquid phase and a density gradient before the end of the pulse envelope. The subsequent hydrodynamic movement delivers reflectivity transients that can be exploited by the proposed double pulse series with adjustable separation or chirped pulses. The use of simplified pulses permits a first intuitive insight into the topological optimization space, as the individual pulse effects can be better followed as the material evolves.

Fig. 3. Spectral intensity enhancement (relative increase or magnification factor) for particular pulse shapes, LP (left column) and DP (right column) in various fluence regimes [low 1.3 J/cm2 (a,b), moderate 2.6 J/cm2 (c,d), and high 4.8 J/cm2 (e,f)]. The emission corresponds to characteristic narrow spectral ranges centered on the observed lines. Neutral Cu-I lines (solid squares) and Zn-I (solid circles) are used, normalized to the SP value. Relative ionic emission increase for a mixed signal comprising Cu-II and Zn-II (open circles and squares) with respect to the SP level are equally shown. Note that in (c,d), in the absence of a measurable ionic signal for SP, the yield was normalized to the detection limit and rescaled for visibility.

3.2. Emissivity enhancement via adaptive optimization

The closed loop mentioned earlier in the experimental section was used to improve the plume fluorescence using a spectrally defined fitness [22

22. M. Guillermin, C. Liebig, F. Garrelie, R. Stoian, A.-S. Loir, and E. Audouard, “Adaptive control of femtosecond laser ablation plasma emission,” Appl. Surf. Sci. 255, 5163–5166 (2009). [CrossRef]

]. A first attempt was made to change the relative ratio of the different chemical species (Cu or Zn) present in the detected region at 4.2 J/cm2 using the lines of Cu-I(d) at 328.27 nm and Zn-I(e) at 330.3 nm. The relatively low sensitivity obtained within the experimental uncertainties allows us to believe that many spectral lines behave in a similar manner, indicating that a common temperature regulates the evolution of the different components. Particular relaxation dynamics that may affect the relative species sensitivity are averaged by the 300 ns detection gating window. This limits the possibility to selectively enhance the individual components by control of relative quantities of ejected material and requires special care in selecting the lines. It also serves as a further indication of the occurrence of homogeneous and congruent ablation in spite of different thermodynamic points and ionization potentials for Cu and Zn, with a selectivity that depends on the recombination dynamics of particular lines. No selective ablation, as usually expected in the case of slow, equilibrium thermal processes, was noticed. As it will be seen in the discussion section, the species may follow different spatial spreading, but behave according to similar temperature gradients.

Fig. 4. Neutral and ionic spectral intensity enhancement (left) for optimized pulse shapes (right) in different fluence regime: (a,b) low fluence 1.2 J/cm2, (c-f) high fluence 4.2 J/cm2. SP-solid lines, OP-dashed lines. The corresponding fitness values were based on absolute neutral yield (a,b), absolute ion yield (c,d) and relative ion yield (e,f) in the given spectral domains (in the 330 nm region for the neutrals, around 493 nm for the ions, and comparative to the 481.05 nm Zn-I line for the relative yield), respectively.

Specifically, this second experimental procedure involves the maximization of different fitness parameters involving neutral lines in low fluence regimes, absolute yield associated to ionic lines at higher fluences, or ionic increase relative to the neutral signal. Figure 4 shows the results of the adaptive approach on neutral and ionic lines in various fluence regimes. An initial regime at F = 1.2 J/cm2 was used to enhance the overall, mainly neutral optical signal, as consequences are relevant for spectroscopy and elemental analysis. A feedback signal based on Cu-I and Zn-I lines [rays (a to f) in Fig. 1(b,c)] was used to measure the global intensity within a spectral window of 66 nm centered at 330 nm [see Fig. 4(a)]. The fitness was chosen in the form of f 1 = (ΣI neutrals)2. The optimized form OP1 is a noisy intensity envelope extending over 14 ps [Fig. 4(b)]. The aspect is dominated by multipeaks separated by a relatively regular ps spacing. The multipeak structure can originate from relatively high amount of spectral phase modulation delivered to the pulse in a discrete manner. The emission increase factor was measured to be around 2.5, similar to that previously observed with LP and DP sequences.

Fig. 5. Thin CuZn films deposited on Si substrates by various pulse shapes: (a) SP, (b) OP1. The fluence was fixed in the low range (F = 1.2 J/cm2) and the exposure time was 52 min at 1 kHz. Note the change in films morphologies. At high fluences resembling particulate distributions are observed.

A final observation relates to the specific threshold for species emission. If the neutral emission threshold seems quasi-similar for the short and optimized sequences (Fig. 2), once in ablation regime, the SP sequence requires more energy to obtain, if possible at all, signal levels comparable with the OP sequences. A factor of two in the energy requirements for ions onset is noticed between SP and OP, suggesting a more efficient energy coupling in the expanding phase for the optimized distribution. This is a relevant aspect for applications requiring a highest signal level with a minimal energy consumption and less invasive to the material such as remote spectroscopy applications.

Fig. 6. Time-of-flight ion mass spectrometry traces indicating ion enhancement from a Cu target under the action of optimal pulses at a fluence 2.8 times higher than the asymptotic multishot ion emission threshold level. The measured signal corresponds to ions with a velocity of 2.6×104 m/s, located in the front part of the plume. Irradiation conditions: F = 0.8 J/cm2, N = 10 pulses per site. Note the increased sensitivity to ions in mass spectrometry as compared to spectral detection, where ions were not easily detected at this fluence (Fig. 2). Inset: the optimal pulse form.

4. Discussion

To attempt an explanation of the observed behavior regarding the spectral emission under short and tailored irradiation, we have employed a non-equilibrium two temperature hydrodynamic code (Esther) [12

12. J. P. Colombier, P. Combis, A. Rosenfeld, I. V. Hertel, E. Audouard, and R. Stoian, “Optimized energy coupling at ultrafast laser-irradiated metal surfaces by tailoring intensity envelopes: Consequences for material removal from Al samples,” Phys. Rev. B 74, 224106/1–16 (2006). [CrossRef]

]. The Lagrangian code treats the material as a continuum two temperature system, solving the fluid equations for the conservation of mass, momentum and energy for electronic and ionic species. The radiation impact is calculated by evaluating the Helmholtz wave equation in the inhomogeneous media. Electronic temperature effects dominate the initial optical transients via the carrier scattering frequency, where typical behaviors are given in [35

35. J. P. Colombier, E. Audouard, P. Combis, A. Rosenfeld, I. V. Hertel, and R. Stoian, “Controlling energy coupling and particle ejection from aluminum surfaces irradiated with ultrashort laser pulses,” Appl. Surf. Sci. 255, 9597–9600 (2009). [CrossRef]

]. The thermodynamic properties of the material (energy, temperature, pressure, density, heat of fusion and vaporization) are described by the Bushman-Lomonosov-Fortov multiphase equation of state (EOS) spanning a large range of densities and temperatures from the cold condensed state to a hot plasma [36

36. A. V. Bushman, I. V. Lomonosov, and V. E. Fortov, “Models of wide-range equations of state for matter under conditions of high energy density,” Sov. Tech. Rev. B: Therm. Phys. Rev. 5, 1 (1993).

]. In the absence of reliable EOS and non-equilibrium properties for brass, the simulation were run for a copper (Cu) sample. Though the approach may seem disputable, we expect that some similar relative tendencies will occur, delivering partial insights into the transformation factors. This expectation is supported by the relative ion increase using optimal pulses from copper targets detected using time-of-flight mass spectrometry [37

37. R. Stoian, H. Varel, A. Rosenfeld, D. Ashkenasi, R. Kelly, and E. E. B. Campbell, “Ion time-of-flight analysis of ultrashort pulsed laser-induced processing of Al2O3,” Appl. Surf. Sci. 165, 44–55 (2000). [CrossRef]

] (Fig. 6) employing this time a higher resolution shaper. Note that, typical to time/velocity resolved spectra [12

12. J. P. Colombier, P. Combis, A. Rosenfeld, I. V. Hertel, E. Audouard, and R. Stoian, “Optimized energy coupling at ultrafast laser-irradiated metal surfaces by tailoring intensity envelopes: Consequences for material removal from Al samples,” Phys. Rev. B 74, 224106/1–16 (2006). [CrossRef]

, 20

20. M. Spyridaki, E. Koudoumas, P. Tzanetakis, C. Fotakis, R. Stoian, A. Rosenfeld, and I. V. Hertel, “Temporal pulse manipulation and ion generation in ultrafast laser ablation of silicon,” Appl. Phys. Lett. 83, 1474–1476 (2003). [CrossRef]

, 21

21. R. Stoian, A. Mermillod-Blondin, N. M. Bulgakova, A. Rosenfeld, I. V. Hertel, M. Spyridaki, E. Koudoumas, P. Tzanetakis, and C. Fotakis, “Optimization of ultrafast laser generated low-energy ion beams from silicon targets,” Appl. Phys. Lett. 87, 124105/1–3 (2005). [CrossRef]

, 37

37. R. Stoian, H. Varel, A. Rosenfeld, D. Ashkenasi, R. Kelly, and E. E. B. Campbell, “Ion time-of-flight analysis of ultrashort pulsed laser-induced processing of Al2O3,” Appl. Surf. Sci. 165, 44–55 (2000). [CrossRef]

], the signal indicates the behavior of ions with a certain velocity, in this case a high velocity (2.6×104 m/s), located approximately to the front of the plume. The comparative prospect is also sustained by the note that, dealing with incoherent processes and thermomechanical dynamics, the class of optimized pulses is rather large (note similar DP effects in [14

14. A. Semerok and C. Dutouquet, “Ultrashort double pulse laser ablation of metals,” Thin. Sol. Films 453–454, 501–505 (2004). [CrossRef]

] in conditions of moderate plasma shielding) and potentially extendable to other materials as well. However, the simulation results should be taken as an indicative parameter as differences between copper and brass exist. We indicate notably a higher thermal conductivity for Cu (approximately four times) and a Cu electron-phonon coupling that strongly augments with the electronic temperature [38

38. Z. Lin, L. V. Zhigilei, and V. Celli, “Electron-phonon coupling and electron heat capacity of metals under conditions of strong electron-phonon nonequilibrium,” Phys. Rev. B 77, 075133/1–17 (2008). [CrossRef]

] while brass may preserve an alloy character. This has consequences on the efficiency of the energy confinement, threshold levels, and, via the electron-phonon interaction increase, on the dynamic optical absorption properties. In the present case the irradiation regimes were calculated relative to the theoretical ablation threshold for Cu of approximately Fth = 0.5 J/cm2.

The simulation results are shown in Fig. 7 for SP and for the experimentally determined optimal shapes (OP1, OP2–1, denoted simply as OP) presented in Fig. 4. The figure indicates time-resolved spatio-temporal density and temperature profiles of the expanding material under shaped and optimal irradiation for a lower fluence regime (6× the calculated threshold), moderate intensities (8× the threshold), and high irradiance (16× the threshold). The three situations are depicted in Fig. 7(a), Fig. 7(b), and Fig. 7(c) respectively, where the scales of simulations were chosen to match as closely as possible the experimental observations (the large dimensional scales imped the clear observations at early times, notably for the temperature maps). At lower fluences we observe the long evolution of ejected dense fluid material with a thickness of few nanometers (seen as evolving nanolayers at liquid density in the left column of Fig. 7), driven by the high initial pressure. This is accompanied by a moderate plume temperature (Fig. 7 right column). The evolving liquid layers can further evaporate or fragment into droplets as a function of the neighboring pressure conditions [39

39. B. Chimier and V. T. Tikhonchuk, “Liquid-vapor phase transition and droplet formation by subpicosecond laser heating,” Phys. Rev. B 79, 184107/1–10 (2009). [CrossRef]

], a situation which is not developed here. A slight decrease of the relative liquid quantity in the plume is observed for higher fluences.

Fig. 7. Ablation density and temperature spatio-temporal profiles above the initial surface in a zt diagram, serving as indication for the temperature and species correlations in the ablation plume. SP and OP conditions are used. The scales were chosen to allow comparison to the experimental detection conditions. Different regimes were tested, a low fluence regime at 6×Fth (a), moderate fluence values at 8×Fth (b), and a high fluence regime at 16×Fth (c). The calculated threshold fluence is 0.5 J/cm2. Smaller particulate content is seen in the lower energy domains for optimized pulses as compared to the observable ejection of nanolayers at liquid density for SP. Increasing the fluence, higher temperature profiles, particularly in the plasma front, are obtained for optimized sequences, suggesting a development of the excitation degree along the temperature gradients. These temperatures may become less sensitive to the pulse form at very high fluence levels, beyond typical ablation regimes. Note the different color scales. The right side depicts the temperature axial profiles at the moment of experiment acquisition (250 ns).

We note that restrictions may apply to these observations due to the real three-dimensional evolution of the plume against the one-dimensional character of the simulation approach, however we believe that the relative behavior remains valid.

An observation that may seem important in the context, also the temperature axial profiles T(z) play an important role (Fig. 7 right column). The temperature spatial gradient along the plume expansion axis that accentuates towards the front determines the excitation efficiencies or the resulting ionization degree as a function of the chemical nature of the species. Provided that the temperature may become high enough and the density does not vary, this leads to distinct regions of different abundances of neutral and ionic species even in the conditions of congruent ablation, creating an impression of spatial segregation. Resuming, the increasing temperature profile together with the corresponding ionization degrees determines the formation of neutral or ionized regions according to the ionization potentials of the individual species, leading as well to a spatially-selective agglomeration of the excited or ionized chemical species. Cu-I, Zn-I species are mainly located in the vicinity of the surface, decoupled from the high temperature regions. Closer to the plasma front the neutral zone is leaded by Cu-II and the Zn-II domains. An experimental demonstration of this particular situation was already indicated [33

33. X. Wang, S. Amoruso, and J. Xia, “Temporally and spectrally resolved analysis of a copper plasma plume produced by ultrafast laser ablation,” Appl. Surf. Sci. 255, 5211–5214 (2009). [CrossRef]

] but was mainly explained based on ambipolar diffusion. This segregation appears to originate in the temperature gradient and follows the axial temperature profiles. Additional influences such as charge effects and specific flight times will further accentuate this tendency.

5. Conclusion

Acknowledgments

This work has been supported in part by the Ultraviolet Laser Facility operating at IESL-FORTH in the frame of the EC project ”Laserlab-Europe” (FP6 Contract RII3-CT-2003-506350 and FP7 Grant Agreement 212025). The support received from the Agence Nationale de la Recherche is equally acknowledged.

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N. M. Bulgakova, I. M. Bourakov, and N. A. Bulgakova, “Rarefaction shock wave: Formation under short pulse laser ablation of solids,” Phys. Rev. E , 63, 046311/1–5 (2001). [CrossRef]

12.

J. P. Colombier, P. Combis, A. Rosenfeld, I. V. Hertel, E. Audouard, and R. Stoian, “Optimized energy coupling at ultrafast laser-irradiated metal surfaces by tailoring intensity envelopes: Consequences for material removal from Al samples,” Phys. Rev. B 74, 224106/1–16 (2006). [CrossRef]

13.

P. Lorazo, L. J. Lewis, and M. Meunier, “Thermodynamic pathways to melting, ablation, and solidification in absorbing solids under pulsed laser irradiation,” Phys. Rev. B 73, 134108/1–22 (2006). [CrossRef]

14.

A. Semerok and C. Dutouquet, “Ultrashort double pulse laser ablation of metals,” Thin. Sol. Films 453–454, 501–505 (2004). [CrossRef]

15.

T. Gunaratne, M. Kangas, S. Singh, A. Gross, and M. Dantus, “Influence of bandwidth and phase shaping on laser induced breakdown spectroscopy with ultrashort laser pulses,” Chem. Phys. Lett. 423, 197–201 (2006). [CrossRef]

16.

S. Singha, Z. Hu, and R. J. Gordon, “Ablation and plasma emission produced by dual femtosecond laser pulses,” J. Appl. Phys. 104, 113520/1–10 (2008). [CrossRef]

17.

T. Donnelly, J. G. Lunney, S. Amoruso, R. Bruzzese, X. Wang, and X. Ni, “Double pulse ultrafast laser ablation of nickel in vacuum,” J. Appl. Phys. 106, 013304/1–5 (2009). [CrossRef]

18.

A. Klini, P. A. Loukakos, D. Gray, A. Manousaki, and C. Fotakis, “Laser Induced Forward Transfer of metals by temporally shaped femtosecond laser pulses,” Opt. Express 16, 11300–11309 (2008). [CrossRef] [PubMed]

19.

A. Santagata, R. Teghil, G. Albano, D. Spera, P. Villani, A. De Bonis, G. P. Parisi, and A. Galasso, “Fs/ns dual-pulse LIBS analytic survey for copper-based alloys,” Appl. Surf. Sci. 254, 863–867 (2007). [CrossRef]

20.

M. Spyridaki, E. Koudoumas, P. Tzanetakis, C. Fotakis, R. Stoian, A. Rosenfeld, and I. V. Hertel, “Temporal pulse manipulation and ion generation in ultrafast laser ablation of silicon,” Appl. Phys. Lett. 83, 1474–1476 (2003). [CrossRef]

21.

R. Stoian, A. Mermillod-Blondin, N. M. Bulgakova, A. Rosenfeld, I. V. Hertel, M. Spyridaki, E. Koudoumas, P. Tzanetakis, and C. Fotakis, “Optimization of ultrafast laser generated low-energy ion beams from silicon targets,” Appl. Phys. Lett. 87, 124105/1–3 (2005). [CrossRef]

22.

M. Guillermin, C. Liebig, F. Garrelie, R. Stoian, A.-S. Loir, and E. Audouard, “Adaptive control of femtosecond laser ablation plasma emission,” Appl. Surf. Sci. 255, 5163–5166 (2009). [CrossRef]

23.

A. Assion, T. Baumert, M. Bergt, T. Brixner, B. Kiefer, V. Seyfried, M. Strehle, and G. Gerber, “Control of Chemical Reactions by Feedback-Optimized Phase-Shaped Femtosecond Laser Pulses,” Science 282, 919–922 (1998). [CrossRef] [PubMed]

24.

A. De Giacomo, M. Dell’Aglio, O. De Pascale, R. Gaudiuso, R. Teghil, A. Santagata, and G. P. Parisi, “ns- and fs-LIBS of copper-based-alloys: A different approach,” Appl. Surf. Sci. 253, 7677–7681 (2007). [CrossRef]

25.

O. V. Borisov, X. L. Mao, A. Fernandez, M. Caetano, and R. E. Russo, “Inductively coupled plasma mass spectrometric study of non-linear calibration behavior during laser ablation of binary Cu-Zn Alloys,” Spectrochim. Acta Part B 54, 1351–1365 (1999). [CrossRef]

26.

R. Hergenröder, O. Samek, and V. Hommes, “Femtosecond laser ablation elemental mass spectrometry,” Mass Spectrom. Rev. 25, 551–572 (2006). [CrossRef] [PubMed]

27.

V. Margetic, A. Pakulev, A. Stockhaus, M. Bolshov, K. Niemax, and R. Hergenröder, “A comparison of nanosecond and femtosecond laser-induced plasma spectroscopy of brass samples,” Spectrochim. Acta Part B 55, 1771–1785 (2000). [CrossRef]

28.

M. Guillermin, “Study of the femtosecond laser ablation plume, control and optimization of processes,” PhD Thesis, Université Jean Monnet, Saint Etienne (2009) (http://tel.archives-ouvertes.fr/tel-00395196/en/).

29.

C. C. Garcia, H. Lindner, A. von Bohlen, C. Vadlab, and K. Niemax, “Elemental fractionation and stoichiometric sampling in femtosecond laser ablation,” J. Anal. At. Spectrom. 23, 470–478 (2008). [CrossRef]

30.

D. Scuderi, O. Albert, D. Moreau, P. P. Pronko, and J. Etchepare, “Interaction of a laser-produced plume with a second time delayed femtosecond pulse,” Appl. Phys. Lett. 86, 071502/1–3 (2005). [CrossRef]

31.

V. Piñon, C. Fotakis, G. Nicolas, and D. Anglos, “Double pulse laser-induced breakdown spectroscopy with femtosecond laser pulses,” Spectrochim. Acta Part B 63, 1006–1010 (2008). [CrossRef]

32.

S. Noël, E. Axente, and J. Hermann, “Investigation of plumes produced by material ablation with two time-delayed femtosecond laser pulses,” Appl. Surf. Sci. 255, 9738–9741 (2009). [CrossRef]

33.

X. Wang, S. Amoruso, and J. Xia, “Temporally and spectrally resolved analysis of a copper plasma plume produced by ultrafast laser ablation,” Appl. Surf. Sci. 255, 5211–5214 (2009). [CrossRef]

34.

Ph. Rohwetter, J. Yu, G. Méjean, K. Stelmaszczyk, E. Salmon, J. Kasparian, J.-P. Wolf, and L. Wöste, “Remote LIBS with ultrashort pulses: characteristics in picosecond and femtosecond regimes,” J. Anal. At. Spectrom. 19, 437–444 (2004). [CrossRef]

35.

J. P. Colombier, E. Audouard, P. Combis, A. Rosenfeld, I. V. Hertel, and R. Stoian, “Controlling energy coupling and particle ejection from aluminum surfaces irradiated with ultrashort laser pulses,” Appl. Surf. Sci. 255, 9597–9600 (2009). [CrossRef]

36.

A. V. Bushman, I. V. Lomonosov, and V. E. Fortov, “Models of wide-range equations of state for matter under conditions of high energy density,” Sov. Tech. Rev. B: Therm. Phys. Rev. 5, 1 (1993).

37.

R. Stoian, H. Varel, A. Rosenfeld, D. Ashkenasi, R. Kelly, and E. E. B. Campbell, “Ion time-of-flight analysis of ultrashort pulsed laser-induced processing of Al2O3,” Appl. Surf. Sci. 165, 44–55 (2000). [CrossRef]

38.

Z. Lin, L. V. Zhigilei, and V. Celli, “Electron-phonon coupling and electron heat capacity of metals under conditions of strong electron-phonon nonequilibrium,” Phys. Rev. B 77, 075133/1–17 (2008). [CrossRef]

39.

B. Chimier and V. T. Tikhonchuk, “Liquid-vapor phase transition and droplet formation by subpicosecond laser heating,” Phys. Rev. B 79, 184107/1–10 (2009). [CrossRef]

OCIS Codes
(140.3390) Lasers and laser optics : Laser materials processing
(310.1860) Thin films : Deposition and fabrication
(320.2250) Ultrafast optics : Femtosecond phenomena
(320.5540) Ultrafast optics : Pulse shaping
(300.6365) Spectroscopy : Spectroscopy, laser induced breakdown

ToC Category:
Ultrafast Optics

History
Original Manuscript: February 12, 2010
Revised Manuscript: March 19, 2010
Manuscript Accepted: March 22, 2010
Published: May 12, 2010

Citation
M. Guillermin, A. Klini, J. P. Colombier, F. Garrelie, D. Gray, C. Liebig, E. Audouard, C. Fotakis, and R. Stoian, "Tuning spectral properties of ultrafast laser ablation plasmas from brass using adaptive temporal pulse shaping," Opt. Express 18, 11159-11172 (2010)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-18-11-11159


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References

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  13. P. Lorazo, L. J. Lewis, and M. Meunier, “Thermodynamic pathways to melting, ablation, and solidification in absorbing solids under pulsed laser irradiation,” Phys. Rev. B 73(13), 134108 (2006). [CrossRef]
  14. A. Semerok, and C. Dutouquet, “Ultrashort double pulse laser ablation of metals,” Thin Solid Films 453–454, 501–505 (2004). [CrossRef]
  15. T. Gunaratne, M. Kangas, S. Singh, A. Gross, and M. Dantus, “Influence of bandwidth and phase shaping on laser induced breakdown spectroscopy with ultrashort laser pulses,” Chem. Phys. Lett. 423(1-3), 197–201 (2006). [CrossRef]
  16. S. Singha, Z. Hu, and R. J. Gordon, “Ablation and plasma emission produced by dual femtosecond laser pulses,” J. Appl. Phys. 104(11), 113520 (2008). [CrossRef]
  17. T. Donnelly, J. G. Lunney, S. Amoruso, R. Bruzzese, X. Wang, and X. Ni, “Double pulse ultrafast laser ablation of nickel in vacuum,” J. Appl. Phys. 106(1), 013304 (2009). [CrossRef]
  18. A. Klini, P. A. Loukakos, D. Gray, A. Manousaki, and C. Fotakis, “Laser induced forward transfer of metals by temporally shaped femtosecond laser pulses,” Opt. Express 16(15), 11300–11309 (2008). [CrossRef] [PubMed]
  19. A. Santagata, R. Teghil, G. Albano, D. Spera, P. Villani, A. De Bonis, G. P. Parisi, and A. Galasso, “Fs/ns dual pulse LIBS analytic survey for copper-based alloys,” Appl. Surf. Sci. 254(4), 863–867 (2007). [CrossRef]
  20. M. Spyridaki, E. Koudoumas, P. Tzanetakis, C. Fotakis, R. Stoian, A. Rosenfeld, and I. V. Hertel, “Temporal pulse manipulation and ion generation in ultrafast laser ablation of silicon,” Appl. Phys. Lett. 83(7), 1474–1476 (2003). [CrossRef]
  21. R. Stoian, A. Mermillod-Blondin, N. M. Bulgakova, A. Rosenfeld, I. V. Hertel, M. Spyridaki, E. Koudoumas, P. Tzanetakis, and C. Fotakis, “Optimization of ultrafast laser generated low-energy ion beams from silicon targets,” Appl. Phys. Lett. 87(12), 124105 (2005). [CrossRef]
  22. M. Guillermin, C. Liebig, F. Garrelie, R. Stoian, A.-S. Loir, and E. Audouard, “Adaptive control of femtosecond laser ablation plasma emission,” Appl. Surf. Sci. 255(10), 5163–5166 (2009). [CrossRef]
  23. A. Assion, T. Baumert, M. Bergt, T. Brixner, B. Kiefer, V. SeyfriedV, M. Strehle, and G. Gerber, “Control of chemical reactions by feedback-optimized phase-shaped femtosecond laser pulses,” Science 282(5390), 919–922 (1998). [CrossRef] [PubMed]
  24. A. De Giacomo, M. Dell’Aglio, O. De Pascale, R. Gaudiuso, R. Teghil, A. Santagata, and G. P. Parisi, “ns- and fs-LIBS of copper-based-alloys: A different approach,” Appl. Surf. Sci. 253(19), 7677–7681 (2007). [CrossRef]
  25. O. V. Borisov, X. L. Mao, A. Fernandez, M. Caetano, and R. E. Russo, “Inductively coupled plasma mass spectrometric study of non-linear calibration behavior during laser ablation of binary Cu-Zn Alloys,” Spectrochim. Acta, B At. Spectrosc. 54(9), 1351–1365 (1999). [CrossRef]
  26. R. Hergenröder, O. Samek, and V. Hommes, “Femtosecond laser ablation elemental mass spectrometry,” Mass Spectrom. Rev. 25(4), 551–572 (2006). [CrossRef] [PubMed]
  27. V. Margetic, A. Pakulev, A. Stockhaus, M. Bolshov, K. Niemax, and R. Hergenröder, “A comparison of nanosecond and femtosecond laser-induced plasma spectroscopy of brass samples,” Spectrochim. Acta, B At. Spectrosc. 55(11), 1771–1785 (2000). [CrossRef]
  28. M. Guillermin, “Study of the femtosecond laser ablation plume, control and optimization of processes,” PhD Thesis, Université Jean Monnet, Saint Etienne (2009) (http://tel.archives-ouvertes.fr/tel-00395196/en/).
  29. C. C. Garcia, H. Lindner, A. von Bohlen, C. Vadla, and K. Niemax, “Elemental fractionation and stoichiometric sampling in femtosecond laser ablation,” J. Anal. At. Spectrom. 23(4), 470–478 (2008). [CrossRef]
  30. D. Scuderi, O. Albert, D. Moreau, P. P. Pronko, and J. Etchepare, “Interaction of a laser-produced plume with a second time delayed femtosecond pulse,” Appl. Phys. Lett. 86(7), 071502 (2005). [CrossRef]
  31. V. Piñon, C. Fotakis, G. Nicolas, and D. Anglos, “Double pulse laser-induced breakdown spectroscopy with femtosecond laser pulses,” Spectrochim. Acta, B At. Spectrosc. 63(10), 1006–1010 (2008). [CrossRef]
  32. S. Noël, E. Axente, and J. Hermann, “Investigation of plumes produced by material ablation with two time delayed femtosecond laser pulses,” Appl. Surf. Sci. 255(24), 9738–9741 (2009). [CrossRef]
  33. X. Wang, S. Amoruso, and J. Xia, “Temporally and spectrally resolved analysis of a copper plasma plume produced by ultrafast laser ablation,” Appl. Surf. Sci. 255(10), 5211–5214 (2009). [CrossRef]
  34. P. Rohwetter, J. Yu, G. Mejean, K. Stelmaszczyk, E. Salmon, J. Kasparian, J.-P. Wolf, and L. Woste, “Remote LIBS with ultrashort pulses: characteristics in picosecond and femtosecond regimesPresented at the Second Euro-Mediterranean Symposium on Laser Induced Breakdown Spectroscopy, Hersonissos, Crete, Greece, September 30th?October 3rd, 2003,” J. Anal. At. Spectrom. 19(4), 437 (2004). [CrossRef]
  35. J. P. Colombier, E. Audouard, P. Combis, A. Rosenfeld, I. V. Hertel, and R. Stoian, “Controlling energy coupling and particle ejection from aluminum surfaces irradiated with ultrashort laser pulses,” Appl. Surf. Sci. 255(24), 9597–9600 (2009). [CrossRef]
  36. A. V. Bushman, I. V. Lomonosov, and V. E. Fortov, “Models of wide-range equations of state for matter under conditions of high energy density,” Sov. Tech. Rev. B: Therm. Phys. Rev. 5, 1 (1993).
  37. R. Stoian, H. Varel, A. Rosenfeld, D. Ashkenasi, R. Kelly, and E. E. B. Campbell, “Ion time-of-flight analysis of ultrashort pulsed laser-induced processing of Al2O3,” Appl. Surf. Sci. 165(1), 44–55 (2000). [CrossRef]
  38. Z. Lin, L. V. Zhigilei, and V. Celli, “Electron-phonon coupling and electron heat capacity of metals under conditions of strong electron-phonon nonequilibrium,” Phys. Rev. B 77(7), 075133 (2008). [CrossRef]
  39. B. Chimier, and V. T. Tikhonchuk, “Liquid-vapor phase transition and droplet formation by subpicosecond laser heating,” Phys. Rev. B 79(18), 184107 (2009). [CrossRef]

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