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Observation of Solid-Density Laminar Plasma Transparencyto Intense 30 Femtosecond Laser Pulses

D. Giulietti,* L. A. Gizzi, A. Giulietti, A. Macchi,† and D. Teychenné*Istituto di Fisica Atomica e Molecolare - CNR, Via del Giardino, 7-56127 Pisa, Italy

P. Chessa, A. Rousse, G. Cheriaux, J. P. Chambaret, and G. DarpentignyLaboratoire d’Optique Appliquée-ENSTA, 91120, Palaieseau cedex, France

(Received 25 March 1997)

Near total transmission of 30 fs laser pulses through0.1 mm plastic foil targets has been observedfor the first time at an intensity of3 3 1018 Wycm2 in absence of precursor plasma. Thislevel of transmittivity is far above the level predicted by current theoretical models or numersimulations. The transmittivity was found to drop by 40 times at an intensity of4 3 1017 Wycm2

and was within the experimental background level at5 3 1016 Wycm2. Our measurements stronglysuggest a new mechanism of propagation of electromagnetic waves through overdense pla[S0031-9007(97)04356-1]

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The interaction of intense optical radiation with plasmas characterized by solid-density and ultrasteep gdients can now be studied by using short pulse lasecapable of delivering up to joules of energy in few tenof femtoseconds. In principle, such laser systems opthe possibility of investigating phenomena produced bhigh intensity radiation in experimentally unexplored conditions, including propagation in plasmas whose densis orders of magnitudes higher than the critical densinc ­ mev

20y4pe2, v0 being the laser frequency.

Recently, penetration of ultraintense, short laser pulsinto overdense plasmas has been extensively investigaboth theoretically and experimentally also in view of itrelevance to the implementation of the fast ignitor conce[1]. Several effects have been considered that predenhanced propagation, including anomalous skin effe[2], self-induced transparency [3], and hole boring [4,5Hole boring and self-induced transparency have bemostly investigated in the interaction of relativistic lasepulses with moderately overdense plasmas. In particulhole boring has been proposed as a possible mechanfor deep energy deposition in overdense plasma regiodue to ponderomotive forces at relativistic intensitieThe importance of relativistic effects is given by thevalue of the normalized relativistic momentum,a0 ­posymoc > 0.85l0

pI0 where pos is the momentum of

the electrons oscillating in the laser field,mo is theelectron mass,c is the velocity of light, andI0 and l0are the laser intensity in units of1018 Wycm2 and thewavelength in microns, respectively. For these effecto give a nonmarginal increase of transmittivity througplasma slabs, either the plasma must be weakly overdeor the intensity must be very high (a0 ¿ 1).

At nonrelativistic intensity the propagation of theelectromagnetic (e.m.) wave into an overdense plasmaexpected to be limited to the skin depthd0. A deeperpenetration (anomalous skin effect) is possible in ve

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hot plasmas, where the electron velocity becomes larthan v0d0 [6]. Recently the anomalous skin effect insolid-density plasmas has been considered both analcally [7] and numerically [8], with attention to the case ointeraction with thin foils.

From an experimental point of view, a serious problem that can prevent interaction of short pulses wisolid-density plasmas may arise from the laser prepuoriginating from amplified spontaneous emission (ASE)the laser chain. If the intensity on target due to the prpulse (typically of nanosecond duration) is higher than tthreshold intensity for plasma formation on target, a prcursor plasma is formed which prevents the main femtosond pulse to interact directly with the solid. In a previouexperiment [9] it was shown that the use of thin plastic fotargets may avoid formation of precursor plasma, enablithe interaction of the main femtosecond pulse with higdensity laminar plasmas. That experiment also providevidence [10] of a substantial transmittivity, of the ordeof a few percent of the incident energy.

In this Letter we report further and novel experimentresults on the transmittivity of solid-density laminar plasmas carried out using the 30 fs (Ti:sapphire) laser of tLaboratoire d’Optique Appliquée, focused onto0.1 mmthick plastic foils. In this experiment we found that whethe laser intensity on target is greater than1017 Wycm2

the transmittivity goes above the experimental backgroulevel of ø1% and increases dramatically with laser intensity, approaching full transparency at an intensity3 3 1018 Wycm2. To our knowledge, this is the first timethat such an important observation is reported in the intaction with a laminar plasma whose sharp boundariesmade possible by the absence of prepulse effects.

The 815 nm, 30 fs laser pulse was focused on targwith an fy7.5 off-axis parabolic mirror, with an angle ofincidence on target of 20±. The laser pulse was linearlyP-polarized. The focal spot was10 mm in diameter.

© 1997 The American Physical Society

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The intensity was varied between5 3 1016 Wycm2 and3 3 1018 Wycm2, by varying the energy in the pulseThe transmitted pulse was studied by using a diffusinscreen placed beyond the target, on the laser propagadirection, at a distance of 1.8 times the focal lengththe focusing optics. A demagnified image of the screwas formed onto a CCD array and onto the entranceof a spectrometer. A second CCD array was placedthe output focal plane of the spectrometer. An additionCCD imaging channel was set up on the specular reflectdirection, with the object plane located at the target plan

The laser system used in our experiment is charactized by an ASE lasting approximately 10 ns that forma “pedestal” to the main pulse. The measured contrratio, i.e., the ratio between the power delivered in thfs pulse and that delivered in the ASE, was$107. Asevere test on the effect of the ASE on target was pformed by firing the laser system, but without injectinthe fs pulse in the amplifier chain. In this condition, wobserved no damage on target over the whole rangeASE intensities. In contrast, when the fs pulse was firea ø100 mm diameter hole was produced in the foil. Thitest is, for two distinct reasons, a proof “a fortiori” thatin full shots the target does not explode before the arrivof the femtosecond pulse. First, since no energy is spin the amplification of the fs pulse, the level of ASE igreater than in the case of operation with fs pulse injetion. Second, only the leading part of the ASE pulse prito the arrival of the main fs pulse is relevant in determining the interaction conditions of the main pulse. On thother hand, within the explored range of intensity of thmain 30 fs pulse, a nearly instantaneous ionizationthe target material (FORMVAR: C5H11O2) is expected tooccur. In this sense, even though direct electron densmeasurements have not been carried out, we can statethe results presented here refer to interaction with a soldensity laminar plasma.

The transmittivity as a function of the incident laser intensity is presented in the plot of Fig. 1. Each data powas obtained by taking into account several interactievents for each laser intensity and by averaging the resuThe background line reported on the graph indicates tlevel at which the transmitted energy is comparable withe ASE energy (close to1% of the main pulse energy),and consequently measurements below this level canbe entirely related to the main pulse. According to thplot of Fig. 1, the transmitted fraction at incident intensties below1017 Wycm2 lies within the experimental back-ground level. However, as the incident intensity increasthe transmitted fraction increases dramatically and the tget becomes basically transparent at3 3 1018 Wycm2.

The diffusing screen also gave information on thintensity distribution in the near field beyond the focuA typical example of such data is shown in Figs. 2(aand 2(b), where the cross section of the transmitted pu2(b) at a laser intensity on target of3 3 1018 Wycm2 is

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FIG. 1. Transmittivity as a function of the intensity of th30 fs laser pulse incident on0.1 mm thick plastic target. Thebackground level indicates the level at which the energythe pedestal (ASE) is comparable with the transmitted energ

compared with that taken without the target 2(a) at tsame intensity. The three perpendicular lines visiblethe images are spatial calibration markers placed ondiffusing screen. A preliminary survey of these resushows that the pulse transmitted through the foil donot suffer major changes. The angular spread appslightly reduced after interaction with respect to the caof free propagation while the transmitted intensity patteis elongated in the horizontal direction.

Very interesting is the comparative spatial Fourianalysis of these patterns with and without the target. Tsquare root of the intensity distribution of the images takby the diffusing screen, i.e., a quantity proportional to telectric field in the near field, was Fourier transformusing a two-dimensional fast Fourier transform algorithThe square of the modulus of the Fourier transfodistribution was then calculated. Within the paraxapproximation, and with appropriate assumption onphase, we obtain the intensity distribution of the laslight in the far field, i.e., on target. The calculation hbeen performed for both patterns 2(a) and 2(b), andlogarithm of the results are shown in Figs. 2(c) and 2(d

The intensity distributions in the near field of the tranmitted pulses without and with the target were normalizto the corresponding average transmitted intensities, i.e1and0.76, in order to allow a direct quantitative comparson of the results. Some high frequency spatial modespresent in the far field image 2(c) obtained from the nefield image 2(a) of the free propagating pulse. The molations (rings) are likely to be due to a sharp radial cut in tamplification/compression chain of the laser system. Tfar field image of 2(d), obtained from the near field ima2(b) taken when interaction with the target occurred, shothat those high spatial frequency modes are basically spressed by the interaction with the target. In other worthe interaction acts as a spatial filter for the high intensultrashort laser pulse. Further evidence of this obsertion is given by the images taken on the reflection chann

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FIG. 2. (a) and (b): Images of the cross section of thtransmitted pulse without (a) and with (b) the0.1 mm thickplastic target taken with the diffusing screen at an intensin the focal plane of3 3 1018 Wycm2. (c) and (d): Two-dimensional fast Fourier transform of the pulse cross sectioof patterns (a) and (b), i.e., without (c) and with (d) the targerespectively. The logarithm of the modulus of the Fouritransforms are shown as gray scale images. (e) Image taon the reflection channel with the object plane located at ttarget plane. A similar pattern is obtained by subtraction of tpattern (d) from the pattern (c).

The image of Fig. 2(e) shows a CCD image of the refleing region of the target that is also the far field of the pulsIt shows that reflection occurs mainly from a region othe target well outside the main spot (ø10 mm diameter)and the reflected pattern has a ringlike shape. By coparing the Fourier transform patterns 2(c) and 2(d) weach other, we can conclude that the pattern of Fig. 2(egenerated by the outer rings in the far field pattern of tincident pulse (filtered out from the transmitted pulse) this specularly reflected by the target surface.

Also interesting is the comparison between the specof the freely propagating pulse and those of interactpulses at different pulse energies, as shown in Fig. 3. Tspectral bandwidth of the pulse is close to the Fourlimit for a pulse with a 30 fs FWHM Gaussian temporaprofile. In agreement with the expected performancethe laser system, these spectra also show a redshift wthe entire amplification chain is fired.

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FIG. 3. Space resolved spectra of the transmitted pulsethree different intensities on target,3 3 1018 Wycm2 (upper),4 3 1017 Wycm2 (middle), and5 3 1016 Wycm2 (lower). Theunperturbed laser spectrum (dotted line) obtained without ttarget at each laser intensity is also plotted for comparison.

The comparative analysis shows that the interactioprocess produces blueshift at moderate and intermediintensities, while producing no shift at high intensity. Inthis latter case the bandwidth is also unaffected by thinteraction. The spectral properties of the transmittepulse were found to be stable shot to shot, exceptintermediate intensity (4 3 1017 Wycm2) where shot toshot variations in shift and width were observed.

Within the entire intensity range explored the targefoil is expected to be ionized during the pulse. In facat the lower and intermediate intensity the blueshift ithe spectrum of the transmitted pulse is a clear signatuof ultrafast ionization. This is well supported also bythe amount of the blueshift, which is about 13 nm a5 3 1016 Wycm2, and about 20 nm at4 3 1017 Wycm2.

VOLUME 79, NUMBER 17 P H Y S I C A L R E V I E W L E T T E R S 27 OCTOBER 1997

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If we assume that the frequency shiftdv is due to self-phase modulation of the laser pulse, i.e., to the ultrafdecrease of the refractive index due to the laser inducionization, we have [11]

dv ­ 2sLycd snmyntdv0 . (1)

Considering a change of the refractive indexnm ø 21as that due to a transition from zero electron densto the critical density, we can use Eq. (1) to evaluathe time scalent of such a transition. By taking theinteraction path equal to the foil thicknessL ­ 0.1 mm,we find nt ø 20 fs and nt ø 13 fs for the low andintermediate intensity, respectively. These values afully consistent with the rise time of the laser pulse. Thconsistency strongly supports the assumption that ultraionization occurs and provides indirect evidence that tplasma thickness is indeed comparable to the original fthickness. The absence of shift in conditions close to tfull transparency at3 3 1018 Wycm2 suggests that, in thiscase, ionization involves a negligible portion of the puls

The transparency is observed at an intensity of3 3

1018 Wycm2 value at whicha0 > 1.2. As the intensity isonly weakly relativistic, and ponderomotive force effecare prevented by enormous electrostatic restoring forcself-induced transparency and hole boring are ruled ou

If we consider in turn the anomalous skin effect, thnumerical simulations of Ref. [8] show that the fraction oenergy transmitted by a0.1 mm thick plasma is expectedto be within the range of1025 to 1026, i.e., many ordersof magnitude lower than the transmittivity measuredour experiment at3 3 1018 Wycm2.

To our knowledge, no theoretical models or numericsimulations reported so far can explain our observatioIt seems that we are facing a novel effect which certaindepends upon laser intensity but may be enhanced byextremely short duration of the laser pulse.

Very recently, the effect of strong static magnetic fieldhas been taken into account [12] in the interaction of itense electric fields with electrons. Preliminary calcultions on this model show that for magnetic fields of thorder of hundreds of MG the motion of electrons alonthe oscillating field is strongly inhibited and the e.m. wavpropagates basically unperturbed. On the other hand,cause of the physical properties of our target and toultrashort pulse duration of 30 fs, highly transient volumionization may occur in our experimental conditions aindicated by the shifts observed in the spectra of Fig.Fast ionization and consequent generation of large crents in the thin layer of material may provide the condtions for production of the sufficiently intense quasistatmagnetic fields necessary to enable transparency.

In conclusion, we studied the propagation of inten30 fs laser pulses through0.1 mm plastic targets. Ourmeasurements show that solid-density plasma traparency takes place at an intensity of3 3 1018 Wycm2.These results suggest that a completely new mec

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nism of propagation of an e.m. wave through a highloverdense plasma is taking place, distinct from othobserved or proposed effects like anomalous skin effeponderomotive hole boring, and relativistic self-inducetransparency. We also observed that in the transmittpulse some high frequency spatial modes generated byamplifier chain were filtered out by selective reflectioon target. This latter effect provides a novel and simpway to perform a difficult task like the spatial filtering ofhigh intensity, ultrashort, aberrated laser pulses. In oopinion, the observation of transparency of solid-densiplasmas to ultrashort pulses at intensities correspondto a0 > 1.2 opens a completely new area of investigationvery promising for applications like the fast ignitorscheme, and challenging for theoretical plasma physics

This work was partially supported by the EuropeaCommunity within the scheme of Access to Large Facilties of the TMR Programme. The participation of twoof the authors (D. T. and P. C.) to this work was supported by two European Networks, namely, SILASI anGAUS-XRP, respectively. We would like to thank J. PGeindre and P. Audebert (LULI-Ecole Polytechnique) fofruitful discussions and A. Barbini (IFAM) and G. Grillon(LOA) for their invaluable technical support during theexperiment.

*Also at Dipartimento di Fisica, Università di Pisa, PiazzaTorricelli, 2, Pisa, Italy.

†Also at Scuola Normale Superiore, Piazza dei CavaliePisa, Italy.

[1] M. Tabak, J. Hammer, M. E. Glinsky, W. L. Kruer, S. C.Wilks, J. Woodworth, E. M. Campbell, M. D. Perry, andR. J. Mason, Phys. Plasmas1, 1626 (1994).

[2] W. Rozmus and V. Tikhonchuk, Phys. Rev. A42, 7401(1990).

[3] E. Lefebvre and G. Bonnaud, Phys. Rev. Lett.74, 2002(1995).

[4] S. C. Wilks, W. L. Kruer, M. Tabak, and A. B. Langdon,Phys. Rev. Lett.69, 1383 (1992); R. Kodama, K. Taka-nashi, K. A. Tanaka, M. Tsukamoto, H. HashimotoY. Kato, and K. Mima, Phys. Rev. Lett.77, 4906 (1996).

[5] G. Malka and J. L. Miquel, Phys. Rev. Lett.77, 75 (1996).[6] E. S. Weibel, Phys. Fluids10, 741 (1967).[7] G. Ma and W. Tan, Phys. Plamas3, 349 (1996).[8] J. P. Matte and K. Aguenou, Phys. Rev. A45, 2558

(1992).[9] L. A. Gizzi, D. Giulietti, A. Giulietti, P. Audebert,

S. Bastiani, J. P. Geindre, and A. Mysyrovicz, Phys. ReLett. 76, 2278 (1996).

[10] D. Giulietti, L. A. Gizzi, V. Biancalana, T. Ceccotti,P. Audebert, J. P. Geindre, and A. Mysyrovicz, inUltra-fast Processes in Spectroscopy,edited by O. Sveltoet al.(Plenum Press, New York, 1996), p. 331.

[11] D. Giulietti, V. Biancalana, M. Borghesi, P. ChessaA. Giulietti, and E. Schifano, Opt. Commun.106, 52(1994).

[12] D. Teychenné, D. Giulietti, A. Giulietti, and L. A. Gizzi(to be published).

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