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GENERATION OF XUV SPECTRA BY POWERFUL PICOSECOND LASER
C. Nam, W. Tighe, S. Suckewer, U. Feldman, J. Seely
To cite this version:
C. Nam, W. Tighe, S. Suckewer, U. Feldman, J. Seely. GENERATION OF XUV SPECTRA BY
POWERFUL PICOSECOND LASER. Journal de Physique Colloques, 1988, 49 (C1), pp.C1-203-C1-
206. �10.1051/jphyscol:1988139�. �jpa-00227459�
JOURNAL DE PHYSIQUE
C o l l o q u e C1, Suppl6ment a u n'3, Tome 49, Mars 1 9 8 8
GENERATION OF X W SPECTRA BY POWERFUL PICOSECOND LASER
C.H. NAM, W. TIGHE, S. SUCKEWER, U. FELDMAN* a n d J. SEELY' P r i n c e t o n U n i v e r s i t y , Plasma P h y s i c s L a b o r a t o r y , P r i n c e t o n . NJ 08544, U . S . A .
' ~ a v a l R e s e a r c h L a b o r a t o r y , Washington, DC 20375, U.S.A.
ABSTRACT
The development of laser action a t wavelengths shorter than those of current X-ray lasers is being investigated along two fronts. In the first case, w e are exploring the possibilities for laser action a t 15.4 nm In Li-like AlXI and 12.9 nm in Li-like SiXIl in a magnetically confined recombining plasma.
Previous work on hydrogen-like carbon, CVI, led to lasing action a t 18.2 nm.
Recently, this has been applied to microscopy and first results from a soft X-ray laser microscope a r e presented. A new technique to generate shorter wavelength X-ray lasing involves the interaction of a high power laser with a preformed plasma.. The Powerful Picosecond Laser (PP-~aser) System with a n output power level of 20-30 GW and focussed power density of loi6
-
1017 w/cm2 has recently become operational The spectra of highly ionized atoms in the XUV region were recorded on a high resolution erazing incidence spectrometer for the PP-Laser beam interacting with different solid targets.Extension of the operating range of soft X-ray lasers1i2 to shorter wavelengths is important for applications such a s microscopy of live biological specimens. Earlier work a t ~ r i n c e t o n ~ - ~ has resulted in a soft X-ray laser with the following characteristics. 18.2 nm, wavelength amplification
-
500;1-3 m J pulse energy; 10-30 nsec pulse duration and 5 mrad beam divergence.
This work has been extended to shorter wavelengths by obtaining a population inversion and gain on the 4f-3d transition for lithium-like ions, AlXI and SiXII.
A s part of the program in which multiphoton excitation is being considered a s a means to provide short wavelength X-ray lasing, w e present the first high-resolution XUV spectra obtained by the interaction of powerful picosecond laser pulses with solid targets. The targets were composed of elements with atomic numbers in the range 2=3 to 2=26 and were irradiated using a KrF*
laser with a pulse duration of 1.2 picosecond and focussed to an intensity of loi6 w/cm2. The spectra in the range of 6
A
to 370A
were recorded by a 3 meter grazing incidence spectrograph with a resolving power sufficient to observe the line profiles of transitions in highly-charged ions.EXPERIMENTAL: SETUP AND RESULTS
In the recombination scheme, the C02 laser is focussed onto an aluminum or silicon disc and creates a plasma column which is confined by a strong (- 50 kG) magnetic field. The maximum laser power density on target is 2 x loi3 w/cm2. The plasma cools rapidly after the laser pulse and fast three-body recombination, followed by cascading processes, provides a high 4f population while the 3d level decays rapidly by the 3d+2p radirtive transitions and in this w a y a 4f-3d population inversion and gain is built up. Maximum gain occurs in the high density, low temperature off-axis regions and a slot in the target disc transmits the axial stimulated emission.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1988139
Cl-204 JOURNAL DE PHYSIQUE
The axial emission is imaged by a grazing-incidence mirror onto the entrance slit of a multichannel soft X-ray spectrometer. A second spectrometer views the plasma transversely. A transverse scan of the axial spectrometer gives information on the relative divergence of the st~mulated emission compared to spontaneous emission lines. In lithium-like ions the 4d-3p and 4p-3s transitions have gA values significantly less than the 4f-3d transition. (g is the statistical weight of the upper level and A is the transition probability).
In the prescence of a n = 4 to m = 3 population inversion, gain is expected to be first apparent in the transition with the highest gA value. A convenient measure of this increase is the 4f-3d intensity relative to the 4d-3p and 4p-3s transitions. The measurements indicate a maximum gain length ( g ~ ) of gL = 3-4 a t 15.4 n m and gl,
=
1-2 at 12.9 n m (~i-like S ~ X I I ) . ~An important new avenue of research at Princeton is the application of the 18.2 n m soft X-ray laser to high resolution contact microscopy. The soft X-ray laser beam is collimated by a rudimentary toroidal mirror onto a biological specimen chamber in a contact microscope. The specimen is isolated from the laser vacuum system by a silicon nitride window, 1000 thick and, during the exposure, the shadow of the specimen is recorded in PMMA-MAA copolymer photoresist. The experiment is in an early stage but experiments a r e planned for the near future using live cells in which the resist will be viewed at high resolution using an electron microscope. Higher quality optics and more advanced microscopes a r e being designed and constructed
A s part of a larger system which is being constructed for the development of short wavelength X-ray lasers and which includes a C02 laser (1 k J energy, 10-50 nsec pulse duration), a powerful picosecond KrF* laser has been developed and used in initial target interaction e ~ ~ e r i m e n t s . ~ > ~ The master oscillator of the PP-laser system w a s a cavity-dumped dye laser tuned to a wavelength of ,648 nm. The dye laser used a hybrid mode-locking scheme and produced 1 psec pulses. The dye laser w a s pumped by the frequency-doubled output of a mode-locked YAG laser. The output of the dye laser w a s injected into a three-stage dye amplifier that w a s pumped by amplified, frequency-doubled pulses from the YAG laser. The amplified 648 nm pulses were frequency-doubled and, then, mixed with 1064 n m YAG pulses. The resulting 248 n m pulses w e r e injected into two KrF* amplifiers. The amplified pulses typically had a n energy of 20 to 25 mJ and a duration of 1 to 1.2 psec. The pulses w e r e focused by an f/5 spherical lens to a n intensity of approximately 1016 ~ / c m 2 ,
The laser pulses were incident on a rotating cylindrical target either composed or else coated with the material under study. The spectra were recorded on Kodak 101 photographic plates, and the spectra produced by up to 7000 laser shots, were integrated onto each exposure.
The spectrum from a solid aluminum target is shown in Fig. l a . Transitions (not shown) occurring in the low-density expansion plasma, have line widths consistent with the 30 mA instrumental broadening (resulting from the 10 pm entrance slit) and Doppler source broadening. The much larger widths of the Li-like n = 2
-
4 and n = 2 - 5 transitions shown in Fig. 1 a r e consistent with quasi-static ion Stark broadening at an electron density In the range 1021-
~ m - . ~ . The same transitions in Li-like fluorine from a teflon target a r e shown in Fig. lb. These line profiles a r e asymmetric and a r e up to 2A
in width.It is well known that the microfield due to the plasma can cause shifts of the energy levels of emitting ions.9 In dense plasmas, when the shifts become comparable to the energy level splittings, the wave functions of neighboring states become mixed, and this permits transitions that a r e not allowed in the absence of the microfield. The ratio of the intensities of the forbidden and allowed transitions increases with plasma density.
11.11111/111,IIlII,,
35 40 5 50 5 5 6 0
WAVELENGTH (I)
- --
90 1 0 0 110 120 130 1 4 0 1 %
WAVELENGTH a)
Figure I : High resolution XUV spectra. Figure Ial i s a r e s u l t from an a2uminz.m t a r g e t showing Li-like AIXI and Be-like AIX. Figure Ibl i s a r e s u l t from a t e f l o n target showing Li--like FVII and Be-ZikeFVI.
The line profiles w e r e modeled using a formalism developed for the calculation of spectral line profiles of multielectron ions in dense plasmas using the quasi-static ion approximation.1° A quantum mechanical relaxation theory w a s used to calculate the piasma electron broadening of the spectral lines. The calculated profile of the F6+ 2p-3d spectral feature showed that a t electron densities greater than 5 x ~ m - ~ , the 2p-3p forbidden component contributes significantly to the red wing of the spectral feature. This i s consistent with the enhanced red wing of the observed 2p-3d spectral feature shown in Fig. 2b.
Similarly, the red wing of the observed 2p-4d feature is enhanced by the 2p-4p forbidden component.
From numerical computations the line width and a s y m m e t r y of the observed
F6+
2p-3d spectral feature a r e consistent w i t h a n electron density of approximately ~ m - ~ . However, not all features show such agreement and cannot be explained in terms of density alone. It is possible that the electric field associated with the intense laser pulse m a y also be influencing the character of the spectral emission Nevertheless, it is of interest to note that the density implied from calculations is more comparable to the critical electron density (2 x ~ m - ~ ) for 248 n m laser radiation than the target density (4 x ~ m - ~ ) . Since the plasma expansion during the picosecond laser pulse is negligible, it seems likely that the observed emission occurs in the expanding plasma immediately after the laser pulse. Furthermore, the time for collisional ionization toF6'
is approximately 1 psec, and the time for n = 2-3 collisional excitation is less than a picosecond. The times for the radiative transitions 3p-2s and 3d-2p a r e 20 psec and 6 psec, respectively, and a r e much larger than the laser pulse duration. Thus it seems reasonable that the excited F6+ ions a r e formed during the picosecond laser pulse, and the bulk of the emission occurs after the laser pulse. The F5+ ionization energy is 160 eV and the F ~ + n = 2-3 excitation energy is 100 eV, and it is possible that multiphoton (nonresonance) processes, requiring a large number of 248 n m (5 eV) photons, contribute to the formation of excited F6+ during the laser pulse.Finally w e note that w i t h the iron target the presence of Na-like
el"
lines, representing the most highly charged ion roduced in plasmas created by a picosecond laser. The He-like ls2
-
ls2p Pi resonance line of aluminumP
a t 7.757
A
(1.6 keV excitation energy) w a s identified in the spectra from the aluminum targets (more details in Ref. 8).CI-206 JOURNAL
DE
PHYSIQUECONCLUSIONS AND FUTURE PLANS
We have demonstrated gain at 15.4 nm and 12.9 nm in a recombining X-ray laser scheme. We hope to achieve still shorter wavelength output by using higher Z elements (e.g. sulphur). Another area in which an intensive effort is being directed is the development of an X-ray laser cavity using soft X-ray mirrors with -, 40% reflectivity.
We have also obtained high resolution XUV spectra from the interaction of the powerful picosecond KrF laser system with several solid targets.
Extremely broad and asymmetric line profiles were observed along with high levels of ionization. Experiments to study the effect of a prepulse on the interaction and with new layered targets a r e beginning. The development of a final stage amplifier For the PP-laser, which will provide much higher power levels, is proceeding.
REFERENCES
1. S. Suckewer, C.A. Skinner, D. Kim, D. Voorhees, and A. Wouters, Phys.
Rev. Lett.
3,
1004 (1986).2. D. Matthews, M. Rosen, S. Brown, N. Ceglio, D. Eder, A. Hawryluk, C.
Keane, R. London, B. MacGowan, S. Maxon, D. Wilson, J. Scofield, and J . Trebes, J. Opt. Soc. Am. B & 575-587 (1987).
3. S. Suckewer, C.H. Skinner, H. Milchberg. C. Keane, and D. Voorhees, Phys. Rev. Lett. 1753-1756 (1985).
4. C.H. Nam, E. Valeo, S. Suckewer, and U. Feldman, J. Opt. Soc. Am. B 3, 1199 (1986).
5. C.H. Skinner and C. Keane, Appl. Phys. Lett. &, 1334 (1986).
6. C.H. Skinner, D. Kim, A. Wouters, D. Voorhees, and S. Suckewer, Proc.
of SPIE 31st Int. Tech, Sym., 831 36 (1987).
7. W.G. Tighe, C.H. Nam, S. Suckewer, and J . Robinson, to be published.
8. C.H. Nam, W.G. Tighe, S. Suckewer, J.F. Seely, U. Feldman, and L.A.
Woltz, Phys. Rev. Lett. 2427 (1987).
9. H.R. Griem, Academic Press, New York, (1974).
10. L.A. Woltz and C.P. Hooper, to be published in Phys. Rev. A. (1987).
