femtosecond vs nanosecond la

8/13/2019 Femtosecond vs Nanosecond LA

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Femtosecond vs. Nanosecond Laser Pulse Duration for Laser Ablation ChemicalAnalysisSpectroscopy, Jan 1, 2013Richard E. Russo,Xianglei Mao,Jhanis J. Gonzalez,Jong Yoo

Laser ablation for direct solid sample chemical analysis has advanced over the past

50 years with applications in many disciplines, including environmental,

geological, medical, energy, security, and others. Although the choice of laser is

still highly dependent on the application requirements, there are distinct

fundamental effects attributed to the laser pulse duration that drive the ablation

sampling process. An overview of nanosecond and femtosecond laser ablation is

presented with respect to analysis based on the optically induced plasma at the

sample surface (such as laser-induced breakdown spectroscopy or laser ablation

molecular isotopic spectrometry) and transport of the ablated mass aerosol to an

inductively coupled plasma.

Laser ablation is widely recognized as a powerful technology for direct solid samplingchemical analysis. Without requiring sample preparation in most cases, any solidsample can be analyzed for elemental and isotopic content within minutes. There aretwo primary approaches for detecting the ablated mass either by measuring the

photons emitted by the optical induced plasma at the sample surface or by entraining the

ablated aerosol into a gas stream with delivery to a secondary source. Laser-inducedbreakdown spectroscopy (LIBS) and laser-ablation molecular isotopic spectroscopy(LAMIS) are based on measuring optical emission in the plasma (110). Theinductively coupled plasma (ICP) with mass spectrometry (MS) or optical emissionspectroscopy (OES) methods are based on the transport of the ablated aerosol (8,1120). The sampling process is the same (laser ablation); detection is dictated by theapplication.

Laser ablation actually dominates in applications other than chemical analysis, such ascutting, welding, micromachining, and laser-assisted in situ keratomileusis (LASIK).Although chemistry is not emphasized in these other applications, their requirements are

similar: efficiently use laser photons to remove mass. Over the past 50 years, researchhas addressed almost every parameter influence on the ablation process. The basis of
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this column is to provide physical concepts on a critical parameter driving efficientmass removal the laser pulse duration. This column describes observations fromnanosecond and femtosecond ablation research with comments related to chemicalanalysis based on both plasma emission and particle detection. Picosecond lasers are notaddressed mainly because they have not been that prevalent in this field; applications

are primarily driven by the availability of commercial lasers. Until about 15 years ago,the majority of research emphasized nanosecond pulsed lasers, which had replacedmicrosecond and longer pulsed lasers. There are early references on long-pulse laserablation (21) but for the most part, chemical analysis would be challenging if not for thedevelopment of short pulsed (nanosecond and femtosecond) lasers. Based on a series ofconcepts established from time-resolved measurements, we provide an overview of the

processes under which ablation occur. Foremost, we admit upfront that much of thepulsed laser ablation research is empirical there is no unified theory that predicts thequantity of mass ablated, the chemistry of the plasma, or the aerosol properties from anablation event. The discussion herein does not delve into specific mechanisms, butinstead presents concepts based on observations from reproducible time-resolved

measurements.

Fundamentals

Laser Beams

The laser beam provides defined photon energy (wavelength) and power (number ofphotons per unit time) that is incident on a sample surface for a specific duration oftime. For the most part, the wavelength of the laser is not resonant with any specificelemental transitions in the sample (although some research has looked into resonance

Figure 1: Initial general concept diagram showing spatial extent for red nanosecondand femtosecond pulsed laser beams aiming at the same sample from right to left.


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excitation). In Figure 1, we show the first step in the process in an illustration of ageneric sample and spatial representation of nanosecond and femtosecond pulses. In thiscase, both lasers have the same color or wavelength. This is a first order approximation,

because the optical bandwidth of each laser is different. In Figure 1, the femtosecondlaser pulse is essentially a delta function in time and space compared to the extended

nanosecond pulse. Common values of the laser pulse duration for commercial Nd:YAGand Ti:sapphire lasers are 6 ns and 100 fs, respectively. At any instant, the 100-fs laserpulse is 30 m long and the 6-ns laser pulse is 1.8 m long. The newer Yb-based fiberlasers have pulse durations of approximately 300 fs. In these cases, the nanosecond laser

pulse is 60,000 times longer than the Ti:sapphire and 20,000 times longer than the Ybfemtosecond laser pulses. The slight differences in the femtosecond pulse durations will

be discussed later; for now, we assume they are similar although on a pure fundamentallevel there can be subtle differences in excitation and the ablation processes.

Optical Penetration Depth

Figure 2 shows where things get interesting for the laser-material interaction. First, theoptical penetration depth of the laser is different even though the sample and laserwavelength are the same. From a linear one-photon Beer's law model, this would not bethe case. However, high peak intensity (energy/area-time) of the laser drives nonlinear

processes, in this case multiphoton (n= 2,3, . . . ) absorption. Therefore, the optical

penetration depth of the energy is less and the energy per unit volume is greater forfemtosecond laser pulses compared to nanosecond laser pulses. The optical penetration

Figure 2: Concept diagram shows the laser pulses entering the same sample from rightto left. Optical penetration depth of laser energy in the sample is different because oflinear (nanosecond) and nonlinear multiphoton absorption (femtosecond) processes.Top image shows shockwaves induced by optical absorption of the nanosecond pulsed

laser inside and outside a glass sample (60). Bottom image represents a transient changein the optical properties from the high intensity femtosecond pulse (26). The irregularvertical line is the glass surface in both images.


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depth depends on the sample and the laser irradiance (energy/area-time). In mostpublications, the parameter of interest is reported as the energy per unit area (fluence),defined by the laser beam spot size on the sample. Although studies show goodcorrelations to fluence, energy per unit volume drives the ablation process (22). Energyis needed to break bonds and move atoms (and particles) from solid to vapor phases.

Again, this is only part of the process, as we ignore changes in reflectivity and otherprocesses that are time dependent. Energy imparts work on or in the sample energyof the photons (laser wavelength), number of photons (pulse energy), and irradiance(power per unit area). Based on an effective optical penetration depth, there is a well-defined volume or number of atoms that have bonds, bond energies, latent processes,and so on, that will play a part in the explosion (ablation) of mass from the irradiatedsample. The energy becomes dissipated into different processes, including electronicexcitation, ionization, heating, shock, and vaporization. Exact mechanisms, from

photons in to kinetic energy of vapor, photon emission (atomic, ionic, and molecular) toaerosol formation are not completely established, and are the basis of many fundamentalendeavors. The images in Figure 2 are an example of shock wave and nonlinear

absorption induced optical index change for nanosecond and femtosecond laser pulseinteractions in a transparent sample.

Femtosecond and Nanosecond Laser Pulses

A second concept presented in Figure 2 is that the femtosecond pulse is effectivelyfinished on this time scale, say 300 fs after firing each laser, whereas the nanosecond

pulse is still irradiating the surface. Ramifications of this behavior are discussed withreference to Figure 3. Keeping with a high-level overview, Figure 3 shows empirical

processes distinct for femtosecond and nanosecond laser pulses. The first thing to noticeis that the plasma (electrons, atoms, molecular, and particles) is propagating into thelaser beam for the nanosecond pulsed laser; laser energy is coupled into the plasma.Even using femtosecond time-resolved imaging above the sample surface, no physicalentity has been measured, leaving the surface on the femtosecond time scale; electrons

Figure 3: Concept diagram shows that the nanosecond laser is still illuminating thesample while the plasma is expanding, whereas the femtosecond pulse is over before

any mass can leave the surface. The heated region is deeper for the nanosecond versusthe femtosecond pulse. Images are photographs of the laser plasma, showing theeffect of laser-plasma heating from the nanosecond plasma (61).
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have been measured picoseconds after the laser pulse (2332). Optical photographs inFigure 3 of laser plasmas established from a copper sample with the same laser energy,spot size, and wavelength confirm the robustness of a nanosecond laser-heated plasmaversus a more delicate femtosecond plasma, which is related to plasma heating. The

proportion of laser energy absorbed or even reflected by this plasma depends on many

conditions, including time, temperature, electron, and mass density. Laser plasmaheating is a likely reason why nanosecond pulsed lasers are more common for LIBSapplications because they offer a more robust, longer persistence plasma. Ideally, theablated mass is a vapor, heated to excite atoms, ions, and molecules for optical emission

by LIBS and LAMIS. Over a short time, as the plasma expands and cools, the vapornucleates and condenses to form particles that are available for entrainment andtransport to the ICP.

Heating

Figure 4: Concept diagram shows the heat affected zone (HAZ) in the sample. HAZ isproportional to the square root of the pulse duration and therefore there is much greaterheating for nanosecond than femtosecond pulses. The graph shows relative decay ratesof measured optical emission from two plasmas initiated with lasers having all

properties equal except the pulse duration (25).


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The concepts presented in Figure 4 illustrate that heating and the heat affected zone

(HAZ), which is proportional to the square root of the pulse duration, are significantlylarger for nanosecond pulses. For a laser pulse duration of 100 fs, the HAZ in an alloy isabout 4 nm compared to approximately 1000 nm for a 6-ns pulse. Figure 4 shows alarge heated and melted region as the energy in the nanosecond pulse has had time todissipate into the sample; phonon modes initiate about 10 ps after the initial laser beaminteraction with a sample, so a 100-fs laser pulse is over before a molecule can vibrate.The HAZ describes part of the mechanism because it is the amount of energy in thiszone as a function of time; how the atoms and molecules respond to this energy per unitvolume and time is an ongoing research program in many disciplines. Heating to a melt

phase is considered detrimental in the ablation process because the sample can undergopreferential vaporization. Heating also drives elemental migration within the bulk,

effectively changing the stoichiometry in the sample. Accuracy of analysis can becompromised by single and successive pulsed laser ablation if heating is sufficient todrive these processes. For accurate analytical chemistry, the ablated mass should bestoichiometric to the sample composition. (The word "should" is used here becausequantitative analytical chemical analysis can be achieved under fractionation conditionsif matrix-matched standards are available and ablate exactly the same as the sample,which is often not the case.) A final comment about Figure 4 is that this is the time

period in which LIBS and LAMIS are performed. The exact time depends on the laserpulse duration and energy. Common times are approximately 1 s for nanosecondpulsed LIBS, several microseconds for LAMIS, and several hundred nanoseconds forfemtosecond pulsed LIBS, all after the laser pulse. The graph in Figure 4 shows thedifferent plasma persistence from nanosecond and femtosecond ablation with the samewavelength, energy, and spot size; pulse duration was the only different parameter.

Producing Particles

Figure 5: Concept diagram representing direct ejection of particles. The particle sizesand size distribution are related to the heated volume. The images at right are examples


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of time-resolved shadowgraphs for ejected particles from a surface (on the left). Thespatial resolution of the images can resolve particles greater than a few hundrednanometers.

Figures 5 and 6 describe the particle (aerosol) character of the ablated mass. We believethere are at least two processes for producing particles: some are ejected from thesurface as distinct particles (fracture, spallation, or melt flush) (Figure 5) and othersundergo nucleation and condensation from vapor to form nanoparticles (Figure 6).

Particles are important for laser-ablation ICP-MS, but deleterious for LIBS and LAMIS.Ideally, there would be no ejected particles, or if there were, they would not originatefrom melt flushing and would be stoichiometric. Ejected particles also can be too largeto be entrained, transported, or digested in the ICP. Particles from the condensate will

be of nanometer size and are better suited for ICP analysis. Ejected particles areobserved shortly after the laser pulse; whereas condensed particles are formed later intime as the plasma expands and cools.

Figure 6: Concept diagram showing the sample and plasma cooling stage. Plasmacooling leads to nucleation and condensation of nanoparticles. The scanning electronmicroscopy images (right) show particles collected from ablation with two lasers havingthe same parameters except for the pulse durations (61)


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We know from differential mobility analysis (DMA) and scanning electron microscopy(SEM) measurements that the particle size distribution is strongly dependent on thesample and laser properties. Sticking with this overview of only laser pulse duration,there are numerous studies showing that the particle size and size distribution are

smaller and narrower, respectively, for femtosecond than for nanosecond laser pulses(3335). This observation holds for both the ejected and nucleated particles. Somereasons for smaller particles from femtosecond ablation include less melting, higherenergy per volume, smaller HAZ, and in many cases, less total mass ablated per pulse,so that the nucleated particles are smaller; nucleation and condensation are based on thevapor density. The higher expansion rate of the femtosecond plasma also supports amore rarified vapor. The shadowgraph images in Figure 5 show ejected particles fromnanosecond laser ablation and only the shockwave from femtosecond ablation; ejected

particles could not be observed in these types of measurements under the energyconditions generally used for femtosecond sampling into the ICP-MS system. SEMimages in Figure 6 show particles from nanosecond and femtosecond ablation when allconditions, except the pulse duration, were constant. Again, these analyses are generalcases; laser parameters can be adjusted to provide smaller or larger particles from anysample. We are addressing the parameter space that is optimized for ablation chemicalanalysis commonly used for LIBS, LAMIS, and laser-ablation ICP-MS applications. Acaveat about particle size measurements using a DMA system: it does not distinguishagglomerated particles from primary particles. As particle sizes become smaller,agglomeration is enhanced. Therefore, DMA measurements should be supported bySEM images.

After the Ablation Process


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Figure 7: Concept diagram shows the sample after all processes are finished. Theaffected region in the sample is related to the pulse duration. Melting leads to rimformation around the crater (ablated mass). The images are white-light interferogramsof copper surfaces after ablation using nanosecond (top) and femtosecond (bottom)

pulses having the same energy, wavelength, and spot size (14).

Figure 7 shows the sample after the ablation process is finished and images of cratersproduced using nanosecond and femtosecond laser ablation with all parameters equalexcept the pulse duration. Again, these figures are only for discussion to demonstrate

commonly observed effects. The HAZ is larger for the nanosecond case and shows arim around the crater; a smaller area of the original sample was influenced by thefemtosecond laser, and the ablated mass was removed by nonthermal photophysical

processes without producing a rim due to melt flushing. The total amount of massremoved (ablated) will strongly depend on the energy in the laser pulse. Generally,microjoules or millijoules of energy per pulse are used for femtosecond or nanosecondlaser ablation, respectively. Therefore, less mass will be removed for the femtosecond

pulse. Nanosecond pulsed lasers in the deep UV can produce clean craters without asignificant rim, and rim formation has been observed under nonoptimal femtosecondcases. Again, many parameters govern the ablation process and can be varied to producea wide range of effects. Irrespective of the rim, the fundamentals of the HAZ remain

intact; using femtosecond laser ablation less heating, more energy per bonds, and moreefficient use of photon energy for ablation are obtained.

Advantages for Analytical Chemistry

Given these attributes of femtosecond laser pulses we ask the following question: Howis analytical chemistry affected? Precision and accuracy have been reported to beimproved in many ICP-MS studies (3639). Improved precision using femtosecondlaser ablation has been attributed to smaller particles and a narrower particle sizedistribution. In addition, the lower amount of mass does not overload the ICP. Onset ofvaporization, ionization, the ion cloud, and chemical interference effects are highly

dependent on the aerosol (dry, in this case). Accuracy is improved by using thefemtosecond laser because of minimized heating of the sample. But there are other


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attributes beyond these performance metrics. Common criticisms of laser ablation areinhomogeneity of the sample, representative sampling, and sensitivity. However, all ofthese concerns can be mitigated by using high repetition rate femtosecond laserablation. High repetition rate femtosecond laser ablation sampling has been shown tosupport bulk sample analysis by faster scanning over small sample areas (4044). The

amount of sample needed will be related to the inhomogeneity, and in most cases, willbe much less than that required for sample dissolution in acid and subsequent liquidnebulization. It is more practical to remove smaller amounts of mass from each low-energy femtosecond laser pulse with high repetition rates to get the mass quantitysufficient for analysis, instead of relying on a single high-energy nanosecond laser pulseto remove the same amount of mass. The ICP benefits from the finer and morecontinuous high repetition rate femtosecond laser ablation process, which is similar to aliquid nebulizer. There are many corollaries for laser ablation with liquid nebulizerssuch as controlling the amount of mass and providing a continuous stream of sample tothe ICP with the right particle distribution. Early nebulizers wasted sample because thedroplet size distribution, which affects precision and accuracy, was not suitable for the

ICP. The same goes for laser ablation: As we improve the particle size and distribution,performance improves.

Laser ablation provides amazingly sensitive absolute detection at the femtogram andattogram levels (37,4552). The reason for the relative insensitivity (compared to liquidnebulization) is the small amount of mass that is sampled during ablation. By using highrepetition rate femtosecond lasers, the amount of mass can be increased to providelimits of detection comparable to those obtained using liquid nebulization, withoutrequiring as much mass as liquid nebulization. Laser ablation is better suited for solidsanalysis (Why use acid digestion and more sample than necessary?) As the lasercontinues to ablate to achieve sufficient mass, elemental migration and preferentialvaporization need to be minimized. These processes are mitigated by using femtosecondlaser pulses with the very small HAZ.

Conclusions

Fundamental processes have been studied for years, and we ignored processes likeshockwaves, spallation, Coulomb explosion, phase explosion, plasma chemistry, spatialgradients in the plasma, and more. Even if we do not fully understand the ablation

processes, reproducible control of the interaction to produce an ideal aerosol for ICP-MS or an ideal plasma for LIBS or LAMIS can be achieved; understanding the

fundamentals drives advancement. In spite of our inability to completely unravelunderlying mechanisms, the beauty of this technology is that the process is reproducibleusing set parameters (just like those of any analytical technique) for laser energy, pulseduration, fluence, and wavelength. Analytical performance metrics are improved usingfemtosecond rather than nanosecond laser ablation. This does not mean that nanosecond

pulsed laser ablation is not a viable solution for many applications. Good analyticalchemistry can be performed with a nanosecond pulsed infrared (IR) laser if matrix-matched standards exist and if the ICP-MS is optimized for such ablation conditions.For most LIBS applications, nanosecond pulsed IR lasers are common to establish arobust long-persistence plasma (1,2). Femtosecond laser ablation provides advantagesfor applications that need excellent precision, high efficiency, and do not have matrix-

matched standards (53). Finally, the femtosecond laser pulse duration is (to the best ofour knowledge) the only way to ablate with nanometer spatial dimensions, both lateral


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and depth (45,5459). Again, the HAZ defines the volume of sample affected by theenergy, allowing imaging and depth profiling on the nanometer scale.

Acknowledgments

The authors would like to thank Dr. Tim Suen for the design of the pulse-durationconcept slides. R.E.R., X.L.M., and J.J.G. acknowledge support from the ChemicalScience Division, Office of Basic Energy Sciences, and the Defense Nuclear

Nonproliferation Research and Development Office of the US Department of Energy(DOE) under contract number DE-AC02-05CH11231 at the Lawrence Berkeley

National Laboratory. J.Y. acknowledges support from the DOE SBIR Program.

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Richard E. Russo iswith the Lawrence Berkeley National Laboratory and Applied Spectra, Inc .

Xianglei Maois with the Lawrence Berkeley National Laboratory. Jhanis J. Gonzalezis with the

Lawrence Berkeley National Laboratory and Applied Spectra, Inc. Jong Yoo is with Applied

Spectra, Inc.

femtosecond vs nanosecond la

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