Observation of VUV radiation at wavelengths in the ωp- and 2ωp-wavelength range emitted from femtosecond laser-plasmas

15 December

1997

OPTKS

C~MMUNICATI~~~~ EISEVIER

Optics Communications

144 (1997) 217-221

Observation of VUV radiation at wavelengths in the ctlP-and 2 w,-wavelength range emitted from femtosecond laser-plasmas U. Teubner ‘,‘, D. Altenbernd a, P. Gibbon a, E. Fijrster ‘, A. Mysyrowicz b, P. Audebert ‘, J.-P. Geindre ‘, J.C. Gauthier ‘, R. Lichters “, J. Meyer-ter-Vehn d ii Ahteilung RBntgenoptik, lnstitut fdrOptik und Quatztenelektronik. Friedrich-Schil~er-Unii,ersitiit

’ ~bor[~toire

Jenu, MUX-Wien-Platz 1. 07743 Jena. Germany h tiborufoire d’opfique Appliq&e. Batterie de l’Yvette, ENSTA, 91120 Palaiseau. France pour ~‘Utilisat~off des Lusers Menses, U~R 100 du Centre Notional de la Recherche Scientifique. Ecolr P&technique. 91128 Palaiseau, France ’ Max-Planck-InstitutJr

Quantenoptik,

Hans-Kopfermann-Strasse

1, 8574X Garching, German)

Received 2 June 1997; revised 7 August 1997: accepted I2 August 1997

Abstract Elec~oma~netic emission close to short-pulse laser-irradiated solid targets. excite Langmuir waves in the overdense harmonics of the plasma frequency wp. radiation may be exploited as a density

from multiples of the plasma frequency wp has been observed experimentally Such radiation may be generated by a two-step process in which hot electron jets region, which then undergo parametric conversion to electromagnetic emission at The observations are supported by particle-in-cell simulations. suggesting that this diagnostic. 0 1997 Elsevier Science B.V.

PACS: 52.40.Nk; 42.6S.ky: 52.65.-y; 33.20.Ni; 52.25.Qt Keywords: Harmonic generation: Plasma waves; Plasma diagnostic; generation

The advance of the technology of the generation of ultrashort laser pulses at terawatt level has lead to numerous applications in many fields of physics [I]. In particular, focusing of femtosecond laser pulses on solid targets to very high intensities (in excess of lOI W/cm’) gives access to laser-matter interaction under unique conditions. It also opens the possibility of radiation generation from laser-plasmas ranging from the far infrared [2] up to MeV energies [3]. In the soft X-ray and VUV region, for instance, beside the well investigated incoherent plasma emission (see, e.g. Refs. [4,5]) special attention has recently been paid to harmonics of the laser wavelength. This kind of radiation originates from the interaction of intense ultrashort laser pulses with a high density plasma

’ E-mail: uteubner~r~ntgen.pbysik.uni-jena.de.

Laser plasma;

High-intensity

femtosecond

laser pulses; Fast electron

from a massive target [6,7]. Harmonics have been observed in the specular direction with respect to the laser light reflected from the plasma surface [S-IO]. Other interesting phenomena like the generation of high harmonics in thin foil targets, but in transmission geometry [ 11,I 21 or emission at the plasma frequency op and its harmonics [ 12, t 31 have been predicted by recent p~icle-in~eil (PIG) simulations. To the best of our knowledge, the present work is the first observation of radiation that may be attributed to emission at wp and its second harmonic from a solid state density plasma. The experiments were carried out with a chirped pulse amplification titanium-sapphire system [14] tuned to a wavelength of h, = 800 nm. The 7L = 130 fs (full width at half maximum, FWHM) laser pulses with an energy of 52 mJ were focused by an f/4 Bowen telescope onto a thick flat glass target. The target was mounted on an

0030-4018/97/$17.00 0 1997 Elsevier Science B.V. All rights reserved. Pfi SOO3~-4018(97)00468-9

218

(1. Teubner et al. /Optics

Communications

xy-stage in a vacuum chamber (pressure below 2 X lop3 mbar) and shifted between consecutive shots so that the incident laser beam always struck a fresh target surface. The size of the elliptically shaped laser profile was 5.3 pm and 8.3 pm in the ho~zont~ and vertical directions respectively. Thus, the intensity on the target surface could be estimated as I, = 5 X lOi W/cm* (at normal incidence). The ratio of the short pulse laser intensity to the background pedestal (partly originating from the amplified spontaneous emission) was measured by third-order autocorrelation, lps before the main pulse, to be better than 108. VUV spectra in the wavelength region from 30 nm to 55 nm and 55 nm to 90 nm have been recorded with a transmission grating spectrograph (TGS). The TGS, which is a modified version of the spectrograph described in Ref. [15], consists of a free standing 1000 lines-per-mm gold grating, a toroidal grazing incidence mirror and a backside illuminated CCD as the detector (512 X 512 pixels, 1 cm X 1 cm). The spectral resolution in the present experiment was 0.25 nm. A 200 nm aluminum foil has been applied in front of the spectrograph to discriminate against stray light from the reflected laser radiation. The measurements were performed with p-polarized laser light at three different angles of incidence, namely o! = 5”, IY= 33” and (Y=45” with respect to the target normal. The TGS has been fixed at the target chamber at an angle of 64” (with respect to the laser axis) and thus leads to an angle of observation of /3 = 59”, @= 31” and /3 = 19” (with respect to the target normal) for the three

144 (1997) 217-221

values of LY.For each measurement 30-100 spectra have been accumulated on the CCD. Fig. 1 shows the result of the measurements for an angle of incidence cy= 33” (the spectra in (b) correspond to the raw data shown overhead in (a)). The wavelength axis in the spectrally dispersed VUV image of the laserplasma on the CCD detector (Fig. la) extends along the (nearly horizontal) line between the atrows and corresponds to the abscissa of the spectrum shown below (same spectral limits). The direction perpendicular to this line is the spatial coordinate of the image. Due to the spatial resolution of the TGS the extension of the image in this direction is four pixels wide. Besides hot spots originating from highly energetic photons and electrons, two strong and distinct peaks are clearly visible in the raw data (Fig. la). The hot spots may easily be discriminated from the two peaks: energetic photons and electrons directly reach the detector and thus lead to a signal of a single pixel per event. Fu~he~ore their spatial distribution for different laser shots is statistically random. On the other hand the VUV emission (i.e. the peaks) that reaches the detector has always passed through the TGS and thus gives a reproducible signal distributed over about 20 pixels (5 pixels in the direction of dispersion). Fig. lb shows the corresponding spectra (integrated in the spatial direction; in arbitrary units) after discrimination of the high energy particle contribution and after background subtraction. The position of the two significant peaks is at a wavelength of h, = (39 i 5) nm (left hand

60

65

70 75 h lnml

80

85

Fig. 1. VUV spectra measured at I,, = 5 X lOI W/cm2, Q = 33”, /3 = 31” with p-polarized laser light. Spectrally dispersed VUV image of the laser-plasma on the CCD detector (raw data) (a) and corresponding spectra (b). The spectral range extends from 29 nm to 56 nm ((a) and {b) on left hand side) and from 56 nm to 86 nm ((a) and (b) on right hand side). The gray scale in (a) corresponds to the signal intensity (dark high, light: low intensity). The accuracy of the wavelength axis is f 13% for the left hand side and f 18% for the right hand side. The peaks at or and 20, are marked by arrows.

0. Teuhner et d/Optics

Communications

side) and A, = (64 + 12) nm (right hand side), respectively. The relative bandwidth of the peaks is A h/A - (56) X IO-’ for both peaks, somewhat larger than the Fourier limit of the relative bandwidth of the laser pulse which is 9 x lo-‘. If the peak at A, is identified with emission at wp = 3 x 10’6 s-1 + 18%, it implies an electron density of n, = 2.7 X 10Z3 crne3 +- 36% which is 150 times the critical electron density n, (n, = 1.7 X IO” cm-’ for 800 nm laser light). In addition, A, is approximately one half of A,. Thus the peak at A, could be identified with emission at 20, (2w,/2 = 2.4 X 10 ” s- ’ k 13%), with a corresponding electron density n, = 1.8 X IO” cm-” f 26% or 100 times the critical electron density. Although these two densities differ, they are consistent with each other within the experimental error. To get a qualitative understanding of the origin of this radiation the laser pulse absorption and suprathermal electron production has to be considered. It is well known that the interaction of ultrashort laser pulses, in particular for long laser wavelength (e.g. 800 nm) with dense solid plasmas may give rise to a large yield of suprathetmal electrons with energies in the keV to MeV range [3,16]. These electrons are produced by collisionless laser pulse absorption mechanisms [ 17-191, particularly for ppolarized light at intermediate angles of incidence LY(e.g.. at cy = 45”). In steep density gradient plasma layers, electrons are extracted from the target into the vacuum during one half-cycle of the strong electric field of the laser pulse. In the following half-cycle they are reinjected into the plasma due to the strong electric field produced by the

80 4

I

I

1 mm1

I

.= s 3 $2

3

r-7

219

charge separation. The electrons gain high velocities and penetrate deeply into the target material. Recently, it has been theoretically demonstrated that these jets of electrons can excite strong plasma waves inside the plasma layer with phase velocities corresponding to the velocity of the fastest particles and frequencies close to the plasma frequency wp [ 131. One can think of this as a high-density analog of the plasma wakefield accelerator in which a train of high phase velocity plasma waves is excited by a relativistic electron beam [20]. However, in the context of solid target interactions. these nonlinear plasma waves undergo Langmuir coalescence leading to emission of light primarily at 2 wp [12.13]. Similar processes have been considered in astrophysics as sources of harmonic radiation at wp and 20, in the so-called solar radio bursts from the solar corona [21,22] (for details of the corresponding theory of this kind of plasma emission see, e.g., Ref. [Zl]). In contrast to the astrophysical case, the emission studied here occurs in the VUV-range, since typical electron densities in laser-plasma experiments are on the order of solid density, i.e. n, N IO” cm-j (experiments performed at nearly identical experimental parameters with the same laser system [23,24] show that the electron density exceeds the solid density, even if plasma expansion is included). Theoretically [ 131, it is expected that the emission regions for the w,, radiation and the 20~ radiation differ somewhat: the or emission originates from a surface layer where the local electron density is relatively large whereas the 2w, emission comes from the more extended bulk material where the average electron density may be lower

80

40

a = 33”

‘3;

144 (I9971 217-221

I

I

I

h b-1

40

I

l- 12

a = 45” - IO -8 -6

Z5 -4 -2

Fig. 2. VUV-spectra obtained from a PIC simulation using the the density n,/n, = 100. The spectrum is time-averaged over normalized to the frequency of the laser fundamental wL. The line) and the experimental 2 op peak (dotted line). Notice the

experimental parameters of the laser pulse and a steplike plasma profile with a period corresponding to 50 laser cycles (or 130 fs) and the frequency o is inset on the left hand side shows details of the shape of the theoretical (solid different scales of the spectra on the left hand side and the right hand side.

V. Teubner et al. / Optics Communications 144 (1997) 217-221

220

We should add that no laser harmonics (at nw,> were seen in this ex~~ment. In fact, the s~ctrometer should in principle have picked up harmonics (n > 10; A < 80 nm) previously observed on the same laser system 191.If these had been present at this intensity (5 X lOI W /cm”), then they were below the detection level of the s~ctrometer and thus did not affect the plasma harmonics. In general however, since the laser harmonics scale strongly with intensity, one could easily have a situation where the 2w, and op signals are “buried” within the overall reflected light spectrum. Indeed, this is what one sees in the PIC simulations at high irradiance in Refs. [6,12,18]. Finally, it may be mentioned that a clear influence of the expe~men~i conditions on the generation of the wpradiation has been observed as well. Measurements performed at three different angles of incidence show that the w,-peak is strongest at (Y= 45” and decreases for smaller angles (Fig. 3). This is also supported by further simulations of the wp and 2 wp emission [13] and certainly warrants more detailed experimental and theoretical investigation. In summary. strong VUV radiation from high density

(due to lower temperature and ionization degree). Higher h~onics of wp may propagate through the overdense plasma whereas the wp radiation cannot. Since @,-light can only be emitted from a thin surface layer, one expects the signal at 20, to be stronger than the signal at wp. These considemtions are illustrated by the foilowing PIC results. Fig. 2 shows spectra from PIC simulations [ 131 of p-polarized laser pulses obliquely incident on a thin step-profile plasma slab. A one-dimensional plasma and laser pulse geometry is assumed, and oblique incidence is incorporated by performing a Lorentz transformation to a moving reference frame with normal incidence. The electron density of the layer was chosen as n, = 1.7 X 10”” cmp3, so that n,,/n, = 100. The PIC simulations show prominent peaks at twice the plasma frequency corresponding to a wavelength A2 = 27rc/(2wp) = 40 nm in vacuum. Their intensity increases from Q = 33” to a! = 45”, as observed in the experimental data. As mentioned before, radiation at wp is much weaker and its intensity may be below the numerical noise level in these simulations. However, further simulations indicate that the wp light may also be strongly emitted under similar conditions [ 111.

40

20

0

z .f-L t 3

40

g

20

i

pp - Peak

3 .b v)

0 40

20

0 60

65

70 h

75

80

85

bml

Fig. 3, VUV spectra measured at three different angles of incidence:

ty = 5” ( p = 59”), (x = 33” f fi = 31”)

and (Y= 45” ( @= 19%

U. Teubner et al/Optics

Communications

plasmas produced by high intensity ultrashort laser pulses has been measured. Although further experimental and theoretical investigations will be necessary the experimental results of the present work exhibit features consistent with theoretical predictions of emission at the plasma frequency and its second harmonic [l I-131. These simulations indicate that the radiation originates from fast-electron-generated Langmuir waves in the overdense plasma, which are subsequently converted to electromagnetic waves, Time-resolved high-order harmonics of the plasma frequency could be used as a density diagnostic of the target bulk during the laser interaction.

Acknowledgements This work was supported by the TMR program of the European Union under contract no. ERBCT95021 and ERBFMXCT960080.

References [I] P. Gibbon, E. Fiirster, Plasma Phys. Control. Fusion 38 (1997) 769. [2] H. Hamster, A. Sullivan, S. Gordon, W. White. R.W. Falcone, Phys. Rev. Lett. 71 (1993) 2725. [3] J.D. Kmetec, C.L. Gordon, J.J. Macklin, B.E. Lemoff, G.S. Brown, S.E. Harris, Phys. Rev. Lett. 68 (1992) 1527. [4] M.M. Murnane, H.C. Kapteyn, M.D. Rosen. R.W. Falcone, Science 25 (1991) 531. [5] U. Teubner, W. Theobald, C. Wilker, E. Forster, Phys. Rev. E 50 (1994) R3334. [6] P. Gibbon, Phys. Rev. Lett. 76 (1996) 50. [7] R. Lichters, J. Meyer-ter-Vehn. A. Pukhov, Phys. Plasmas 3 (1996) 3425.

144 (19971217-221

[a1 S. Kohlweyer,

321

G.D. Tsakiris, C-G. Wahlstriim, C. Tillman, I. Mercer, Optics Comm. 117 (1995) 431. 191D. von der Linde. T. Engers. G. Jenke, P. Agostini. G. Grillon, E. Nibbering, A. Mysyrowicz. A. Antonetti, Phys. Rev. A 52 (1995) R25. [lOI P. Norreys et al., Phys. Rev. Lett. 76 (1996) 1832. [ill P. Gibbon, D. Altenbemd, U. Teubner, E. Fdrster, P. Audebert, J.P. Geindre. J.C. Gauthier, A. Mysyrowicz, Phys. Rev. E 55 (1997) R6325. in: Multiphoton Processes [El R. Lichters, J. Meyer-ter-Vehn, 1996 (Institute of Physics Publishing, Bristol and Philadelphia. 1997) p. 221 [131 R. Lichters, Ph.D. thesis. Technical University Miinchen (1997): MPQ Report 219. Max-Planck-Institut fur Quantenoptik, D-85748 Garching (1997). [141 C. LeBlanc, G. Grillon, J.P. Chambaret, A. Migus. A. Antonetti, Optics Lett. 18 (1993) 140. [I51 J. Jasny, U. Teubner. W. Theobald. C. Wilker. J. Bergmann, F.P. Schafer, Rev. Sci. Instrum. 65 (1994) 1631. [161 A. Rousse, P. Audebert, J.P. Geindre, F. Fallies, J.-C. Gauthier. A. Mysyrowicz. G. Grillon, A. Antonetti. Phys. Rev. E 50 (1994) 2200. 1171 F. Brunei. Phys. Rev. Lett. 59 (1987) 52. [I81 P. Gibbon, A.R. Bell. Phys. Rev. Lett. 68 (1992) 1535. [191 H. Ruhl. J. Opt. Sot. Am B 13 (1996) 388: Phys. Plasmas 3 (1996) 3129. ml P. Chen, I.M. Dawson, R.W. Huff, T. Katsouleas. Phys. Rev. Lett. 54 (1985) 693. [21] A. Willes, P.A. Robinson. D.B. Melrose, Phys. Plasmas 3 (1996) 149. [22] V.L. Ginzbug, V.V. Zhelezniakov, Sov. Astron. J. 2 (1958) 653. [23] J.-C. Gauthier. J.-P. Geindre, P. Audebert, S. Bastiani. C. Quoix. G. Grillon, A. Mysyrowicz, A. Antonetti, R.C. Mancini. Phys. Plasmas 4 (1997) 18 11. [24] D. Altenbemd, U. Teubner, P. Gibbon, E. FGrster, P. Audebet-t. J.P. Geindre, J.C. Gauthier, G. Grillon, A. Antonetti, Soft X-ray Brilliance of Femtosecond and Picosecond LaserPlasmas, J. Phys. B 30 (1997), in press.