Ž .Applied Surface Science 127–129 1998 1035–1040

Diagnostic of an expanding laser-produced lithium plasma usingICCD frame photography and shadowgraphy

William Whitty, Jean-Paul Mosnier )

School of Physical Sciences, Dublin City UniÕersity, GlasneÕin, Dublin 9, Ireland

Abstract

The expansion of a laser-produced lithium plasma is characterized using two different high-speed imaging techniques.Firstly, a sequence of frames of the luminous plume is recorded using an interference filterrgated ICCD cameracombination. Expansion velocities are estimated from these images. The conditions, in which the radial distributions ofemitters could be recovered using Abel inversion, are discussed. Secondly, shadowgraphs obtained with a synchronizedtunable dye laser light source are recorded at different probe wavelengths in the vicinity of the Li0 670.7-nm resonance. Thefringe patterns observed in these images are interpreted in terms of strong refractive index gradients within the plasma. Theeffect of anomalous dispersion is observed and strongly modifies the appearance of the shadowgraphs. q 1998 ElsevierScience B.V.

PACS: 52.70 La; 32.70 Jz

Keywords: Laser plasma; High-speed imaging; Anomalous dispersion; Shadowgraphy; Lithium

1. Introduction

Laser-produced plasmas are characterized by steepand rapidly varying electron densityrrefractive indexand temperature gradients. Their spatio-temporalevolution is driven by collisional processes betweenelectrons, ions, neutrals and photons. Laser plasmasare intense sources of line and continuum electro-magnetic radiation due to the relative strength of theradiative processes. Thus, the motion of the excitedplasma-emitting light by de-excitation or recombina-tion in ions or neutrals can be determined usinghigh-speed imaging. It provides information on the‘local’ structure and dynamics of the constituentparticles provided that a radiation model linking

) Corresponding author. Tel.: q44-353-1-704-5303; fax: q44-353-1-704-5384; mosnierjp@dcu.ie.

observed light intensities to particle distributions ex-ist. At least one severe complication in this recoveryprocess arises from the fact that the recorded 2D-in-tensities are necessarily integrated along the ob-served plasma depth, hence, requiring an appropriatetransformation of the data. In the present work, twodifferent high-speed imaging techniques, namely,framing camera imaging and shadowgraphy, are used.We now present their essential features.

High-speed photographic recording of expandinglaser plasmas using framing image-converter cam-

w xeras is a well-established technique 1,2 . More re-cently, the technique has been refined by the intro-

Žduction of gated ICCD Intensified Charge Coupled.Device cameras which allow direct digital record-

ing, thus, greatly facilitating computer processing ofthe images as well as providing improved sensitivity.Such imaging devices have been widely used in the

0169-4332r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved.Ž .PII S0169-4332 97 00603-X

( )W. Whitty, J.-P. MosnierrApplied Surface Science 127–129 1998 1035–10401036

Ž .area of pulsed laser deposition PLD of materialsŽ w x.see, e.g., Ref. 3 . Despite the technique’s ease ofimplementation, the retrieval of quantitative informa-tion characterizing the plume expansion from theimages is usually complex. By narrowing down thespectral range of the imaged light to that of a knownemission line, it becomes possible to track the evolu-tion of the corresponding excited state. This can becarried out by placing a tuned interference filter infront of the camera, in which case, the measuredintensities are integrated over the line profile. Spec-trally-resolved images can also be obtained if an

w ximaging spectrometer is used 4 .The shadowgraph technique consists of passing a

pencil of light through the test section and letting itfall directly, or via an imaging lens, onto a recordingdevice such as a photographic plate or a CCD ma-trix. It has long been used in the study of compress-

w xible gas flow 5 and plasmas, including laser-pro-Ž w xduced plasmas see, e.g., Ref. 2 and references

.therein . One can show that the intensity modulationsin the shadowgraph depend upon the second deriva-tive of the refractive index of the traversed mediumw x5 . The technique is thus suitable for the study ofexpanding laser plasmas where the refractive indexexhibits very rapid changes. Intense pulsed laserlight sources proved particularly useful to record theshadowgraphs of bright laser plasmas and the tech-nique has commonly been applied over the years.Numerous references to such early work in the 60s

w xand 70s can be found in Ref. 2 . Michaelis and Williw x6 showed that when probing a laser plasma of steeprefractive index gradient with a pencil of laser light,the rays traversing the denser regions suffer greaterrefraction and may interfere with the rays that didnot interact with the plasma, thus producing a patternof bright and dark fringes called refractive fringes.The authors showed, using an optical path ray analy-sis, that such refractive fringes could be used todetermine the electron density distributions of laser-

Ž w xproduced plasmas see also Refs. 7,8 for improved.analysis . More recently, the use of tunable dye laser

light as the probe beam brought a further refinementto the technique and was frequently applied as adiagnostic tool in the area of Pulsed Laser Deposi-

Ž . w xtion PLD of materials 9–11 . One should point outhere that shadowgraphs obtained with laser light maybe regarded as single beam holograms of laser plas-

mas. Holograms of laser plasmas were first recordedŽ w x.in the 60s see, e.g., Ref. 2 .

The strength of the interaction between electro-magnetic radiation of angular frequency v and mat-ter, is embodied in the value of the index of refrac-tion n . The value of n largely determines thev v

intensity distribution of light in the images obtainedwith the methods just described, and is related to theparticle densities of the probed medium, therebyexplaining the principle of the diagnostic. It is well-known that the index of refraction of a plasma atoptical frequencies is largely due to the free electron

w xcontribution 12 . However, in the vicinity of anatomic resonance, significant additions must be madeto the refractive index. Since we are concerned with

Ž .such a situation see Section 2 , we write the com-plex index of refraction nsny ik of a plasma in˜the vicinity of an atomic resonance of angular fre-quency v , weighed oscillator strength f and width0

w xG in the form 12,13 :

1 4p Nfe2 12n y1s˜ 2 2ž /4p´ m v yv q i Gv0 e 0

qn2 y1 1Ž .e

where N is the number of atoms per unit volume andm the electron mass. The free electron contributione

is equal to:

2vp2n s 1y 2Ž .(e 2v

where v is the plasma frequency for an electronp

density N such that:e

1 4p N e2e2v s . 3Ž .p ž /4p´ m0 e

Ž . Ž .From Eqs. 1 and 2 , we see that the electron gascontribution to the phase index of refraction is nega-tive and slightly less than unity. On the other hand,the complex term gives rise to the effect of anoma-

Ž .lous dispersion see Section 3 for details .In Sections 2 and 3, we give a brief description of

the experimental set-up used to record frames andshadowgraphs of a laser-produced lithium plasmaexpanding either in vacuum or in a low-pressureargon atmosphere. We then present typical images

( )W. Whitty, J.-P. MosnierrApplied Surface Science 127–129 1998 1035–1040 1037

obtained in both configurations and provide someelements of their physical interpretation.

2. Experimental and results

The basic configuration used for both types ofexperiments is shown in Fig. 1a.

The arrangement for shadowgraphy is illustratedin Fig. 1b. The lithium plasma is created by a

Ž .Nd:YAG laser 0.4 J, 15 ns using a slightly defo-Ž .cused 120-mm uncorrected plano-convex lens FL ,

providing a target irradiance of ;3=1010 Wrcm2

Ž 2 .;450 Jrcm . The dye laser beam used to probeŽ .the plasma is initially expanded BE to cover the

entire sensitive area of the CCD. The optical FWHMŽ .Full-Width Half Maximum is 8 ns, thus, introduc-ing temporal resolution. The experiment is synchro-

Ž .nized using a master pulse from a PC PC486 to aŽ .digital delay generator DG , thus controlling the

Ž .inter-laser triggering delay DT to an accuracy ofless than 3 ns. Background light emission from theplasma is eliminated by the introduction of neutral

Ždensity filters andror a tuned interference filter ND.q IF .

Blocking the dye laser beam and substituting theCCD detector for a gated image intensifierrCCD

Žcombination coupled to a zoom lens while retaining

Ž . Ž .Fig. 1. a Basic experimental configuration: CCD Charge Cou-Ž . Ž .pled Device, PC486 486 Personal Computer, DG Delay Gener-

Ž .ator. b Detailed view of the apparatus used for shadowgraphy:Ž . Ž . Ž .BE Beam Expander, FL Focusing Lens, NDqIF NeutralDensity FiltersqTuned Interference Filter.

0 Ž . Ž .Fig. 2. Temporal and spatial evolution of Li 670.7 nm . a,b,cŽ .Expanding into vacuum. d,e,f Expanding into 200 mTorr of

Ž . Ž . Ž .argon. a,d DT s10 ns. b,e DT s100 ns. c,f DT s250 ns.All units are in millimeter.

the neutral density filters and tuned interference fil-.ter provides the arrangement for frame photography.

Ž .An uncorrected plano convex lens FL of 190-mmfocal length was used to create the plasma in thiscase. The spot size was ;5 mm in diameter, result-

8 2 Žing in an irradiance of ;2.7=10 Wrcm ;42 .Jrcm . Synchronization is achieved as before. Suc-

cessive frames of the lithium plasma are obtained byvarying the triggering delay between the laserŽ .Nd:YAG 0.7 J; 15 ns used to create the plasma and

Ž .the gating pulse 8 ns FWHM to the image intensi-fier.

The temporal and spatial evolution of a lithiumplume expanding into vacuum as well as into a low

( )W. Whitty, J.-P. MosnierrApplied Surface Science 127–129 1998 1035–10401038

0 Ž . Ž .Fig. 3. Shadowgraphs of Li 670.7 nm at DT s70 ns. aŽ . Ž .l s669.7 nm. b l s670.7 nm. c l s672.5 nm.probe probe probe

Ž .pressure gas 200 mTorr of argon is shown in Fig. 2for different time delays. The camera is tuned to the

Ž . 02s–2p 670.7 nm Li transition using an interfer-ence filter centered at 670.7 nm with a 9.3 nmŽ .FWHM spectral window. Shadowgraphs at differ-ent probe wavelengths in the vicinity of the sametransition are presented in Fig. 3.

3. Analysis and discussion

First, we consider the sequence of frames pre-sented in Fig. 2a,b,c depicting the expansion invacuo of the laser-produced lithium plasma. Charac-

teristic is the strong spatial anisotropy of the earlyw xexpansion 1 : the plume remains mostly confined

around the axis constituting the target normal with aŽ .velocity component longitudinal along this axis

Ž .larger than the velocity component radial perpen-dicular to the target normal. The density contourshows a certain asymmetry in the very early stageprobably due to inhomogeneities in the energy distri-bution of the laser beam. In order to estimate thelongitudinal velocity of neutral lithium, we follow

w xthe procedure first described in Ref. 14 . At a fixeddistance from the target on the axis of expansionŽ .ys0 mm , we record the integrated intensity varia-tions of the Li0 670.7-nm emission line as a functionof time. This procedure is then repeated for different

Ž .positions d along the axis Fig. 4 . A plot of d as afunction of the time corresponding to the peak of thedistribution curve yields the desired quantity. A value

0 Ž .Fig. 4. The integrated intensity variations of Li 670.7 nm atŽ . Ž .fixed distances d mm as a function of time ns .

( )W. Whitty, J.-P. MosnierrApplied Surface Science 127–129 1998 1035–1040 1039

of ;2.4=106 cm sy1 is obtained. It should becompared with the advance velocity of the plasmafront which is recovered by measuring the positionof the leading luminous edge of the plume as afunction of time, corresponding to frames obtained

Ž .without the interference filter not shown here . Thesubsequent velocity shows the linearity typical of afree expansion with a measured slope of ;3.5=106

cm sy1. An identical procedure was applied to theleading edge of the plume in the direction normal tothe expansion axis, i.e., parallel to the target surface.A value of ;5=105 cm sy1 was obtained. In the

Žcase of Fig. 2d,e,f, expansion in 200 mTorr of.argon the physical picture is as follows: the leading

edge of the luminous plume advances linearly as aŽ 6 y1.function of time Õf3.3=10 cm s until about

200 ns after the creation of the plasma. Then, theisointensity contour lines appear to pile up at thefront edge in a manner reminiscent of the sphericalwavefronts of a shock wave. In this regime, we fittedthe time evolution of the plasma leading edge to the

n Žpower law xskt and obtained ns0.42 see, e.g.,w x.Ref. 15 .

We also note in Fig. 2a,b,c from the isointensitycontour lines that the shape of the intensity leveldistribution along the expansion axis remains fairlyconstant with time. This suggests a self-similar ex-pansion for the plume, i.e., the form of the densityprofile does not change with time. However, thiswould only apply if a linear relationship betweenlight intensity levels and atomic densities could beestablished.

We now quantify the relationship between themeasured 2D images and the 3D distribution ofemitters within the plume. In our experimental condi-

Ž .tions, the light intensity recorded by a pixel x, y isŽgiven by see Figs. 1 and 2 for the orientation of the

.axes :

qzr2I s ´ r d zdn 4Ž . Ž .H HŽ x , y. n

yzr2 line

Ž .in which ´ r is the emission coefficient at then

photon frequency n , and z the total length of theemitting plasma chord. The radial distribution of

Ž .emitters N r could be obtained by Abel inversion

Ž . Ž .of the integral Eq. 4 provided that: a the observedŽ .light rays are virtually paraxial, b the plasma is

optically thin, in which case the emission coefficientŽ . Ž . Ž .´ r is linearly proportional to N r , and c theren

Ž .is radial symmetry in the y, z plane. It is necessarythat these three conditions be met simultaneously forthe Abel inversion procedure to be applicable. From

Ž .the images presented in Fig. 2, the condition c ofradial symmetry seems readily satisfied. As regardsŽ . Ž .a and b , further experimental work would berequired to establish their validity. Furthermore, evenif the necessary conditions were satisfied, the Abeltransform procedure would still only provide thedistribution of the corresponding excited atomic den-sities in the plume. An appropriate radiation modelfor the plasma would then be required to obtainground state population densities and possibly elec-tronic densities.

We now provide a qualitative interpretation of theshadowgraphs of Fig. 3a,b,c in terms of a modelbased on the refraction of light rays through theplasma. The value of the complex phase index of

Ž .refraction is given by Eq. 1 , from which we extractŽ .n and k Fig. 5 for typical values of the atomic and

electronic densities corresponding to the early phaseŽ .of the plume expansion. At 669.7 nm Fig. 3a , the

electronic and atomic contributions add up to lessthan unity and the observed pattern is identical to

Ž . Ž .Fig. 5. The real n and imaginary k parts of the plasmaŽ . Žrefractive index as a function of wavelength nm . G f0.4 nm,

Ns1=1018 cmy3 , N s1=1019 cmy3 , f s0.75, l s670.7e 0.nm.

( )W. Whitty, J.-P. MosnierrApplied Surface Science 127–129 1998 1035–10401040

w xthose reported previously in Refs. 6,7 . Steep den-sity gradients refract the incident beam toward re-gions of higher index and the bright and dark fringesŽ .refractive fringes on the outer part of the shadow-graph are due to optical interferences between theundisturbed part of the incident beam and the re-fracted rays. The situation changes dramatically at

Ž . Ž .670.7 nm on resonance Fig. 3b and 672.5 nmŽ .Fig. 3c . The atomic contribution to the real part ofthe index dominates and becomes greater than unity.The steep density gradients steer the beam in thedirection opposite to that of Fig. 3a since the regionsof higher index now correspond to the regions ofhigher density. The effect is particularly pronouncedin Fig. 3c, as can be seen from the position of thetarget surface. In order to obtain a rough estimate ofthe atomic density, the method used by El-Astal and

w xMorrow 16 was followed. From Fig. 3c and otherŽ .data not shown here , the maximum deflection angle

was estimated at ;7 mrad. For a plume opticallength of 4 mm, this corresponds to a uniform refrac-tive index gradient of 1.75 my1. This value along

w xwith an oscillator strength of 0.748 17 correspondsto a Li atom density gradient of ;6=1025 my4.Complete modelling of the present experiment iscurrently under construction, from which improvedatomic density estimates are expected. Nonetheless,the series of shadowgraphs of Fig. 3 provide directevidence of the preponderant role played by refrac-tion when scanning an absorption resonance in alaser plasma plume with an external tunable source.Strong distortion of the intrinsic line shape is likely

Ž w x.to result see, e.g., Ref. 16 .

Acknowledgements

The authors wish to thank Forbairt for financialassistance, M. Hopkins for the loan of the dye laser,and E.T. Kennedy for reading the manuscript.

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