Temporally and spatially resolved spectroscopy of laser-inducedplasma from a droplet
Jia-biao Zheng,* Wen-Feng Hsieh, Shu-chi Chen,t and Richard K. Chang
Section of Applied Physics and Center for Laser Diagnostics, Yale University, New Haven, Connecticut 06520
Received March 2, 1988; accepted March 30, 1988
A new diagnostic technique provides temporally and spatially resolved information at two settable wavelengths
within the plasma-emission profile of a single transparent water droplet (40 ,um in radius and containing 4 M NaCl)
irradiated by a high-intensity laser beam. Data from this technique are compared with those from conventional
techniques that separately provide time-averaged spatially resolved spectra or spatially averaged temporallyresolved spectra of the plasma emission from droplets.
Various spectroscopic techniques have providedpiecemeal information on the plasma resulting fromthe laser-induced breakdown (LIB) of transparentdroplets irradiated by a high-intensity laser beam.More complete plasma information is needed beforecomparing the measured results and theoretical mod-els that incorporate the hydrodynamic, thermody-namic, and, more recently, electrodynamic processestaking place within the droplet and in the externalplasma plumes. -5
Spatially resolved (along the z axis parallel to thelaser direction) but time-averaged spectroscopy(X(z))t has been applied to the study of the plasmaplumes ejected from the shadow and illuminated facesof a transparent droplet. These (X(Z))t results, ob-tained by using an optical multichannel analyzer con-sisting of a spectrograph and a vidicon, provide infor-mation on the location of LIB initiation,6' 7 the inho-mogeneity of the plasma plume,8 and the asymmetrydetected in the plasma-emission profiles of the shad-ow and illuminated faces.8 A spectrometer and a pho-tomultiplier were used for the time-resolved but spa-tially averaged measurements9 (X(t))z. These (X(t))zresults indicate that the emission maximum of theionized species is reached before that of the neutralspecies, that the decay times of the various emittingspecies are different, and that the intensity ratio of thetwo emitting species is time dependent. By using astreak camera with its slit along the laser direction, wedetermined the time-resolved and space-resolvedmeasurements of the plasma emission within a broadwavelength region10 AX(z, t). These measurementsprovide the propagation velocities of the plasma eject-ed from the shadow face, propagated internally fromthe shadow face toward the illuminated face, andejected from the illuminated face.
We report a new diagnostic technique that providesboth temporally and spatially resolved information onthe LIB-generated plasma at two settable wave-lengths, X1(z, t) and X2 (Z, t). Knowledge of the plasmaevolution in both the Eulerian and Lagrangian frame-work is provided by the simultaneously determined
temporally and spatially resolved spectral data ob-tained during and after one laser shot.
The schematic of the new technique is shown in Fig.1. A linear stream of monodispersed water droplets(with radius n40 gum and containing 4 M NaCl) isproduced by a Berglund-Liu generator. The incidentradiation (from the second harmonic of a Q-switchedlaser with X0 = 532 nm) is focused to a diameter of-150 Arm and has an intensity of 0.4 GW/cm2 . Thetemporal and spatial behavior of the LIB-generatedplasma emission at two wavelengths, X1(z, t) and X2 (Z,t), and the temporal behavior of Xo(t) are measured inthe following manner: (1) the laser beam travelingalong the horizontal z axis is aimed at a single dropletflowing along the vertical y axis (the y and z axes arerotated by 90° in Fig. 1); (2) the images of the dropletand the plasma plumes are magnified 12X and focused
Droplet
Time
Fig. 1. Schematic of the experimental configuration thatcan provide temporally and spatially resolved plasma-emis-sion spectra at two settable wavelengths X, and X2. Theinset illustrates that the spatial sensitivity of the spectro-graph and the fiber ribbons is reasonably uniform. Thecentral peak in the inset is from light channeled through thesingle fiber without first passing through the spectrograph.
onto the entrance slit (parallel to the z axis) of a low-dispersion spectrograph (with 12 nm/mm and rotatedby 900 in Fig. 1); (3) the ends of two fiber ribbons (6mm X 250 ptm) are located where Xi and X2 appear atthe spectrograph exit plane; (4) the other two ends ofthe fiber ribbons are placed along the vertical entranceslit of the streak camera; (5) a single fiber channelssome of the laser radiation at X0 onto the central por-tion of the streak-camera entrance slit; and (6) a vid-icon camera reads the information presented at thestreak-camera output and stores this information in acomputer. The output of the streak camera consistsof XA(z, t), X2(z, t), and Xo(t), which are simultaneouslydetected during and after one laser pulse.
The inset in Fig. 1 illustrates that the spatial varia-tion of the two fiber ribbons is reasonably smoothwhen a tungsten source uniformly illuminates thespectrograph entrance slit. The central spike is fromthe single fiber that intercepts the tungsten radiationwithout passing through the spectrograph. For this
(a)
a) )0
I o
Co I ,
, 1
(b)SRS
H- Na a()
S I sI
Laser
Fig. 2. Single-shot data of the temporally and spatiallyresolved spectra from a single water droplet (-40 gm inradius and containing 4 M NaCl) that has undergone laser-induced breakdown. The horizontal axis corresponds tospace along the laser direction (indicated by the arrow), andthe droplet illuminated face and shadow face are labeled Iand S, respectively. The vertical axis corresponds to time,which is increasing in (a) and decreasing in (b). The curvesout of the space-time axes represent the relative intensity.The right-hand portions of (a) and (b) show X, set at the Naresonance line Na(I). The left-hand portions of (a) and (b)show X2 set at the Balmer line H-a and the stimulated Ramanscattering (SRS) of H20. The central portion shows thetime profile of the laser radiation channeled to the streakcamera through the single fiber shown in Fig. 1.
measurement, the streak camera is not swept in time,i.e., it operates in the focused mode.
Data from one laser shot are plotted with the timeincreasing [Fig. 2(a)] and the time decreasing [Fig.2(b)] along the vertical axis. The spatial informationis along the horizontal axis, with the right-hand por-tion of Fig. 2 corresponding to Xl(z, t) [set at Xi = 589nm, the Na resonance wavelength Na(I)] and the left-hand portion of Fig. 2 corresponding to X2(z, t) [set atX2 = 656 nm, the Balmer H-a emission and the stimu-lated Raman scattering (SRS) of the 0-H stretchingmode of water]. Both Xi(z, t) and X 2 (Z, t) unavoidablycontain some contribution from the plasma continu-um emission. The labels I and S on the horizontalaxes designate the droplet illuminated face and shad-ow face, respectively. The laser direction is indicatedby the arrow, and the time profile of the laser pulse isshown in the central portion of Fig. 2, i.e., X0(t) con-tains only the temporal information, not spatial infor-mation, indicating that there are four main peakswithin a single pulse. The intensities at Xo(t), Xl(z, t),and X2 (Z, t) are plotted in the third dimension in Fig. 2.
As time increases, the following sequence of note-worthy events can be extracted from X2(z, t) [Fig. 2(a)]:(1) SRS is observed only near I and S, consistent withthe fact that the SRS circumvents the droplet rim,where the optical feedback is large.11 (2) The disap-pearance of the signal near I indicates that SRS isquenched by the internal plasma created after LIB isinitiated within S, where the internal fields are thehighest.12 (3) The H-a emission (from H produced bythe plasma dissociation of H20) appears within S. (4)The propagation velocity of the H-a emission outsideS is 3.5 km/sec, and the internal propagation velocityfrom S toward I is 5.3 km/sec. The internal H-aemission barely reaches I and exhibits four peaks,which correlate to the time profile of Xo(t).
Similarly, the following sequence of events can beextracted from Xi(z, t) as time increases [Fig. 2(a)]: (1)Na(I), like H-a, first appears near S after the SRS isquenched; and (2) Na(I) emerges from S at a velocityof 5.3 km/sec (faster than that of H-a) and propagatestoward I with a velocity of 5.3 km/sec (equal to theinternal velocity of H-a). The internal Na(I) emis-sion actually reaches I and exhibits four main peaks,which are correlated with the time profiles of the inter-nal H-a and Xo(t).
Figure 2(b) shows the same data as Fig. 2(a) but withtime decreasing. Several new pieces of informationcan be extracted from such a rotated plot: (1) Theexternal velocities of Na(I) and H-a are constant dur-ing the input laser pulse and distinctly slow down afterthis pulse ends. (2) The internal velocities of Na(I)and H-a are constant during the input pulse. Afterthis pulse ends, the Na(I) intensity also ends, while theH-a emission continues to be detectable and switchesthe direction of propagation, i.e., the emission propa-gates from I to S with a velocity of 12 km/sec. (3)After the internal H-a emission associated with thelast laser peak reaches S, the H-a emission is againejected from S and appears to merge into the slowedH-a emission that was ejected during the previouslaser peaks. This difference between the behavior of
July 1988 / Vol. 13, No. 7 / OPTICS LETTERS 561
(a)A,
<A2(zt)>t <A,(Z,t)>t
H-a DIPH-ce ~~~~Na(1co)-t 1
s I s I
-7 Laser-
(b)
. .
._C 0
0
Nal)\\ <A,(z,t)>z
A,(t)
SR -I - z. .i i
10 nsec
Time
Fig. 3. The X1(z, t) and X2(z, t) results in Fig. 2 are (a)temporally averaged and (b) spatially averaged. The (Xl(z,t))t and (X2(z, t))t curves in (a) mimic the technique thatusesaspectrographanddavidicon. The (X1(zt))zand (X2(Z,t) ), curves in (b) mimic the technique that uses a monochro-mator and a photomultiplier. Xo(t) corresponds to the inputlaser radiation channeled through the single fiber.
Xj(z, t) and X2 (Z, t) within the droplet indicates thatmost of the internal emission is from the discrete emis-sion of Na(I) and H-a and not from the continuum,which is expected to exhibit the same spatial and tem-poral behavior at X, and X2. The present results con-tradict our-previous (X(z) ) t data, which indicated thatthe time-averaged spectra inside the droplet consistonly' of the continuum and are devoid of discrete emis-sion peaks of Na(I) and HI-ai.6 -8
To mimic-the spatially resolved but temporally-av-eraged spectra [(X(z)) t 'dispersed by a spectrographand detected by a vidicon],6 -8 the results shown in Fig.2 can be integrated in time [Fig. 3(a) for (X(z, t) )tl.The' characteristic dip in intensity of (X2(z, t) )t atabout one radius in front of S is consistent with theintensity dip observed in photographing the' plasmaplume through a red filter and in the previous (X (z) )tmeasurements.6 -8 The spatial profiles of' (X1(z, t))tand (X2(Z, t))t are noticeably different, and the (X1(z,t) ) curve does not exhibit an intensity dip.
To mimic the temporally resolved but spatially av-
eraged data at one wavelength [(X(t)), dispersed by amoriochromator and detected by a photomultiplier],9the results shown in Fig. 2 can be integrated along thez axis [Fig. 3(b)]. The (X1(z, t))z curve indicates thatthe intensity peaks of Na(I) can be correlated with theinput pulse shown by Xo(t). The Na(I) intensity grad-ually decays after Xo(t) ends. The (X2 (Z, t)), curveindicates that there is a time delay between the inten-sity maxima of the SRS and the H-a emission. Thereis also a time delay between the intensity maxima ofNa(I) and H-a. After X0(t) ends, the decay time of H-ctis different from that of Na(I).
The contrast between the information that can beextracted from the Xj(z, t) curves and from the (X1(z,t) )t or (Xi(z, t) ) , curves illustrates the potential of thenew diagnostic technique to investigate LIB interac-tions with single transparent droplets. Detailed stud-ies of the temporally and spatially resolved emissionprofiles from single droplets as a function of laserinput intensity are under way.
We gratefully acknowledge helpful discussions withB. T. Chu, as well as the partial support of this re-search by the U.S. Air Force Office of Scientific Re-search (grant no. 88-0100) and the U.S. Army Re-search Office (contract no. DAAL03-88-K-0040).
* On leave from the Department of Physics, FudanUniversity, Shanghai, China.
t On leave from the Department of Physics, HongKong Baptist College, Hong Kong.
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