128 IEEE TRANSACTIONS ON PLASMA SCIENCE, VOL. 27, NO. 1, FEBRUARY 1999

Three-Dimensional Electron Number Densitiesin a Titanium PLD Plasma Using Interferometry

L. A. Doyle, G. W. Martin, T. P. Williamson, A. Al-Khateeb, I. Weaver, D. Riley, M. Lamb,T. Morrow, and C. L. S. Lewis

Abstract— Interferometry has been used to investigate thespatio-temporal evolution of the electron number density in theinitial stages of expansion following 248 nm ablation of a titaniumtarget. Three-dimensional electron number densities are obtainedfrom an interferogram of the plasma plume using the Abelinversion technique.

Index Terms—Ablation, density measurement, optical interfer-ometry, plasma properties.

I NTERFEROMETRY has been used to investigate thespatio-temporal evolution of the electron number density

in the initial stages of expansion following 248 nm ablationof a titanium target. Three-dimensional (3-D) electron numberdensities are obtained from an interferogram of the plasmaplume using the Abel inversion technique.

Pulsed laser deposition (PLD) has been shown to be a usefultechnique for the stoichometric deposition of thin films ofvarious materials [1]. Measurement of the plasma parametersunder well-controlled conditions is essential to achieve greaterphysical insight and to provide data for direct comparisonwith computer simulations of the ablation process. In thispaper we illustrate the use of time-resolved interferometry toinvestigate the spatio-temporal evolution of the free electronnumber density in the early stages of plasma expansion intovacuum.

The experimental configuration used in the present studyhas been described previously [2]. Briefly, the output from aLamda Physik 201i KrF excimer laser [ nm,ns full width at half maximum (FWHM)] is focussed ontothe surface of a stationary titanium target in vacuum (510 torr). A circular spot ( 1.5 mm) is obtained usinga focussing system consisting of two cylindrical lenses anda random phase plate (RPP). The RPP, a binary diffractiveoptic, produces a smoothed azithumally symmetric spot whichimproves the homogeneity of the plasma plume and alsosimplifies data interpretation [3], [4]. The resulting plasmaplume is located in one arm of a Mach–Zehnder interferometerilluminated by a Nd:YAG pumped dye laser (Lamda PhysikScanmate 2C, 5 ns FWHM, 0.15 cm ). Theinterferometer is initially adjusted to give closely spacedinterference fringes parallel to the target surface which areimaged onto a gated intensified charge-coupled device (ICCD)(Oriel Instruments INSTASPEC V). The temporal resolution

Manuscript received June 29, 1998; revised September 10, 1998. This workwas supported by the Engineering and Physical Science Research Council andthe Department for Education for Northern Ireland.

The authors are with the School of Mathematics and Physics, The Queen’sUniversity of Belfast, Belfast, BT7 1NN, U.K. (e-mail: l.doyle@qub.ac.uk).

Publisher Item Identifier S 0093-3813(99)02418-2.

Fig. 1. Spatial energy distribution on target of a typical excimer laserablation pulse.

is determined by the 2 ns optical gate of the ICCD which issynchronous with the peak of the dye laser and can be delayedwith respect to the excimer pulse.

Free electrons in the plume cause a reduction in the refrac-tive index and therefore the optical path length through theplume resulting in a shift in the positions of the interferencefringes. If the dye laser wavelength is tuned away from anyatomic resonances, then the ratio of the observed fringeshift to fringe spacing is related to the electron number density

by

(1)

where nm is the probe wavelength, is the criticalelectron density, and is the path length through the plume.The measured fringe shifts are integrated along the line of sightbut if the plume has cylindrical symmetry, as is the case here,then the technique of Abel inversion can be used to calculatethe 3-D electron number density.

Fig. 1 shows the spatial energy distribution of the excimerablation spot as monitored with an equivalent plane monitor(EPM) which has an overall resolution of 18 m. TheEPM uses a 4% reflection of the excimer pulse incident

0093–3813/99$10.00 1999 IEEE

IEEE TRANSACTIONS ON PLASMA SCIENCE, VOL. 27, NO. 1, FEBRUARY 1999 129

Fig. 2. Interferogram recorded during the ablation of titanium at an ir-radiance of 415� 54 MWcm�2 at a delay of 54 ns after the ablatingpulse.

Fig. 3. Electron number densityN plotted as a function of distance fromtargetZ and radial distance from the axis of symmetryR calculated fromthe interferogram in Fig. 2.

on a plate of glass positioned at exactly the same distancefrom the beamsplitter as the target and whose fluorescenceis imaged onto a charge-coupled device (CCD) detector. Thehigh spatial frequency speckle in the spot produced by theRPP smoothes out during the laser/target interaction timeas opposed to lower spatial frequency hotspots found intypical rectangular type spots obtained with standard opticsand apertures. This is illustrated in Fig. 2 which shows aninterferogram with smoothly varying fringes obtained duringthe ablation of titanium at an irradiance of 41554 MWcm

Fig. 4. Simulated interferogram reconstructed from the electron numberdensities shown in Fig. 2.

and a delay of 54 ns after the excimer is first incident onthe target. Application of the above equation to Abel-invertedfringe shifts produces the electron number density distributionshown in Fig. 3 where the electron number density is plottedas a function of distance from the target and radial distancefrom the symmetry axis. The number densities fall off withaxial and radial distance from target from a peak measuredvalue of 5 10 cm . The lower detection limit ofthe system, 5 10 cm at the probe wavelength, isdetermined by the minimum measurable fringe shift, which isCCD pixel limited. The upper limit is determined by refraction,solid angle collection effects, and the spatial resolution ofthe imaging system. Fig. 4 shows a simulated interferogram,reconstructed by integrating the measured electron densitiesin Fig. 3. The accuracy of the Abel inversion is illustrated bythe good agreement between Figs. 2 and 4. The technique hasbeen applied to investigation of electron densities as a functionof incident power density and for different target materials [2].

ACKNOWLEDGMENT

The authors wish to thank W. A. Montgomery for specialistlaser support. The random phase plates were provided by C.Danson and D. Pepler at the EPSRC Central Laser Facility,CLRC Rutherford Appleton Laboratory.

REFERENCES

[1] D. B. Chrisey and G. K. Hubler, Eds.,Pulsed Laser Deposition of ThinFilms. New York: Wiley, 1994.

[2] L. A. Doyle, G. W. Martin, A. Al-Khateeb, I. Weaver, D. Riley, M.J. Lamb, T. Morrow, and C. L. S. Lewis, “Electron number densitymeasurements in magnesium laser produced plumes,”Appl. Surf. Sci.,vol. 127–129, no. 1–4, pp. 716–720, 1998.

[3] D. A. Pepler, C. N. Danson, R. Bann, I. N. Ross, R. M. Stevenson, M.J. Norman, M. Desselberger, and O. Willi, “Focal spot smoothing andtailoring for high-power laser applications,”SPIE Proc., vol. 1870, pp.76–87, 1993.

[4] Y. Kato, K. Mima, N. Miyanaga, S. Arinaga, Y. Kitagawa, M. Nakat-suka, and C. Yamanaka, “Random phasing of high-power lasers foruniform target acceleration and plasma-instability suppression,”Phys.Rev. Lett., vol. 53, no. 11, pp. 1057–1060, 1984.